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This novel mosaic model has demonstrated that when a fraction of podocytes is injured, other podocytes are subjected to secondary injury. This spreading of injury may occur ubiquitously irrespective of the primary cause of podocyte injury, leading to end-stage renal failure. Understanding the molecular mechanism of secondary podocyte injury and its prevention is important for the treatment of progressive kidney diseases. This model will be a powerful tool for studying the indirect podocyte injury.

Background LMB-100 is an antibody-toxin conjugate with an antimesothelin Fab linked to a 24-kilodalton portion of Pseudomonas exotoxin A with mutations that decrease immunogenicity. The objective of the current first-in-human phase 1 study was to determine the maximum tolerated dose (MTD) and safety in patients with advanced solid tumors expressing mesothelin. Methods Cohorts of 1 to 7 patients received intravenous LMB-100 at 7 dose levels from 40 µg/kg to 250 µg/kg intravenously on days 1, 3, and 5 of a 21-day cycle. Results Of the 25 patients accrued, 17 had mesothelioma, 3 each had ovarian or pancreatic cancer, and 2 patients had gastric cancer. Dose-limiting toxicities occurred in 2 of 4 patients treated at a dose of 250 µg/kg (capillary leak syndrome) and in 3 of 7 patients treated at a dose of 170 µg/kg (creatinine increase). The MTD of LMB-100 was 140 µg/kg. Of the 10 patients with mesothelioma who were treated at doses of 170 µg/kg or 140 µg/kg, 8 had stable disease and 2 developed progressive disease. Peak LMB-100 plasma concentrations were dose-dependent during cycle 1. The development of antidrug antibodies decreased LMB-100 blood levels in 8 of 21 patients (38%) who received cycle 2 and 9 of 11 patients (81.8%) who received cycle 3. Conclusions The MTD for single-agent LMB-100 was found to be 140 µg/kg given on a schedule of every other day for 3 doses every 3 weeks. Although less immunogenic than the first-generation antimesothelin immunotoxin SS1P, the majority of patients developed antidrug antibodies after 2 cycles, indicating that LMB-100 has limited antitumor efficacy as a single agent. Phase 2 studies of LMB-100 plus pembrolizumab currently are ongoing for patients with mesothelioma and lung cancer. Lay summary Mesothelin, a cell surface antigen, is an attractive target for cancer therapy given its limited expression in normal human tissues and high expression in many human cancers. LMB-100 is a recombinant antimesothelin immunotoxin consisting of a humanized antimesothelin antibody fragment fused to a truncated Pseudomonas exotoxin A. In the current study, the authors determined the safety, maximum tolerated dose, and pharmacokinetics of LMB-100, as well as the generation of antidrug antibodies. Ongoing phase 2 clinical trials are evaluating the combination of LMB-100 plus pembrolizumab in patients with treatment-refractory mesothelioma and non-small cell lung cancer.

Background and aims Treatment of hepatocellular carcinomas using our glypican-3 (GPC3)-targeting human nanobody (HN3) immunotoxins causes potent tumor regression by blocking protein synthesis and down-regulating the Wnt signaling pathway. However, immunogenicity and a short serum half-life may limit the ability of immunotoxins to transition to the clinic. Approach and results To address these concerns, we engineered HN3-based immunotoxins to contain various deimmunized Pseudomonas exotoxin (PE) domains. This included HN3-T20, which was modified to remove T-cell epitopes and contains a PE domain II truncation. We compared them to our previously reported B-cell deimmunized immunotoxin (HN3-mPE24) and our original HN3-immunotoxin with a wild-type PE domain (HN3-PE38). All of our immunotoxins displayed high affinity to human GPC3, with HN3-T20 having a KD value of 7.4 nM. HN3-T20 retained 73% enzymatic activity when compared with the wild-type immunotoxin in an adenosine diphosphate-ribosylation assay. Interestingly, a real-time cell growth inhibition assay demonstrated that a single dose of HN3-T20 at 62.5 ng/mL (1.6 nM) was capable of inhibiting nearly all cell proliferation during the 10-day experiment. To enhance HN3-T20's serum retention, we tested the effect of adding a streptococcal albumin-binding domain (ABD) and a llama single-domain antibody fragment specific for mouse and human serum albumin. For the detection of immunotoxin in mouse serum, we developed a highly sensitive enzyme-linked immunosorbent assay and found that HN3-ABD-T20 had a 45-fold higher serum half-life than HN3-T20 (326 minutes vs. 7.3 minutes); consequently, addition of an ABD resulted in HN3-ABD-T20-mediated tumor regression at 1 mg/kg. Conclusion These data indicate that ABD-containing deimmunized HN3-T20 immunotoxins are high-potency therapeutics ready to be evaluated in clinical trials for the treatment of liver cancer.

We investigated the role of mesothelin (Msln) and thymocyte differentiation antigen 1 (Thy1) in the activation of fibroblasts across multiple organs and demonstrated that Msln-/- mice are protected from cholestatic fibrosis caused by Mdr2 (multidrug resistance gene 2) deficiency, bleomycin-induced lung fibrosis, and UUO (unilateral urinary obstruction)-induced kidney fibrosis. On the contrary, Thy1-/- mice are more susceptible to fibrosis, suggesting that a Msln-Thy1 signaling complex is critical for tissue fibroblast activation. A similar mechanism was observed in human activated portal fibroblasts (aPFs). Targeting of human MSLN+ aPFs with two anti-MSLN immunotoxins killed fibroblasts engineered to express human mesothelin and reduced collagen deposition in livers of bile duct ligation (BDL)-injured mice. We provide evidence that antimesothelin-based therapy may be a strategy for treatment of parenchymal organ fibrosis.

Despite encouraging clinical results with immune checkpoint inhibitors and other types of immunotherapies, the rate of failure is still very high. The development of proper animal models which could be applied to the screening of effective preclinical antitumor drugs targeting human tumor antigens, such as mesothelin, is a great need. Mesothelin (MSLN) is a 40 kDa cell-surface glycoprotein which is highly expressed in a variety of human cancers, and has great value as a target for antibody-based therapies. The present study reports the establishment of an immunocompetent transgenic mouse expressing human MSLN (hMSLN) only in thyroid gland by utilizing an expression vector containing a thyroid peroxidase (TPO) promoter. These mice do not reject genetically modified tumor cells expressing hMSLN on the cell membrane, and tolerate high doses of hMSLN-targeted immunotoxin. Employing this TPO-MSLN mouse model, we find that combination treatment of LMB-100 and anti-CTLA-4 induces complete tumor regression in 91% of the mice burdened with 66C14-M tumor cells. The combination therapy provides a significant survival benefit compared with both LMB-100 and anti-CTLA-4 monotherapy. In addition, The cured mice reject tumor cells when rechallenged, indicating the development of long-term antitumor immunity. This novel TPO-MSLN mouse model can serve as an important animal tool to better predict tumor responses to any immunomodulatory therapies that target MSLN.

Multiple myeloma (MM) is a B cell malignancy for which new treatments are urgently needed. The B cell maturation antigen (BCMA) is a lineage-restricted differentiation protein highly expressed on myeloma. Recombinant immunotoxins (RITs) are proteins composed of the Fv or Fab portion of an antibody fused to a bacterial toxin. We previously treated H929 myeloma s.c. tumors with anti-BCMA immunotoxins, very active on killing cultured cells, and observed tumor growth inhibition but not complete tumor responses. To determine if immunotoxins were more active against cells growing in the bone marrow (BM), the normal location of myeloma cells, we developed a BM mouse model that is more relevant to human disease. H929 cells were transfected with luciferase and GFP, enriched by flow, recycled through the BM of a mouse, and injected IV into nonobese diabetic scid γ mice (NSG) mice. A second myeloma mouse model used the MM.1S-GFP-luc cell line. Mice were treated IV with immunotoxins, and the tumor burden was assessed using bioluminescence imaging. We achieved complete durable remissions when treating mice with H929-GFP-luc cells with anti-BCMA RITs both leptomycin B-75 (LMB-75) [anti-BCMA-disulfide-stabilized (ds)-Fv-PE24] (where PE represents Pseudomonas exotoxin A) or LMB-70 (anti-BCMA-Fab-PE24) given every other day for 5-d (QOD×5) doses beginning on day 4 or day 8. Mice were disease free at 3 months; untreated mice became moribund around day 40. We also achieved long-term responses using the MM.1S-GFP-luc myeloma cell line. Treatment with an 1.5 mg/kg LMB-75 QOD×5 anti-BCMA RIT beginning on day 4 caused the complete disappearance of tumors for 80 days. To summarize, LMB-75 and LMB-70, our anti-BCMA RITs, induced complete durable responses in two myeloma models.

LMB-100 is a recombinant immunotoxin being developed for cancer treatment that is composed of a Fab that binds to mesothelin and a portion of Pseudomonas exotoxin A. LMB-100 is in clinical trials for the treatment of mesothelioma and pancreatic cancer. To determine if check point modulating antibodies enhance the formation of anti-drug antibodies (ADA) against LMB-100, we treated mice with LMB-100 and four different immune modulating monoclonal antibodies that have different mechanisms of action; anti-CTLA4, anti-OX40, anti-PD-1 and anti-PDL-1. We found that anti-PD-1 and anti PDL-1 do not increase the formation of ADA, but anti-CTLA-4 and anti-OX-40 do increase the onset of ADA. These results indicate that combining anti-CTLA-4 and anti-OX-40 with antibodies and other protein-based therapeutics may enhance ADA formation in humans.

