The promoting effects of sodium salts of N-acyl-L-phenylalanines, p-substituted benzoic acids, saturated straight-chain fatty acids, saturated straight-chain α-bromofatty acids and N-acyl-N-methylglycines on the rectal absorption of sodium ampicillin (ABPC Na) were investigated in rats. The absorption-promoting effect of each carboxylic acid sodium salt was found to be parabolically correlated with its lipo-hydrophilic character (log P). The optimal log P value (log P0) of carboxylic acids exerting the maximum rectal absorption-promoting effect was in the range of 4.2-4.8. Among these carboxylic acids, sodium caprate was selected for examination of its rectal absorption-promoting action on four penicillins (ampicillin, sulbenicillin, piperacillin and mezlocillin) and eight cephalosporins (cephacetrile, ceftizoxime, cephalothin, cefazolin, cefmetazole, cefotiam, cefoperazone and cefpiramide). The extents of rectal absorption of the β-lactam antibiotics coadministered with sodium caprate were found to correlate well with the permeability of the drugs to cellulose membrane.
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Nishimura et al. (1985) studied this question.