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Background: Resistance to venetoclax, a BCL2 inhibitor, has been shown to be mediated by co-expression of other BCL2 proteins including BCLXL and MCL1 in relapsed/refractory AML (R/R AML). Therapeutic agents reducing resistance to BCL2 inhibitors and exhibiting anti-tumor activity could synergize with venetoclax to increase response rates further. One approach to enhance the potency of venetoclax is through combination with targeted radiotherapy. Studies in tumor cells have shown that DNA damage reduces the level of MCL1, a known mediator of venetoclax resistance. The monoclonal antibody radioconjugate, lintuzumab-Ac225, is a highly cytotoxic alpha-radiation emitter that selectively targets CD33, a cell surface antigen expressed on majority of AML cells. The high-energy alpha-particle emissions from lintuzumab-Ac225 as a single agent elicits single and double-strand DNA breaks in targeted tumor cells as demonstrated from prior clinical studies. (ASH 2017, 2018). In venetoclax-resistant cell lines in vitro, lintuzumab-Ac225 promotes MCL1 degradation through DNA damage resulting in increased cell sensitivity to venetoclax. Similarly, mouse xenograft models with venetoclax-resistant AML tumor lines demonstrated tumor regression and increased survival in mice receiving both venetoclax and lintuzumab-Ac225. The aims of this phase I/II study are to assess the safety, tolerability and efficacy of lintuzumab-Ac225 in combination with venetoclax in R/R AML. Study Design: The Phase I portion of the study uses a 3+3 dose-escalation design to determine the maximum tolerated dose (MTD) of lintuzumab-Ac225 when given in combination with venetoclax. The dose levels for lintuzumab-Ac225 are 0.5, 0.75, and 1.0 µCi/kg. The Phase II portion of the study will enroll up to an additional 20 patients and treat with the recommended phase II dose (RP2D) with venetoclax to determine the best overall response (CR+CRh+CRi) up to 6 months after starting treatment. Eligible patients include R/R-AML patients aged 18 years and older with adequate organ function, ECOG Performance Status 0-2, and more than 25% CD33 positive of leukemic blasts by flow cytometry. Patients with antecedent MDS, MPNs, or therapy-related AML are eligible. During Cycle 1, venetoclax dosing will be ramped up during the first 4 days in order to minimize the risk of tumor lysis syndrome (TLS). After Cycle 1, all patients will receive venetoclax at 400 mg/day PO on Days 1 to 21 of each cycle. Lintuzumab-Ac225 is administered as a single dose on Day 5 of each cycle. Results: Ten R/R AML patients were treated with lintuzumab-Ac225 at 0.5 µCi/kg (n=3), 0.75 µCi/kg (n=6) and 1.0 µCi/kg (n=1) in combination with venetoclax in the phase I dose escalation portion as of 1-August-2021. The median age was 67 years (range 49-83) with 5/10 (50%) refractory; 8/10 (80%) had unfavorable risk (ELN). Three patients were enrolled on the first cohort at dose level 0.5 µCi/kg of lintuzumab-Ac225 and showed clinically acceptable safety profile with no DLTs. In addition, one patient achieved CRi at the end of cycle 1. Results from the first cohort were encouraging and acceptable for dose escalation. An intermediate Cohort 2B at dose level 0.75 µCi/kg, was implemented. To date, 6 patients were enrolled on cohort 2B, and based on the Safety Committee recommendation, dose level at 1.0 µCi/kg may be resumed. In preliminary data on all patients, lintuzumab-Ac225 treatment-related grades 3/4 adverse events (AEs) include hematologic AEs and only one count of non-hematologic event (hyponatremia) (Table 1). There were no early deaths (≤30d) in any cohort. Overall, lintuzumab-Ac225 can be combined with venetoclax with a manageable toxicity profile. While sample size is limited, ORR (CR/CRi) for all R/R AML patients and TP53-mutant R/R patients were 20% (2/10) and 67% (2/3), respectively. Updated safety and response data will be presented at ASH2021. Conclusion: Combining lintuzumab-Ac225 with venetoclax in patients with R/R AML has an acceptable clinical safety profile with 0% 30-day mortality rate. 67% ORR is particularly encouraging in TP53-mutant R/R AML. These data support the further evaluation of lintuzumab-Ac225 in combination with venetoclax in patients with R/R AML, especially in TP53-mutant R/R patients. Figure 1 Figure 1. Disclosures Schiller: FujiFilm: Research Funding; Forma: Research Funding; Arog: Research Funding; Gamida Cell Ltd.: Research Funding; Karyopharm: Research Funding; Kite/Gilead: Honoraria, Research Funding, Speakers Bureau; Geron: Research Funding; Mateon: Research Funding; Onconova: Research Funding; Pfizer: Current equity holder in publicly-traded company, Research Funding; PrECOG: Research Funding; Regimmune: Research Funding; Delta-Fly: Research Funding; Actinium Pharmaceuticals, Inc: Research Funding; Actuate: Research Funding; Samus: Research Funding; Constellation Pharmaceuticals: Research Funding; Deciphera: Research Funding; Incyte: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Abbvie: Research Funding; Takeda: Research Funding; Stemline Therapeutics, Inc.: Honoraria, Research Funding, Speakers Bureau; Sangamo: Research Funding; Genentech-Roche: Research Funding; Celator: Research Funding; BMS/Celgene: Consultancy, Current equity holder in publicly-traded company, Research Funding, Speakers Bureau; Astellas: Honoraria, Research Funding, Speakers Bureau; Trovagene: Research Funding; Tolero: Research Funding; Daiichi-Sankyo: Research Funding; Agios: Consultancy, Research Funding, Speakers Bureau; Amgen: Consultancy, Current equity holder in publicly-traded company, Honoraria, Research Funding, Speakers Bureau; Jazz: Consultancy, Honoraria, Research Funding, Speakers Bureau; Elevate: Research Funding; Bio: Research Funding; Ono-UK: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Sanofi: Honoraria, Research Funding, Speakers Bureau; Pharma: Consultancy; Johnson & Johnson: Current equity holder in publicly-traded company; Biomed Valley Discoveries: Research Funding; Eli Lilly: Research Funding; ASH foundation: Other: Chair-unpaid; Sellas: Research Funding; Ono: Consultancy; Incyte: Consultancy; Ariad: Research Funding; AstraZeneca: Consultancy; Kaiser Permanente: Consultancy; Cyclacel: Research Funding; MedImmune: Research Funding; Ambit: Research Funding; Leukemia & Lymphoma Society: Research Funding; Bluebird Bio: Research Funding; Boehringer-Ingleheim: Research Funding; Cellerant: Research Funding; CTI Biopharma: Research Funding; Janssen: Research Funding; Kura Oncology: Research Funding; Pharmacyclics: Honoraria, Speakers Bureau; Millennium: Research Funding; National Marrow Donor Program: Research Funding; NIH: Research Funding; Onyx: Research Funding; Pharmamar: Research Funding; UC Davis: Research Funding; UCSD: Research Funding; Evidera: Consultancy; NCI: Consultancy; Novartis: Speakers Bureau. Finn: Jazz: Consultancy, Speakers Bureau; BMS: Consultancy, Speakers Bureau; BeiGene: Consultancy, Speakers Bureau; ADC Therapeutics: Consultancy, Speakers Bureau. Roboz: Mesoblast: Consultancy; Jazz: Consultancy; Astellas: Consultancy; Janssen: Consultancy; Actinium: Consultancy; Blueprint Medicines: Consultancy; Jasper Therapeutics: Consultancy; Daiichi Sankyo: Consultancy; Celgene: Consultancy; Glaxo SmithKline: Consultancy; Bayer: Consultancy; AbbVie: Consultancy; AstraZeneca: Consultancy; Amgen: Consultancy; Astex: Consultancy; Helsinn: Consultancy; MEI Pharma - IDMC Chair: Consultancy; Agios: Consultancy; Novartis: Consultancy; Bristol Myers Squibb: Consultancy; Otsuka: Consultancy; Janssen: Research Funding; Pfizer: Consultancy; Roche/Genentech: Consultancy. Orozco: Actinium Pharmaceuticals, Inc.: Other: site PI for clinical trial(s) sponsored by Actinium, Research Funding. Lin: Actinium Pharmaceuticals, Inc.: Current Employment. Chen: Actinium Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. OffLabel Disclosure: Venetoclax use in relapsed-refractory AML

CD33 is a transmembrane protein that is found on cells of myeloid lineage. It is also intensely expressed on acute myeloid leukemia (AML) progenitor cells but not on normal stem cells. It internalizes on binding and dimerization, making it a specific and ideal target for AML therapeutics and drug delivery. Several targeted therapies have been tested and many are still currently in development. Gemtuzumab ozogamicin was the first and only CD33-directed antibody-drug conjugate to be US Food and Drug Administration approved for AML. Other targeted agents have not achieved such success. Promising new strategies include cellular therapy mechanisms and linker molecules. This is an exciting target that requires a considerable amount of precision to yield clinical benefit.

Background: Lintuzumab-Ac225 (Actimab-A) is a humanized CD33 antibody, lintuzumab, conjugated to an alpha emitting isotope, actinium (Ac225), with potent single agent activity against AML. We hypothesized that a low dose infusion of lintuzumab-Ac225, after salvage chemotherapy, would effectively eliminate residual leukemia and improve remission rates, and depth of remission. Patients and Methods: Adult patients with relapsed/refractory AML (RR-AML), and with adequate organ function and performance status were eligible for screening. On screening, we assessed CD33 expression, greater than 25% of leukemic blasts must have expressed CD33 antigen by flow cytometry for inclusion. Induction consisted of G-CSF, 300mcg/d, given D1-6, cladribine 5mg/m2, given D2-6, cytarabine 2g/m2, given D2-6, and mitoxantrone 10mg/m2, given D2-4. Lintuzumab-Ac225 was administered as a single dose on either day 7, 8, or 9. This trial enrolled to 4 cohorts, administering lintuzumab-ac225 at a dose of 0.25uCi/kg, 0.50uCi/kg, 0.75uCi/kg, and 1.0uCi/kg. Treatment consisted of one induction cycle. Disease assessment, including MRD assessment by flow, was performed between Days 29-42. Results: Sixteen patients with a median age of 64 yrs were evaluable for toxicity and response. Three pts were treated in cohort 1 (0.25uCi/kg), nine pts were treated in cohort 2 (0.5uCi/kg), three pts were treated in cohort 3 (0.75uCi/kg), and one pt was treated in cohort 4 (1.0uCi/kg). Patient characteristics include 63% (n=10) pts with adverse cytogenetics, mutations involving TP53 in 44% (n=7) pts, and secondary/therapy-related AML in 50% (n=8) of patients. Patients received a median 2 lines of therapy, and 50% (n=8) received a venetoclax combination regimen prior to enrollment. Table 1 summarizes patient characteristics. No undue non-hematologic toxicities were observed in this trial. Commonest AEs were neutropenic fever and infection. Among responders, an ANC>1000 was achieved in 9/10 patients, at a median of 33 days, and a platelet count >50k was achieved in 6/10 patients at a median of 37 days. Of note, two patients proceeded to HCT prior to platelet recovery. Overall, among patients enrolled in cohorts 1-3, CR/CRi was observed in 67% (n=10) pts. One patient enrolled in cohort 4 achieved an aplastic marrow by D+42, with subsequent stem cell boost and neutrophil recovery. Of responders, 70% (7/10) had no measurable disease (MRD(-)) by flow cytometry. Among patients receiving Conclusions: Lintuzumab-Ac225 at a low dose appears to be safe in combination with the salvage chemotherapy regimen CLAG-M. Furthermore, this combination has demonstrated promising efficacy results among high risk RR-AML patients, namely patients with adverse molecular/cytogenetic features and patients previously receiving venetoclax. Updated Cohort 4 results will be presented at ASH. Upon determination of MTD, we plan to explore the combination of lintuzumab-Ac225 with other salvage regimens (e.g. FLAG, FLAG-Ida, MEC) to determine whether adding this novel agent can generally improve salvage outcomes in hard to treat patients. Figure 1 Figure 1. Disclosures Abedin: Helsinn: Research Funding; Agios: Honoraria; AltruBio: Research Funding; Pfizer: Research Funding; Amgen: Honoraria; Actinium: Research Funding; Astellas Pharma Inc.: Research Funding. Guru Murthy: Guidepoint: Consultancy; Cancerexpertnow: Honoraria; Techspert: Consultancy; Qessential: Consultancy; Cardinal Health Inc.: Honoraria; TG therapeutics: Other: Advisory board. Hamadani: Sanofi, Genzyme, AstraZeneca, BeiGene: Speakers Bureau; Takeda, Spectrum Pharmaceuticals and Astellas Pharma: Research Funding; Janssen, Incyte, ADC Therapeutics, Omeros, Morphosys, Kite: Consultancy. Atallah: Pfizer: Consultancy, Research Funding; BMS: Honoraria, Speakers Bureau; Takeda: Consultancy, Research Funding; Abbvie: Consultancy, Speakers Bureau; Amgen: Consultancy; Novartis: Consultancy, Honoraria, Research Funding.

