Application of USP Apparatus 4 and In Situ Fiber Optic Analysis to Microsphere Release Testing

Introduction In the past 15 years,the number of novel drug dosage forms such as controlled release (CR) parenterals (microspheres, liposomes,stents,and other implants) entering to the U.S. market has increased (1). These products can deliver drugs at a desired rate over periods of several days to months. It is essential to develop discriminatory in vitro release testing methods and standards for these delivery systems. In vitro release methods are necessary to monitor batch-to-batch variability,evaluate any process or manufacturing change,ensure the clinical performance of the drug i.e. safety and efficacy and determine a relationship between the in vitro and in vivo release data. Standard dissolution methods are inappropriate for CR parenterals since:these were designed for oral and transdermal products; they utilize large volumes of media,and often require separation of the delivery system from the release media for analysis. Methods of in vitro release testing that are currently used for CR parenterals include membrane diffusion,sample-and-separate, in situ and continuous flow (2). USP apparatus 1 (basket),2 (paddle) and 4 (flow through) have been used for microspheres and other dispersed systems. Sample-and-separate methods have been used where samples are taken from USP Apparatus 2 and the dispersed phase is isolated by centrifugation or filtration. In addition to the official USP Apparatus,other methods such as miniaturized methods and dialysis sacs have been used for CR dispersed systems. Since most of these products are small volume parenterals,miniaturized methods have been designed where the samples are placed into small vials instead of one-liter vessels. Problems with this method include violation of sink conditions,sample aggregation due to limited agitation,and disruption of the delivery system during centrifugation or filtration (e.g. coalescence of emulsion droplets,or aggregation of liposomes) in sample-and-separate methods leading to inaccuracy in the release data. A recent AAPS-EUFEPS workshop report (2003) on “Assuring Quality and Performance of Sustained and Controlled Release Parenterals”stated that there is a need for standards for in vitro release testing methods for controlled release parenterals with respect to apparatus, media,sampling interval,and temperature,and also acknowledged that guidance on in vivo release testing of these products and development of in vitro-in vivo relationships/prediction is necessary (1). In vitro release testing methods for poly (lactic-co-glycolic acid) (PLGA) microspheres are discussed herein. The 2003 AAPS-EUFEPS workshop report recommended the use of USP Apparatus 4 for in vitro release testing of CR microspheres (1). USP Apparatus 4,which is a flow through method that includes a pump,flow-through cells,water bath and media reservoir,was designed as an in vitro dissolution method for controlled release oral powders,granules,and solid dispersions. The specifications with respect to cell size of this apparatus are described in the US Pharmacopeia (3). USP Apparatus 4 can be operated under different conditions such as open or closed system mode;different flow rates and temperatures. The diversity of available cell types allows application to a wide range of dosage forms. Since the reservoir volume is not fixed,media volume can be decreased to accommodate systems where the concentration of drug released would otherwise be below the detection limit (e.g. systems with low drug loading) or increased to allow ease of maintenance of sink conditions for poorly soluble compounds. USP Apparatus 4 also offers flexibility in monitoring release via in situ UV analysis. Such analysis can be achieved without any correction for scattering by the dispersed system or particle accumulation on the probes since the microspheres or other delivery system are isolated Abstract The objective of this study was to determine the applicability of USP Apparatus 4 and in situ fiber optic UV analysis to in vitro release testing of poly (lactic-co-glycolic acid) (PLGA) microspheres. The release of dexamethasone from microspheres prepared with both high and low Mw PLGA was evaluated. Dexamethasone release profiles obtained using USP apparatus 4 were compared with those obtained using a sample-and-separate method. Cumulative release obtained using USP Apparatus 4 was approximately 16% higher over a 30-day period when compared to that obtained using the sample-and-separate method. This difference was attributed to loss of microspheres during separation as well as to microsphere aggregation in the sample-and-separate method. The versatility of USP Apparatus 4 with respect to alteration of flow rate allowed demonstration of diffusion-controlled release from low Mw PLGA microspheres. Fiber optic UV probes used in conjunction with USP Apparatus 4 enabled complete characterization of the initial burst release of drug from the microsphere systems.