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 .

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.

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 Conventionally, breast cancer (BC) prognosis and prediction of response to therapy are based on TNM staging, histological and molecular subtype, as well as genetic alterations. The role of various epigenetic factors has been elucidated in carcinogenesis. However, it is still unknown to what extent miRNAs affect the response to neoadjuvant chemotherapy (NACT). This pilot study is focused on evaluating the role of miR-34a, miR-124a, miR-155, miR-137 and miR-373 in response to NACT. Methods That was a prospective study enrolling 34 patients with histologically confirmed BC of II-III stages. The median age of patients was 53 (47–59.8) years old, 70.6% of whom were HR-positive. MiRs levels were measured in the primary tumor before and after NACT. The response to therapy was assessed after surgery using the Miller-Payne scoring system. To establish the role of miRs in modulating response to NACT the Cox model was applied for analysis. Results BC demonstrated a great variability of miRs expression before and after NACT with no strong links to tumor stage and molecular subtype. Only miR-124a and miR-373 demonstrated differential expression between malignant and normal breast tissues before and after therapy though these distinctions did not impact response to NACT. Besides miR-124a and miR-137 levels after NACT were found to be dependent on HR status. While miR-124a levels increased (p = 0.021) in the tumor tissue, the expression of miR-137 was downregulated (p = 0.041) after NACT in HR positive BC. Conclusions The study revealed differences in miR-124a and miR-373 expression after NACT in primary BC tissues. Although miRs levels did not impact the response to NACT, we found miR-124a and miR-137 levels to be related to hormonal sensitivity of BC.

Background: Dysregulated miRNA expression has been found in nearly all the stages of cancer process compared to normal tissue. Certain miRNAs expression level is closely related to the histopathological parameters and molecular subtypes of breast cancer (BC) as well as the response to treatment and prognosis.Methods: This prospective research has evaluated the level of expression of 5 miRNAs (miR-124a, miR-137, miR-34a, miR-155, miR-373) in the primary tumor before and after chemotherapy in patients with operable BC. The patients received 2-4 cycles neoadjuvant chemotherapy (NACT) and then underwent surgery – mastectomy or breast conservation with axillary lymph node dissection. Assessment of all mRNAs was analyzed in selected pathology specimens obtained during the biopsy and the targeted area taken by the surgery. Results: A total of 34 patients were treated between February 2016 and December 2017 with median age of 53 years (23 – 75). 70.6% were HR-positive and 29.4% were HR-negative. AntioncomiR-124a expression was mostly downregulated in primary BC tissue (p=0.001). The restored expression of miR-124 (p=0.004) was observed after chemotherapy having reached the indistinguishable level from non-tumor tissue. The dynamics of miR-373 expression considered to be oncomiR was a 7.7-fold decrease level in the primary tumor with continued suppress of expression compared to non-tumor tissue (p=0.006). The expression levels of miR-34a, miR-137, miR-155 in the pathology specimens did not show the differences compared to the surrounding non-tumor tissue (p>0.05). The difference in the dynamics of the two suppressor miRNAs (miR-124a and miR-137) expression in HR-positive BC was identified. While the expression of miR-124a in the tumor tissue increases after NACT (p=0.021), the expression of miR-137 continues to decrease (p=0.041) and does not differ from the levels of non-tumor tissue (p=0.146). There was no relation between the expression of the 5 studied miRNAs with histopathological differentiation of the primary tumor and the pathomorphological response after NACT. A Cox proportional hazards model did not confirm the relationship between miRNAs expression level neither in primary tumor nor in clinicopathological BC characteristics.Conclusions: The differential regulatory effects of miRNA cannot be explained by the differences in expression alone as paradoxical behavior of miR-373 shows. Taking into account the heterogeneity of the breast tumor, the detected inconsistency of changes in the expression of two suppressor miRNAs (miR-124a and miR-137) can be explained by their different regulatory functions. An accurate picture of the functioning of microRNA requires further research due to its dynamic nature.