Metastatic melanoma is a malignant tumor of melanocyte origin that has spread to other organs. The basis of treatment is systemic therapy, most commonly anti-programmed death-1 (PD-1) and anti- cytotoxic T-lymphocyte-associated protein 4 (CTLA- 4) immunotherapy, although for some patients, thyrosine kinase inhibition is also a treatment option. Survival of metastatic melanoma patients has been significantly extended over the last decade, and the use of immunotherapy has led to a median overall survival of 72 months and a response rate of up to 60%. However, not all cancer patients respond equally to immunotherapy, and the variation in response may be attributed to the individual's microbiome, which is heavily influenced by dietary habits. Over 2/3 of patients will progress during the 6.5 years of follow-up and currently there is a limited number prognostic and predictive parameters to assess the response to immunotherapy. However, understanding and manipulating the microbiome through dietary interventions offers a potential avenue for enhancing immunotherapy outcomes in cancer patients and, consequently, may serve as a complementary therapeutic strategy. Due to previous data, as well as investigators' previous publication on the importance of gut microbiome as a co-denominator for immunotherapy response, and aiming towards constructing a protocol on dietary intervention for enhancing immunotherapy response in metastatic melanoma patients, the investigators evaluated a systematic search for the data showing association with an improved immunotherapy response.

A diet with moderately elevated sodium use (2.3-4 g daily), diet rich in fiber (20-25 g daily), fruits and vegetables (≥5 servings daily), vitamin D, 23 omega-3 fatty acids and whole grains (≥3 servings daily) has been associated with a more diverse and beneficial microbiome, which in turn promotes the production of short-chain fatty acids (SCFA). All the listed foods belong to the Mediterranean diet (MD), which is based on fresh fruits and vegetables, legumes, olive oil, and unrefined fiber. MD also includes a lower intake of meat, eggs, processed foods, sugars, and saturated fatty acids, which may all lead to a less diverse and potentially less favorable microbiome and health outcomes. Various clinical studies already demonstrated the beneficial effect of MD in non-oncology patients. As for oncology patients, the benefit of the MD in reducing the incidence of cancer has been known for many years, especially for breast cancer patients. Furthermore, a published cohort study with 52 patients receiving immune checkpoint blockade (ICB) treatment for various solid tumors revealed that higher fecal SCFA concentrations were associated with longer progression-free survival. Another cohort study comprising patients with advanced melanoma in the UK and the Netherlands, demonstrated that a stronger adherence to the MD principles was linked to a higher likelihood of responding positively to ICB treatment. However, there is a scarcity of randomized trials evaluating the effect of the nutritional intervention based on the MD in cancer patients undergoing treatment. As far as the investigators are aware, there are no current clinical trials applying a nutritional intervention based on the MD in patients with metastatic melanoma; however, there is an ongoing randomized DIET study - NCT04645680 (with expected ending in 2024), aiming to evaluate effectiveness of fiber-enriched diet within the melanoma setting. The potential effect of the MD based nutritional intervention could result in a greater benefit compared to trials focusing only on one nutrient. Considering the issues learned from COVID-19 pandemic, the main goal of the trial is to determine the effectiveness and applicability of a remote personalized nutritional intervention based on the MD to increase the intake of micronutrients (flavones, anthocyanins, omega-3 fatty acids, vitamin D and fiber) previously associated with a positive response to immunotherapy.