Disposition and ultimate fate of a PLA-PEG sustained release polymer after intra-articular injection

Abstract

Purpose: Chronic inflammation after injury and surgical intervention drives catabolic metabolism and many downstream events that lead to osteoarthritis. Pain, altered gait or muscle recruitment tissue destruction is some of the consequences that occur in both animals and human patients. Oral Non-steroidal anti-inflammatory drugs (NSAIDs) are associated with cardiovascular, hepatic, renal and gastrointestinal complications. NSAIDs also alter hemostasis, so they are often avoided when anti-coagulant therapy is used. Intra-articular injection of joints with drugs and proteins utilizing a sustained release carrier is a relatively novel method of delivering active pharmacological ingredients to the synovial environment. This method could be cost effective and safer than daily oral NSAIDs. The purpose of the current experiments was to gain a better understanding of the fate of a polymer carrier after intra-articular injection and potential mechanisms by which it delivers active ingredients to the synovial environment. Methods: Eighteen female adult sheep were assigned to 6 groups of n = 3 and their right stifle joint was injected with 0.6 mL of a biodegradable mix of two polymers comprising D,L-lactide and polyethylene glycol (PDL-PEG) proprietary polymer (BEPOTM) combined with 102 mg of celecoxib and a biocompatible green dye (D&C #6). The contralateral limb was injected with 0.6 mL of physiologic saline USP. Plasma and synovial fluid were collected at times 0,1, 7,14 28, 42 and 90 days post injection. The sheep were sacrificed in groups of three at 0, 1, 7, 14 28, 42 and 90 days to collect articular cartilage, synovial membrane, menisci and any residual polymer in the synovial cavity. Macroscopic and histological observations (OARSI scoring) were conducted to characterize the polymer formulation and its interaction with synovial tissues. Plasma, synovial fluid and synovial tissue were analyzed for celecoxib concentration using HPLC. Automated counting to determine the synovial fluid leukocyte and protein concentration was measured using the total solids method. Results: Sheep that received polymer injections into the stifle joint experienced mild effusion for 3 days without lameness throughout the study. There was mild elevation in SF leukocyte concentrations (11.0 and 4.1 109/L) on days 1 and 7 respectively but protein concentration did not normalize until day 28. Very low (<10 ng/mL) levels of celecoxib were detectable in plasma on Day 1 only. Higher levels of celecoxib above the estimated therapeutic level were detected in the synovial fluid on Day 1 and declined to zero over 28 days, but celecoxib was detectable in synovial tissue until the end of the study at 90 days. Macroscopically, small polymer particles were found in all compartments, cul de sacs and gutters of the stifle joint demonstrating that the original polymer depot had been morselized and widely distributed. Most particles were adherent to the synovial membrane by day 7 and increasingly incorporated into the synovial tissues over time. Histology of the synovial membrane demonstrated that larger polymer particles were engulfed within folds of the synovial lining and hypertrophied synovial lining cells appeared to phagocytose smaller particles (Fig 1- left panel). Interestingly, where polymer was found within the synovial membrane a typical foreign body reaction was observed dominated by macrophages and giant cells resulting in degradation of the polymer particles (Fig. 1 right panel). The reactions were focal and localized to polymer; areas of synovial membrane lacking polymer displayed normal morphology. By day 28 there were minimal reactions to polymer particles and by day 90, no polymer was detected. There were no adverse effects on other synovial tissues. Conclusions: This study shows that the primary target of this sustained release drug formulation is likely to be the synovial membrane where the polymer matrix undergoes cellular degradation and hydrolysis whilst releasing the active ingredient. Celecoxib exposure in the circulation was minimal eliminating the potential for systemic complications, but celecoxib persisted in the synovial membrane longer than in synovial fluid. This mechanism of uptake and degradation suggests that the synovial membrane can be a depot for release of active pharmacological ingredients into the synovial environment. (Figure Presented).