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17-(Allylamino)-17-demethoxygeldanamycin (17-AAG) is a semisynthetic antitumor agent, which has entered phase I/II clinical trials. Melanoma cell lines in the NCI in vitro screen (mean GI50=84 nM) were relatively sensitive to the agent, which was therefore tested in vivo in four s.c. growing human melanoma xenografts (MEXF 276, 989, 462 and 514) in athymic mice. 17-AAG markedly inhibited tumor growth at doses of 80 (maximum tolerated dose) and 60 mg/kg/day in a qd×5 (h: 0, 6; i.p.) schedule in two of four xenograft models. Cell lines derived from the 17-AAG-sensitive MEXF 276 and -resistant MEXF 514 melanomas, MEXF 276L and 514L, were chosen to study the effects of 17-AAG on its target Hsp90 as well as the Hsp90 ‘client’ protein c-Raf-1 in vitro. Cells were exposed to drug concentrations which just cause total growth inhibition (total growth inhibition=375 nM in MEXF 276L and 10 μM in MEXF 514L). Pharmacokinetic determinations confirmed that 17-AAG concentrations producing growth inhibition invitro are readily achievable in vivo at the MTD (AUC0–∞ 1068 μM·min). Whilst 17-AAG treatment did not affect Hsp90 expression in the relatively resistant MEXF 514L cells, it caused a rapid transient decline in the markedly sensitive MEXF 276L cell line. In contrast, Hsp72 expression increased. Following Hsp90 depletion at 2–8 h in MEXF 276L cells, down-regulation of c-Raf-1 was seen starting at 16 h after drug addition. In MEXF 276 xenograft tissues treated with effective dose levels, loss of Hsp90 was seen and was associated with occurrence of apoptotic figures. The apoptotic index rose from 9% after 48 h, greater than 12% at 72 h to 45% at 10 days. These data support the hypothesis that in some melanoma models, a very good response (e.g. with tumor regressions) to 17-AAG may be associated with modulation of Hsp90 expression. The expression of this target should be followed in clinical studies with 17-AAG.