Background: Lintuzumab Ac225 is a radiolabeled anti-CD33 antibody, composed of an alpha emitting isotope, Ac225, conjugated with the humanized anti-CD33 monoclonal antibody lintuzumab. As monotherapy, lintuzumab Ac225 was studied as upfront therapy for unfit AML, and nearly 70% of pts achieved a CR/CRi at the highest dose level (2uCi/kg) without significant non-hematologic toxicity. Therefore, we hypothesized that lintuzumab Ac225, when added to the salvage chemotherapy regimen CLAG-M (cladribine, cytarabine, G-CSF, and mitoxantrone), would be tolerable and would improve remission rates in the treatment of relapsed/refractory AML (RR-AML). This investigator-initiated phase I study is the first study to combine radioimmunotherapy with salvage chemotherapy in pts with RR-AML. Patients and Methods: Medically fit pts with RR-AML, aged 18 years and older were eligible. Eligibility also required that more than 25% of leukemic blasts must have been CD33 positive by flow cytometry. Induction consisted of G-CSF, 300mcg/d, given D1-6, cladribine 5mg/m2, given D2-6, cytarabine 2g/m2, given D2-6, and mitoxantrone 10mg/m2, given D2-4. Lintuzumab Ac225 was administered as a single dose on either day 7, 8, or 9. This trial had three cohorts, administering lintuzumab Ac225 at a dose of 0.25uCi/kg, 0.50uCi/kg, or 0.75uCi/kg. Treatment consisted of one induction cycle, with subsequent therapy up to physician discretion. Results: Fifteen pts were evaluable, with a median age of 61 yrs (Figure 1). Slightly less than half of the pts had previously received venetoclax/HMA and slightly more than half had previously had allogeneic HCT. Three pts enrolled into cohort 1 (0.25uCi/kg), nine pts enrolled into cohort 2 (0.5uCi/kg), and three pts enrolled into cohort 3 (0.75uCi/kg). Grade 3 or greater AEs irrespective of causality included febrile neutropenia (n=12), infection (n=8), maculopapular rash (n=2), nausea (n=2), and one patient each with QTc prolongation and tumor lysis syndrome. Of 15 pts, CR/CRi was observed in 10 (67%) pts. Excluding pts receiving >3 prior lines of therapy, 10/12 (83%) achieved CR/CRi. In cohort 1, 1 patient (33%) achieved remission (1 CR). In cohort 2, 6 pts (67%) achieved remission (3 CR, 2 CRp, 1 CRi). In cohort 3, 3 pts (100%) achieved remission (1 CR, 2 CRp). 7 pts of the 10 with CR/CRi (70%) achieved MRD negativity by flow (Table 2). Two of the 3 patients in cohort 3 were MRD- after therapy and the third had a small number of AML cells detected (0.2%). All pts who were independent for platelet transfusions pre-treatment recovered platelets. Overall, median time to ANC recovery ≥500 was 33 days, and median time to platelet recovery ≥50k was 35 days. 6 pts proceeded to allogeneic HCT after therapy. Conclusion: We conclude that lintuzumab Ac225 in combination with CLAG-M chemotherapy has a clinically acceptable safety profile. Dose escalation yielded highly encouraging efficacy results for RR-AML. With acceptable safety at 0.75uCi/kg, we have amended this protocol to study a 4th dose level at 1.0uCi/kg. Overall, this regimen represents a safe and potentially effective therapy for medically fit RR-AML pts, particularly as a bridge to allogeneic HCT. Disclosures Abedin: Helsinn Healthcare: Honoraria; Actinium Pharmaceuticals: Research Funding; Helsinn Healthcare: Research Funding; Pfizer: Research Funding; Jazz Pharmaceuticals: Honoraria; Agios: Honoraria. Hamadani:Takeda Pharmaceutical Company; Spectrum Pharmaceuticals; Astellas Pharma: Research Funding; ADC Therapeutics: Membership on an entity's Board of Directors or advisory committees; Janssen R&D; Incyte Corporation; ADC Therapeutics; Celgene Corporation; Pharmacyclics, Omeros, AbGenomics, Verastem, TeneoBio: Consultancy; Sanofi Genzyme, AstraZeneca: Speakers Bureau. Michaelis:Jazz Pharmaceuticals: Research Funding. Atallah:Abbvie: Consultancy; Genentech: Consultancy; Jazz: Consultancy; Pfizer: Consultancy; Takeda: Consultancy, Research Funding; Novartis Pharmaceutical Corporation: Consultancy.

The short range and high linear energy transfer of α-particles offer the potential for efficient tumor killing while sparing normal bystander cells. Hematologic malignancies are ideally suited to targeted α-particle therapy (TAT) due to easy accessibility of malignant cells in blood, bone marrow, lymph nodes, and spleen as well as their radiosensitivity. Most clinical trials using α-particle therapy for hematologic malignancies have focused on acute myeloid leukemia (AML); however, preclinical studies have shown activity against other diseases such as non-Hodgkin's lymphoma and multiple myeloma. To date, the short-lived radionuclide bismuth-213 (213Bi) and its parent actinium-225 (225Ac) have been used clinically, but trials with astatinie-211 (211At) have recently begun, and thorium-227 (227Th) has shown promising preclinical results. Lintuzumab is a humanized monoclonal antibody that targets the cell surface antigen CD33, which is expressed on the vast majority of AML cells. Initial studies showed that 213Bi-labeled lintuzumab had antileukemic activity and could produce remissions after partial cytoreduction with cytarabine. An initial phase I trial demonstrated that a single infusion of 225Ac-lintuzumab could be given safely at doses upto 111 kBq/kg with antileukemic activity across all dose levels. A second phase I study showed that fractionated-dose 225Ac-lintuzumab could be safely combined with low-dose cytarabine and produced objective responses in 28% of older patients with untreated AML. In a phase II study, treatment with 225Ac-lintuzumab monotherapy for a similar patient population resulted in remission in 69% of patients receiving two fractions of 74 kBq/kg and 22% of patients receiving two 55.5-kBq/kg fractions. Additionally, TAT may be useful in intensifying antileukemic therapy prior to hematopoietic cell transplantation, and pretargeting strategies offer the possibility for improved tumor-to-normal organ dose ratios.

The clinical efficacy of the first approved alpha pharmaceutical, Xofigo (radium-223 dichloride, 223RaCl2), has stimulated significant interest in the development of new alpha-particle emitting drugs in oncology. Unlike radium-223 (223Ra), the parent radionuclide thorium-227 (227Th) is able to form highly stable chelator complexes and is therefore amenable to targeted radioimmunotherapy. We describe the preparation and use of a CD33-targeted thorium-227 conjugate (CD33-TTC), which binds to the sialic acid receptor CD33 for the treatment of acute myeloid leukemia (AML). A chelator was conjugated to the CD33-targeting antibody lintuzumab via amide bonds, enabling radiolabeling with the alpha-emitter 227Th. The CD33-TTC induced in vitro cytotoxicity on CD33-positive cells, independent of multiple drug resistance (MDR) phenotype. After exposure to CD33-TTC, cells accumulated DNA double-strand breaks and were arrested in the G2 phase of the cell cycle. In vivo, the CD33-TTC demonstrated antitumor activity in a subcutaneous xenograft mouse model using HL-60 cells at a single dose regimen. Dose-dependent significant survival benefit was further demonstrated in a disseminated mouse tumor model after single dose injection or administered as a fractionated dose. The data presented support the further development of the CD33-TTC as a novel alpha pharmaceutical for the treatment of AML. Mol Cancer Ther; 15(10); 2422–31. ©2016 AACR.

Context Resistance to venetoclax, a BCL2 inhibitor, has been shown to be mediated by co-expression of other BCL2 proteins, including BCLXL and MCL1. Therapeutic agents that reduce resistance to BCL2 inhibitors and themselves exhibit anti-tumor activity may synergize with venetoclax to increase response rates. One approach to enhance the potency of venetoclax is through combination with targeted radiotherapy. Studies have shown that DNA damage reduces the level of MCL1, a known mediator of venetoclax resistance, in tumor cells. The monoclonal antibody radioconjugate lintuzumab-Ac225 is a highly cytotoxic alpha-radiation emitter that selectively targets CD33, a cell surface antigen expressed on the majority of AML cells. The high-energy alpha-particle emissions from lintuzumab-Ac225 elicit single and double-strand DNA breaks in targeted tumor cells, and prior clinical studies have demonstrated single-agent anti-leukemic activity. In venetoclax-resistant cell lines in vitro, lintuzumab-Ac225 promotes MCL1 degradation through DNA damage, resulting in increased cell sensitivity to venetoclax. Similarly, mouse xenograft models with venetoclax-resistant AML tumor lines demonstrated tumor regression and increased survival in mice receiving both venetoclax and lintuzumab-Ac225. Objective Determine the maximum tolerated dose (MTD) of lintuzumab-Ac225 when given in combination with venetoclax; Establish preliminary efficacy at MTD. Design Multicenter, open-label, dose-finding Phase I study followed by a Phase II trial. Setting Ongoing in the USA. Patients Patients aged >18 years with relapsed/refractory AML. Interventions Lintuzumab-Ac225 in combination with venetoclax. Main outcome measures Phase I: Assess dose-limiting toxicities (DLTs) to establish the MTD; Phase II: Determine the best overall response (CR+CRh) of the MTD 6 months after starting treatment. Results and Conclusions Ongoing study, no results available yet.

The CD33 single-nucleotide polymorphism (SNP) rs3865444 has been associated with the risk of Alzheimer's disease (AD). Rs3865444 is in linkage disequilibrium with rs12459419 which has been associated with efficacy of an acute myeloid leukemia (AML) chemotherapeutic agent based on a CD33 antibody. We seek to evaluate the extent to which CD33 genetics in AD and AML can inform one another and advance human disease therapy. We have previously shown that these SNPs are associated with skipping of CD33 exon 2 in brain mRNA. Here, we report that these CD33 SNPs are associated with exon 2 skipping in leukocytes from AML patients and with a novel CD33 splice variant that retains CD33 intron 1. Each copy of the minor rs12459419T allele decreases prototypic full-length CD33 expression by ∼ 25% and decreases the AD odds ratio by ∼ 0.10. These results suggest that CD33 antagonists may be useful in reducing AD risk. CD33 inhibitors may include humanized CD33 antibodies such as lintuzumab which was safe but ineffective in AML clinical trials. Here, we report that lintuzumab downregulates cell-surface CD33 by 80% in phorbol-ester differentiated U937 cells, at concentrations as low as 10 ng/ml. Overall, we propose a model wherein a modest effect on RNA splicing is sufficient to mediate the CD33 association with AD risk and suggest the potential for an anti-CD33 antibody as an AD-relevant pharmacologic agent.

Objectives Due to the shorter range and higher linear energy transfer of α-particles compared to β-particles, targeted α-particle therapy may produce more efficient tumor killing while sparing neighboring healthy cells. We will review the clinical studies using α-particle therapy for Acute Myeloid Leukemia (AML). Methods A series of clinical trials were conducted to assess the safety, feasibility, and anti-leukemic effects of lintuzumab, an anti-CD33 humanized monoclonal antibody, labeled with the α-emitters bismuth- 213 (213Bi) and actinium-225 (225Ac). Results An initial phase I study conducted in 18 patients with relapsed or refractory AML demonstrated the safety and antitumor effects of 213Bi-lintuzumab. Subsequently, 213Bi-lintuzumab produced remissions in AML patients after partial cytoreduction with cytarabine in phase I/II trial. The 46- minute half-life of 213Bi and need for an onsite generator has limited its utility. Therefore, a secondgeneration construct was developed using 225Ac, a radiometal that yields four α-particle emissions. A phase I trial demonstrated that a single infusion of 225Ac-lintuzumab could be given safely at doses up to 111 kBq/kg with anti-leukemic activity across all dose levels studied. In a second phase I study, 28% of older patients with untreated AML had objective responses after receiving fractionated-dose 225Aclintuzumab and low-dose cytarabine. Conclusion Based upon the encouraging results seen in phase I trials of 225Ac-lintuzumab, a phase II study of 225Ac-lintuzumab monotherapy for older patients with untreated AML is now in progress and is also being studied in a subset of patients with CD33-positive multiple myeloma.

