Pharmaco-microdynamics (PMD): Redefining dose, exposure, and control for living drug carriers

Living drug delivery systems including probiotics, engineered microbial therapeutics, and live biotherapeutic products represent a rapidly emerging therapeutic modality whose behavior fundamentally diverges from the assumptions underlying classical pharmacokinetics and pharmacodynamics (PK/PD). Unlike chemically defined, non-replicating drugs, living therapeutics persist, replicate, adapt, and generate bioactive molecules in situ, such that therapeutic exposure is not externally imposed but biologically generated over time. As a result, administered dose functions only as an initiating condition, while realized exposure emerges from population dynamics, ecological establishment, spatial localization, and regulated functional output. These properties render concentration-based PK/PD frameworks insufficient for predicting efficacy, safety, and controllability of living drug carriers. We introduce pharmaco-microdynamics (PMD) as a quantitative delivery-science framework designed to define, measure, and control exposure for living therapeutics. PMD is operationalized through a set of formal metrics including the functional exposure integral (F-AUC), colonization efficiency (CE), residence-time-weighted activity (RTWA), effective functional concentration (EFC₅₀), and the genetic stability index (GSI) that serve as living-system analogues of AUC, bioavailability, mean residence time, EC₅₀, and product-identity specifications. PMD reconceptualizes exposure as a time-integrated biological process governed by four interdependent axes: population kinetics, functional output kinetics, spatial pharmacology, and evolutionary dynamics. By integrating principles from pharmacology, microbial ecology, synthetic biology, biomaterials science, and systems modeling, PMD provides an operational vocabulary for translating adaptive biological agents into predictable and engineerable delivery systems. We further delineate PMD from adjacent frameworks such as quantitative systems pharmacology (QSP) and ecological microbiome modeling, and critically discuss boundary conditions under which classical PK/PD remains applicable to non-replicating or transient microbial interventions. This review critically examines the limitations of classical PK/PD in modeling living drug carriers, formalizes the core principles of PMD, and illustrates them through three quantitative case studies: SYNB1618 for phenylketonuria, synchronized-lysis bacterial tumor therapies, and fecal microbiota transplantation for recurrent Clostridioides difficile infection. Regulatory and clinical implications are addressed, emphasizing the need to shift from dose- and concentration-centric evaluation toward functional biomarkers, persistence metrics, and model-informed assessment of biological activity. Collectively, pharmaco-microdynamics establishes a unifying conceptual and quantitative foundation for the rational development of living medicines.