Beyond CTLA-4 and PD-1 Inhibition: Novel Immune Checkpoint Molecules for Melanoma Treatment

SIMPLE SUMMARY: Although the incorporation of immune checkpoint inhibitors (ICIs) in melanoma treatment has significantly improved prognosis for patients with advanced disease, a substantial proportion of patients have limited clinical benefit from ICIs’ administration. Except for the well-studied PD-1 and CTLA-4 immune checkpoints, the expression of several other molecules has been associated with immune resistance and T-cell exhaustion. In this overview, we present the functional characteristics of presently described immune checkpoints, including LAG-3, TIGIT, TIM-3, VISTA, IDO1/IDO2/TDO, CD27/CD70, CD39/73, HVEM/BTLA/CD160 and B7-H3, with an emphasis on early clinical and preclinical data over the novel therapeutic agents that target these molecules. The aim of our review is to enrich the understanding of the dynamic interplay between melanoma, immune checkpoints and immune cells, and to provide an update on currently investigated ICIs, beyond anti-PD-1 and anti-CTLA-4 agents. ABSTRACT: More than ten years after the approval of ipilimumab, immune checkpoint inhibitors (ICIs) against PD-1 and CTLA-4 have been established as the most effective treatment for locally advanced or metastatic melanoma, achieving durable responses either as monotherapies or in combinatorial regimens. However, a considerable proportion of patients do not respond or experience early relapse, due to multiple parameters that contribute to melanoma resistance. The expression of other immune checkpoints beyond the PD-1 and CTLA-4 molecules remains a major mechanism of immune evasion. The recent approval of anti-LAG-3 ICI, relatlimab, in combination with nivolumab for metastatic disease, has capitalized on the extensive research in the field and has highlighted the potential for further improvement of melanoma prognosis by synergistically blocking additional immune targets with new ICI-doublets, antibody–drug conjugates, or other novel modalities. Herein, we provide a comprehensive overview of presently published immune checkpoint molecules, including LAG-3, TIGIT, TIM-3, VISTA, IDO1/IDO2/TDO, CD27/CD70, CD39/73, HVEM/BTLA/CD160 and B7-H3. Beginning from their immunomodulatory properties as co-inhibitory or co-stimulatory receptors, we present all therapeutic modalities targeting these molecules that have been tested in melanoma treatment either in preclinical or clinical settings. Better understanding of the checkpoint-mediated crosstalk between melanoma and immune effector cells is essential for generating more effective strategies with augmented immune response.