As part of an ongoing effort to discover novel small-molecule
antifolates combining the
enzyme-binding species selectivity of
trimethoprim (
TMP) with the potency of
piritrexim (PTX), 10 previously unreported 2,4-diamino-5-(2'-methoxy-5'-substituted)benzylpyrimidines (2-11) containing a carboxyl group at the distal end of the 5'-substituent were synthesized and tested as inhibitors of
dihydrofolate reductase (DHFR) from Pneumocystis carinii (Pc), Toxoplasma gondii (Tg), and Mycobacterium avium (Ma), three of the opportunistic pathogens frequently responsible for life-threatening illness in people with impaired immune systems as a result of
HIV infection or immunosuppressive
chemotherapy. The selectivity index of DHFR inhibition was evaluated by comparing the potency of each compound against the parasite
enzymes with its potency against rat liver DHFR. 2,4-Diamino-5-[5'-(5-carboxy-1-pentynyl)-2'-methoxybenzyl]
pyrimidine (3) inhibited Pc DHFR with a selectivity index of 79 and was 430 times more potent than
TMP. 2,4-Diamino-5-[5'-(4-carboxy-1-butynyl)-2'-methoxybenzyl]
pyrimidine (2), with one less
carbon than 3 in the side chain, had a selectivity index of 910 against Ma DHFR and was 43 times more potent than
TMP. 2,4-Diamino-5-[5'-(5-carboxypentyl)-2'-methoxybenzyl]
pyrimidine (6) had a selectivity index of 490 against Tg DHFR and was 320 times more potent than
TMP. 2,4-Diamino-5-[5'-(6-carboxy-1-hexynyl)-2'-methoxybenzyl]
pyrimidine (4), with one more
carbon than 3, was less potent against all three of the parasite
enzymes than either 3 or 6 and also had a lower selectivity index than 3 against the Pc
enzyme. However, 4 was the only member of the series with a selectivity index of >300 against both Tg and Ma DHFR. Given that PTX is at least 10 times more potent against rat DHFR than against P. carinii or T. gondii DHFR and that the selectivity index of several of the compounds matches or exceeds that of
TMP as well as PTX, our results suggest that it may be possible to develop clinically useful nonclassical
antifolates that are both potent and selective against the major opportunistic pathogens of
AIDS.