Long-acting injectable drug products are commonly interpreted through their nominal release-controlling material, particle attributes, or in vitro release profile. Such descriptions are useful but incomplete because the administered product does not remain in its pre-injection state. In situ forming systems undergo solvent exchange, phase separation, and structural consolidation; polymeric microspheres hydrate, reorganize, degrade, and erode; crystalline suspensions dissolve within a changing extracellular and cellular environment. Meanwhile, the surrounding tissue supplies fluid, buffering capacity, proteins, extracellular matrix, inflammatory cells, and potentially a foreign-body response that can modify the microenvironment through which drug must escape. This Mechanistic Perspective argues that long-acting injectable performance should therefore be interpreted through three coupled domains: formation and transformation of the depot, escape of drug from that evolving depot, and biological response of the local tissue. Evidence from phase-inverting implants, PLGA microspheres, crystalline suspensions, imaging studies, dissolution methodologies, cellular experiments, and mechanistic modeling indicates that control can be conditional and time dependent rather than attributable to a single invariant rate-limiting step. The perspective develops the basis for a proposed Depot–Escape–Tissue Coupling Model in which formulation attributes determine an initial state but subsequent release depends on reciprocal changes in matrix structure, drug physical state, fluid access, cellular interaction, and tissue response. The model is intended as a hypothesis-organizing and formulation-design framework rather than an empirically validated universal mechanism. Its value lies in identifying which state variables must be measured, challenged, and translated before an apparently stable release profile can be treated as mechanistically understood.
IJPRAS
