Studies on Absorption of Suppositories. VII. Effect of the Amount of Base on Absorption of Sulfonamides from Rabbit Rectum

To investigate the effect of the amount of a base on the absorption of drugs from the rectum, amount of rectal fluid and distribution region of a drug in the rectum after administration of a suppository were determined. The distribution region of a drug in the rectum increased in proportion to the volume of suppository, both in PEG and cacao butter base, The rectal fluid increased with increasing volume of the PEG base, but concentration of the base in rectal fluid was always about 30%, independent of the base volume, at 2 hr after administration. On the other hand, there was no detectable amount of the fluid but a little mucus in the rectum after administration of cacao butter suppository. The results of blood concentration studies showed that both the rate and extent of absorption of sparingly soluble sulfonamides were enhanced by the increasing amount of PEG and cacao butter base. The absorbability of water-soluble sulfonamide was reduced by the increased volume of PEG base, and a better absorbability of a soluble sulfonamide was obtained from cacao butter than from PEG base. The results of in vitro membrane permeation experiment with isolated gut segments served sufficiently to explain the influence of the amount of base on in vivo rectal absorption of sulfonamides. From the results of in vivo and in vitro experiments, it was concluded that rectal absorption of sulfonamides from increased base volume of suppository depended mainly on the increment of the distribution area, that is, absorption area in rectum.