Changes in human skin after topical PDT with hexyl aminolevulinate

Background: Photodynamic therapy (PDT) induces physiological changes in human skin, but details and kinetics are not known. Methods: Changes in human skin induced by PDT with red light in the presence of topically applied cream with the hexyl aminolevulinate (HAL) were investigated in the skin of five healthy volunteers. In addition to testing the effects of HAL-PDT three control studies were performed on the volunteers: (A) the HAL containing cream was applied to the skin without light exposure; (B) the cream without HAL was applied to the skin; (C) the skin was exposed to light in the absence of the cream. Reflectance spectra of the skin were measured in the wavelength region 300-600 nm before and after treatment. An advanced and new inverse radiative transfer model was used to determine changes induced in a number of skin parameters. Results: The main discoveries were that the dermal blood concentration increased immediately after PDT, reached a maximum after 1-2 days, and then decreased. The blood oxygenation increased significantly immediately after PDT and then decreased. After PDT the melanosome concentration in the upper epidermis increased steadily. No such changes were observed in the control sites. Conclusions: Our results imply that HAL-PDT leads to increased vascularisation, oxygenation and melanin formation.