Alastair J. King

Demonstration of a Genetic Therapeutic Index for Tumors Expressing Oncogenic BRAF by the Kinase Inhibitor SB-590885

Oncogenic BRAF alleles are both necessary and sufficient for cellular transformation, suggesting that chemical inhibition of the activated mutant protein kinase may reverse the tumor phenotype. Here, we report the characterization of SB-590885, a novel triarylimidazole that selectively inhibits Raf kinases with more potency towards B-Raf than c-Raf. Crystallographic analysis revealed that SB-590885 stabilizes the oncogenic B-Raf kinase domain in an active configuration, which is distinct from the previously reported mechanism of action of the multi-kinase inhibitor, BAY43-9006. Malignant cells expressing oncogenic B-Raf show selective inhibition of mitogen-activated protein kinase activation, proliferation, transformation, and tumorigenicity when exposed to SB-590885, whereas other cancer cell lines and normal cells display variable sensitivities or resistance to similar treatment. These studies support the validation of oncogenic B-Raf as a target for cancer therapy and provide the first evidence of a correlation between the expression of oncogenic BRAF alleles and a positive response to a selective B-Raf inhibitor.

Increased cyclin D1 expression can mediate BRAF inhibitor resistance in BRAF V600E–mutated melanomas

Recent studies have shown that there is a considerable heterogeneity in the response of melanoma cell lines to MEK and BRAF inhibitors. In the current study, we address whether dysregulation of cyclin-dependent kinase 4 (CDK4) and/or cyclin D1 contribute to the BRAF inhibitor resistance of melanoma cells. Mutational screening identified a panel of melanoma cell lines that harbored both a BRAF V600E mutation and a CDK4 mutation: K22Q (1205Lu), R24C (WM39, WM46, and SK-Mel-28), and R24L (WM902B). Pharmacologic studies showed that the presence of a CDK4 mutation did not alter the sensitivity of these cell lines to the BRAF inhibitor. The only cell line with significant BRAF inhibitor resistance was found to harbor both a CDK4 mutation and a CCND1 amplification. Array comparative genomic hybridization analysis showed that CCND1 was amplified in 17% of BRAF V600E–mutated human metastatic melanoma samples, indicating the clinical relevance of this finding. As the levels of CCND1 amplification in cell lines are lower than those seen in clinical specimens, we overexpressed cyclin D1 alone and in the presence of CDK4 in a drug-sensitive melanoma line. Cyclin D1 overexpression alone increased resistance and this was enhanced when cyclin D1 and CDK4 were concurrently overexpressed. In conclusion, increased levels of cyclin D1, resulting from genomic amplification, may contribute to the BRAF inhibitor resistance of BRAF V600E–mutated melanomas, particularly when found in the context of a CDK4 mutation/overexpression. [Mol Cancer Ther 2008;7(9):2876–83]

Dabrafenib; Preclinical Characterization, Increased Efficacy when Combined with Trametinib, while BRAF/MEK Tool Combination Reduced Skin Lesions

Mitogen-Activated Protein Kinase (MAPK) pathway activation has been implicated in many types of human cancer. BRAF mutations that constitutively activate MAPK signalling and bypass the need for upstream stimuli occur with high prevalence in melanoma, colorectal carcinoma, ovarian cancer, papillary thyroid carcinoma, and cholangiocarcinoma. In this report we characterize the novel, potent, and selective BRAF inhibitor, dabrafenib (GSK2118436). Cellular inhibition of BRAFV600E kinase activity by dabrafenib resulted in decreased MEK and ERK phosphorylation and inhibition of cell proliferation through an initial G1 cell cycle arrest, followed by cell death. In a BRAFV600E-containing xenograft model of human melanoma, orally administered dabrafenib inhibited ERK activation, downregulated Ki67, and upregulated p27, leading to tumor growth inhibition. However, as reported for other BRAF inhibitors, dabrafenib also induced MAPK pathway activation in wild-type BRAF cells through CRAF (RAF1) signalling, potentially explaining the squamous cell carcinomas and keratoacanthomas arising in patients treated with BRAF inhibitors. In addressing this issue, we showed that concomitant administration of BRAF and MEK inhibitors abrogated paradoxical BRAF inhibitor-induced MAPK signalling in cells, reduced the occurrence of skin lesions in rats, and enhanced the inhibition of human tumor xenograft growth in mouse models. Taken together, our findings offer preclinical proof of concept for dabrafenib as a specific and highly efficacious BRAF inhibitor and provide evidence for its potential clinical benefits when used in combination with a MEK inhibitor.

