Recurrent somatic mutations in the BRG1/BRM-associated factor (BAF) chromatin remodeling complex subunit ARID1A occur frequently in advanced urothelial, endometrial, and ovarian clear cell carcinomas, creating an alternative chromatin state that may be exploited therapeutically. The histone methyltransferase EZH2 has been previously identified as targetable vulnerability in the context of ARID1A mutations. In this study, we describe the discovery of tulmimetostat, an orally available, clinical stage EZH2 inhibitor, and it elucidates the aspects of its application potential in ARID1A mutant tumors. Tulmimetostat administration achieved efficacy in multiple ARID1A mutant bladder, ovarian, and endometrial tumor models and improved cisplatin response in chemotherapy-resistant models. Consistent with its comprehensive and durable level of target coverage, tulmimetostat demonstrated greater efficacy than other PRC2-targeted inhibitors at comparable or lower exposures in a bladder cancer xenograft mouse model. Tulmimetostat mediated extensive changes in gene expression, in addition to a profound reduction in global H3K27me3 levels in tumors. Phase I clinical pharmacokinetic and pharmacodynamic data indicated that tulmimetostat exhibits durable exposure and profound target engagement. Importantly, a tulmimetostat controlled gene expression signature identified in whole blood from a cohort of 32 patients with cancer correlated with tulmimetostat exposure, representing a pharmacodynamic marker for the assessment of target coverage for PRC2-targeted agents in the clinic. Collectively, these data suggest that tulmimetostat has the potential to achieve clinical benefit in solid tumors as a monotherapy but also in combination with chemotherapeutic agents, and may be beneficial in various indications with recurrent ARID1A mutations. Significance: The EZH2 inhibitor tulmimetostat achieves comprehensive target inhibition in ARID1A mutant solid tumor models and cancer patients that can be assessed with a pharmacodynamic gene signature in peripheral blood.

PURPOSE: Patients with metastatic urothelial carcinoma have poor prognosis after failure of standard first-line chemotherapy. Immune check point programmed death 1-programmed death ligand 1 antibodies have low response rates and thus there exists a major unmet need. MATERIALS AND METHODS: In this phase II trial, patients with metastatic urothelial carcinoma that recurred or progressed after platinum-based chemotherapy received soluble EphB4-human serum albumin (sEphB4-HSA) in combination with pembrolizumab. The primary end points were tolerability and overall survival (OS). The secondary end points were progression-free survival (PFS), objective response rate (ORR), duration of response, and toxicity. The expression of sEphB4-HSA target EphrinB2 was correlated with outcomes. RESULTS: Seventy patients were enrolled. The median follow up was 22.9 months (range, 1.3-54.7). The regimen had acceptable toxicity. In the intent-to-treat analysis (N = 70), the median OS was 14.6 months (95% CI, 9.2 to 21.5). Twenty-six (37%) patients had an objective response (95% CI, 26 to 48). The median PFS was 4.1 (95% CI, 1.5 to 5.7) months. Forty-six (66%) patients expressed EphrinB2, and among them, the median OS was 21.5 months (95% CI, 12.4 to not reached), the ORR was 52% (95% CI, 37 to 67), including a complete response rate of 24% (11 of 46; 95% CI, 12 to 36). The median PFS was 5.7 (95% CI, 2.7 to 27.9) months. Response was maintained at 6, 12, and 24 months in 88%, 74%, and 69% of the patients, respectively. CONCLUSION: The combination of sEphB4-HSA and pembrolizumab appears synergistic with improved OS and ORR compared with historical data for programmed death 1/programmed death ligand 1 monotherapy.

Purpose: Resistance to androgen receptor (AR) pathway inhibitors necessitates the development of regimens to improve outcomes in metastatic prostate cancer (mPC). This phase Ib clinical trial evaluated the safety/tolerability and pharmacokinetics of bavdegalutamide, an oral proteolysis-targeting chimera AR degrader, combined with abiraterone in patients with mPC experiencing prostate-specific antigen (PSA) progression on abiraterone. Patients and Methods: Patients received bavdegalutamide 420 mg, abiraterone ≤1,000 mg, and corticosteroid daily. Assessments included safety [dose-limiting toxicities (DLT); treatment-emergent adverse events (TEAE)], pharmacokinetics, PSA control rate, ≥50% PSA decline (PSA50), and radiographic progression-free survival (rPFS). Results: Forty-five patients were treated; seven had AR ligand-binding domain (LBD) mutations. The median duration of exposure was 35.7 weeks; 13 patients were continuing bavdegalutamide at the data cutoff. No DLTs were observed. There were no clinically significant effects of bavdegalutamide on abiraterone pharmacokinetics. Bavdegalutamide 420 mg daily was selected as the recommended phase II dose. TEAEs were mostly grade 1/2 (82.2%), and 17.8% were grade 3. Dose modifications or discontinuations due to TEAEs occurred in 48.9% and 13.3% of patients, respectively. The most common bavdegalutamide-related TEAEs were fatigue (48.9%) and nausea (46.7%). The PSA control rate was 48.9% [95% confidence interval (CI), 33.7–64.2]. The PSA50 response rate was 24.4% (95% CI, 12.9–39.5), including in five out of seven patients with AR LBD mutations. The median time to PSA progression was 2.8 months (95% CI, 1.5–6.5). The median rPFS was 16.3 months (95% CI, 8.2–not calculable). Conclusions: Bavdegalutamide combined with abiraterone demonstrated a manageable safety profile with no major drug–drug interactions and evidence of clinical activity in mPC.

As is the case with most solid tumors, the heterogeneity of the disease biology of prostate cancer presents clinicians managing this disease with daily challenges. However, in contrast to other common cancers such as breast, lung, and colorectal cancers, there are unique challenges in prostate cancer management, including the variety of clinicians who manage aspects of the disease (urologists, medical oncologist, radiation oncologists) and the striking absence of prospective comparative data to inform the optimal sequence of systemic therapy in patients with metastatic castration-resistant disease. The purpose of this review is to attempt to assist practicing oncologists with sorting through the myriad of prostate cancer disease subsets and the challenges in making therapeutic decisions in multiple data-free zones given the absence of level 1 comparative clinical trials in the metastatic hormone-sensitive and castration-resistant states.