Antitumor activity of BRAF inhibitor vemurafenib in preclinical models of BRAF-mutant colorectal cancer

The protein kinase BRAF is a key component of the RAS-RAF signaling pathway which plays an important role in regulating cell proliferation, differentiation, and survival. Mutations in BRAF at codon 600 promote catalytic activity and are associated with 8% of all human (solid) tumors, including 8% to 10% of colorectal cancers (CRC). Here, we report the preclinical characterization of vemurafenib (RG7204; PLX4032; RO5185426), a first-in-class, specific small molecule inhibitor of BRAF(V600E) in BRAF-mutated CRC cell lines and tumor xenograft models. As a single agent, vemurafenib shows dose-dependent inhibition of ERK and MEK phosphorylation, thereby arresting cell proliferation in BRAF(V600)-expressing cell lines and inhibiting tumor growth in BRAF(V600E) bearing xenograft models. Because vemurafenib has shown limited single-agent clinical activity in BRAF(V600E)-mutant metastatic CRC, we therefore explored a range of combination therapies, with both standard agents and targeted inhibitors in preclinical xenograft models. In a BRAF-mutant CRC xenograft model with de novo resistance to vemurafenib (RKO), tumor growth inhibition by vemurafenib was enhanced by combining with an AKT inhibitor (MK-2206). The addition of vemurafenib to capecitabine and/or bevacizumab, cetuximab and/or irinotecan, or erlotinib resulted in increased antitumor activity and improved survival in xenograft models. Together, our findings suggest that the administration of vemurafenib in combination with standard-of-care or novel targeted therapies may lead to enhanced and sustained clinical antitumor efficacy in CRCs harboring the BRAF(V600E) mutation.     

Resistance to selective BRAF inhibition can be mediated by modest upstream pathway activation

A high percentage of patients with BRAF(V600E) mutant melanomas respond to the selective RAF inhibitor vemurafenib (RG7204, PLX4032) but resistance eventually emerges. To better understand the mechanisms of resistance, we used chronic selection to establish BRAF(V600E) melanoma clones with acquired resistance to vemurafenib. These clones retained the V600E mutation and no second-site mutations were identified in the BRAF coding sequence. Further characterization showed that vemurafenib was not able to inhibit extracellular signal-regulated kinase phosphorylation, suggesting pathway reactivation. Importantly, resistance also correlated with increased levels of RAS-GTP, and sequencing of RAS genes revealed a rare activating mutation in KRAS, resulting in a K117N change in the KRAS protein. Elevated levels of CRAF and phosphorylated AKT were also observed. In addition, combination treatment with vemurafenib and either a MAP/ERK kinase (MEK) inhibitor or an AKT inhibitor synergistically inhibited proliferation of resistant cells. These findings suggest that resistance to BRAF(V600E) inhibition could occur through several mechanisms, including elevated RAS-GTP levels and increased levels of AKT phosphorylation. Together, our data implicate reactivation of the RAS/RAF pathway by upstream signaling activation as a key mechanism of acquired resistance to vemurafenib, in support of clinical studies in which combination therapy with other targeted agents are being strategized to combat resistance.     

New therapeutic strategies for BRAF mutant colorectal cancers

Oncogenic BRAF mutations are found in ~10% of colorectal cancers (CRCs) and predict poor prognosis. Although BRAF inhibitors have demonstrated striking efficacy in BRAF mutant melanomas, BRAF inhibitor monotherapy is ineffective in BRAF mutant CRC. Over the past few years, studies have begun to define the molecular mechanisms underlying the relative resistance of BRAF mutant CRC to BRAF inhibitors, leading to the development of novel therapeutic strategies that are showing promising clinical activity in initial clinical trials. Our current understanding of the mechanisms of BRAF inhibitor resistance in BRAF mutant CRC and the therapeutic approaches currently in clinical trials for BRAF mutant CRC are reviewed herein.     

p53 Reactivation by PRIMA-1Met (APR-246) sensitises V600E/KBRAF melanoma to vemurafenib

