Mechanistic understanding of LAI suspension intramuscular depot formation: A physicochemical perspective

Long-acting injectable (LAI) suspensions have emerged as a promising class of drug delivery systems offering sustained drug release, improved patient adherence, and reduced dosing frequency. However, their in vivo performance remains poorly understood due to the complex interplay of physicochemical and physiological factors governing depot formation at the intramuscular (IM) injection site. The present study provides a mechanistic understanding of intramuscular depot formation and drug release of Depo-Provera® (medroxyprogesterone acetate) injectable suspension and its qualitatively and quantitatively (Q1/Q2) equivalent formulations from a physicochemical perspective. Characterization of polyethylene glycol (PEG3350) polymers from two different suppliers revealed notable differences in molecular weight and polydispersity. These molecular-scale variations appeared to influence the strength of particle-polymer agglomerates, with the extent of this effect likely depending on both the polymer characteristics and the surface chemistry of the drug substance. These differing interactions subsequently contribute to variations in both in vitro and in vivo drug release behavior. In vivo studies in female New Zealand White rabbits demonstrated formulation-dependent differences in depot morphology and excipient retention. Depots formed by Depo Provera® and its Q1/Q2 equivalents with formulation differences retained more than 60% of PEG3350 after 14 days and were gel-like in appearance. Whereas, Depo Provera® Q1/Q2 equivalents with manufacturing differences showed reduced PEG retention and formed granular depots. Scanning electron microscopy (SEM) analyses further revealed distinct patterns of particle evolution: compact depots primarily underwent surface dissolution and largely preserved the particle morphology, while loosely packed granular depots dissolved from both the surface and the core, resulting in reduced particle size and altered morphology. Differential scanning calorimetry showed that medroxyprogesterone acetate in the formulations with manufacturing differences maintained crystallinity, whereas those with formulation differences demonstrated reduced melting points consistent with plasticization and hydration effects. Histopathological and immunohistochemical analyses showed that depot morphology directly influenced the local tissue response. Compact depots primarily induced macrophage accumulation at the periphery. In contrast, sparse, loosely structured, granular depots allowed deeper macrophage infiltration and elicited a more pronounced inflammatory response. Co-administration of dexamethasone microspheres markedly attenuated inflammation and reduced CD68-positive cell density across the formulations. However, no clear correlation was observed between particle size and in vivo drug release under the study conditions, potentially due to confounding factors affecting systemic clearance. Instead, a potential association was identified between depot structure, local immune response, and drug release kinetics, highlighting depot morphology and tissue interactions as key determinants of the in vivo performance.