Grant McArthur

Melanoma treatment is undergoing a fundamental change due to the success of both BRAF targeted and immune therapies; however, both have their limitations. Typically, targeted therapies are associated with short-term responses due to acquisition of therapy resistance in patients; in contrast, immunotherapies have a lower patient response rate, but have long-term responses.

In preclinical studies using melanoma cell lines and in vivo models, we have demonstrated remarkably prolonged responses to a combination of targeted inhibitors to mutant BRAF and CDK4. We are now investigating how the combination of BRAF/MEK/ERK and CDK4 inhibitors induce durable responses. Furthermore, we are investigating the potential mechanism behind the development of resistance to each of these therapies. Understanding mechanisms of resistance is now an essential part of targeted therapy development, as it can provide both a biomarker for early detection of treatment failure and options for alternative subsequent treatments.

We have recently identified Protein Arginine Methyltransferase 5 (PRMT5) as a potential new target for CDK4 driven cancers including melanoma, pancreatic and oesophageal cancer. PRMT5 through methylation of histones and many other proteins regulates a diverse array of cellular functions and its activity is regulated by CDK4.

Our preclinical studies have identified novel PRMT5 inhibitor combinational therapies that are effective in pre-clinical CDK4 driven cancer models. Using both genomic and proteomic approaches we aim to identify potential downstream targets and pathways that are required for sensitivity to PRMT5 inhibitors and thus potential biomarkers of sensitivity and mechanisms of resistance to this class of inhibitor. In addition, a comparison across melanoma, pancreatic and oesophageal cancer cell lines will provide insight into common and likely fundamental mediators involved in PRMT5 signalling and also those that are unique to each cancer type.

Metabolic reprogramming is a recognised hallmark of cancer. To support continued proliferation and growth, tumour cells must metabolically adapt to balance their bioenergetic and biosynthetic needs. To achieve this, cancer cells switch from mitochondrial oxidative phosphorylation to predominantly rely on glycolysis, a process known as the Warburg effect. The BRAF oncogene, mutated in approximately 50 per cent of melanoma patients, has recently emerged as a critical regulator of this process, bringing to the fore the importance of metabolic reprogramming in the pathogenesis and treatment of melanoma.

To further explore regulation of glycolysis by BRAF, we have performed a genome-wide RNAi screen. This approach has identified a network of novel protein complexes and pathways that may play a role in coupling oncogenic BRAF signalling to metabolic reprogramming. Significantly, depletion of components of this network specifically synergise with vemurafenib to potently suppress glycolysis and survival. These novel regulators of BRAF-driven metabolic reprogramming are under further investigation, using metabolomics and Seahorse Extracellular Flux analysis, for their potential as therapeutic targets in the context of BRAF-mutant melanoma.

Melanoma is considered highly immunogenic but has evolved mechanisms that disable immune recognition and attack. Many small molecule inhibitors including BRAF, MEK and CDK4 inhibitors affect the tumour immune microenvironment, either promoting or diminishing the anti-tumour immune response. Understanding the impact of targeted therapy on both tumour immunogenicity and directly on immune cells is essential in advancing these agents, as well as their combination with immune therapies, into the clinic.

Using human melanoma cell lines, in vivo mouse models and patient samples, we are currently investigating the direct effect of targeted therapies on immune cell function and tumour immunogenicity. To do this we are using a variety of techniques including single cell RNA sequencing, flow cytometry and multiplex immunohistochemistry.  In addition, we are assessing how these targeted therapies can be successfully combined with immune therapies to sustain tumour regression.

The amazing initial response rate (87-90%) to targeted therapies in BRAF mutant melanoma patients is undermined by the development of resistance and disease progression that occurs in ~70% of patients. Strategies to prolong or maintain these initial responses would be a clear game changer in the treatment of melanoma. Several recent retrospective studies have shown increased overall survival in obese (BMI:>30) and overweight (BMI:25-30) melanoma patients treated with either immune- or targeted-therapy but not chemotherapy. This is surprising given the abundance of evidence linking obesity with increased risk of cancer. In our novel syngeneic mouse model, we are able to replicate the human data showing that a high fat diet increased the efficacy of BRAF/MEK inhibitors. Thus we have a model system to address the mechanism(s) underpinning the association of body fat with improved outcomes to targeted therapy in melanoma. By understanding the biological basis for this association we will further understand how to enhance these initial responses and design rational novel therapeutic strategies (targeted, hormonal, immune, metabolic, dietary) that will improve the outcomes for patients and importantly, avoid the adverse health effects associated with being overweight.