Mechanisms underlying inhibition of melanoma brain metastases upon immune checkpoint targeting

Melanoma is the deadliest form of skin cancer. Once melanoma has spread throughout the body, it is known as metastatic melanoma. At this stage melanoma becomes very difficult to treat and the standard treatment is effective only in a very small proportion of patients. In recent years new drugs have been approved for the treatment of metastatic melanoma. These drugs inhibit the molecules called PD-1 and CTLA-4 that are present on a subpopulation of white blood cells called T lymphocytes. Inhibition of PD-1 and CTLA-4 helps the immune system to attack the cancer. Although these drugs significantly extend lives of melanoma patients, complete responses upon combined inhibition of PD-1 and CTLA-4 are seen only in 11.5 % of the patients. It is therefore important to gain a better understanding of how these drugs work in order to be able to develop approaches that further improve their efficacy.
Notably, the immune system works in different ways within different organs in the body. It is therefore important to understand how the drugs targeting PD-1 and CTLA-4 work within the organs to which melanoma most commonly spreads. Our goal is to understand how the efficacy of PD-1 and CTLA-4 blockade could be improved in the brain, to which cancer spreads in up to 60% of metastatic melanoma patients. The resulting tumours are called brain metastases (BrM) and they are particularly difficult to treat. In comparison to the melanoma in general, we know very little about BrM; this is because - despite their high incidence - patients with BrM are mostly excluded from clinical trials and BrM are experimentally strongly understudied. Notably, brain has a very distinct cellular composition and the presence of the blood-brain barrier restricts access of drugs and immune cells into the tumour. Ignoring these specifics of the brain poses a danger that - despite a progress in the treatment of melanoma in other parts of the body - treatment of BrM once again lacks behind and BrM become a limiting factor in patient survival. It is therefore critical to identify the mechanisms involved in the action of drugs targeting PD-1 and CTLA-4 in BrM in a timely manner.
There are to date no experimental studies investigating how the drugs targeting PD-1 and CTLA-4 work in BrM. To study the latter, we established an in vivo model of melanoma BrM and demonstrated that a combined targeting of CTLA-4 and PD-1 significantly inhibits growth of BrM and prolongs the survival. This was mainly mediated by a subpopulation of T lymphocytes called Cytotoxic T lymphocytes (CTLs) and by another type of white blood cells called natural killer cells. CTLs accumulated in tumours following therapy. Therefore our goal is to understand how CTLs travel to BrM and to determine how they kill cancer cells in the context of this therapy. We also observed increased accumulation of white blood cells of so-called myeloid lineage in tumours. We therefore aim to determine whether these cells are also required for activity of drugs targeting PD-1 and CTLA-4 in the brain.
Understanding how CTLs travel to BrM will enable the development of strategies that can enhance CTL accumulation within the tumour in the brain and are therefore expected to potentiate the efficacy of therapy targeting PD-1 and CTLA-4. If our study determines that white blood cells of myeloid lineage are involved in inhibition of BrM following targeting of PD-1 and CTLA-4, this will provide a rational for improved therapies combining targeting of PD-1/CTLA-4 and myeloid cells. At least part of the newly gained knowledge is expected to be applicable to melanoma at sites other than the brain. Thus, the knowledge emerging from the proposed research has a potential to contribute towards improved outcomes of patients with BrM and those with metastatic melanoma in general.

Technical summary

Despite the great promise of anti-PD-1 and anti-CTLA-4 therapies in melanoma, only 11.5% of the patients show a complete response after anti-PD-1/anti-CTLA-4 combination. Thus, a better understanding of the underlying mechanisms is required in order to enable the improvement of therapy. This should include improvement of the efficacy in the brain, as brain metastases (BrM) develop in up to 60% of metastatic melanoma patients and are associated with very poor prognosis. Recent clinical trials provided initial evidence for the efficacy of anti-PD-1 and anti-CTLA-4 in melanoma BrM. Due to the distinct characteristics of the tumour microenvironment in the brain and the blood-brain barrier that limits access of drugs, the molecular mechanisms need to be addressed in the specific context of BrM.

