| The objective of this research is to develop potential agents to treat opportunistic infections associated with acquired immune deficiency syndrome (AIDS), such as Pneumocystis carinii pneumonia and toxoplasmic encephalitis. Generally, antifolate agents are used for treatments and prophylaxis of these infections. The commonly used drugs include trimethoprim, pyrimethamine, pyritrexim, and trimetrexate, all of which are inhibitors of dihydrofolate reductase (DHFR). However, all of them suffer various side effects, sometimes too severe to be tolerated by patients. More effective treatments with fewer side effects are desired for daily clinical management of AIDS patients.; Our approach of developing more effective antifolate agents was to selectively inhibit DHFR of Pneumocystis carinii and Toxoplasma gondii, an essential enzyme involved in de novo biosynthetic pathway of pathogen DNA. Based on the three-dimensional X-ray structure of the target enzyme, we designed a series of 5-arylalkyl-2,4,6-triaminopyrimidine and 5-methyl-6-arylalkyl-2,4-diamino[2,3-d]pyrimidine nonclassical antifolates with functional groups intended to interact with specific amino acid side chains at the active site of the enzyme. We hypothesized that these structural alterations would lead to selectivity of inhibition. One of the target amino acid residues was Phe69 in P. carinii DHFR. These antifolates were synthesized and tested for their inhibitory activities against DHFR of various opportunistic pathogens and their mammalian counterparts.; The two monocyclic analogues 1 and 2 showed weak activity and poor selectivity. Two of the bicyclic antifolates 3 and 5 were found to possess moderate selectivity against P. carinii DHFR. Activities of homologs of 5-arylalkyl-2,4-diaminothieno[2,3- d]pyrimidine series were compared. The bridge length of three carbon atoms was determined as optimal for potent inhibitory activity. A conformationally restrained compound (11) was also synthesized and evaluated for its activities. Based on results with 11, we proposed a possible mechanism of optimal inhibition and differential binding. Phe69 was deemed to be an attractive target for developing selectivity. A possible cooperativity of binding with NADPH may be involved in the better activity of some compounds. |
