Inhibition of Pneumocystis carinii, Toxoplasma gondii, and Mycobacterium avium dihydrofolate reductases by 2,4-diamino-5-[2-methoxy-5-(ω-carboxyalkyloxy)benzyl]pyrimidines: Marked improvement in potency relative to trimethoprim and species selectivity relative to piritrexim

A. Rosowsky, R. A. Forsch, S. F. Queener

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A series of previously undescribed 2,4-diamino-5-[2-methoxy-5-alkoxybenzyl]pyrimidines (3a-e) and 2,4-diamino-5-[2-methoxy-5-(ω-carboxyalkyloxy)benzyl]pyrimidines (3f-k) with up to eight CH2 groups in the alkoxy or ω-carboxyalkyloxy side chain were synthesized and tested for the ability to inhibit partially purified dihydrofolate reductase (DHFR) from Pneumocystis carinii (Pc), Toxoplasma gondii (Tg), Mycobacterium avium (Ma), and rat liver in comparison with two standard inhibitors, trimethoprim (1) and piritrexim (2). The latter drug is known to be extremely potent but shows a marked preference for binding to mammalian DHFR, whereas the former is very selective for the parasite enzymes but is a much weaker inhibitor. The underlying strategy for the synthesis of compounds 3a-k was that a hybrid structure embodying some features of both 1 and 2 might possess a more favorable combination of potency and selectivity than either parent drug. The choice of analogues 3f-k was based on the idea that the acidic ω-carboxyl group might interact preferentially with a basic center in the active site of DHFR from any of the parasite species relative to the active site of mammalian DHFR. In addition, the ω-carboxyl group was expected to improve water solubility relative to 1 or 2. In standardized spectrophotometric assays with dihydrofolate as the substrate and NADPH as the cofactor, 2,4-diamino-5-[(2-methoxy-4-carboxybutyloxy)benzyl]pyrimidine (3g) inhibited Pc DHFR with an IC50 of 0.049 μM and rat DHFR with IC50 of 3.9 μM. Its potency against Pc DHFR was 140-fold greater than that of 1 and close to that of 2, and its selectivity index, defined as the ratio IC50(rat liver)/IC50(P. carinii), was 8-fold higher than that of 1 and > 104fold higher than that of 2. Although it was less potent and less selective against Tg than Pc DHFR, it was very potent as well as highly selective against Ma DHFR, with an IC50 of 0.0058 μM and an IC50(rat liver)/IC50(M. avium) ratio of > 600. Because of this favorable combination of potency and selectivity relative to 1 and 2, compound 3g may be viewed as a promising lead in the search for new antifolates with potential clinical activity against P. carinii and other opportunistic pathogens in patients with AIDS.

Original languageEnglish (US)
Pages (from-to)233-241
Number of pages9
JournalJournal of Medicinal Chemistry
Issue number1
StatePublished - Jan 3 2002


ASJC Scopus subject areas

  • Molecular Medicine
  • Drug Discovery

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