Background: Niraparib maintenance therapy significantly prolonged progression-free survival (PFS) versus placebo in patients with newly diagnosed advanced ovarian cancer (aOC), but its antitumor activity remains unclear in those with measurable residual disease (MRD) after first-line platinum-based chemotherapy (1LCT). This study aims to report the efficacy, including antitumor activity, and safety of niraparib maintenance therapy in patients with MRD after 1LCT from the phase 3 PRIME trial (NCT03709316). Methods: In PRIME, adults with newly diagnosed aOC who had received cytoreductive surgery and responded to 1LCT were randomized 2:1 to receive niraparib or placebo with stratification by receipt of neoadjuvant chemotherapy, response to 1LCT, status of germline BRCA mutations, and tumor homologous recombination deficiency status. Tumor assessment was conducted at baseline and every 12 weeks thereafter by blinded independent central review (BICR) according to RECIST, version 1.1. Between 29 June 2018 and 11 November 2019, 384 patients were randomized (255 niraparib, 129 placebo). The data cut-off date was 30 September 2021. This post-hoc analysis reports BICR-assessed objective response rate (ORR) and PFS in patients with MRD at baseline. An initial response was confirmed ≥4 weeks later. Results: In total, 73 (19.0%) patients (47 niraparib, 26 placebo) had MRD at baseline. Baseline characteristics were well balanced between the two MRD groups. Complete and partial responses, both confirmed, were observed in 12 (25.5%) and 15 (31.9%) niraparib-treated patients and 3 (11.5%) and 5 (19.2%) placebo-treated patients, respectively, leading to a confirmed ORR of 57.4% with niraparib and 30.8% with placebo (odds ratio, 3.20; 95% confidence interval [CI], 1.11–9.11). ORRs by biomarker status are provided in the table. Median PFS (95% CI) was 22.3 (8.7–not estimable) months with niraparib versus 8.3 (5.6–11.0) months with placebo (hazard ratio, 0.36; 95% CI, 0.19–0.71). Treatment-emergent adverse events (TEAEs) of grade ≥3 occurred in 28 (59.6%) niraparib-treated patients and 7 (26.9%) placebo-treated patients. TEAEs led to treatment discontinuation in three (6.4%) niraparib-treated patients and one (3.8%) placebo-treated patient. Conclusions: In patients with newly diagnosed aOC who had MRD after 1LCT, niraparib maintenance therapy tended to induce additional antitumor activity and led to a clinically meaningful increase in PFS versus placebo. Clinical trial information: NCT03709316.

CR, complete response; ORR, objective response rate; PR, partial response.