Knecht, John George

e13124 Background: Bria-IMT, an allogeneic whole-cell cancer vaccine (SV-BR-1-GM), was administered w/ an anti PD1 checkpoint inhibitor (CPI) to evaluate its safety and efficacy in advanced/metastatic breast cancer (MBC). SV-BR-1-GM breast cancer cells were engineered to directly stimulate anti-tumor immunity through expression of tumor associated antigens and secretion of GM-CSF to enhance endogenous dendritic cell activation. Addition of a CPI potentiates SV-BR-1-GM action by overcoming the immune suppressive tumor microenvironment and may have a differential impact vs monotherapy SV-BR-1-GM. Methods: In this exploratory analysis of the Phase 1 + randomized Phase 2, HLA matching status was assessed to explore potential correlations w/ clinical outcomes. Pts were stratified by overall HLA match by Class I or II (0 vs ≥1 matches), formulation and MBC subtype to assess objective response rate (ORR), clinical benefit rate (CBR), progression free survival (PFS), and overall survival (OS). Partial HLA matching (≥1 Class I and ≥1 Class II allele) was also analyzed. Results: In the total pt population, the ORR was 10%, CBR 56%, PFS 3.2 months and OS 9.9 months. In those pts treated w/ the phase 3 formulation, ORR was 15%, CBR 63%, PFS 3.6 months and OS 13.4 months. Using available HLA data (n = 52), 25% had 0, 34.6% 1, 32.6% 2, and 8% ≥3 matches. While OS and PFS were greater (Undefined vs 8.5 mos ; p = .02) in pts without an HLA match compared w/ those w/ ≥1 match (see Table), ORR and CBR were similar. This was also the case for pts who had 0 HLA Class I matches vs those w/ ≥1 Class I match (15.6 vs 7.2 mos; p = .002). ORR, CBR, PFS and OS were generally similar for pts who had 0 Class II HLA matches vs those w/ ≥1 match. Pts who matched at least at 1 Class I and 1 Class II allele had a higher ORR and CBR than those who matched at a single Class I or a single Class II allele although PFS and OS were similar between the groups. Similar observations were seen for pts treated w/ the phase 3 formulation. Conclusions: Matching at Class I seemed to decrease PFS & OS, but did not have a similar effect on ORR and CBR. Both matched and non matched pts demonstrated favorable OS and response rates. These findings support further investigation into specific mechanisms and the clinical significance of HLA matching in immunotherapy response. A randomized phase 3 trial is ongoing comparing Bria-IMT vs treatment of physician's choice (NCT06072612). Clinical trial information: NCT03328026 . n (Evaluable) % of Cohort Best ORR Best CBR PFS OS Full Cohort 52 (41) 10% 56% 3.2 9.9 No Match (Overall) 13 (10) 25% 20% 60% 4.2 Undefined Any Match (Overall) 39 (31) 75% 6% 55% 3 8.5 MHC Class I No Match / Any Match 27 (20) / 25 (21) 52% / 48% 10% /10% 55% /57% 3.5 / 2.7 15.6 / 7.23 MHC Class II Any Match 27 (19) 52% 11% 63% 3.2 11.4 MHC Class I&II Match 13 (9) 25% 22% 77% 2.8 8.6 Phase 3 Formulation (N=35) 35 (27) 15% 63% 3.57 13.43 No Match (Phase 3 Formulation) 11 (9) 31% 22% 67% 5.6 Undefined MHC Class I&II Match (Phase 3 Formulation) 11 (7) 31% 29% 71% 2.7 7.23

3553 Background: Vilastobart (XTX101) is tumor-activated, high affinity, Fc-enhanced aCTLA-4 designed to focus activity toward the tumor and minimize systemic adverse events. Fc-enhancement augments FcγR co-engagement on antigen presenting cells and has been linked with efficacy of aCTLA-4 combinations in patients (pts) with microsatellite stable (MSS) colorectal cancer (CRC) and other tumors [Fakih ASCO GI 2025]. Vilastobart was generally well-tolerated and demonstrated evidence of anti-tumor activity in pts with immunologically “cold” advanced solid tumors, both as a monotherapy and in combination with atezolizumab [Davar