1999 Volume 22 Issue 3 Pages 305-309
In vitro simultaneous transport and metabolism of three ester prodrugs of nicotinic acid (NA), methyl nicotinate (MN), ethyl nicotinate (EN) and butyl nicotinate (BN) were studied using excised skin from hairless mouse. Hydrolysis studies of these esters with and without skin homogenate were also done at 37°C. Both the ester and NA were detected in all receiver solutions in permeation studies, and no chemical hydrolysis of the esters was found, indicating that the esters were hydrolyzed during the skin permeation process. The total (ester+NA) flux from a saturated solution of ester prodrugs was higher than that of NA and was highest for MN, followed by EN and BN, whereas the total permeability coefficient of ester prodrugs increased from MN to BN. A difference in the NA/total flux ratio was found among these prodrugs; thus, esterase activity was also dependent on the alkyl chain length of the esters. The total flux from each ester solution increased linearly with the donor concentration. NA flux from MN and EN solutions increased with an increase in the donor concentration and reached a plateau at the high concentration range, suggesting that metabolic saturation occurred. NA fluxes at the plateau were similar among ester prodrugs and corresponded to the Vmax estimated from the hydrolysis experiment. The order of donor concentration at which NA reched a plateau also corresponded to the order of Km. It was confirmed that a difference in alkyl chain length of the ester prodrugs affected not only permeability but also metabolism in the skin permeation process.
