AbstractThe treatment of melanoma has fundamentally changed over the past 10 years with the discovery of targeted and immune therapies. Despite this, the majority of patients with late stage melanoma have a median survival of less than 3 years. Resistance and toxicity to therapy remain the key challenges in the era of targeted and immune treatments. In this thesis, I investigate different approaches to overcoming resistance and exploiting vulnerabilities in melanoma biology through the use of patient-derived samples. I investigate how the brain microenvironment interacts with melanoma cells resulting in resistance to therapy. In a patient with acral melanoma who initially had a complete response to nivolumab an anti-PD-1 therapy, I show that resistance to immune therapy is associated with infiltration of M2-switched macrophages/microglia in the brain microenvironment. Furthermore, factors present in cerebrospinal fluid result in increased phosphatidylinositol 3- kinase/protein kinase B (PI3K/AKT) signalling in brain-derived melanoma cells resulting in resistance to the BRAF inhibitor, dabrafenib. The novel pan-RAF inhibitor CCT3833 has been shown to be effective in preclinical models of BRAF and NRAS mutant melanoma. I aimed to investigate if melanoma cells could overcome the inhibitory effects of the drug and explored combination strategies targeting potential mechanisms of emerging resistance. Resistance to CCT3833 was associated with upregulation of the mitogen-activated protein kinase (MAPK) or PI3K/AKT pathways. Combining CCT3833 with taselisib, a PI3K inhibitor resulted in decreased melanoma growth in vitro and in vivo. Furthermore, I examined the mechanisms underlying drug addiction, which can develop in targeted therapy resistance. In CCT3833 resistant cells, which had up-regulated the MAPK pathway, ERK and JunB were hyperactivated when drug was withdrawn, resulting 17 in decreased cell growth in vitro and in vivo. I showed that this effect could be augmented through the addition of a protein kinase C (PKC) agonist, which increases ERK hyperactivation. An important strategy in overcoming resistance may be to treat early when melanoma is less complex and tumour burden is reduced. Therefore, I developed a liquid biopsy based on circulating tumour DNA (ctDNA) that identified patients at highest risk of relapse following curative intent surgery. Selecting patients on the basis of detectable ctDNA levels enables intensification of treatment for high-risk patients, whilst those at lower risk may be spared potential toxicity. Finally, I designed clinical trials to test the use of ctDNA as a dynamic approach to monitoring tumour burden in patients with stage IV disease and identifying early relapse in patients with stage IIB/C melanoma following surgery. Thus, in order to be most effective, precision medicine must anticipate both melanoma cell intrinsic and extrinsic mechanisms of resistance to therapy. CCT3833 is a promising new MAPK inhibitor and combination strategies will optimise the duration of response. The level of ERK activation is finely tuned in melanoma and resistance to targeted therapy results in vulnerability to ERK hyperactivation and decreased growth. Therefore scheduling strategies may further extend the efficacy of targeted therapy. Monitoring with ctDNA enables a more dynamic approach to identification of treatment response, relapse and emerging mechanisms of resistance, allowing early therapeutic decisions to be made. Ultimately, precision medicine approaches in melanoma will enable targeting of patients at highest risk of disease progression whilst sparing others from potential toxicities from treatments that will provide little benefit.
|Date of Award||1 Aug 2019|
|Supervisor||Paul Lorigan (Supervisor) & Richard Marais (Supervisor)|
- targeted therapy
- circulating tumour DNA
- precision medicine