The majority of human melanomas bears BRAF mutations and thus is treated with inhibitors of BRAF, such as vemurafenib.,While patients with BRAF mutations often demonstrate an initial dramatic response to vemurafenib, relapse is extremely common.,Thus, novel agents are needed for the treatment of these aggressive melanomas.,Honokiol is a small molecule compound derived from Magnolia grandiflora that has activity against solid tumors and hematopoietic neoplasms.,In order to increase the lipophilicity of honokiol, we have synthesized honokiol DCA, the dichloroacetate ester of honokiol.,In addition, we synthesized a novel fluorinated honokiol analog, bis-trifluoromethyl-bis-(4-hydroxy-3-allylphenyl) methane (hexafluoro).,Both compounds exhibited activity against A375 melanoma in vivo, but honokiol DCA was more active.,Gene arrays comparing treated with vehicle control tumors demonstrated induction of the respiratory enzyme succinate dehydrogenase B (SDHB) by treatment, suggesting that our honokiol analogs induce respiration in vivo.,We then examined its effect against a pair of melanomas, LM36 and LM36R, in which LM36R differs from LM36 in that LM36R has acquired vemurafenib resistance.,Honokiol DCA demonstrated in vivo activity against LM36R (vemurafenib resistant) but not against parental LM36.,Honokiol DCA and hexafluoro inhibited the phosphorylation of DRP1, thus stimulating a phenotype suggestive of respiration through mitochondrial normalization.,Honokiol DCA may act in vemurafenib resistant melanomas to increase both respiration and reactive oxygen generation, leading to activity against aggressive melanoma in vivo.
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.