Research paper
Oxazolidines: prodrugs for the delivery of β-aminoalcohols through human skin from aqueous solution

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Abstract

Ephedrine (I) is delivered through human skin, in vitro, significantly faster from aqueous solutions of 3,4-dimethyl-5-phenyl-oxazolidine (II) with pH values between 7.0 and 10.88 than from corresponding solutions of I. The difference in penetration rates was most marked at pH 7.0 where 917 μg and < 10 μg of I was delivered through 1.3 cm2 of skin in 24 h from 1% w/v solutions of II and I, respectively. No detectable amounts of I were delivered through 1.3 cm2 of skin in 24 h from 1% w/v solutions of I or II in propylene glycol and delivery of I was faster from 1% w/v solutions of I in liquid paraffin than from corresponding solutions of II. Compound II has a lower pKa value than compound I (ca. 5.5 vs 9.63) and a higher partition coefficient between water and liquid paraffin (5.6 vs 1.0). Although II rapidly hydrolyzed to I plus formaldehyde (t50% < 1 min at pH values between 6 and 8), the system rapidly came to an equilibrium which increasingly favoured compound II as the initial concentration of II or the pH value was increased. Hence, it can be concluded that compound II and possibly other oxazolidines are potentially useful prodrugs for promoting the delivery through skin of I and other β-aminoalcohols from aqueous solutions with pH values close to 7.

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