Intrinsic and acquired resistance of metastatic melanoma to ^V600E/KBRAF and/or MEK inhibitors, which is often caused by activation of the PI3K/AKT survival pathway, represents a major clinical challenge. Given that p53 is capable of antagonising PI3K/AKT activation we hypothesised that pharmacological restoration of p53 activity may increase the sensitivity of BRAF-mutant melanoma to MAPK-targeted therapy and eventually delay and/or prevent acquisition of drug resistance. To test this possibility we exposed a panel of vemurafenib-sensitive and resistant (innate and acquired) ^V600E/KBRAF melanomas to a ^V600E/KBRAF inhibitor (vemurafenib) alone or in combination with a direct p53 activator (PRIMA-1^Met/APR-246). Strikingly, PRIMA-1^Met synergised with vemurafenib to induce apoptosis and suppress proliferation of ^V600E/KBRAF melanoma cells in vitro and to inhibit tumour growth in vivo. Importantly, this drug combination decreased the viability of both vemurafenib-sensitive and resistant melanoma cells irrespectively of the TP53 status. Notably, p53 reactivation was invariably accompanied by PI3K/AKT pathway inhibition, the activity of which was found as a dominant resistance mechanism to BRAF inhibition in our lines. From all various combinatorial modalities tested, targeting the MAPK and PI3K signalling pathways through p53 reactivation or not, the PRIMA-1^Met/vemurafenib combination was the most cytotoxic. We conclude that PRIMA-1^Met through its ability to directly reactivate p53 regardless of the mechanism causing its deactivation, and thereby dampen PI3K signalling, sensitises ^V600E/KBRAF-positive melanoma to BRAF inhibitors.     

Protein kinase D3 sensitizes RAF inhibitor RAF265 in melanoma cells by preventing reactivation of MAPK signaling

RAS mutations occur in more than 30% of all human cancers but efforts to directly target mutant RAS signaling as a cancer therapy have yet to succeed. As alternative strategies, RAF and MEK inhibitors have been developed to block oncogenic signaling downstream of RAS. As might be expected, studies of these inhibitors have indicated that tumors with RAS or BRAF mutations display resistance RAF or MEK inhibitors. In order to better understand the mechanistic basis for this resistance, we conducted a RNAi-based screen to identify genes that mediated chemoresistance to the RAF kinase inhibitor RAF265 in a BRAF (V600E) mutant melanoma cell line that is resistant to this drug. In this way, we found that knockdown of protein kinase D3 (PRKD3) could enhance cell killing of RAF and MEK inhibitors across multiple melanoma cell lines of various genotypes and sensitivities to RAF265. PRKD3 blockade cooperated with RAF265 to prevent reactivation of the MAPK signaling pathway, interrupt cell cycle progression, trigger apoptosis, and inhibit colony formation growth. Our findings offer initial proof-of-concept that PRKD3 is a valid target to overcome drug resistance being encountered widely in the clinic with RAF or MEK inhibitors.     

Mitochondrial oxidative stress is the achille's heel of melanoma cells resistant to Braf-mutant inhibitor

Vemurafenib/PLX4032, a selective inhibitor of mutant BRAFV600E, constitutes a paradigm shift in melanoma therapy. Unfortunately, acquired resistance, which unavoidably occurs, represents one major limitation to clinical responses. Recent studies have highlighted that vemurafenib activated oxidative metabolism in BRAFV600E melanomas expressing PGC1α. However, the oxidative state of melanoma resistant to BRAF inhibitors is unknown. We established representative in vitro and in vivo models of human melanoma resistant to vemurafenib including primary specimens derived from melanoma patients. Firstly, our study reveals that vemurafenib increased mitochondrial respiration and ROS production in BRAFV600E melanoma cell lines regardless the expression of PGC1α. Secondly, melanoma cells that have acquired resistance to vemurafenib displayed intrinsically high rates of mitochondrial respiration associated with elevated mitochondrial oxidative stress irrespective of the presence of vemurafenib. Thirdly, the elevated ROS level rendered vemurafenib-resistant melanoma cells prone to cell death induced by pro-oxidants including the clinical trial drug, elesclomol. Based on these observations, we propose that the mitochondrial oxidative signature of resistant melanoma constitutes a novel opportunity to overcome resistance to BRAF inhibition.     

