Drug accumulation dynamics designates the quantitative description of how intracellular drug concentration evolves over time as a net balance between drug entry and exit rates. Drug enters via passive diffusion or carrier-mediated uptake, while removal occurs through passive leakage and active efflux transporters such as P-glycoprotein ^[1][2][3]. Under given conditions, these opposing processes approach a steady-state intracellular concentration at which entry equals removal ^[2][3]. The kinetic description comprises uptake and efflux rate constants, the transmembrane concentration gradient, intracellular binding capacity, and efflux pump saturation kinetics ^[4]. Within drug transport and resistance mechanisms, the concept is defined by its explanatory role: acquired resistance in cancer cells is frequently characterised by reduced steady-state intracellular drug accumulation, arising from diminished uptake or enhanced efflux capacity ^[2]. The term thus designates the formal, rate-based account of intracellular drug exposure—spanning influx, efflux, binding, and approach to steady state—as the kinetic determinant of cytotoxic response and resistance ^[1][2].