A Genome-Scale RNA Interference Screen Implicates NF1 Loss in Resistance to RAF Inhibition
Steven R. WhittakerJean‐Philippe TheurillatEliezer M. Van AllenNikhil WagleJessica HsiaoGlenn S. CowleyDirk SchadendorfDavid E. RootLevi A. Garraway
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Abstract:
RAF inhibitors such as vemurafenib and dabrafenib block BRAF-mediated cell proliferation and achieve meaningful clinical benefit in the vast majority of patients with BRAF(V600E)-mutant melanoma. However, some patients do not respond to this regimen, and nearly all progress to therapeutic resistance. We used a pooled RNA interference screen targeting more than 16,500 genes to discover loss-of-function events that could drive resistance to RAF inhibition. The highest ranking gene was NF1, which encodes neurofibromin, a tumor suppressor that inhibits RAS activity. NF1 loss mediates resistance to RAF and mitogen-activated protein kinase (MAPK) kinase kinase (MEK) inhibitors through sustained MAPK pathway activation. However, cells lacking NF1 retained sensitivity to the irreversible RAF inhibitor AZ628 and an ERK inhibitor. NF1 mutations were observed in BRAF-mutant tumor cells that are intrinsically resistant to RAF inhibition and in melanoma tumors obtained from patients exhibiting resistance to vemurafenib, thus showing the clinical potential for NF1-driven resistance to RAF/MEK-targeted therapies.Keywords:
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We report on a patient with an adenocarcinoma of the lung harbouring a BRAF V600E mutation who benefited from combination therapy with dabrafenib-trametinib after developing resistance to vemurafenib. To our knowledge, our report shows, for the first time, that combination therapy with dabrafenib-trametinib can overcome vemurafenib resistance in a BRAF V600E-mutated adenocarcinoma of the lung.
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BRAF V600E mutations are detected in 3%–10% of patients with multiple myeloma (MM) and are associated with more aggressive disease, higher frequency of extramedullary growth and shorter survival. Monotherapy with the BRAF inhibitor vemurafenib has been disappointing in MM. In patients with BRAF-mutated melanoma, MEK and BRAF inhibition has been a successful approach. Here we describe a very good partial response and possible mechanisms of resistance to a combination of the BRAF inhibitor dabrafenib and the MEK inhibitor trametinib in a patient with BRAF V600E-mutant refractory MM.
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The treatment of melanoma has been revolutionized over the past decade with the development of effective molecular and immune targeted therapies. The great majority of patients with melanoma have mutations in oncogenes that predominantly drive signaling through the mitogen activated protein kinase (MAPK) pathway. Analytic tools have been developed that can effectively stratify patients into molecular subsets based on the identification of mutations in oncogenes and/or tumor suppressor genes that drive the MAPK pathway. At the same time, potent and selective inhibitors of mediators of the MAPK pathway such as RAF, MEK, and ERK have become available. The most dramatic example is the development of single-agent inhibitors of BRAF (vemurafenib, dabrafenib, encorafenib) and MEK (trametinib, cobimetinib, binimetinib) for patients with metastatic BRAF V600 -mutant melanoma, a subset that represents 40% to 50% of patients with metastatic melanoma. More recently, the elucidation of mechanisms underlying resistance to single-agent BRAF inhibitor therapy led to a second generation of trials that demonstrated the superiority of BRAF inhibitor/MEK inhibitor combinations (dabrafenib/trametinib; vemurafenib/cobimetinib) compared to single-agent BRAF inhibitors. Moving beyond BRAF V600 targeting, a number of other molecular subsets—such as mutations in MEK, NRAS, and non-V600 BRAF and loss of function of the tumor suppressor neurofibromatosis 1 ( NF1)—are predicted to respond to MAPK pathway targeting by single-agent pan-RAF, MEK, or ERK inhibitors. As these strategies are being tested in clinical trials, preclinical and early clinical trial data are now emerging about which combinatorial approaches might be best for these patients.
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Since 2011, the treatment options for metastatic malignant melanoma have significantly changed. In that year, ipilimumab, an anti-CTLA4 monoclonal antibody, and vemurafenib, a potent inhibitor of mutated-BRAF (V600E and V600K), were approved by the U.S. Food and Drug Administration (FDA). In 2013, dabrafenib, another inhibitor of mutated-BRAF, and trametinib, a MEK inhibitor, were approved by the FDA. Most recently, combination therapy with dabrafenib and trametinib was approved. This article will describe a patient with metastatic malignant melanoma with BRAFV600E who has responded very well to vemurafenib monotherapy. We will then explore the molecular basis, pharmacologic development and clinical outcomes of inhibition of the mitogen-activated protein (MAP) kinase pathway in patients with metastatic malignant melanoma with oncogenic BRAF (V600E and V600K).
