Resistance to BRAF Inhibitors Is More Complicated Than Thought

Two new studies show that several different genetic mutations can make melanoma tumors resist drugs known as BRAF inhibitors, complicating treatment. These mutations are in genes that are part of the ‘MAPK pathway.’ The first study was on BRAF-inhibitor resistant melanomas from 45 people. In about half of the tumors, one of a set of three genes (MEK1, MEK2, MITF) was abnormal, and in three of the tumors more than one was abnormal.

The second study compared melanomas before and after resistance to combination treatment with both BRAF and MEK inhibitors. Tumors from three of the five people in the study developed genetic abnormalities that were not seen before treatment. On a positive note, when cells from resistant melanomas with both BRAF and MEK mutations were grown in the laboratory, they responded to a drug that inhibits a related protein called ERK.

The mutations in this study were all found in genes that code for proteins in the MAPK pathway, a particular group of proteins in a cell that work together to control cell multiplication that can lead to tumor growth. Knowing exactly which mutations a melanoma has will help doctors target it with the right combination of treatments.

A Melanocyte Lineage Program Confers Resistance to MAP Kinase Pathway Inhibition

“Malignant melanomas harbouring point mutations (Val600Glu) in the serine/threonine-protein kinase BRAF (BRAF(V600E)) depend on RAF–MEK–ERK signalling for tumour cell growth1. RAF and MEK inhibitors show remarkable clinical efficacy in BRAF(V600E) melanoma23; however, resistance to these agents remains a formidable challenge24. Global characterization of resistance mechanisms may inform the development of more effective therapeutic combinations. Here we carried out systematic gain-of-function resistance studies by expressing more than 15,500 genes individually in a BRAF(V600E) melanoma cell line treated with RAF, MEK, ERK or combined RAF–MEK inhibitors. These studies revealed a cyclic-AMP-dependent melanocytic signalling network not previously associated with drug resistance, including G-protein-coupled receptors, adenyl cyclase, protein kinase A and cAMP response element binding protein (CREB). Preliminary analysis of biopsies from BRAF(V600E) melanoma patients revealed that phosphorylated (active) CREB was suppressed by RAF–MEK inhibition but restored in relapsing tumours. Expression of transcription factors activated downstream of MAP kinase and cAMP pathways also conferred resistance, including c-FOSNR4A1NR4A2 and MITF. Combined treatment with MAPK-pathway and histone-deacetylase inhibitors suppressed MITF expression and cAMP-mediated resistance. Collectively, these data suggest that oncogenic dysregulation of a melanocyte lineage dependency can cause resistance to RAF–MEK–ERK inhibition, which may be overcome by combining signalling- and chromatin-directed therapeutics.”

MITF and PAX3 Play Distinct Roles in Melanoma Cell Migration; Outline of a “Genetic Switch” Theory Involving MITF and PAX3 in Proliferative and Invasive Phenotypes of Melanoma

“Melanoma is a very aggressive neoplasm with a propensity to undergo progression and invasion early in its evolution. The molecular pathways underpinning invasion in melanoma are now just beginning to be elucidated, but a clear understanding of the transition from non-invasive to invasive melanoma cells remains elusive. Microphthalmia-associated transcription factor (MITF), is thought to be a central player in melanoma biology, and it controls many aspects of the phenotypic expression of the melanocytic lineage. However, recently the paired box transcription factor PAX3 was shown to transcriptionally activate POU3F2/BRN2, leading to direct repression of MITF expression. Here we present a theory to explain melanoma phenotype switching and discuss the predictions that this theory makes. One prediction is that independent and opposing roles for MITF and PAX3 in melanoma would be expected, and we present empirical evidence supporting this: in melanoma tissues PAX3 expression occurs independently of MITF, and PAX3 does not play a key role in melanoma cell proliferation. Furthermore, we show that knockdown of PAX3 inhibits cell migration in a group of “lower MITF” melanoma cell lines, while knockdown of MITF promotes cell migration in a complementary “higher MITF” group of melanoma cell lines. Moreover, the morphological effects of knocking down PAX3 versus MITF in melanoma cells were found to differ. While these data support the notion of independent roles for MITF and PAX3, additional experiments are required to provide robust examination of the proposed genetic switch theory. Only upon clear delineation of the mechanisms associated with progression and invasion of melanoma cells will successful treatments for invasive melanoma be developed.”

