Identification of the MEK1(F129L) activating mutation as a potential mechanism of acquired resistance to MEK inhibition in human cancers carrying the B-RafV600E mutation

MEK inhibitors as a chemotherapeutic intervention in multiple myeloma

The Ras/Raf/MEK/extracellular signal regulated kinase (ERK) (Ras/mitogen-activated protein kinases (MAPK)) signal transduction pathway is a crucial mediator of many fundamental biological processes, including cellular proliferation, survival, angiogenesis and migration. Aberrant signalling through the Ras/MAPK cascade is common in a wide array of malignancies, including multiple myeloma (MM), making it an appealing candidate for the development of novel targeted therapies. In this review, we explore our current understanding of the Ras/MAPK pathway and its role in MM. Additionally, we summarise the current status of small molecule inhibitors of MEK under clinical evaluation, and discuss future approaches required to optimise their use.

Mechanisms of acquired resistance to ERK1/2 pathway inhibitors

The ERK1/2 (extracellular signal-regulated kinase 1 and 2) pathway, comprising the protein kinases RAF (v-raf-1 murine leukemia viral oncogene homolog 1), MEK1/2 (mitogen-activated protein kinase or ERK kinase 1 and 2) and ERK1/2 is frequently de-regulated in human cancers, due to mutations in RAS or BRAF (v-raf-1 murine leukemia viral oncogene homolog B1). New, highly selective inhibitors of BRAF and MEK1/2 have shown promise in clinical trials, including in previously intractable diseases such as melanoma. However, drug-resistant tumour cells invariably emerge leading to disease progression. It is important to understand the mechanisms underlying such acquired resistance since this may lead to the development of rational strategies either to delay its onset or to overcome it once established. It also offers unique insights into the plasticity of signalling pathways, which may in turn inform our understanding of the basic biology of these pathways and lead to the validation of new drug targets. Several recent reports have identified diverse mechanisms of acquired resistance to MEK1/2 or BRAF inhibitors. In this article, we review these studies, discuss the different mechanisms, identify common themes and consider their therapeutic implications.

Integrating new therapies in the treatment of advanced melanoma

Treatments for advanced melanoma have evolved rapidly based on improved understanding of the pathways that determine T-cell responses and knowledge of growth-related mutations, which can be targeted with new classes of pharmacologic agents. The FDA approved ipilimumab and vemurafenib for advanced melanoma in 2011. Our practice is to evaluate all tumors from patients with metastatic disease for the presence of a BRAF mutation (Fig. 1). More than 20 years of follow-up show that responders to IL-2 can be cured of their melanoma. Therefore, we recommend high-dose IL-2 as first line therapy for patients with excellent functional status and normal cardiopulmonary reserve regardless of their BRAF mutation status. We use ipilimumab, which can induce durable tumor regressions and improved survival, as initial therapy for patients who refuse or are not candidates for IL-2, also regardless of their BRAF mutation status. Ipilimumab can be used as salvage therapy for patients with advanced disease after IL-2 or vemurafenib. Targeted therapies such as vemurafenib or imatinib can be offered to patients whose melanomas express the BRAF V600E or C-Kit mutations. Vemurafenib is particularly useful for patients whose disease is progressing rapidly, as clinical improvement can be obtained within days of starting therapy and response rates may be as high as 70 %. The major reason we do not recommend vemurafenib as first line treatment in all patients whose tumors have BRAF mutations is the short median duration of response of approximately 7 months. Enrollment in a clinical trial should always be considered for patients with metastatic melanoma. The clinical trial focus has changed from finding any agent with activity in melanoma, to overcoming mechanisms of resistance and enhancing the immunomodulatory activity of these new agents that confer therapeutic benefit. Selected patients can benefit from surgical resection or radiation to manage oligometastatic disease.

Mechanisms of acquired resistance to targeted cancer therapies

Drugs that target genomically defined vulnerabilities in human tumors have now been clinically validated as effective cancer therapies. However, the relatively rapid acquisition of resistance to such treatments that is observed in virtually all cases significantly limits their utility and remains a substantial challenge to the clinical management of advanced cancers. As molecular mechanisms of resistance have begun to be elucidated, new strategies to overcome or prevent the development of resistance have begun to emerge. In some cases, specific mutational mechanisms contribute directly to acquired drug resistance, and in other cases it appears that nonmutational and possibly epigenetic mechanisms play a significant role. This article discusses the various genetic and nongenetic mechanisms of acquired drug resistance that have been reported in the context of 'rationally targeted' drug therapies.

