Pembrolizumab drug and anti-drug antibody (ADA) levels in a clinical cohort. Data generated from patients with melanoma treated with pembrolizumab (n=41). (A) Pembrolizumab drug level at baseline (B/L), just prior to Cycle 2 (T1) and last available timepoint (T2; Median Cycle 7 or 21 weeks IQR Cycle 4-13). (B) pembrolizumab ADA were detectable at B/L, T1 and T2. (C) Clinical correlates to pembrolizumab drug level just prior to Cycle 2 (T1). Pembrolizumab drug level did not significantly correlate with sex, age, body surface area or progressive disease at 6 months. Patients with an ADA at T1 had higher pembrolizumab drug levels (*p<0.05).