Background: Lintuzumab Ac225 is a radiolabeled anti-CD33 antibody with demonstrated single-agent activity in AML. The drug is composed of an alpha emitting isotope, Ac225, conjugated with the humanized anti-CD33 monoclonal antibody lintuzumab. In a prior study, lintuzumab Ac225 monotherapy was safely administered to elderly/unfit AML patients with toxicities primarily limited to prolonged myelosuppression. Nearly 70% of patients achieved a CR/CRi at the highest dose level (2uCi/kg). We hypothesized that lintuzumab Ac225, when added to the salvage chemotherapy regimen CLAG-M (cladribine, cytarabine, G-CSF, and mitoxantrone), would be tolerable and would improve remission rates in the treatment of relapsed/refractory AML (RR-AML). CLAG-M salvage was selected based on favorable institutional outcomes (CR/CRi: 54%, Mushtaq et al, ASH 2018). This novel investigator-initiated phase I study is the first study to combine radioimmunotherapy and intensive chemotherapy in patients with RR-AML. Patients and Methods: Eligible patients include medically fit, RR-AML patients aged 18 years and older, with adequate organ function. In addition, more than 25% of leukemic blasts must have been CD33 positive by flow cytometry. Induction consisted of G-CSF, 300mcg/d, given D1-6, cladribine 5mg/m2, given D2-6, cytarabine 2g/m2, given D2-6, and mitoxantrone 10mg/m2, given D2-4. Lintuzumab Ac225 was administered as a single dose on either days 7, 8, or 9. Cohort 1 received lintuzumab Ac225 at a dose of 0.25uCi/kg, and cohort 2 received a dose of 0.50uCi/kg. Only one induction course was administered, and subsequent treatments were up to physician discretion. Results: Nine patients with a median age of 59 years (range 47-73 yrs) have been enrolled. Patients had a median of 2 (range 1-4) prior, anti-leukemic treatments, including 4 patients who relapsed following allogeneic HCT. Four patients (44%) had intermediate risk cytogenetics, and five patients (56%) had adverse risk features. Median blast CD33 expression was 73% (range 32-100%). All patients completed one cycle of lintuzumab Ac225 with CLAG-M. Three patients enrolled into cohort 1 (0.25mcg/kg), and six patients enrolled into cohort 2 (0.5mcg/kg). Among all enrolled patients, Grade 3 or greater AEs include febrile neutropenia (n=4), infection (n=4), QTc prolongation (n=1), hypophosphatemia (n=1), hyponatremia (n=1), and tumor lysis syndrome (n=1). In two responding patients enrolled to cohort 2, one patient had prolonged time to neutrophil recovery (ANC>500 at 53 days), and a second patient, who was post-HCT, had an ANC recovery to 300 by day 42, and then received donor CD34+ stem cells to aide in count recovery. Figure 1 summarizes toxicities. All responding patients with a platelet count>50k prior to therapy, had a post-treatment platelet count >50k within 42 days. No mortalities were observed on study. Among cohort 1 patients (n=3), one patient achieved CR, one patient had a >50% blast reduction to 6.2% blasts with neutrophil recovery and platelet improvement to 70k, and the third patient had no response. Overall, 33% (1/3) achieved a CR. Among patients enrolled into cohort 2, two patients achieved CR, two patients achieved CRp, one patient achieved CRi, and one patient had no response. Overall, 83% (5/6), achieved CR/CRp/CRi. After, three patients on study subsequently went onto allogeneic HCT, two patients initiated donor lymphocyte infusions (DLI), and one patient underwent CD34+ stem cell boost. Conclusion: Combining lower doses of lintuzumab Ac225 with salvage CLAG-M chemotherapy appears to have a clinically acceptable safety profile. With dose escalation, increased myelosuppression has been noted alongside, but also highly encouraging efficacy results for RR-AML. As two patients in Cohort 2 met DLT criteria due to prolonged ANC recovery, this study will be amended to mandate the addition of G-CSF two weeks after Lintuzumab Ac225 treatment. Tentatively, this study will plan to extend the ANC recovery observation period to 60 days. Depending on safety and further efficacy data, this may lead to further dose escalation, or potentially an expanded number of patients in a Phase 2 study. Disclosures Abedin: Actinium Pharmaceuticals: Research Funding; Jazz Pharmaceuticals: Honoraria; Agios: Honoraria; Helsinn Healthcare: Research Funding; Pfizer Inc: Research Funding. Runaas:Agios: Honoraria; Blueprint Medicine: Honoraria. Michaelis:Incyte: Consultancy, Research Funding; Novartis: Consultancy; BMS: Research Funding; TG Therapeutics: Consultancy, Research Funding; JAZZ: Other: Data Safety Monitoring Board, uncompensated, Research Funding; Macrogeneics: Research Funding; Millenium: Research Funding; ASTEX: Research Funding; Pfizer: Equity Ownership, Research Funding; Celgene: Consultancy, Research Funding; Bioline: Research Funding; Janssen: Research Funding. Atallah:Helsinn: Consultancy; Jazz: Consultancy; Takeda: Consultancy, Research Funding; Pfizer: Consultancy; Jazz: Consultancy; Helsinn: Consultancy; Novartis: Consultancy. Hamadani:Celgene: Consultancy; Merck: Research Funding; Sanofi Genzyme: Research Funding, Speakers Bureau; Otsuka: Research Funding; Janssen: Consultancy; Medimmune: Consultancy, Research Funding; ADC Therapeutics: Consultancy, Research Funding; Takeda: Research Funding; Pharmacyclics: Consultancy.

Acute myeloid leukemia (AML) is the most common leukemia among adults and is associated with a poor prognosis, especially in patients with adverse prognostic factors, older age, or relapsed disease. The last decade has seen a surge in successful immune-based therapies in various solid tumors; however, the role of immune therapies in AML remains poorly defined. This chapter describes the rationale, clinical data, and toxicity profiles of immune-based therapeutic modalities in AML including naked and conjugated monoclonal antibodies, bispecific T-cell engager antibodies, chimeric antigen receptor (CAR)-T cells, and checkpoint blockade via blockade of PD1/PDL1 or CTLA4. Monoclonal antibodies commonly used in AML therapy target highly expressed “leukemia” surface antigens and include (1) naked antibodies against common myeloid markers such as anti-CD33 (e.g., lintuzumab), (2) antibody-drug conjugates linked to either, (a) a highly potent toxin such as calicheamicin, pyrrolobenzodiazepine, maytansine, or others in various anti-CD33 (gemtuzumab ozogamicin, SGN 33A), anti-123 (SL-401), and anti-CD56 (lorvotuzumab mertansine) formulations, or (b) radioactive particles, such as 131I, 213Bi, or 225Ac-labeled anti-CD33 or CD45 antibodies. Novel monoclonal antibodies that recruit and promote proximity-induced cytotoxicity of tumor cells by T cells (bispecific T-cell engager [BiTE] such as anti CD33/CD3, e.g., AMG 330) or block immune checkpoint pathways such as CTLA4 (e.g., ipilimumab) or PD1/PD-L1 (e.g., nivolumab) unleashing the patients T cells to fight leukemic cells are being evaluated in clinical trials in patients with AML. The numerous ongoing clinical trials with immunotherapies in AML will improve our understanding of the biology of AML and allow us to determine the best approaches to immunotherapy in AML.

Lintuzumab, a humanized anti-CD33 antibody, targets myeloid leukemia cells and has modest activity against acute myeloid leukemia (AML). To increase the antibody's potency yet avoid nonspecific cytotoxicity seen with β-emitting isotopes, lintuzumab was conjugated to the α-emitters bismuth-213 (213Bi) and actinium-225 (225Ac). The 46-minute half-life of 213Bi limits its widespread use. Therefore, 225Ac was also conjugated to various antibodies using DOTA-SCN. We conducted a phase I trial of 213Bi-lintuzumab and subsequently administered cytarabine with 213Bi-lintuzumab in a phase I/II study. The toxicity and biological activity of 225Ac-linutuzumab in patients with relapsed/refractory AML in a phase I dose-escalation trial was determined. An initial phase I trial demonstrated the feasibility, safety and antileukemic activity of 213Bi-lintuzumab. 213Bi-lintuzumab produced responses in 24% of AML patients receiving doses ≥ 37 MBq/kg after partial cytoreduction with cytarabine. 225Ac-labeled immunoconjugates killed in vitro at doses at least 1,000 times lower than 213Bi analogs. Eighteen patients with relapsed/refractory AML received 18.5 to 148 kBq/kg of 225Ac-lintuzumab in a phase I study. Dose-limiting toxicities were myelosuppression lasting >35 days in one patient and death due to sepsis in two patients. The maximum tolerated dose (MTD) was 111 KBg/kg. Bone marrow blast reductions were seen across all dose levels. Targeted α- particle immunotherapy with 213Bi- and 225Ac-lintuzumab is safe, has significant antileukemic effects, and can produce remissions after partial cytoreduction. How to cite this article Jurcic JG. Targeted Alpha-Particle Immunotherapy with Bismuth-213 and Actinium-225 for Acute Myeloid Leukemia. J Postgrad Med Edu Res 2013;47(1): 14-17.

Purpose: Leukemias with MLL gene rearrangement are associated with a poor prognosis. Natural killer (NK) cell therapy is a potential treatment, but leukemia cells may be resistant. Here, we sought to determine the susceptibility of MLL-rearranged leukemia cells to NK cell lysis and to develop a novel immunotherapeutic approach to optimize NK cell therapy, including the use of an antibody against leukemia-associated antigen and the elimination of killer-cell immunoglobulin-like receptor (KIR)–mediated inhibition. Experimental Design: Three MLL-rearranged leukemia cell lines (RS4;11, SEM, and MV4-11) and primary leukemia blasts were assessed for surface phenotype and susceptibility to NK cell lysis with or without antibodies against CD19 (XmAb5574), CD33 (lintuzumab), or KIR ligands. Results: All three cell lines were resistant to NK cell lysis, had some inhibitory KIR ligands and protease inhibitor-9, and expressed low levels of NKG2D activating ligands and adhesion molecules. After treatment with XmAb5574 or lintuzumab, MLL-rearranged leukemia cells were efficiently killed by NK cells. The addition of pan–major histocompatibility complex class I antibody, which blocked inhibitory KIR-HLA interaction, further augmented degranulation in all three KIR2DL1, KIR2DL2/3, and KIR3DL1 subsets of NK cells based on the rule of missing-self recognition. A mouse model showed a decreased rate of leukemia progression in vivo as monitored by bioluminescence imaging and longer survival after antibody treatment. Conclusion: Our data support the use of a triple immunotherapy approach, including an antibody directed against tumor-associated antigen, KIR-mismatched NK cell transplantation, and inhibitory KIR blockade, for the treatment of NK cell–resistant MLL-rearranged leukemias. Clin Cancer Res; 18(22); 6296–305. ©2012 AACR.

CD33, a 67-kDa glycoprotein expressed on the majority of myeloid leukemia cells as well as on normal myeloid and monocytic precursors, has been an attractive target for monoclonal antibody (mAb)-based therapy of acute myeloid leukemia (AML). Lintuzumab, an unconjugated, humanized anti-CD33 mAb, has modest single-agent activity against AML but failed to improve patient outcomes in two randomized trials when combined with conventional chemotherapy. Gemtuzumab ozogamicin, an anti-CD33 mAb conjugated to the antitumor antibiotic calicheamicin, improved survival in a subset of AML patients when combined with standard chemotherapy, but safety concerns led to US marketing withdrawal. The activity of these agents confirms that CD33 remains a viable therapeutic target for AML. Strategies to improve the results of mAb-based therapies for AML include antibody engineering to enhance effector function, use of alternative drugs and chemical linkers to develop safer and more effective drug conjugates, and radioimmunotherapeutic approaches.

