SUMMARY Targeted intervention of the B-Raf V600E gene product that is prominent in melanoma has been met with modest success. Here, we characterize the pharmacological properties of PLX4032, a next-generation inhibitor with exquisite specificity against the V600E oncogene and striking anti-melanoma activity. PLX4032 induces potent cell cycle arrest, inhibits proliferation, and initiates apoptosis exclusively in V600E-positive cells in a variety of in vitro experimental systems; follow-up xenograft studies demonstrate extreme selectivity and efficacy against melanoma tumors bearing the V600E oncoproduct. The collective data support further exploration of PLX4032 as a candidate drug for patients with metastatic melanoma; accordingly, validation of PLX4032 as a therapeutic tool for melanoma patients is now underway in advanced human (Phase III) clinical trials.
Purpose To investigate the roles of melanoma-associated macrophages in melanoma resistance to BRAF inhibitors (BRAFi). Experimental Design An in vitro macrophage and melanoma cell co-culture system was used to investigate whether macrophages play a role in melanoma resistance to BRAFi. The effects of macrophages in tumor resistance were examined by proliferation assay, cell death assay and western blot analyses. Furthermore, two mouse preclinical models were used to validate whether targeting macrophages can increase the anti-tumor activity of BRAFi. Finally, the number of macrophages in melanoma tissues was examined by immunohistochemistry. Results We demonstrate that in BRAF mutant melanomas, BRAFi paradoxically activate the MAPK pathway in macrophages to produce VEGF, which reactivates the MAPK pathway and stimulates cell growth in melanoma cells. Blocking the MAPK pathway or VEGF signaling, then reverses macrophage-mediated resistance. Targeting macrophages increases the anti-tumor activity of BRAFi in mouse and human tumor models. The presence of macrophages in melanomas predicts early relapse after therapy. Conclusions Our findings demonstrate that macrophages play a critical role in melanoma resistance to BRAFi, suggesting that targeting macrophages will benefit patients with BRAF mutant melanoma.
Purpose Inhibitor of apoptosis proteins (IAPs) promote cancer cell survival and confer resistance to therapy. We report on the ability of second mitochondria-derived activator of caspases (SMAC) mimetic, birinapant, which acts as antagonist to cIAP1 and cIAP2, to restore the sensitivity to apoptotic stimuli such as tumor necrosis factor (TNF)-α in melanomas Experimental Design Seventeen melanoma cell lines, representing five major genetic subgroups of cutaneous melanoma, were treated with birinapant as a single agent or in combination with TNF-α. Effects on cell viability, target inhibition, and initiation of apoptosis were assessed and findings were validated in in 2D, 3D spheroid and in vivo xenograft models. Results When birinapant was combined with TNF-α, strong combination activity, i.e. neither compound was effective individually but the combination was highly effective, was observed in twelve out of eighteen cell lines. This response was conserved in spheroid models, whereas in vivo birinapant inhibited tumor growth without adding TNF-α in in vitro resistant cell lines. Birinapant combined with TNF-α inhibited the growth of a melanoma cell line with acquired resistance to BRAF inhibition to the same extent as in the parental cell line. Conclusions Birinapant in combination with TNF-α exhibits a strong anti-melanoma effect in vitro. Birinapant as a single agent shows in vivo anti-tumor activity, even if cells are resistant to single agent therapy in vitro. Birinapant in combination with TNF-α is effective in a melanoma cell line with acquired resistance to BRAF inhibitors.
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