Moxetumomab pasudotox (Moxe) is a bacterial toxin–based drug consisting of an anti-CD22 immunoglobulin variable domain genetically joined to Pseudomonas exotoxin (PE38) [1]. The drug was approved i...

Chimeric toxins are generated by removing or crippling the receptor-binding domains of bacterial or plant toxins and substituting in their place proteins or peptides that bind mammalian cells. The cell-binding protein will then dictate which cell type the chimeric toxin will bind and kill. Gene fusions are then introduced into an appropriate expression system and recombinant chimeric toxins produced. Recombinant chimeric toxins have been made with diphtheria toxin and Pseudomonas exotoxin in combination with a variety of hormones, antibodies, and other cell-binding proteins. To be cytotoxic, chimeric toxins must have binding, translocating, and enzymatic activities. Of the chimeric toxins examined, most bind 3to 10-fold less well than the corresponding native ligands (34,38,45). The chimeric toxin IL-2-PE40 is toxic for cells and cell lines that display IL-2 receptors on their surface. IX A major clinical focus for the use of chimeric toxins is the treatment of cancer and immunological disorders.

Immunotoxins are cytolytic fusion proteins developed for cancer therapy, composed of an antibody fragment that binds to a cancer cell and a protein toxin fragment that kills the cell. Pseudomonas exotoxin A (PE) is a potent toxin that is used for the killing moiety in many immunotoxins. Moxetumomab Pasudotox (Lumoxiti) contains an anti-CD22 Fv and a 38 kDa portion of PE. Lumoxiti was discovered in the Laboratory of Molecular Biology at the U.S. National Cancer Institute and co-developed with Medimmune/AstraZeneca to treat hairy cell leukemia. In 2018 Lumoxiti was approved by the US Food and Drug Administration for the treatment of drug-resistant Hairy Cell Leukemia. Due to the bacterial origin of the killing moiety, immunotoxins containing PE are highly immunogenic in patients with normal immune systems, but less immunogenic in patients with hematologic malignancies, whose immune systems are often compromised. LMB-100 is a de-immunized variant of the toxin with a humanized antibody that targets mesothelin and a PE toxin that was rationally designed for diminished reactivity with antibodies and B cell receptors. It is now being evaluated in clinical trials for the treatment of mesothelioma and pancreatic cancer and is showing somewhat diminished immunogenicity compared to its un modified parental counterpart. Here we review the immunogenicity of the original and de-immunized PE immunotoxins in mice and patients, the development of anti-drug antibodies (ADAs), their impact on drug availability and their effect on clinical efficacy. Efforts to mitigate the immunogenicity of immunotoxins and its impact on immunogenicity will be described including rational design to identify, remove, or suppress B cell or T cell epitopes, and combination of immunotoxins with immune modulating drugs.

Immunotoxins are a class of targeted cancer therapeutics in which a toxin such as Pseudomonas exotoxin A (PE) is linked to an antibody or cytokine to direct the toxin to a target on cancer cells. While a variety of PE-based immunotoxins have been developed and a few have demonstrated promising clinical and preclinical results, cancer cells frequently have or develop resistance to these immunotoxins. This review presents our current understanding of the mechanism of action of PE-based immunotoxins and discusses cellular mechanisms of resistance that interfere with various steps of the pathway. These steps include binding of the immunotoxin to the target antigen, internalization, intracellular processing and trafficking to reach the cytosol, inhibition of protein synthesis through ADP-ribosylation of elongation factor 2 (EF2), and induction of apoptosis. Combination therapies that increase immunotoxin action and overcome specific mechanisms of resistance are also reviewed.

Two renal-risk variants in the apolipoprotein L1 gene (APOL1) in African American (AA) deceased donors (DD) are associated with shorter renal allograft survival after transplantation. To identify additional genes contributing to allograft survival, a genome-wide association study was performed in 532 AA DDs. Phenotypic data were obtained from the Scientific Registry of Transplant Recipients. Association and single-nucleotide polymorphism (SNP)-by-APOL1 interaction tests were conducted using death-censored renal allograft survival accounting for relevant covariates. Replication and inverse-variance-weighted meta-analysis were performed using data from 250 AA DD in the Genomics of Transplantation study. Accounting for APOL1, multiple SNPs near the Nudix Hydrolase 7 gene (NUDT7) showed strong independent effects (P = 1.6 × 10(−8)−2.2 × 10(−8)). Several SNPs in the Translocation protein SEC63 homolog (SEC63; P = 2 × 10(−9)-3.7 × 10(−8)) and plasmacytoma variant translocation 1 (PVT1) genes (P = 4.0 × 10(−8)-7 × 10(−8)) modified the effect of APOL1 on allograft survival. SEC63 is expressed in human renal tubule cells and glomeruli, and PVT1 is associated with diabetic kidney disease. Overall, associations were detected for 41 SNPs (P = 2 × 10(−9)-5 × 10(−8)) contributing independently or interacting with APOL1 to impact renal allograft survival after transplantation from AA DD. Given the small sample size of the discovery and replication sets, independent validations and functional genomic efforts are needed to validate these results.

Mesothelin (MSLN) is a lineage restricted cell surface protein expressed in about 30% of human cancers and high MSLN expression is associated with poor survival in several different cancers. The restricted expression of MSLN in normal tissue and its frequent expression in cancers make MSLN an excellent target for antibody-based therapies. Many clinical trials with agents targeting MSLN have been carried out but to date none of these agents have produced enough responses to obtain FDA approval. MSLN shedding is an important factor that may contribute to the failure of these therapies, because shed MSLN acts as a decoy receptor and allows release of antibodies bound to cell-surface MSLN. We have investigated the mechanism of shedding and show here that members of the ADAM, MMP and BACE families of proteases all participate in shedding, that more than one protease can produce shedding in the same cell, and that inhibition of shedding greatly enhances killing of cells by an immunotoxin targeting MSLN. Our data indicates that controlling MSLN shedding could greatly increase the activity of therapies that target MSLN., Liu et al find that mesothelin (MSLN), a candidate target for antibody-based cancer therapy, is targeted by several proteases, and that inhibition of shedding facilitates killing of cells by an immunotoxin targeting MSLN.

Purpose: LMB-100 is a recombinant immunotoxin (iTox) consisting of a mesothelin-binding Fab for targeting and a modified Pseudomonas exotoxin A payload. Preclinical studies showed that combining taxanes with iTox results in synergistic antitumor activity. The objectives of this phase I/II study were to determine the MTD of LMB-100 when administered with nanoalbumin bound (nab)-paclitaxel to patients with previously treated advanced pancreatic adenocarcinoma and to assess the objective response rate. Patients and Methods: Patients (n = 20) received fixed-dose nab-paclitaxel (125 mg/m2 on days 1 and 8) with LMB-100 (65 or 100 μg/kg on days 1, 3, and 5) in 21-day cycles for 1–3 cycles. Results: Fourteen patients were treated on the dose escalation and an additional six in the phase II expansion. MTD of 65 μg/kg was established for the combination. Dose-limiting toxicity resulting from capillary leak syndrome (CLS) was seen in two of five patients treated at 100 μg/kg and one of six evaluable phase I patients receiving the MTD. Severity of CLS was associated with increases in apoptotic circulating endothelial cells. LMB-100 exposure was unaffected by anti-LMB-100 antibody formation in five of 13 patients during cycle 2. Seven of 17 evaluable patients experienced >50% decrease in CA 19-9, including three with previous exposure to nab-paclitaxel. One patient developed an objective partial response. Patients with biomarker responses had higher tumor mesothelin expression. Conclusions: Although clinical activity was observed, the combination was not well tolerated and alternative drug combinations with LMB-100 will be pursued.

Recombinant immunotoxins (RITs) are genetically engineered proteins being developed to treat cancer. They are composed of an Fv that targets a cancer antigen and a fragment of a bacterial toxin that kills tumor cells. Because the toxin is a foreign protein, it is immunogenic. The clinical success of RITs in patients with a normal immune system is limited by their immunogenicity. In this review, we discuss our progress in therapeutic protein deimmunization and the balancing act between immunogenicity and therapeutic potency. One approach is to prevent the activation of B cells by mapping and elimination of B-cell epitopes. A second approach is to prevent helper T-cell activation by interfering with major histocompatibility complex II presentation or T-cell recognition. Immunizing mice with RITs that were deimmunized by elimination of the murine B- or T-cell epitopes showed that both approaches are effective. Another approach to control immunogenicity is to modify the host immune system. Nanoparticles containing synthetic vaccine particles encapsulating rapamycin can induce immune tolerance and prevent anti-drug antibody formation. This treatment restores RIT anti-tumor activity that is otherwise neutralized because of immunogenicity.

