Variation of the bridge linking the heterocyclic ring and p-aminobenzoyl-L-glutamate portions of our previously described classical 2,4-diaminofuro[2,3-d]pyrimidines 1 and 2 are reported as inhibitors of dihydrofolate reductase (DHFR) and thymidylate synthase (TS) and as antitumor agents. Specifically -CH2CH2- and -CH2NHCH2- bridged analogues, N-[4-[2-(2,4-diaminofuro[2,3-d]pyrimidin-5-yl) ethyl]benzoyl]-L-glutamic acid (3) and N-[4-[[N-[(2,4-diaminofuro[2,3-d]pyrimidin-5-yl) methyl]amino]methyl]benzoyl]-L-glutamic acid (4), respectively, were synthesized. Compound 3 was obtained via a Wittig reaction of the tributylphosphonium salt of 2,4-diamino-5-(chloromethyl)furo[2,3-d]pyrimidine (5) and methyl 4-formylbenzoate (6) followed by reduction and coupling with the diethyl ester of L-glutamic acid. Compound 4 was synthesized by the nucleophilic displacement of 5 with diethyl N-[4-(aminomethyl)benzoyl]-L-glutamate (15) and saponification. Both analogues were evaluated in vitro as inhibitors of DHFRs from (recombinant) human, human CCRF-CEM cells, and Lactobacillus casei. Compound 3 showed moderate activity (IC50 10(-6)-10(-7) M). Compound 4 was essentially inactive (IC50 10(-5) M, CCRF-CEM). The compounds were also evaluated against TS from (recombinant) human and L. casei and were of low activity (IC50 10(-5) M). The three-atom-bridged analogue 4 was somewhat more inhibitory to human TS than methotrexate (MTX). Compound 3 inhibited the growth of tumor cells in culture (IC50 10(-7) M) while 4 showed a low level of growth inhibitory activity. The inhibition of the growth of leukemia CCRF-CEM cells by both compounds parallels their inhibition of CCRF-CEM DHFR. Analogue 3 was a good substrate for human folylpolyglutamate synthetase (FPGS) derived from CCRF-CEM cells (Km 8.5 microM). Further evaluation of the growth inhibitory activity of 3 against the MTX-resistant subline of CCRF-CEM cells (R30dm) with decreased FPGS indicated that poly-gamma-glutamylation was important for its action. Protection studies with 3 in the FaDu squamous cell carcinoma cell line indicated that inhibition was completely reversed by leucovorin [(6R,S-5-formyltetrahydrofolate] or by a combination of thymidine and hypoxanthine, suggesting an antifolate effect directed at DHFR.