Adverse event profiles of apalutamide, enzalutamide, and darolutamide in SPARTAN, PROSPER, and ARAMIS: How confident are we about which drug is safest?

Abstract

318 Background: Apalutamide, enzalutamide, and darolutamide were approved for non-metastatic castration-resistant prostate cancer (nmCRPC) based on three randomized placebo-controlled trials that showed similar efficacy. Adverse event (AE) profiles have been invoked to differentiate the drugs but have only been informally compared. Methods: We accounted for baseline characteristics, AE collection and reporting, and statistical uncertainty when comparing AE risks in the drug and placebo arms of SPARTAN (NCT01946204), PROSPER (NCT02003924), and ARAMIS (NCT02200614). Results: Patients in SPARTAN, PROSPER, and ARAMIS had a similar median age (74 years in all studies), median PSA doubling time (3.7-4.7 months) and ECOG performance status (68-80% with ECOG 0). AEs were gathered consistently using CTCAE 4.0 or 4.03. However, PROSPER and ARAMIS reported AEs occurring in ≥5% of patients; SPARTAN reported AEs occurring in ≥15%. Trials also reported 5 to 14 ‘AEs of interest.’ Of 34 AE types reported overall, only 10 were reported in all three trials. Absolute risks of adverse events in the placebo arms differed considerably. Compared to the placebo arm of SPARTAN, AEs were on average 44% less common in the placebo arm of PROSPER (95% CI, 28-56%) and 54% less common in the placebo arm of ARAMIS (95% CI, 41-64%), a difference not explained by length of follow-up. With lower event numbers, relative risk estimates were less precise with wider confidence intervals. For example, comparing treatment vs. placebo arms, the relative risk for fatigue, the most common AE, was 1.45 (95% CI, 1.16-1.80) in SPARTAN, 2.37 (95% CI, 1.86-3.04) in PROSPER, and 1.39 (95% CI, 1.01-1.91) in ARAMIS. Across all AE types, compared to SPARTAN, relative risks from PROSPER were 23% less precise and relative risks from ARAMIS were 30% less precise. Conclusions: While conducted in similar patient populations, these trials had remarkable differences in AE reporting and in absolute AE risks between placebo arms. Rather than indicating better safety, low absolute adverse event numbers decrease confidence in AE profiles. Published data are insufficient to differentiate the AE profiles of these agents in nmCRPC.