SITC 2024]. Methods: Phase 2 of the NCT04896697 study evaluated the initial recommended Phase 2 dose of vilastobart 100 mg Q6W in combination with atezolizumab 1200 mg Q3W in pts with MSS CRC who had at least 1 prior chemotherapy regimen in the metastatic setting, excluding pts with prior immune checkpoint inhibitors. Pts with (LM) and without liver metastasis (NLM) were eligible. Tumor biopsies were obtained before and during treatment for translational analyses. Results: As of January 13, 2025, 40 pts were dosed in Phase 2. Median age was 55 (25-82) and 70% of pts had 3 or more prior lines of therapy. Of the 24 enrolled NLM pts, 11 were response-evaluable with an available on-treatment scan as of the data cut. In these 11 pts, two confirmed and one unconfirmed partial response (PR) were reported, all accompanied by significant decreases in ctDNA and serum tumor marker CEA and with each pt ongoing on therapy, for a preliminary ORR of 27%. One additional pt (with peritoneal metastasis) had a 24% reduction in target lesions (first scan) and was ongoing on therapy. Of the 16 enrolled LM pts, 7 were response evaluable, with one stable disease and another pt reporting a mixed response with significant serum tumor marker reductions, both ongoing on therapy. Of note, one LM pt in Phase 1C dose escalation treated with vilastobart (150 mg Q6W) combination had a confirmed PR with LM resolution. Six pts (15%) reported G3+ treatment-related adverse events (TRAEs), with two G4 laboratory TRAEs and no G5 TRAEs. Only three pts discontinued therapy for TRAEs. TRAEs occurring in ≥10% (all grade) or ≥5% (G3) of pts are summarized in the Table. Conclusions: The combination of vilastobart, a novel tumor-activated, Fc-enhanced a-CTLA-4, and atezolizumab demonstrated initial evidence of anti-tumor activity in late line, metastatic MSS CRC where immune checkpoint blockade has historically been relatively ineffective. Vilastobart was observed to have a differentiated safety profile distinct from systemically active a-CTLA-4, consistent with tumor-selective activation. Clinical trial information: NCT04896697 . AE term All Graden (%) Grade 3n (%) Fatigue 12 (30%) 0 Diarrhea 8 (20%) 0 Infusion related reactions 5 (13%) 0 Pyrexia 4 (10%) 0 ALT increased 4 (10%) 0 AST increased 4 (10%) 1 (3%) Colitis 2 (5%) 2 (5%)

1096 Background: Bria-IMT is a combination immunotherapy consisting of allogeneic whole cell cancer vaccine (SV-BR-1-GM) administered w/ immune checkpoint inhibitor (CPI). SV-BR-1-GM breast cancer cells are engineered to directly stimulate anti tumor immunity via expression of tumor associated antigens and secretion of GM-CSF to enhance dendritic cell activation. Addition of CPI potentiates SV-BR-1-GM to overcome the immune suppressive tumor microenvironment. Methods: This Ph I/randomized Ph II study evaluated the Bria-IMT regimen in pts w/ metastatic breast cancer; CTX (300 mg/m²) on day -2/-3, SV-BR-1-GM and CPI on Day 0, w/ low dose peg interferon α at inoculation sites on day 2 (±1) . Phase II pts were randomized 1:1 to receive CPI at cycle 1 or cycle 2. Two SV-BR-1-GM formulations (w/ vs w/o IFNγ incubation) were evaluated. Biomarkers included cancer-associated macrophage-like cells, circulating tumor cells, PD-L1 scores, and delayed-type hypersensitivity skin tests. Results: 54 pts (22 Ph I, 32 Ph II) enrolled; 11 received pembrolizumab, 44 retifanlimab (1 crossover). 