Mechanisms Involved in the Potentiation of Paclitaxel or 5-FU by the Hypoxic Cytotoxin NLCQ-1 (NSC 709257) In Vitro

Abstract

Background: 4-[3-(2-Nitro-1-imidazolyl)-propylamino]-7-chloroquinoline hydrochloride (NLCQ-1) is a novel, weakly DNA-intercalating hypoxia selective cytotoxin, which significantly potentiates the antitumor effect, but not the systemic toxicity, of commonly used chemotherapeutic agents. In the present study, we investigated if the same schedule-dependent synergism seen in vivo exists in vitro as well, between NLCQ-1 and paclitaxel or 5-fluorouracil (5FU), and some of the mechanisms involved in such interactions. Materials and Methods: V79 cells, while in monolayers, were exposed to paclitaxel or 5FU under aerobic conditions at various time-intervals before or after their hypoxic exposure to NLCQ-1, while in suspension. Survival was assessed by the clonogenic assay and synergistic interactions were evaluated by using the fractional product analysis. Cells treated as above were examined for apoptosis, DNA damage and repair, as well as DNA, RNA and protein syntheses inhibition. Results: A schedule-dependent synergistic interaction existed between NLCQ-1 and paclitaxel/5FU, similar to that seen in transplanted EMT6 murine tumors. Optimal potentiation was observed when NLCQ-1 was administered 2 or 4 h after the antimitotic paclitaxel or the thymidylate synthase inhibitor 5FU. Apoptosis induced by paclitaxel or 5FU alone was enhanced by NLCQ-1 up to 2.5- and 1.7-fold, respectively. DNA damage was detected as single-strand breaks (ssbs) in the combination treatments and was unrepairable at least up to 24 h post treatment. DNA, RNA and protein syntheses were inhibited by paclitaxel/5FU alone ca. 50% immediately post treatment and this inhibition was persistent up to 24 h post treatment. In combination with NLCQ-1, a slight synergistic and persistent inhibition was observed in all three syntheses. Conclusion: Enhancement in apoptosis, unrepairable DNA damage and inhibition of DNA, RNA and protein syntheses are some of the mechanisms involved in the potentiation of paclitaxel or 5FU by NLCQ-1.