DESIGN OF AN ANTIFUNGAL METHIONINE INHIBITOR NOT ANTAGONIZED BY METHIONINE

Only a few biosynthetic pathways in fungal cells have been used as ant ifungal targets. Therefore, the number of antifungals has been limited , and a cross-drug resistance among them has emerged in the therapy of mycoses. Under such circumstances, the identification of an antifunga l with a new mode of action is highly desirable. By infecting mice wit h a mutant of C. albicans deficient in the sulfate assimilation pathwa y, we have discovered a new target for the discovery of antifungal age nts. We have proven that azoxybacilin inhibits the sulfate assimilatio n pathway by showing its inhibitory activity for [S-35]SO4 incorporati on into proteins. We have also demonstrated that azoxybacilin was take n up into fungal cells via an active transport system specific for met hionine. This sharing of the uptake system with methionine may explain the mechanism by which the antifungal activity of azoxybacilin is ant agonized by methionine, and led us to design azoxybacilin derivatives that lack the structural feature of amino acids and, at the same time, have increased hydrophobicity to give higher non-specific permeabilit y through the cell membrane. As a result, we have found that ester der ivatives of azoxybacilin were not antagonized by methionine in their u ptake, and that they showed antifungal activity independent of methion ine. The benzyl ester of azoxybacilin was the same as azoxybacilin in its mode of action, but was not markedly antagonized by methionine at concentrations up to 1 mg/ml. These results suggest that azoxybacilin may not merely interfere with the sulfate assimilation pathway.