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Purpose:HER2/neu is overexpressed in up to 30% of gastroesophageal adenocarcinomas (GEA) and linked to poor prognosis. Recombinant mAbs to treat HER2/neu-overexpressing cancers are effective with limitations, including resistance and toxicity. Therefore, we developed a therapeutic B-cell epitope vaccine (IMU-131/HER-Vaxx) consisting of three fused B-cell epitopes from the HER2/neu extracellular domain coupled to CRM197 and adjuvanted with Montanide. This phase Ib study aimed to evaluate the optimal/safe dose leading to immunogenicity and clinical responses (https//clinicaltrials.gov/ct2/show/NCT02795988).Patients and Methods:A total of 14 patients with HER2/neu-overexpressing GEA were enrolled, and dose escalation (10, 30, 50 μg) was performed in three cohorts (C). Immunogenicity was evaluated by HER2-specific Abs and cellular responses, clinical responses by CT scans according to RECIST version 1.1.Results:IMU-131 was safe without vaccine-related significant local/systemic reactions or serious adverse events. A total of 11 of 14 patients were evaluable for changes in tumor size and vaccine-specific immune responses. One patient showed complete, 5 partial responses, and 4 stable diseases as their best response. HER2-specific IgG levels were dose dependent. In contrast to patients in C1 and C2, all patients in C3 mounted substantial HER2-specific Ab levels. In addition, cellular vaccine responses, such as Th1-biased cytokine ratios and reduced regulatory T cell numbers, were generated. Progression-free survival was prolonged in C3, correlating with the vaccine-specific humoral and cellular responses.Conclusions:IMU-131 was well tolerated and safe. The induced HER2-specific Abs and cellular responses were dose dependent and correlated with clinical responses. The highest dose (50 μg) was recommended for further evaluation in a phase II trial, with chemotherapy + IMU-131 or chemotherapy alone, which is currently ongoing.

Supp. Table 1; Supp. Table 2; Supp. Table 3 A,B; Supp. Table 4; Supp. Table 5; Supp. Table 6; Supplementary Data 1; Supplementary Data 2

Approved CDK4/6 inhibitors have demonstrated significant improvements in progression free survival when combined with fulvestrant in patients with HR+/HER2- advanced breast cancer, though limited by neutropenia and gastrointestinal side effects. Lerociclib is a potent selective CDK4/6 inhibitor with a differentiated PK/PD profile from currently approved CDK4/6 inhibitors that has demonstrated clinical proof-of-concept (Phase 1b data ASCO 2018). This Phase 1b/2a study assesses lerociclib dose escalation in combination with 500 mg fulvestrant using a 3+3 design followed by dose expansion in patients with metastatic or locally advanced HR+/HER2- breast cancer that had progressed following endocrine therapy. Up to two prior chemotherapies in Phase 1b and one prior in Phase 2a in the advanced setting are allowed. Prior fulvestrant was excluded for all patients; prior CDK4/6 inhibitors were excluded in Phase 2a only. Lerociclib is administered daily without a drug holiday. The objectives are to evaluate DLTs, safety, tolerability, PK, and tumor response and to determine the recommended dose and schedule (QD or BID) of lerociclib administered with fulvestrant for randomized trials. The BID dosing regimen demonstrated optimal safety, tolerability, and antitumor activity. To date, 35 patients (6 patients @ 100 mg BID, 9 patients @ 150 mg BID, 17 patients @ 200 mg BID, and 3 patients @ 250 mg BID) with mean age of 54 years, ECOG of 0 (86%) or 1 (14%), and a median of two prior anticancer therapies in the advanced setting have received lerociclib + fulvestrant for up to 804 days. A dose proportional increase in drug exposure has been observed. The most common lerociclib-related adverse events were neutropenia (63%), leukopenia (57%), nausea (37%), anemia (29%), diarrhea (20%), and thrombocytopenia (20%). The rates of lerociclib-related Grade 3 and Grade 4 neutropenia were 29% and 9%, respectively. Following an initial decline, ANC plateaued beginning at week 4. There were no reports of Grade 3 or greater nausea, vomiting, or diarrhea. One DLT of dose interruption due to Grade 2 fatigue and nausea was observed at 200 mg BID. There have been no reports of venous thromboembolism, QT prolongation or drug-induced liver injury. Twenty patients were evaluable for tumor response based on RECIST v1.1. Five patients (25%) had a confirmed PR; 8 patients (40 %) had stable disease; 6 patients (30 %) had PD. The CBR (CR + PR + SD ≥ 24 weeks) was 67% (12 of 18 patients either had SD at the week 24 tumor assessments or achieved an objective response). A population PK/PD/efficacy model was developed, and simulation data are concordant with existing clinical results. Additional patients are being enrolled at 150 mg BID and 200 mg BID, and updated data will be presented. Lerociclib, dosed BID with no drug holiday, has a favorable safety profile with low rates of gastrointestinal AEs and Grade 3/4 neutropenia, as well as encouraging antitumor activity in patients with HR+/HER2- advanced breast cancer. Phase 2b is ongoing to confirm the BID dose (150 mg or 200 mg) for randomized clinical trials. Clinical trial information: NCT02983071. Citation Format: Iurie Bulat, Marina Maglakelidze, Carmen Murias, Boris Krastev, Richard D Baird, Andrew M Wardley, Rebecca Roylance, Adrian Crijanovschi, Maia Gogiladze, Yili Pritchett, Amy McCullough, Chao Li, Jessica A Sorrentino, Rajesh Malik, Andrew P Beelen. Dose escalation and expansion study of lerociclib (G1T38), an oral CDK4/6 inhibitor, dosed with no drug holiday in combination with fulvestrant in patients with HR+/HER2- advanced breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-19-17.