Tac (CD25) is expressed on multiple hematologic malignancies and is a target for cancer therapies. LMB-2 is an extremely active anti-Tac recombinant immunotoxin composed of an Fv that binds to Tac and a 38-kDa fragment of Pseudomonas exotoxin A (PE38). Although LMB-2 has shown high cytotoxicity toward Tac-expressing cancer cells in clinical trials, its efficacy was hampered by the formation of anti-drug antibodies against the immunogenic bacterial toxin and by dose-limiting off-target toxicity. To reduce toxin immunogenicity and nonspecific toxicity, we introduced six point mutations into domain III that were previously shown to reduce T-cell immunogenicity and deleted domain II from the toxin, leaving only the 11aa furin cleavage site, which is required for cytotoxic activity. Although this strategy has been successfully implemented for mesothelin and CD22-targeting immunotoxins, we found that removal of domain II significantly lowered the cytotoxic activity of anti-Tac immunotoxins. To restore cytotoxic activity in the absence of PE domain II, we implemented a combined rational design and screening approach to isolate highly active domain II–deleted toxin variants. The domain II–deleted variant with the highest activity contained an engineered disulfide-bridged furin cleavage site designed to mimic its native conformation within domain II. We found that this approach restored 5-fold of the cytotoxic activity and dramatically improved the MTD. Both of these improvements led to significantly increased antitumor efficacy in vivo. We conclude that the next-generation anti-Tac immunotoxin is an improved candidate for targeting Tac-expressing malignancies. Mol Cancer Ther; 17(7); 1486–93. ©2018 AACR.

Recombinant immunotoxins (RITs) are chimeric proteins being developed for cancer treatment. They are composed of an Ab fragment that targets a cancer Ag and a cytotoxic portion of Pseudomonas exotoxin A. They are effective for patients with hematologic malignancies with defective immunity, but their efficacy against solid tumors is limited by anti-drug Ab (ADA) responses in immune-competent patients. Pre-existing Abs or immune memory owing to previous toxin exposure represent additional hurdles because they induce rapid and strong ADA responses. Here, we evaluated the efficacy of methotrexate (MTX) to prevent ADA formation against the mesothelin-targeting RIT LMB-100 in naive mice and in mice with pre-existing Abs. We found that low-dose MTX combined with LMB-100 completely suppressed the formation of ADAs in a dose- and frequency-dependent manner. Suppression of the immune response restored blood levels of LMB-100 and prevented its neutralization. Furthermore, combination of MTX with LMB-100 did not compromise the immune response against a second Ag given after stopping MTX, indicating specific immune tolerance. Adoptive transfer of splenocytes suppressed Ab responses to LMB-100 in recipient mice, indicating a durable immune tolerance. We conclude that combination of MTX and LMB-100 is effective at preventing immune responses in a durable, Ag-specific manner. We propose combining low-dose MTX in immune-competent cancer patients receiving RIT therapy to prevent immunogenicity. This approach could be applied to other immunogenic therapeutic agents and to proteins for which there is pre-existing immunity.

Chronic glomerular injury is associated with eventual development of tubulointerstitial fibrosis. Here we aimed to assess whether, and how, mild chronic tubulointerstitial injury affects glomeruli. For this, we generated mice expressing different toxin receptors, one on their proximal tubular epithelial cells (diphtheria toxin receptor [DTR]) and the other only on podocytes (human CD25 [IL-2R] driven by the nephrin promoter [Nep25]), allowing serial induction of tubule-specific and glomerular (podocyte)-specific injury, respectively. Six weeks after diphtheria toxin injection, mild interstitial fibrosis was found in Nep25 + /DTR + , but not in Nep25 + /DTR – mice. However, atubular glomeruli and neuronal nitric oxide synthase, a mediator of tubuloglomerular feedback, were higher in Nep25 + /DTR + than in DTR – mice and these atubular glomeruli had less podocyte density as assessed by WT-1 biomarker expression. Peritubular capillary density, hypoxia-inducible factor-1 and -2, and cyclooxygenase 2 expression were similar at week six in the two groups. At week seven, all mice were given the immunotoxin LMB-2, which binds to CD25 to induce podocyte injury. Ten days later, proteinuria, podocyte injury, and glomerulosclerosis were more severe in Nep25 + /DTR + than Nep25 + /DTR – mice with more severe sclerosis in the tubule-connected glomeruli. This supports the concept that even mild preexisting tubulointerstitial injury sensitizes glomeruli to subsequent podocyte-specific injury. Thus, increased atubular glomeruli and abnormal tubuloglomerular feedback significantly contribute to the crosstalk between the tubulointerstitium and glomeruli.

Recombinant immunotoxins targeting B-cell maturation antigen are cytotoxic to myeloma cell lines and myeloma cells from patients

Renal allografts from deceased African American donors with two apolipoprotein L1 gene (APOL1) renal-risk variants fail sooner than kidneys from donors with fewer variants. The Kidney Donor Risk Index (KDRI) was developed to evaluate organ offers by predicting allograft longevity and includes African American race as a risk factor. Substituting APOL1 genotype for race may refine the KDRI. For 622 deceased African American kidney donors, we applied a 10-fold cross-validation approach to estimate contribution of APOL1 variants to a revised KDRI. Cross-validation was repeated 10 000 times to generate distribution of effect size associated with APOL1 genotype. Average effect size was used to derive the revised KDRI weighting. Mean current-KDRI score for all donors was 1.4930 versus mean revised-KDRI score 1.2518 for 529 donors with no or one variant and 1.8527 for 93 donors with two variants. Original and revised KDRIs had comparable survival prediction errors after transplantation, but the spread in Kidney Donor Profile Index based on presence or absence of two APOL1 variants was 37 percentage points. Replacing donor race with APOL1 genotype in KDRI better defines risk associated with kidneys transplanted from deceased African American donors, substantially improves KDRI score for 85-90% of kidneys offered, and enhances the link between donor quality and recipient need.

// Shingo Inaguma 1, 2 , Zengfeng Wang 1 , Jerzy Lasota 1 , Masanori Onda 3 , Piotr Czapiewski 4, 5 , Renata Langfort 6 , Janusz Rys 7 , Joanna Szpor 8 , Piotr Waloszczyk 9 , Krzysztof Okon 8 , Wojciech Biernat 4 , Hiroshi Ikeda 2 , David S. Schrump 10 , Raffit Hassan 10 , Ira Pastan 3 , Markku Miettinen 1 1 Laboratory of Pathology, National Cancer Institute, Bethesda, MD, USA 2 Department of Pathology, Aichi Medical University School of Medicine, Nagakute, Japan 3 Laboratory of Molecular Biology, National Cancer Institute, Bethesda, MD, USA 4 Department of Pathomorphology, Medical University of Gdansk, Gdansk, Poland 5 Department of Pathology, Otto-von-Guericke University Magdeburg, Magdeburg, Germany 6 Department of Pathology, National Tuberculosis and Lung Diseases Research Institute, Warsaw, Poland 7 Department of Tumor Pathology, Centre of Oncology, Maria Sklodowska-Curie Memorial Institute, Krakow Branch, Poland 8 Department of Pathomorphology, Jagiellonian University, Krakow, Poland 9 Independent Laboratory of Pathology, Zdunomed, Szczecin, Poland 10 Thoracic and GI Oncology Branch, National Cancer Institute, Bethesda, MD, USA Correspondence to: Shingo Inaguma, email: inaguma@aichi-med-u.ac.jp Keywords: mesothelin (MSLN), immunohistochemistry, 5B2, MN-1, malignant pleural mesothelioma Received: October 05, 2016 Accepted: February 15, 2017 Published: March 01, 2017 ABSTRACT Mesothelin (MSLN) is a glycophosphatidylinositol (GPI)-linked cell surface protein highly expressed in several types of malignant tumors sometimes in association with increased tumor aggressiveness and poor clinical outcome. In the present study, 1562 tumors were immunohistochemically analyzed for mesothelin expression using two different types of mouse monoclonal antibodies (5B2 and MN-1) to determine the clinical usefulness of mesothelin immunohistochemistry as well as to pinpoint potential targets for future anti-mesothelin therapy. Also, characterization of selected mesothelin-positive tumors was performed by immunohistochemistry and oncogene sequencing. Among the tumors analyzed, the highest frequencies of mesothelin-positivity were detected in ovarian serous carcinoma (90% in 5B2 and 94% in MN-1). Both antibodies showed frequent positivity in pancreatic adenocarcinoma (71% using 5B2 and 87% using MN-1) and malignant pleural mesothelioma (75% using 5B2 and 78% using MN-1). In malignant mesothelioma, overall survival was significantly longer in the cohort of patients with diffuse membranous expression of mesothelin ( P < 0.001). Both antibodies showed positive staining in thymic carcinoma (77% in 5B2 and 59% in MN-1), however, no expression was detected in thymoma. No correlation was detected between mesothelin expression and mismatch repair system deficient phenotype or gene mutation ( BRAF and RAS ) status in gastrointestinal adenocarcinomas. Mesothelin immunohistochemistry may assist the differential diagnosis of thymoma vs. thymic carcinoma as well as prognostication of mesothelioma patients. Our results demonstrate that patients with solid tumors expressing mesothelin could be targeted by anti-mesothelin therapies.