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At present, two selective BRAF inhibitors (dabrafenib and vemurafenib) and one MEK inhibitor (trametinib) are FDA-approved for treating stage IV BRAFV600E -mutated metastatic melanoma. Single-agent dabrafenib but not vemurafenib has been tested against combination dabrafenib/trametinib. Vemurafenib monotherapy is in current use; these investigators studied its efficacy relative to combination dabrafenib/trametinib.
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Oncogenic BRAF mutations are present in approximately 40-50% of patients with metastatic melanoma. Targeting BRAF mutations with either small molecule inhibitors of BRAF or one of the downstream mediators of oncogenic BRAF - MEK - is associated with improved outcomes compared with chemotherapy and has led to the US FDA approval of two BRAF inhibitors - vemurafenib and dabrafenib - and the MEK inhibitor trametinib. Further, the combination of dabrafenib and trametinib is well tolerated and associated with higher responses and improved survival compared with single-agent BRAF inhibitors.
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Abstract: The treatment of melanoma has improved markedly over the last several years with the advent of more targeted therapies. Unfortunately, complex compensation mechanisms, such as those of the mitogen-activated protein kinase (MAPK) pathway, have limited the clinical benefit of these treatments. Recently, a better understanding of melanoma resistance mechanisms has given way to intelligently designed multidrug regimes. Herein, we review the extensive pathways of BRAF inhibitor (vemurafenib and dabrafenib) resistance. We also review the advantages of dual therapy, including the addition of an MEK inhibitor (cobimetinib or trametinib), which has proven to increase progression-free survival when compared to BRAF inhibitor monotherapy. Finally, this review touches on future treatment strategies that are being developed for advanced melanoma, including the possibility of triple therapy with immune checkpoint inhibitors and the work on optimizing sequential therapy. Keywords: cobimetinib, trametinib, vemurafenib, dabrafenib, BRAF inhibitor, MAPK pathway
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Melanoma is the most lethal form of skin cancer. Melanoma is usually curable with surgery if detected early, however, treatment options for patients with metastatic melanoma are limited and the five-year survival rate for metastatic melanoma had been 15–20% before the advent of immunotherapy. Treatment with immune checkpoint inhibitors has increased long-term survival outcomes in patients with advanced melanoma to as high as 50% although individual response can vary greatly. A mutation within the MAPK pathway leads to uncontrollable growth and ultimately develops into cancer. The most common driver mutation that leads to this characteristic overactivation in the MAPK pathway is the B-RAF mutation. Current combinations of BRAF and MEK inhibitors that have demonstrated improved patient outcomes include dabrafenib with trametinib, vemurafenib with cobimetinib or encorafenib with binimetinib. Treatment with BRAF and MEK inhibitors has met challenges as patient responses began to drop due to the development of resistance to these inhibitors which paved the way for development of immunotherapies and other small molecule inhibitor approaches to address this. Resistance to these inhibitors continues to push the need to expand our understanding of novel mechanisms of resistance associated with treatment therapies. This review focuses on the current landscape of how resistance occurs with the chronic use of BRAF and MEK inhibitors in BRAF-mutant melanoma and progress made in the fields of immunotherapies and other small molecules when used alone or in combination with BRAF and MEK inhibitors to delay or circumvent the onset of resistance for patients with stage III/IV BRAF mutant melanoma.
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Introduction: Malignant melanoma (MM) is an aggressive disease with a rapidly rising incidence due to neoplasm of melanocytes. Molecular targeted therapies have demonstrated lower toxicity and improved overall survival versus conventional therapies of MM. The revealing of mutations in the BRAF/MEK/ERK pathway has led to the development of BRAF inhibitors such as vemurafenib and dabrafenib for the treatment of cutaneous MM. Though, progression of resistance to these agents has prompted attempts to target downstream proteins in this pathway. Trametinib, a MEK1/2 inhibitor, was approved in 2013 for the treatment of BRAF V600E/K mutation-positive unresectable or metastatic cutaneous melanoma patients.Areas covered: The aim of the current review is to present an update on the role of MEK in progressive melanomas and summarize latest results of clinical studies with innovative MEK inhibitors and/or combined approaches with other kinase inhibitors such as BRAF inhibitors in the treatment of MM.Expert opinion: Two combined treatments (i.e. trametinib plus dabrafenib and vemurafenib plus cobimetinib) target two different kinases in the BRAF/MEK/ERK pathway. The simultaneous prohibition of both MEK and BRAF is associated with more durable response rate than BRAF monotherapy and can overcome acquired resistance.
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