33 (61%) pts were ER+/PR+/HER2-, 18 (33%) TNBC, 3 (6%) HER2+. Median OS, PFS, ORR, and CBR were evaluated against two pivotal Ph 3 trials, ASCENT 1 (SG in TNBC) and TROPiCS-02 2 (SG in HR+/HER2- MBC) (see Table 1). In randomized pts, C1 vs C2 CPI had PFS (3.7 vs 3.2 mos, P=0.09) and OS (11.4 vs 7.4 mos, P=0.19). Pts receiving Ph 3 formulation (w/o IFNγ; N=37) had greater PFS (3.6 vs 2.6 mos, P=0.01) and OS (13.4 vs 6.9 mos, P=0.01). Bria-IMT was well tolerated w/ no Tx related D/Cs. Conclusions: The Bria-IMT Ph 3 formulation cohort OS was comparable to ASCENT and TROPiCS-02 (13.43 vs 11.8, 14.4 mos), exceeding TPC arms (6.9, 11.2 mos). CBR (61%) compared favorably to ASCENT (40%) and TROPiCS-02 (34%); ORR (14%) matched or exceeded TPC arms (4%, 14%). These outcomes were observed in a more heavily pretreated population, demonstrating Bria-IMT’s clinical activity. Randomized Ph 2 results suggest efficacy and safety in heavily pretreated MBC, w/ no significant OS difference between C1 and C2 CPI initiation and 22% of pts still in active survival follow up. Superior outcomes w/ the Ph 3 formulation support its continued evaluation. A randomized Ph 3 trial is ongoing, comparing Bria-IMT vs treatment of physician’s choice (NCT06072612). Clinical trial information: NCT03328026 . Trial (Cohort) Age (Median, Range) Prior Therapies (Median) OS (Median, mos) PFS (Median, mos) ORR (%) CBR (%) Bria-IMT (Overall Cohort) 61 (38-81) 6 (2-13) 9.9 (1.8-30.3) 3.6 10% 55% Bria-IMT (Ph 3 Formulation) 62 (44-80) 6 (2-13) 13.43 (1.8-30.3) 3.6 (1.8-16.5) 14% 61% ASCENT (SG) 54 (27-82) 4 (2-17) 11.8 4.8 31% 40% ASCENT (TPC) 53 (27-81) 4 (2-14) 6.9 1.7 4% 8% TROPiCS-02 (SG) 57 (49-65) 3 14.4 5.5 21% 34% TROPiCS-02 (TPC) 55 (48-63) 3 11.2 4 14% 22% References: Bardia A et al. J Clin Oncol. 2024 May 20;42(15):1738-1744Rugo, Hope S et al. The Lancet, Volume 402, Issue 10411, 1423 – 1433.

206 Background: XTX101 is an investigational tumor-activated, Fc-enhanced, high affinity binding aCTLA-4 designed to block CTLA-4 and deplete regulatory T cells upon activation in the tumor by proteases. XTX101 also contains mutations designed to enhance Fcγ receptor binding. Fc enhancement augments FcγR co-engagement on antigen presenting cells and has been linked with efficacy of aCTLA-4 combinations in patients (pts) with “cold tumors”, including microsatellite stable (MSS) colorectal cancer (CRC). Parts 1A/1B of the study evaluated XTX101 monotherapy in pts with advanced solid tumors. XTX101 was generally well tolerated with limited peripheral XTX101 activation (~13%) compared to evidence of XTX101 activation in the tumor microenvironment (73-96%, in n=2 on-treatment tumor biopsies). A durable partial response (PR) and resolution of hepatic metastases was observed in a pt with PD-L1 negative NSCLC at the monotherapy recommended phase 2 dose (RP2D) of 150 mg once every six weeks (Q6W) [1]. Methods: Part 1C evaluated the safety and feasibility of XTX101 in combination with atezolizumab in pts with advanced solid tumors using 3+3 dose escalation to establish the RP2D. Phase 2 is enrolling pts with MSS CRC with at least 1 prior regimen for metastatic CRC. Initial safety and anti-tumor activity data from Phase 2 in ~20 pts will be presented. Results: As of July 15, 2024, 15 pts were enrolled in Part 1C: MSS CRC (n=12), esophageal cancer, NSCLC, and ampullary carcinoma (n=1 each). Median age 69 and 3 prior lines of therapy (1-12). Across all XTX101 dose levels (75 mg to 150 mg), treatment-related adverse events (TRAEs) of any grade with >10% incidence included infusion reactions (n=4), fatigue and diarrhea (n=2 each), no G4 or G5 TRAEs were reported. In Part 1C, 2 pts experienced a dose limiting toxicity: 1 pt had G3 colitis, and 1 pt had G3 liver enzyme elevation (both were at the XTX101 150 mg dose level and resolved with immunosuppression). Two unconfirmed PRs were reported: in a pt with MSS CRC, including resolution of a liver target lesion, and in a pt with ampullary carcinoma (accompanied by a Ca 19-9 decrease from 700 to 