Screening existing drugs for new biological activities is emerging as an expedient approach to the identification of lead compounds for new drug development. As part of a high-throughput screen for small molecules that cause yeast cell lysis, we identified tamoxifen (TAM) and its closely related analog clomiphene as active fungilytic agents. TAM is used clinically as an estrogen receptor antagonist in the treatment of breast cancer but it was known to have activity against both C. albicans and S. cerevisiae has (MIC 6-8 mcg/mL). We found it also has similar levels of activity against Cryptococus neoformans. The molecular mechanism of the antifungal activity of TAM is not known. Since TAM inhibits calmodulin function in human cells, we hypothesized that the antifungal activity of TAM may be due to inhibition of calmodulin. Based on studies in S. cerevisiae, we have developed five lines of evidence supporting calmodulin as the yeast target of TAM. First, TAM treated yeast cells phenocopy cell morphologic, nuclear separation and actin cytoskeletal defects displayed by calmodulin mutants. Second, calmodulin mutants are hypersensitive to TAM. Third, overexpression of wild type calmodulin in yeast induces resistance to TAM. Fourth, calmodulin is mislocalized in TAM-treated yeast cells. Fifth, co-immunoprecipitation studies indicate that TAM disrupts the binding of proteins to yeast calmodulin. In C. albicans, TAM treatment inhibits germ tube formation, a known effect of calmodulin inhibitors. Since the structure of calmodulin in yeast has diverged significantly from human calmodulin, our results suggest that TAM and, more generally, triarylethylenes may represent an attractive class of lead compounds for the development of calmodulin-targeted antifungal drugs.
Full conference title:
48th Annual ICAAC/46th Annual IDSA
- ICAAC 48th/IDSA 46th