Microbiome–nitric oxide axis in cancer immunotherapy: a qualitative systematic review
The review identified clinical associations between gut microbiome composition and ICI outcomes but found no direct evidence that the microbiome–nitric oxide axis improves oncological outcomes in humans.
| Population | Cancer patients receiving ICIs (clinical studies) and animal/cellular models with experimental cancers |
|---|---|
| Intervention | Gut microbiome composition, microbial metabolites (SCFAs, nitrate/nitrite) and nitric oxide synthase pathways |
| Comparator | Distinct microbiomes, no intervention, or unexposed controls (variable by study) |
| Outcome | Microbiome–ICI response association (clinical studies); Microbial transfer and antitumor immune response (animal models); Non-randomized FMT and ICI outcomes (preliminary clinical studies); NO/NOS pathway and antitumor immune function (in vitro/animal); Dietary nitrate and circulating nitrate/nitrite levels (humans, outside oncology); Direct link between microbial nitrate reduction and ICI outcomes; Anticancer efficacy or ICI synergy of microbiome–NO axis interventions |
Summary of findings
| Outcome | Effect | 95% CI | Certainty | Clinical relevance | Notes |
|---|---|---|---|---|---|
| Microbiome–ICI response association (clinical studies) | narrative association only; in the pooled RR/OR or 95% CI reported | — | Low | — | 21 studies |
| Microbial transfer and antitumor immune response (animal models) | narrative proof-of-concept; in the pooled effect size or 95% CI reported | — | Very low | — | 19 studies |
| Non-randomized FMT and ICI outcomes (preliminary clinical studies) | narrative only; in the pooled effect size or 95% CI reported | — | Low | — | |
| NO/NOS pathway and antitumor immune function (in vitro/animal) | narrative mechanistic evidence only; in the pooled effect size or 95% CI reported | — | Very low | — | 14 studies |
| Dietary nitrate and circulating nitrate/nitrite levels (humans, outside oncology) | narrative: increased circulating nitrate/nitrite with measurable vascular effects; in the oncological endpoint effect size reported | — | Low | — | |
| Direct link between microbial nitrate reduction and ICI outcomes | no data available; evidence absent | — | Very low | — | |
| Anticancer efficacy or ICI synergy of microbiome–NO axis interventions | no data available; evidence absent in humans | — | Very low | — |
Context
Immune checkpoint inhibitors (ICIs) produce heterogeneous responses; identifying modifiable predictors is a clinical priority. Nitric oxide (NO) regulates antitumor immunity and tumor vascular biology, and the gut microbiome is a potential source of nitrogenous substrates. The review tests whether sufficient evidence exists to support interventions targeting this axis.
What the study showed
Clinical cohorts associated enrichment of Akkermansia muciniphila, Faecalibacterium prausnitzii and Bifidobacterium spp. with better ICI outcomes, but microbial signatures varied across cohorts and analytical platforms. Microbial-transfer animal experiments and non-randomized fecal microbiota transplantation (FMT) trials provide proof-of-concept that microbiome composition can influence antitumor immune responses, without yet constituting robust clinical evidence. Dietary nitrate studies in humans outside oncology demonstrate increased circulating nitrate and nitrite with measurable vascular effects but do not establish anticancer or ICI efficacy. Direct evidence linking microbial nitrate reduction to ICI outcomes is absent.
How it was done
Qualitative systematic review searching five databases (January 2000–August 2025). Fifty-four eligible primary studies: 21 clinical, 19 animal and 14 in vitro mechanistic. Narrative synthesis by study type and thematic domain; no quantitative meta-analysis.
Effect magnitude
No pooled effect size was calculated; the review is qualitative and reports no 95% CIs or aggregated estimates. The clinical effect magnitude of the microbiome–NO axis on ICI outcomes remains undefined.
Risk of bias
Qualitative design without meta-analysis precludes effect estimates; heterogeneity in metagenomic platforms and ICI response criteria across cohorts limits comparisons. Included clinical studies are predominantly observational and non-randomized, with high confounding risk (risk-of-bias tool not formally reported). Narrative synthesis is subject to extraction and selection bias.
What this study does NOT prove
This study does not prove that manipulating the microbiome or NO pathway improves ICI response or survival in cancer patients. Associations between microbial signatures and ICI outcomes do not establish causality and are not generalizable across tumor types, treatments or populations.
In clinical practice
There is no basis to recommend interventions targeting the microbiome–NO axis to improve ICI response outside research protocols. The gut microbiome is an associative biomarker of ICI response, not a validated therapeutic target. FMT and dietary nitrate supplementation require evaluation in randomized clinical trials before any clinical application.
Limitations
Qualitative design without meta-analysis precludes effect estimates; heterogeneity in metagenomic platforms and ICI response criteria across cohorts limits comparisons. Included clinical studies are predominantly observational and non-randomized, with high confounding risk (risk-of-bias tool not formally reported). Narrative synthesis is subject to extraction and selection bias.
What is still missing
RCTs evaluating FMT or dietary microbiome modulation in patients receiving ICIs with primary oncological endpoints. Studies establishing the direct mechanistic link between microbial nitrate reduction, NO production and antitumor immune response in humans.
Technical appendix
Version history
- 1.0 · 2026-10-10 — Auto-generated under Evidence Standard v1.0