289 Background: Active immunization with the B-lymphocyte stimulating HER2 vaccine, HER-Vaxx (IMU-131), has shown clinical response correlated with HER2-specific antibodies (Wiedermann et. al., Clinical Cancer Research, 2021). The HER-Vaxx HERIZON study is based on the landmark ToGA study (Bang et. al., The Lancet, 2010) and included patients with HER2/neu over-expressing metastatic or advanced gastric/GEJ adenocarcinoma who were naïve to HER2 therapy. Methods: Thirty-six patients were randomized to either HER-Vaxx plus standard chemotherapy or standard chemotherapy alone. The primary endpoint was overall survival (OS). HER-Vaxx plus chemotherapy treated patients received 50 µg dose of HER-Vaxx by intra-muscular injection at Days 0, 14, 35, 77 and every 63 days until disease progression. Both groups received chemotherapy starting at Day 0 and then every 21 days for a maximum of 6 cycles or until disease progression. Standard chemotherapy consisted of cisplatin + 5FU or capecitabine, or oxaliplatin + capecitabine. Statistical analysis pre-specified a 1-sided false positive probability of 0.10. Results: Of 36 patients randomized (19 treated with HER-Vaxx plus chemotherapy and 17 with chemotherapy alone), 32 patients had a survival event (15 and 17 respectively) at the time of final analysis. All patients received oxaliplatin + capecitabine chemotherapy. Analysis showed a 42% survival benefit for patients treated with HER-Vaxx plus chemotherapy compared to chemotherapy alone. This translated into an OS HR of 0.580 (80% 2-sided CI: 0.362, 0.927) with a statistically significant p-value of 0.066. The median OS for patients receiving HER-Vaxx plus chemotherapy was 13.9 (7.5, 14.3) months, compared to 8.3 (6.0, 9.6) months in patients treated with chemotherapy alone. Median duration of response was 30 vs 19 weeks in favor of the HER-Vaxx arm. There was no difference in safety between the two treatment arms, indicating HER-Vaxx does not add toxicity to standard chemotherapy. HER-Vaxx induced persistent HER2 specific antibodies which correlated with clinical response. Additional response parameters including DOR and biomarker data will be presented at the meeting. Conclusions: These data demonstrate that in patients with HER2 over-expressing gastric/GEJ cancer active HER2 immunization with HER-Vaxx is safe and provides relevant clinical benefit over standard of care chemotherapy. Clinical trial information: NCT02795988 .