Background: Older patients (pts) with AML unfit for intense induction chemotherapy have a poor prognosis with a 5 year survival of 6 weeks and 5 CRi patients did not reach an ANC of ≥ 500/µL. Therefore, the activity level was reduced to 1.5 µCi/kg/dose for further evaluation. Methods: This study enrolled older pts with untreated AML who were considered to be unfit for standard induction chemotherapy. Pts aged 60 to 74 years were required to have significant co-morbidities, while all pts ≥ 75 years were eligible. Other eligibility criteria included ECOG PS 0-2, CD33 expression on > 25% of blasts, and a peripheral blast count Results: 18 pts were included in this prespecified Interim Analysis. An additional 9 pts were treated at this dose and will be included in an updated analysis at the ASH meeting. The median age of the 18 pts was 73.5 years (range 60-87) and median ECOG Performance Status was 1 (0 in 3 pts, 1 in 9 pts, & 6 in 2 pts). 11 pts had prior AHDs (6 MDS, 2 CMML, 2 NHL, 1 myelofibrosis), and 9 had prior treatment for AHDs. Of the pts with known cytogenetic and molecular genetic results, 1 had favorable-risk, 3 had intermediate-risk, and 5 had adverse-risk AML using NCCN guidelines. The median baseline BM blast % was 40.5% (range, 22-66%) with a median CD33 expression of 62% (range, 26-100%) of AML cells. Objective responses were seen in 4 pts (22%): 3 complete remissions with incomplete platelet count recovery (CRp) and 1 complete remission with incomplete hematologic recovery (CRi). Among the responders with known cytogenetics, 1 pt had adverse genetics and 1 had Intermediate-risk genetics.Of the pts with remission, 3 had CT genotype for the CD33 splicing polymorphism SNP rs12459419 (including 2 CRp) and 1 had CC genotype. 3 of the responders are in follow up at Days 59, 169 and 266 without further treatment. Myelosuppression was seen in all pts including Grade 4 thrombocytopenia with marrow aplasia for > 6 weeks after the first dose in 4 pts. 1 pt with prior MDS had pancytopenia for > 4 months. The 3 pts with CRp achieved an ANC ≥ 1000/µL at Days 28, 38, and 40 from the first dose of 225Ac-lin. The pt with CRi did not reach an ANC of 500/µL. Non hematologic Grade ≥3 Treatment-Emergent adverse events (AEs) that were at least possibly related were 4 patients with febrile neutropenia, as well as 1 patient each with fungal pneumonia, acute respiratory failure, pulmonary edema, chest pain, sepsis, gastric hemorrhage, generalized muscle weakness, atrial fibrillation with rapid ventricular response and typhilitis. No pts had an infusion-related reaction requiring dose interruption. Veno-occlusive disease did not occur. The 30-day mortality rate was 16.7% (1 cardiac arrest and 2 with multi organ failure). Conclusions: Preliminary data from this Interim Analysis of 225Ac-lin monotherapy in older AML pts unfit for intensive therapy indicate a lower rate of myelosuppression at 1.5 µCi/kg/dose but also a lower response rate than was seen at 2.0 µCi/kg/dose. Although the study met the prespecified response criteria for continuing enrollment, it will be closed to further accrual in recognition that targeted radiation, like other therapies, will likely have the best outcomes when used in combination with other therapies in pts with active AML. An extensive development program in AML, MDS, and multiple myeloma is planned. One study will utilize 225Ac-lin in combination with salvage chemotherapy and another with venetoclax. 225Ac-lin will be used as a single agent for AML postremission therapy. In addition, 225Ac-lin will be used as targeted conditioning prior to hematopoietic stem cell transplant in pts with MDS with a complex karyotype and as a conditioning agent prior to autologous transplant in MM. Table. Table. Disclosures Atallah: Jazz: Consultancy; Novartis: Consultancy; Pfizer: Consultancy; Abbvie: Consultancy; BMS: Consultancy. Orozco:Actinium Pharmaceuticals: Research Funding. Craig:Novartis: Research Funding; Actinium Pharmaceuticals: Research Funding; Celgene: Research Funding. Levy:Takeda (Millennium Pharmaceuticals, Inc.): Consultancy. Finn:Ochsner Clinic Foundation: Employment. Perl:NewLink Genetics: Membership on an entity's Board of Directors or advisory committees; Actinium Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees; Arog: Consultancy; Astellas: Consultancy; Novartis: Membership on an entity's Board of Directors or advisory committees; Daiichi Sankyo: Consultancy; Pfizer: Membership on an entity's Board of Directors or advisory committees. Park:Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Juno Therapeutics: Consultancy, Research Funding; Shire: Consultancy; AstraZeneca: Consultancy; Adaptive Biotechnologies: Consultancy; Kite Pharma: Consultancy; Pfizer: Consultancy; Novartis: Consultancy. Roboz:Astex Pharmaceuticals: Consultancy; Bayer: Consultancy; Aphivena Therapeutics: Consultancy; Celltrion: Consultancy; Celgene Corporation: Consultancy; Novartis: Consultancy; Celgene Corporation: Consultancy; Pfizer: Consultancy; Argenx: Consultancy; Roche/Genentech: Consultancy; AbbVie: Consultancy; Cellectis: Research Funding; Aphivena Therapeutics: Consultancy; Roche/Genentech: Consultancy; Sandoz: Consultancy; Bayer: Consultancy; Sandoz: Consultancy; Jazz Pharmaceuticals: Consultancy; Otsuka: Consultancy; Astex Pharmaceuticals: Consultancy; Daiichi Sankyo: Consultancy; Orsenix: Consultancy; Janssen Pharmaceuticals: Consultancy; AbbVie: Consultancy; Jazz Pharmaceuticals: Consultancy; Argenx: Consultancy; Janssen Pharmaceuticals: Consultancy; Eisai: Consultancy; Otsuka: Consultancy; Pfizer: Consultancy; Eisai: Consultancy; Orsenix: Consultancy; Cellectis: Research Funding; Daiichi Sankyo: Consultancy; Novartis: Consultancy; Celltrion: Consultancy. Tse:Brown Cancer Center, University of Louisville School of Medicine: Employment; Jazz: Consultancy; Amgen: Consultancy; Amgen: Honoraria; Amgen: Honoraria. Mawad:Swedish Cancer Institute: Employment. Rizzieri:Novartis: Consultancy; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Jazz: Consultancy, Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Teva: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Arog: Consultancy, Membership on an entity's Board of Directors or advisory committees; Gilead: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Berger:Actinium Pharmaceuticals: Employment, Equity Ownership. Jurcic:Astellas: Research Funding; Daiichi-Sankyo: Research Funding; Incyte: Consultancy; Celgene: Research Funding; AbbVie: Consultancy, Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees; Forma Therapeutics: Research Funding; Syros Pharmaceuticals: Research Funding; Actinium Pharmaceuticals, Inc: Research Funding; Genetech: Research Funding; Kura Oncology: Research Funding.

Methods to generate monoclonal antibodies to antigens of neoplastic cells have revolutionized our understanding of cancer cell growth and differentiation, diagnosis, and treatment. Monoclonal antibodies derived by immunizing animals (mostly mice) with mammalian cells or molecules have been critical reagents for the discovery and characterization of many key molecules involved in the behavior of neoplastic cells. Now, over 30 years later, monoclonal antibodies are widely used in the differential diagnosis of cancer and are key elements in the treatment of many forms of cancer. This review will focus on the roles that monoclonal antibodies play in the treatment of hematological malignancies. In particular, we will focus on acute myeloid leukemia and mature B-cell neoplasms.

Purpose of review This review addresses use of monoclonal antibodies and immunoconjugates to treat acute myeloid leukemia. Recent findings Monoclonal antibodies used in acute myeloid leukemia have been directed against the antigens CD33, CD45, and CD66. Unconjugated monoclonal antibodies such as lintuzumab have modest activity against overt acute myeloid leukemia but can eliminate minimal residual disease in acute promyelocytic leukemia. Most experience with immunoconjugates is with gemtuzumab ozogamicin, an anti-CD33 monoclonal antibody linked to the potent antitumor antibiotic calicheamicin. Gemtuzumab ozogamicin has shown activity both singly, particularly in acute promyelocytic leukemia, and combined with conventional cytotoxic chemotherapy. Radiolabeled monoclonal antibodies against CD45 and CD66 have also been used to intensify the conditioning regimen before stem cell transplantation. The most promising results were obtained with radiolabeled anti-CD45 antibodies. Antibodies reactive with CD66 have been used to deliver targeted radiation to hematopoietic tissues in patients with advanced myeloid malignancies. Summary Both unlabeled monoclonal antibodies and immunoconjugates appear to have a limited role if used as single agents to treat acute myeloid leukemia. These agents hold promise as potentially useful additions to conventional therapy, but the optimal dosing and timing remain to be defined.

MAb have become an important treatment modality in cancer therapy. Genetically engineered chimeric and humanized Ab have demonstrated activity against a variety of tumors. While the humanized anti-CD33 MAb lintuzumab has only modest single-agent activity against overt AML, it can eliminate minimal residual disease detectable by reverse transcription-PCR in acute promyelocytic leukemia. Targeted chemotherapy with the anti-CD33–calicheamicin construct gemtuzumab ozogamicin has produced remissions in patients with relapsed AML and appears promising when used in combination with standard chemotherapy in the treatment of newly diagnosed AML.

Purpose Lintuzumab (HuM195) is an unconjugated humanized murine monoclonal antibody directed against the cell surface myelomonocytic differentiation antigen CD33. In this study, the efficacy of lintuzumab in combination with induction chemotherapy was compared with chemotherapy alone in adults with first relapsed or primary refractory acute myeloid leukemia (AML). Patients and Methods Patients with relapsed or primary resistant AML (duration of first response, zero to 12 months) were randomly assigned to receive either mitoxantrone 8 mg/m2, etoposide 80 mg/m2, and cytarabine 1 g/m2 daily for 6 days (MEC) in combination with lintuzumab 12 mg/m2, or MEC alone. Overall response, defined as the rate of complete remission (CR) and CR with incomplete platelet recovery (CRp), was the primary end point of the study, with additional analyses of survival time and toxicity. Results A total of 191 patients were randomly assigned from November 1999 to April 2001. The percent CR plus CRp with MEC plus lintuzumab was 36% v 28% in patients treated with MEC alone (P = .28). The overall median survival was 156 days and was not different in the two arms of the study. Apart from mild antibody infusion–related toxicities (fever, chills, and hypotension), no differences in chemotherapy-related adverse effects, including hepatic and cardiac dysfunction, were observed with the addition of lintuzumab to induction chemotherapy. Conclusion The addition of lintuzumab to salvage induction chemotherapy was safe, but did not result in a statistically significant improvement in response rate or survival in patients with refractory/relapsed AML.

The monoclonal antibodies M195 and HuM195 target CD33, a glycoprotein found on myeloid leukemia cells. When labeled with iodine-131 ((131)I), these antibodies can eliminate large disease burdens and produce prolonged myelosuppression. We studied whether (131)I-labeled M195 and HuM195 could be combined safely with busulfan and cyclophosphamide (BuCy) as conditioning for allogeneic BMT. A total of 31 patients with relapsed/refractory acute myeloloid leukemia (AML) (n=16), accelerated/myeloblastic chronic myeloid leukemia (CML) (n=14), or advanced myelodysplastic syndrome (n=1) received (131)I-M195 or (131)I-HuM195 (122-437 mCi) plus busulfan (16 mg/kg) and cyclophosphamide (90-120 mg/kg) followed by infusion of related-donor bone marrow (27 first BMT; four second BMT). Hyperbilirubinemia was the most common extramedullary toxicity, occurring in 69% of patients during the first 28 days after BMT. Gamma camera imaging showed targeting of the radioisotope to the bone marrow, liver, and spleen, with absorbed radiation doses to the marrow of 272-1470 cGy. The median survival was 4.9 months (range 0.3-90+ months). Three patients with relapsed AML remain in complete remission 59+, 87+, and 90+ months following bone marrow transplantation (BMT). These studies show the feasibility of adding CD33-targeted radioimmunotherapy to a standard BMT preparative regimen; however, randomized trials will be needed to prove a benefit to intensified conditioning with radioimmunotherapy.

HuM195 is a humanized, unconjugated, anti-CD33 monoclonal antibody. Fifty adult patients with relapsed or refractory AML were randomized to receive HuM195 at a dose of 12 or 36 mg/m(2) by intravenous infusion over 4 h on days 1-4 and 15-18. Patients with stable or responding disease received two additional cycles on days 29-32 and 43-46. HuM195 was given as first salvage therapy in 24 patients and as second or subsequent salvage therapy in 26 patients. Pretreatment blast percentage in the marrow was between 5 and 30% in 20 patients with the others having blast counts greater than 30%. The median age of patients was 62 years (range 26-86) and CD33 was detected in 95% of patients for whom immunophenotyping was available. Of 49 evaluable patients, two complete and one partial remission were observed. All three responses were in patients treated at the 12 mg/m(2) dose level and all had baseline blast percentages less than 30%. Decreases in blast counts ranging from 30 to 74% were seen in nine additional patients. Infusion-related events of fever and chills occurred in the majority of patients and were generally mild and primarily related to the first dose of antibody. No hepatic, renal or cardiac toxicities were observed and other adverse events such as nausea, vomiting, mucositis and diarrhea were uncommon or felt to be unrelated to HuM195. In addition, anti-HuM195 responses were not detected. HuM195 as a single agent has minimal, but observable, anti-leukemic activity in patients with relapsed or refractory AML and activity is confined to patients with low burden disease. No significant differences in clinical efficacy or toxicity were seen between the two dose levels of antibody. HuM195 was well tolerated with infusion-related fevers and chills the predominant toxicities seen. Meaningful clinical efficacy of this unconjugated monoclonal antibody may be realized only in patients with minimal residual disease, or in combination with chemotherapy.