BRAF Inhibitor Vemurafenib Improves the Antitumor Activity of Adoptive Cell Immunotherapy

Combining immunotherapy with targeted therapy blocking oncogenic BRAF(V600) may result in improved treatments for advanced melanoma. In this study, we developed a BRAF(V600E)-driven murine model of melanoma, SM1, which is syngeneic to fully immunocompetent mice. SM1 cells exposed to the BRAF inhibitor vemurafenib (PLX4032) showed partial in vitro and in vivo sensitivity resulting from the inhibition of MAPK pathway signaling. Combined treatment of vemurafenib plus adoptive cell transfer therapy with lymphocytes genetically modified with a T-cell receptor (TCR) recognizing chicken ovalbumin (OVA) expressed by SM1-OVA tumors or pmel-1 TCR transgenic lymphocytes recognizing gp100 endogenously expressed by SM1 resulted in superior antitumor responses compared with either therapy alone. T-cell analysis showed that vemurafenib did not significantly alter the expansion, distribution, or tumor accumulation of the adoptively transferred cells. However, vemurafenib paradoxically increased mitogen-activated protein kinase (MAPK) signaling, in vivo cytotoxic activity, and intratumoral cytokine secretion by adoptively transferred cells. Taken together, our findings, derived from 2 independent models combining BRAF-targeted therapy with immunotherapy, support the testing of this therapeutic combination in patients with BRAF(V600) mutant metastatic melanoma. Cancer Res; 72(16); 3928-37. ?2012 AACR.     

Oncogenic activation of the RAS/RAF signaling pathway impairs the response of metastatic colorectal cancers to anti-epidermal growth factor receptor antibody therapies

Monoclonal antibodies (mAbs) against the extracellular domain of the epidermal growth factor receptor (EGFR) have been introduced for the treatment of metastatic colorectal cancer (mCRC). We have reported recently that increased copy number of the EGFR can predict response to anti-EGFR mAbs and that patients might be selected for treatment based on EGFR copy number. Here, we show that mutations activating the RAS/RAF signaling pathway are also predictive and prognostic indicators in mCRC patients, being inversely correlated with response to anti-EGFR mAbs. In cellular models of CRCs, activation of the RAS signaling pathway by introduction of an activated K-RAS allele (Gly(12)Val) impairs the therapeutic effect of anti-EGFR mAbs. In cancer cells carrying constitutively active RAS, the pharmacologic inhibition of the mitogen-activated protein kinase (MAPK) signaling cascade improves anti-EGFR treatment based on mAbs. These results have implications for the identification of patients who are likely to respond to anti-EGFR treatment. They also provide the rationale for combination therapies, targeted simultaneously to the EGFR and RAS/RAF/MAPK signaling pathways in CRC patients.     

Overexpression of miR-145 increases the sensitivity of vemurafenib in drug-resistant colo205 cell line

Vemurafenib is a selective and potent small molecule inhibitor of the V600 mutant form of the BRAF protein used in the treatment of melanoma and colorectal cancer. However, vemurafenib has less effect in BRAF mutant colorectal cancer due to the resistance of tumor cell to vemurafenib. To verify whether or not miR-145, a short RNA molecule of microRNA which has been supposed to be a tumor suppressor, is involved in this process, we established vemurafenib-resistant cell line colo205/V and found that the miR-145 expression was significantly downregulated in colo205/V cells compared to normal colo205 cells. Moreover, the overexpression of miR-145 could increase the sensitivity of colo205/V cells to vemurafenib both in vitro and in vivo. In conclusion, miR-145 might be used as a therapeutic target in the treatment of colorectal cancer patients with BRAF V600E mutation.     

