DNA repair mechanisms are involved in the hypoxia-dependent toxicity of NLCQ-1 (NSC 709257) and its synergistic interaction with alkylating agents
Papadopoulou, M.V.; Bloomer, W.D.
In Vivo 21(2): 175-180
2007
ISSN/ISBN: 0258-851X PMID: 17436565 Document Number: 613856
Background: The hypoxia selective cytotoxin NLCQ-1 significantly potentiates the antitumor effect of several alkylating agents in vitro and in vivo. Synergy in mice requires administration of NLCQ-1 ca. 1 h before the alkylating agent, a fact that may be related to an in vitro hypoxic pretreatment effect. Since NLCQ-1 targets DNA through weak intercalation, the induction of DNA lesions upon reductive metabolism may be a reasonable mechanism that predisposes cells to further damage by a subsequent alkylating agent. Materials and Methods: To indirectly identify such lesions, cell lines defective in specific DNA repair genes (EM9 and UV41) and their repair-proficient parental AA8, were exposed to NLCQ-1 +/- L-PAM/cisDDP under hypoxic/aerobic conditions and appropriate administration routes, and assessed for clonogenicity. Selected comparisons with tirapazamine (TPZ) were also performed. Results: EM9 cells, which lack the functional XRCC1 gene involved in base excision repair, and thus are unable to efficiently repair DNA single-strand breaks (ssbs), were 3.7-fold and 4.5-fold more sensitive to NLCQ-1 and TPZ, respectively, than the parental AA8 cells, under hypoxic conditions. UV41 cells, which are defective in repairing DNA interstrand cross-links (ERCC4/XPF), were 4.5-fold more sensitive than AA8 cells to NLCQ-1. In potentiation studies with melphalan or cisplatin, synergy was observed in AA8 cells but not in EM9 or UV41 cells, with either NLCQ-1 or TPZ. Conclusion: These results suggest that NLCQ-1 is involved in the formation of DNA ssbs and interstrand crosslinks under hypoxic conditions. The synergistic interaction of NLCQ-1 with L-PAM or cisDDP is probably due to an enhancement in the LPAM/cisDDP-induced DNA interstrand cross-links, possibly as a result of an inhibited repair mechanism of these lesions.