Cellular pharmacology of chloroquinoxaline sulfonamide and a related compound in murine B16 melanoma cells

Chloroquinoxaline sulfonamide (CQS), a chlorinated derivative of sulfaquinoxaline (SQ), inhibited proliferation of murine B16 melanoma cells, but only when relatively high drug concentrations (1 mM) were used. The inhibition of cell growth by CQS was at least partially reversible by incubation in drug-free medium. Incubation of melanoma cells with CQS was associated with an arrest of the cell cycle in G0/G1 as measured by flow cytometry. The drug slightly decreased uptake of radiolabeled deoxyuridine and thymidine after 24- and 48-hr incubation periods but increased nucleoside incorporation at 72 hr. No evidence of intercalation with DNA was found. Because SQ previously was reported to inhibit an aspect of folate metabolism, we investigated the possibility that CQS limits tumor cell growth by altering folate homeostasis. This appears unlikely, however, in view of the following observations: (1) the cytotoxic effects of CQS could not be reversed by folinic acid; (2) deoxyuridine suppression of thymidine incorporation was not affected by CQS treatment; (3) CQS did not inhibit dihydrofolate reductase from mammalian or Bacterial sources; and (4) CQS toxicity in mice was not reduced by folinic acid. Experiments performed with analogues modified in the quinoxaline and para-amino phenyl functions indicated that tumor cell inhibition did not require preservation of the conventional sulfonamide structure.