Modulation of rifampicin release from spray-dried microspheres using combinations of poly-(DL-lactide)

Microspheres containing 20% w/w rifampicin (RIF) with smooth morphology have been readily prepared from combinations of low, R104 (Mw, 2000) and moderate, R202H (Mw, 9000), molecular weight poly(D,L-lactide) (PDLLA) as a means to modulate drug release from either polymer when used alone. These have been characterized with respect to their drug loading, granulometry, in vitro drug release and thermal behaviour. Particle size distributions were Gaussian, whereby mean microsphere diameter was found to increase from 2.11 to 2.98 microns as the proportion of more viscous R202H increased, whilst > 95% of particles were < 10 microns, irrespective of the polymer blend used. Use of a reduced inlet temperature for spray-drying gave uncharacteristically high production yields in the range of 55.8-80.7% for the process. Encapsulation efficiencies were quantitative with the weight proportion of drug co-dissolved (p < 0.05), yielding microspheres of high and predictable RIF loading. In vitro drug release revealed a dramatic shift in release profile between 40 and 60% R104. Closer examination in this range showed the predicted pattern of increased release rate as the fraction of more hydrophilic R104 increased. However, disproportionate differences were evident between 44 and 48% R104. From the apparent temperature dependent drug release, the criticality of matrix composition was attributed to the coincidence of matrix softening with the dissolution medium temperature and consequent hydration, which, at a finite composition, resulted in a controlled auto-hydration mechanism. Dramatic dependence of release rate with dissolution methodology was accountable to the fact that drug release was considerably quicker where microspheres remained suspended and individualized with the USP paddle method as opposed to aggregated with the shaking bath methodology. In conclusion, the utility of blending racemic PDLLA to modulate drug release and the convenience of spray-drying as a technique to produce microspheres of predictable character have been demonstrated. The temperature-dependent release exhibited may have application in the site-specific delivery of drugs where local increased biochemical activity promotes drug release in response to an increased pharmacological need.