Long-acting injectable (LAI) suspensions provide sustained drug release, improved adherence, and reduced dosing frequency. Despite clinical utility, the in vivo behavior of LAI suspensions is incompletely understood because depot formation at the intramuscular (IM) injection site reflects a complex interplay of physicochemical and physiological factors. This study focuses on mechanistic determinants of depot formation and release for Depo‑Provera® (medroxyprogesterone acetate) and qualitatively/quantitatively equivalent (Q1/Q2) formulations, analyzed from a physicochemical perspective.
The investigation aimed to link formulation attributes to depot morphology, excipient retention, drug physical state, local tissue response, and drug release kinetics. Key approaches included polymer characterization, in vivo implantation in female New Zealand White rabbits, scanning electron microscopy (SEM) to monitor particle evolution, differential scanning calorimetry (DSC) to assess drug crystallinity and thermal behavior, and histopathological plus immunohistochemical analyses to evaluate local immune responses. The effect of anti-inflammatory co-administration was tested using dexamethasone microspheres.
Polyethylene glycol (PEG3350) used as an excipient was sourced from two different suppliers and characterized for molecular weight and polydispersity. Notable differences at the molecular scale were observed between suppliers. These polymer differences altered the strength of particle–polymer agglomerates, an effect that also depended on the surface chemistry of medroxyprogesterone acetate. The study links these excipient-driven variations in agglomeration strength to downstream differences in both in vitro and in vivo drug release behavior.
In rabbit studies, depot morphology varied by formulation. Depots formed by Depo‑Provera® and certain Q1/Q2 equivalents with formulation differences appeared gel-like and retained a majority of PEG3350; specifically, more than 60% of PEG3350 remained present after 14 days. By contrast, Q1/Q2 equivalents with manufacturing differences produced granular depots with reduced PEG retention. Thus, both formulation composition and manufacturing processes influenced excipient persistence and gross depot appearance at the IM site.
SEM analyses identified distinct patterns of particle evolution tied to depot microstructure. Compact, densely packed depots predominantly exhibited surface-limited dissolution and largely preserved initial particle morphology over time. Loosely packed, granular depots underwent dissolution at both surface and core, producing reductions in particle size and notable morphological changes. These divergent dissolution behaviors reflect how depot packing and excipient distribution modulate local exposure of particles to surrounding fluid and cells.
Differential scanning calorimetry revealed formulation-dependent differences in the physical state of the drug. Formulations with manufacturing differences retained medroxyprogesterone acetate crystallinity, whereas formulations with specific formulation differences exhibited reduced melting points consistent with plasticization and hydration effects. These thermodynamic signatures suggest that excipient interactions can alter the solid-state environment of the drug within the depot.
Depot morphology directly influenced local tissue reactions. Compact depots primarily provoked macrophage accumulation at the depot periphery, while sparse, loosely structured granular depots permitted deeper macrophage infiltration into the depot and elicited a more pronounced inflammatory response. Immunohistochemical assessment included CD68-positive cell density as a marker of macrophage presence, correlating spatially with depot structure.
Co-administration of dexamethasone microspheres reduced local inflammation across formulations and decreased CD68-positive cell density. This finding demonstrates that modulation of the local immune environment can attenuate the inflammatory component of tissue response to intramuscular depots, though the study does not report further systemic or pharmacokinetic consequences beyond local inflammatory markers.
Under the study conditions, no clear correlation was observed between particle size and in vivo drug release, potentially due to confounding factors affecting systemic clearance. Instead, a potential association emerged linking depot microstructure and the character of the local immune response to observed drug release kinetics. Compact depots that preserved particle morphology tended to support different release behavior than loosely packed depots that experienced deeper immune cell infiltration and accelerated morphological change.
The data position depot morphology and tissue interactions as central determinants of LAI in vivo performance. Molecular differences in excipients such as PEG3350 can alter agglomeration strength and thereby influence depot structure, excipient retention, drug physical state, and local immune responses. These factors together shape drug release kinetics more strongly than particle size alone in this model system. The findings underline the importance of characterizing polymer attributes, manufacturing effects, and host responses when developing or assessing LAI suspensions. Details on specific quantitative in vitro–in vivo correlations, pharmacokinetic outcomes, and longer-term effects were not reported in the abstract and would require consultation of the full text for additional data and methodological specifics.