Abstract A01: Distinct mechanism of action of DNA-damaging agent dianhydrogalactitol (VAL-083) suggests combination therapy with PARP inhibitors
Authors: Bacha Jeffrey A. , He Guangan , Xie Xiaolei , Steino Anne , Brown Dennis M. , Siddik Zahid H. 
Published: August 2018

Abstract
DNA damage response (DDR) is a network of intracellular pathways designed to minimize the impact of DNA damage. Multiple DNA repair pathways are known, including mismatch repair (MMR), O6-methylguanine DNA methyltransferase (MGMT), non-homologous end joining (NHEJ), and homologous recombination (HR), which act either by repairing the damage, arresting cell growth or, if necessary, promoting cell death. DDR defects are a hallmark of cancer development, rendering the cancer cells highly sensitive to targeted DNA-damaging agents. Ovarian cancer is normally treated with platinum (Pt)-based chemotherapy, which forms predominantly intrastrand DNA crosslinks; however, most advanced ovarian cancer patients develop episodes of recurrent disease with progressively shorter disease-free intervals. Dysfunctional p53 diminishes the therapeutic cytotoxicity of many DNA-targeting drugs, including Pt-based agents, comprising a therapeutic challenge in high-grade serous ovarian cancer (HGSOC), where p53 is almost universally mutated (~96%). Attempts to overcome Pt-resistance in HGSOC include PARP inhibitors (PARPi), which target tumors with deficiencies in the HR pathway (50% of HGSOC), responsible for the repair of DNA double-strand breaks (DSBs). Unfortunately, PARPi resistance frequently arises, leading to a 5-year survival rate of only 40% in this cancer. VAL-083 is a first-in-class DNA-damaging agent that has demonstrated clinical activity against a range of tumor types, including ovarian cancer, in historical NCI-sponsored clinical studies. VAL-083 rapidly induces interstrand crosslinks at guanine-N7, causing DNA DSBs that, if left unrepaired, are lethal to the cancer cell. We have shown that VAL-083 treatment leads to DNA damage that activates HR and is independent of common DNA repair pathways, including MGMT, MMR, and NHEJ. Notably, VAL-083 induces persistent S/G2 phase cell cycle arrest through two parallel pathways--one p53-independent and one p53-dependent. Taken together, these data propose the potential for targeting HR-impaired tumors and for combination treatment with S-phase specific DNA-targeting agents, including PARPi. We have previously seen VAL-083 cytotoxicity against a panel of ovarian cancer cells with differing p53 status and superadditivity between VAL-083 and PARPi olaparib, in vitro. The purpose of this study was to expand on these studies by examining the cytotoxicity of VAL-083 against HR-impaired ovarian cancer cells, the impact of p53 status, and the potential for VAL-083 combination with PARPi as part of a treatment strategy for HR-impaired, p53 dysfunctional HGSOC. VAL-083 cytotoxicity was investigated using the 5-day MTT assay in A2780 ovarian cancer cells. HR-impaired A2780 were examined using BRCA1 siRNA oligos prior to VAL-083 exposure. Combinations with PARPi veliparib (catalytic) and talazoparib (trapping) were examined in HR-proficient and HR–impaired A2780. The impact of p53 status was investigated by CRISPR/cas 9 knockout of p53 in the wildtype (wt) A2780 cells and in p53-mutated OVCAR3 cancer cells. We report increased VAL-083 cytotoxicity against HR-impaired A2780 cancer cells, further supporting HR as the main repair pathway for VAL-083-induced DNA damage, and suggesting HR-impaired HGSOC as a target for VAL-083 treatment. We further report superadditivity between VAL-083 and PARPi veliparib and talazoparib. Significantly, VAL-083 cytotoxicity against wt and knockout p53 A2780 cells and against p53-mutated OVCAR3 cells differed only by a factor of ~2, supporting a p53-independent mechanism of action for VAL-083. In conclusion, our results demonstrate a distinct DNA-damaging mechanism for VAL-083, resulting in the ability to target HR-impaired tumors and overcome MGMT- , MMR- , and NHEJ-related chemoresistance. VAL-083 activity was independent of p53 status, and superadditivity with PARPi was identified in HR-proficient A2780. Studies in HR-impaired A2780 are ongoing and will also be presented.
Citation Format: Jeffrey A. Bacha, Guangan He, Xiaolei Xie, Anne Steino, Dennis M. Brown, Zahid H. Siddik. Distinct mechanism of action of DNA-damaging agent dianhydrogalactitol (VAL-083) suggests combination therapy with PARP inhibitors. [abstract]. In: Proceedings of the AACR Conference: Addressing Critical Questions in Ovarian Cancer Research and Treatment; Oct 1-4, 2017; Pittsburgh, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(15_Suppl):Abstract nr A01.