D2C7-(scdsFv)-PE38KDEL (D2C7-IT) is a novel recombinant Pseudomonas exotoxin A-based immunotoxin (IT), targeting both wild-type epidermal growth factor receptor (EGFRwt) and mutant EGFR variant III (EGFRvIII) proteins overexpressed in glioblastomas. Initial pre-clinical testing demonstrated the anti-tumor efficacy of D2C7-IT against orthotopic glioblastoma xenograft models expressing EGFRwt, EGFRvIII, or both EGFRwt and EGFRvIII. A good laboratory practice (GLP) manufacturing process was developed to produce sufficient material for a phase I/II clinical trial. D2C7-IT was expressed under the control of the T7 promoter in Escherichia coli BLR (λ DE3). D2C7-IT was produced by a 10-L batch fermentation process and was then purified from inclusion bodies using anion exchange, size exclusion, and an endotoxin removal process that achieved a yield of over 300 mg of purified protein. The final vialed batch of D2C7-IT for clinical testing was at a concentration of 0.12 ± 0.1 mg/mL, the pH was at 7.4 ± 0.4, and endotoxin levels were below the detection limit of 10 EU/mL (1.26 EU/mL). The stability of the vialed D2C7-IT has been monitored over a period of 42 months through protein concentration, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), isoelectric focusing, size exclusion chromatography, cytotoxicity, sterility, and pH measurements. The vialed D2C7-IT is currently being tested in a phase I/II clinical trial by intratumoral convection-enhanced delivery for 72 h in patients with recurrent glioblastoma (NCT02303678, D2C7 for Adult Patients with Recurrent Malignant Glioma; clinicaltrials.gov ).

LMB-100 is an immunotoxin targeting the cell surface protein mesothelin, which is highly expressed in many cancers including mesothelioma. Having observed that patients receiving pembrolizumab off protocol after LMB-100 treatment had increased tumor responses; we characterized these responses and developed animal models to study whether LMB-100 made tumors more responsive to antibodies blocking programmed cell death protein 1 (PD-1). The overall objective tumor response in the 10 patients who received PD-1 inhibitor (pembrolizumab, 9; nivolumab, 1) after progression on LMB-100 was 40%, and the median overall survival was 11.9 months. Of the seven evaluable patients, four had objective tumor responses, including one complete response and three partial responses, and the overall survival for these patients was 39.0+, 27.7, 32.6+, and 13.8 months. When stratified with regard to programmed death ligand 1 (PD-L1) expression, four of five patients with tumor PD-L1 expression had objective tumor response. Patients with positive tumor PD-L1 expression also had increased progression-free survival (11.3 versus 2.1 months, P = 0.0018) compared with those lacking PD-L1 expression. There was no statistically significant difference in overall survival (27.7 versus 6.8 months, P = 0.1). LMB-100 caused a systemic inflammatory response and recruitment of CD8(+) T cells in patients’ tumors. The enhanced antitumor effects with LMB-100 plus anti–PD-1 antibody were also observed in a human peripheral blood mononuclear cell–engrafted mesothelioma mouse model and a human mesothelin–expressing syngeneic lung adenocarcinoma mouse model. LMB-100 plus pembrolizumab is now being evaluated in a prospective clinical trial for patients with mesothelioma.

Previously, we found that mild tubulointerstitial injury sensitizes glomeruli to subsequent injury. Here, we evaluated whether stabilization of hypoxia-inducible factor-α (HIF-α), a key regulator of tissue response to hypoxia, ameliorates tubulointerstitial injury and impact on subsequent glomerular injury. Nep25 mice, which express the human CD25 receptor on podocytes under control of the nephrin promotor and develop glomerulosclerosis when a specific toxin is administered were used. Tubulointerstitial injury, evident by week two, was induced by folic acid, and mice were treated with an HIF stabilizer, dimethyloxalylglycine or vehicle from week three to six. Uninephrectomy at week six assessed tubulointerstitial fibrosis. Glomerular injury was induced by podocyte toxin at week seven, and mice were sacrificed ten days later. At week six tubular injury markers normalized but with patchy collagen I and interstitial fibrosis. Pimonidazole staining, a hypoxia marker, was increased by folic acid treatment compared to vehicle while dimethyloxalylglycine stimulated HIF-2α expression and attenuated tubulointerstitial hypoxia. The hematocrit was increased by dimethyloxalylglycine along with downstream effectors of HIF. Tubular epithelial cell injury, inflammation and interstitial fibrosis were improved after dimethyloxalylglycine, with further reduced mortality, interstitial fibrosis, and glomerulosclerosis induced by specific podocyte injury. Thus, our findings indicate that hypoxia contributes to tubular injury and consequent sensitization of glomeruli to injury. Hence, restoring HIFs may blunt this adverse crosstalk of tubules to glomeruli.

AIMS: To characterize the pharmacokinetics (PK) of moxetumomab pasudotox, an anti‐CD22 recombinant immunotoxin, in adults with relapsed or refractory hairy cell leukaemia, we examined data from a phase 1 study (Study 1001; n = 49) and from the pivotal clinical study (Study 1053; n = 74). METHODS: Data from both studies were pooled (n = 123) to develop a population PK model. Covariates included demographics, disease state, liver and kidney function, prior treatment, and antidrug antibodies (ADAs). Exposure–response and exposure–safety were analysed separately by study. A 1‐compartment model with linear elimination from the central compartment and 2 clearance (CL) rates was developed. RESULTS: Moxetumomab pasudotox was cleared more rapidly after cycle 1, day 1 (CL(1) = 24.7 L/h) than subsequently (CL(2) = 3.76 L/h), with high interindividual variability (116 and 109%, respectively). In Study 1053, patients with ADA titres >10 240 showed ~4‐fold increase in CL. Higher exposures (≥median) were related to higher response rates, capillary leak syndrome and increased creatinine (Study 1053 only), or grade ≥3 adverse events (Study 1001 only). Clinical benefits were still observed in patients with lower exposure or high ADA titres. CONCLUSION: Despite a high incidence of immunogenicity with increased clearance, moxetumomab pasudotox demonstrated efficacy in hairy cell leukaemia.

Mesothelin (MSLN) is a glycophosphatidylinositol (GPI)-linked cell surface protein that is highly expressed in several types of malignant tumor, including malignant pleural mesothelioma, ovarian cancer and pancreatic adenocarcinoma. Recently, a comprehensive immunohistochemical study using MN-1 monoclonal antibody identified a significant number of colorectal tumors in which MSLN was expressed. However, the clinicopathological profiles and survival of patients with MSLN-positive colorectal cancer have not been fully analyzed. In the current study, the expression of MSLN in 270 primary and 44 metastatic colorectal tumors was immunohistochemically analyzed to determine the clinical usefulness of MSLN immunohistochemistry and to identify potential candidates for future anti-MSLN therapy. In vitro experiments using colon cancer cell lines were performed to investigate the biological significance of MSLN expression in tumors. The results of univariate analyses identified a significant correlation between MSLN expression and females (P=0.0042). Furthermore, an inverse correlation between MSLN expression and solid/sheet-like proliferation (P=0.014) was also revealed. Additionally, overall survival was significantly shorter in patients with diffuse luminal/membranous expression of MSLN (P=0.018). Multivariable Cox hazards regression analysis revealed diffuse MSLN expression (hazard ratio, 2.26; 95% confidence interval, 1.04-4.91; P=0.039) as a potential risk factor. When comparing primary CRCs and the metastasis of each, a weakly positive correlation was identified for MSLN positivity (% positive cells; R=0.484; P

Podocytes are known to express various complement factors including complement factor H (CFH) and to promote the removal of both subendothelial and subepithelial immune complex (IC) deposits. Using podocyte-selective injury model NEP25 mice and an IgG3-producing hybridoma clone 2B11.3 established by MRL/lpr mice, the present study investigated the role of podocyte complement regulation in only subendothelial IC deposition. In immunotoxin (LMB2) induced fatal podocyte injury (NEP25/LMB2) at day 12, glomerular CFH and C3a receptor (C3aR) expression was decreased as compared with NEP25/vehicle mice. In contrast, in sublytic podocyte injury 5 days after LMB2, glomerular CFH and C3aR expression was increased as compared with NEP25/vehicle mice. Intra-abdominal injection of 2B11.3 hybridoma to NEP25 mice (NEP25/hybridoma) caused IC deposition limited to the subendothelial area associated with unaltered CFH expression. NEP25/hybridoma mice with sublytic podocyte injury (NEP25/hybridoma/LMB2) resulted in increased glomerular CFH expression (1.7-fold) accompanied by decreased subendothelial IC deposition, as compared with NEP25/hybridoma. Immunostaining revealed that CFH was dominantly expressed in podocytes of NEP25/hybridoma/LMB2. In addition, puromycin-induced sublytic podocyte injury promoted CFH expression in immortalized mouse podocytes in vitro. These results suggest that in response to sublytic levels of injury, podocyte induced CFH expression locally and clearance of subendothelial IC deposits.