A biochemical rationale for the anticancer effects of Hsp90 inhibitors: Slow, tight binding inhibition by geldanamycin and its analogues

Heat shock protein (Hsp)90 is emerging as an important therapeutic target for the treatment of cancer. Two analogues of the Hsp90 inhibitor geldanamycin are currently in clinical trials. Geldanamycin (GA) and its analogues have been reported to bind purified Hsp90 with low micromolar potency, in stark contrast to their low nanomolar antiproliferative activity in cell culture and their potent antitumor activity in animal models. Several models have been proposed to account for the ≈100-fold-greater potency in cell culture, including that GA analogues bind with greater affinity to a five-protein Hsp90 complex than to Hsp90 alone. We have determined that GA and the fluorescent analogue BODIPY-GA (BDGA) both demonstrate slow, tight binding to purified Hsp90. BDGA, used to characterize the kinetics of ligand–Hsp90 interactions, was found to bind Hsp90α with koff = 2.5 × 10−3 min−1, t1/2 = 4.6 h, and Ki* = 10 nM. It was found that BDGA binds to a functional multiprotein Hsp90 complex with kinetics and affinity identical to that of Hsp90 alone. Also, BDGA binds to Hsp90 from multiple cell lysates in a time-dependent manner with similar kinetics. Therefore, our results indicate that the high potency of GA in cell culture and in vivo can be accounted for by its time-dependent, tight binding to Hsp90 alone. In the broader context, these studies highlight the essentiality of detailed biochemical characterization of drug–target interactions for the effective translation of in vitro pharmacology to cellular and in vivo efficacy.

Identification of a Novel Subgroup of Melanomas with KIT/Cyclin-Dependent Kinase-4 Overexpression

Although many melanomas harbor either activating mutations in BRAF or NRAS, there remains a substantial, yet little known, group of tumors without either mutation. Here, we used a genomic strategy to define a novel group of melanoma cell lines with co-overexpression of cyclin-dependent kinase 4 (CDK4) and KIT. Although this subgroup lacked any known KIT mutations, they had high phospho-KIT receptor expression, indicating receptor activity. Quantitative PCR confirmed the existence of a similar KIT/CDK4 subgroup in human melanoma samples. Pharmacologic studies showed the KIT/CDK4-overexpressing subgroup to be resistant to BRAF inhibitors but sensitive to imatinib in both in vitro and in vivo melanoma models. Mechanistically, imatinib treatment led to increased apoptosis and G(1) phase cell cycle arrest associated with the inhibition of phospho-ERK and increased expression of p27(KIP). Other melanoma cell lines, which retained some KIT expression but lacked phospho-KIT, were not sensitive to imatinib, suggesting that KIT expression alone is not predictive of response. We suggest that co-overexpression of KIT/CDK4 is a potential mechanism of oncogenic transformation in some BRAF/NRAS wild-type melanomas. This group of melanomas may be a subpopulation for which imatinib or other KIT inhibitors may constitute optimal therapy.

The essential role of fibroblasts in esophageal squamous cell carcinoma-induced angiogenesis.

BACKGROUND & AIMS Esophageal squamous cell carcinoma (ESCC) is known to be a highly angiogenic tumor. Here, we investigated the role of the stromal fibroblasts in the ESCC-induced angiogenic response using a novel 3-dimensional model. METHODS A novel assay was developed where cocultures of ESCC and esophageal fibroblasts induced human microvascular endothelial cell (HMVEC) vascular network formation in a 3-dimensional collagen gel. Biochemical studies showed that the ESCC-induced activation of the fibroblasts was required to induce vascular network formation via a transforming growth factor (TGF)-beta and vascular endothelial growth factor (VEGF)-dependent pathway. RESULTS Conditioned media from a panel of 4 ESCC lines transdifferentiated normal esophageal fibroblasts into myofibroblasts via TGF-beta signaling. The presence of fibroblasts was essential for efficient HMVEC network formation, and the addition of ESCC cells to these cultures greatly enhanced the angiogenic process. The role of TGF-beta in this process was shown by the complete inhibition of network formation following TGF-beta inhibitor treatment. Finally, we showed that ESCC-derived TGF-beta regulates angiogenesis through the release of VEGF from the fibroblasts and that the VEGF release was blocked following TGF-beta inhibition. CONCLUSIONS This study shows the essential role of fibroblasts in the ESCC angiogenic-induced response and suggests that the pharmacologic targeting of the TGF-beta signaling axis could be of therapeutic benefit in this deadly disease.

