Hematopoietic stem cell transplantation (HSCT) is effective for hematological disorders. Its success partly depends on the efficacy of the conditioning regimen. To identify efficacious regimens, we exposed cells to various drug combinations, analyzed their cytotoxicity and identified their molecular mechanisms of interaction. We have shown the synergistic cytotoxicity of DNA alkylating agents (AA) and nucleoside analogs (NA) and proposed a mechanistic model -the “loop of death”. NA initiates DNA damage resulting in chromatin remodeling and makes genomic DNA more susceptible to DNA alkylation. DNA damage response (DDR) is activated and the loop of DNA damage, chromatin remodeling, and DNA alkylation continues until the tumor cells commit to apoptosis. Using this model, we hypothesized that epigenetic modifiers would amplify the loop of death. Indeed, inhibitors of histone deacetylases (HDACi) and DNA methyl transferases (DNMTi), which facilitate relaxation of chromatin, were found to be synergistic with AA+NA. Since active DNA repair may contribute to decreased efficacy of these drug combinations, we also examined the inclusion of DNA repair inhibitor olaparib. Addition of olaparib to a [AA+NA] combination caused significant apoptosis by activation of the DDR, inhibition of PARP activity and DNA repair, production of reactive oxygen species and depolarization of the mitochondrial membranes. Most of our pre-clinical studies have been translated to the clinic and results from some of our clinical trials will be presented. Overall, our pre-clinical and clinical results suggest that the conditioning regimen for HSCT may be optimized by combining drugs that provide synergistic cytotoxicity based on their molecular mechanisms of action.

Ben Valdez obtained his PhD in Biochem at Louisiana State Univ. He did his post-doctoral training at Baylor College of Medicine and became an Assistant Professor. His laboratory cloned the genes and corresponding cDNAs for RNA helicase II/Gu α and β and discovered their functions. His lab identified the functions of treacle, encoded by the TCOF1 gene, in the expression and methylation of pre-ribosomal RNA. He moved to MD Anderson Cancer Center in 2005 where he is now an Associate Professor. His current research focuses on the identification of safe and efficacious conditioning regimen for HSCT for patients with blood disorders. He conceptualized and proved the efficacy of combined DNA alkylators, nucleoside analogs, and epigenetic modifiers in leukemia, lymphoma and multiple myeloma, and proposed a model called the “loop of death” to explain their cytotoxic synergism. He developed an assay for cellular efflux of chemotherapeutic drugs which is relevant to understanding drug interactions. The results of his pre-clinical studies have been used as bases for several clinical trials at MD Anderson Cancer Center.