Background: Rintodestrant (G1T48) is a potent oral selective estrogen receptor degrader (SERD) that competitively binds to the estrogen receptor (ER) and blocks ER signaling in tumors resistant to other endocrine therapies. Preliminary results from Part 1 dose escalation showed robust target engagement on 18F-fluoroestradiol positron emission tomography (FES-PET), a favorable safety profile, and encouraging antitumor activity in patients with heavily pretreated ER+/HER2- advanced breast cancer (ABC), including those with ESR1 mutations (Dees et al., ESMO 2019 [abstract #3587]). Here, we present updated results from dose escalation and expansion (Parts 1 and 2). Methods: This Phase 1, first-in-human, open-label study evaluated rintodestrant monotherapy in women with ER+/HER2- ABC after progression on endocrine therapy. Part 1 was a 3+3 dose escalation (200-1000 mg once daily [QD]); Part 2 expanded 600 and 1000 mg QD; and Part 3 was added to assess rintodestrant with palbociclib in patients in earlier lines in the advanced setting. Primary objectives included dose-limiting toxicities (DLTs), maximum tolerated dose (MTD), safety, and recommended Phase 2 dose. Secondary objectives included pharmacokinetics and antitumor activity (RECIST v1.1). Exploratory objectives included pharmacodynamic inhibition of ER target engagement (FES-PET), mutation profiling (cell-free DNA [cfDNA]), and change in ER expression from baseline to on-treatment tumor biopsies. Results: As of May 13, 2020, 67 patients (Part 1: n = 26; Part 2: n = 41) were treated, with a median age of 61 years (range 34-83) and ECOG PS of 0 (49%) or 1 (51%). Median number of prior lines in the advanced setting was 2 (range 0-9), including prior fulvestrant (64%), CDK4/6 inhibitor (69%), mTOR inhibitor (22%), and/or chemotherapy (46%). Median number of prior lines of endocrine therapy in the advanced setting was 2 (range 0-5), with 61% of patients having received ≥2 lines. Treatment-related adverse events (TRAEs) were reported in 70% of patients. The most common TRAEs in ≥10% of patients included hot flush (24%), fatigue (21%), nausea (19%), diarrhea (18%), and vomiting (10%), mostly grade 1 or 2. No DLTs were reported and MTD was not reached. Dose reduction due to TRAEs occurred in 1 patient (1%), with elevated transaminases (grade 3 ALT and grade 2 AST) at 600 mg. Serious TRAEs occurred in 2 patients at 1000 mg (grade 5 cerebral hemorrhage in the setting of low molecular weight heparin and grade 2 upper abdominal pain). Two patients (3%) discontinued treatment due to TRAEs. Overall, the frequency of patients with TRAEs at 800 mg was comparable with that at 600 mg (57% vs 63%) and less than that at 1000 mg (81%). Of 67 patients, 16 were on study treatment for ≥24 weeks and 3 (n = 1 at 600 mg; n = 2 at 1000 mg, including 1 with ESR1 mutation) had a confirmed partial response (clinical benefit rate [CBR]: 28%). FES-PET standard uptake values decreased at week 4 with a mean reduction of 87% (±8%) at doses ≥ 600 mg. Of 59 patients tested for baseline cfDNA, 41% harbored ≥1 ESR1 mutation, with a similar CBR in both groups (33% in ESR1 mutant and 29% in ESR1 wild-type). Seven of 9 patients had a decrease in ER immunohistochemistry H-score at both 600 and 1000 mg (median [range]: -27.8% [-33.8%, -3.4%]), irrespective of ESR1 mutation status. Based on safety, efficacy, and ER degradation, 800 mg was selected as the optimal dose for further study. Conclusions: Rintodestrant continues to demonstrate an excellent safety/tolerability profile across all doses, with promising antitumor activity in patients with heavily pretreated ER+/HER2- ABC, including those with tumors harboring ESR1 mutations. Part 3 of this study, evaluating rintodestrant 800 mg QD with palbociclib in a more endocrine-sensitive population, is ongoing (NCT03455270). Citation Format: Philippe Aftimos, Patrick Neven, Mark Pegram, Catharina Willemien Menke-van der Houven van Oordt, E. Claire Dees, Carolien Schröder, Agnes Jager, Iurie Bulat, Linnea Chap, Marina Maglakelidze, Erika Hamilton, Massimo Cristofanilli, Susanna Ulahannan, Jorianne Boers, Ramsha Iqbal, Adrian Crijanovschi, Curt D Wolfgang, Wenli Tao, Christina Sipes, Rajesh Malik, Sarika Jain. Rintodestrant (G1T48), an oral selective estrogen receptor degrader in ER+/HER2- locally advanced or metastatic breast cancer: Updated phase 1 results and dose selection [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS12-04.