Directed Phenotype Switching as an Effective Antimelanoma Strategy

“Therapeutic resistance in melanoma and other cancers arises via irreversible genetic, and dynamic phenotypic, heterogeneity. Here, we use directed phenotype switching in melanoma to sensitize melanoma cells to lineage-specific therapy. We show that methotrexate (MTX) induces microphthalmia-associated transcription factor (MITF) expression to inhibit invasiveness and promote differentiation-associated expression of the melanocyte-specific Tyrosinase gene. Consequently, MTX sensitizes melanomas to a tyrosinase-processed antifolate prodrug 3-O-(3,4,5-trimethoxybenzoyl)-(−)-epicatechin (TMECG), that inhibits the essential enzyme DHFR with high affinity. The combination of MTX and TMECG leads to depletion of thymidine pools, double-strand DNA breaks, and highly efficient E2F1-mediated apoptosis in culture and in vivo. Importantly, this drug combination delivers an effective and tissue-restricted antimelanoma therapy in vitro and in vivo irrespective of BRAFMEK, or p53 status.”

The Anti-Melanoma Activity of Dinaciclib, a Cyclin-Dependent Kinase Inhibitor, Is Dependent on p53 Signaling

Although cyclin dependent kinase (CDK)-2 is known to be dispensable for the growth of most tumors, it is thought to be important for the proliferation of melanoma cells, where its expression is controlled by the melanocyte-lineage specific transcription factor MITF. Treatment of a panel of melanoma cells with the CDK inhibitor dinaciclib led to a concentration-dependent inhibition of growth under both 2D adherent and 3D organotypic cell culture conditions…”

Dabrafenib Bests Chemotherapy and May be Safer than Vemurafenib

A clinical trial found that dabrafenib, a BRAF inhibitor, was far more effective in treating melanomas that have BRAF mutations than the chemotherapy drug dacarbazine, according to a report at an American Society of Clinical Oncology meeting. Patients treated with this drug lived without getting worse for 70% longer than those treated with dacarbazine (5.1 vs. 2.7 months, respectively). Moreover, compared to those treated with vemurafenib in other studies, dabrafenib-treated patients had less risk of another kind of skin cancer called squamous cell carcinoma. This suggests that dabrafenib, which is experimental, could be safer than vemurafenib, which is FDA approved.

Vemurafenib Benefits People Previously Treated for Melanoma

A New England Journal of Medicine study found that vemurafenib, which was approved by the FDA in 2011, controlled melanomas in about half of people who had been previously treated for this disease. The trial included 132 repeat patients; tumors shrank in 47% of the patients and were not evident in 6% during the course of the trial. Vemurafenib is a BRAF inhibitor and about half of melanoma patients have BRAF mutations. While 26% of patients developed another kind of skin cancer called squamous cell carcinoma, these lesions were successfully removed surgically.

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Scans May Reveal Early Drug Resistance in Melanomas

Preliminary results suggest that an imaging technique can give early signs of drug resistance in melanomas. A Journal of Clinical Oncology study found that positron-emission tomography (PET)/computed tomography (CT) scans correlated with standard measures of tumor response in seven melanoma patients treated with vemurafenib. The scans also showed that during the third and fourth weeks of treatment, tumors in three patients began to take up and metabolize more of a sugar. This is a sign of cell activity, suggesting that these tumors were starting to resist the drug.

Vemurafenib Boosts Immunotherapy Against Melanoma in Mice

Vemurafenib increases the effectiveness of a treatment that uses immune system cells modified to target cancer cells, according to a study in Cancer Research. When combined with vemurafenib, which targets melanomas with the most common BRAF mutations (V600), this immunotherapy treatment killed more melanoma cells in mice. The combination treatment was also more successful than vemurafenib alone. The researchers conclude that their work supports testing this combination treatment in people with melanomas that have BRAF V600 mutations.