Complexation of the sunscreen agent, phenylbenzimidazole sulphonic acid with cyclodextrins: effect on stability and photo-induced free radical formation.

The interaction between the sunscreen agent, phenylbenzimidazole sulphonic acid (PBSA) and hydrophilic alpha-, beta-, and gamma-cyclodextrin derivatives was investigated under acidic conditions (pH 4.0) by phase-solubility analysis. Among the available cyclodextrins, hydroxypropyl-beta-cyclodextrin (HP-beta-CD) and random methyl-beta-cyclodextrin (RM-beta-CD) had the greatest solubilizing activity. The complexation of the sunscreen agent with HP-beta-CD and RM-beta-CD was confirmed by nuclear magnetic resonance spectroscopy. Solid-phase characterization of the PBSA/cyclodextrin systems by X-ray diffractometry defined the most appropriate method (co-evaporation) and cyclodextrin concentration (10-fold molar excess) for the preparation of a stable complexed form of PBSA. Long-term stability studies demonstrated that the decrease of the sunscreen level in emulsion preparations (pH 4.0) was almost completely suppressed by HP-beta-CD, RM-beta-CD being less effective. Moreover, the irradiation-induced decomposition of PBSA in the emulsion vehicle was markedly reduced by complexation with HP-beta-CD (the extent of degradation was 3.9% for the complex compared to 9.1% for uncomplexed PBSA), whereas RM-beta-CD had no significant influence. In addition, electron paramagnetic resonance (EPR) spin-trapping studies showed that the inclusion of the sunscreen agent into the HP-beta-CD cavity completely inhibited the formation of free-radicals generated by PBSA on exposure to simulated sunlight, thereby suppressing its photosensitising potential.