Bessudo, Alberto

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%)

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 .

2579 Background: HA, a glycosaminoglycan, accumulates in ~25% of solid tumors and is associated with poor prognosis. Tumors that accumulate HA develop high interstitial fluid pressure (IFP) and become resistant to chemotherapy. In animal models, PEGPH20 reduced tumor-associated HA and increased efficacy of chemotherapy. Methods: This ongoing phase 1 study was designed to assess safety, tolerability, PK, PD, and the maximum tolerated dose (MTD) of PEGPH20 as a single agent in patients with treatment-refractory solid tumors. IV PEGPH20 was administered in a twice weekly or a once weekly dosing schedule with oral dexamethasone (dex) pre-and post dose. Biomarkers included tumor histochemistry to detect HA depletion, plasma HA catabolites, and DCE-MRI or FDG-PET. Results: Fifteen patients have been enrolled into the study. Dose administration ranged from 0.5 µg/kg to 5 µg/kg. Musculoskeletal events (MSEs) were the most common and dose limiting toxicities. To date, up to 3 µg/kg twice weekly IV infusions of PEGPH20 were well tolerated with manageable Grade 1 MSEs in most patients. MTD has not been reached. Plasma concentrations of PEGPH20 showed dose proportional exposure with a terminal half life of ~2 days. Enzymatic activity of PEGPH20 was reflected by dose-dependent HA catabolites in plasma. Pre- and post-dose tumor biopsies from 2 colon patients demonstrated 23-60% reduction in tumor HA after 4 weeks of dosing. Serial DCE-MRI and FDG-PET demonstrated increased tumor perfusion (Ktrans) and decreased metabolic activity 8 and 24hrs post-PEGPH20 in one patient each. Doses of ≤3 µg/kg were tolerated with intermittent Grade 1-2 MSE. Enrollment at dose levels > 3 µg/kg is ongoing. Conclusions: Repeated IV PEGPH20 infusion resulted in dose-dependent systemic exposure and is tolerated with concomitant dex treatment. HA catabolites in plasma demonstrated the in vivo enzymatic activity of PEGPH20. Reduced HA in tumor biopsies and serial DCE-MRI and FDG-PET images confirmed PD activity. Increased tumor perfusion by PEGPH20 may enhance drug delivery to tumor. Combination studies of PEGPH20 with cytotoxic chemotherapy are planned.

Abstract Enzymatic degradation of hyaluronan (HA), a key component of the extracellular matrix (ECM), can enhance tumor perfusion, improve delivery of chemotherapeutics, and enhance the effects of anti-tumor agents. HA accumulation has been correlated with local invasion, the presence of distal metastasis, higher tumor grade, and poorer overall survival. PEGPH20, a pegylated form of recombinant human hyaluronidase PH20, is an investigational therapeutic agent under clinical development for the treatment of patients with solid tumors that may accumulate HA. Preclinical studies have demonstrated that sustained HA removal, accomplished with PEGPH20, inhibits tumor growth and enhances chemotherapeutic activity in HA-rich xenografts and genetically engineered mouse tumor models. Twenty-six patients with advanced solid tumors were enrolled in a Phase 1 multi-center trial. Patients received weekly or twice weekly doses of IV PEGPH20 (0.5 - 5 μg/kg) for the first 4-week cycle, followed by once-weekly dosing for the subsequent 4-week cycles. Serial blood samples were drawn from each patient, and plasma concentrations were measured by an ultrasensitive hyaluronidase activity assay to assess PEGPH20 pharmacokinetics (PK). Plasma levels of HA catabolites were measured using a quantitative HPLC method to characterize PEGPH20 pharmacodynamics (PD). Other measures of PD activity were evaluated in selected patients, including DCE-MRI, DW-MRI, 18FDG-PET, textural analysis on CT, and histochemical staining of HA in tumor biopsies collected both before and after PEGPH20 treatment. Peak plasma PEGPH20 concentrations increased with dose, and plasma pharmacokinetics (PK) were well described by a linear PK model. Pre-treatment plasma HA levels were typically <1 μg/mL and increased in a dose-dependent fashion after PEGPH20 treatment. Increased plasma HA corresponded with decreased HA staining in patients with available tumor biopsies. DCE-MRI analysis indicated early (24-48 hr) and rapid increase in tumor perfusion compared to baseline in 4 patients with serial MRI assessments. Pharmacodynamic activity was also demonstrated when pre-treatment 18FDG-PET scans were compared with images collected at scheduled times after PEGPH20 dosing. Reduction in FDG uptake exceeding 25% was demonstrated in 3 of 4 patients after one cycle of treatment, consistent with a partial metabolic response. These data are consistent with the mechanism of action reported in preclinical models, and results support continued evaluation of PEGPH20 in combination with anti-tumor agents via modification of the ECM. Citation Format: Ping Jiang, Daniel C. Maneval, Ramesh K. Ramanathan, Jeffrey R. Infante, Mitesh Borad, Alberto Bessudo, Patricia LoRusso, Barry J. Sugarman, Deborah Carson, Marie A. Printz, Curtis B. Thompson, Paneer Selvam, Joy Zhu, Ronald Korn, H Michael Shepard, Gregory I. Frost. Phase 1 pharmacodyamic activity of multiple-dose PEGylated hyaluronidase PH20 (PEGPH20) in patients with solid tumors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3375. doi:10.1158/1538-7445.AM2013-3375