Effect of suppository bases on rectal absorption rate of drugs
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Thein vitro release rate andin vivo absorption rate from various suppository bases was measured for benzoic acid (soluble in lipid, moderately soluble in water) and its sodium salt (very soluble in water, insoluble in lipid). Differences in the absorption rate due to a supposed unequal spreading of the basein vivo could not be detected.
Micronised fractions of sodium benzoate were absorbed at the same rate from the various bases, indicating that the transport of the particles through the bases and across the lipid-water interface, which is rate limiting in this case, is not influenced by the chemical composition of the base. Absorption of a coarse fraction of sodium benzoate was significantly faster from Witepsol H 15 and Estarinum B suppository bases than from cocoabutter and Massuppol.
It is concluded that in the case of sodium benzoate an interfacial resistance between the suppository base and the rectal fluid is present. This is probably caused by a combined effect of the aggregated state of the coarse sodium benzoate particles and an elastic emulsion layer formed at the interface of cocoabutter and Massuppol, since a coarse size fraction of sodium salicylate did not show this effect.
KeywordsLipid Benzoic Acid Salicylate Size Fraction Absorption Rate
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