The cytotoxicity of two novel folate cycle inhibitors with quinoxalinic structure,
3-methyl-7-trifluoromethyl-2(R)-[3,4,5-trimethoxyanilino]-quinoxaline (453R) and 3-piperazinilmethyl-2[4(oxymethyl)-phenoxy]quinoxaline
(311S), was tested against a panel of both cisplatin(cDDP)-sensitive and -resistant
carcinoma cell lines. Interestingly, the cisplatin-resistant human ovarian line, C13
cells, exhibited collateral sensitivity towards the two compounds when compared to
its sensitive parental 2008 cells. In this resistant line, which showed elevated expression
of the folate cycle enzymes, thymidylate synthase (TS) and dihydrofolate reductase
(DHFR), due to cisplatin-resistance phenotype, collateral sensitivity correlated with
the greater reduction of enzyme expression. In addition, TS and DHFR expression of
the other resistant lines, the human ovarian carcinoma A2780/CP cells and the human
breast cancer MDA/CH cells, were decreased in accordance with the similar sensitivity
or the low level of cross-resistance to these compounds in comparison to their respective
parental lines. Noteworthy, unlike 5-fluorouracil, both drugs reduced the level of
TS without inducing ternary complex formation with the co-substrate and the nucleotide
analogue. Median effect analysis of the interactive effects of cisplatin with the
two quinoxalines mainly showed additive or synergistic cell killing, depending on
schedules of drug combinations. In particular, synergistic effects were more often
obtained, even on the resistant cells, when cisplatin was added at the beginning of
the treatment. These results indicate that, despite the possibility of other mechanisms
being involved, inhibition of TS cycle enzymes plays an important role in the pharmacology
of these compounds, which might also represent a useful component in drug treatment
protocols against cDDP-resistant cells.