Kinetics of drug release from hydrophobic polybasic gels: effect of buffer acidity

The present work studies the variability of drug release rates from hydrophobic polybasic gels, due to the presence of acidic buffer species. Release kinetics of the model drug caffeine from crosslinked poly(methyl methacrylate-co-N,N-dimethylaminoethyl methacrylate) hydrogels were measured as a function of buffer concentration, buffer acidity (pK), and solution pH, with total ionic strength held constant. Results show that the release rate of caffeine is determined by the concentration of buffering species in the nonionized, conjugate acid form, and not by the pH itself. These results correlate well with the buffer effects on swelling rate that have been demonstrated elsewhere [8]. Since it is difficult to control the concentration and composition of weak acids in the stomach fluid, precise pH-modulated gastric controlled release from hydrophobic polybasic gels may be difficult to achieve. These gels may be more suitable in cases where pH-triggered release is desired, but precise rate control is not warranted.