Discovery of Dabrafenib: A Selective Inhibitor of Raf Kinases with Antitumor Activity against B-Raf-Driven Tumors.

Hyperactive signaling of the MAP kinase pathway resulting from the constitutively active B-Raf(V600E) mutated enzyme has been observed in a number of human tumors, including melanomas. Herein we report the discovery and biological evaluation of GSK2118436, a selective inhibitor of Raf kinases with potent in vitro activity in oncogenic B-Raf-driven melanoma and colorectal carcinoma cells and robust in vivo antitumor and pharmacodynamic activity in mouse models of B-Raf(V600E) human melanoma. GSK2118436 was identified as a development candidate, and early clinical results have shown significant activity in patients with B-Raf mutant melanoma.

Abstract B88: A selective Raf kinase inhibitor induces cell death and tumor regression of human cancer cell lines encoding B‐RafV600E mutation

Activation of the Ras‐Raf‐MEK‐ERK pathway has been implicated in a large range of human cancers. Growth factor receptor stimulation by extracellular ligands activates Ras, which then sets in motion a signal transduction cascade through the Raf, MEK and ERK serine/threonine kinases. Mutation of the B‐Raf kinase constitutively activates MAPK signalling, thus bypassing the need for upstream stimuli. This has been genetically associated with several human cancers, especially occurrence of the B‐Raf V600E mutant and its high prevalence in melanoma, colorectal carcinoma, ovarian cancer, papillary thyroid carcinoma, and cholangiocarcinoma. The ability to selectively and potently inhibit B‐Raf should provide a potential therapy for patients with mutant B‐Raf tumors, for which addictive dependency on this pathway is observed. We have identified a novel, potent, and selective Raf kinase inhibitor that is capable of inhibiting the kinase activity of wild‐type B‐Raf, B‐Raf V600E and c‐Raf with IC 50 values of 3.2, 0.8, and 5.0 nM, respectively. Kinase panel screening for over 270 kinases has indicated that this inhibitor is selective for Raf kinase, with ∼400 fold selectivity towards B‐Raf over 91% of the other kinases tested. Specific cellular inhibition of B‐Raf V600E kinase by this inhibitor leads to decreased ERK phosphorylation and inhibition of cell proliferation by an initial arrest in the G1 phase of the cell cycle, followed by cell death. This inhibition is selective for cancer cells that specifically encode the mutation for B‐Raf V600E . Oral compound administration inhibits the growth of B‐Raf V600E mutant melanoma (A375P) and colon cancer (Colo205) human tumor xenografts, growing subcutaneously in immuno‐compromised mice. This cell‐specific B‐Raf V600E inhibitor is currently being evaluated in a human Phase I clinical trial. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B88.

Development of potent B-RafV600E inhibitors containing an arylsulfonamide headgroup.

A potent series of inhibitors against the B-Raf(V600E) kinase have been developed that show excellent activity in cellular assays and good oral bioavailability in rats. The key structural features of the series are an arylsulfonamide headgroup, a thiazole core, and a fluorine ortho to the sulfonamide nitrogen.

A Robust, Target-Driven, Cell-Based Assay for Checkpoint Kinase 1 Inhibitors

Checkpoint kinase 1 (Chk1), a serine/threonine kinase, plays an important role in DNA damage checkpoint control and is an attractive target for cancer treatment. To develop a Chk1-specific cell-based assay, stable clones were established in which Chk1 kinase domain fused at its N-terminus with p53 through 4 tandem repeats of Gly-Gly-Gly-Gly-Ser was expressed in an inducible manner. Chk1 kinase specificity of the phosphorylation of fused p53 was confirmed by the experiments with a kinase-inactive Chk1. Only in the presence of an inducer molecule was phosphorylation of p53 at Ser-15 in the stable clones induced. Furthermore, its assay performance proved acceptable for high-throughput screening applications, judging from the Z′ factor values (> 0.77). Finally, the cell-based assay thus established yielded structure-activity relationship data for a small set of test inhibitors of Chk1 within cells. Collectively, these results demonstrate that the established cell-based assay provides a novel and highly sensitive cellular platform for Chk1 inhibitor discovery. (Journal of Biomolecular Screening 2007:809-817)