(A and B) Mice (n = 7 per group) were injected i.p. with either 200 mg/kg EC102 or equivalent vehicle once daily for 7 days. Following the final injection, 5 mice were killed within 1 hour, the 2 remaining mice were harvested 24 hours later, and their brain homogenates were assessed by Western blot (A) and those of the 5 animals harvested after 1 hour were quantitated by densitometry normalized to GAPDH (B). Hsp70 levels showed a significant increase compared with vehicle-treated mice that persisted for 24 hours. Conversely, Hsp90 levels were significantly reduced only 1 hour after EC102 administration, but were indistinguishable from vehicle-treated animals within 24 hours. Values represent optical density ± SD. (C and D) Two cohorts of four 13- to 14-month-old Htau mice were injected i.p. with either EC102 or vehicle as above, all tissue was harvested 24 hours following the final injection, and Western blot (C) and densitometric quantitation for total tau (D) were performed. Dramatic reductions in phosphorylated S396/S404 and S202/T205 immunoreactivity in EC102-treated mice were observed. Hsp70 levels were significantly elevated in EC102-treated animals. Only the high–molecular weight species of tau (60–65 kDa), presumably p-tau species, were reduced by EC102 treatment; normal tau species (45–55 kDa) remained unaffected. Levels of Hsp90 clients Cdk5 and Akt were unaltered by Hsp90 inhibition. Error bars represent SD. *P < 0.05; **P < 0.001 versus vehicle.