Fecal Transplants Are Being Tested to Fix Cancer Immunotherapy’s Side Effects

Immune checkpoint inhibitors have transformed treatment for melanoma, kidney cancer, and non-small-cell lung cancer, but they carry a stubborn problem: severe immune-related side effects that can force patients off treatment just as it starts working. A review published in Nature Communications by researchers Marina Ninkov and Saman Maleki Vareki now pulls together five clinical trials testing a strategy that sounds unlikely but keeps producing real numbers: transplanting a healthy person's gut bacteria into cancer patients before or during immunotherapy.

TACITO Trial Links Donor Stool to Longer Progression-Free Survival

The most rigorous evidence comes from TACITO, a randomized, placebo-controlled phase 2 trial in 45 patients with treatment-naive metastatic kidney cancer (renal cell carcinoma) who received pembrolizumab plus axitinib. Patients were randomly assigned to receive a fecal microbiota transplant (FMT) from a healthy donor, delivered by colonoscopy and then oral capsules, or a placebo transplant. According to the trial data summarized in the review, median progression-free survival reached 24.0 months in the donor-FMT group versus 9.0 months with placebo. The trial's primary endpoint, 12-month progression-free survival, was not statistically significant, but the review notes that early detection of donor-derived bacterial strains, including Bifidobacterium adolescentis and Akkermansia massiliensis, tracked with better outcomes. Safety data published in the original TACITO trial report in Nature Medicine show the transplant was well tolerated, with severe side effect rates comparable between the donor and placebo groups.

Melanoma, Kidney, and Lung Cancer Patients Are the Focus So Far

The stakes are highest for patients facing the toughest checkpoint-inhibitor regimens. Drugs combining anti-PD-1 and anti-CTLA-4 antibodies, such as ipilimumab plus nivolumab, produce grade 3-4 immune-related side effects in roughly 30 to 40 percent of patients across pooled analyses cited in the review, effects that can hit the gut, lungs, liver, heart, and hormone system hard enough to require immunosuppressive drugs that blunt the cancer treatment itself. In the phase 1 PERFORM trial, published separately in Nature Medicine, kidney cancer patients receiving oral FMT capsules alongside checkpoint inhibitors saw an objective response rate of 50 percent, and 89 percent of responders avoided severe immune-related side effects. In FMT-LUMINate, a phase 2 trial spanning melanoma and non-small-cell lung cancer, objective response rates reached 80 percent in the lung cancer patients and 75 percent in the melanoma patients who received donor FMT alongside their checkpoint inhibitors.

A Two-Stage Model: Clearance First, Then Stabilization

Because these trials used different donors, delivery routes, and cancer types, the review's real contribution is a proposed mechanism to explain why results vary so much. The authors describe what they call a two-stage ecological-functional model. In the first stage, "ecological clearance," the transplant appears to work partly by displacing a patient's own resident bacteria rather than by donor strains simply moving in. In the FMT-LUMINate trial, responders showed significantly greater loss of their pre-treatment bacterial species than non-responders did. TACITO data show a similar pattern: strain-specific microbial turnover within one week of the transplant correlated with better progression-free survival at 12 months, more so than the total amount of donor bacteria that took hold. In the second stage, "functional stabilization," patients who kept producing donor-associated metabolites over time, including short-chain fatty acids and specific amino acids like histidine, tended to keep their immune systems in a more regulated, tolerant state. In the earlier phase 1 MIMIC melanoma trial, an objective response rate of 65 percent was reported, and higher body mass index was associated with more efficient donor-strain engraftment, a detail the review flags as a candidate marker for predicting which patients will benefit most.

Small Trials and Correlation, Not Yet Cause and Effect

None of this amounts to a ready-to-use protocol, and the review is explicit about the gaps. Most of the supporting trials are small, single-arm, phase 1 or phase 2 studies. TACITO is the only placebo-controlled trial in the group, and even it missed its primary endpoint of 12-month progression-free survival. Two earlier melanoma trials that used stool from patients who had already responded well to anti-PD-1 therapy, rather than from healthy donors, restored treatment sensitivity in only 30 percent of patients in one study (led by Baruch and colleagues) and produced a 20 percent objective response rate in another (led by Davar and colleagues), both in checkpoint-refractory patients. The associations the review highlights, between donor-strain persistence, metabolite shifts, and clinical outcomes, are correlational. The authors are careful to say their two-stage model is "hypothetical" and "requires direct mechanistic validation." Effects also vary by cancer type, individual donor, and how patients are prepared for the transplant (bowel cleansing versus antibiotic pretreatment), so there is no single protocol yet that reliably reproduces the benefit.

What Comes Next for Donor Selection and Trial Design

The review's authors argue the field's next step is turning these correlations into predictive tools, using metabolite signatures and early strain-turnover data to select donors and flag which patients are likely to develop severe side effects before they happen. An ongoing phase 2 trial, ME.17, is currently testing FMT alongside various checkpoint inhibitor combinations in advanced melanoma. Until trials like it report results, and until placebo-controlled designs like TACITO's become the norm rather than the exception, fecal transplants for cancer immunotherapy will remain a promising but investigational strategy, not a standard one.

References

1. Marina Ninkov, Saman Maleki Vareki (2026). Microbial engraftment and immune regulation during fecal microbiota transplantation and immune checkpoint inhibitor therapy. Nature Communications. https://doi.org/10.1038/s41467-026-76051-z
2. Yuntao Yao, Yifan Liu, Yuanan Li, Bingnan Lu, Xingang Cui, Xiuwu Pan (2026). Re: Fecal Microbiota Transplantation plus Pembrolizumab and Axitinib in Metastatic Renal Cell Carcinoma: The Randomized Phase 2 TACITO Trial. European Urology. https://doi.org/10.1016/j.eururo.2026.04.003
3. Yuntao Yao, Yifan Liu, Yuanan Li, et al. (2026). Re: Fecal Microbiota Transplantation plus Immunotherapy in Metastatic Renal Cell Carcinoma: The Phase 1 PERFORM Trial. European Urology. https://doi.org/10.1016/j.eururo.2026.05.015
4. Danial Khan Hadi, Kelly J Baines, Behnam Jabbarizadeh, et al. (2025). Improved survival in advanced melanoma patients treated with fecal microbiota transplantation using healthy donor stool in combination with anti-PD1: final results of the MIMic phase 1 trial. Journal for ImmunoTherapy of Cancer. https://doi.org/10.1136/jitc-2025-012659