Unlike β particle–emitting isotopes, α emitters can selectively kill individual cancer cells with a single atomic decay. HuM195, a humanized anti-CD33 monoclonal antibody, specifically targets myeloid leukemia cells and has activity against minimal disease. When labeled with the β-emitters 131I and 90Y, HuM195 can eliminate large leukemic burdens in patients, but it produces prolonged myelosuppression requiring hematopoietic stem cell transplantation at high doses. To enhance the potency of native HuM195 yet avoid the nonspecific cytotoxicity of β-emitting constructs, the α-emitting isotope 213Bi was conjugated to HuM195. Eighteen patients with relapsed and refractory acute myelogenous leukemia or chronic myelomonocytic leukemia were treated with 10.36 to 37.0 MBq/kg 213Bi-HuM195. No significant extramedullary toxicity was seen. All 17 evaluable patients developed myelosuppression, with a median time to recovery of 22 days. Nearly all the 213Bi-HuM195 rapidly localized to and was retained in areas of leukemic involvement, including the bone marrow, liver, and spleen. Absorbed dose ratios between these sites and the whole body were 1000-fold greater than those seen with β-emitting constructs in this antigen system and patient population. Fourteen (93%) of 15 evaluable patients had reductions in circulating blasts, and 14 (78%) of 18 patients had reductions in the percentage of bone marrow blasts. This study demonstrates the safety, feasibility, and antileukemic effects of 213Bi-HuM195, and it is the first proof-of-concept for systemic targeted α particle immunotherapy in humans.

Targeted therapy of acute myeloid leukemia (AML) with monoclonal antibodies (MoAbs) has thus far been mostly directed at CD33, CD45, and CD66. The notion that some AMLs may predominantly or entirely involve committed CD33+ myeloid precursors provided the rationale for eradicating underlying stem cells with anti-CD33 antibodies. Emerging data demonstrating efficacy of the anti-CD33 immunoconjugate, gemtuzumab ozogamicin, in acute promyelocytic leukemia and other favorable- and intermediate-risk AMLs validate CD33 as drug target. Unlike CD33, CD45 and CD66 are noninternalizing antigens and have primarily been targeted to deliver radionuclides to sites of hematopoietic tissue, particularly to augment pre-transplant conditioning regimens. Early studies document the feasibility of this approach in selected patients, although future controlled studies will need to assess whether this strategy indeed leads to improved outcomes. Given this encouraging clinical experience, the use of MoAbs is likely expanding significantly in the future with identification of additional AML (stem) cell-associated antigens.

Because alpha-particles have a shorter range and a higher linear energy transfer (LET) compared with beta-particles, targeted alpha-particle immunotherapy offers the potential for more efficient tumor cell killing while sparing surrounding normal cells. To date, clinical studies of alpha-particle immunotherapy for acute myeloid leukemia (AML) have focused on the myeloid cell surface antigen CD33 as a target using the humanized monoclonal antibody lintuzumab. An initial phase I study demonstrated the safety, feasibility, and antileukemic effects of bismuth-213 (213Bi)-labeled lintuzumab. In a subsequent study, 213Bi-lintuzumab produced remissions in some patients with AML after partial cytoreduction with cytarabine, suggesting the utility of targeted alpha-particle therapy for small-volume disease. The widespread use of 213Bi, however, is limited by its short half-life. Therefore, a second-generation construct containing actinium-225 (225Ac), a radiometal that generates four alpha-particle emissions, was developed. A phase I trial demonstrated that 225Ac-lintuzumab is safe at doses of 3 μCi/kg or less and has antileukemic activity across all dose levels studied. Fractionated-dose 225Ac-lintuzumab in combination with low-dose cytarabine (LDAC) is now under investigation for the management of older patients with untreated AML in a multicenter trial. Preclinical studies using 213Bi- and astatine-211 (211At)-labeled anti-CD45 antibodies have shown that alpha-particle immunotherapy may be useful as part conditioning before hematopoietic cell transplantation. The use of novel pretargeting strategies may further improve target-to-normal organ dose ratios.

A number of new agents in acute myeloid leukemia (AML) have held much promise in recent years, but most have failed to change the therapeutic landscape. Indeed, with the exception of gemtuzumab ozogamicin (which was subsequently voluntarily withdrawn from the commercial market), no new agent has been approved for acute myeloid leukemia (AML) beyond the 7 + 3 regimen, which was has been in use for over 40 years. This review touches upon the potential reasons for these failures and explores the newer therapeutic approaches being pursued in AML.

Background: 225Ac-lintuzumab is a radioimmunoconjugate composed of 225Ac (t½=10 days), which emits 4 α-particles, linked to a humanized anti-CD33 monoclonal antibody. Short-ranged (50-80 µm), high-energy (~100 keV/µm) α particle-emitting isotopes such as 225Ac may result in more specific tumor cell kill and less damage to normal tissues than β-emitters. An initial phase I trial in 20 patients with relapsed/refractory AML showed that a single infusion of 225Ac-lintuzumab is safe at doses ≤ 3 µCi/kg and has anti-leukemic activity (Jurcic et al. ASH, 2011). We conducted a multicenter, phase I dose-escalation trial to determine the maximum tolerated dose (MTD), toxicity, and biological activity of fractionated-dose 225Ac-lintuzumab in combination with LDAC. Patients and Methods: Patients ≥ 60 years with untreated AML not candidates for standard induction therapy (e.g., antecedent hematologic disorder, poor-risk cytogenetic or molecular features, and significant comorbidities) were eligible. Patients received LDAC 20 mg twice a day for 10 days every 4-6 weeks for up to 12 cycles. During Cycle 1, 2 fractions of 225Ac-lintuzumab were given 1 week apart, beginning 4-7 days following completion of LDAC. To prevent radiation-induced nephrotoxicity, patients were given furosemide while receiving 225Ac-lintuzumab then spironolactone for 1 year afterward. Four dose levels of 225Ac-lintuzumab were studied using a 3+3 design. Dose escalation proceeded if < 33% of patients in a cohort experienced dose-limiting toxicity (DLT). Results: Eighteen patients (median age, 77 years; range, 68-87 years) completed therapy. Twelve (67%) had prior myelodysplastic syndrome (MDS), for which 10 (83%) received therapy with hypomethylating agents (n=9) or allogeneic hematopoietic cell transplantation (n=1). One patient (6%) had chronic myeloid leukemia in molecular remission prior to developing AML. Eleven patients (61%) had intermediate-risk and 7 (39%) had poor-risk disease by NCCN criteria. Median CD33 expression was 81% (range, 30-100%). 225Ac-lintuzumab was given at 0.5 (n=3), 1 (n=6), 1.5 (n=3), or 2 (n=6) μCi/kg/fraction. Up to 4 cycles of LDAC were administered. Two patients experienced DLT (grade 4 thrombocytopenia with marrow aplasia for > 6 weeks following therapy), one each in the 1 and 2 µCi/kg/fraction cohorts. Although the MTD was not reached, 2 µCi/kg/fraction was chosen as the phase II dose to limit prolonged myelosuppression. Hematologic toxicities included grade 4 neutropenia (n=5) and thrombocytopenia (n=9). Grade 3/4 non-hematologic toxicities included febrile neutropenia (n=6), pneumonia (n=5), other infections (n=3), atrial fibrillation/syncope (n=1), transient creatinine increase (n=1), generalized fatigue (n=1), hypokalemia (n=1), mucositis (n=1), and rectal hemorrhage (n=1). Thirty- and 60-day mortality rates were 0% and 17%, respectively. Eleven of 14 patients (79%) evaluated after Cycle 1 had bone marrow blast reductions (mean reduction, 66%; range, 19-100%). Objective responses (2 CR, 1 CRp, 2 CRi) were seen in 5 of the 18 patients (28%), but only at doses ≥ 1 µCi/kg/fraction (Table 1). One of the responders received 15 cycles of azacitidine for prior MDS. All responses occurred after 1 cycle of therapy, in contrast to historical data with LDAC alone, where the median time to response was 3 cycles. Median progression-free survival (PFS) for all patients was 2.7 months (range, 1.0-31.8+ months). Median overall survival (OS) was 5.6 months (range, 1.6-32+ months). Median response duration was 5.6 months (range, 4.9-32+ months). Peripheral blood blast counts were a strong predictor of response. Among 38 patients treated in the current and initial phase I trials, responses were seen in 8 of 19 patients (42%) with blast counts < 200/µL, compared with 0 of 17 patients with blast counts ≥ 200/µL (P=0.002). This difference is likely due to decreased marrow targeting in patients with higher circulating blast counts when the subsaturating antibody doses used in this trial are given. Conclusions: Fractionated-dose 225Ac-linutuzmab can be safely combined with LDAC and induce remission in older patients with untreated AML. A phase II trial of 225Ac-lintuzumab monotherapy at 2 µCi/kg/fraction using hydroxyurea, if needed, to lower peripheral blast counts prior toadministration will be undertaken to determine response rate, PFS, and OS in this patient population. Disclosures Jurcic: Forma Therapeutics: Research Funding; Seattle Genetics: Research Funding; Kura Oncology: Research Funding; Celgene: Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees; Bayer: Consultancy; Alexion Pharmaceuticals: Consultancy; Merck & Co.: Consultancy; Astellas: Research Funding; Actinium Pharmaceuticals, Inc.: Research Funding; Daiichi-Sankyo: Research Funding. Levy:Janssen: Speakers Bureau; Actinium Pharmaceuticals, Inc.: Research Funding; Seattle Genetics: Research Funding; Amgen: Speakers Bureau; Takeda Pharmaceuticals International Co.: Speakers Bureau. Park:Amgen: Consultancy; Genentech/Roche: Research Funding; Juno Therapeutics: Consultancy, Research Funding. Ravandi:Actinium Pharmaceuticals, Inc.: Research Funding. Perl:Actinium Pharmaceuticals, Inc.: Research Funding; Seattle Genetics: Consultancy. Smith:Actinium Pharmaceuticals, Inc.: Research Funding. Cicic:Actinium Pharmaceuticals, Inc.: Employment, Equity Ownership. Scheinberg:Actinium Pharmaceuticals, Inc.: Equity Ownership, Patents & Royalties: Ac-225-Lintuzumab.

Meditope Biosciences’ SnAP (Site-specific novel Antibody Platform) technology provides a novel and specific way to create antibody drug conjugates (ADCs). Cyclic peptides, termed ‘meditopes’, have been designed to selectively bind to a ‘meditope binding site’ engineered into the Fab of an antibody. The process of conferring meditope binding is referred to as “meditope enablement”. Enabling antibodies for meditope binding does not interfere with antigen recognition or antibody integrity due to the unique location of the binding site. Meditope peptide binding can be utilized to attach cytotoxic payloads to an enabled antibody in a specific and consistent manner, and has the potential to dramatically improve the consistency of ADC production. For advanced hematological cancers such as CD33 positive acute myeloid leukemia (AML), using an anti-CD33 monoclonal antibody conjugated to a potent cytotoxic agent may be a useful tumor reduction strategy, especially when patients have failed other treatments. To this end, meditope enablement of lintuzumab and gentuzumab permits a consistent, site-specific attachment of cytotoxins with a highly controllable drug antibody ratio (DAR). Structural and biophysical characterization of the meditope enabled anti CD33 antibodies has provided key insight to the meditope enablement process. Experiments confirm that payload conjugation and meditope peptide binding does not interfere with antibody integrity or antigen recognition. The meditope peptide drug complexes were tested in cell-based assays for CD33-targeted cell killing. Using X-ray crystallographic analysis and guided docking, we propose modifications to further enhance affinity of the meditope peptide conjugate : CD33 antibody interaction. Citation Format: Calin D. Dumitru, Michael Matho, Elisabeth Gardiner, John C. Williams, Krzysztof Bzymek. Meditope enablement and structural analysis of anti-CD33 antibodies. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 869.

Acute myelogenous leukemia (AML) is characterized by uncontrolled proliferation of the cells of myeloid origin. It can present at all ages, but is more common in adults. It is one of the most common leukemias in adults and continues to pose significant challenge in diagnosis and long-term management.

Improving outcomes in older adults with acute myeloid leukemia remains a formidable challenge. Lintuzumab (SGN-33; HuM195) is a humanized monoclonal antibody directed against CD33, which is expressed on the majority of myeloblasts in acute myeloid leukemia. The primary objective of this randomized, double-blinded, placebo-controlled trial was to determine whether addition of lintuzumab to low-dose cytarabine would increase overall survival in adults aged 60 years and over with untreated acute myeloid leukemia. Randomization was stratified by age, previous hematologic disorder, and performance status. All patients received cytarabine (20 mg subcutaneously twice daily) on Days 1-10 of each 28-day cycle. Patients received lintuzumab (600 mg) or placebo intravenously once weekly in Cycle 1 and once every other week in Cycles 2-12. A total of 211 patients (107 lintuzumab, 104 placebo) were randomized. Median age was 70 years (range 60-90). Survival was not significantly prolonged with lintuzumab treatment (hazard ratio 0.96; 95% confidence interval (CI) 0.72-1.28; P=0.7585). Median survival was similar between treatment arms (4.7 months lintuzumab vs. 5.1 months placebo) and in the subgroup of patients with high-risk cytogenetics (4.5 months). Infusion-related reactions, predominantly Grades 1-2, occurred more commonly in the lintuzumab arm (51% vs. 7% placebo); no other clinically significant difference in safety was noted. These results confirm that lintuzumab in combination with low-dose cytarabine did not prolong survival and that low-dose cytarabine remains a valid comparator for trials of non-intensive therapies in older patients with acute myeloid leukemia, regardless of cytogenetic profile.

