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 patientmonths 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 lowgrade 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.

No abstract

Objective: Enzalutamide is effective in treating metastatic castrate-resistant prostate cancer (mCRPC) but can have side effects that require treatment breaks (TB). We conducted a retrospective analysis of outcomes of patients who had extended TB due to toxicity compared to continuous dosing. Methods: Patients prescribed enzalutamide for mCRPC from September 2011 to February 2018 were included. TB was defined as an interruption of four weeks or more. Overall survival (OS) from enzalutamide start, time to prostate-specific antigen failure (TTF) and total enzalutamide treatment time (TTT) were analysed for TB and continuous responders (>50% PSA drop), and a significance level of 0.05 was assigned. Results: A total of 110 patients were continuous responders, and 29 had TB. The median number of interruptions was one (range 1–7), and time on treatment was 70% in the TB group. The TB group had significantly improved OS (100 vs. 60 months; hazard ratio=1.8, 95% confidence interval 1.17–2.77, p=0.02), prolonged TTF (median 11 vs. 6 months; p=0.008) and TTT (median 15 vs. 8 months; p=0.0001). Conclusion: Extended TB do not seem to impact OS or treatment duration adversely in patients who are responding and experiencing toxicity, and may be a useful option in managing toxicity. Prospective trials could explore this further. Level of evidence: Level 2c.

Background: We investigated our institution’s mCRPC enzalutamide or abiraterone patients examining PSA responses and impact of sequencing of these drugs. Methods: All enzalutamide / abiraterone mCRPC patients (2011-2018) were included. Rates of PSA >50% response (PSA50) were compared. Time to treatment failure (TTF) and overall survival (OS) was analysed as per lines of previous therapy and timing of chemotherapy. Results: 363 patients included (Enza n=236, Abi n=127), with 15.6 months median follow-up. PSA50 was greater in enzalutamide group (58% vs 31% p<0.0005) but TTF was similar for both groups (median Abi 4.2m vs Enza 6m, p=0.965). There was no significant median OS difference between the groups (Enza 13.8m vs Abi 12.5m p=0.065). Number of lines of prior therapy (p=0.735) or timing of chemotherapy before or after Abi/Enza (p=0.21) had no significant OS impact. Conclusion: Enzalutamide showed higher PSA50 response than abiraterone. Previous lines of therapy or sequencing of chemotherapy with abiraterone / enzalutamide showed no significant survival differences indicating no detriment in either treatment sequence.

81 Background: Intermittent hormone manipulation in castrate-sensitive prostate cancer can improve quality of life whilst maintaining comparable disease outcomes with continuous scheduling. Enzalutamide is effective in metastatic castrate-resistant prostate cancer (mCRPC) treatment but can have significant side-effects. We conducted a retrospective analysis of patients treated with intermittent enzalutamide compared with continuous dosing. Methods: Patients prescribed enzalutamide for mCRPC at Royal Cornwall Hospital from September 2011 to February 2018 were included. Data was collected from electronic medical records, selecting patients with at least a 1 month treatment break. Kaplan-Meier analysis of overall survival from enzalutamide start (OS), time to PSA failure (TTF) and total enzalutamide treatment time (TTT) was calculated for intermittent and continuous responders (>50% PSA drop), assigned significance level of 0.05. Results: 243 patients received enzalutamide, 110 (45%) were continuous responders and 29 (12%) had intermittent dosing. All patients treated intermittently had a PSA response prior to first treatment break, which was most commonly for fatigue (60%). 25% were still receiving enzalutamide. Median number of breaks was 1 (range 1-7), time on treatment was 70% and time to first break was 5 months. The intermittent group had significantly improved OS with median not reached, median OS for continuous responders was 19 months (HR 2.39, 95% CI 1.53-3.76, p=0.002). The intermittent group had prolonged TTF (median 13 vs 6 months, p=0.001) and TTT (median 30 vs 10 months, p=0.0003). Conclusions: Intermittent dosing of enzalutamide in these mCRPC patients does not adversely impact OS, increasing time patients remain on treatment. However, this was a small, retrospective, single-centre study; prospective trials are necessary to clarify the role of intermittent enzalutamide.[Table: see text]