Synthesis, specificity, and antifungal activity of inhibitors of the Candida albicans Δ²⁴-sterol methyltransferase

A series of side chain modified analogues of cholesterol and lanosterol (1-10) have been synthesized and evaluated as inhibitors of the Candida albicans Δ²⁴-sterol methyltransferase. Two sterol substrate analogues 1 and 2 which contained a 24-thia substituent were relatively modest inhibitors of the enzyme (K(i) = 1.5-72 μM). Compounds which mimic the carbocation intermediates proposed for the methyltransferase reaction, including sulfonium salts 4-6, amidines 7 and 8, and imidazoles 9 and 10 were substantially more potent inhibitors (K(i) = 5-500 nM). All of the sterol analogues examined displayed less than 10-fold selectivity for inhibition of the methyltransferase versus the rat liver Δ²⁴-sterol reductase. The sterol analogues were tested for in vitro antifungal activity against C. albicans, Candida tropicalis, and Torulopsis glabrata. The minimum inhibitory concentrations versus C. albicans correlated well with the K(i) values for methyltransferase inhibition, and the potency of several compounds approached that of amphotericin B, although only modest fungicidal activity was observed.