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

Four diastereomeric propranolol ester prodrugs (1S2S, 1S2R, 1R2S, 1R2R) wer e 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 racemiza tion of propranolol enantiomers during synthesis and hydrolysis studies. A reversed phase HPLC method was also developed to simultaneously analyze pro pranolol and the ester prodrug. Hydrolysis of these esters was studied in d ifferent rat tissue homogenates, i.e., liver, intestine, plasma, skin, brai n, 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 standar d side-by-side diffusion cells at 37 degrees C. The disappearance of the di astereomeric ester prodrugs in rat tissue homogenates followed apparent fir st-order kinetics and was stereoselective. The ratio of brain to plasma hyd rolytic 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 a nd is stereoselective. The permeability coefficients [K-p 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 i n liver and intestinal tissues while it exhibits fairly rapid conversion in plasma. The results also suggest the configuration on the second chiral ca rbon atom to be the determinant in the rate of hydrolysis of all the diaste reomeric prodrugs in all biological media examined. The K-p of all four pro drugs markedly increased compared to that of the parent drug, with 1S2S sho wing a 30-fold increase in skin permeability, the highest among all four pr odrugs.