| Abstract
| - In a continuing effort to design small-molecule inhibitors of dihydrofolate reductase (DHFR)that combine the enzyme-binding selectivity of 2,4-diamino-5-(3‘,4‘,5‘-trimethoxybenzyl)pyrimidine (trimethoprim, TMP) with the potency of 2,4-diamino-5-methyl-6-(2‘,5‘-dimethoxybenzyl)pyrido[2,3-d]pyrimidine (piritrexim, PTX), seven previously undescribed 2,4-diamino-5-[2‘-methoxy-5‘-(substituted benzyl)]pyrimidines were synthesized in which the substituentat the 5‘-position was a carboxyphenyl group linked to the benzyl moiety by a bridge of two orfour atoms in length. The new analogues were all obtained from 2,4-diamino-5-(5‘-iodo-2‘-methoxybenzyl)pyrimidine via a Sonogashira reaction, followed, where appropriate, by catalytichydrogenation. The new analogues were tested as inhibitors of DHFR from Pneumocystis carinii(Pc), Toxoplasma gondii (Tg), and Mycobacterium avium (Ma), three life-threatening pathogensoften found in AIDS patients and individuals whose immune system is impaired as a result oftreatment with immunosuppressive chemotherapy or radiation. The selectivity index (SI) ofeach compound was obtained by dividing its 50% inhibitory concentration (IC50) against Pc,Tg, or Ma DHFR by its IC50 against rat DHFR. 2,4-Diamino-[2‘-methoxy-5‘-(3-carboxyphenyl)ethynylbenzyl]pyrimidine (28), with an IC50 of 23 nM and an SI of 28 in the Pc DHFR assay,had about the same potency as PTX and was 520 times more potent than TMP. As an inhibitorof Tg DHFR, 28 had an IC50 of 5.5 nM (510-fold lower than that of TMP and similar to that ofPTX) and an SI value of 120 (2-fold better than TMP and vastly superior to PTX). Against MaDHFR, 28 had IC50 and SI values of 1.5 nM and 430, respectively, compared with 300 nM and610 for TMP. Although it had 2.5-fold lower potency than 28 against Ma DHFR (IC50 = 3.7nM) and was substantially weaker against Pc and Tg DHFR, 2,4-diamino-[2‘-methoxy-5‘-(4-carboxyphenyl)ethynylbenzyl]pyrimidine (29), with the carboxy group at the para rather thanthe meta position, displayed 2200-fold selectivity against the Ma enzyme and was the mostselective 2,4-diamino-5-(5‘-substituted benzyl)pyrimidine inhibitor of this enzyme we haveencountered to date. Additional bioassay data for these compounds are also reported.
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