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Clinical efficacy of a RAF inhibitor needs broad target blockade in BRAF-mutant melanoma

Abstract

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.

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Figure 1: Three-dimensional structure of PLX4032 binding to B-RAF(V600E).
Figure 2: Effect of PLX4032 on COLO205 xenograft tumour growth.
Figure 3: Semi-quantitative immunohistochemistry (IHC) in paired tumour biopsies.
Figure 4: Representative PET scans for patients taken pre-dose and following 2 weeks of dosing with PLX4032.

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Protein Data Bank

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Atomic and structural data have been deposited in Protein Data Bank under accession number 3OG7.

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Acknowledgements

We thank L. Andries and M. Knaapen from HistoGeneX for evaluating paired biopsies, and also our colleagues at the Molecular Imaging Research division of Charles River Labs for conducting the xenograft studies. We also thank D. Heimbrook, S. Cheng, L. Burdette and B. Lestini for helpful comments on the manuscript. This research was funded in part by NIH grants to K.L.N.

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Contributions

G.B., P.H., C.Z., K.L.N. and K.N. designed studies, interpreted data and wrote the manuscript. J.T., G.H., E.A.B., B.W., G.T., B.L.W., B.P., R.S., A.M., H.N., F.S., and B.H. conducted or managed biochemical or biological studies. J.Z., P.N.I., H.C., W.S., D.R.A. and R.I. designed and conducted chemistry and formulation experiments. Y.Z. and K.Y.J.Z. conducted and interpreted structural studies. J.S. helped interpret data and write the manuscript. K.D., A.K., M.S. and X.X. designed, managed and interpreted biomarker studies. P.S.L., R.J.L., J.G., I.P., K.B.K., A.R., G.A.M., J.A.S., P.B.C. and K.T.F. managed or conducted clinical and translational studies.

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Correspondence to Gideon Bollag.

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Competing interests

[Many of the authors are employed by the biotechnology company Plexxikon or the pharmaceutical company Roche, as indicated in the author affiliations.]

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This file contains Supplementary Methods, Supplementary Results, additional references and Supplementary Tables 1-3. (PDF 182 kb)

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Bollag, G., Hirth, P., Tsai, J. et al. Clinical efficacy of a RAF inhibitor needs broad target blockade in BRAF-mutant melanoma. Nature 467, 596–599 (2010). https://doi.org/10.1038/nature09454

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