Overcoming Acquired BRAF Inhibitor Resistance in Melanoma via Targeted Inhibition of Hsp90 with Ganetespib

Abstract

Activating BRAF kinase mutations serve as oncogenic drivers in over half of all melanomas, a feature that has been exploited in the development of new molecularly targeted approaches to treat this disease. Selective BRAF V600E inhibitors, such as vemurafenib, typically induce initial, profound tumor regressions within this group of patients; however, durable responses have been hampered by the emergence of drug resistance. Here, we examined the activity of ganetespib, a small-molecule inhibitor of Hsp90, in melanoma lines harboring the BRAF V600E mutation. Ganetespib exposure resulted in the loss of mutant BRAF expression and depletion of mitogen-activated protein kinase and AKT signaling, resulting in greater in vitro potency and antitumor efficacy compared with targeted BRAF and MAP-ERK kinase (MEK) inhibitors. Dual targeting of Hsp90 and BRAF V600E provided combinatorial benefit in vemurafenibsensitive melanoma cells in vitro and in vivo. Importantly, ganetespib overcame mechanisms of intrinsic and acquired resistance to vemurafenib, the latter of which was characterized by reactivation of extracellular signal-regulated kinase (ERK) signaling. Continued suppression of BRAF V600E by vemurafenib potentiated sensitivity to MEK inhibitors after acquired resistance had been established. Ganetespib treatment reduced, but not abolished, elevations in steady-state ERK activity. Profiling studies revealed that the addition of a MEK inhibitor could completely abrogate ERK reactivation in the resistant phenotype, with ganetespib displaying superior combinatorial activity over vemurafenib. Moreover, ganetespib plus the MEK inhibitor TAK-733 induced tumor regressions in vemurafenib-resistant xenografts. Overall these data highlight the potential of ganetespib as a single-agent or combination treatment in BRAF V600E -driven melanoma, particularly as a strategy to overcome acquired resistance to selective BRAF inhibitors. Mol Cancer Ther; 13(2); 353-63. Ó2014 AACR.

The Quinone Methide Aurin Is a Heat Shock Response Inducer That Causes Proteotoxic Stress and Noxa-dependent Apoptosis in Malignant Melanoma Cells

“…6, A and B) as already indicated by array analysis (Fig. 2, A and B) (34,37,63). Consistent with an aurin-induced up-regulation of heat shock response gene expression downstream of PCR; n ϭ 3).…”

“…Small molecule inhibitors of Hsp90 are, therefore, an important class of experimental and investigational chemotherapeutics that have reached the stage of clinical testing. Hsp90-directed inhibitory activity of aurin seems to be of particular relevance in the context of melanoma chemotherapy, as Hsp90 inhibition has emerged as a particularly promising therapeutic strategy based on the finding that V600E B-Raf is an obligatory Hsp90 client protein essential to the clonal evolution of melanoma tumors and that Hsp90 inhibitors cause the degradation of V600E B-Raf (34,37,63). Moreover, it has now been observed that V600E BRAF melanoma cells that have acquired resistance to vemurafenib due to a variety of molecular changes retain sensitivity to Hsp90 inhibition suggesting feasibility of FIGURE 8.…”

“…Specifically, Hsp90 serves as an essential factor stabilizing oncogenic V600E BRAF in malignant melanoma cells, and its inhibition has emerged as a promising strategy for antimelanoma intervention (34 -37). Therefore, strategies that aim at increasing proteotoxic stress through pharmacological modulation of proteasomal, autophagic-lysosomal, or heat shock response functions are now pursued for experimental and investigational chemotherapeutic intervention targeting malignant melanoma (27)(28)(29)(30)(31)(32)(33)(37)(38)(39)(40).…”

HSP90 empowers evolution of resistance to hormonal therapy in human breast cancer models

“…In contrast, we now report that very modest, sustained inhibition of HSP90, at well-tolerated levels that do not deplete clients on their own or activate HSF1, reduces heterogeneity in the response of tumors to antiestrogens and prolongs the duration of disease control by these agents. Focusing on these striking effects of low-level, noncytotoxic HSP90 inhibition, the current study provides a new perspective, one very different from the many previous reports describing interesting activity for high-level HSP90 inhibition against recurrent drug-resistant cancers (33)(34)(35)(36) or synergistic interaction with other chemotherapeutics to increase their activity (37)(38)(39). From a translational perspective, the recent development of orally bioavailable HSP90 inhibitors should make sustained, low-level inhibitor exposure feasible in patients.…”

HSP90 Inhibition Enhances the Anti-Tumor Efficacy of Combination Chemo Immunotherapy Targeting DNA Repair Proteins

“…In the melanoma setting, DNA-RP (such as ATR, MRN complex proteins (MRE11, Rad50, NBN), Rad51, PRKDC) are transcriptionally over expressed in a disease stage-associated manner in association with chemotherapy-resistance and poor overall survival [11][12][13][14][15][16][17][18]. Notably, tumor cell (over) expression of DNA-RP may also be posttranslationally stabilized via the action of heat shock protein-90 HSP90; [11,[13][14][15][16][17]19], a molecular chaperone that is highlyabundant in the cancer proteome [20]. HSP90 forms the core of a super-chaperone machine consisting of HSP70, HSP40, HIP and HOP, which extends the molecular lifespan of a growing list of client proteins, including signaling protein kinases, transcription factors, DNA-RP and other cytosolic or nuclear proteins in their functionally mature and active conformations [19].…”