Abstract 1843: Melflufen efficacy in multiple myeloma with TP53 aberrations

Introduction Multiple myeloma (MM) is an incurable plasma cell malignancy characterized by clonal evolution and heterogeneous genetic abnormalities. Deletion of the short arm of chromosome 17 (del17p) harboring TP53 is a high-risk abnormality associated with aggressive disease and therapy resistance. Here, we tested in vitro and ex vivo efficacy of a peptide-drug conjugate melflufen, currently in phase 3 clinical trials for relapsed/refractory multiple myeloma (RRMM), in an isogenic myeloma p53-abrogated cell line model and patient samples. Methods Efficacy of melflufen versus melphalan was tested in the AMO-1 cell line, which displays biallelic wildtype TP53 and retains aspects of a functional p53 system, and AMO-1 clones with either complete loss of p53 or expressing point-mutated p53 protein (R282W hotspot mutation), created using the CRISPR/Cas9 system and modified via Sleeping Beauty vectors. We assessed toxicity and apoptosis using Annexin V and Alamar blue assays. Ex vivo sensitivity to the drugs was examined by flow cytometry in CD138+ plasma cells isolated from MM patients with confirmed del17p or TP53 mutations. We also analyzed response rates in a subpopulation of patients with confirmed del17p from OP-106 HORIZON, an ongoing phase 2 study evaluating the efficacy of melflufen plus dexamethasone in patients with RRMM (NCT02963493). Results Whereas loss of p53 functionality strongly impaired melphalan induced cytotoxicity in the AMO-1 MM cell line model system, treatment with melflufen showed superior efficacy in the parental cells, and was effective independent of the respective TP53 status. Although at low dosages wild-type cells were still more sensitive, in contrast to melphalan this effect was overcome by slight dose increases of melflufen, due to the much steeper dose-effect relationships in the p53 deficient sublines. In contrast to melphalan, the melflufen effect was only partially blocked by combined pre-treatment with inhibitors of apoptosis and necroptosis. Similar results were observed in patient-derived material with melflufen displaying superior activity towards TP53 mutated plasma cells. Analysis of OP-106 HORIZON trial data showed comparable ORR of 20% (CI 95% 4.3-48.1) in the del17p subpopulation (n=15) to 26.2% (CI 95% 18.8-34.6) in the total population (n=136). Conclusions Melflufen is able to trigger myeloma cell death regardless of p53 status, and thus overcome p53-deficiency-mediated melphalan resistance. It demonstrates better ex vivo efficacy in MM patient derived plasma cells harboring del17p or TP53 mutations. Melflufen response rates in the del17p Citation Format: Ana Slipicevic, Umair Munawar, Thorsten Stuhmer, Johan Aschan, Fredrik Lehmann, Nina N. Nupponen, Juho Miettinen, Maiju-Emilia Huppunen, Caroline A. Heckman, Maria-Victoria Mateos, Paula Rodriguez Otero, Paul Richardson, Ralf Bargou. Melflufen efficacy in multiple myeloma with TP53 aberrations [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1843.