A randomized trial reveals that while gut microbiome diversity at the time of neutrophil engraftment does not predict non-relapse mortality (NRM) in patients undergoing allogeneic hematopoietic cell transplantation (allo-HCT), baseline microbial diversity remains a significant indicator of overall survival.
The findings, published in Nature, clarify the complex ecological shifts occurring during standard GVHD prophylaxis regimens.
The research, which tracked 304 participants, suggests that the specific prophylactic regimen chosen does not necessarily dictate clinical outcomes through the mechanism of microbiome disruption at the point of engraftment. Instead, the patient’s initial microbial health—the baseline state before conditioning—appears to be a more reliable marker for long-term prognosis.
- Baseline Matters: A higher diversity of gut bacteria before beginning transplant conditioning is linked to better overall survival and improved recovery outcomes.
- Engraftment Diversity: Contrary to some previous expectations, the diversity of gut bacteria specifically at the time of neutrophil engraftment (when white blood cells recover) did not reliably predict whether a patient would experience non-relapse mortality.
- Regimen Impact: Patients receiving the cyclophosphamide-based (PTCy) regimen experienced a more pronounced decline in microbial diversity and absolute microbial load compared to those on the standard tacrolimus and methotrexate (Tac/MTX) regimen, particularly between 14 and 28 days post-transplant.
The BMT CTN 1801 study, a companion to the BMT CTN 1703 trial, sought to quantify the ecological impact of two distinct graft-versus-host disease (GVHD) prophylaxis strategies. Researchers collected 2,575 stool samples from 304 participants, analyzing them through metagenomic sequencing to establish species-level taxonomic profiles. The primary diseases among the cohort were acute myelogenous leukemia and myelodysplastic syndrome.
While the study found no significant association between engraftment-phase diversity and NRM—using both Shannon’s index tertiles and continuous linear models—the trajectory of the microbiome differed significantly by treatment arm. The PTCy arm, which utilizes post-transplant cyclophosphamide, saw a sharper reduction in both alpha diversity and 16S rRNA gene copy number (a proxy for total microbial load) compared to the Tac/MTX cohort.
The research shows the clinical importance of the gut microbiome prior to the initiation of conditioning. Participants in the lowest tertile of baseline microbial diversity faced significantly shorter overall survival compared to those in the highest diversity group.
The study also identified a clear link between antimicrobial exposure and clinical outcomes. Patients who received a higher number of antibacterial doses during the first 30 days post-transplant faced an increased risk of developing acute GVHD (grade II–IV). This correlation aligns with previous reports, which have hypothesized that antibiotic-induced “dysbiosis”—a state of microbial imbalance—exacerbates inflammatory responses following transplantation.
| Metric | Tac/MTX Arm | PTCy Arm |
|---|---|---|
| Participants (N) | 147 | 157 |
| Mean Engraftment Day | 14.6 | 15.3 |
| Microbial Load Drop (Days 14-28) | Lower | Higher (2x smaller) |
| Primary Outcome (Engraftment Diversity vs NRM) | No Association | No Association |
Patients undergoing allo-HCT are strictly monitored for cardiac and gastrointestinal adverse events, which were the most frequent complications reported in this trial.
The trial, conducted between July 2019 and June 2021, highlights that while we are gaining precision in understanding the gut environment, current clinical protocols for GVHD must remain focused on established pharmacological standards.
Despite the lack of a direct link between engraftment diversity and NRM, the study confirms that the gut environment is significantly altered by the specific drugs used to prevent GVHD.
- Effect of two common GVHD prophylaxis regimens on the gut microbiome in a randomized phase 3 trial (Nature, 2024).
