Influence of Macrogol Bases on the Bioavailability of Bacampicillin after Rectal Administration in Rabbits

Rectal administration of bacampicillin hydrochloride (BAPC) in Witepsol H-15 base suppository provided a fairly good absorption in rabbits; the extent of bioavaliability was 44%. Since BAPC has high lipophilicity compared to the parent compound (ampicillin, ABPC), rectal absorption from hydrophilic bases (macrogol 1000 and/or 4000) was examined. The highest bioavailability (88%) was obtained with macrogol 1000 base, but the bioavailability devreased with the invrease of macrogol 4000 base addition. It is known that macrogol bases increase the secretion of rectal fluid while oleaginous bases fo not change the amount of the fluid. The solubility of BAPC, however, was constant regardless of the amount and kind of the macrogol bases. In contrast, the partition coefficients (toluene/water) were reduced by the addition of both kinds of macrogol bases. Furthermore, the release rates of BAPC from the suppositories were little affected by the addition of the bases. Therefore, macrogol 1000 base seemes to improve the bioavalability by prolonging absorption with an invrease of the distribution area in the rectum. On the other hand, macrogol 4000 base is assumed to reduce the bioavailability by taking up secreted rectal fluid to dissolve and/or by forming a highly viscous solution.