IPI-504, a selective inhibitor of Hsp90, exhibits potent anti-tumor activity against metastatic melanoma cell lines that harbor B-Raf or N-Ras mutations

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The MAP kinase pathway is constitutively active in cutaneous melanomas due primarily to activating mutations in B-Raf (60-70%) or N-Ras (5-33%). In addition to the MAP kinase pathway, the phosphoinositol 3 kinase (PI3K)-Akt pathway is also activated in a high percentage of melanomas through the loss of PTEN. Previously, it has been shown that B-Raf and C-Raf are client proteins of the Hsp90 complex. These observations suggest that melanomas with mutant B-Raf or N-Ras may be susceptible to IPI-504 mediated inhibition of Hsp90 complex. In this study, we investigated the ability of the IPI-504 to modulate key client proteins and their down-stream signaling molecules in melanoma cell lines. IPI-504 exhibited potent anti-proliferative effects on all cell lines tested harboringB-Raf or N-Ras mutations. For mutated B-Raf (V600E), the IC₅₀ of variouscell lines ranged from 24 to 283 nM. The N-Ras mutated (Q61R), SKMel-2 melanoma line was equally sensitive in this assay with an IC₅₀ of 30 nM. Similarly, as assessed by a nucleosome release assay, IPI-504 was also effective in inducing apoptosis of all cell lines tested. The EC₅₀ for apoptosis was found to be 89 to 157 nM for all cell lines tested except for the A375M cell line, which showed a 4-fold less sensitive (EC₅₀ of 426 nM). In athymic nude mice, we observed dose-dependent and schedule-dependent tumor growth inhibition with IPI-504 in the A375 (mutated B-Raf) melanoma xenograft model. At the end of study, mice receiving 100 mg/kg IPI-504 showed a 70% inhibition of tumor growth. Interestingly, when assessing plasma and tumor levels of IPI-504, we demonstrated that IPI-504 exhibited a short half life in plasma but accumulated in tumor tissues, with an extended half life in tumor tissues. These data indicate that Hsp90 inhibition has significant anti-tumor activity in metatstatic melanoma and provides the framework for an ongoing Phase 2 clinical trial of this novel class of agents in patients with metastatic melanoma.