Abstract
Bioavailability studies on aspirin suspensions in rabbits using an 8 × 8 latin square design of experiment have shown that significant differences in the amount of drug absorbed, but not in its rate of absorption, occur when different macromolecular suspending agents are used. Linear relationships between the logarithm of the apparent viscosity of the suspension medium at a shear rate of 100 s −1, pH 1.2 and 37°C and the amount of drug absorbed in 9 h and its peak concentration in the blood have been established. It is suggested that the effect of viscosity on the gastric emptying rate is the reason for the variation in the bioavailability of aspirin from the suspensions. Results obtained using suspensions of salicyclic acid in 1% w/v dispersions of sodium carboxymethylcellulose and xanthan gum in a two-way cross-over test in 8 rabbits substantiate those obtained with the aspirin suspensions and indicate that the effect of viscosity on the hydrolysis of aspirin in the gastro-intestinal tract is not important from a bioavailability points of view. The unsuitability of traditional flask-stirrer and dialysis techniques as methods capable of providing results for satisfactory correlation with the in vivo bioavailability parameters obtained in this study is discussed.
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