41). In addition, a decrease in intra-tumoral Tregs was observed in paired tumor biopsies from a pt. The combination RP2D was established as XTX101 100 mg Q6W and atezolizumab 1200 mg once every three weeks (Q3W), and enrollment in the Phase 2 part of the study for pts with MSS CRC is ongoing at the RP2D. Conclusions: In Part 1C dose escalation, the combination of XTX101, a tumor-activated, Fc-enhanced aCTLA-4, and atezolizumab was generally well tolerated in pts with advanced solid tumors and demonstrated initial evidence of anti-tumor activity in cold tumors. These data support the initiation of an ongoing Phase 2 study evaluating the combination in pts with MSS CRC with and without liver metastases. 1. Davar ESMO IO 2023. Clinical trial information: NCT04896697 .

1022 Background: The Bria-IMT regimen is a combination immunotherapy using an allogeneic whole cell cancer vaccine combined with checkpoint inhibitors (CPIs). Intradermally administered irradiated SV-BR-1-GM breast cancer cells secrete GM-CSF and present tumor associated antigens (TAAs) on HLA-I and –II molecules to directly stimulate anti-tumor immunity. Anti-PD1 CPI further stimulates T-cells and synergizes with SV-BR-1-GM. Here we report the results of the phase 1/2 trial (NCT03328026) of Bria-IMT in metastatic/advanced breast cancer evaluating sequencing of SV-BR-1-GM inoculation and CPI. Methods: Analysis of the phase 1 with expansion phase 2 cohort randomized by sequence using SV-BR-1-GM, checkpoint inhibitor (pembrolizumab or retifanlimab), low dose cyclophosphamide (Day –2) and post-dose local pegylated interferon α, with cycles every 3 weeks. In the 1:1 randomized cohort, CPI is either dosed at C1 prior to SV-BR-1-GM (original sequence) or delayed to C2 post-SV-BR-1-GM (alternative sequence). Cancer-associated macrophage-like cells (CAMLs) and circulating tumor cells (CTCs) were monitored as biomarkers. Results: A total of 54 patients were dosed: 11 with pembrolizumab, 44 with retifanlimab (1 patient received pembrolizumab and later retifanlimab). Median age=61 (38-81); Median prior regimen=6 (2-13); 72% ER+/PR+/HER2-; 6% HER2+ and 22% triple negative. Bria-IMT regimen was well tolerated with only 9% drop-out rate due to AE. AEs related to IPs were seen in 74% of patients; the majority (91%) being grade 1-2, and few (1.7%) grade 4 cases including pulmonary embolism, dehydration and increased lipase. Among 30 patients with post-dose CAML/CTC data and evaluable outcome, CAML count <5 associates with disease control (p=0.06, Chi square test). Patients who developed DTH to SV-BR-1-GM had significantly better PFS and higher OS. Patients treated with phase III formulation had an increase in PFS. Quality of life improvement correlates with disease outcome and survival. Among the patients treated with the phase III formulation of SV-BR-1-GM (n=23 with evaluable outcomes), objective response rate (ORR) is 13% and disease control rate (DCR) is 61%, with median PFS and OS are 4.1 and 13.3 months, respectively. In the randomized portion, the original sequence cohort had higher DCR (43% vs 36%) and OS (P=0.08, HR 0.30, 95%CI 0.08-1.17) compared to the alternative sequence cohort. Conclusions: BRIA-IMT has shown a favorable safety profile while providing benefits to heavily treated aMBC patients. Superior outcomes and survival were seen in patients treated with the phase III formulation and regimen now being used in the ongoing Ph3 trial, confirming the improvement in efficacy of the novel regimen. The results from this randomized phase 2 study informed the study design of the ongoing Ph3 trial comparing BRIA-IMT to standard of care physician choice treatments (NCT06072612). Clinical trial information: NCT03328026 .

