Background: Development of resistance to standard treatments for RRMM highlights the need for novel therapies. Melphalan flufenamide (melflufen) is a first-in-class peptide-drug conjugate (PDC) that leverages aminopeptidases and rapidly releases alkylating agents inside tumor cells. Melflufen + dex showed clinical activity and an acceptable safety profile in HORIZON (Richardson et al. J Clin Oncol. 2020 Dec 9 [Epub]). This is an update of the BTZ arm of the phase 1/2a ANCHOR study (NCT03481556). Methods: Patients (pts) with RRMM were intolerant or refractory to a prior IMiD, with 1-4 prior lines of therapy (LoTs). Prior treatment with a proteasome inhibitor (PI) was allowed, but pts could not be refractory to PIs in the last LoT. Melflufen (30, 40, or 20 mg intravenously; d 1 of each 28-d cycle) was administered with BTZ (1.3 mg/m² subcutaneous) + oral dex (20 mg on d 1, 4, 8, and 11 and 40 mg on d 15 and 22; dex dose reduced if aged ≥ 75 y). The primary objective in phase 1 was to determine the optimal phase 2 dose of melflufen for this combination. Results: As of the data cutoff date (October 19, 2020), 13 pts received melflufen (30 mg, n = 6; 40 mg, n = 7) + dex and BTZ. In the 30 mg and 40 mg cohorts, respectively, median age was 78.5 y (range, 70-82) and 70.0 y (range, 61-76); median prior LoTs was 3.5 (range, 2-4) and 2.0 (range, 1-4); 33% and 50% of evaluable pts had high-risk cytogenetics; 83% and 71% were refractory to last LoT; 100% and 86% received a prior PI; 33% and 14% were refractory to PIs. In the 30 mg and 40 mg cohorts, respectively, median treatment duration was 6.5 mo (range, 1.4-29.0) and 8.7 mo (range, 2.1-19.6); 4 (67%) and 4 pts (57%) were still on treatment; 2 and 3 pts discontinued (30 mg: progressive disease [PD] and other [1 pt each]; 40 mg: adverse event [AE], lack of efficacy, and PD [1 pt each]). Confirmed overall response rate in the 30 mg and 40 mg cohorts, respectively, was 50% (1 very good partial response [VGPR] and 2 partial response [PR]) and 71% (1 complete response, 3 VGPR, and 1 PR). Most common grade 3/4 treatment-related AEs (TRAEs) were thrombocytopenia (30 mg: 50%; 40 mg: 100%) and neutropenia (30 mg: 33%; 40 mg: 71%); grade 3/4 nonhematologic TRAEs were infrequent; 3 pts discontinued study treatment due to treatment-emergent AEs (30 mg: cardiac failure chronic and osteolysis [1 pt each]; 40 mg: thrombocytopenia [1 pt]). Serious TRAEs occurred in 2 pts (33%) in the 30 mg cohort (neutropenia and pneumonia [1 pt], syncope [1 pt]) and 1 pt (14%) in the 40 mg cohort (thrombocytopenia and neutropenia). No dose-limiting toxicities occurred at either dose level. Fatal AEs occurred in 1 pt in the 30 mg cohort (cardiac failure chronic; unrelated to study treatment). Conclusions: ANCHOR determined that the optimal dose of melflufen is 30 mg + dex and BTZ; results showed clinical activity in heavily pretreated pts. Recruitment is ongoing; updated data will be presented. Clinical trial information: NCT03481556