In Vitro and In Vivo Antitumor Activity of a Novel Alkylating Agent, Melphalan-Flufenamide, against Multiple Myeloma Cells

Purpose: The alkylating agent melphalan prolongs survival in patients with multiple myeloma; however, it is associated with toxicities and development of drug-resistance. Here, we evaluated the efficacy of melphalan-flufenamide (mel-flufen), a novel dipeptide prodrug of melphalan in multiple myeloma. Experimental Design: Multiple myeloma cell lines, primary patient cells, and the human multiple myeloma xenograft animal model were used to study the antitumor activity of mel-flufen. Results: Low doses of mel-flufen trigger more rapid and higher intracellular concentrations of melphalan in multiple myeloma cells than are achievable by free melphalan. Cytotoxicity analysis showed significantly lower IC50 of mel-flufen than melphalan in multiple myeloma cells. Importantly, mel-flufen induces apoptosis even in melphalan-and bortezomib-resistant multiple myeloma cells. Mechanistic studies show that siRNA knockdown of aminopeptidase N, a key enzyme mediating intracellular conversion of mel-flufen to melphalan, attenuates anti-multiple myeloma activity of mel-flufen. Furthermore, mel-flufen-induced apoptosis was associated with: (i) activation of caspases and PARP cleavage; (ii) reactive oxygen species generation; (iii) mitochondrial dysfunction and release of cytochrome c; and (iv) induction of DNA damage. Moreover, mel-flufen inhibits multiple myeloma cell migration and tumor-associated angiogenesis. Human multiple myeloma xenograft studies showed amore potent inhibition of tumor growth in mice treated withmel-flufen than mice receiving equimolar doses of melphalan. Finally, combining mel-flufen with lenalidomide, bortezomib, or dexamethasone triggers synergistic anti-multiple myeloma activity. Conclusion: Our preclinical study supports clinical evaluation of mel-flufen to enhance therapeutic potential of melphalan, overcome drug-resistance, and improve multiple myeloma patient outcome. (C) 2013 AACR.