Drug release from poly(dl-lactide) microspheres controlled by gamma-irradiation

The use of gamma-irradiation as a method for controlling the drug release from poly(dl-lactide) microspheres was studied. Polymer decomposition caused by the gamma-irradiation of microspheres was utilized to adjust the glass transition temperature (T-g) of microspheres which determines the drug release rate, without altering the physical properties of the microspheres such as surface area and particle size other than the molecular weight of polymer. The initial release rate of progesterone from dl-PLA microspheres increased as T-g decreased in response to gamma-irradiation. The decrease in T-g depended on the irradiation dose, indicating that the release rate can be controlled by irradiation dose. dl-PLA microspheres irradiated at a relatively low dose exhibited a two-phase release profile; a slower initial release followed by a rapid release after the T-g was lowered below 37 degrees C. The time period before the start of rapid release could be controlled by altering the irradiation dose.