Dietary Fiber to Induce Gut Microbiota-mediated Response to Immunotherapy in Melanoma.

Inclusion Criteria: * Histologically confirmed cutaneous melanoma classified as irresectable stage III or stage IV disease * Measurable disease according to RECIST 1.1 criteria * Age ≥ 18 years * Starting standard-of-care treatment with ICI in first line * Written informed consent must be given * Able to read and understand Dutch or English * Able to comply with study procedures (e.g. willing to provide fecal samples) Exclusion Criteria: * Symptomatic brain metastases * Received prior immunotherapy; previous (neo)adjuvant treatment is allowed if the last administration is at least 6 months ago * Use of systemic immunosuppressive medications * Use of laxatives up to 1 week prior to start of ICI * Use of supplements that alter bowel function or gut microbiota such as fibers/prebiotics, probiotics, synbiotics, or postbiotics up to 1 month prior to start of ICI * Use of antibiotics up to 1 months prior to start of ICI * Use of proton pumps inhibitors (PPI's) up to 3 months prior to start of ICI * Pregnant or lactating * Current participation in another clinical trial requiring the use of study medication * Has a known allergy to plants such as lettuce, sage, tarragon, chicory, artichoke, chamomile, daisy, or sunflower. * Has a medical history of gastrointestinal resection (appendectomy is allowed) * Has active inflammatory bowel disease