Overall survival in metastatic melanoma correlates with pembrolizumab exposure and T cell exhaustion markers

Trial data support an absence of an exposure–survival relationship for pembrolizumab. As these relationships remain unexamined in a real‐world setting, we determined them in metastatic melanoma prospectively in an observational study. Translational objectives included identifying biomarkers of progressive disease (PD). Checkpoint blockade naïve patients receiving 2 mg/kg Q3W pembrolizumab had pharmacokinetic and clinical outcome data collected. Trough, a valid surrogate for drug exposure, was assessed using ELISA. T‐cell exhaustion and chemokine markers were determined using flow cytometry. Geometric means of exposures and biomarkers were tested against objective response groups using one‐way ANOVA. The cohort was split by the median into high versus low pembrolizumab exposure groups. Kaplan–Meier progression‐free survival (PFS) and overall survival (OS) curves were estimated for high versus low exposure, compared using the log rank test. The high pembrolizumab exposure group (n = 14) experienced substantially longer median OS (not reached vs. 48 months, p = .014), than the low exposure group (n = 14). A similar positive exposure PFS relationship was found (median not reached vs. 48 months, p = .045). The frequency of TIM‐3 expression on CD4(+) T cells was significantly higher in PD (mean 27.8%) than complete response (CR) (13.38%, p = .01) and partial response (12.4%, p = .05). There was a near doubling of CXCR6 and TIM‐3 co‐expression on CD4(+) T cells in PD (mean 23.3%) versus CR (mean 11.4, p = .003) and partial response (9.8%, p = .0001). We describe positive exposure‐PFS and exposure‐OS relationships for pembrolizumab in metastatic melanoma. TIM‐3, alongside co‐expression of CXCR6 and TIM‐3 on circulating CD4(+) T cells are potential bio markers of treatment failure.