High affinity, potent and selective Hsp90 inhibitor (Ki = 0.2 nM and IC50 = 1.1 nM). Exhibits selectivity for Hsp90 over Grp94 and TRAP1 (Ki values are 61 and 255 nM respectively) and has no effect (IC50 >10 μM) against a panel of 285 kinases. Degrades Her-2 in MCF-7 breast cancer cells (EC50 = 14 nM). Blocks tumor growth and induces partial tumor regression in an N87 gastric tumor mouse model. Also inhibits LPS-induced TNFα release in an LPS shock mouse model and suppresses disease development in a rat model of collagen-induced arthritis. Exhibits <20-fold efficacy over BIIB 021 (Cat. No. 4608) in mice. Orally available and brain penetrant. Purity 98%. MW (observed): 413.9