Dissolution of Solid Dosage Form. VII. Effect of Shape on the Dissolution of Nondisintegrating Single Component Tablet under Non-sink Condition

Abstract

Effect of tablet shape on dissolution was examined as an example of the application of the z-law and Ln-z equations. The tablet shape can be altered systematically by changing the diameter and weight of tablet, since these equations were applicable for dissolutions with various optional initial amounts within the amount required to saturate the solution.It was confirmed that the z-law and Ln-z equations efficiently treated the dissolution of tablet irrespective of the weight and size, i.e., the diameter and thickness as well as crystalline particles. Both dissolution equations gave almost the same dissolution rate constants for a tablet of a fixed weight and size.The dissolution rate constant changed with the tablet weight and size, and it was suggested that the latter factor probably had greater effect. The tablet size was then converted to the degree of isometricity or shape factor. Then, the effect of shape on the dissolution rate constants of tablets (k_z) was estimated using the z-law equation. The k_z-value of a long cylindrical tablet was large, and it decreased to reach a minimum value with decrease in tablet thickness and increase in diameter. The k_z-value increased to reach almost a fixed value of shallow cylindrical tablets as if it had been obtained by a rotating disk method.The difference in the dissolution rate constants was examined using a specially devised tablet of which the flat-faced surface or curved surface was coated with wax. Dissolutions with these tablets suggested that the dissolution rate constant of the long cylindrical tablet was the larger of the two. Also, the way in which the flat-faced and curved surfaces contribute to the k_z-value was examined as a function of the ratio of flat-faced surface area (2S_d) to the whole surface area (S_o), i.e., 2S_d/S_o. The k_z-value decreased almost straightly within the 2S_d/S_o-value reached to around 0.63, showing a close similar value where the 2S_d/S_o-value was larger than around 0.73. Thus, it was suggested that the tablet form should be taken into consideration when necessary.