The objective of the present study was to investigate the effect of isopropyl myristate (IPM) on the in vitro permeation of testosterone through human cadaver skin from carbopol gels. Six testosterone ...

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The objective of the present study was to investigate the effect of isopropyl myristate (IPM) on the in vitro permeation of testosterone through human cadaver skin from carbopol gels. Six testosterone gel formulations were prepared using different IPM contents of 0%, 0.4%, 0.7%, 1%, 2%, and 3%. The gels were characterized for drug permeation, matrix morphology, pH, kinetics of ethanol evaporation, and viscosity. Mass balance studies were performed to estimate testosterone distribution among the compartments of diffusion cells. All formulations exhibited pH values of 5.1 and viscosities of 1.25-1.75 Pa.s depending on IPM contents. Under occlusive condition, testosterone flux was found to increase significantly (p < 0.05) by increasing IPM content. Gels containing 2% IPM exhibited 11-fold increase in flux compared with formulation devoid of IPM. Ethanol was found to have a synergistic effect with IPM in enhancing testosterone flux. Mass balance analysis showed that testosterone was in a saturated state in the skin. Conducting permeation experiments under nonocclusive condition was nondiscriminating because of the evaporation of alcohol and consequent precipitation of drugs. Based on demonstrated effect of IPM on product performance, the final IPM concentration should be controlled with minimal variation during manufacturing and shelf life of drug product (1).

Isopropyl myristate (IPM) is a widely used penetration enhancer in pharmaceutical formulations, however, its mechanism of action on a molecular scale is still not completely understood. Previous work using a quaternary Stratum Corneum (SC) lipid model in bulk suggested the incorporation of isopropyl myristate into the SC lipid matrix, phase separation, and perturbation of the multilamellar lipid assembly. Here, we used 2D Langmuir monolayers of a ternary SC lipid model, containing ceramide AP C18:18, stearic acid and cholesterol in a molar ratio of [1:1:0.7], respectively, to shed light on the mechanism of action of this important lipophilic penetration enhancer. To do so, the synthesis of chain deuterated isopropyl myristate was successfully performed in order to study the different coupling possibilities between the hydrogenated and deuterated IPM and the alkyl chains of the SC molecules. Our results indicate that only a small portion of IPM is able to mix with our SC model leading to a limited fluidizing effect (small increase of the wavenumber of CH2 stretching vibration, increase of the SC layer flexibility), but will be squeezed out at higher lateral pressures. Furthermore, the deuteration of IPM enhances the miscibility with this SC model, probably due to a different coupling between the alkyl chains or the alkyl and deuterated chains. Additionally, using the pure D-form of CER[AP] in the SC model amplifies the obtained results (2).

References___________________________________
(1) Effect of Isopropyl Myristate on Transdermal Permeation of Testosterone From Carbopol Gel.
Zidan AS, Kamal N, Alayoubi A, Seggel M, Ibrahim S, Rahman Z, Cruz CN, Ashraf M.
J Pharm Sci. 2017 Jul;106(7):1805-1813. doi: 10.1016/j.xphs.2017.03.016. Epub 2017 Mar 21.
(2) Oliveira JS, Lange S, Dobner B, Brezesinski G.
Chem Phys Lipids. 2017 Apr;204:1-9. doi: 10.1016/j.chemphyslip.2017.02.002. Epub 2017 Feb 7

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