A phase 2 basket study of talabostat, a small-molecule inhibitor of dipeptidyl peptidases, administered in combination with pembrolizumab in patients with advanced solid cancers

Background: Talabostat, an oral small molecule inhibitor of dipeptidyl peptidases (DPP4 and DPP8/9), has shown synergistic activity with immune checkpoint inhibitors in preclinical studies. This open label, phase 2 basket trial assessed the antitumor activity of combining talabostat and pembrolizumab (anti–programmed death-1 antibody) in advanced solid tumor patients. Methods: The primary objective was assessment of dose-limiting toxicity (DLT) rates in the first six patients (lead-in stage) and response rate (efficacy stage; included cohort A [checkpoint inhibitor (ICI) naive] and cohort B [ICI pretreated]) for the study treatment using the Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 and immune RECIST (iRECIST). Efficacy was assessed using a Bayesian optimal phase 2 design. Results: A total of 31 patients enrolled in this trial (14 in cohort A, 17 in cohort B). The median age was 61 years; 17 (55%) patients were male and 21 (68%) patients were White. Among 19 (61%) patients evaluable for response, the best response was stable disease in nine patients, unconfirmed progressive disease in seven patients, and clinical progressive disease in three patients based on iRECIST. Disease control rate was 47%. One patient with programmed death-ligand 1 negative, microsatellite stable endometrial cancer had unconfirmed partial response. Median progression-free survival was 2.7 months; median overall survival was 20.5 months. One patient (cohort A) experienced a grade 4 hypotension as a DLT and treatment discontinuation. The most common toxicities were hypotension (22.6%), fatigue (9.7%), diarrhea, rash, thrombocytopenia, vomiting, syncope, general disorders and administration site conditions-other, and skin and subcutaneous tissue disorders-other, each in 6.5% of patients. Conclusions: This study of the combination of talabostat and pembrolizumab in patients with advanced solid tumors demonstrated predictable adverse events and limited activity. The combination was shown to be safe. Efficacy data shows immune stable disease in nine of 19 evaluable patients, and an unconfirmed immune partial response in a patient with endometrial cancer.