Phosphoinositide-dependent kinase-1 (PDK-1) is a serine/threonine protein kinase that phosphorylates members of the conserved AGC kinase superfamily, including AKT and PKC, and is implicated in important cellular processes including survival, metabolism and tumorigenesis.,In large cohorts of nevi and melanoma samples, PDK1 expression was significantly higher in primary melanoma, compared with nevi, and was further increased in metastatic melanoma.,PDK1 expression suffices for its activity, due to auto-activation, or elevated phosphorylation by phosphoinositide 3'-OH-kinase (PI 3-K).,Selective inactivation of Pdk1 in the melanocytes of BrafV600E::Pten−/− or BrafV600E::Cdkn2a−/−::Pten−/− mice delayed the development of pigmented lesions and melanoma induced by systemic or local administration of 4-HT.,Melanoma invasion and metastasis were significantly reduced or completely prevented by Pdk1 deletion.,Administration of the PDK1 inhibitor GSK2334470 (PDKi) effectively delayed melanomagenesis and metastasis in BrafV600E::Pten−/− mice.,Pdk1−/− melanomas exhibit a marked decrease in the activity of AKT, P70S6K and PKC.,Notably, PDKi was as effective in inhibiting AGC kinases and colony forming efficiency of melanoma with Pten WT genotypes.,Gene expression analyses identified Pdk1-dependent changes in FOXO3a-regulated genes and inhibition of FOXO3a restored proliferation and colony formation of Pdk1−/− melanoma cells.,Our studies provide direct genetic evidence for the importance of PDK1, in part through FOXO3a-dependent pathway, in melanoma development and progression.
B-RAF is the most frequently mutated protein kinase in human cancers.1 The finding that oncogenic mutations in BRAF are common in melanoma2 followed by the demonstration that these tumors are dependent on the RAF/MEK/ERK pathway3 offered hope that inhibition of B-RAF kinase activity could benefit melanoma patients.,Herein, we describe the structure-guided discovery of PLX4032 (RG7204), a potent inhibitor of oncogenic B-RAF kinase activity.,Preclinical experiments demonstrated that PLX4032 selectively blocked the RAF/MEK/ERK pathway in BRAF mutant cells and caused regression of BRAF mutant xenografts.4 Toxicology studies confirmed a wide safety margin consistent with the high degree of selectivity, enabling Phase 1 clinical trials using a crystalline formulation of PLX4032.5 In a subset of melanoma patients, pathway inhibition was monitored in paired biopsy specimens collected before treatment initiation and following two weeks of treatment.,This analysis revealed substantial inhibition of ERK phosphorylation, yet clinical evaluation did not show tumor regressions.,At higher drug exposures afforded by a new amorphous drug formulation,4,5 greater than 80% inhibition of ERK phosphorylation in the tumors of patients correlated with clinical response.,Indeed, the Phase 1 clinical data revealed a remarkably high 81% response rate in metastatic melanoma patients treated at an oral dose of 960 mg twice daily.5 These data demonstrate that BRAF-mutant melanomas are highly dependent on B-RAF kinase activity.