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