Drug targets and predictive biomarkers in the management of metastatic melanoma

Melanoma is the leading cause of fatal skin cancer, and in the past few decades, there has been an increase in the incidence of and mortality from metastatic melanoma. Until recently, the therapeutic options for treatment of metastatic melanoma were limited. The approval of ipilimumab (an anti-CTLA-4 antibody) and vemurafenib (mutant B-RAF^V600E kinase inhibitor) by the Federal Drug Administration has led to a new era in melanoma treatment, and additional promising drugs and drug combinations are currently being investigated. As the choices of treatment for melanoma have expanded, the need to identify predictive biomarkers to tailor treatment strategies to individual tumor or immune system characteristics has become necessary. Such strategies have the potential of maximizing antitumor effect while minimizing toxicity and improving clinical benefit. In this article, we review the currently approved targeted therapies in melanoma and discuss the future of personalized therapy for this disease.

BRAFV600E negatively regulates the AKT pathway in melanoma cell lines

Cross-feedback activation of MAPK and AKT pathways is implicated as a resistance mechanism for cancer therapeutic agents targeting either RAF/MEK or PI3K/AKT/mTOR. It is thus important to have a better understanding of the molecular resistance mechanisms to improve patient survival benefit from these agents. Here we show that BRAFV600E is a negative regulator of the AKT pathway. Expression of BRAFV600E in NIH3T3 cells significantly suppresses MEK inhibitor (RG7167) or mTORC1 inhibitor (rapamycin) induced AKT phosphorylation (pAKT) and downstream signal activation. Treatment-induced pAKT elevation is found in BRAF wild type melanoma cells but not in a subset of melanoma cell lines harboring BRAFV600E. Knock-down of BRAFV600E in these melanoma cells elevates basal pAKT and downstream signals, whereas knock-down of CRAF, MEK1/2 or ERK1/2 or treatment with a BRAF inhibitor have no impact on pAKT. Mechanistically, we show that BRAFV600E interacts with rictor complex (mTORC2) and regulates pAKT through mTORC2. BRAFV600E is identified in mTORC2 after immunoprecipitation of rictor. Knock-down of rictor abrogates BRAFV600E depletion induced pAKT. Knock-down of BRAFV600E enhances cellular enzyme activity of mTORC2. Aberrant activation of AKT pathway by PTEN loss appears to override the negative impact of BRAFV600E on pAKT. Taken together, our findings suggest that in a subset of BRAFV600E melanoma cells, BRAFV600E negatively regulates AKT pathway in a rictor-dependent, MEK/ERK and BRAF kinase-independent manner. Our study reveals a novel molecular mechanism underlying the regulation of feedback loops between the MAPK and AKT pathways.

Tumour micro-environment elicits innate resistance to RAF inhibitors through HGF secretion

Drug resistance presents a challenge to the treatment of cancer patients. Many studies have focused on cell-autonomous mechanisms of drug resistance. By contrast, we proposed that the tumour micro-environment confers innate resistance to therapy. Here we developed a co-culture system to systematically assay the ability of 23 stromal cell types to influence the innate resistance of 45 cancer cell lines to 35 anticancer drugs. We found that stroma-mediated resistance is common, particularly to targeted agents. We characterized further the stroma-mediated resistance of BRAF-mutant melanoma to RAF inhibitors because most patients with this type of cancer show some degree of innate resistance. Proteomic analysis showed that stromal cell secretion of hepatocyte growth factor (HGF) resulted in activation of the HGF receptor MET, reactivation of the mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-OH kinase (PI(3)K)-AKT signalling pathways, and immediate resistance to RAF inhibition. Immunohistochemistry experiments confirmed stromal cell expression of HGF in patients with BRAF-mutant melanoma and showed a significant correlation between HGF expression by stromal cells and innate resistance to RAF inhibitor treatment. Dual inhibition of RAF and either HGF or MET resulted in reversal of drug resistance, suggesting RAF plus HGF or MET inhibitory combination therapy as a potential therapeutic strategy for BRAF-mutant melanoma. A similar resistance mechanism was uncovered in a subset of BRAF-mutant colorectal and glioblastoma cell lines. More generally, this study indicates that the systematic dissection of interactions between tumours and their micro-environment can uncover important mechanisms underlying drug resistance.

