BackgroundMAGNITUDE (NCT03748641) demonstrated favourable outcomes with niraparib plus abiraterone acetate plus prednisone (+AAP) versus placebo+AAP in patients with BRCA1/2-altered metastatic castration-resistant prostate cancer (mCRPC). Imbalances in prognostic variables were reported between arms, which impacts estimation of both the clinical benefit and cost‑effectiveness of niraparib+AAP for healthcare systems. A pre-specified multivariable analysis (MVA) demonstrated improved overall survival (OS) with niraparib+AAP. Here, we used an inverse probability of treatment weighting (IPTW) model to adjust for covariate imbalances and assess time-to-event outcomes. MethodsIPTW analysis of time-to-event outcomes was conducted using data from patients with BRCA1/2-altered mCRPC (N = 225) in MAGNITUDE. Patients received niraparib+AAP or placebo+AAP. OS, radiographic progression-free survival, time to symptomatic progression, time to initiation of cytotoxic chemotherapy and time to prostate-specific antigen progression were assessed. Weighted Kaplan-Meier curves were generated for each endpoint, and adjusted hazard ratios (HR) were obtained from a weighted Cox model. ResultsImprovements in survival outcomes were estimated for niraparib+AAP versus placebo+AAP: unadjusted median OS was 30.4 months versus 28.6 months, respectively (HR: 0.79; 95 % confidence interval [CI]: 0.55, 1.12; p = 0.183). Following IPTW, median OS increased to 34.1 months with niraparib+AAP versus a decrease to 27.4 with placebo (HR: 0.65; 95 % CI: 0.46, 0.93; p = 0.017). Similar improvements were observed for other time-to-event endpoints. ConclusionsIPTW adjustment provided a more precise estimate of the clinical benefit of niraparib+AAP versus placebo+AAP in patients with BRCA1/2-altered mCRPC. Results were consistent with the pre-specified MVA, and further demonstrated the value of adjusting for baseline imbalances, particularly in smaller studies. Trial registrationNCT03748641 (MAGNITUDE)