Purpose: Thymoquinone (TQ) is a quinone-based phytochemical and was first identified in 1963 in Nigella sativa (black cumin seed) by El-Dakhakhany. Based on the ideal characteristics of transdermal delivery, TQ can be an attractive candidate for TDDS (transdermal drug delivery systems). The purpose of this study was to investigate for the first time the feasibility of transdermal delivery of thymoquinone (TQ) and to assess the effect of ethanol and propylene glycol (PG) as solvents together with the effects of selected chemical penetration enhancers on the in vitro human skin deposition and permeation of TQ.
Methods: Formulations were prepared by adding an excess of TQ in 5 mL solvents (ethanol or PG) with 5% of a chemical penetration enhancer (Azone (laurocapram), Transcutol® P (Tc), oleic acid, ethanol, Polysorbate 80 (Tween 80), and N-methyl-pyrrolidone (NMP)). The mixtures were agitated at 37°C for 48 hours. Permeation of saturated solutions of TQ in both solvents were evaluated using vertical Franz diffusion cells mounted with dermatomed human cadaver skin. Phosphate buffered saline (pH 7.4) receptor media was used and the cells were maintained at 37°C at 600 rpm continuous stirring. At each time point (3,4,6,8,10,12 and 24 hours) 300µL of receptor were withdrawn and immediately replaced with an equal volume of PBS (pH 7.4). At the end of 24 hours, receptor aliquots of 300µL were analyzed using a validated HPLC method. The flux of TQ was determined from the slope of the linear portion of the cumulative amount of TQ permeated per unit skin surface area (µg/cm2) versus time (hours) plot. Individual permeation profiles were generated to calculate average TQ flux. The lag time was calculated from extrapolation of the linear portion to the x-axis intercept of the permeation profile. Results are reported as mean ± SD (n=5). The statistical analysis of the data was performed by using one-way Anova and Student’s t - test, and p-values < 0.05 were considered significant. Skin drug content was quantified by analyzing the extracted compound TQ from skin homogenates using HPLC.
Results: There were significant differences in the penetration parameters for TQ in presence of the chemical enhancers and vehicles. The rank order for the TQ flux together with enhancer in PG was as follows: Azone + oleic acid>Tc>control + Tween 80>ethanol>NMP. In addition, the rank order for the TQ flux with enhancers from ethanol was as follows: Tc>oleic acid>Azone>control>Tween 80 + NMP. Comparing the penetration parameters for the two vehicles it was found that there was significant increase (p<0.05) in TQ flux of control, Tween 80 and NMP formulations from ethanol. Also, there was significant increase (p<0.05) in TQ flux in the Azone and oleic acid PG formulation compared with ethanol. The data obtained for both Azone and oleic acid in PG can be explained by the fact that there probably is a synergistic effect between the enhancer and PG. Such synergism has been reported previously by other authors. Although ethanol was able to increase the solubility of TQ it was not able to increase the TQ flux. From the above findings, it can be postulated that thermodynamic activity of TQ in the formulation was modified most probably by the rapid permeation and evaporation of ethanol vehicle. The rank order for TQ skin deposition in PG using the enhancers is as follows: Azone>oleic acid> ethanol>control>Tc>NMP>Tween 80. Additionally, the rank order for TQ skin deposition in ethanol is as follows: oleic acid>Azone>control>Tc>Tween 80> NMP. These data show that both Azone and oleic acid were able to provide a skin reservoir resulting in higher TQ skin deposition.
Conclusion: Skin penetration of TQ was influenced by the physicochemical properties of the vehicle and by the presence of permeation enhancers. It can be concluded that transdermal permeation and skin deposition of TQ can be achieved by using penetration enhancers and different vehicles. Azone, oleic acid and Tc at 5% was able to provide measurable TQ flux and are good penetration enhancers to further develop a novel transdermal formulation of TQ. These penetration enhancers were also able to result in TQ skin reservoirs that may contribute to a sustained release of TQ from the skin.
Bozena Michniak-Kohn– Professor of Pharmaceutics, Rutgers University Ernest Mario School of Pharmacy, New Jersey