Synthesis, stereoselective enzymatic hydrolysis, and skin permeation of diastereomeric propranolol ester prodrugs

Four diastereomeric propranolol ester prodrugs (1S2S, 1S2R, 1R2S, 1R2R) were synthesized by treating pure R- and S-propranolol hydrochloride with pure enantiomers R- and S-phenylbutyryl chloride. A HPLC technique using alpha-1 acid glycoprotein (chiral AGP) column was developed to study the racemization of propranolol enantiomers during synthesis and hydrolysis studies. A reversed phase HPLC method was also developed to simultaneously analyze propranolol and the ester prodrug. Hydrolysis of these esters was studied in different rat tissue homogenates, i.e., liver, intestine, plasma, skin, brain, and pure plasma cholinesterases, i.e., butyryl cholinesterase (EC 3.1.1.8) and acetyl cholinesterase (EC 3.1.1.7). In vitro percutaneous permeation studies across full thickness shaved rat skin were performed using standard side-by-side diffusion cells at 37 degrees C. The disappearance of the diastereomeric ester prodrugs in rat tissue homogenates followed apparent first-order kinetics and was stereoselective. The ratio of brain to plasma hydrolytic rate constants are 27.8, 5.58, 6.07, and 2.97 for 1S2S, 1R2R, 1R2S, and 1S2R esters, respectively. Hydrolysis of all four diastereomeric ester prodrugs was faster by acetyl cholinesterase than butyryl cholinesterase and is stereoselective. The permeability coefficients [Kp x 10(3) (cm h-1)] are 1.40 +/- 0.30, 1.41 +/- 0.27, 42.20 +/- 1.24, 29.26 +/- 3.41, 16.27 +/- 3.12, 12.99 +/- 2.84 for (R)-propranolol, (S)-propranolol, 1S2S, 1R2S, 1S2R, and 1R2R ester prodrugs, respectively. The results indicate that the 1R2S diastereomeric ester prodrug of propranolol shows greatest stability in liver and intestinal tissues while it exhibits fairly rapid conversion in plasma. The results also suggest the configuration on the second chiral carbon atom to be the determinant in the rate of hydrolysis of all the diastereomeric prodrugs in all biological media examined. The Kp of all four prodrugs markedly increased compared to that of the parent drug, with 1S2S showing a 30-fold increase in skin permeability, the highest among all four prodrugs.