Gut dysbiosis in difficult-to-treat rheumatoid arthritis: hypothesized microbial endotypes, persistent inflammation, and pharmacological treatment resistance.

Difficult-to-treat rheumatoid arthritis (D2T RA) remains a major clinical problem despite treat-to-target care and an expanding range of disease-modifying antirheumatic drugs (DMARDs). D2T RA includes biologically distinct states, particularly persistent inflammatory refractory rheumatoid arthritis (PIRRA) and non-inflammatory refractory rheumatoid arthritis (NIRRA). Direct microbiome evidence in D2T RA is sparse, but studies in established RA, at-risk populations, and treatment-response cohorts suggest that intestinal ecological disruption may sustain inflammation through barrier failure, persistent microbial-product sensing, and impaired metabolite-mediated immune regulation. Gut microorganisms may also influence drug response, especially to methotrexate, while antirheumatic therapy can remodel the microbial ecosystem. We therefore synthesize the literature into three hypothesis-generating functional states, provisionally termed inflammation-amplifying, immune-tolerance-deficient, and poor-drug-response-associated microbial endotypes. These states are not validated patient classes and may overlap or evolve over time. We also propose a microbiota-treatment resistance feedback loop, explicitly as an inferential model rather than an established causal pathway. Microbiota-directed adjuncts are evaluated according to evidence maturity, mechanistic fit, and safety. Overall, the gut microbiota is best considered a potential upstream modifier linking mucosal immunity, persistent inflammation, and pharmacological response, with the most coherent mechanistic relevance to PIRRA. Prospective D2T RA cohorts with objective inflammatory phenotyping, longitudinal multi-omics, and detailed drug-exposure data are needed before these concepts can inform clinical stratification or treatment.