The present study aims at elucidating the intricate nature of the drug release and absorption following intramuscular (i.m.) injection of sustained-release prodrug nanocrystals/microcrystals. A paliperidone palmitate (PPP) long-acting suspension was characterized with regard to particle size (Dv,50 = 1.09 μm) and morphology prior to i.m. injection in rats. The local disposition was rigorously investigated by means of (immuno)histochemistry and transmission electron microscopy while the concurrent multiphasic pharmacokinetics was linked to the microanatomy. A transient (24 h) trauma-induced inflammation promptly evolved into a subclinical but chronic granulomatous inflammatory reaction initiated by the presence of solid material. The dense inflammatory envelope (CD68(+) macrophages) led to particle agglomeration with subsequent drop in dissolution rate beyond 24 h postinjection. This was associated with a decrease in apparent paliperidone (PP) absorption (near-zero order) until 96 h and a delayed time of occurrence of observed maximum drug plasma concentration (168 h). The infiltrating macrophages phagocytosed large fractions of the depot, thereby influencing the (pro)drug release. Radial angiogenesis (CD31(+)) was observed throughout the inflammatory rim from 72 h onwards and presumably contributed to the sustained systemic PP concentrations by maintaining a sufficient absorptive capacity. No solid-state transitions of the retrieved formulation were recorded with X-ray diffraction analysis. In summary, the initial formulation-driven prodrug (PPP) dissolution and drug (PP) absorption were followed by a complex phase determined by the relative contribution of formulation factors and dynamic physiological variables.
Keywords: biocompatibility; disposition; histopathology; inflammation; muscle; nanoparticles; paliperidone palmitate; pharmacokinetics; prodrugs; sustained release.
© 2014 Wiley Periodicals, Inc. and the American Pharmacists Association.