Spectroscopic characterization and photobleaching kinetics of hypericin-N-methyl pyrrolidone formulations

Hypericin (HY) is a promising photosensitizer in photodynamic therapy (PDT). It was recently reported that appropriate use of N-methyl pyrrolidone (NMP) enhanced in vivo PDT efficacy of HY and enhanced in vivo delivery of HY. This present study further investigates the use of NMP and other known non-toxic pharmaceutical additives, polyvinylpyrrolidone (PVP, K29/32) and copolyvidonum (S630), for formulating HY to enhance its delivery with photodynamic activity as a goal in mind. Hence, the first objective of this study was to characterize the solubilization of HY by NMP, K29/32 and S630. Thermodynamic considerations were used to explain the solvation process. Photobleaching is another important property of photosensitizers. There is no report on the photostability of HY in pharmaceutical formulations used for PDT. Therefore, the second objective of this study was to investigate the photobleaching of HY in these formulations. The fluorescence of HY was found to increase significantly in higher concentrations of NMP or when 5% of polymer was co-mixed with 5% of NMP solution. The photobleaching of HY in these formulations followed first-order kinetics. The loss of fluorescence paralleled to the loss of absorption of HY. The formulation of HY with 40% NMP was found to be the most stable.