THE IMMUNOSUPPRESSANT FK506 AND ITS NONIMMUNOSUPPRESSIVE ANALOG L-685,818 ARE TOXIC TO CRYPTOCOCCUS-NEOFORMANS BY INHIBITION OF A COMMON TARGET PROTEIN

The immunosuppressant FK506 (tacrolimus) is an antifungal natural prod uct macrolide that suppresses the immune system by blocking T-cell act ivation. In complex with the intracellular protein FKBP12 FK506 inhibi ts calcineurin, a Ca2+-calmodulin-dependent serine-threonine protein p hosphatase, We recently reported that growth of the opportunistic fung al pathogen Cryptococcus neoformans is resistant to FK506 at 24 degree s C but sensitive at 37 degrees C and that calcineurin, the target of FKBP12-FK506, is required for growth at 37 degrees C in vitro and path ogenicity in vive, These findings identify calcineurin as a potential antifungal drug target, In previous studies the calcineurin inhibitor cgclosporin A (CsA) was effective against murine pulmonary infections but exacerbated cryptococcal meningitis in rabbits and mice, likely be cause CsA does not cross the blood-brain barrier, Although we find tha t FK506 penetrates the CNS, FK506 also exacerbates crSptococcal mening itis in rabbits. Thus, FK506 immunosuppression outweighs antifungal ac tion in r?ir o. Like FIi506, the nonimmunosuppressive FK506 analog L-6 85,818 is toxic to C. neoformans in vitro at 37 degrees C but not at 2 4 degrees C, and FK506-resistant mutants are resistant to L-685.818, i ndicating a similar mechanism of action, Fluconazole-resistant C. neof ormans clinical isolates were also found to be susceptible to both FK5 06 and L-685,818. Our findings identify calcineurin as a novel antifun gal drug target and suggest the nonimmunosuppressive FK506 analog L-68 51818 or other congeners warrant further consideration as antifungal d rugs for C. neoformans.