Quinazoline antifolates as inhibitors of growth, dihydrofolate reductase, and thymidylate synthetase of mouse neuroblastoma cells in culture

Correlations between inhibition of growth and of enzyme activity by several quinazoline analogues of folic acid have been made with two lines of C1300 mouse neuroblastoma cells, one sensitive and one resistant to quinazoline analogues. The quinazoline analogues studied fell into two classes: 2,4 diamino and 2 amino 4 hydroxy derivatives. The 2,4 diamino quinazoline analogues DAQ (N [p [[(2,4 diamino 6 quinazolinyl)methyl]methylamino]benzoyl] L glutamic acid) and methasquin (N [p [(2,4 diamino 5 methylquinazolinyl)methylamino]benzoyl] L aspartic acid) effectively inhibited both cell growth and dihydrofolate reductase activity, whereas the 2 amino 4 hydroxyquinazoline analogue AHQ (N [p [[(2 amino 4 hydroxy 6 quinazolinyl)methyl]methylamino]benzoyl] L glutamic acid) was less potent. Nevertheless, AHQ was more inhibitory toward thymidylate synthetase than DAQ. With the 2,4 diaminoquinazoline analogues there was a good correlation between inhibition of growth and dihydrofolate reductase, but with AHQ growth inhibition seemed to depend upon the inhibition of both dihydrofolate reductase and thymidylate synthetase. Sensitive cells could be completely protected against the toxic effects of AHQ by either leukovorin or thymidine; the concentration of leukovorin (0.56 μM) necessary to provide 50% protection against a lethal dose of AHQ (30 μM) was 38 times less than the amount of thymidine (21 μM) required for 50% protection. Because of its unique potency toward thymidylate synthetase, AHQ may be useful in the chemotherapy of neuroblastoma.