Hairy cell leukemia (HCL) is an indolent B-cell malignancy characterized by high initial sensitivity to purine analog chemotherapy, minimal residual disease (MRD) frequently accompanying complete remission (CR), and relapses requiring additional treatment. Repeat chemotherapy shows decreasing efficacy and increasing toxicity with each course. Newer therapies targeting BRAF/MEK or Bruton's tyrosine kinase are effective but generally leave MRD. Rituximab has modest activity as a single agent and can achieve MRD-negative CR in combination with purine analogs, but there is significant toxicity from the chemotherapy. Moxetumomab pasudotox-tdfk (Moxe) is a biologic containing an antibody fragment (Fv) binding to CD22, attached to a portion of Pseudomonas exotoxin A. Binding to CD22 enables the toxin to enter and kill cells. Moxe is administered by 30-minute infusions on days 1, 3, and 5 of up to six cycles spaced 4 weeks apart. In phase I testing, 64% of 33 patients at the highest dose level achieved CR, most without MRD. Lack of MRD correlated with prolonged CR duration; of 11 MRD-negative CRs, 10 were still in CR after a median of 42 months of observation. In pivotal testing, 75% of 80 patients had a hematologic response, 41% with CR; 82% (27/33) of CRs were MRD-negative, and only 4 of the 27 MRD-negative patients relapsed during the follow-up period. Hemolytic uremic syndrome and capillary leak syndrome were each observed in 9% of patients, all reversible. In September 2018, the U.S. Food and Drug Administration approved Moxe for the treatment of relapsed/refractory HCL after ≥2 prior therapies. Moxe is undergoing further development in combination with rituximab. IMPLICATIONS FOR PRACTICE: Hairy cell leukemia (HCL) has effective treatments including purine analogs with and without rituximab, and oral inhibitors of BRAF, MEK and Bruton's tyrosine kinase (BTK). Despite these therapies, relapse occurs, and moxetumomab pasudotox has an important role in relapsed and refractory HCL because of its ability to achieve high rates of complete remissions (CRs) without chemotherapy; most of these CRs are without minimal residual disease (MRD). CR duration is enhanced in patients who achieve eradication of MRD. To improve the efficacy of this recombinant immunotoxin, a phase I trial is underway in combination with rituximab to reduce tumor burden and decrease immunogenicity.

Funding: This research was funded by MakCHS–UCBerkeley-Yale Pulmonary Complications of AIDS Research Training program however initial data was obtained from the East African Public Health Laboratory Networking (EAPHLN) Project which was funded by the World Bank. This research was also funded by the Office of AIDS Research (US) NIH D43TW009607 to IN. Background: Determining mycobacterial burden is important in assessing severity of disease, evaluating infectiousness and predicting patient treatment outcomes. Mycobacterial burden assessed by smear microscopy grade and time to culture positivity is clearly interpretable by most physicians. GeneXpert (Xpert) has been recommended by WHO as a first line tuberculosis (TB) diagnostic test as an alternative to smear microscopy. Xpert gives cycle threshold (Ct) values as a potential measure for mycobacterial burden. For physicians to clearly interpret Ct values as measures of mycobacterial burden, this study compared the Xpert quantification capabilities with those of smear microscopy and culture. The study also determined a linear relationship between Xpert Ct values and MGIT culture time to positivity (MGIT-TTP) and associated factors. A cut off Ct value which best predicts smear positivity was also determined using the Receiver Operator Curve analysis method. Results: Excluding missing results and rifampicin resistant TB cases, a moderately strong correlation of 0.55 between Xpert Ct value and smear grade was obtained. A weak correlation of 0.37 was obtained between Xpert Ct values and MGIT time to positivity while that between Xpert Ct values and LJ culture was 0.34. The Xpert Ct values were found to increase by 2.57 for every unit increase in days to positive and HIV status was significantly associated with this relationship. A cut off Ct value of 23.62 was found to best predict smear positivity regardless of HIV status. Conclusion: Our study findings show that GeneXpert Ct values are comparable to smear microscopy as a measure of M. tuberculosis burden and can be used to replace smear microscopy. However, given the low correlation between Xpert Ct value and culture positivity, Xpert Ct values cannot replace culture as a measure of M. tuberculosis burden among TB patients. Publisher PDF

3051 Background: LMB-100 recombinant immunotoxin consists of a mesothelin-binding Fab for targeting a modified Pseudomonas exotoxin A payload to tumors. Previous clinical trials demonstrated that almost all patients formed anti-drug-antibodies (ADAs) to LMB-100 that made administration beyond cycle 2 ineffective. Tofacitinib is an oral JAK inhibitor that prevented formation of ADAs against a closely related immunotoxin in pre-clinical studies. The primary objective of the dose escalation cohort was assessment of safety and tolerability of LMB-100 given with tofacitinib to patients with mesothelin-expressing solid tumors. The primary objective of the expansion cohort was to determine whether co-administration of tofacitinib delays formation of neutralizing LMB-100 ADAs. Methods: Patients (n = 13) with pancreatic adenocarcinoma and other mesothelin-expressing solid tumors (n = 3; cholangiocarcinoma, appendix, cystadenocarcinoma) were treated for up to 3 cycles with LMB-100 as a 30-minute infusion on days 4, 6, and 8 at two dose levels (100 and 140 mcg/kg) and co-treated with oral tofacitinib for the first 10 days of the cycle (10 mg BID). Results: Dose level 1 of LMB-100 was started at 100 mcg/kg one dose level below the single agent MTD. Dose escalation to 140 mcg/kg (dose level 2) resulted in DLTs in 2 of the 3 patients treated: grade 3 cardiac toxicity and grade 4 hyponatremia, both attributed to capillary leak syndrome. Ultimately, 7 patients were treated at dose level 1 without DLTs and 100 mcg/kg was chosen as the LMB-100 dose for the expansion cohort. The last of 6 patients treated in the expansion cohort developed grade 4 pericardial effusion leading to early closure of the study for toxicity. No objective responses were seen. Of the 8 patients who received two cycles of treatment at MTD, 4 met prespecified criteria for ADA prevention, and 2 patients who went on to receive cycle 3 had detectable LMB-100 plasma drug levels after administration. Conclusions: LMB-100 was unable to be co-administered safely with tofacitinib. ADA formation was prevented in 2 patients through 3 cycles, a rare occurrence. Clinical trial information: NCT04034238.

7036 Background: Anti-CD22 recombinant immunotoxin moxetumomab pasudotox (Moxe) is FDA-approved for hairy cell leukemia (HCL) patients who have received at least two prior systemic therapies including a purine nucleoside analog. In phase 3 testing the complete remission (CR) rate was 41%, and response was higher in patients with lower tumor burden and lower titers of antidrug antibodies (ADA). Phase 1 testing indicated that most CRs were without minimal residual disease (MRD) and eradication of MRD was associated with prolonged CR duration. Monoclonal antibody (Mab) rituximab binds to CD20 on HCL cells and induces apoptosis or immune-mediated killing, but as a single-agent achieved only 13% CRs in relapsed HCL requiring therapy. In a phase 1 trial to determine safety, rituximab was combined with Moxe, with the goal to help reduce tumor burden and to prevent or delay ADA by killing normal B-cells. Methods: To allow rituximab sufficient time to accomplish both goals, it was infused 3 days before day 1 of cycle 1 at 375 mg/m2, and Moxe was given by 30-minute infusion on days 1, 3 and 5. On repeat cycles of Moxe days 1, 3 and 5, rituximab was given on day 1. Cycles were generally spaced 4 weeks apart. Moxe was begun at a lower dose, 30 rather than the 40 mcg/kg dose used in phase 3 in case the rituximab would increase its toxicity. Bone marrow aspirate flow cytometry, which can detect 0.002% HCL cells, was the most sensitive test used for MRD detection, much more sensitive than BRAF V600E digital droplet PCR (ddPCR) or bone marrow biopsy immunohistochemistry (IHC). Patients could receive 4 cycles past MRD-free CR, but not more than 8 cycles. Results: Three patients received Moxe at 30 mcg/Kg/dose and 6 received 40 mcg/Kg/dose, all without dose limiting toxicity (DLT). There was no evidence of hemolytic uremic syndrome or capillary leak syndrome. To prevent intravascular hypovolemia due to expected third spacing, patients were encouraged to drink one cup per hour of water or other fluid from days 1 to 8 and take dexamethasone 4 mg orally if headache or nausea prevented good oral hydration. Of the 9 patients, 7 (78%) achieved CR after 2 (n = 6) or 3 (n = 1) cycles, and achieved MRD-free CR after 2 (n = 3), 4 (n = 3) or 6 (n = 1) cycles. No patients became infected with COVID-19. Conclusions: This phase 1 trial met its primary endpoint of determining whether rituximab could be safely combined with Moxe and will enroll 4 additional patients to further access clinical activity. Further testing will determine whether addition of a CD20 Mab to Moxe significantly improves clinical outcome compared to Moxe alone, particularly long-term MRD-free CR rate. Clinical trial information: NCT03805932.

TPS452 Background: LMB-100 recombinant immunotoxin consists of a mesothelin-binding Fab for targeting with a modified Pseudomonas exotoxin A payload. Formation of anti-drug antibodies (ADAs) is thought to contribute significantly to limited clinical efficacy of LMB-100 seen in prior clinical trials. Most patients develop clinically meaningful ADAs after 1-2 cycles of LMB-100, resulting in rapid neutralization of LMB-100 during subsequent cycles and undetectable plasma drug levels. Tofacitinib is an oral Janus Kinase-1 and -3 (JAK) inhibitor approved by the FDA for the treatment of rheumatoid arthritis and ulcerative colitis. Pre-clinical studies have shown that tofacitinib can prevent the formation of ADAs against recombinant immunotoxin (Onda et al. Journal of Immunology 2014), and that co-administration of tofacitinib with LMB-100 increases immunotoxin serum half- life in mice and anti-tumor efficacy (Simon et al. JCI Insight 2019). We hypothesize that co-administration of tofacitinib with LMB-100 will prevent or delay the formation of high titer ADAs to LMB-100, such that 2 effective cycles of immunotoxin can be administered to patients. Methods: This phase I clinical trial consists of a dose escalation phase using 3+3 design to determine the maximum tolerated dose (MTD) of LMB-100 that can be administered with tofacitinib in participants (n = 18 max) with mesothelin-expressing solid tumors, followed by a dose expansion phase at the MTD for participants (n = 15) with pancreatic adenocarcinoma or extrahepatic cholangiocarcinoma. The primary objective of the expansion phase is to determine whether co-administration of tofacitinib delays formation of neutralizing anti-LMB-100 ADAs for cycle 2 of treatment as measured by LMB-100 plasma drug levels. A positive outcome will be reached if percent of participants achieving threshold LMB-100 drug levels during cycle 2 increases from 50% to 80% (83.6% probability if >10 of 15 evaluable participants meet this milestone). Plasma drug levels during cycle 3 will also be analyzed as a secondary endpoint. Key inclusion criteria include adults with histologically confirmed previously treated solid tumor malignancies. Participants will receive tofacitinib 10 mg twice daily on days 1-10 and LMB-100 at 65, 100, or 140 mcg/kg on days 4, 6 and 8 of a 21-day cycle. The dose escalation phase has been completed and enrollment onto dose expansion phase is ongoing. Clinical trial information: NCT04034238.

