Effect of particle size on the available surface area of nifedipine from nifedipine-polyethylene glycol 6000 solid dispersions

Solid dispersions containing 5%, 10%, 20%, 30% and 50% of nifedipine were prepared with polyethylene glycol (PEG) 6000 as carrier, respectively, by the fusion method. Drug release from four different size fractions of nifedipine-polyethylene glycol 6000 solid dispersions were examined. The probability parameters of Weibull distribution or log-normal distribution could be obtained from linear regression of the dissolution data. The effects of particle size on the dissolution rate of nifedipine were evaluated in terms of the time course of the available surface area (S(t)). The X-ray diffraction patterns showed that nifedipine was dispersed homogeneously in an amorphous state in the solid dispersions with nifedipine concentration up to 10%. The initial values and faster rate of decrease of S(t) during the dissolution process were markedly enhanced in the solid dispersions with lower contents of nifedipine (5% and 10%) due to the formation of high energy metastable (amorphous) states of the drug and differences in the particle sizes had little effect on the values of the available surface area and the dissolution of the drug. Values of available surface area were particle size dependent for the solid dispersions with higher contents of nifedipine (20%, 30% and 50%) and the rate of decrease of S(t) was enhanced as the particle size reduced.