The characterization and release kinetics evaluation of baclofen microspheres designed for intrathecal injection

Baclofen, a water soluble drug advocated for the treatment of spinal spasticity, was microencapsulated, using the oil/water emulsion extraction process in an attempt to identify the appropriate experimental conditions capable of producing microspheres releasing baclofen over 2-4 weeks. Individual microspheres ranging in size from 15 to 30 microns were formed exhibiting smooth surfaces at low drug payload (12.8% w/w), irregular and rough surface at high drug content (33.9% w/w). The microencapsulation yield remained practically unchanged (85-90%) up to theoretical payloads of 37.5% w/w, and decreased markedly to 70% when the initial theoretical payload was 50% w/w. The in vitro release profile of baclofen from the poly(D,L-lactide-co-glycolide) microspheres was biphasic only for the high drug payload microspheres with a rapid release of 70% within 48 h, followed by a slower release rate over at least 25 days. In contrast, the microspheres containing low baclofen contents (12.8% w/w) exhibited a gradual and progressive release rate over the course of the experiment. The baclofen release data did not fit either the general equation which describes the diffusional release of dispersed tiny drug particles from spherical micromatrices, or to the kinetic equations which describe the release of dissolved drug from monolithic microspherical devices. It appears that the release of baclofen from the present microspheres is not governed by a unique mechanism. This should be attributed either to the presence of some uncoated drug particles or to the large size of the embedded drug particles compared with the relatively small size of the spherical micromatices, or to some polymeric erosion occurring after several days incubation in the release medium.