Epigenetic modifications alter transcriptional activity and contribute to the effects of environment on the individual risk of obesity and Type 2 Diabetes (T2D). Here, we have estimated the in vivo effect of a fat-enriched diet (HFD) on the expression and the epigenetic regulation of the Ankyrin repeat domain 26 (Ankrd26) gene, which is associated with the onset of these disorders. In visceral adipose tissue (VAT), HFD exposure determined a specific hyper-methylation of Ankrd26 promoter at the −436 and −431 bp CpG sites (CpGs) and impaired its expression. Methylation of these 2 CpGs impaired binding of the histone acetyltransferase/transcriptional coactivator p300 to this same region, causing hypo-acetylation of histone H4 at the Ankrd26 promoter and loss of binding of RNA Pol II at the Ankrd26 Transcription Start Site (TSS). In addition, HFD increased binding of DNA methyl-transferases (DNMTs) 3a and 3b and methyl-CpG-binding domain protein 2 (MBD2) to the Ankrd26 promoter. More importantly, Ankrd26 down-regulation enhanced secretion of pro-inflammatory mediators by 3T3-L1 adipocytes as well as in human sera. Thus, in mice, the exposure to HFD induces epigenetic silencing of the Ankrd26 gene, which contributes to the adipose tissue inflammatory secretion profile induced by high-fat regimens.

OBJECTIVES: Advanced thymic epithelial tumors (TETs) lack adequate treatment options in part due to absence of well characterized tumor-specific antigens. Mesothelin, a cell surface antigen, has been used successfully as a target for tumor-directed therapy. We sought to determine tumor expression and serum levels of mesothelin in patients with TETs. PATIENTS AND METHODS: Tissue samples were obtained from 71 patients with histologically confirmed, unresectable advanced TETs and evaluated for mesothelin expression by immunohistochemistry. The evaluation was blinded for clinical data and outcome. Mesothelin expression and its association with clinico-pathological parameters and survival were assessed. RESULTS: Thymic carcinoma, thymoma, and thymic neuroendocrine tumors (NETs) accounted for 34 (48%), 29 (41%), and 8 (11%) cases respectively. Mesothelin expression was seen in a significantly larger proportion of thymic carcinoma (27/34, 79%) than thymoma (3/29, 10%) (P50% of tumor cells) had significantly improved overall survival (median not reached, n=19) compared to patients with no or low mesothelin expression (1.60 years; 95% CI: 1.24 to 4.94 years; n=15; HR=4.46, 95% CI: 1.55 to 12.80; p=0.0026). CONCLUSION: Mesothelin expression is frequently observed in advanced thymic carcinomas, infrequently in thymomas and is absent in thymic NETs. Due to strong, membranous expression mesothelin is a potential therapeutic target in thymic carcinoma.

Synthetic lethal screens are used to discover new combination treatments for cancer. In traditional high-throughput synthetic lethal screens, compounds are tested at a single dose, and hit selection is based on threshold activity values from the variance of the efficacy of the compounds tested. The limitation of the single-dose screening for synthetic lethal screens is that it does not allow for the robust detection of differential activities from compound collections with a broad range of potencies and efficacies. There is therefore a need to develop screening approaches that enable the identification of compounds with synthetic lethal effects based on changes in both potency and efficacy. Here we describe the implementation of a dose response–based synthetic lethal screen to find drugs that enhance or mitigate the cytotoxic effect of an immunotoxin protein (HA22). We developed a data analysis framework for the selection of compounds with enhancing or mitigating cytotoxic activities based on the use of dose-response parameters. The data analysis framework includes an ensemble ranking approach that allows the use of multiple dose-response parameters in a nonparametric fashion. Quantitative high-throughput screening (HTS) enables the identification of compounds with synthetic lethal activity not identified by single-dose HTS.

Mesothelin overexpression in lung adenocarcinomas correlates with the presence of activating KRAS mutations and poor prognosis. Hence SS1P, a mesothelin‐targeted immunotoxin, could offer valuable treatment options for these patients, but its use in solid tumor therapy is hampered by high immunogenicity and non‐specific toxicity. To overcome both obstacles we developed RG7787, a de‐immunized cytotoxic fusion protein comprising a humanized SS1 Fab fragment and a truncated, B‐cell epitope silenced, 24 kD fragment of Pseudomonas exotoxin A (PE24). Reactivity of RG7787 with sera from immunotoxin‐treated patients was >1000 fold reduced. In vitro RG7787 inhibited cell viability of lung cancer cell lines with picomolar potency. The pharmacokinetic properties of RG7787 in rodents were comparable to SS1P, yet it was tolerated up to 10 fold better without causing severe vascular leak syndrome or hepatotoxicity. A pharmacokinetic/pharmacodynamic model developed based on NCI‐H596 xenograft studies showed that for RG7787 and SS1P, their in vitro and in vivo potencies closely correlate. At optimal doses of 2–3 mg/kg RG7787 is more efficacious than SS1P. Even large, well established tumors (600 mm3) underwent remission during three treatment cycles with RG7787. Also in two patient‐derived lung cancer xenograft models, Lu7336 and Lu7187, RG7787 showed anti‐tumor efficacy. In monotherapy two treatment cycles were moderately efficacious in the Lu7336 model but showed good anti‐tumor activity in the KRAS mutant Lu7187 model (26% and 80% tumor growth inhibition, respectively). Combination of RG7787 with standard chemotherapies further enhanced efficacy in both models achieving near complete eradication of Lu7187 tumors.

Mesothelin (MSLN) is a differentiation antigen that is highly expressed in many epithelial cancers. MSLN is an important therapeutic target due to its high expression in cancers and limited expression in normal human tissues. Although it has been assumed that shed antigen is a barrier to immunotoxin action, a modeling study predicted that shed MSLN may enhance the action of MSLN-targeting recombinant immunotoxins such as SS1P and similar therapeutics by facilitating their redistribution within tumors. We aimed to determine whether shed MSLN enhances or reduces the antitumor effect of MSLN-targeting immunotoxins SS1P and RG7787. We engineered a cell line, A431/G9 (TACE mutant) that expresses a mutant form of MSLN in which the TNF-converting enzyme protease site is replaced with GGGS. We compared the response of the TACE-mutant cells with immunotoxins SS1P and RG7787 with that of the parental A431/H9 cell line. We show that TACE-mutant cells shed 80% less MSLN than A431/H9 cells, that TACE-mutant cells show a 2- to 3-fold increase in MSLN-targeted immunotoxin uptake, and that they are about 5-fold more sensitive to SS1P killing in cell culture. Tumors with reduced shedding respond significantly better to treatment with SS1P and RG7787. Our data show that MSLN shedding is an impediment to the antitumor activity of SS1P and RG7787. Approaches that decrease MSLN shedding could enhance the efficacy of immunotoxins and immunoconjugates targeting MSLN-expressing tumors. Mol Cancer Ther; 15(7); 1648–55. ©2016 AACR.

// Ronit Mazor 1 , Masanori Onda 1 , Dong Park 1,3 , Selamawit Addissie 1 , Laiman Xiang 1,* , Jingli Zhang 2 , Raffit Hassan 2 and Ira Pastan 1 1 Laboratory of Molecular Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland, USA 2 Thoracic and GI Oncology Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland, USA 3 New Business Development Department, Medytox Inc., Bundang-gu, Seongnam-si, Gyeonggi-do, South Korea * Retired Correspondence to: Ira Pastan, email: // Keywords : epitope, immunogenicity, rational design, mesothelioma, pancreatic cancer Received : March 22, 2016 Accepted : April 22, 2016 Published : May 04, 2016 Abstract Recombinant immunotoxins (RITs) are genetically engineered proteins being developed to treat cancer. They are composed of an Fv that targets a cancer antigen and a portion of a protein toxin. Their clinical success is limited by their immunogenicity. Our goal is to produce a new RIT that targets mesothelin and is non-immunogenic by combining mutations that decrease B- and T-cell epitopes. Starting with an immunotoxin that has B-cell epitopes suppressed, we added mutations step-wise that suppress T-cell epitopes. The final protein (LMB-T14) has greatly reduced antigenicity as assessed by binding to human anti-sera and a greatly decreased ability to activate helper T-cells evaluated in a T-cell activation assay. It is very cytotoxic to mesothelioma cells from patients , and to cancer cell lines. LMB-T14 produces complete remissions of a mesothelin expressing cancer (A431/H9) xenograft. The approach used here can be used to de-immunize other therapeutic foreign proteins.