Maen Hussein,1 Marina Maglakelidze,2 Donald A Richards,3 Marielle Sabatini,4 Todd A Gersten,5 Keith Lerro,6 Ivan Sinielnikov,7 Alexander Spira,8,9 Yili Pritchett,10 Joyce M Antal,10 Rajesh Malik,10 J Thaddeus Beck11 1Florida Cancer Specialists, Leesburg, FL, USA; 2LLC Arensia Exploratory Medicine, Tbilisi, Georgia; 3Texas Oncology-Tyler, US Oncology Research, Tyler, TX, USA; 4Saint Leon Hospital, Bayonne, France; 5Florida Cancer Specialists, West Palm Beach, FL, USA; 6Regional Medical Oncology Center, Wilson, NC, USA; 7Volyn Regional Oncology Center, Lutsk, Ukraine; 8Virginia Cancer Specialists, Fairfax, VA, USA; 9US Oncology Research, The Woodlands, TX, USA; 10G1 Therapeutics, Inc., Research Triangle Park, NC, USA; 11Highlands Oncology Group, Fayetteville, AR, USACorrespondence: Maen HusseinFlorida Cancer Specialists, Leesburg, FL, USATel +1 352-787-9448Email mhussein@flcancer.comPurpose: Trilaciclib is an intravenous cyclin-dependent kinase 4/6 inhibitor indicated to decrease the incidence of chemotherapy-induced myelosuppression (CIM) by protecting hematopoietic stem and progenitor cells and immune system function from chemotherapy-induced damage (myeloprotection). Here, we investigated the myeloprotective effects of trilaciclib among patients at increased risk of CIM.Patients and Methods: Data were pooled from three randomized, double-blind, placebo-controlled, phase 2 clinical studies of trilaciclib administered prior to chemotherapy in patients with extensive-stage small cell lung cancer (ES-SCLC). Myeloprotective outcomes were evaluated in patient subgroups based on age (< 65 or ≥ 65 years), risk of chemotherapy-induced febrile neutropenia (FN), and risk of anemia or red blood cell (RBC) transfusions. For the FN and anemia analyses, risk factors were identified from published literature and used to classify patients into FN and anemia risk categories. Subgroup analysis based on age was also performed on patient reported outcome (PRO) measures.Results: In total, 123 patients received trilaciclib and 119 patients received placebo. Myeloprotective benefits of trilaciclib were observed regardless of age, with greater effects observed among patients aged ≥ 65 years. Across FN risk factors and categories, trilaciclib had beneficial effects on neutrophil-related endpoints vs placebo, with greater effects observed in patients at higher risk of FN. Effects on RBC-related endpoints favored trilaciclib vs placebo, regardless of anemia risk factors and categories. Improvements in PROs with trilaciclib were observed irrespective of age group, but with greater improvements and less deterioration from baseline observed in older patients.Conclusion: By both decreasing the incidence of CIM and improving quality of life, trilaciclib has the potential to allow patients receiving chemotherapy for ES-SCLC, including patients who are older or more vulnerable to CIM, to receive chemotherapy on schedule and at standard-of-care doses, and to improve the experience for patients receiving chemotherapy to treat ES-SCLC.Clinical Trial Numbers: NCT02499770; NCT03041311; NCT02514447.Keywords: trilaciclib, myelosuppression, myeloprotection, myelopreservation, chemotherapy, small cell lung cancer

Purpose: HER2/neu is overexpressed in up to 30% of gastroesophageal adenocarcinomas (GEA) and linked to poor prognosis. Recombinant mAbs to treat HER2/neu-overexpressing cancers are effective with limitations, including resistance and toxicity. Therefore, we developed a therapeutic B-cell epitope vaccine (IMU-131/HER-Vaxx) consisting of three fused B-cell epitopes from the HER2/neu extracellular domain coupled to CRM197 and adjuvanted with Montanide. This phase Ib study aimed to evaluate the optimal/safe dose leading to immunogenicity and clinical responses (https//clinicaltrials.gov/ct2/show/NCT02795988). Patients and Methods: A total of 14 patients with HER2/neu-overexpressing GEA were enrolled, and dose escalation (10, 30, 50 μg) was performed in three cohorts (C). Immunogenicity was evaluated by HER2-specific Abs and cellular responses, clinical responses by CT scans according to RECIST version 1.1. Results: IMU-131 was safe without vaccine-related significant local/systemic reactions or serious adverse events. A total of 11 of 14 patients were evaluable for changes in tumor size and vaccine-specific immune responses. One patient showed complete, 5 partial responses, and 4 stable diseases as their best response. HER2-specific IgG levels were dose dependent. In contrast to patients in C1 and C2, all patients in C3 mounted substantial HER2-specific Ab levels. In addition, cellular vaccine responses, such as Th1-biased cytokine ratios and reduced regulatory T cell numbers, were generated. Progression-free survival was prolonged in C3, correlating with the vaccine-specific humoral and cellular responses. Conclusions: IMU-131 was well tolerated and safe. The induced HER2-specific Abs and cellular responses were dose dependent and correlated with clinical responses. The highest dose (50 μg) was recommended for further evaluation in a phase II trial, with chemotherapy + IMU-131 or chemotherapy alone, which is currently ongoing.