The current treatment of patients with acute myeloid leukaemia yields poor results, with expected cure rates in the order of 30–40% depending on the biological characteristics of the leukaemic clone. Therefore, new agents and schemas are intensively studied in order to improve patients’ outcomes. This review summarizes some of these new paradigms, including new questions such as which anthracycline is most effective and at what dose. High doses of daunorubicin have shown better responses in young patients and are well tolerated in elderly patients. Monoclonal antibodies are promising agents in good risk patients. Drugs blocking signalling pathways could be used in combination with chemotherapy or in maintenance with promising results. Epigenetic therapies, particularly after stem cell transplantation, are also discussed. New drugs such as clofarabine and flavopiridol are reviewed and the results of their use discussed. It is clear that many new approaches are under study and hopefully will be able to improve on the outcomes of the commonly used ‘7+3’ regimen of an anthracycline plus cytarabine with daunorubicin, which is clearly an ineffective therapy in the majority of patients.

Purpose: Lintuzumab (HuM195), a humanized anti-CD33 antibody, targets myeloid leukemia cells and has modest single-agent activity against acute myeloid leukemia (AML). To increase the potency of the antibody without the nonspecific cytotoxicity associated with β-emitters, the α-particle–emitting radionuclide bismuth-213 (213Bi) was conjugated to lintuzumab. This phase I/II trial was conducted to determine the maximum tolerated dose (MTD) and antileukemic effects of 213Bi-lintuzumab, the first targeted α-emitter, after partially cytoreductive chemotherapy. Experimental Design: Thirty-one patients with newly diagnosed (n = 13) or relapsed/refractory (n = 18) AML (median age, 67 years; range, 37-80) were treated with cytarabine (200 mg/m2/d) for 5 days followed by 213Bi-lintuzumab (18.5-46.25 MBq/kg). Results: The MTD of 213Bi-lintuzumab was 37 MB/kg; myelosuppression lasting >35 days was dose limiting. Extramedullary toxicities were primarily limited to grade ≤2 events, including infusion-related reactions. Transient grade 3/4 liver function abnormalities were seen in five patients (16%). Treatment-related deaths occurred in 2 of 21 (10%) patients who received the MTD. Significant reductions in marrow blasts were seen at all dose levels. The median response duration was 6 months (range, 2-12). Biodistribution and pharmacokinetic studies suggested that saturation of available CD33 sites by 213Bi-lintuzumab was achieved after partial cytoreduction with cytarabine. Conclusions: Sequential administration of cytarabine and 213Bi-lintuzumab is tolerable and can produce remissions in patients with AML. Clin Cancer Res; 16(21); 5303–11. ©2010 AACR.

Despite therapeutic advances, the poor prognoses for acute myeloid leukemia (AML) and intermediate and high-risk myelodysplastic syndromes (MDS) point to the need for better treatment options. AML and MDS cells express the myeloid marker CD33, making it amenable to CD33-targeted therapy. Lintuzumab (SGN-33), a humanized monoclonal anti-CD33 antibody undergoing clinical evaluation, induced meaningful responses in a Phase 1 clinical trial and demonstrated anti-leukemic activity in preclinical models. Recently, it was reported that 5-azacytidine (Vidaza™) prolonged the overall survival of a group of high risk MDS and AML patients. To determine whether the combination of lintuzumab and 5-azacytidine would be beneficial, a mouse xenograft model of disseminated AML was used to evaluate the combination. There was a significant reduction in tumor burden and an increase in overall survival in mice treated with lintuzumab and 5-azacytidine. The effects were greater than that obtained with either agent alone. As the in vivo anti-leukemic activity of lintuzumab was dependent upon the presence of mouse effector cells including macrophages and neutrophils, in vitro effector function assays were used to assess the impact of 5-azacytidine on lintuzumab activity. The results show that 5-azacytidine significantly enhanced the ability of lintuzumab to promote tumor cell killing through antibody-dependent cellular cytotoxicity (ADCC) and phagocytic (ADCP) activities. These results suggest that lintuzumab and 5-azacytidine act in concert to promote tumor cell killing. Additionally, these findings provide the rationale to evaluate this combination in the clinic.

A multi-institutional, phase 1 dose-escalation trial of lintuzumab (humanized anti-CD33 antibody; SGN-33, HuM195) was performed in patients with CD33-positive myeloid malignancies. In this study, higher doses than previously tested and prolonged duration of treatment for responding patients were evaluated. Over the dose range of 1.5-8 mg/kg/week, lintuzumab was well tolerated, and a maximum tolerated dose was not defined. The most common adverse event was transient chills with the initial lintuzumab infusion (39%). Responses were observed in 7 of 17 patients with acute myeloid leukemia: morphologic complete remission (n = 4), partial remission (n = 2), and morphologic leukemia-free state (n = 1). Of 14 patients with myelodysplastic syndrome or myeloproliferative diseases, 1 patient had major hematologic improvement and 9 patients had stable disease. In contrast to aggressive conventional chemotherapy, lintuzumab was administered in an ambulatory clinic setting with acceptable toxicity.

Despite therapeutic advances, the long-term survival rates for acute myeloid leukemia (AML) are estimated to be 10% or less, pointing to the need for better treatment options. AML cells express the myeloid marker CD33, making it amenable to CD33-targeted therapy. Thus, the in vitro and in vivo anti-tumor activities of lintuzumab (SGN-33), a humanized monoclonal anti-CD33 antibody undergoing clinical evaluation, were investigated. In vitro assays were used to assess the ability of lintuzumab to mediate effector functions and to decrease the production of growth factors from AML cells. SCID mice models of disseminated AML with the multi-drug resistance (MDR)-negative HL60 and the MDR(+), HEL9217 and TF1-alpha, cell lines were developed and applied to examine the in vivo antitumor activity. In vitro, lintuzumab significantly reduced the production of TNFalpha-induced pro-inflammatory cytokines and chemokines by AML cells. Lintuzumab promoted tumor cell killing through antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis (ADCP) activities against MDR(-) and MDR(+) AML cell lines and primary AML patient samples. At doses from 3 to 30 mg/kg, lintuzumab significantly enhanced survival and reduced tumor burden in vivo, regardless of MDR status. Survival of the mice was dependent upon the activity of resident macrophages and neutrophils. The results suggest that lintuzumab may exert its therapeutic effects by modulating the cytokine milieu in the tumor microenvironment and through effector mediated cell killing. Given that lintuzumab induced meaningful responses in a phase 1 clinical trial, the preclinical antitumor activities defined in this study may underlie its observed therapeutic efficacy in AML patients.

Ten patients with myeloid leukemias were treated in a phase I trial with escalating doses of mouse monoclonal antibody (mAb) M195, reactive with CD33, a glycoprotein found on myeloid leukemia blasts and early hematopoietic progenitor cells but not on normal stem cells. M195 was trace-labeled with iodine-131 (131I) to allow detailed pharmacokinetic and dosimetric studies by serial sampling of blood and bone marrow and whole-body gamma-camera imaging. Total doses up to 76 mg were administered safely without immediate adverse effects. Absorption of M195 onto targets in vivo was demonstrated by biopsy, pharmacology, flow cytometry, and imaging; saturation of available sites occurred at doses greater than or equal to 5 mg/m2. The entire bone marrow was specifically and clearly imaged beginning within hours after injection; optimal imaging occurred at the lowest dose. Bone marrow biopsies demonstrated significant dose-related uptake of M195 as early as 1 hour after infusion in all patients, with the majority of the dose found in the marrow. Tumor regressions were not observed. An estimated 0.33 to 1.0 rad/mCi 131I was delivered to the whole body, 1.1 to 6.1 rad/mCi was delivered to the plasma, and up to 34 rad/mCi was delivered to the red marrow compartment. 131I-M195 was rapidly modulated, with a majority of the bound immunoglobulin G (IgG) being internalized into target cells in vivo. These data indicate that whole bone marrow ablative doses of 131I-M195 can be expected. The rapid, specific, and quantitative delivery to the bone marrow and the efficient internalization of M195 into target cells in vivo also suggest that the delivery of other isotopes such as auger or alpha emitters, toxins, or other biologically important molecules into either leukemia cells or normal hematopoietic progenitor cells may be feasible.

Since the approval of rituximab in 1997, monoclonal antibodies have come to play an important role in the therapy of hematological malignancies. Rituximab, gemtuzumab ozogamicin, and alemtuzumab are US Food and Drug Administration-approved for treatment of B-cell lymphomas, acute myeloid leukemia, and chronic lymphocytic leukemia, respectively. Multiple monoclonal antibodies directed against new and not-so-new cellular antigens are undergoing development and investigation all over the world. Most of these new compounds have undergone primatization or humanization, improving their specificity and decreasing their antigenicity when compared to earlier murine or chimeric products. This review will focus on three major aspects of monoclonal antibody therapy: 1) new therapeutic approaches with currently approved agents; 2) preclinical and clinical experience accumulated on new agents in the last few years; discussion will include available phase I, II, and III data on ofatumumab, epratuzumab, CMC-544, HeFi-1, SGN-30, MDX-060, HuM195 (lintuzumab), galiximab, lumiliximab, zanolimumab, and apolizumab; and 3) the role of naked and radiolabeled monoclonal antibodies in the hematopoietic stem cell transplantation setting.

Background: The use of short-ranged (50-80 µm), high-energy (~100 keV/µm) α particle-emitting isotopes for radioimmunotherapy may result in more specific tumor cell kill and less damage to normal tissues than β-emitters. 225Ac-lintuzumab consists of a radiometal that emits four α-particles linked to an anti-CD33 antibody. A phase I trial showed that 225Ac-lintuzumab is safe at doses ≤ 3 µCi/kg and has anti-tumor activity against relapsed/refractory AML across all dose levels studied (Jurcic et al. ASH, 2011). We are conducting a multicenter, phase I dose-escalation trial to determine the maximum tolerated dose (MTD), toxicity, and biological activity of fractionated-dose 225Ac-lintuzumab in combination with LDAC. Patients and Methods: Patients ≥ 60 years with untreated AML not suitable for standard induction chemotherapy (e.g., antecedent hematologic disorder, unfavorable cytogenetic or molecular abnormalities, and significant comorbidities) were eligible. Patients received LDAC 20 mg twice daily for 10 days every 4-6 weeks for up to 12 cycles. During Cycle 1, two fractions of 225Ac-lintuzumab were given one week apart, beginning 4-7 days following completion of LDAC. To prevent radiation-induced nephrotoxicity, patients were given furosemide while receiving 225Ac-lintuzumab and spironolactone for one year afterward. 225Ac doses were escalated using a 3+3 design. Four dose levels were studied with a total accrual of up to 24 patients. In planned analyses, dose escalation proceeded if < 33% of patients in a cohort experienced dose-limiting toxicity (DLT). Results: Fourteen patients (median age, 77 years; range, 68-87 years) completed therapy. An additional patient received only one of two planned fractions of 225Ac-lintuzumab due to technical issues and is excluded from analysis. Nine (64%) had prior myelodysplastic syndrome, for which seven received prior therapy with hypomethylating agents (n=6) or allogeneic hematopoietic cell transplantation (n=1). One patient (7%) had chronic myeloid leukemia in molecular remission prior to development of AML. Nine patients (64%) had intermediate-risk and five (36%) had unfavorable cytogenetics. Median CD33 expression was 81% (range, 45-100%). 225Ac-lintuzumab was given at 0.5 (n=3), 1 (n=6), 1.5 (n=3), or 2 (n=2) μCi/kg/fraction. Up to 4 cycles of LDAC were administered. DLT was seen in one patient at 1 µCi/kg/fraction who had grade 4 thrombocytopenia with marrow aplasia for more than 6 weeks following therapy. Hematologic toxicities included grade 4 neutropenia (n=4) and thrombocytopenia (n=6). Grade 3/4 non-hematologic toxicities included febrile neutropenia (n=7), pneumonia (n=4), bacteremia (n=1), cellulitis (n=1), transient creatinine increase (n=1), hypokalemia (n=1), rectal hemorrhage (n=1), and generalized weakness (n=2). Eight of 11 patients (73%) evaluated after Cycle 1 had bone marrow blast reductions (mean reduction, 72%; range, 34-100%). Seven (64%) had blast reductions of at least 50%. Objective responses (1 CR, 1 CRp, 2 CRi) were seen in four of the 14 patients (29%) after one cycle of therapy (Table 1). Responses were seen only at doses ≥ 1 µCi/kg/fraction (4 of 11 patients, 36%). Median progression-free survival (PFS) was 2.7 months (range, 1.7-16.9 months). Median overall survival (OS) was 5.5 months (range, 2.2-24 months). Conclusions: Fractionated-dose 225Ac-linutuzmab can be safely combined with LDAC and produce remission in older patients with untreated AML. Dose escalation continues to define the MTD. Additional patients will be treated at the MTD in the phase II portion of this trial to determine response rate, PFS, and OS. Table 1. Objective Responses Response Dose Level (μCi/kg/fraction) Total (n=14) 0.5 (n=3) 1 (n=6) 1.5 (n=3) 2 (n=2) CR 0 0 1 (33%) 0 1 (7%) CRp 0 0 0 1 (50%) 1 (7%) CRi 0 1 (17%) 1 (33%) 0 2 (14%) Overall Response 0 1 (17%) 2 (67%) 1 (50%) 4 (29%) Abbreviations: CR, complete remission; CRp, CR with incomplete platelet recovery; CRi, CR with incomplete count recovery. Disclosures Jurcic: Ambit Biosciences: Research Funding; Astellas Pharma US, Invc.: Research Funding; Tetralogic Pharmaceuticals: Research Funding; Sunesis Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Novartis Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy; Bayer Pharmaceuticals: Consultancy; Merck and Co.: Consultancy; Celgene Corp.: Research Funding; Actrinium Pharmaceuticals, Inc.: Membership on an entity's Board of Directors or advisory committees, Research Funding. Off Label Use: Ac-225-lintuzumab is an investigational agent being developed for the treatment of acute myeloid leukemia.. Pagel:Actinium Pharmacetuicals, Inc.: Equity Ownership. Park:Actinium Pharmaceuticals, Inc.: Research Funding; Juno Therapeutics: Consultancy. Levy:Takeda: Consultancy. Perl:Actinium Pharmaceuticals, Inc.: Research Funding. Earle:Actinium Pharmaceuticals, Inc.: Employment. Cicic:Actinium Pharmaceuticals, Inc.: Employment, Equity Ownership. Scheinberg:Actinium Pharmaceuticals, Inc.: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties.

