Abstract 5667: Mechanism-based combinations of agents impacting the homologous recombination and nucleotide excision repair pathways

Sapacitabine, a prodrug of the nucleoside analog CNDAC [2′-C-cyano-2′-deoxy-1-β-D-arabino-pentofuranosylcytosine], has the advantage of oral bioavailability. CNDAC has a unique action mechanism of inducing single strand breaks following its incorporation into DNA through a β-elimination process. Such nicks caused by sapacitabine/CNDAC are partially repaired through the transcription-coupled nucleotide excision repair (NER) pathway [Cancer Res. 2008, 68:3881-89]. However, DNA replication across the unrepaired nicks generates double strand breaks which are mainly repaired by the ATM-dependent homologous recombination (HR) mechanism [Blood, 2010, 116:1737-46]. Sapacitabine is undergoing clinical trials with encouraging outcomes in AML or MDS patients [J. Clin.Oncol., 2010, 28:285-91]. A trial of sapacitabine in combination with cyclophosphamide and rituximab (SCR regimen) in chronic lymphocytic leukemia (CLL) relapsed from first line therapies has been initiated in populations lacking ATM function. Other candidates for clinical combinations with sapacitabine include bendamustine, an alkylating agent, and oxaliplatin, which are currently used for treatment of CLL. Primary adducts caused by each of these compounds are repaired by NER. To explore mechanistic interactions between sapacitabine/CNDAC and agents impacting the NER pathway, we used median effect analysis to compare cellular clonogenic survival of drug combinations with survival after single agents alone. First, cells deficient in HR components (Rad51D, Xrcc3 and Brca2) were all sensitized to 4-hydroperoxycyclophosphamide (4-HC, readily converted to the active metabolite of cyclophosphamide, 4-hydroxycyclophosphamide), bendamustine, ciplatin and oxaliplatin by 10- to 50-fold, whereas cells lacking the key NER endonuclease XPF were more sensitive to these agents. This indicates that HR is responsible for residual lesions not repaired by NER. Second, CNDAC in combination with 4-HC presented additive to mildly synergistic effect (combination index CI ≤ 1) in cells defective in Xrcc3, Rad51D, Brca2 or XPF. Third, combination of bendamustine with CNDAC showed additive loss of clonogenicity (CI ∼ 1) in cells lacking Rad51D or Xrcc3. Fourth, combination of cisplatin or oxaliplatin with CNDAC also exerted additive effects (CI ∼ 1) in HR- as well as NER-deficient cells. Finally, cisplatin/oxaliplatin combined with CNDAC, both at relatively high concentration ranges lethal to HR-defective cells, had no synergy in wild type cells either. Together these results suggest that agents causing DNA adducts, which are mainly repaired by NER process, are additive in their interaction with sapacitabine/CNDAC. Thus, this study provides rationales for sapacitabine-based combinational and targeted therapeutic strategies for translation into the design of clinical trials. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5667. doi:1538-7445.AM2012-5667