Hydroquinone (HQ) is an anti-hyperpigmentation agent with poor physicochemical\nstability. HQ formulations are currently elaborated by compounding in local pharmacies.\nVariability in the characteristics of HQ topical formulations can lead to remarkable differences in\nterms of their stability, efficacy, and toxicity. Four different semisolid O/W formulations with 5%\nHQ were prepared using: i) Beeler´s base plus antioxidants (F1), ii) Beeler´s base and dimethyl\nisosorbide (DMI) as solubiliser (F2), iii) olive oil and DMI (F3), and iv) Nourivan®, a skinmoisturising\nand antioxidant base, along with DMI (F4). Amongst the four formulations, F3 showed\nthe greatest physicochemical stability with less tendency to coalescence but with marked chromatic\naberrations. An inverse correlation was established by multivariate analysis between the mean\ndroplet size in volume and the steady-state flux, which explains why F3, with the smallest droplet\nsize and the most hydrophobic excipients, exhibited the highest permeation across both types of\nmembranes with enhancement ratios of 2.26 and 5.67-fold across Strat-M® and mouse skin,\nrespectively, compared to F1. It is crucial to understand how the HQ is formulated, bearing in mind\nthat the use of different excipients can tune the transdermal delivery of HQ significantly.
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