The effect of conformational restriction of the C9-N10 bridge on inhibitory potency and selectivity of trimetrexate against dihydrofolate reductase, was studied. Specifically three nonclassical tricyclic 1, 3-diamino-8-(3',4',5'-trimethoxybenzyl)-7,9-dihydro-pyrrolo[3,4-c]pyrido [2,3-d]pyrimidin-6(5H,8H)-one (4), 1,3-diamino-8-(3',4',5'-trimethoxybenzyl)-9-hydro-pyrrolo [3,4-c]pyrido [2,3-d]pyrimidin-6-(8H)-one (5) and 1,3-diamino-(8H)-(3',4',5'-trimethoxybenzyl)-7,9-dihydro-pyrrolo [3,4-c]pyrido [2,3-d]pyrimidine (7) antifolates were synthesized. The tricyclic analogues 4 and 5 were obtained via the regiospecific cyclocondensation of the β-keto ester 17 with 2,4,6-triaminopyrimidine. The analogue 7 was obtained via reduction of the lactam 4 with borane in tetrahydrofuran. Compounds 4, 5 and 7 were evaluated as inhibitors of dihydrofolate reductase from Pneumocystis carinii, Toxoplasma gondii and rat liver. All three compounds were more selective than trimetrexate against Pneumocystis carinii dihydrofolate reductase and significantly more selective than trimetrexate against Toxoplasma gondii dihydrofolate reductase compared with rat liver dihydrofolate reductase. Currently available treatments for Pneumocystis carinii (P. carinii) and Toxoplasma gondii (T. gondii), which are often fatal opportunistic infections in AIDS patients, are usually a combination of agents [2][3][4]. The use of trimethoprim and sulfamethoxazole is considered to be the most effective combination for the treatment and prophylaxis of P. carnii infections [5]. T. gondii infections are treated by the first line combination of pyrimethamine and sulfadiazine [6]. Both these combinations utilize a selective dihydrofolate reductase inhibitor trimethoprim or pyrimethamine along with a dihydropteroate synthase inhibitor that is the sulfa drug. Though these combinations are effective, a significant number of patients suffer side effects primarily attributed to the sulfonamide drug which limits the dose and in many cases forces a discontinuation of therapy [7,8]. The dihydrofolate reductase inhibitors in these combinations are weak ineffective agents when used as monotherapy and require the sulfonamide to afford a synergistic, clinically effective combination [9,10]. In an attempt to circumvent the use of the sulfonamide in these combinations, due to its toxicity which necessitates discontinuation of treatment, considerable effort has been expended to design and synthesize pathogen selective dihydrofolate reductase inhibitors that also possess increased potencies against P. carinii dihydrofolate reductase and T. gondii dihydrofolate reductase. Such a potent and selective dihydrofolate reductase inhibitor would preclude the necessity of the sulfonamide and hence the toxicity associated with the combination and could be clinically viable monotherapy for P. carinii and T. gondii infections.The 3,000 to 10,000 fold selectivity of trimethoprim (TMP) for bacterial dihydrofolate reductase over the mammalian enzyme has been attributed, in part, to the adoptio...
