Effect of loading on swelling-controlled drug release from hydrophobic polyelectrolyte gel beads

The effect of oxprenolol HCl loading on the kinetics of polymer swelling and drug release from suspension-polymerized poly(methyl methacrylate-co-methacrylic acid) (PMMA/MAA) beads has been studied in detail. Within the range of variables studied, the polymer swelling rate increases with buffer pH and concentration. And an ionization-controlled swelling mechanism (analogous to the relaxation-controlled mechanism) mechanism) seems to become more rate-limiting at higher buffer concentrations. At oxprenolol HCL loading levels below 17.8%, the drug release and associated dimensional changes (in pH 7.4) exhibit an extended quasi-linear region despite the inherent limitation of spherical geometry. At higher loading levels, the drug release becomes faster and first-order in nature. This is apparently a result of the transition from a dissolved to a dispersed system above a percolation threshold (15-18% loading in the present study). As a result of competition from processes such as the reduction of bead dimension due to drug release and the dimensional increase due to polymer swelling and osmotic contributions from the drug, the transient bead diameter increases monotonically during drug release at loading levels up to 25.6%, whereas upon further increasing the drug loading, the bead diameter goes through a maximum during the early drug release, which eventually increases again as a result of the slow but continuous increase in polymer swelling due to further ionization. In all cases, both the drug release and the dimensional changes approach completion as the penetrating ionization fronts meet at the center, indicating a true swelling-controlled behavior.