Efficacy of Melflufen in Patients with Relapsed/Refractory Multiple Myeloma and Mutated or Deleted TP53

Clinical challenges remain for patients with relapsed/refractory multiple myeloma (MM), especially for high-risk patients with del(17p) and TP53 mutation who have poor response and significantly shorter survival compared to patients without these aberrations. Here, we present the mechanism of action of melphalan flufenamide (melflufen) in TP53 wild type and double mutant (TP53-/-) isogenic myeloma cells in relation to DNA damage, apoptosis kinetics, and mitochondrial function. Melflufen demonstrated superior in vitro efficacy in comparison to melphalan or cyclophosphamide. Differential gene expression and DNA damage pathway analysis upon treatment of TP53 wild type and TP53-/- MM cell lines revealed qualitative differences between melflufen and melphalan. In addition, we investigated response of primary bone marrow (BM) patient samples with del(17p)/TP53 mutation to melflufen. In accordance with the cell line data, ex vivo sensitivity to melflufen was independent of BM TP53 mutation status. Gene set enrichment analysis of single cell transcriptome data from MM cells highly or less sensitive to melflufen revealed differences related to p53 and DNA damage repair pathways. These findings provide molecular understanding of results from post-hoc analysis of the del(17p) patient population from the OCEAN randomized, open label, head-to-head, phase III clinical trial. From this analysis, we observed favorable progression free survival in the del(17p) subgroup treated with melflufen plus dexamethasone compared to the pomalidomide plus dexamethasone arm. Our insights into the molecular mechanisms of melflufen activity in mutant TP53 MM support its clinical efficacy and application in the del(17p) and mutant TP53 patient population. Overall design: Myeloma plasma cells from viably frozen myeloma patient bone marrow mononuclear cells were isolated using Fluoresence-activated cell sorting (FACS) according to the presence or absence of CD138 cell surface marker. scRNAseq analyis was performed to CD138 positive and CD138 negative cells that were mixed in 1:1 ratio.