Fluorescence investigation of the binding of model PDT drugs to nonionic and zwitterionic surfactants.

The binding of two model photodynamic therapy drugs, chlorin p(6) and purpurin 18, with surfactants has been studied using steady-state and time-resolved fluorescence techniques. The surfactants used are amphiphilic nonionic surfactant (Tween 80 and Tween 40) and zwitterionic surfactant (HAPS). These have applications in drug delivery. The studies have been performed at pH 7 and 5 for chlorin p(6) and at pH 7 for purpurin 18. The binding constants have been estimated from the change in fluorescence parameters and have been compared with those for Cremophor EL and human serum albumin. Chlorin p(6) is found to bind to the surfactants to a greater extent at pH 5 than at pH 7. The same ionic species of chlorin p(6) is found to exist at the maximum concentrations of the surfactants.