ERK inhibition overcomes acquired resistance to MEK inhibitors

The RAS/RAF/MEK pathway is activated in more than 30% of human cancers, most commonly via mutation in the K-ras oncogene and also via mutations in BRAF. Several allosteric mitogen-activated protein/extracellular signal-regulated kinase (MEK) inhibitors, aimed at treating tumors with RAS/RAF pathway alterations, are in clinical development. However, acquired resistance to these inhibitors has been documented both in preclinical and clinical samples. To identify strategies to overcome this resistance, we have derived three independent MEK inhibitor-resistant cell lines. Resistance to allosteric MEK inhibitors in these cell lines was consistently linked to acquired mutations in the allosteric binding pocket of MEK. In one cell line, concurrent amplification of mutant K-ras was observed in conjunction with MEK allosteric pocket mutations. Clonal analysis showed that both resistance mechanisms occur in the same cell and contribute to enhanced resistance. Importantly, in all cases the MEK-resistant cell lines retained their addiction to the mitogen-activated protein kinase (MAPK) pathway, as evidenced by their sensitivity to a selective inhibitor of the ERK1/2 kinases. These data suggest that tumors with acquired MEK inhibitor resistance remain dependent on the MAPK pathway and are therefore sensitive to inhibitors that act downstream of the mutated MEK target. Importantly, we show that dual inhibition of MEK and ERK by small molecule inhibitors was synergistic and acted to both inhibit the emergence of resistance, as well as to overcome acquired resistance to MEK inhibitors. Therefore, our data provide a rationale for cotargeting multiple nodes within the MAPK signaling cascade in K-ras mutant tumors to maximize therapeutic benefit for patients.

Combinations of BRAF, MEK, and PI3K/mTOR inhibitors overcome acquired resistance to the BRAF inhibitor GSK2118436 dabrafenib, mediated by NRAS or MEK mutations

Recent results from clinical trials with the BRAF inhibitors GSK2118436 (dabrafenib) and PLX4032 (vemurafenib) have shown encouraging response rates; however, the duration of response has been limited. To identify determinants of acquired resistance to GSK2118436 and strategies to overcome the resistance, we isolated GSK2118436 drug-resistant clones from the A375 BRAF(V600E) and the YUSIT1 BRAF(V600K) melanoma cell lines. These clones also showed reduced sensitivity to the allosteric mitogen-activated protein/extracellular signal-regulated kinase (MEK) inhibitor GSK1120212 (trametinib). Genetic characterization of these clones identified an in-frame deletion in MEK1 (MEK1(K59del)) or NRAS mutation (NRAS(Q61K) and/or NRAS(A146T)) with and without MEK1(P387S) in the BRAF(V600E) background and NRAS(Q61K) in the BRAF(V600K) background. Stable knockdown of NRAS with short hairpin RNA partially restored GSK2118436 sensitivity in mutant NRAS clones, whereas expression of NRAS(Q61K) or NRAS(A146T) in the A375 parental cells decreased sensitivity to GSK2118436. Similarly, expression of MEK1(K59del), but not MEK1(P387S), decreased sensitivity of A375 cells to GSK2118436. The combination of GSK2118436 and GSK1120212 effectively inhibited cell growth, decreased ERK phosphorylation, decreased cyclin D1 protein, and increased p27(kip1) protein in the resistant clones. Moreover, the combination of GSK2118436 or GSK1120212 with the phosphoinositide 3-kinase/mTOR inhibitor GSK2126458 enhanced cell growth inhibition and decreased S6 ribosomal protein phosphorylation in these clones. Our results show that NRAS and/or MEK mutations contribute to BRAF inhibitor resistance in vitro, and the combination of GSK2118436 and GSK1120212 overcomes this resistance. In addition, these resistant clones respond to the combination of GSK2126458 with GSK2118436 or GSK1120212. Clinical trials are ongoing or planned to test these combinations.

BRAF in Melanoma: Pathogenesis, Diagnosis, Inhibition, and Resistance

Since the initial discovery that a subset of patients with cutaneous melanoma harbor BRAF mutations, substantial research has been focused on determining the pathologic consequences of BRAF mutations, optimizing diagnostic techniques to identify these mutations, and developing therapeutic interventions to inhibit the function of this target in mutation-bearing tumors. Recently, advances have been made which are revolutionizing the standard of care for patients with BRAF mutant melanoma. This paper provides an overview on the pathogenic ramifications of mutant BRAF signaling, the latest molecular testing methods to detect BRAF mutations, and the most recent clinical data of BRAF pathway inhibitors in patients with melanoma and BRAF mutations. Finally, emerging mechanisms of resistance to BRAF inhibitors and ways of overcoming this resistance are discussed.