Lipophilic gamma-monoamide derivatives of aminopterin (AMT) were synthesized in high overall yield from 4-amino-4-deoxy-N10-formylpteroic acid and gamma-N-tert-alkyl-, gamma-N-aralkyl-, or gamma-N-arylamides of alpha-benzyl L-glutamate via a modification of the mixed carboxylic-carbonic anhydride coupling method. Coupling was also accomplished with p-nitrophenyl 4-amino-4-deoxy-N10-formylpteroate. Compounds obtained in this manner included the gamma-tert-butylamide, gamma-(1-adamantylamide), gamma-benzylamide, gamma-(3,4-dichlorobenzylamide), gamma-(2,6-dichlorobenzylamide), gamma-anilide, gamma-(3,4-methylenedioxyanilide), and gamma-(3,4-dihydroxanilide) derivatives of AMT. Also prepared, from 4-amino-4-deoxy-N10-methylpteroic acid via diethyl phosphorocyanidate coupling, was the gamma-(3,4-methylenedioxyanilide) of MTX. The methylenedioxyanilides were cleaved smoothly to dihydroxyanilides with boron tris(trifluoroacetate) in trifluoroacetic acid. All the gamma-monoamides were tested as inhibitors of purified dihydrofolate reductase (DHFR) from murine L1210 leukemia cells and as inhibitors of the growth of wild-type L1210 cells and a subline (L1210/R81) with high-level resistance to MTX and AMT based mainly on a defect in drug uptake via active transport. Several compounds were also tested against human leukemic lymphoblasts (CEM cells) and a resistant subline (CEM/MTX) whose resistance is likewise based on uptake. The IC50 of the gamma-monoamides against DHFR was 1.5- to 5-fold higher than that of the parent acids, but the IC50 against cultured cells varied over a much broader range, suggesting that uptake and/or metabolism rather than DHFR binding are principal determinants of in vitro growth inhibitory activity for these compounds. gamma-N-Aryl and gamma-N-aralkyl derivatives appeared to be more potent than gamma-N-tert-alkyl derivatives. Where comparison could be made, AMT gamma-monoamides were more potent than MTX gamma-monoamides. Several of the gamma-monoamides showed potency comparable to that of the parent acid against wild-type L1210 and CEM cells; all of them were more potent than MTX against the L1210/R81 subline; and some of the AMT gamma-monoamides were also more potent than the parent acid against resistant CEM/MTX cells. As a group, however, the gamma-monoamides were considerably more active against the murine cells than against the human cells, suggesting that the former may take up the amides better or may be able to metabolize them more efficiently than the parent acids. All the gamma-monoamides were tested in vivo against L1210 leukemia in mice.(ABSTRACT TRUNCATED AT 400 WORDS)