The identification of new therapeutic targets is of profound importance if we are to improve outcomes in gastroesophageal cancer. This study assessed the rate of mesothelin expression in tumors of western patients with upper gastrointestinal tract carcinomas. In addition, the AGS gastric cancer cell line was tested for sensitivity to SS1(dsFv)PE38, a mesothelin-targeting immunotoxin. Previously constructed tissue microarrays containing samples from 127 patients with gastroesophageal adenocarcinomas were examined by immunohistochemistry (IHC) for mesothelin expression. Labeling for HER2-neu, E-cadherin, and c-met were also assessed. Tumors were considered positive for mesothelin if at least moderate cytoplasmic/membranous or luminal staining was present in minimum 10% of the neoplastic cells. The AGS gastric cancer cell line was assessed for surface mesothelin expression by flow cytometry and the viability of cells treated with SS1P was measured. Gastroesophageal cancers were mesothelin positive in 64 of 127 tumors [50.4%; 95% confidence interval (CI), 41.4%-59.4%], whereas only 9 carcinomas (7.1%; 95% CI, 3.3%-13.0%) were HER2-neu IHC 3+ positive and 8 (6.6%; 95% CI, 2.9%-12.5%) were c-met positive. Mesothelin expression increased from stage I to stage IV tumors (37.5% to 56.3%, respectively, P=0.10). The AGS gastric cancer cell line was sensitive to the immunotoxin with an EC50 value in the low picomolar range (0.4 ng/mL). A gastric cancer cell line derived from a western patient was exquisitely sensitive to the mesothelin-targeted immunotoxin SS1P. Clinical trials involving novel mesothelin targeted immunotherapeutics in gastroesophageal cancer are currently in development.

Hairy cell leukemia (HCL) is an indolent B-cell malignancy effectively treated but not often cured by purine analog therapy; after multiple courses of purine analogs, patients can become purine analog resistant and in need of alternative therapies. Complete remission to single-agent purine analog is often accompanied by minimal residual disease (MRD), residual HCL cells detectable by immunologic methods, considered a risk factor for eventual relapse. Several different non-chemotherapy approaches are being used to target relapsed and refractory HCL, including inhibitors of BRAF, but so far only monoclonal antibody (MAb)-based approaches have been reported to eliminate MRD in a high percentage of patients. One of the MAb-based options for HCL currently under clinical investigation involves recombinant immunotoxins, containing a fragment of a MAb and a bacterial toxin. The bacterial toxin, a highly potent fragment from Pseudomonas exotoxin, catalytically ADP-ribosylates elongation factor 2 (EF2), resulting in protein synthesis inhibition and apoptotic cell death. Recombinant immunotoxins tested in HCL patients include LMB-2, targeting CD25, and BL22, targeting CD22. An affinity matured version of BL22, termed moxetumomab pasudotox (formerly HA22 or CAT-8015) achieved high CR rates in phase I, and is currently undergoing multicenter Phase 3 testing. Phase I testing was without dose-limiting toxicity, although 2 patients had grade 2 hemolytic uremic syndrome (HUS) with transient grade 1 abnormalities in platelets and creatinine. Preclinical work is underway to identify residues on moxetumomab pasudotox leading to immunogenicity. Moxetumomab pasudotox is undergoing pivotal testing for relapsed and refractory HCL.

3553 Background: LMB-100 recombinant immunotoxin consists of a mesothelin-binding Fab for targeting a modified Pseudomonas exotoxin A payload. Previous Phase 1 clinical testing of a 30-minute LMB-100 “short” infusion format identified a serum half-life of ~1 hour. Pre-clinical data suggested that extending infusion time could improve anti-tumor efficacy by increasing tumor cell duration of exposure to LMB-100. The primary objective of this study was to determine the safety and tolerability of administering LMB-100 in a long infusion format over 24-48 hours alone or with nab-paclitaxel chemotherapy in patients with mesothelin-expressing solid tumors. Methods: Patients (n = 15) with pancreatic adenocarcinoma and other mesothelin-expressing solid tumors (n = 3; mesothelioma, colon, and ampullary cancers) treated on 3 dose levels received long infusion of LMB-100 (65 or 100 mcg/kg/day) for 24 hour on Days 1 and 4 (n = 6) or 48 hour on Day 1 (n = 9) with or without a loading dose (40 mcg/kg over 30 minutes) for up to 2 cycles. In the second arm, patients (n = 5) with pancreatic adenocarcinoma were treated with LMB-100 over 24 hours on Day 1 concurrently with nab-paclitaxel (125 mg/m2) for up to 3 cycles. Results: DLT of proteinuria (grade 3) in one patient and acute kidney injury (grade 1) in one patient were observed amongst patients receiving 100 mcg/kg/day over 48 hours and 24 hours, respectively. No objective responses were seen but all patients receiving nab-paclitaxel had > 50% decrease in CA 19-9. Patients at all single agent dose levels (8 of 10 evaluable) developed high titer anti-drug antibodies (ADAs) against LMB-100. Those with ADAs (8 of 8) had undetectable cycle 2 peak plasma LMB-100 concentration. Development of high titer ADAs occurred more frequently with long infusion than seen previously with “short” infusion LMB-100. Most long infusion patients (19 of 20) developed increased serum IL-6 within 24 hours of LMB-100 infusion. However, the systemic inflammatory response to LMB-100 (as measured by increased serum CRP) which occurs in most “short” infusion patients was not observed. Conclusions: Long infusion format LMB-100 is generally well tolerated but immunogenicity limits treatment to 1 effective cycle. No anti-tumor efficacy of the single agent was observed. Clinical trial information: NCT02810418 .

Acute Lymphoblastic Leukemia (ALL) remains the most frequent cause of cancer-related mortality in children and novel therapies are needed for the treatment of relapsed/refractory childhood ALL. One approach is the targeting of ALL blasts with the Pseudomonas immunotoxin CAT-8015. Although CAT-8015 has potent anti-leukemia activity, with a 32% objective response rate in a phase 1 study of childhood ALL, haemolytic-uremic syndrome (HUS) and vascular leak syndrome (VLS), major dose-limiting toxicities, have limited the use of this therapeutic approach in children. Investigations into the pathogenesis of CAT-8015-induced HUS/VLS are hindered by the lack of an adequate model system that replicates clinical manifestations, but damage to vascular endothelial cells (ECs) and blood cells are believed to be major initiating factors in both syndromes. Since there is little evidence that murine models replicate human HUS/VLS, and CAT-8015-induced HUS/VLS predominantly affects children, we developed human models and used novel methodologies to investigate CAT-8015 interactions with red blood cells (RBCs) from pediatric ALL patients and ECs of excised human mesenteric arteries. We provide evidence that CAT-8015 directly interacts with RBCs, mediated by Pseudomonas toxin. We also show correlation between the electrical properties of the RBC membrane and RBC susceptibility to CAT-8015-induced lysis, which may have clinical implication. Finally, we provide evidence that CAT-8015 is directly cytototoxic to ECs of excised human mesenteric arteries. In conclusion, the human models we developed constitutes the first, and very important, step in understanding the origins of HUS/VLS in immunotoxin therapy and will allow further investigations of HUS/VLS pathogenesis.

SS1P is an anti-mesothelin immunotoxin composed of a targeting antibody fragment genetically fused to a truncated fragment of Pseudomonas exotoxin A. Delayed responses reported in mesothelioma patients receiving SS1P suggest that anti-tumor immunity is induced. The goal of this study is to evaluate if SS1P therapy renders mesothelioma tumors more sensitive to cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) immune checkpoint blockade. We evaluated the ability of SS1P to induce adenosine triphosphate (ATP) secretion and calreticulin expression on the surface of AE17M mouse mesothelioma cells. Both properties are associated with immunogenic cell death. Furthermore, we treated these tumors with intra-tumoral SS1P and systemic CTLA-4. We found that SS1P increased the release of ATP from AE17M cells in a dose and time-dependent manner. In addition, SS1P induced calreticulin expression on the surface of AE17M cells. These results suggest that SS1P promotes immunogenic cell death and could sensitize tumors to anti-CTLA-4 based therapy. In mouse studies, we found that the combination of anti-CTLA-4 with intra-tumoral SS1P induced complete regressions in most mice and provided a statistically significant survival benefit compared to monotherapy. The surviving mice were protected from tumor re-challenge, indicating the development of anti-tumor immunity. These findings support the use of intra-tumoral SS1P in combination with anti-CTLA-4.

