Abstract 962: Mechanism-based drug combinations with the DNA-strand-breaking nucleoside analog CNDAC
Authors: Liu Xiaojun , Nowak Billie , Matsuda Akira , Plunkett William 
Published: June 2012

Abstract
Sapacitabine, an orally bioavailable prodrug of CNDAC (2’-C-cyano-2’-deoxy-1-β-D-arabino-pentofuranosylcytosine), is undergoing clinical trials with encouraging outcomes [J. Clin.Oncol., 2010, 28(2): 285-91]. CNDAC has a unique action mechanism; following its incorporation into DNA, it induces single strand breaks through a β-elimination process, which also generates the chain-terminating analog CNddC. Subsequent processing or DNA replication across the unrepaired nicks generates double strand breaks which are primarily repaired through the ATM-dependent homologous recombination (HR) pathway [Blood, 2010, 116(10): 1737-46]. We hypothesized that agents directly affecting or interacting with the HR pathway may be synergistic with CNDAC, which will provide a rationale for clinical combinations. This study used clonogenic survival assays to explore mechanistic interactions between CNDAC and other chemotherapeutic agents. First, inhibition of ATM kinase by a specific inhibitor, KU55933, greatly sensitized the human myeloid cell line OCI-AML3, as well as primary cells from acute myeloid leukemia patients, to CNDAC. Second, colon carcinoma HCT116 cells pre-treated with the c-Abl kinase inhibitor, imatinib, were sensitized 4-5 fold to CNDAC compared to those exposed to CNDAC alone. This is likely due to inhibition of Rad51 recombinase, the key component of HR, as a consequence to suppression of c-Abl. In contrast, no sensitization resulted from inhibition of DNA-PK (NU7441) or Chk1 (UCN-01). Third, co-incubation of the poly(ADP-ribose) polymerase 1 inhibitor, AZD2281, and CNDAC exhibited greater than additive cell killing selectively in hamster cells deficient in Brca2, a breast cancer susceptibility protein associated with Rad51 function. The mild synergy may result from a synthetic lethal interaction between base excision repair and HR pathways. Fourth, temozolomide, an alkylating agent which causes base methylation that is repaired partially by base excision repair, had greater than additive effect with CNDAC specifically in hamster cells lacking the Rad51 paralogs Rad51D or Xrcc3, based on a median effect analysis (CI &lt; 1). This again suggests a mechanistic interaction selectively targeting HR-defective cells. Finally, combination of the alkylating agent bendamustine with CNDAC showed no synergy (CI &gt; = 1) in cells lacking Rad51D or Xrcc3, although HR-deficient cells were more sensitive to bendamustine alone compared to HR-proficient cells. This suggests that nucleotide excision process by which bendamustine-caused DNA adducts are repaired does not generate a synthetic lethal interaction with HR. Together these results indicate rationales for CNDAC-based combinational and targeted therapeutic strategies for translation into the design of clinical trials.
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 962. doi:10.1158/1538-7445.AM2011-962