Clinical Efficacy of a RAF Inhibitor Needs Broad Target Blockade in BRAF-mutant Melanoma

PLX4032, a small-molecule inhibitor being developed by Plexxikon of California and Roche Pharmaceuticals in New Jersey ( http://go.nature.com/QnVGQx ), selectively targets B-RAFV600E, a mutant form of the B-RAF protein kinase common in several human cancers. In this issue of Nature, Gideon Bollag and colleagues report promising results for PLX4032 in an early clinical trial in melanoma patients who carry this B-RAF mutation. They also describe the structure and function of PLX4032 and present translational data from a phase I trial to show that clinical efficacy requires a drug concentration that is sufficient to cause a substantial degree of inhibition of the ERK pathway downstream of B-RAF. The study demonstrates how the design of early clinical trials based on the biological mechanisms underlying tumour formation has the potential to speed up the process by which anticancer drugs can reach the clinic. PLX4032 is a selective inhibitor of the B-RAF protein that has shown promising results in an early clinical trial in melanoma patients with an activating mutation in B-RAF. Now the structure and function of this inhibitor are described. Translational data from a phase I trial show that clinical efficacy requires a substantial degree of inhibition of the ERK pathway downstream of B-RAF. The data also show that BRAF-mutant melanomas are highly dependent on B-RAF activity. B-RAF is the most frequently mutated protein kinase in human cancers1. The finding that oncogenic mutations in BRAF are common in melanoma2, followed by the demonstration that these tumours 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 xenografts4. 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 (ref. 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 tumour regressions. At higher drug exposures afforded by a new amorphous drug formulation4,5, greater than 80% inhibition of ERK phosphorylation in the tumours 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 daily5. These data demonstrate that BRAF-mutant melanomas are highly dependent on B-RAF kinase activity.