Transdermal delivery enhancement of carvacrol from Origanum vulgare L. essential oil by microemulsion

Carvacrol has been reported for analgesic and anti-inflammatory activity by cyclooxygenase inhibition but it could induce gastrointestinal toxicity because of its non-selective inhibition. Therefore, the present study aimed to develop transdermal microemulsion from Origanum vulgare essential oil to deliver carvacrol into and through the skin which would overwhelm the gastrointestinal problems. O. vulgare essential oil was extracted by hydrodistillation and its carvacrol content was determined using high performance liquid chromatography. Pseudoternary phase diagrams were constructed using water dilution method to investigate the suitable microemulsion components. Microemulsions were then characterized for external appearance, particle size, size distribution, zeta potential, electrical conductivity, refractive index, viscosity, transmittance, pH, and stability. Additionally, the irritation property of microemulsions were investigated by hen's egg on the chorioallantoic membrane assay. The release profile, percutaneous absorption, and skin retention were investigated using dialysis bag and Franz diffusion cell, respectively. The interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) were investigated using the enzyme-linked immunosorbent assay. The results remarked that carvacrol was a major component of O. vulgare essential oil with high concentration of 83.7%. The most suitable microemulsion (ME 1), composing of 5% w/w O. vulgare essential oil, 25%w/w Tween 60, 25%w/w butylene glycol, and 45% w/w deionized water, had the smallest internal droplet size (179.5 +/- 27.9 nm), the narrowest polydispersity index (0.30 +/- 0.07), the highest transmittance (93.13 +/- 0.04%), and Newtonian flow behavior with low viscosity of 0.30 +/- 0.07 Pas. ME 1 could reduce the irritation effect of O. vulgare essential oil since ME 1 (IS = 3.1 +/- 0.10) exhibited significantly lower irritation effect than its blank formulation (IS = 4.8 +/- 0.02) and O. vulgare oil solution (IS = 5.0 +/- 0.01) (p < 0.05). Furthermore, ME 1 sustain released carvacrol from the formulation, remarkedly deliver more carvacrol through the skin layer (2.6 +/- 2.2%) and significantly retained carvacrol in the skin layer (2.60 +/- 1.25%). Additionally, ME 1 significantly enhanced IL-6 inhibition of O. vulgaris oil and carvacrol (p < 0.05). Therefore, O. vulgaris oil microemulsion was suggested to be used for the transdermal delivery and anti-inflammatory activities enhancement of carvacrol.