Background: HER2/neu is overexpressed in up to 30% of gastroesophageal adenocarcinoma (GEA) patients and associated with poor prognosis. Recombinant mAbs to treat HER2/neu-amplified malignancies are effective with limitations so alternatives are needed. Therefore, we developed a B-cell epitope immunotherapy vaccine (IMU-131/HER-Vaxx) consisting of 3 fused B-cell epitopes (P467) from the HER2/neu extracellular domain coupled to CRM197 and adjuvanted with Montanide. This open-label multicenter Phase 1b study aimed to evaluate the optimal/safe IMU-131 dose leading to immunogenicity and clinical responses.Patients & Methods: Fourteen patients with HER2/neu overexpressing GEA were enrolled and dose escalation with 10, 30, and 50µg was performed in 3 cohorts. Immunogenicity was evaluated by P467- and HER2-specific Ab levels and cellular responses, and clinical response rates by CT/MRI scan evaluated according to RECISTv1.1. Results: IMU-131 was safe with no vaccine-related significant local or systemic reactions or SAEs. 11/14 patients were evaluable for changes in tumor size and vaccine-specific immune responses. One patient showed complete response, 5 showed partial response and 4 stable disease as their best response. HER2-specific IgG levels were dose-dependent. While in cohort 1 and 2 only 1/3 and 2/5 patients respectively responded with moderate to high Ab titers, all patients in cohort 3 mounted moderate to high HER2-specific IgGs. Additionally, cellular vaccine responses were induced, e. g. Th1-biased cytokine ratios and reduction of Tregs. The clinical response (progression free survival) was significantly prolonged in cohort 3 and showed a clear association with the magnitude of humoral and cellular vaccine responses. Conclusions: IMU-131 was well tolerated and safe. The vaccine-induced HER2-specific Abs and cellular responses were dose-dependent and correlated with clinical responses. The highest dose (50 µg) was recommended for further evaluation in a Phase II trial with two arms, chemotherapy with IMU-131 vaccine or chemotherapy alone, which is currently ongoing. https://clinicaltrials.gov/ct2/show/NCT02795988 Citation Format: Marina Maglakelidze, Dinara Ryspayeva, Iurie Bulat, Zoran Andric, Ivan Nikolic, Tanuj Chawla, Rajnish Nagarkar, Vaibhav Chourdhary, Giri Venkata, Rajesh Kumar Singh, Davorin Radosavljevic, Zoran Petrovic, Ursula Wiedermann, Leslie Chong, Nicholas J. Ede, Rita Laeufle, Anthony Good. A PHASE 1B/2 OPEN-LABEL STUDY WITH RANDOMIZATION IN PHASE 2 OF IMU-131 HER2/NEU PEPTIDE VACCINE PLUS STANDARD OF CARE CHEMOTHERAPY IN PATIENTS WITH HER2/NEU OVEREXPRESSING METASTATIC OR ADVANCED ADENOCARCINOMA OF THE STOMACH OR GASTROESOPHAGEAL JUNCTION [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr CT107.

