1868P Real-world safety of the 6-weekly pembrolizumab regimen in cancer patients treated in South East London

Background: Pembrolizumab (pembro) is a PD-1 inhibitor indicated for the treatment (tx) of a several malignancies. Most clinical trials used a 3-weekly tx (Q3W) but a 6-weekly 400mg regimen (Q6W) is now approved, based on pharmacokinetic data, also supported by the KEYNOTE-555 trial. The real world tolerability of the Q6W remains unknown. The COVID-19 pandemic led to rapid adoption of the Q6W tx, usually in patients (pts) previously receiving Q3W tx, as it facilitates less hospital visits. We report the toxicity profile of pts treated with Q6W pembro and the comparison of the preceding Q3W tx. Methods: We retrospectively analysed adverse events for non-small cell lung cancer (NSCLC), urothelial cancer (UC) and melanoma pts, who received at least 1 cycle of Q6W pembro. Pts that received pembro in combination tx were excluded. Previous Q3W monotherapy was permitted. Toxicity was graded as per CTCAE v5.0. Results: We included 94 pts (melanoma=39, NSCLC=38, UC=17). Median number of Q6W cycles received was 3 (range 1-6). 71% received the Q3W regimen (median 7 cycles, range 1-32) prior to switching to Q6W. New toxicity of any severity was recorded in 52% (49/94) during Q6W versus 70% (47/67) during Q3W tx. G 3/4 toxicities occurred in 15% (15) during Q6W versus 0% during previous Q3W tx. Of the 27 who started de novo with Q6W, 4 (15%) developed G 3/4 toxicities. G 3/4 toxicities were: GI (4% [colitis = 2, gastritis =1, oral lichen planus = 1]), nephritis (3%), arthralgia (2%), skin (2%), myositis/CK rise (2%), anaemia (1%), myocarditis (1%). Steroids for management of toxicity were initiated in 22% (21) pts, including 8 with G2 toxicity. Three (3%) pts switched back to Q3W administration. None of them received Q6W again. In total 9% of pts in Q6W discontinued tx due to toxicity. Conclusions: In our cohort of pts, the majority were previously treated with the Q3W regimen without significant toxicity. Switch to Q6W or de novo Q6W pembro led to a 15% rate of G 3/4 toxicity and 9% discontinuation rate. In pts pre-treated with Q3W, we cannot distinguish whether this was due to cumulative toxicity or due to switch to Q6W. More studies are required to ascertain the safety of the Q6W schedule. In the COVID-19 context, any Q6W toxicity concern should be weighed against the advantages of fewer hospital visits. Legal entity responsible for the study: The authors. Funding: Has not received any funding. Disclosure: All authors have declared no conflicts of interest.