TPS633 Background: Ovarian function suppression (OFS) when combined with tamoxifen or an aromatase inhibitor is the standard of care for premenopausal estrogen receptor positive (ER+) human epidermal growth factor receptor 2 negative (HER2-) breast cancer (BC) based on SOFT (Francis 2022) and TEXT (Pagani 2022) clinical trials and confirmed in a metanalysis (EBCTCG 2022). However, estrogen deprivation in premenopausal women is associated with significant morbidity and up to 40% of premenopausal women with ER+/HER2- BC are intolerant of OFS. For these women, tamoxifen monotherapy is the only FDA approved option. Therefore, there is substantial room to improve endocrine therapy for premenopausal women with ER+/HER2- BC. (Z)-endoxifen is one of the most active anti-estrogen tamoxifen metabolites, with preclinical data demonstrating superiority to tamoxifen and letrozole in aromatase expressing xenograft models and evidence for antitumor activity in early phase clinical trials in patients with prior progression on endocrine therapy including tamoxifen. In addition to its ability to potently block ERa, (Z)-endoxifen at clinically achievable concentrations inhibits protein kinase C beta 1 (PKCβ1), resulting in downstream AKT inhibition and apoptosis. It is hypothesized that (Z)-endoxifen, when delivered as monotherapy at doses that dually target both ERα and PKCβ1, will potently inhibit BC proliferation while obviating the need for OFS in premenopausal women. If successful, this innovative approach could spare women with endocrine sensitive BC from the side effects of OFS. Methods: To further study (Z)-endoxifen in premenopausal women, a multicenter study in the United States, with possible expansion to other countries was designed (EVANGELINE; NCT05607004). This is a Phase 2, open-label, randomized, neoadjuvant study in premenopausal women with Stage IIA or IIB ER+/HER2- BC. Subjects are randomized 1:1 to either exemestane plus goserelin or (Z)-endoxifen. The primary objective is to determine whether the endocrine sensitive disease (ESD) rate (defined as Ki-67 ≤ 10% after 4 weeks of neoadjuvant therapy) with (Z)-endoxifen is non-inferior to exemestane plus goserelin. Subjects will be enrolled with the intent of surgical treatment in the involved breast(s) after completing 6 months of neoadjuvant treatment. Safety, tolerability, response rates and surgical outcomes will be assessed. Disease progression will be monitored throughout study participation. Paired tumor biopsies and blood sampling will allow for a robust biomarker evaluation. Prior to initiating the Treatment Cohort (162 subjects), up to 12 subjects will be enrolled in a pharmacokinetic (PK) Run-in Cohort to identify the optimal dose for the Treatment Cohort. Enrolment in the PK Run-in cohort began in February 2023. Clinical trial information: NCT05607004 .

