Pharmacomicrobiomics in blood cancers: How the gut microbiome and its metabolites shape drug efficacy and toxicity.

Clinical management of hematologic cancers is frequently complicated by marked, unpredictable inter-patient variation in both therapeutic benefit and adverse effects. Beyond host genetics, accumulating mechanistic and translational data implicate the gut microbiota and its small-molecule metabolome as active modifiers of drug chemistry, host immune tone, and clinical outcomes. In this review we synthesize three recurring, actionable findings: (1) discrete microbial enzymes, most prominently bacterial β-glucuronidases, repeatedly re-activate hepatic drug-glucuronides in the gut and amplify local gastrointestinal toxicity (e.g., irinotecan and mycophenolate), a pathway successfully targeted in preclinical and translational studies. (2) loss of anaerobic short-chain fatty acid production, especially butyrate, is a recurrent, causal mediator of mucosal injury and aggravated graft-versus-host disease after allogeneic hematopoietic cell transplantation. (3) reduced gut microbial diversity and the depletion of key functional taxa predict worse transplant outcomes (including mortality), arguing that baseline ecosystem state is a prognostic biomarker that merits routine consideration in trial design. Building on these syntheses, we propose a compact clinical framework for hematology trials and practice: (A) baseline ecosystem phenotyping (shotgun metagenome + targeted metabolite panel including butyrate and measured bacterial β-glucuronidase activity), (B) risk-stratified interventions (enzyme-targeted inhibitors, defined consortia, or metabolite replacement for high-risk patients), and (C) embedded holo-omic endpoints to confirm on-target biochemical modulation and link molecular change to clinical benefit. Together, these elements move Pharmacomicrobiomics beyond descriptive cataloging toward mechanistic stratification and testable interventions that can reduce unexplained variability in drug response and toxicity for patients with leukemia, lymphoma and myeloma.