1063 Background: Rintodestrant, a potent, oral selective estrogen receptor degrader, competitively binds and degrades the estrogen receptor (ER), thus blocking ER signaling in tumors resistant to other endocrine therapies (ET). Results from parts 1 and 2 dose-escalation/expansion indicate that once-daily (QD) rintodestrant has a favorable safety profile and antitumor activity in patients (pts) with heavily pretreated ER+/HER2– advanced breast cancer (ABC), including those with ESR1 variants (Aftimos et al. SABCS 2020 [PS12-04, PD8-07]). The optimal dose of rintodestrant was 800 mg. Here, we present part 3, combining rintodestrant with the CDK4/6 inhibitor palbociclib. Methods: This open-label study evaluated rintodestrant in pts with ER+/HER2– ABC after progression on ET (NCT03455270). Part 3 assessed rintodestrant 800 mg QD + palbociclib 125 mg QD for 21 days every 28 days. Key eligibility criteria included ≤1 line of chemotherapy and/or ≤1 line of ET in the advanced setting, with ≥6 months of ET in the advanced setting and/or ≥24 months in the adjuvant setting. Prior CDK4/6 inhibitor therapy was not allowed. Primary objectives included safety and efficacy. Secondary objectives included pharmacokinetics and antitumor activity (RECIST v1.1). Exploratory objectives included mutation profiling (cell-free DNA) at baseline and cycle 1 day 15. Results: Enrollment occurred Jul–Oct 2020. As of Dec 9, 2020, 40 pts were treated, with a median age of 58 years (35–76) and ECOG PS of 0 (70%) or 1 (30%); 20% had de novo stage 4 disease, 10% bone-only, and 68% visceral metastases. Median number of visceral sites was 1 (0–3): 30% of pts with lung and 40% with liver involvement. Median number of prior lines in the advanced setting was 1 (0–2), including chemotherapy (48%), fulvestrant (15%), and aromatase inhibitors (50%). Most recent ET was given in the adjuvant and metastatic settings in 28% and 73% of pts, respectively. Rintodestrant-related adverse events (AEs) were reported in 8% of pts—all nonserious and grade 2—and included nausea (3%), vomiting (3%), and neutropenia (3%). The most common (≥10%) treatment-related AEs (rintodestrant and/or palbociclib) were neutropenia (88%), leukopenia (45%), anemia (10%), and thrombocytopenia (10%); grade 3/4 neutropenia was 38%/15%, in line with the safety profile of palbociclib. No deaths or treatment discontinuations due to AEs were reported. At data cutoff (median treatment duration of 3 months [1.5–4.6]), 28 pts (70%) remained on study treatment, 2 (5%) had a confirmed partial response, and 27 (68%) had stable disease. Additional efficacy and pharmacodynamic data will be presented. Conclusions: Rintodestrant, as monotherapy or combined with palbociclib, continues to demonstrate an excellent safety/tolerability profile with promising antitumor activity in pts with ER+/HER2– ABC, including those with ESR1 variants. Clinical trial information: NCT03455270 .

e16065 Background: The phase 2 part of the study hypothesizes HER-Vaxx (IMU-131) will provide treatment benefits consistent with traditional monoclonal antibodies that target HER2 in patients with confirmed Her2+ advanced or metastatic gastric cancer with a corresponding adaptive immune response without added toxicity. In the phase 1b dose finding part of the study tumor response of patients who received 50ug dose strongly correlated with antibody levels with 50ug selected as the phase 2 dose (Wiedermann et. al., 2019). Phase 2 is a randomized two arm study of either HER-Vaxx plus standard chemotherapy or standard chemotherapy alone. The primary endpoint is overall survival, with progression-free survival, safety and immune related changes in humoral and cellular immunogenicity secondary endpoints. Methods: Patients received SOC chemotherapy for a maximum of 6 cycles with the HER-Vaxx group also receiving 50ug dose of IMU-131 at Baseline/Day 0, Day 14, Day 35, Day 77 and then every 63 days until disease progression. The per protocol pre-planned first interim analysis (OS, PFS and safety) in a total of 27 patients after 15 progression events was reviewed by the independent data monitoring committee (IDMC). Results: Within ITT analysis, 8 of 27 patients died on the control arm and 4 on the HER-Vaxx plus SOC chemotherapy arm with an overall survival HR of 0.418 (2 sided 80% CI: 0.186, 0.942) and a 1-sided p-value of 0.083. Out of 27 patients, 9 patients progressed on the control arm while 6 patients progressed on the HER-Vaxx plus SOC chemotherapy arm with a HR of 0.532 (2 sided 80% CI: 0.267, 1.060) and a 1-sided p-value of 0.086. A robust HER2 specific antibody response developed in the HER-Vaxx arm compared to the control arm whereas there was no difference in safety between the two treatment arms. Conclusions: The IDMC confirmed that the safety of the study was favorable with no added toxicity of HER-Vaxx and SOC chemotherapy with a favorable risk-benefit for the combination. The study has completed enrollment and final data is expected in late 2021. Clinical trial information: NCT02795988.

