Abstract 3867: DNA repair processes involved with the hypoxia-activated prodrug TH-302: comparison to cisplatin and temozolomide
Authors: Meng Fanying , Bhupathi Deepthi , Hart Charles P. 
Published: August 2015

Abstract
Therapeutic regimens which include platinum compounds (Pt) or temozolomide (Tmz) can be affected by the presence or acquisition of specific DNA repair processes conferring relative resistance to these agents. TH-302 is a hypoxia-activated prodrug that is reduced at its nitroimidazole group and selectively, under hypoxic conditions, releases Br-IPM, a DNA cross-linker. TH-302 is currently being investigated in multiple clinical trials for the treatment of cancer including indications and settings where resistance to Pt (2nd line non-small cell lung cancer; NCT01403610) or Tmz (recurrent glioblastoma; NCT02093962) is an important consideration. Here we investigate potential DNA damage response and repair pathways of the three agents. Two complementary approaches were employed: (1) paired isogenic cell lines in which specific DNA repair factors were deficient in one of the lines; and (2) the use of pharmacological inhibitors of specific DNA repair factors. The cytotoxicity of TH-302 and cisplatin (CisPt) was enhanced in the CHO cell line deficient in the HDR gene XRCC3 and the nucleotide excision repair (NER) factors that are also involved in HDR (ERCC1 and ERCC4). For CisPt, but not TH-302, enhanced cytotoxicity was also observed in cell lines deficient in factors specifically involved in NER (ERCC2 and ERCC5). An increase of cytotoxicity for both TH-302 and CisPt was also observed in cells deficient in the specific non-homologous end joining (NHEJ) factor Ku80. The cytotoxicity of TH-302 and CisPt was unchanged in cells deficient in the specific base excision repair (BER) factor XRCC1, whereas the cytotoxicity of Tmz was enhanced. Tmz cytotoxicity was not affected by deficiency of HDR or NER. The role of O6-methylguanine-DNA-methyltransferase (MGMT) repair was investigated using its specific inhibitor, O6-benzylguanine (OBG). OBG enhanced the cytotoxicity of Tmz in cell lines expressing MGMT but had no effect on TH-302 or CisPt cytotoxicity. Involvement of mismatch repair (MMR) was tested using a MSH6 deficient cell line. Neither TH-302 nor CisPt was affected by MMR deficiency, whereas Tmz activity was. Single-strand break (SSB) repair pathway mediated by poly (ADP-ribose) polymerase (PARP) was assessed using PARP inhibitors. Tmz cytotoxicity, but not TH-302 or CisPt, was enhanced by the addition of the PARP inhibitors. In conclusion, TH-302-induced lesions are repaired by HDR or NHEJ. Cisplatin-induced lesions are repaired by the same HDR and NHEJ pathways as TH-302 but can additionally be repaired by NER. Thus, cisplatin-induces a more complex spectrum of DNA lesions than TH-302. TMZ-induced lesions are repaired by BER, MGMT, MMR and PARP. In consequence, since MGMT-expressing gliomas are resistant to TMZ and since TH-302-induced lesions are repaired by different mechanisms, TH-302 or cisplatin may be effective in this subset of gliomas.
Citation Format: Fanying Meng, Deepthi Bhupathi, Charles P. Hart. DNA repair processes involved with the hypoxia-activated prodrug TH-302: comparison to cisplatin and temozolomide. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3867. doi:10.1158/1538-7445.AM2015-3867