BRAF and RAS mutations in human lung cancer and melanoma

BRAF encodes a RAS-regulated kinase that mediates cell growth and malignant transformation kinase pathway activation. Recently, we have identified activating BRAF mutations in 66% of melanomas and a smaller percentage of many other human cancers. To determine whether BRAF mutations account for the MAP kinase pathway activation common in non-small cell lung carcinomas (NSCLCs) and to extend the initial findings in melanoma, we screened DNA from 179 NSCLCs and 35 melanomas for BRAF mutations (exons 11 and 15). We identified BRAF mutations in 5 NSCLCs (3%; one V599 and four non-V599) and 22 melanomas (63%; 21 V599 and 1 non-V599). Three BRAF mutations identified in this study are novel, altering residues important in AKT-mediated BRAF phosphorylation and suggesting that disruption of AKT-induced BRAF inhibition can play a role in malignant transformation. To our knowledge, this is the first report of mutations documenting this interaction in human cancers. Although >90% of BRAF mutations in melanoma involve codon 599 (57 of 60), 8 of 9 BRAF mutations reported to date in NSCLC are non-V599 (89%; P < 10(-7)), strongly suggesting that BRAF mutations in NSCLC are qualitatively different from those in melanoma; thus, there may be therapeutic differences between lung cancer and melanoma in response to RAF inhibitors. Although uncommon, BRAF mutations in human lung cancers may identify a subset of tumors sensitive to targeted therapy.     

Vemurafenib and BRAF inhibition: a new class of treatment for metastatic melanoma

The U.S. Food and Drug Administration recently approved vemurafenib for the treatment of BRAF valine in exon 15, at codon 600 (V600E) mutant metastatic melanoma. Vemurafenib is a competitive small-molecule serine-threonine kinase inhibitor that functions by binding to the ATP-binding domain of mutant BRAF. Compared with dacarbazine chemotherapy, vemurafenib significantly improved the 6-month overall survival of patients from 64% to 84% and exhibited a response rate of approximately 50%. Median progression-free survival was also significantly improved with vemurafenib as compared with dacarbazine (5.3 versus 1.6 months, respectively), and this was consistent among groups analyzed, including age, sex, geography, Eastern Cooperative Oncology Group status, disease stage, and serum lactate dehydrogenase. The success of targeting melanoma genomics has created a paradigm shift for future drug development. Currently, the elucidation of resistant mechanisms to vemurafenib therapy remains an important area of active investigation that will shape rational drug treatments for melanoma. The development of vemurafenib, the role of BRAF targeting, and the changing landscape of treatment for melanoma provide a new foundation for clinical investigation.     

Targeting TRAP1 as a downstream effector of BRAF cytoprotective pathway: A novel strategy for human BRAF-driven colorectal carcinoma

The HSP90 chaperone TRAP1 is translational regulator of BRAF synthesis/ubiquitination, since BRAF down-regulation, ERK signaling inhibition and delay of cell cycle progression occur upon TRAP1 silencing/inhibition. Since TRAP1 is upregulated in human colorectal carcinomas (CRCs) and involved in protection from apoptosis and as human BRAF-driven CRCs are poorly responsive to anticancer therapies, the relationship between TRAP1 regulation of mitochondrial apoptotic pathway and BRAF antiapoptotic signaling has been further evaluated. This study reports that BRAF cytoprotective signaling involves TRAP1-dependent inhibition of the mitochondrial apoptotic pathway. It is worth noting that BRAF and TRAP1 interact and that the activation of BRAF signaling results in enhanced TRAP1 serine-phosphorylation, a condition associated with resistance to apoptosis. Consistently, a BRAF dominant-negative mutant prevents TRAP1 serine phosphorylation and restores drug sensitivity in BRAFV600E CRC drug-resistant cells with high TRAP1 levels. In addition, TRAP1 targeting by the mitochondria-directed HSP90 chaperones inhibitor gamitrinib induces apoptosis and inhibits colony formation in BRAF-driven CRC cells. Thus, TRAP1 is a downstream effector of BRAF cytoprotective pathway in mitochondria and TRAP1 targeting may represent a novel strategy to improve the activity of proapoptotic agents in BRAF-driven CRC cells.     

