Pembrolizumab is an anti-programmed cell death protein 1 immune checkpoint inhibitor with a dosing schedule of 200 mg 3 weekly (q3w). Dose of 400 mg 6 weekly (q6w) was approved based on simulation of dose/exposure relationships and predicted no difference in toxicity. We present real-world comparative toxicity data. Patients receiving pembrolizumab for any indication between March and December 2019 were included across 3 regional centers. Toxicity data were collected retrospectively using Common Terminology Criteria for Adverse Events, v5.0. Clinically significant immune-related adverse events (CSirAE) were defined as immune-related events and grade ≥3 rash. Data were analyzed using incidence (Poisson distribution) and incidence ratio. Overall, 63 patients started on q6w and 110 patients received q3w. There were 3 (q6w) and 8 (q3w) grade 3-5 CSirAE and 13 (q6w) and 31 (q3w) grade 1-2 CSirAE. The incidence of grade 3-5 CSirAE was 0.77 (95% confidence interval: 0.16-2.24) per 100 patient-months in q6w and 0.68 (95% confidence interval: 0.29-1.34) per 100 patient-months in q3w (incidence ratio of 1.13; 95% confidence interval: 0.19-4.70). Low-grade toxicity was common (fatigue, pruritus, rash; q6w 46%, q3w 42%). Incidence of CSirAEs was low but low-grade toxicity was common. Despite a limited number of events, there is the suggestion that the q6w schedule has a similar toxicity profile to q3w and therefore consideration should be given to the reduced burden to patients and health services when deciding treatment.
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