Abstract Background: Bria-IMT (SV-BR-1-GM) is an off-the-shelf whole tumor cell therapeutic vaccine engineered to express class I & class II HLAs, secrete GM-CSF, and function as antigen-presenting cells; with subsequent enhancements improving in-vitro characteristics. By expressing cancer antigens such as HER2/Neu and PRAME, SV-BR-1-GM also serves as the reservoir of cancer antigens to activates patient’s anti-tumor immune responses. The safety and efficacy of the ongoing clinical trial of SV-BR-1-GM in combination with a PD1 inhibitor are reported. We also report post-hoc exploratory data for patients with advanced metastatic breast cancer (aMBC) treated with SV-BR-1-GM regimens. Methods: Ongoing prospective, phase 1-2, with an expansion randomized phase 2 cohort (NCT03328026; 2018-present) using SV-BR-1-GM with a PD1 inhibitor (pembrolizumab or retifanlimab) with cycles every 3 weeks (16 patients dosed to date). SV-BR-1-GM “monotherapy” (NCT03066947; 2013-8), a completed prospective phase 1-2 results are also presented. Results: A total of 51 patients received any SV-BR-1-GM regimen; 27 in monotherapy (SV) and 24 in combination with a PD-1 inhibitor (CO) currently ongoing: 4 subjects from SV continued on with CO. Median prior regimens = 5; 61% ER or PR+; 6% ER or PR-/HER2+ and 29% triple negative. SV-BR-1-GM was well tolerated. Disease control rate was 40% with SV and 41% with CO in subjects with available assessment(s). Median PFS was 2.7 months with 6 having PFS &gt;150 days (150-226) for SV and 2.6 months with 5 &gt; 150 days (188-308) for CO. Per protocol, 23 patients had anergy testing prior to treatment. Among those with a normal immunologic response (n=13, 56%), disease outcomes included 4 SD and 2 PR (46% CBR) vs anergic patients (n=7, 30%) with 1 SD (14% CBR); 3 patients (13%) with unknown anergy testing had 0% CBR. All 7 anergic patients developed delayed-type hypersensitivity (DTH) injection site responses to SV-BR-1-GM. Subjects with DTH (injection site induration/erythema) had a significant reduction in the risk of progression or death (HR=0.096, 95%CI:0.02-0.50, p&lt;0.05). Subjects with a reduction in circulating tumor cells (CTC) also had an improvement in PFS (HR 0.08, p=.0012). Median OS (uncensored) was 11.2 months in CO (n = 8). 7 out of 20 (35%) for SV and 9 out of 22 subjects (41%) for CO with available data, exceeded prior PFS. Among HLA matched subjects 4/7 (57%) and 6/9 (67%), respectively, had an increase in PFS. PFS ratio improved independent of prior number of lines, HER2 or HR status. The greatest improvement in PFS on a SV-BR-1-GM was in a HLA matched subject (&gt;6mo). TEAEs occurred in 20/24 (83%) of subjects in CO including 9/24 (38%) with injection site AEs. There were no AEs of special interest eg cytokine storm. There were no Grade 5 and only 6 grade 4 AEs in CO. Conclusions: In an ongoing randomized phase 2, anergic patients had worse outcomes. SV-BR-1-GM was able to illicit an immune response regardless. Responses, stable disease and CBR were seen and associated with immune responses including injection site erythema/induration (DTH), CTC changes and tumor grade I/II. Patients with disease control had better quality of life. SV-BR-1-GM may be able to alter an anergic immune system for clinical benefit. Future pivotal trials will be based upon the ongoing confirmatory randomized clinical trial. Citation Format: Saranya Chumsri, Minal Barve, Chaitali Nangia, Bonnie L. Guerin, Jarrod Holmes, Karim Mohammed, John Knecht, Shaker Dakhil, William V. Williams, Charles Wiseman, Mingjin Chang, Miguel Lopez-Lago, John G. Knecht, Kendrith Rowland, Ralph V. Boccia, Giuseppe Del Priore, Carmen Calfa. Whole cell antigen presenting immune stimulating cells (Bria-IMT) for the treatment of metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT143.