Phase 2 study of talabostat, a small molecule inhibitor of dipeptidyl peptidases (DPP), administered in combination with pembrolizumab in patients with small cell neuroendocrine prostate cancer

Background: Small cell neuroendocrine prostate cancer (SCNC) is a lethal subset of prostate cancer with limited treatment options. Talabostat, an oral small molecule inhibitor of dipeptidyl peptidases (DPP4 and DPP 8/9) triggers the inflammasome to alert and prime immune cells, leading to induction of interleukin (IL)-18 and IL-1ß, bridging innate and adaptive immunity. Talabostat was evaluated in a phase 2 study in combination with pembrolizumab in patients with SCNC. Methods: Patients were required to have centrally confirmed histologic evidence of de novo or treatment-emergent SCNC and progression on ≥1 prior line of systemic therapy. Patients received pembrolizumab (200 mg intravenous every 21 days) + BXCL701 0.2 mg orally two times per day for a week, with step-up to 0.3 mg two times per day on days 8-14, and 0.3 mg two times per day on days 1-14 of subsequent cycles. The primary endpoint was the composite response rate (CRR). Biomarkers including levels of DPP 8/9 expression and association with clinical outcomes were analyzed in a post hoc fashion. Results: 34 patients were enrolled, including 21 (62%) with visceral metastases. Patients had received a median of 3 prior lines of systemic treatment, including 19 (56%) and 18 (53%) of patients who received prior platinum and taxane chemotherapy, respectively. In the response evaluable subset (n=30), the CRR was 20% (95% CI 7.7% to 38.6%) and the objective response rate was 13% (95% CI 3.8% to 30.7%). The median duration of objective response was 9.0 months. All responders had tumors with low tumor mutational burden and/or microsatellite stability. The median radiographic progression-free survival was 2.1 months (95% CI 1.9 to 4.2). The median overall survival (OS) was 13.7 months (95% CI 7.0 to unevaluable) and the 12-month OS rate was 54.1% (95% CI 34.2% to 70.3%). The most frequently occurring treatment-related adverse events of any grade severity were fatigue (41%), hypotension (29%), dizziness (21%), pruritus (24%), nausea (15%), and diarrhea (15%). Baseline levels of DPP9 tumor stromal expression were associated with response. Conclusions: Talabostat plus pembrolizumab demonstrates preliminary anti-tumor activity in patients with relapsed SCNC. Further evaluation in a randomized study is warranted to assess the contribution of talabostat in this high-risk disease subset. Trial registration number: NCT03910660.