Evaluation of pembrolizumab monotherapy in patients with previously treated advanced salivary gland carcinoma in the phase 2 KEYNOTE-158 study

Aim: We evaluated pembrolizumab monotherapy in patients with advanced salivary gland carcinoma on the phase 2 KEYNOTE-158 study (NCT02628067). Methods: Eligible patients had histologically/cytologically confirmed advanced salivary gland carcinoma with prior failure or intolerance to standard therapy, measurable disease per Response Evaluation Criteria in Solid Tumours (RECIST) v1.1., and ECOG performance status 0–1. Patients were enrolled irrespective of tumour PD-L1 expression. Patients received pembrolizumab 200 mg Q3W for up to 35 cycles (∼2 years). Radiographic imaging occurred every 9 weeks through month 12, then every 12 weeks. PD-L1 positivity was defined as combined positive score ≥1 (evaluated using PD-L1 IHC 22C3 pharmDx). The primary endpoint was objective response rate per RECIST v1.1. Results: In total, 109 patients were enrolled (PD-L1–positive, 25.7%). At the data cutoff (October 5, 2020), median follow-up was 53.3 (range, 50.8–56.3) months. Objective response rate was 4.6% (95% CI, 1.5–10.4%) among all patients (complete response, n = 1; partial response, n = 4) and was 10.7% (95% CI, 2.3–28.2%) in patients with PD-L1–positive disease and 2.6% (95% CI, 0.3–9.1%) in patients with PD-L1–negative disease. Duration of response was ≥24 months for all 5 responders; median duration of response was not reached (range, 25.1–49.8+ months). Median progression-free survival and overall survival were 4.0 (95% CI, 2.6–4.2) and 21.1 (95% CI, 15.9–25.5) months, respectively. Treatment-related adverse events occurred in 75.2% (grade 3–4, 15.6%; grade 5, 0%) of patients. Immune-mediated adverse events occurred in 22.0% of patients (grade 3, 5.5%; grade 4–5, 0). Conclusions: A small subset of patients with advanced salivary gland carcinoma treated with pembrolizumab had a response; all had response duration ≥2 years. The safety profile of pembrolizumab was manageable.