On the photodegradation of dithranol in different topical formulations: use of SLN to increase the stability of the drug

The purpose of this study was to improve the stability of dithranol, an effective drug for topical treatment of psoriasis. The influence of several formulations (microemulsions, O/W emulsion, gel emulsion, and gel) on the photodegradation kinetics of dithranol was investigated. The photodegradation rate was found to be related with the initial drug concentration and the nature of the vehicle. Solid lipid nanoparticles (SLN) were prepared by solvent injection technique to investigate whether the inclusion in the lipid matrix could increase the stability of the drug. Physicochemical characterization of the particles by optical microscopy, photon correlation spectroscopy (PCS) and differential scanning calorimetry (DSC) revealed that solvent injection is a suitable approach for dithranol-loaded SLN preparation. The obtained particle sizes were between 230 and 270 nm; up to 92% of drug was entrapped in the SLN. The photodegradation kinetic constants (kc) of dithranol in SLN were related with the medium in which the particles were dispersed. The stability over time of dithranol was also investigated storing the samples at 25°C and the results showed that the drug inclusion in SLN dispersed in gel emulsion reduced its rate of degradation.