7050 Background: 225Ac-lintuzumab consists of a radiometal that emits 4 α-particles linked to an anti-CD33 antibody. A phase I trial showed safety and efficacy of 225Ac-lintuzumab in relapsed AML. ...

Background: Lintuzumab, a humanized anti-CD33 monoclonal antibody, targets myeloid leukemia cells but has only modest activity in AML. To increase the antibody’s potency yet avoid nonspecific cytotoxicity of β-emitting isotopes, 225Ac (t½=10 d), a radiometal that yields 4 α-particles, was conjugated to lintuzumab. A phase I trial showed that 225Ac-lintuzumab is safe at doses ≤ 3 µCi/kg and has anti-leukemic activity across all dose levels studied (Jurcic et al. ASH, 2011). We are conducting a multicenter, phase I dose-escalation trial to determine the maximum tolerated dose (MTD), toxicity, and biological activity of fractionated-dose 225Ac-lintuzumab in combination with LDAC. Patients and Methods: Patients ≥ 60 yrs who had untreated AML with poor prognostic factors, e.g., an antecedent hematologic disorder, unfavorable cytogenetic or molecular abnormalities, and significant comorbidities, were eligible. Patients received LDAC 20 mg twice daily for 10 d every 4-6 wks for up to 12 cycles. During Cycle 1, beginning 4-7 days after completion of LDAC, two doses of 225Ac-lintuzumab were given approximately one week apart. To prevent radiation-induced nephrotoxicity, patients were given furosemide while receiving 225Ac-lintuzumab and spironolactone for one year afterward. Results: Nine patients (median age, 76 yrs; range, 73-81 yrs) were treated. Seven patients (78%) had a history of myelodysplastic syndromes (MDS), for which five (56%) received prior therapy with hypomethylating agents (n=4) or allogeneic hematopoietic cell transplantation (n=1). One patient (11%) had chronic myeloid leukemia in a molecularly undetectable state at the time of AML diagnosis. Six patients (67%) had intermediate-risk cytogenetics, and three (33%) had unfavorable cytogenetics. The median CD33 expression was 76% (range, 45-100%). Patients received 225Ac-lintuzumab at doses of 0.5 (n=3) or 1 (n=6) μCi/kg/fraction. Total administered activity ranged from 68-199 μCi. The median number of cycles administered was 2 (range, 1-4). Dose-limiting toxicity was seen in one patient receiving 1 µCi/kg/fraction who had grade 4 thrombocytopenia with bone marrow aplasia persisting > 6 wks after receiving 225Ac-lintuzumab. Hematologic toxicities included grade 4 neutropenia (n=1) and thrombocytopenia (n=3). Grade 3/4 non-hematologic toxicities included febrile neutropenia (n=6), pneumonia (n=2), bacteremia (n=1), cellulitis (n=1), transient increase in creatinine (n=1), hypokalemia (n=1), and generalized weakness (n=1). Bone marrow blast reductions were seen in 5 of 7 patients (71%) evaluated after Cycle 1. Mean blast reduction was 61% (range, 34-100%). Three of the 7 patients (43%) had marrow blast reductions of ≥ 50%; however, no remissions were observed. Median progression-free survival (PFS) was 2.5 mos (range, 1.7-15.7+ mos). Median overall survival (OS) from study entry was 5.4 mos (range, 2.2-24 mos). For the 7 patients with prior MDS, median OS was 9.1 mos (range 2.3-24 mos). Conclusions: Fractionated-dose 225Ac-linutuzmab in combination with LDAC is feasible, safe, and has anti-leukemic activity. Dose escalation continues to define the MTD, with planned doses up to 2 µCi/kg/fraction. Additional patients will be treated at the MTD in the phase II portion of this trial to determine response rate, PFS, and OS. Disclosures Ravandi: Actinium Pharmaceuticals, Inc.: Research Funding. Pagel:Actinium Pharmaceuticals, Inc.: Equity Ownership, Research Funding. Park:Actinium Pharmaceuticals, Inc.: Research Funding. Wahl:Actinium Pharmaceuticals, Inc.: Research Funding. Earle:Actinium Pharmaceuticals, Inc.: Employment, Equity Ownership. Cicic:Actinium Pharmaceuticals, Inc.: Employment, Equity Ownership. Scheinberg:Actinium Pharmaceuticals, Inc.: Equity Ownership, Research Funding.

Background Lintuzumab, a humanized anti-CD33 antibody, targets myeloid leukemia cells and has modest activity against AML. To increase the antibody’s potency yet avoid nonspecific cytotoxicity seen with β-emitting isotopes, the isotope generator 225Ac (t½=10 days), which yields 4 α-particles, was conjugated to lintuzumab. A phase I trial demonstrated that 225Ac-lintuzumab is safe at doses ≤ 3 μCi/kg and has anti-leukemic activity across all dose levels studied (Jurcic et al. ASH, 2011). We are conducting a multicenter, phase I dose escalation trial to determine the maximum tolerated dose (MTD), toxicity, and biological activity of fractionated-dose 225Ac-lintuzumab in combination with LDAC. Patients and Methods Patients ≥ 60 yrs who had untreated AML with poor-prognostic factors, such as an antecedent hematologic disorder (AHD), unfavorable cytogenetic or molecular abnormalities, and significant comorbidities, were eligible. Patients received LDAC 20 mg bid for 10 days every 4-6 weeks. During cycle 1, beginning 4-7 days after LDAC, two doses of 225Ac-lintuzumab were given approximately one week apart. Results Seven patients (median age, 76 yrs; range, 72-80 yrs) were treated, all of whom had AHDs. Five (71%) had intermediate-risk cytogenetics, and two (29%) had unfavorable cytogenetics. The median CD33 expression was 76% (range, 69-95%). Prior therapy for myelodysplastic syndrome included hypomethylating agents (n=4) and allogeneic hematopoietic cell transplantation (n=1). Patients received 225Ac-lintuzumab at doses of 0.5 (n=3) or 1 (n=4) μCi/kg/fraction, two fractions per patient (total administered activity, 68-199 μCi). Dose-limiting toxicity was seen in one patient receiving 1 μCi/kg/fraction who had grade 4 thrombocytopenia in the setting of an aplastic bone marrow that persisted > 6 wks after completing the second fraction of 225Ac-lintuzumab. Other toxicities included grade 3 febrile neutropenia (n=5), bacteremia (n=1), pneumonia (n=1), cellulitis (n=1), transient increase in creatinine (n=1), and generalized weakness (n=1). Bone marrow blast reductions were seen in 4 of 6 evaluable patients (67%) after cycle 1 (mean blast reduction, 58%; range, 34-100%). No CRs, however, were observed. The median number of cycles administered was 2 (range, 1-4), and the median time to progression was 2.5 months (range, 2-7+ months). Conclusions Fractionated-dose 225Ac-linutuzmab in combination with LDAC is feasible, safe, and has anti-leukemic activity. Accrual continues to define the MTD, with planned dose levels up to 2 μCi/kg/fraction. Additional patients will be treated at the MTD in the phase II portion of this trial to determine response rate, progression-free survival, and overall survival. Disclosures: Jurcic: Actinium Pharmaceuticals, Inc.: Membership on an entity’s Board of Directors or advisory committees. Ravandi:Actinium Pharmaceuticals, Inc.: Research Funding. Pagel:Actinium Pharmaceuticals, Inc.: Equity Ownership, Membership on an entity’s Board of Directors or advisory committees, Research Funding. Park:Actinium Pharmaceuticals, Inc.: Research Funding. Douer:Actinium Pharmaceuticals, Inc.: Research Funding. Estey:Actinium Pharmaceuticals, Inc.: Membership on an entity’s Board of Directors or advisory committees. Cicic:Actinium Pharmaceuticals, Inc.: Employment, Equity Ownership. Scheinberg:Actinium Pharmaceuticals, Inc.: Ac-225-Lintuzumab, Ac-225-Lintuzumab Patents & Royalties, Equity Ownership, Membership on an entity’s Board of Directors or advisory committees.

S380 Context: Lintuzumab, a humanized anti-CD33 antibody, has modest activity against AML. To increase the antibody's potency yet avoid nonspecific cytotoxicity seen with b-emitting isotopes, Ac (t1/2 1⁄410 days), which yields 4 a-emitting isotopes, was conjugated to lintuzumab. Objectives: We sought to determine the MTD, toxicity, pharmacokinetics, and antileukemic activity of Ac-lintuzumab. Design: We conducted a first-in-man, phase I, 3+3, dose-escalation trial from 2006-2012. Setting: This was a single-institution study performed at a tertiary cancer center. Patients: Eighteen patients (median age, 64 yrs; range, 45-80 yrs) with relapsed (n1⁄411) or refractory (n1⁄47) AML were treated. Intervention: Patients received a single infusion of Ac-lintuzumabat doses of 0.5 (n1⁄43), 1 (n1⁄44), 2 (n1⁄43), 3 (n1⁄46) or 4 (n1⁄42) mCi/kg. Main Outcome Measure: MTD was defined as the highest dose for which the incidence of DLT was 35 days in 1 patient receiving 4 mCi/kg and death from sepsis in 2 patients receiving 3 and 4 mCi/kg. The MTD was 3 mCi/kg. Myelosuppression was the most common toxicity. Median time to resolution of grade 4 leukopenia was 27 days (range, 0-71 days). Significant extramedullary toxicities were limited to transient grade 3 liver function abnormalities in 3 patients. We analyzed plasma pharmacokinetics by gamma counting at energy windows for 2 daughters of Ac, francium-221 and bismuth-213. Two-phase elimination kinetics were seen with mean plasma t1/2 -a and t1/2 -b of 1.9 and 38 hours, respectively, similar to other lintuzumab constructs containing long-lived radionuclides. Peripheral blasts were eliminated in 10 of 16 evaluable patients (63%), but only at doses of 1 mCi/kg. Marrow blast reductions were seen in 10 of 15 evaluable patients (67%) at 4 weeks, including 8 patients (53%) who had blast reductions of 50%. Three patients receiving 1, 3, and 4 mCi/kg achieved marrow blasts of 5%. Conclusions: This is the first study to show that therapy with a targeted a-particle generator is feasible in humans. Ac-lintuzumab had antileukemic activity across all dose levels and is now being investigated in a multicenter phase I/II trial in combination with low-dose cytarabine for older AML patients.

