Controlled release of progesterone from polyethylene oxide-silicone rubber matrix

Bioabsorbable polymers for implantable therapeutic systems

For a long time, subcutaneous implantable drug pellets using nondegradable polymers have been used for long-term, continuous drug administration. The procedure requires surgical implantation and removal of the drug-containing devices or polymeric matrices, which has a significant negative impact on the acceptability of the product candidate. In addition, the release profile from such devices is neither constant nor readily controlled in terms of precision of rate of release and duration of action. These facts have led to the research and development of novel, controllable, nonirritating, noncarcinogenic, biocompatible, and bioabsorbable drug delivery systems for overcoming the drawbacks of nondegradable implantable pellets for prolonged continuous release. Biodegradable implantable systems release the drug over a long period of time with simultaneous or subsequent degradation in the tissue of the polymer to harmless constituents, thus avoiding removal once the therapy is complete. This approach has considerably improved patient acceptability and patient compliance. Various bioabsorbable polymers have been evaluated for controlled implantable drug delivery, including hydrogels, copolymers of polylactic and polyglycolic acids, polylactic acid, poly(orthoesters), polyanhydrides, poly(E-caprolactone), and polyurethanes. Their characteristics have been studied using a variety of drugs, like anticancer agents, hormone agonists and antagonists, nonsteroidal anti-inflammatory agents, neuroleptics, contraceptives, and others. The present paper describes the current research on implantable therapeutic systems, the bioabsorbable polymers, and the biologically active agents being used in this approach.

Controlled drug release from implantable matrices based on hydrophobic polymers

Reports on the controlled release of drugs, including macromolecular drugs, from silicone elastomers and ethylene-vinyl acetate copolymers, based on the formation of channels and cracks in the polymer, are reviewed. Aqueous interconnected pores are produced by osmotically active additives or by using loads of water-soluble drugs exceeding the percolation threshold. The release is generally proportional to the square root of time (t1/2). Nevertheless, pseudo-zero-order release kinetics can be obtained by adequately controlling the formulation variables. The factors controlling the release pattern and rate are discussed. In vivo applications of these types of systems are also considered.