Epithelial Tissue Hyperplasia Induced by the RAF Inhibitor PF-04880594 Is Attenuated by a Clinically Well-Tolerated Dose of the MEK Inhibitor PD-0325901

A Novel RAF Kinase Inhibitor with DFG-Out-Binding Mode: High Efficacy in BRAF-Mutant Tumor Xenograft Models in the Absence of Normal Tissue Hyperproliferation

BI 882370 is a highly potent and selective RAF inhibitor that binds to the DFG-out (inactive) conformation of the BRAF kinase. The compound inhibited proliferation of human BRAF-mutant melanoma cells with 100× higher potency (1-10 nmol/L) than vemurafenib, whereas wild-type cells were not affected at 1,000 nmol/L. BI 882370 administered orally was efficacious in multiple mouse models of BRAF-mutant melanomas and colorectal carcinomas, and at 25 mg/kg twice daily showed superior efficacy compared with vemurafenib, dabrafenib, or trametinib (dosed to provide exposures reached in patients). To model drug resistance, A375 melanoma-bearing mice were initially treated with vemurafenib; all tumors responded with regression, but the majority subsequently resumed growth. Trametinib did not show any efficacy in this progressing population. BI 882370 induced tumor regression; however, resistance developed within 3 weeks. BI 882370 in combination with trametinib resulted in more pronounced regressions, and resistance was not observed during 5 weeks of second-line therapy. Importantly, mice treated with BI 882370 did not show any body weight loss or clinical signs of intolerability, and no pathologic changes were observed in several major organs investigated, including skin. Furthermore, a pilot study in rats (up to 60 mg/kg daily for 2 weeks) indicated lack of toxicity in terms of clinical chemistry, hematology, pathology, and toxicogenomics. Our results indicate the feasibility of developing novel compounds that provide an improved therapeutic window compared with first-generation BRAF inhibitors, resulting in more pronounced and long-lasting pathway suppression and thus improved efficacy.

Extracellular signal-regulated kinases 1/2 and Akt contribute to triclosan-stimulated proliferation of JB6 Cl 41-5a cells

Triclosan is a broad spectrum anti-bacterial agent widely used in many personal care products, household items, medical devices, and clinical settings. Human exposure to triclosan is mainly through oral and dermal routes. In previous studies, we found that sub-chronic dermal exposure of B6C3F1 mice to triclosan induced epidermal hyperplasia and focal necrosis; however, the mechanisms for these responses remain elusive. In this study, using mouse epidermis-derived JB6 Cl 41-5a cells, we found that triclosan stimulated cell growth in a concentration- and time-dependent manner. Enhanced cell proliferation was demonstrated by a substantial increase in the percentage of BrdU-positive cells, an elevation in the protein levels of cyclin D1 and cyclin A, and a reduction in the protein level of p27(Kip1). Western blotting analysis revealed that triclosan induced the activation of extracellular signal-regulated kinases 1/2 (ERK1/2), c-Jun N-terminal kinases (JNK), p38, and Akt. Pre-treatment of the cells with PD184352, an inhibitor of the upstream kinase MEK1/2, or with wortmannin, an inhibitor of phosphoinositide 3-kinase, blocked triclosan-mediated phosphorylation of ERK1/2 and Akt, respectively, and substantially suppressed triclosan-stimulated cell proliferation, whereas the JNK inhibitor SP600125 or the p38 inhibitor SB203580 had little to no effect on triclosan-stimulated cell proliferation. The phosphorylation activation of ERK1/2 and Akt was further confirmed on the skin of mice dermally administered triclosan. These data suggest that the activation of ERK1/2 and Akt is involved in triclosan-stimulated proliferation of JB6 Cl 41-5a cells.

Dabrafenib and its potential for the treatment of metastatic melanoma

The purpose of this study is to review the development of BRAF inhibitors, with emphasis on the trials conducted with dabrafenib (GSK2118436) and the evolving role of dabrafenib in treatment for melanoma patients. Fifty percent of cutaneous melanomas have mutations in BRAF, resulting in elevated activity of the mitogen-activated protein kinase signaling pathway. Dabrafenib inhibits the mutant BRAF (BRAF(mut)) protein in melanomas with BRAF(V600E) and BRAF(V600K) genotypes. BRAF(V600E) metastatic melanoma patients who receive dabrafenib treatment exhibit high clinical response rates and compared with dacarbazine chemotherapy, progression-free survival. Efficacy has also been demonstrated in BRAF(V600K) patients and in those with brain metastases. Dabrafenib has a generally mild and manageable toxicity profile. Cutaneous squamous cell carcinomas and pyrexia are the most significant adverse effects. Dabrafenib appears similar to vemurafenib with regard to efficacy but it is associated with less toxicity. It is expected that new combinations of targeted drugs, such as the combination of dabrafenib and trametinib (GSK1120212, a MEK inhibitor), will provide higher response rates and more durable clinical benefit than dabrafenib monotherapy.

Mechanisms of intrinsic and acquired resistance to kinase-targeted therapies

Cancer drugs that target pivotal signaling molecules required for malignant cell survival and growth have demonstrated striking antitumor activities in appropriately selected patient populations. Unfortunately, however, therapeutic responses are often of limited duration, typically 6-12 months, because of emergence of drug-resistant subclones of tumor cells. In this review, we highlight several of the mechanisms of emergent resistance to several kinase-targeted small molecule therapies used in melanoma, non-small cell lung cancer (NSCLC) and other solid tumors as illustrative examples. We discuss the implications of these findings for the development of new treatment strategies to delay or prevent the onset of drug resistance.