Novel 6,5-fused ring heterocyclic antifolates: biochemical and biological characterization

Six novel antifolates with 2,4-diaminopyrimidine-fused five-membered rings containing either pyrrole or cyclopentene rings were characterized at the cellular and biochemical level. Five of these antifolates were more growth inhibitory to the CCRF-CEM human leukemia cell line than methotrexate [MTX; drug concentration effective at inhibiting cell growth by 50% relative to untreated control (EC50), 12 nM], the antifolate used in the clinic, and two were more potent than 10-ethyl-10-deazaaminopterin (EC50, 2.7 nM); similar patterns of response were obtained in the FaDu and A253 squamous carcinoma cell lines. In addition, the growth inhibitory potency of these antifolates was generally less dependent on exposure time than was MTX. Growth inhibitory effects could be reversed by leucovorin, indicating an antifolate mechanism. These antifolates targeted dihydrofolate reductase (DHFR) based on direct human DHFR inhibition assays [drug concentration inhibiting enzyme activity by 50% (IC50), 0.6-28 nM; MTX IC50, 0.8 nM] and the cross-resistance of MTX-resistant CCRF-CEM cells containing elevated DHFR. Inhibition of human thymidylate synthase was generally weak. These 6,5-fused ring heterocyclic antifolates utilized the reduced folate/MTX transporter for uptake, based on the cross-resistance of MTX uptake-impaired CCRF-CEM cells, and were efficient substrates for this uptake system, based on inhibition of [3H]MTX uptake (IC50, 0.3-5.8 microM; aminopterin IC50, 2.6 microM). These analogues were substrates for CCRF-CEM folylpolyglutamate synthetase, with several being among the most active substrates now known (highest Vrel/Km 0.73; MTX and 10-ethyl-10-deazaaminopterin, 0.013 and 0.24, respectively). Substrate activity for murine intestinal folylpolyglutamate synthetase was also assayed, and a different specificity pattern was observed. These new antifolates are apparently not substrates for aldehyde oxidase. Analogues containing the fused cyclopentene ring are preferred to those containing the fused pyrrole ring based on growth inhibitory potency, effectiveness against decreased uptake mutants and apparent affinity for transport, and inhibition of DHFR. In addition, fused cyclopentene-containing analogues are efficiently polyglutamylated. The data indicate that antifolates with 2,4-diaminopyrimidine-fused five-membered rings, especially those containing the fused cyclopentene ring, are an important new class of antifolates which warrant further exploration at the synthetic and preclinical levels.