Development of a potent small‐molecule degrader against oncogenic BRAFV600E protein that evades paradoxical MAPK activation

BRAF mutations are frequently observed in melanoma and hairy‐cell leukemia. Currently approved rapidly accelerated fibrosarcoma (RAF) kinase inhibitors targeting oncogenic BRAF V600 mutations have shown remarkable efficacy in the clinic, but their therapeutic benefits are occasionally hampered by acquired resistance due to RAF dimerization–dependent reactivation of the downstream MAPK pathway, which is known as paradoxical activation. There is also a concern that paradoxical activation of the MAPK pathway may trigger secondary cancer progression. In this study, we developed chimeric compounds, proteolysis targeting chimeras (PROTACs), that target BRAF^V600E protein for degradation. CRBN(BRAF)‐24, the most effective chimera, potently degraded BRAF^V600E in a ubiquitin‐proteasome system (UPS)‐dependent manner and inhibited the proliferation of BRAF^V600E‐driven cancer cells. In BRAF wild‐type cells, CRBN(BRAF)‐24 induced neither BRAF^WT degradation nor paradoxical activation of the MAPK pathway. Biochemical analysis revealed that CRBN(BRAF)‐24 showed more potent and sustained suppression of MAPK signaling than a BRAF^V600E inhibitor, PLX‐8394, in BRAF^V600E‐driven cancer cells. Targeted degradation of BRAF^V600E by CRBN(BRAF)‐24 could be a promising strategy to evade paradoxical activation of the RAF‐MAPK pathway.     

MAPK pathway in melanoma part II—secondary and adaptive resistance mechanisms to BRAF inhibition

BRAF mutation can be identified in about 45% of the patients with metastatic melanoma. In these patients, BRAF and MEK inhibitors are able to induce rapid responses and to prolong survival. However, a significant percentage of patients will develop resistance to targeted therapy and will have progressive disease.MAPK pathway is the most important pathway involved in BRAF/MEK inhibition resistance, particularly MAPK pathway reactivation.Resistance mechanisms can be classified as 1) primary or intrinsic characterised by no response to therapy, 2) secondary or acquired with MAPK pathway reactivation after a time of tumour regression and 3) as adaptive with initial response and early resistance.BRAF inhibition also alters the immune response. Several publications have described immune effects of BRAF inhibition in melanoma tumours, showing that combining targeted and immunotherapy can improve response, despite a possible cross-resistance.Here, we continue the review on resistance mechanisms to BRAF/MEK inhibition and focus on the secondary and adaptive mechanisms.     

Colorectal cancer with mutation in BRAF, KRAS, and wild-type with respect to both oncogenes showing different patterns of DNA methylation.

PURPOSE: BRAF mutations are common in sporadic colorectal cancers (CRCs) with a DNA mismatch repair (MMR) deficiency that results from promoter methylation of hMLH1, whereas KRAS mutations are common in MMR proficient CRCs associated with promoter methylation of MGMT. The aim of this study was to further investigate the link between genetic alterations in the RAS/RAF/ERK pathway and an underlying epigenetic disorder. PATIENTS AND METHODS: Activating mutations of BRAF and KRAS were identified and correlated with promoter methylation of 11 loci, including MINT1, MINT2, MINT31, CACNA1G, p16(INK4a), p14(ARF), COX2, DAPK, MGMT, and the two regions in hMLH1 in 468 CRCs and matched normal mucosa. RESULTS: BRAF V599E mutations were identified in 21 (9%) of 234 CRCs, and KRAS mutations were identified in 72 (31%) of 234 CRCs. Mutations in BRAF and KRAS were never found in the same tumor. CRCs with BRAF mutations showed high-level promoter methylation in multiple loci, with a mean number of methylated loci of 7.2 (95% CI, 6.6 to 7.9) among 11 loci examined (P < .0001). Tumors with KRAS mutations showed low-level promoter methylation, and CRCs with neither mutation showed a weak association with promoter methylation, with an average number of methylated loci of 1.8 (95% CI, 1.5 to 2.1) and 1.0 (95% CI, 0.79 to 1.3), respectively. CONCLUSION: In CRC, the methylation status of multiple promoters can be predicted through knowledge of BRAF and, to a lesser extent, KRAS activating mutations, indicating that these mutations are closely associated with different patterns of DNA hypermethylation. These changes may be important events in colorectal tumorigenesis.     

