Fiber-Boost Randomized Controlled Trial

Inclusion Criteria: * Age ≥18 years * Written informed consent according to Swiss law and ICH/GCP regulations before inclusion and prior to any trial-specific procedures * Histologically or cytologically confirmed NSCLC * Advanced or recurrent NSCLC not amenable to curative treatment * PD-L1 expression ≥50% (TPS) determined by an approved IHC test * No actionable genetic alterations in genes such as EGFR, ALK, ROS1, HER2, BRAF, RET, MET, NTRK * First-line palliative PD-1/PD-L1-targeted monotherapy at a 3-weekly schedule indicated per local investigator * Willingness and ability to undergo study interventions * ECOG performance status 0-2 * Adequate organ function: * Hemoglobin ≥70 g/L, platelet count ≥50 G/L, granulocytes ≥1 G/L * Bilirubin, ALT, AST ≤3 x ULN * Glomerular filtration rate (Cockroft-Gault) ≥30 mL/min/1.73m² * Measurable or evaluable disease per RECIST 1.1 * Patients with CNS metastases are eligible, provided there is no requirement for corticosteroids as therapy for CNS disease and no evidence of clinical progression * Women with child-bearing potential use effective contraception (two independent methods), are not currently pregnant or lactating, and agree to not become pregnant during the trial treatment and during 3 months thereafter. A negative pregnancy test in either urine or blood is required for women with child-bearing potential before trial inclusion. * Men who are not sterile agree to use contraceptive methods (condoms) or abstain from sexual intercourse during the trial treatment and 3 months thereafter. Exclusion Criteria: -History of malignancy, unless in remission for at least 3 years before inclusion with the exception of pT1-2 prostate cancer Gleason score \<6, adequately treated cervical carcinoma in situ or localized non-melanoma skin cancer Adjuvant or additive systemic ICB treatment within 6 months prior to inclusion * Systemic treatment with an antibiotic within 10 days of ICB treatment/HFD intervention start * Concomitant immunosuppressive drugs including corticosteroids at a daily dose of ≥10mg prednisone equivalents, methotrexate, azathioprine, TNF-α inhibitors * Concurrent treatment with other experimental drugs or other anticancer therapy * Major surgical procedures within 14 days prior to inclusion as judged by the investigator * Active autoimmune disease requiring systemic immunosuppressive treatment that is seen as contraindication for the use of PD-1/PD-L1-targeted monoclonal antibodies * Uncontrolled diabetes mellitus * Severe or uncontrolled cardiovascular disease * Any other serious underlying medical, psychiatric, psychological, familial or geographical condition, which in the judgment of the investigator may interfere with the planned staging, treatment and follow-up, affect patient compliance or place the patient at high risk from treatment-related complications * Had an allogeneic tissue/solid organ transplant * Ongoing supplementation with OptiFibre® or another fiber supplement