Gut microbiome (GM) and its either pro-tumorigenic or anti-tumorigenic role is intriguing and constitutes an evolving landscape in translational oncology. It has been suggested that these microorganisms may be involved in carcinogenesis, cancer treatment response and resistance, as well as predisposition to adverse effects. In melanoma patients, one of the most immunogenic cancers, immune checkpoint inhibitors (ICI) and MAPK-targeted therapy—BRAF/MEK inhibitors—have revolutionized prognosis, and the study of the microbiome as a modulating factor is thus appealing. Although BRAF/MEK inhibitors constitute one of the main backbones of treatment in melanoma, little is known about their impact on GM and how this might correlate with immune re-induction. On the contrary, ICI and their relationship to GM has become an interesting field of research due to the already-known impact of immunotherapy in modulating the immune system. Immune reprogramming in the tumor microenvironment has been established as one of the main targets of microbiome, since it can induce immunosuppressive phenotypes, promote inflammatory responses or conduct anti-tumor responses. As a result, ongoing clinical trials are evaluating the role of fecal microbiota transplant (FMT), as well as the impact of using dietary supplements, antibiotics and probiotics in the prediction of response to therapy. In this review, we provide an overview of GM’s link to cancer, its relationship with the immune system and how this may impact response to treatments in melanoma patients. We also discuss insights about novel therapeutic approaches including FMT, changes in diet and use of probiotics, prebiotics and symbiotics. Finally, we hypothesize on the possible pathways through which GM may impact anti-tumor efficacy in melanoma patients treated with targeted therapy, an appealing subject of which little is known.

Metastatic melanoma (MM) is a pathological entity with a very poor prognosis that, until a few decades ago, had a low response rate to systemic treatments. Fortunately, in the last few years, new therapies for metastatic melanoma have emerged. Currently, targeted therapy and immunotherapy are the mainstays of the therapeutic arsenal available for patients with unresectable or metastatic melanoma. However, both clinical evolution and drug efficacy in melanoma patients are very different depending on the stage at which it is diagnosed. In fact, the aggressiveness of melanoma is different depending on whether it debuts directly as metastatic disease or if what occurs is a relapse after a first diagnosis at an early stage, although the biological determinants are largely unknown. Another key aspect in the clinical management of metastatic melanoma at first diagnosis strives in the different prognosis of melanoma of unknown primary (MUP) compared to melanoma of known primary (MPK). Understanding the mechanisms behind this, and the repercussion of implementing targeted and immune therapies in this specific form is crucial for designing diagnosis and treatment decision algorithms that optimize the current strategies. In this review article, we recapitulate the information available thus far regarding the epidemiology and response to immunotherapy treatments or targeted therapy in patients diagnosed with metastatic melanoma as a first diagnosis, with especial emphasis on the emerging specific information of the subpopulation formed by MUP patients.

Background: Immune checkpoint inhibitors (ICIs) have revolutionized melanoma treatment, with programmed cell death protein 1 (PD-1) inhibitors—alone or in combination with cytotoxic T-lymphocyte–associated protein 4 or lymphocyte-activation gene 3 inhibitors—demonstrating significant efficacy. However, there is a critical lack of robust data to determine the optimal sequencing of these therapies for individual patients. In particular, the role of relatlimab+nivolumab (rela/nivo) within treatment sequences remains poorly defined. Choosing the right sequence is strategic, as an inappropriate order may compromise the effectiveness of subsequent treatments and limit long-term benefits. Methods: This multicenter retrospective and prospective study evaluated 190 patients across three treatment arms: rela/nivo followed by ipilimumab+nivolumab (ipi/nivo) (arm A, N=40), ipi/nivo followed by rela/nivo (arm B, N=71), and anti-PD-1 followed by rela/nivo (arm C, N=79). The study assessed the impact of treatment sequencing on outcomes including response rate, progression-free survival, and overall survival (OS). Results: The overall response rate to second treatment was highest in arm C (30.4%), followed by arm B (28.1%) and arm A (17.5%). Patients with secondary resistance to first-treatment ICIs had better responses to second-treatment ICIs than those with primary resistance, particularly in arm B (p=0026). Median OS from date of first ICI treatment was significantly longer in arms B (40.9 months) and C (42.5 months) compared with arm A (16.3 months). Conclusions: Our findings indicate that rela/nivo may remain active following anti-PD-1 or ipi/nivo therapy. Additionally, our results suggest that sequencing ipi/nivo before rela/nivo may yield better outcomes than starting with rela/nivo. Patients who respond to the first combination regimen appear to derive greater benefit from the second. Further efforts are needed to optimize sequencing strategies in advanced melanoma, and future studies should consider the impact of prior treatment outcomes.