The Gut Microbiome Dependency Continuum in Drug Discovery: A Unified Pharmacology Framework Linking Clinical Drugs, Natural Products, and Engineered Microbial Therapeutics

Highlighting its pivotal role in modern pharmacology, the gut microbiome is emerging as a key determinant of drug efficacy, toxicity, and bioavailability. This review proposes the Gut Microbiome Dependency Continuum, a four-layer framework describing progressively deeper levels of microbiome involvement in drug discovery and therapeutic function. The first layer, intact functional microbiome-dependent therapeutics and includes interventions such as faecal microbiota transplantation and defined microbial consortia. The second layer, microbiome-modulated approved drugs include widely used therapeutics whose pharmacokinetics or pharmacodynamics are strongly influenced by microbial metabolism. Examples include metformin, irinotecan, levodopa, and digoxin, where gut microbial interactions influence efficacy, toxicity, and inter-individual variability in treatment outcomes. The third layer, microbiota-transformable natural products, encompasses dietary and plant-derived compounds such as polyphenols, ginsenosides, alkaloids, fibres, isoflavones, lignans, and glucosinolates. Their biological activity depends on microbial biotransformation into bioactive metabolites. The fourth layer, engineered microbiome therapeutics, includes synthetic biology approaches such as programmable microbial systems, engineered probiotics, CRISPR-based microbiome editing, and microbiome-responsive drug delivery systems. It also includes synthetic microbial consortia, enabling targeted sensing, therapeutic delivery, and ecological reprogramming of gut microbial communities. Altogether, these layers define a continuum in which the gut microbiome evolves from a passive modulator to an essential metabolic organ and ultimately a programmable therapeutic platform. The article provides an integrated framework for microbiome-informed drug discovery. It also supports the development of precision, ecology-aware, and engineered microbial therapeutics.