Bioequivalence of two formulations of didanosine, encapsulated enteric-coated beads and buffered tablet, in healthy volunteers and HIV-infected subjects.

Didanosine is an acid labile drug and hence has been given with buffering agents. To avoid the need for concurrent administration with antacids, an encapsulated enteric-coated bead formulation of didanosine was developed. The objective of this study was to assess the bioequivalence of the encapsulated enteric-coated beads compared to the buffered tablet. Two separate open-label, randomized, two-way crossover studies were conducted, one in healthy subjects and the other in HIV-infected subjects (with CD4 cell counts > 200 cells/mm3). All subjects received a 400-mg dose of the buffered tablet (reference formulation) and the encapsulated enteric-coated beads (test formulation). Blood samples were collected over 12 hours, and plasma levels of didanosine were determined using a validated assay. The 90% confidence interval (CI) of the ratio of the geometric means of log-transformed Cmax and AUCinfinity values were used to assess bioequivalence between the two formulations using the equivalence interval of 0.80 and 1.25. In healthy volunteers (n = 46), the point estimate and 90% CI of the ratios of Cmax and AUCinfinity values were 0.58 (0.52, 0.64) and 1.02 (0.95, 1.01), respectively. In HIV-infected subjects (n = 30), the point estimate and 90% CI of the ratios of Cmax and AUCinfinity values were 0.64 (0.56, 0.72) and 0.95 (0.86, 1.06), respectively. Median t(max) value increased significantly from 0.67 hours for the buffered tablet in both studies to 2.33 hours (in healthy subjects) or 2.0 hours (in HIV-infected subjects) for the enteric-coated beads. The mean half-life of didanosine was similar between treatments and ranged between 1.60 and 1.70 hours across healthy and HIV-infected subjects. It was concluded that the encapsulated enteric-coated bead formulation of didanosine is equivalent to the buffered tablet in the extent of exposure but differs in the rate of absorption. The pharmacokinetic profile of the enteric formulation appears to be similar in healthy and HIV-infected subjects.