Abstract 2239: Buthionine sulfoximine enhanced melphalan cytotoxic activity against multiple myeloma cell lines

Abstract

Abstract Multiple myeloma (MM) is a neoplasm of terminally differentiated antibody secreting B cells commonly treated with myeloablative doses of the alkylating agent melphalan (L-PAM) and supported by stem cell transplantation (SCT). Relapse after SCT is common and a second SCT with L-PAM for patients achieving less than a complete response or at time of relapse is usually only modestly effective. Glutathione (GSH) is ubiquitous intracellular tripeptide that acts as antioxidant and protects cells against reactive oxygen species (ROS) and xenobiotics. Conjugation of GSH to alkylating agents is known to be one mechanism of melphalan resistance. Buthionine sulfoximine (BSO) inhibits synthesis of GSH and enhances the activity of melphalan in preclinical and clinical studies of neuroblastoma. The potent hematopoietic toxicity of BSO + L-PAM led us to conduct an ongoing phase I trial in recurrent neuroblastoma of escalating L-PAM + BSO given with SCT. To explore the potential for a similar trial in MM, we have undertaken preclinical studies of BSO + L-PAM in MM. We determined if buthionine sulfoximine (BSO) enhanced the activity of melphalan (L-PAM) against MM cell lines. In-vitro cytotoxicity mediated by BSO (0-400 micro molar) alone and in combination with L-PAM (0-50 micro molar) was assessed in a panel of six MM cell lines in both “standard cell culture conditions” (20% O2) and “bone marrow level hypoxia” (5% O2) using the DIMSCAN digital image fluorescence assay. Single-strand DNA (ssDNA) damage by F7-26 antibody, apoptosis by TdT labeling (TUNEL) and mitochondrial depolarization by JC-1 were measured by flow cytometry. At clinically achievable concentrations, BSO consistently caused two to four logs synergistic enhancement of L-PAM cytotoxicity (combination index < 1) in all six multiple myeloma cell lines at both 5% and 20% O2. BSO + L-PAM caused a significant increase in single strand DNA damage (p<0.001), mitochondrial depolarization (p<0.001), and apoptosis (p<0.001) compared with L-PAM alone. Thus BSO can enhance L-PAM activity and overcome L-PAM resistance in multiple myeloma cell lines and BSO/L-PAM synergy is also achieved in bone marrow level hypoxia. Obtaining additional data on this novel combination approach to therapy of multiple myeloma is warranted. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2239. doi:10.1158/1538-7445.AM2011-2239