Characterization of naproxen salts with amino acid esters and their application in topical skin preparations.

In the study, the modification of naproxen (NAP) with esters of four amino acids (AAs): glycine (GlyOiPr), L-proline (ProOiPr), L-leucine (LeuOiPr), and L-serine (SerOiPr) isopropyl ester was performed to improve water solubility and enhance the permeation of the drug through the skin in comparison to the parent NAP. The NAP derivatives were prepared using the equimolar ratio of the components. In-depth NMR and FTIR analysis revealed that the salts formed with the proton transfer from the carboxylic group of NAP to the amine group of the appropriate AA ester. The NAP salts exhibited improved solubility in water and PBS solution (pH 7.4) when compared to parent NAP. The values of the partition coefficient (log P _O/W ) for prepared salts were lower than for NAP, however, the salts maintained hydrophobic character determined by the positive values of log P. The In vitro permeation through the pig skin performed in Franz diffusion cells showed that all NAP salts exhibited a higher cumulative mass of permeated NAP (Q _24h ) than the parent acid. The highest permeation value was noted for [ProOiPr][NAP], with a pseudo-steady state flux (J _ss ) 32.5 µg NAP cm ^-2 h ^-1 , and Q _24h  = 246.4 µg NAP cm ^-2 , it was 2.5 % of the applied dose. Moreover, topical preparations with [ProOiPr][NAP] and NAP were prepared based on two vehicles - Celugel® and Pentravan®- approved in pharmacy recipes. The permeation experiments through the Strat-M® showed, that both the J _ss and Q _24h of permeated drug from preparations containing [ProOiPr][NAP], were statistically several times greater than from the respective preparations with the unmodified acid. Additionally, preparations with [ProOiPr][NAP] provided significantly improved permeation of NAP than two commercial preparations, one of which contained naproxen as the acid and the other - as the sodium salt.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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