Validation of a cage implant system for assessing invivo performance of long-acting release microspheres.

Abstract

Here we describe development of a silicone rubber/stainless steel mesh cage implant system, much like that used to assess biocompatibility of biomaterials [1], for easy removal of injectable polymer microspheres invivo. The sterile cage has a type 316 stainless steel mesh size (38μm) large enough for cell penetration and free fluid flow invivo but small enough for microsphere retention, and a silicone rubber shell for injection of the microspheres. Two model drugs, the poorly soluble steroid, triamcinolone acetonide, and the highly water-soluble luteinizing hormone-releasing hormone (LHRH) peptide superagonist, leuprolide, were encapsulated in PLGA microspheres large enough (63-90μm) to be restrained by the cage implant invivo. The invitro release from both formulations was followed by ultra-performance liquid chromatography (UPLC) with and without the cage in a standard release media, PBS pH 7.4+0.02% Tween 80+0.05% sodium azide, at 37°C. Pharmacokinetics (PK) in rats was assessed after SC injection or SC in-cage implantation of microspheres with plasma analysis by LC-MS/MS or EIA. Tr-A and leuprolide invitro release was largely unaffected after the initial burst irrespective of the cage or test tube incubation vessel and release was much slower than observed invivo for both drugs. Moreover, Tr-A and leuprolide pharmacokinetics with and without the cage were highly similar during the 2-3 week release duration before a significant inflammatory response was caused by the cage implant. Hence, the PK-validated cage implant provides a simple means to recover and evaluate the microsphere drug carriers invivo during a time window of at least a few weeks in order to characterize the polymer microsphere release and erosion behavior invivo. This approach may facilitate development of mechanism-based invitro/invivo correlations and enable development of more accurate and useful invitro release tests.