Background: Rintodestrant is an orally bioavailable, potent and selective estrogen receptor degrader (SERD) that inhibits estrogen receptor (ER) gene transcription, degrades the ER, and delays tumor proliferation in preclinical models. Preliminary results from Part 1 dose escalation (200-1000 mg once daily) demonstrated that rintodestrant has a favorable safety profile and encouraging antitumor activity in patients (pts) with heavily pretreated ER+/HER2- advanced breast cancer (ABC) (Dees et al., ESMO 2019 [abstract #3587]). Here, we report the pharmacodynamic (PD) analysis in peripheral blood and tumor biopsies from pts who received rintodestrant in Part 1 and 2 (600 and 1000 mg dose expansion) to characterize the pt population and mechanisms of response. Methods: This Phase 1, first-in-human, open-label study evaluated rintodestrant in women with ER+/HER- ABC after progression on endocrine therapy. PD analysis included inhibition of ER target engagement with 18F-fluoroestradiol positron emission tomography (FES-PET), mutational profiling (cell-free DNA [cfDNA]), and circulating tumor cell (CTC) enumeration. Tumor biopsies sampled at baseline and 6 weeks on treatment were evaluated for ER degradation (immunohistochemistry [IHC]) and proliferation (Ki67, IHC) to understand the on-target effects of rintodestrant. Results: As of May 13, 2020, 67 pts had been treated. FES-PET data were obtained in 14 pts and showed a decrease in all pts, with maximum standard uptake values (SUVmax) ranging from 70% to 98% after 4 weeks of rintodestrant monotherapy across all doses. Fifty-nine pts were tested for cfDNA at baseline; 95% (n = 56) harbored ≥1 somatic variant (median = 3 mutations per pt). Among pts with somatic variants, 41% had ESR1 mutations, with D538G being the most common (58%). Additionally, 46% and 42% of pts harbored mutations in TP53 and PIK3CA, respectively, and 10% had mutations in both ESR1 and PIK3CA. Similar clinical benefit rates were observed in wild-type vs ESR1 and/or PIK3CA mutant tumors. An analysis of change of variant allele fraction (VAF) in 55 pts between baseline and 2 weeks of treatment revealed that 58% had a decrease in mean VAF, with a decrease in ESR1 VAF in 16/20 pts that had ESR1 mutations at baseline. Furthermore, of 24 pts who had samples collected at baseline and progression, 16 (67%) developed additional variants (median [range]: 2 [1, 15]), including EGFR, ERBB2, TP53, and ESR1. CTC analysis (n = 45) showed the mean value of Epi+CD45- CTCs decreased from 2.8 cells/mL to 1.8 cells/mL after 8 weeks of treatment. Tumor biopsies were collected in 9 pts (5 received 600 mg and 4 received 1000 mg) at baseline and 6 weeks on treatment. Of the 7/9 pts that had a decrease in the ER H-score (median [range]: -27.8% [-33.8%, -3.4%]), 4 had ≥1 variant in ESR1 at baseline. Overall, 4 pts had a decrease in Ki67, with reductions mostly observed in pts who received 600 mg rintodestrant. Additional analyses, including correlations with clinical response, are ongoing and will be presented. Conclusions: Rintodestrant demonstrated robust ER target engagement on FES-PET, as well as substantial decreases in ER H-score, cfDNA VAF, and Epi+CD45- CTCs. These data, along with promising clinical benefit in pts with heavily pretreated ER+/HER2- ABC, regardless of ESR1 or PIK3CA mutation status, warrant additional investigation of rintodestrant (NCT03455270). Citation Format: Philippe Aftimos, Marina Maglakelidze, Andor WJM Glaudemans, Erika Hamilton, Linnea Chap, Elisabeth de Vries, Catharina Willemien Menke-van der Houven van Oordt, Agnes Jager, E. Claire Dees, Massimo Cristofanilli, Mark Pegram, Susanna Ulahannan, Patrick Neven, Iurie Bulat, Ruhi Rai, Wenli Tao, Sarika Jain, Andrew P Beelen, Jessica A Sorrentino. Pharmacodynamic analysis from a phase 1 study of rintodestrant (G1T48), an oral selective estrogen receptor degrader, in ER+/HER2- locally advanced or metastatic breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD8-07.

