Novel biomarkers for melanoma immunotherapy

Abstract

Traditionally, metastatic melanoma had a dismal prognosis, but the recent advent of immune checkpoint inhibitors (ICI) has extended survival from months to years for some patients. There is an urgent need to identify prognostic and predictive biomarkers for melanoma patients treated with ICI, given that only a minority of patients respond, coupled with the potential treatment related toxicities. This thesis aimed to investigate clinical factors, functional PET imaging and tumour immune profiling as candidate biomarkers for ICI in patients with melanoma.

Firstly, Chapter 3 focused on baseline performance status as a biomarker for outcome following anti-PD-1. The hypothesis was that unlike cytotoxic chemotherapy, baseline performance status was not correlated with outcome following ICI, owing to its distinct mechanism of action. However, in the cohort of 91 patients treated with anti-PD-1 at Peter MacCallum Cancer Centre, poor performance status was correlated with poor survival and low response rate to anti-PD-1. Furthermore, patients with poor performance status were more likely to be hospitalised and more likely to die in hospital.

Patient characteristics and blood parameters were further examined in Chapter 4, but specific to a cohort of patients with melanoma brain metastases. Melanoma commonly metastasise to the central nervous system and this is associated with extremely poor survival. Recently, combination ICI has resulted in intracranial responses and durable survival. Most of the existing literature in biomarkers in melanoma brain metastases also predates the introduction of ICI, therefore investigation of biomarkers in patients with melanoma brain metastases treated with ICI is needed. A post-hoc analysis of patients with melanoma brain metastases as part of the phase II Anti-PD1 Brain Collaboration study was performed to identify possible predictors of clinical outcome or toxicity. In this study, patients were randomised to receive either nivolumab monotherapy or nivolumab in combination with ipilimumab. High C-reactive protein, a marker of systemic inflammation, was correlated with poor survival. Treatment with combination ICI, hypernatraemia and increased body mass index were associated with higher likelihood of severe toxicity at 120 days, whereas CRP was not associated with higher toxicity.

The thesis then went on to examine the role of FDG PET functional imaging as a source of biomarkers for outcome following ICI in Chapter 5. Baseline pre-treatment tumoural FDG-PET avidity (measured by SUVmax or metabolic tumour volume) as well as FDG-avidity in the immune system (measured by spleen to liver ratio) were assessed in relation to survival outcomes. Interestingly, tumoural PET avidity was not correlated with survival, whereas high spleen to liver ratio was correlated with poor survival after ipilimumab. This was subsequently validated in a combined cohort of patients from two separate European centres. High spleen to liver ratio was correlated with low albumin in a multivariate analysis, thus suggesting a possible association with systemic inflammation. Early on-treatment PET (EOT PET) were assessed in a small subset of 16 patients, and several challenges were identified that may limit the use of FDG PET in this early juncture as a biomarker for outcome after ICI.

In-depth characterisation of tumoural immune landscape is crucial to improving the understanding of melanoma immuno-biology, with potential implications for biomarker development. Chapter 6 aimed to compare the immune profile of UV related skin cancers (melanoma, cutaneous squamous cell carcinoma and Merkel cell carcinoma) using orthogonal methods of bulk RNA-sequencing and multi-spectral immunohistochemistry. The three skin cancers showed distinct immune landscapes, with melanoma having a significantly higher intratumoural T cell infiltrate compared to Merkel cell carcinoma, whereas PD-L1 density was highly variable across three skin cancers. Transcriptomic analyses of melanoma samples with high PD-L1 density were associated with upregulation of genes related to leucocyte proliferation, migration and adaptive immune responses, in contrast to MCC samples with high PD-L1 density, where such a signature was not observed.

Lastly, an in-depth case study of six patients highlighted how multi-factorial biomarkers such as clinical factors, functional PET imaging, baseline blood parameters, and multi-spectral immunohistochemistry can be applied together.

In conclusion, this thesis evaluated multi-factorial biomarkers including clinical, functional imaging and tumoural immune profiling biomarkers. These studies add to the evolving literature on biomarkers associated with ICI treatment. It is envisaged that with time, these complementary methods of understanding the patient and tumoural immune environment can aid rational selection of immune based therapies for patients with advanced melanoma.