Effects of the Ester Moiety on Stereoselective Hydrolysis of Several Propranolol Prodrugs in Rat Tissues

The stereochemical characteristics of the hydrolysis of several ester-type prodrugs of propranolol, O-acetyl, O-propionyl, O-butyryl, O-pivaloyl and succinyl esters, were studied in phosphate buffer (pH 7.4), rat plasma and rat tissue homogenates. In phosphate buffer, no differences were observed in the hydrolysis rate between the esters of (R)- and (S)-propranolol. The effects of the ester moieties on the hydrolysis rate in phosphate buffer were in the following order: acetate > propionate > butyrate > succinate > pivalate. In rat plasma and tissue homogenates, the hydrolysis of the esters was accelerated, and stereoselective hydrolysis was observed. In plasma, the hydrolysis of the (R)-isomer was faster than that of the (S)-isomer except for the succinate ester. On the other hand, in the liver and intestine homogenates, the (S)-isomer was hydrolyzed faster than the (R)-isomer except for the succinate and pivalate esters in the liver homogenate. Also, the ratio of the hydrolysis rates (S/R) changed with the type of prodrug. As the length of the alkyl chain of the ester increased, the S/R ratio approached unity in liver and intestine homogenates but stayed almost constant in plasma. For the pivalate ester, stereoselective hydrolysis was observed in plasma and intestine homogenate but not in liver homogenate. The stereoselective hydrolysis of the succinate ester was observed only in intestine homogenate, but the S/R ratio was almost 1 in plasma, liver and intestine homogenates. No interconversion between (R)- and (S)-isomer was observed during the hydrolysis of any of the ester prodrugs. These results indicate that hydrolysis of ester-type prodrugs of propranolol occurs stereoselectively in rat tissues, and that its selectivity is dependent on the kind of tissue and prodrug.