Importance of hydrolysis of amino acid moiety in water-soluble prodrugs of disodium cromoglycate for increased oral bioavailability.

The relationship between physicochemical properties and oral absorption was investigated in prodrugs for the oral delivery of disodium cromoglycate (DSCG). To improve the lipophilicity of DSCG, various lipophilic moieties were introduced into the twin carboxyl groups. However, this did not lead to improved oral absorption in rabbits because of loss of water solubility, in spite of improved lipophilicity. Water-soluble prodrugs, in which an amino acid was introduced into a hydroxy group by ester linkage in addition to ethyl residues at twin carboxyl groups of DSCG, were synthesized and examined for oral absorption in rabbits and rats. The oral absorption of these prodrugs was affected by the species of amino acids introduced as a water-soluble moiety. Therefore, we examined the relation between the oral absorption of water-soluble prodrugs and the hydrolysis rate of the amino acid moiety. A good linear correlation was obtained between the oral absorption and the hydrolysis rate constant catalyzed by digestive enzymes, trypsin or alpha-chymotrypsin. It was thus concluded that the amino acid moiety of water-soluble prodrugs must be rapidly hydrolyzed to a permeable lipophilic prodrug still possessing the ethyl moiety at twin carboxyl groups in the small intestinal tract for good oral absorption.