Purpose: LMB-100 recombinant immunotoxin (iTox) consists of a mesothelin-binding Fab for targeting and a modified Pseudomonas exotoxin A payload. Preclinical studies showed that combining taxanes with iTox results in synergistic antitumor activity. The primary objectives of this phase I/II study were to determine the maximum tolerated dose (MTD) of LMB-100 when given with nanoalbumin bound (nab)-paclitaxel to participants with previously treated advanced pancreatic adenocarcinoma and to assess the objective response rate. Patients and Methods: Patients (n = 20) received fixed-dose nab-paclitaxel (125 mg/m2 on Days 1 and 8) with LMB-100 (65 or 100 mcg/kg on Days 1, 3, and 5) in 21-day cycles. Treatment was limited to 2-3 cycles by protocol. Safety and tolerability, pharmacokinetics, immunogenicity, and relationship of activity to tumor mesothelin expression were assessed. Results: Dose-limiting toxicity (DLT) of Grade 3 myalgia was observed in 2 of 5 patients treated with 100 mcg/kg. DLT of Grade 3 edema from capillary leak syndrome (CLS) was observed in 1 of 6 patients at MTD of 65 mcg/kg. Severe toxicities of Grade 4 left ventricular dysfunction and hypotension from CLS were observed in 1 patient in the phase II expansion, resulting in study closure. LMB-100 anti-drug antibody formation limited LMB-100 exposure in 5 of 13 patients during cycle 2. Seven of 17 patients with evaluable CA19-9 experienced >50% decrease of this tumor marker, and one patient developed a partial response. Response was associated with higher tumor mesothelin expression and Cycle 2 LMB-100 exposure. Conclusions: The combination caused serious toxicity. Despite some observed clinical activity, further testing of this combination will not be pursued. This abstract is also being presented as Poster B01. Citation Format: Christine C. Alewine, Mehwish Ahmad, Cody J. Peer, Zishuo Hu, Min-jung Lee, Akiro Yuno, Jessica Kindrick, Anish Thomas, Seth Steinberg, Jane B. Trepel, William D. Figg, Raffit Hassan, Ira Pastan. Phase I/II study of mesothelin-targeted immunotoxin LMB-100 with nab-paclitaxel for patients with advanced pancreatic adenocarcinoma [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr PR07.

Glypican-3 (GPC3) is a cell surface glypican that serves as a co-receptor for Wnt. Our laboratory has generated two high affinity antibodies targeting GPC3, HN3 and YP7. The HN3 human nanobody recognizes the N-lobe of GPC3, whereas the YP7 antibody recognizes the C-lobe of GPC3. Mutating residue F41 on the N-lobe of GPC3 inhibits binding of Wnt and the HN3 nanobody and inhibits activation of β-catenin. Both antibodies are fused to a fragment of Pseudomonas exotoxin A (PE38) to create recombinant immunotoxins. Interestingly, the HN3-derived immunotoxin (HN3-PE38) has superior antitumor activity as compared with the YP7-derived immunotoxin (YP7-PE38). Intravenous administration of HN3-PE38 causes regression of hepatocellular carcinoma (HCC) tumor xenografts in mice. Our study establishes GPC3 as a promising target for immunotoxin-based liver cancer therapy and demonstrates immunotoxin-induced tumor regression via dual mechanisms: inactivation of Wnt signaling via the HN3 nanobody and inhibition of protein synthesis via the PE bacteria toxin. However, immunogenicity and a short serum half-life may limit the ability of immunotoxins to transition to the clinic. To address these issues, we have recently engineered HN3-based immunotoxins to contain various deimmunized PE toxins. These new immunotoxins include HN3-T20, which is modified to remove domain II of the PE toxin and six T-cell epitopes. All of our immunotoxins display high affinity to human GPC3, with HN3-T20 having a KD value of 7.4 nM. A real-time cell growth inhibition assay demonstrates that a single dose of HN3-T20 at 1.6 nM is capable of inhibiting nearly all cell proliferation during the 10-day experiment. To enhance HN3-T20’s serum retention, we further engineer the HN3-T20 by adding a streptococcal albumin binding domain (ABD) and a llama single-domain antibody fragment (ALB1) specific for serum albumin. For the detection of immunotoxin in mouse serum, we develop a highly sensitive ELISA and find that HN3-ABD-T20 has a 45-fold higher serum half-life than HN3-T20 (326 min vs 7.3 minutes); consequently, HN3-ABD-T20 shows the best serum retention and remains detectible even after a 24-hour period in mice. Furthermore, addition of an albumin binding domain results in HN3-ABD-T20 mediated tumor regression in mice at a low dose (1 mg/kg). These data show that the albumin binding deimmunized immunotoxins are high potency therapeutics ready to be evaluated in clinical trials for the treatment of liver cancer. Citation Format: Mitchell Ho, Bryan D Fleming, Tim F Greten, Ira Pastan. Immunotoxins targeting GPC3 for liver cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr CN07-02. doi:10.1158/1535-7163.TARG-19-CN07-02

The functionality of eukaryotic translation elongation factor 2 (eEF2) is modulated by phosphorylation, eEF2 is simultaneously the molecular target of ADP-ribosylating toxins. We analyzed the interplay between phosphorylation and diphthamide-dependent ADP-ribosylation. Phosphorylation does not require diphthamide, eEF2 without it still becomes phosphorylated. ADP-ribosylation not only modifies the H715 diphthamide but also inhibits phosphorylation of S595 located in proximity to H715, and stimulates phosphorylation of T56. S595 can be phosphorylated by CDK2 and CDK1 which affects EEF2K-mediated T56-phosphorylation. Thus, ADP-ribosylation and S595-phosphorylation by kinases occur within the same vicinity and both trigger T56-phosphorylation. Diphthamide is surface-accessible permitting access to ADP-ribosylating enzymes, the adjacent S595 side chain extends into the interior. This orientation is incompatible with phosphorylation, neither allowing kinase access nor phosphate attachment. S595 phosphorylation must therefore be accompanied by structural alterations affecting the interface to ADP-ribosylating toxins. In agreement with that, replacement of S595 with Ala, Glu or Asp prevents ADP-ribosylation. Phosphorylation (starvation) as well as ADP-ribosylation (toxins) inhibit protein synthesis, both affect the S595/H715 region of eEF2, both trigger T57-phosphorylation eliciting similar transcriptional responses. Phosphorylation is short lived while ADP-ribosylation is stable. Thus, phosphorylation of the S595/H715 ‘modifier region’ triggers transient interruption of translation while ADP-ribosylation arrests irreversibly.

Moxetumomab pasudotox is a fusion protein of a CD22-targeting antibody and Pseudomonas exotoxin. Minutes of exposure to Moxetumomab achieves similar cell killing than hours of exposure to a novel deimmunized variant against some acute lymphoblastic leukemia (ALL). Because blood levels fall quickly, Moxetumomab is more than 1000-fold more active than the deimmunized variant in vivo. We aimed to identify which part of Moxetumomab increases in vivo efficacy and generated five immunotoxins, tested time-dependent activity, and determined the efficacy in a KOPN-8 xenograft model. Full domain II shortened the time cells had to be exposed to die to only a few minutes for some ALL; deimmunized domain III consistently extended the time. Against KOPN-8, full domain II accelerated time to arrest protein synthesis by three-fold and tripled PARP-cleavage. In vivo efficacy was increased by more than 10-fold by domain II and increasing size, and therefore half-life enhanced efficacy two- to four-fold. In summary, in vivo efficacy is determined by the time cells have to be exposed to immunotoxin to die and serum half-life. Thus, domain II is most critical for activity against some ALL treated with bolus doses; however, immunotoxins lacking all but the furin-cleavage site of domain II may be advantageous when treating continuously.

Recombinant immunotoxins (RITs) are chimeric proteins consisting of a Fv that binds to a cancer cell and a portion of a protein toxin. One of these, Moxetumomab pasudotox, was shown to be effective in treating patients with some leukemias, where the cells are readily accessible to the RIT. However, their short half-life limits their efficacy in solid tumors, because penetration into the tumors is slow. Albumin and agents bound to albumin have a long half-life in the circulation. To increase the time tumor cells are exposed to RITs, we have produced and evaluated variants that contain either an albumin-binding domain (ABD) from Streptococcus or single-domain antibodies from Llama. We have inserted these ABDs into RITs targeting mesothelin, between the Fv and the furin cleavage site. We find that these proteins can be produced in large amounts, are very cytotoxic to mesothelin-expressing cancer cell lines, and have a high affinity for human or mouse serum albumin. In mice, the RIT containing an ABD from Streptococcus has a longer half-life and higher antitumor activity than the other two. Its half-life in the circulation of mice ranges from 113 to 194 min compared with 13 min for an RIT with no ABD. Cell uptake studies show the RIT enters the target cell bound to serum albumin. We conclude that RITs with improved half-lives and antitumor activity should be evaluated for the treatment of cancer in humans.

Moxetumomab pasudotox (Moxe) is a chimeric protein composed of an anti-CD22 Fv fused to a portion of Pseudomonas exotoxin A and kills CD22-expressing leukemia cells. It is very active in hairy-cell leukemia, but many children with relapsed/refractory acute lymphoblastic leukemia (ALL) either respond transiently or are initially resistant. Resistance to Moxe in cultured cells is due to low expression of diphthamide genes (DPH), but only two of six ALL blast samples from resistant patients had low DPH expression. To develop a more clinically relevant model of resistance, we treated NSG mice bearing KOPN-8 or Reh cells with Moxe. More than 99.9% of the cancer cells were killed by Moxe, but relapse occurred from discrete bone marrow sites. The resistant cells would no longer grow in cell culture and showed major chromosomal changes and changes in phenotype with greatly decreased CD22. RNA deep sequencing of resistant KOPN-8 blasts revealed global changes in gene expression, indicating dedifferentiation toward less-mature B cell precursors, and showed an up-regulation of myeloid genes. When Moxe was combined with 5-azacytidine, resistance was prevented and survival increased to over 5 months in the KOPN-8 model and greatly improved in the Reh model. We conclude that Moxe resistance in mice is due to a new mechanism that could not be observed using cultured cells and is prevented by treatment with 5-azacytidine.