RUSTMICIN, A POTENT ANTIFUNGAL AGENT, INHIBITS SPHINGOLIPID SYNTHESISAT INOSITOL PHOSPHOCERAMIDE SYNTHASE

Rustmicin is a 14-membered macrolide previously identified as an inhib itor of plant pathogenic fungi by a mechanism that was not defined. We discovered that rustmicin inhibits inositol phosphoceramide synthase, resulting in the accumulation of ceramide and the loss of all of the complex sphingolipids, Rustmicin has potent fungicidal activity agains t clinically important human pathogens that is correlated with its sph ingolipid inhibition, It is especially potent against Cryptococcus neo formans, where it inhibits growth and sphingolipid synthesis at concen trations <1 ng/ml and inhibits the enzyme with an IC50 of 70 pM. This inhibition of the membrane-bound enzyme is reversible; moreover, rustm icin is nearly equipotent against the solubilized enzyme. Rustmicin wa s efficacious in a mouse model for cryptococcosis, but it was less act ive than predicted from its in vitro potency against this pathogen. St ability and drug efflux were identified as two factors limiting rustmi cin's activity. in the presence of serum, rustmicin rapidly epimerizes at the C-2 position and is converted to a gamma-lactone, a product th at is devoid of activity. Rustmicin was also found to be a remarkably good substrate for the Saccharomyces cerevisiae multidrug efflux pump encoded by PDR5.