Abstract 3815 Background: The activity of DNA methyltransferase inhibitor (DNMTI) monotherapy is suboptimal for most patients (pts) with myelodysplastic syndromes (MDS) and acute myelogenous leukemia (AML). DNMTI combinations studied to date have not convincingly produced higher response rates compared to DNMTI monotherapy, although time to response appears to be improved in some studies. We combined decitabine (DAC) with tretinoin (ATRA), an RAR ligand which can recruit histone acetyltransferases to gene regulatory regions, as a means to upregulate gene expression, and to induce apoptosis or differentiation, as is seen in in vitro studies of non-M3 AML. Our previous phase 1 study established 65 mg/m2/day of ATRA as the MTD (headache was DLT at 85 mg/m2/d) when given on days 10–19 following standard dose decitabine (20 mg/m2/d), given on days 1–5 of a 28-day cycle (Klimek VM, et al. Leuk Res, 2011;35:S70-S). Materials and Methods: MDS pts with any FAB or WHO subtype were enrolled if they had an IPSS score ≥ 0.5, were ineligible for allogeneic stem cell transplant (AlloSCT) at study entry, and had adequate hepatic and renal function. Prior DAC or 5-azacytidine (5-aza) responders who then progressed off treatment, and pts whose MDS progressed on 5-aza were eligible. Pts received up to 10 cycles of DAC (given on days 1–5) and tretinoin (65 mg/m2/d on days 10–19 of a 28-day cycle), allowing for delays due to infection or to allow count recovery as deemed necessary by the treating physician. Those with an ongoing response after 10 cycles continued DAC alone off-study. The primary endpoint is efficacy, assessed after even-numbered cycles using the 2006 Modified International Working Group MDS response criteria. Kaplan-Meier methodology was used to estimate duration of best response due to the censoring for patients undergoing AlloSCT. Results: 36 eligible pts (27M, 9F; median age 66, range 45–84 yrs) were enrolled in the phase 2 cohort as of 6/2012. FAB/WHO subtypes included: RA/RCMD, n=3 (8%); RARS/RCMD-RARS, n=1 (3%); RAEB/RAEB-1 & 2, n= 24 (67%); RAEBt/AML, n= 6 (17%); CMML/CMML-1, n=2 (6%). IPSS risk categories included: Int-1, n=5 (14%); Int-2, n=16 (44%); High Risk, n=11 (31%), and pts with ≥ Int-1 (n=3) or ≥ Int-2 (n=1) risk disease who could not be definitively classified using the IPSS. Most pts (n=20) had IPSS poor risk cytogenetics (CG), 11 had IPSS good risk CG (all with normal karyotype), 3 had intermediate risk CG, and CG results were unknown in 2 pts. 15/36 (42%) had therapy-related disease (t-MDS/AML), all with IPSS poor risk CG and either IPSS Int-2 (n=9) or High Risk (n=6) disease. Marrow blasts were ≥ 5% in 26/36 pts: (5–10%, n=13; 11–20%, n=13; 21–30%, n=6). Prior MDS therapy included lenalidomide (n=2), thalidomide (n=1), 5-aza (n=3), DAC (n=1), and lintuzumab (n=1). Pts received a median of 4 cycles of therapy (range, 1–10), and started therapy 0.6–180 months (median, 1.42 months) from the time of diagnosis. Two pts were classified as treatment failures since they died during the first cycle of therapy. One pt with CMML-1 at baseline progressed to AML during the screening period, and was deemed ineligible. Two pts on study have not yet had a full response assessment. Best responses in the evaluable intent to treat cohort (n=33) include: CR, n=7 (21%); PR, n=1 (3%); mCR, n=3 (9%); mCR+HI, n=3 (9 %); HI alone, n=3 (9 %); Stable disease, n=10 (30%); Disease progression, n= 3 (9%). The composite CR rate (CR+mCR±HI) was 39% (13/33), and the overall response rate (CR+PR+mCR±HI+HI) is 51% (17/33). Median time from diagnosis to start of therapy for responders was 1.7 months (range, 0.8–180). The ORR for the 13 t-MDS/AML pts was 46% (6/13), including 5 pts with monosomy 17 and/or p53 loss by FISH. The median time to initial response and best response is 1.1 months and 2.3 months, respectively. The median response duration is 7.8 months, incorporating the 7 pts censored at the time they came off study to undergo AlloSCT. Conclusions: DAC/ATRA is active in IPSS Int-2 and High Risk MDS and in t-MDS/AML, which is characterized by poor risk cytogenetics and an increased frequency of p53 or chromosome 17p loss. Although our interim ORR appears similar to DAC and 5-aza monotherapy trials, our results were achieved in a higher risk cohort, and the CR rate is equal to or greater than some earlier monotherapy studies with a shorter median time to response. Ongoing and planned correlative studies may define pre-treatment biomarkers for response. Disclosures: Off Label Use: Tretinoin.

Abstract 1363 Introduction: Acute myeloid leukemia (AML), the most frequent form of adult acute leukemia, is a heterogeneous disease with respect to presentation and clinical outcome. Although there has been progress made in treatment for some subgroups of AML patients, there is still a high medical need to develop new anti-leukemic therapies. CD33 is a myeloid lineage-specific antigen which is expressed on the cell surface of normal myeloid cells and AML blasts. CD33 has been investigated as a target for antibody-based therapies for over a decade, leading to transient approval of Mylotarg®, an antibody-calicheamicin drug conjugate. Mylotarg®has been shown to substantially improve outcome of patients with AML in combination with chemotherapy, however is associated with additional toxicity. Here we describe a novel, unconjugated Fc-engineered antibody to CD33, mAb 33.1, which mediates potent antibody dependent cellular cytotoxicity (ADCC) against AML derived cell lines and malignant AML blasts. Methods: MAb 33.1 is a fully human, Fc-engineered, IgG1-type of antibody which has increased affinity to human Fc-gamma receptor IIIa and binds CD33 with low nanomolar affinity as determined by FACS scatchard analysis. ADCC of mAb 33.1 was determined on a panel of myeloid cell lines and primary AML cells. ADCC assays were performed using peripheral blood mononuclear cells (PBMC) from healthy donors as effector cells and cytotoxicity was determined by LDH release or FACS quantification of 7-AAD-positive cells. Internalization kinetics was assessed by FACS analysis using fluorochrome labelled antibody. Epitope mapping of mAb 33.1 was performed by direct competition experiments and hydrogen/deuterium exchange mass spectrometry (HXMS). Results: Antibody mAb 33.1 is able to mediate potent ADCC on a panel of AML derived cell lines and primary AML blasts in vitro. Direct comparison of mAb 33.1 to the humanized CD33 mAb lintuzumab (HuM195) demonstrates superior cytolytic activity on AML cell lines and primary AML cells. ADCC activity of mAb 33.1 is also evident on AML cell lines and primary AML blasts that do not respond to lintuzumab treatment in vitro. In contrast to lintuzumab, mAb 33.1 shows significantly slower internalization kinetics on HL60 cells, leading to prolonged exposure of antigen/antibody complexes on the cell surface. In direct competition experiments mAb 33.1 is competing with lintuzumab for antigen binding. HXMS experiments revealed that mAb 33.1 recognizes a novel epitope on CD33 different to that of lintuzumab. Conclusion: MAb 33.1 represents a novel, Fc-engineered antibody specific for CD33 which shows decelerated internalization compared to lintuzumab, and which recognizes a novel epitope on CD33 different to that of lintuzumab. MAb 33.1 displays strongly enhanced ADCC against leukemia cell lines and primary AML cells, resulting in significantly higher cytolysis than lintuzumab. MAb 33.1 is considered as a promising candidate for treatment of patients with CD33-positive myeloid malignancies and phase I clinical trials are currently under preparation. Disclosures: Heider: Boehringer Ingelheim: Employment. Konopitzky:Boehringer Ingelheim: Employment. Ostermann:Boehringer Ingelheim: Employment. Lamche:Boehringer Ingelheim: Employment. Kuepcue:Boehringer Ingelheim: Employment. Koessl:Boehringer Ingelheim: Employment. Adam:Boehringer Ingelheim: Employment. Borges:Boehringer Ingelheim: Employment.

Abstract 1521 The treatment for acute myeloid leukemia (AML) has not changed dramatically for the last 20 years. Anti-CD33 monoclonal antibody therapy, the most prominent of which is gemtuzumab ozogamicin (GO), has been used over the last decade, in order to improve the results of patients with AML. It has been studied in a variety of contexts; at induction, consolidation and as re-induction after disease relapse. Several randomized controlled trials have evaluated the addition of anti-CD33 to chemotherapy as compared to chemotherapy alone for the treatment of AML. Systematic review and meta-analysis of randomized controlled trials comparing addition of anti-CD33 to chemotherapy with chemotherapy alone in patients with AML, for either induction or post-remission therapy. Patients under 18 years old or those with acute promyelocytic leukemia were excluded. The Cochrane Library, MEDLINE, conference proceedings and references were searched until July 2012. Two reviewers appraised the quality of trials and extracted data. Outcomes assessed were: all-cause mortality at 30 days, all-cause mortality at the end of follow up, complete response and relapse rate. Relative risks (RR) were estimated and pooled. Our search yielded 11 included trials, five of them published as abstracts, including 5239 patients. Most trials assessed GO and two trials assessed lintuzumab as the anti-CD33 agent. Dose and schedule differed between trials. Eight trials examined the effect of anti-CD33 in the induction setting: six assessed the addition of anti-CD33 to intensive induction (daunorubicin and cytarabine based) and two used low dose cytarabine in elderly patients. Three trials examined the addition of anti-CD33 in the post remission setting (one as consolidation, one as maintenance and one after relapse). When analyzing the addition of anti-CD33 in all settings together, there was no difference in all-cause mortality at 30 days or at the end of follow up between chemotherapy with anti-CD33 and chemotherapy alone (RR 1.15 [95% CI, 0.96–1.37, 7 trials], RR 0.96 [95% CI, 0.92–1.00, 8 trials] respectively). In the subgroup of favorable and intermediate risk AML, the addition of anti-CD33 had no effect on all-cause mortality, RR 0.94 (95% CI, 0.87–1.02, 3 trials)], however when analyzing the subgroup of favorable risk patients only, mortality was significantly reduced with the use of anti-CD33 (RR 0.60, 95% CI, 0.42–0.85, 2 trials). Treatment with anti-CD33 had no effect on complete remission rate, RR 1.05 (95% CI 0.96–1.14, 7 trials). Yet, it significantly reduced relapse rate, RR 0.90 (95% CI 0.84–0.96, 4 trials). There was not enough data to evaluate the incidence of veno-occlusive disease. In conclusion, the addition of anti-CD33 to chemotherapy significantly decreased relapse rate, with no effect on all cause mortality. Yet, in the favorable risk subgroup, the use of anti-CD33 significantly reduced mortality. Further research is needed to confirm the beneficial effect in the favorable risk AML patients. Disclosures: No relevant conflicts of interest to declare.

There has been little improvement of standard of care for AML patients over the past 20 years, creating a large demand for therapies with improved benefit/risk profile and potential to cure. The CD19/CD3-bispecific, T cell-engaging (BiTE) antibody blinatumomab has shown high response rates and durable remissions in patients with acute lymphocytic leukemia and non-Hodgkin lymphoma. In order to employ this novel therapeutic modality for the treatment of patients with CD19-negative AML, we have constructed a panel of CD33/CD3-bispecific BiTE antibodies and selected MT114 as lead candidate for pre-clinical development for its superior pharmacological and pharmaceutical properties. Its target antigen CD33 (SIGLEC-3) is frequently expressed on AML blasts and leukemic stem cells, and extensive clinical experience has been gained by the CD33-targeting antibodies Mylotarg®, lintuzumab and AVE9633. Here, we report on the biological activities of MT114. MT114 recognized epitopes in the V domain of CD33 and of CD3 epsilon that are conserved between human and macaque antigen, allowing pharmacological and nonclinical safety investigations of the BiTE antibody in macaques as a relevant primate species. MT114-induced redirected lysis of 10 human AML cell lines by unstimulated peripheral blood mononuclear cell (PBMC) at half-maximal concentrations of MT114 between 23 and 328 pg/ml (0.4-7 pM). The 10 AML lines expressed CD33 at 20,000-50,000 molecules/cell. Redirected lysis was specific for CD33-expressing cells, reached completion after 28-h of co-culture, and was effective even at PBMC-to-target ratios of α1:2. MT114 induced the expression of activation markers CD69 and CD25 on T cells and the transient release of IFN-γ, TNF-α and IL-2, -6 and -10 by T cells, but only in the presence of CD33+ target cells. Redirected lysis and T cell activation by MT114 was not affected by up to 100 ng/ml of shed CD33, serum levels of which were found to be associated with AML progression. Neoexpression of CD33 on BiTE-activated T cells did not exceed 8% of total activated T cells when tested with 10 human T cell donors. The BiTE antibody significantly delayed growth of the subcutaneously injected human AML cell line HL60, and induced a robust infiltration of T cells into tumor tissue. MT114 is a highly active novel BiTE antibody with promise for the treatment of AML. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3523. doi:1538-7445.AM2012-3523