Melflufen plus dexamethasone (dex) in patients (pts) with relapsed/refractory multiple myeloma (rrmm) exposed/refractory to prior alkylators - a pooled analysis of the o-12-m1 and horizon studies

8048 Background: Melphalan flufenamide (melflufen) is a first-in-class peptide-drug conjugate (PDC) that leverages aminopeptidases and rapidly releases alkylating agents inside tumor cells. Melflufen has a mechanism of action distinct from other alkylating agents (Slipicevic et al. AACR 2020. Abs. 1843). In the O-12-M1 (NCT01897714) and HORIZON (OP-106; NCT02963493) studies, melflufen plus dex showed meaningful efficacy and a clinically manageable safety profile in pts with RRMM (Richardson et al. Lancet Haematol. 2020;7:5; Richardson et al. J Clin Oncol. 2020;Dec 9 [Epub]). This pooled analysis examines pts from these studies exposed to prior alkylators. Methods: Both the O-12-M1 and HORIZON studies included pts with RRMM who received ≥ 2 prior lines of therapy (LoTs) and had a primary endpoint of overall response rate (ORR). Secondary endpoints included progression-free survival (PFS) and safety. Data from the 2 studies were pooled and analyzed according to previous exposure and refractoriness to alkylators before study entry. Refractoriness to prior alkylator therapy was defined as disease that failed to achieve a minimal response or progressed while on therapy, or within 60 d of last therapy. Results: Of 202 pts (HORIZON: n = 157, cutoff January 14, 2020; O-12-M1: n = 45, cutoff October 29, 2019), 178 (88%) had been exposed to alkylators in ≥ 1 prior LoT (see Table for subgroups). Pts exposed and refractory to alkylators in ≥ 2 LoTs had the highest number of pts refractory to an alkylator in the last LoT (61%), and 82% were refractory to an alkylator within 12 mo of study entry. Meaningful response rates were seen in all subgroups, except for pts who were exposed and refractory to alkylators in ≥ 2 prior LoTs (see Table). PFS trended toward being shorter with higher exposure and refractoriness to prior alkylators. Results should be interpreted with caution due to limited pt numbers. Grade 3/4 adverse events (AEs) were similar between pts exposed to prior alkylators (O-12-M1: 85%; HORIZON: 89%) and the overall population (O-12-M1: 84%; HORIZON: 89%). The most common AEs were hematologic, but were mostly reversible and clinically manageable. Nonhematologic AEs were infrequent and primarily grade 1/2. Conclusions: Melflufen in combination with dex showed meaningful efficacy and a clinically manageable safety profile in pts with RRMM exposed/refractory to prior alkylators. Clinical trial information: NCT02963493 and NCT01897714. [Table: see text]