Nanosuspensions of hesperetin: preparation and characterization.

Nanosuspensions are a smart formulation principle for dermal applications, as they increase the penetration of poorly soluble substances into the skin. Because microbial stability is a pre-requisite for dermal formulations, water containing formulations need to be preserved. Preservatives are known to possibly impair the physical stability of disperse systems, i.e. by causing agglomeration. These aggregation phenomena might occur during storage of the final product, but can already occur during the production process itself. Therefore, in this study the influence of six different preservatives on the diminution efficiency during the production of hesperetin nanocrystals was investigated. Particles with and without the addition of preservatives were produced by high pressure homogenization (HPH) and the final particle size was analysed and compared to the non-preserved suspension. All preservatives influenced the diminution progress during production and the final particle sizes obtained. The non-preserved suspension yielded a particle size of about 300 nm. Preservation with Hydrolite, Euxyl PE9010, Rokonsal and Phenonip led to sizes of about 400 nm. Preservation with Caprylyl glycol and MultiEx did not lead to nanoparticles (size > 1 microm) and caused a slight agglomeration of the nanosuspensions. Based on zeta potential measurements it was found that the impairment is related to the lipophilicity of the presverative, i.e. the lower the lipophilicity, the less is the impairment. In conclusion, preservatives impair the diminution efficacy during the production of nanosuspensions. Therefore, if possible, preservatives should be added to nanosuspensions after the production process. If preservatives are required during production, highly hydrophilic preservatives, e.g. Hydrolite E, should be used.