Inhibition of dihydrofolate reductases from Toxoplasma gondii, Pneumocystis carinii, and rat liver by rotationally restricted analogues of pyrimethamine and metoprine.

Twenty-one conformationally restricted tricyclic pyrimethamine and metoprine analogues with one or two chlorine atoms, or other substituents, at different positions of the phenyl ring were tested for potency and species selectivity against dihydrofolate reductase (DHFR) from Toxoplasma gondii, Pneumocystis carinii, and rat liver. Heterocyclic systems studied included indeno[2,1-d]pyrimidines, benzo[f]quinazolines, and benzo[3,4]cyclohepta[1,2-d]pyrimidines. All but one of the analogues were more potent against T. gondii and rat liver DHFR than against P. carinii DHFR, and those with a one-carbon (CH2) bridge were generally less potent than those with a two-carbon (CH2CH2, CH=CH) or three-carbon (CH2CH2CH2) bridge. Although a number of compounds with a two- and three-carbon bridge were more potent than pyrimethamine against P. carinii DHFR, and especially T. gondii DHFR, none of them were selective for the P. carinii versus the mammalian enzyme, and only those with a one-carbon bridge showed selectivity approaching that of pyrimethamine for the T. gondii enzyme. Computer-simulated docking into the active site pocket of P. carinii and human DHFR suggested that, as a group, the rotationally restricted tricyclic structures are at a disadvantage relative to pyrimethamine and metoprine, in that torsional relief of unfavorable steric interactions between the chlorine atoms and two critical serine and threonine residues in the active site is prevented by the bridge.