Background: HER2/neu is overexpressed in 15-25% of gastric cancer patients and associated with poor prognosis. Alternative treatments to monoclonal antibodies are needed due to cost and global availability issues of mAbs. Thus a B-cell peptide vaccine (IMU-131) was developed, consisting of 3 fused B-cell epitopes (p467) from the HER2/neu extracellular domain coupled to CRM197 applied with the adjuvant Montanide. The current study evaluated the optimal and safe vaccine dose leading to immunogenicity and clinical responses. Material & Methods: In an open-label multicenter Phase Ib trial in SE-Asia and Eastern Europe, 14 patients with HER2/neu overexpressing (++/+++) gastric or gastroesophageal junction adenocarcinoma were recruited to receive 3 injections of IMU-131 (days 0, 14, 35) in combination with chemotherapy (CT) every 21 days. Dose escalation with 10 µg, 30 µg and 50 µg was performed in 3 cohorts. Safety, immunogenicity and clinical responses were evaluated. Results: No SAEs related to administration of IMU-131 were reported. Eleven of 14 patients were evaluable for vaccine-specific immune responses and 10 for tumor growth assessment. Higher p467- and Her-2 specific IgG levels were observed in cohort 2 (30 µg/dose) compared to cohort 1 (10 µg/dose). Two of five patients in cohort 2 showed minimal antibody titers. In contrast, all patients in cohort 3 (50µg/dose) responded to the vaccine with equally high antibody levels. Response rate was an exploratory endpoint and of 10 evaluable patients, 5 patients showed partial response and 4 patients showed stable disease. In cohort 3 the high antibody levels correlated with clinical response, while in cohort 2 only moderate correlations between humoral and clinical responses were observed. In cohort 1 partial response did not correlate with Ab levels, but rather with a high percentage of CD8 T-cells and increased inflammatory cytokine levels (high IFN-γ and TNF-α/IL-10 ratio). Conclusions: The vaccine was well tolerated and safe with antibody responses at the highest dose (50 µg) showing a strong correlation with clinical responses. Thus, a dose of 50 µg was recommended for further evaluation in Phase II, featuring two arms of either IMU 131 plus CT or CT alone. We propose that this vaccine might be of significant medical benefit and further trials are warranted. Citation Format: Ursula Wiedermann, Erika Garner-Spitzer, Yee Chao, Iurie Bulat, Arunee Dechaphunkul, Wichit Arpornwirat, Chaiyut Charoentum, Chia-Jui Yen, Thomas Cheung Yau, Marina Maglakelidze, Suebpong Tanasanvimon, Jedzada Maneechavakajorn, Aumkhae Sookprasert, Li-Yuan Bai, Wen-Chi Chou, Teerapat Ungtrakul, Christoph C. Zielinski, Leslie Chong, Nick Ede, Anthony J Good. A Phase Ib open label multicenter study with a HER2/neu peptide vaccine administered with cisplatin and 5-fluorouracil or capecitabine chemotherapy shows safety, immunogenicity and clinical response in patients with HER2/Neu overexpressing advanced cancer of the stomach [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr CT059.

TPS176 Background: Gastric cancer is the 5th most frequently diagnosed cancer and the 3rd leading cause of cancer deaths. HER2/neu is overexpressed in 15% to 25% of patients with gastric cancer and associated with a poor prognosis. Monoclonal antibodies against HER2/neu have been shown to be effective but alternative treatments are needed due to cost and global availability issues. IMU-131 is a B-cell peptide vaccine composed of 3 B cell epitopes derived from the extracellular domain of HER2/neu. Polyclonal antibodies against IMU-131 peptides binding 3 separate regions (DIII, IV) of HER2/neu have been shown to elicit antitumor activity in vitro and a phase I study demonstrated safety and immunogenicity in Her-2 +/++ metastatic breast cancer patients. Fusion of the single peptides into a hybrid peptide conjugated to CRM197 in conjunction with the adjuvant Montanide (P467-CRM-Montanide) improved formulation and stability of the vaccine. With the present Phase 1b/2 trial performed in patients with HER2/neu overexpressing gastric or gastroesophageal junction (GEJ) adenocarcinoma, we hypothesized that administration of IMU-131 in addition to chemotherapy is safe and immunogenic, and will prolong survival and may delay tumor progression and/or reduce tumor burden. Methods: This study is an international open-label multicenter study performed in 16 Asian and Eastern European sites with a maximum of 18 patients enrolled in Phase 1b. This dose escalation study is designed to assess safety, tolerability, immunogenicity and recommended phase 2 dose (RP2D) of IMU-131. Each patient is administered three injections of IMU-131, at a single dose level on Days 0, 14, and 35, accompanied by chemotherapy cycles of cisplatin and 5-fluorouracil or capecitabine every 21 days. The RP2D will be evaluated in the dose expansion Phase 2 study with 68 patients being enrolled. Results: The study is ongoing with the completion of the phase 1b portion in 4Q18. Conclusions: No conclusions can be drawn at this time. Clinical trial information: NCT02795988.

1061Background: CDK4/6 inhibition has demonstrated significant improvements in PFS when combined with fulvestrant (F) in patients with breast cancer (BC). G1T38 (38) is a potent, selective oral CDK...