Vemurafenib in combination with cobimetinib in relapsed and refractory extramedullary multiple myeloma harboring the BRAF V600E mutation

BRAF mutations are present in a variety of cancers and cause constitutive activation of the Ras‐Raf–MEK‐ERK signaling pathway. In cutaneous malignant melanoma, combined treatment with BRAF and MEK inhibitors is associated with high response rates and has been shown to improve progression free as well as overall survival compared to BRAF inhibition alone. In multiple myeloma, BRAF mutations are detectable only in a minority of patients. Only few data are available regarding the clinical activity of BRAF inhibitors in BRAF‐positive multiple myeloma patients, including some anecdotal reports on remarkable responses in individuals being resistant to all other available anti‐myeloma treatment approaches. We here present the first report on the combination of vemurafenib and cobimetinib in a young patient with highly resistant and rapidly progressing multiple myeloma harboring the BRAF V600E mutation who achieved a rapid and sustained response to this combination therapy.     

Crosstalk signaling in targeted melanoma therapy

Inhibition of the BRAF/MAPK pathway belongs to the standard therapies for patients with activating BRAF^V600E/K mutations. However, even in well-responding tumors, anti-tumorigenic effect and clinical benefit are only transient, and the original tumors often relapse. This demonstrates that there are remaining residual tumors, which have withstood therapy-induced apoptosis and which have the potential to resume growth. Although BRAF mutant melanoma cells seem to depend on BRAF/MAPK signaling, the inhibition of this pathway triggers several events, which modulate the tumor as well as the tumor niche. After a certain adaptation period, this can turn out to be beneficial for tumor growth and metastasis—even in cases of good initial tumor response. This review sheds light on the biology of BRAF/MEK inhibitor-sensitive melanoma cells, which survive targeted therapy and will address the crosstalk signaling events occurring in BRAF mutant melanomas when the BRAF/MAPK pathway is fully blocked. The knowledge of these events is important for potential future drug combinations, which enhance the inhibitory effect of BRAF/MEK inhibition, particularly in patients not eligible for immune therapy.     

Detection of BRAF gene mutation in primary choroidal melanoma tissue

Numerous BRAF mutations have been detected in melanoma biopsy specimens and cell lines. In contrast, several studies report lack of BRAF mutations in uveal melanoma including primary and metastatic choroidal and ciliary body melanomas. To our knowledge, for the first time, here we report a case of choroidal melanoma harboring the BRAF mutation (V600E). The activation of RAF/MEK/ERK pathway, although independent of BRAF mutation, was reported in uveal melanoma. The presence of V600E mutation indicates that the RAF/MEK/ERK pathway, in addition to cutaneous melanoma progression, may play a role in the choroidal melanoma development.     

Oligonucleotide microarray analysis of distinct gene expression patterns in colorectal cancer tissues harboring BRAF and K-ras mutations

Various types of human cancers harbor BRAF somatic mutations, leading researchers to seek molecular targets for BRAF inhibitors. A mutually exclusive relationship has been observed between the BRAF-V600E mutation and K-ras mutations, suggesting that the BRAF-V600E mutation may differ from the other BRAF mutant types. Here, we used microarray analysis to examine differences between the BRAF and K-ras mutant colorectal samples and within the BRAF group (V600E versus non-V600E), in the hope that the identified gene sets could form the basis for new target development. Eleven colorectal cancers (CRCs) with BRAF mutations and nine with K-ras mutations were examined by high-density microarray analysis. We also tested whether other significant genetic or clinical status involved in CRC development, such as APC and TP53 mutations, MSI and TNM-Duke's staging, were related with the observed BRAF- or K-ras associated expression profiles. Unsupervised two-way hierarchical clustering and multidimensional scaling revealed that the differentially expressed genes clustered according to the mutation status of BRAF and K-ras, and that samples with the BRAF-V600E and non-V600E mutants could be distinguished from each other by gene profiling. Examination of TNM-Duke's staging, MSI and mutations in APC and TP53 revealed that these significant mutations could not account for the hierarchical clustering results observed in our study. We herein identified distinct gene expression patterns and gene sets that may form the basis for identification of BRAF-targeting molecules or provide researchers with a better understanding of the molecular pathogenesis underlying RAS-RAF signaling.