Hydroquinone loaded solid lipid nanoparticles comprised of stearic acid and ionic emulsifiers: Physicochemical characterization and in vitro release study

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• SLNs of stearic acid and ionic emulsifiers encapsulating hydroquinone are highly stable. • The particles provide different release profiles for the drug hydroquinone. • DPV study reveals the sustained release of drug form SLNs stabilized with CTAB. • SLNs with high concentration of SLS gives burst release. Solid lipid nanoparticles (SLNs) were successfully prepared using stearic acid and cationic/anionic emulsifiers by ultrasonication method. The model drug hydroquinone (HQ) which is used for the treatment of hyperpigmentation was loaded into SLNs. HQ loaded SLNs help to overcome drawbacks of hydroquinone including instability due to its rapid oxidation and its severe side effects on the skin. Six free SLN formulations (without HQ) and six HQ loaded SLN formulations were prepared using stearic acid and various concentrations (5%, 10% and 15%) of cationic emulsifier cetyltrimethylammonium bromide (CTAB) and anionic emulsifier sodium lauryl sulphate (SLS). The detailed physicochemical characterization of SLNs was performed using a combination of light scattering, microscopic, calorimetric and spectroscopic techniques. All the prepared SLNs were in nanometer range (150-275 nm) and contain stearic acid in stable polymorphic form. The drug in vitro release behaviour from SLNs was studied using novel electrochemical method based on differential pulse voltammetry (DPV). HQ entrapment increases with increase in emulsifier concentration. CTAB stabilized SLNs displayed continuous prolonged release of HQ whereas, SLS stabilized SLN at higher concentration (15%) gives burst release of HQ. The burst release can be correlated with smallest particle size of HQ-SLNs, lowest crystallinity and partitioning of drug into aqueous emulsifier phase at higher SLS concentration. Thus validated electrochemical DPV method proved to be highly applicable for quantification of HQ in SLN and release behaviour of HQ from SLNs.