Biodegradable polymers in controlled drug delivery.

ABSTRACT

Erosion mechanisms are divided into three types and drug release within each type is described. Type I erosion involves hydrolysis of hydrogels and these are useful in the controlled release of macromolecules entangled within their network structure. Type II erosion involves solubilization of water-insoluble polymers by reactions involving groups pendant from the polymer backbone. Of particular interest are polymers that solubilize by ionization of carboxylic acid groups, and the utilization of those systems is described. Type III erosion involves cleavage of hydrolytically labile bonds within the polymer backbone and four distinct polymer systems within this category are under development. One system involves the diffusion of drugs from a reservoir through a bioerodible membrane, another system utilizes microcapsules, a third system utilizes monolithic devices, and the fourth system utilizes drugs chemically bound to a bioerodible polymer.

ABSTRACT

PIP Controlled-release methodologies can be classified on the basis of the mechanism that controls the release of the active agent from the delivery device diffusion, osmosis, or polymer erosion. This paper presents an extensive review of the research on erosional devices. The various polymer erosion mechanisms are of 3 basic types. Type I erosion refers to water-soluble polymers that have been insolubilized by covalent cross-links and that solubilize as the cross-links (type IA) or backbone (type IB) undergo a hydrolytic cleavage. In type II erosion, polymers that are initially water insoluble are solubilized by hydrolysis, ionization, or pronation of a pendant group. In type III erosion, hydrophobic polymers are converted to small water-soluble molecules by backbone cleavage. The choice of a particular erosion mechanism is dictated by the specific application. In addition, there are 2 mechanisms of polymer release from bioerodible polymers one approach involves surrounding the drug core with a rate-controlling bioerodible membrane, while the other involves dispersing the drug within a polymer to form a bioerodible monolithic device. The use of biodegradable systems for the sustained release of fertility-regulating agents is based on type III erosion. Polymer erosion tends to lead drug release, and there is some indication that drug release from the implant is controlled by rate of solubilization of the highly water-insoluble steroid.