Release characteristics of polymethacrylate nanospheres containing coumarin-6

Sustained release nanospheres were prepared from the polymethacrylates Eudragit^r` S₁₀₀ and E₁₀₀ containing a water insoluble dye by a salting-out method. Coumarin-6 was used as a model for insoluble analgesics to ascertain uptake and release properties dependent on polymer characteristics and pH. Morphology and particle size were characterized by scanning electron microscopy (SEM). Particles were smooth, spherical and uniform with diameters ranging from 0.6–0.8 μm. Yield was 38% and 86% for E₁₀₀and S₁₀₀, respectively, and encapsulation of coumarin-6 efficiency was 58% and 75%, respectively. Coumarin-6 was stable within the polymer matrix at temperatures from -20$C to 45$C for 4 months. Release was most efficient from S₁₀₀ polymers in phosphate buffer at pH 7.4 and 8.0 reaching a maximum ～ 5 hours prior to samples at pH 7.0 and 9.0. Release was biphasic and concentration as a function of the square root of time produced linear data suggesting a Higuchi type diffusion from a polymer matrix. Release from E₁₀₀ was 65% lower than that from S₁₀₀ and was not solely dependent upon the ionization of polymer but most likely due to a combination of factors including buffer ionization.