We have established a tumour transplantation model of melanoma BrM in which clinically observed intracranial efficacy of anti-PD-1/anti-CTLA-4 combination could be faithfully recapitulated. CD8+ T cells and anti-PD-1/anti-CTLA-4-induced enhancement of their chemokine receptor-dependent trafficking to BrM were critical for the intracranial therapeutic efficacy. The latter also required NK cells and correlated with a significant intra-tumoural increase in microglia and macrophages. Building upon these findings, our goals are to further elucidate molecular mechanisms underlying T cell trafficking to BrM, identify critical NK and T cell effector functions, and determine whether myeloid cells, including microglia and macrophages, are functionally implicated in intracranial efficacy of combined PD-1/CTLA-4 blockade. While primarily based in a tumour transplantation model, our studies will be further validated in a spontaneous BrM model and on human tissue. This is expected to reveal potential strategies for the enhancement of anti-PD-1/anti-CTLA-4 efficacy.

Our ultimate goal is to facilitate the improvement of the immune checkpoint inhibitor efficacy in melanoma patients by generating knowledge about molecular mechanisms involved in this therapy. Beneficiaries of our research are:

Commercial private sector:
Many chemokine receptor agonists/antagonists have been developed by pharmaceutical companies and many of them are being tested in clinical trials. Knowledge about the importance of a specific chemokine/chemokine receptor for the therapeutic efficacy of immune checkpoint inhibitors, which is expected to emerge from our study, will inform the repurposing of existing chemokine receptor agonists/antagonists and therefore has a potential to financially benefit pharmaceutical companies (please refer to the "Pathways to impact" for details on how this will be achieved). Similarly, myeloid cells emerged as an important target in cancer and their repolarization or depletion as therapeutic strategies are being pursued. Examples of drugs targeting macrophages include Trabectedin and CSF1R inhibitors (already used in clinical trials). Understanding the role of myeloid cells in the context of immune checkpoint inhibition is expected to pave the way for rational combination therapies with myeloid cell repolarizing/depleting drugs. These new therapeutic applications also have a potential to benefit pharmaceutical companies.

Public sector and wider public:
If our findings can be successfully translated into the clinic, then this may have a direct impact on the health and wellbeing of patients in medium/long-term. Our particular focus is on the improvement of the therapy in the brain due to the high incidence of brain metastases (BrM) in patients with metastatic melanoma (60%) and a particularly poor prognosis of this patient group. Majority of therapies that are achieving responses outside the central nervous system are unsuccessful in the brain. It is widely recognised that organ-specific tumour microenvironment critically contributes to the tumour growth and therapeutic responses. Ignoring specifics of BrM poses a danger that - despite a progress in treatment of extracranial disease - BrM once again become a limiting factor in patient survival. This would affect more than half of the patients with metastatic melanoma. We therefore reason that mechanisms that could lead to the improvement of the immune checkpoint inhibitor therapy in the brain should be investigated now in parallel to the mechanisms that are relevant in the context of the systemic disease.
As we are in parallel aiming to analyse extracranial tumours, our findings are expected to also facilitate improvement of the anti-CTLA-4/anti-PD-1 therapy for the extracranial disease, targeting an even larger population of patients. Improvement of the life quality through the improved therapy would also benefit the carers of these patients; often the family members are forced to cut their full time employment to part time in order to be able to care for the sick relatives, and therefore improvement of the therapy and patient's quality of life would also improve the economic status of their carers and potentially also benefit their employers. Since we are validating our findings on human tissue, this may result in opportunities for the development of predictive biomarkers that would benefit clinicians by enabling them to identify patients that are most likely to benefit from the therapy. Focusing treatment on the patients that are likely to respond would also reduce the treatment costs and thus benefit the healthcare sector. Our research is also benefiting general public through our active participation in education of lay people and dissemination of our data to the brain tumour and cancer charities, the patients and their carers (please refer to the "Pathways to impact" for details and examples).