THE BENZOQUINONE ANSAMYCIN 17-ALLYLAMINO-17-DEMETHOXYGELDANAMYCIN BINDS TO HSP90 AND SHARES IMPORTANT BIOLOGIC ACTIVITIES WITH GELDANAMYCIN

Purpose: Benzoquinone ansamycins are antibiotics with anticancer poten tial. First described as tyrosine kinase inhibitors, they are now freq uently used to target HSP90 chaperone function. While herbimycin A and geldanamycin (GA) have been widely used in preclinical studies, both drugs are poor candidates for clinical trials owing to their in vivo t oxicity and lack of stability. We therefore examined the biologic effe cts of 17-allylamino-17-demethoxygeldanamycin (17-AG), an ansamycin de rivative with lower in vivo toxicity than GA. Methods: Binding of 17-A G to HSP90 was studied in vitro using a GA-affinity beads competition assay. We analyzed the drug-induced destabilization of p185erbB2, Raf- l and mutant p53 in SKBR3 breast cancer cells by Western blotting. The antiproliferative activities of 17-AG and GA were compared using the MTT assay. Results: We found that, in a similar manner to GA itself, 1 7-AG bound specifically to HSP90. It also led to degradation of the re ceptor tyrosine kinase p185(erbB2), th, serine/threonine kinase Raf-1 and mutant p53. Both GA and 17-AG displayed comparable antiproliferati ve effects in SKBR3 and MCF7 cells. Even though HSP90 binding by 17-AG was weaker than by GA, 17-AG and GA caused biologic effects in tumor cells at similar doses. Conclusion: 17-AG shares the important biologi c features of its parent compound GA. Since 17-AG has a better toxicit y profile than GA, it is an interesting candidate benzoquinone ansamyc in for clinical development.