How is photodynamic therapy used in dermatology?

Photodynamic therapy is a two-step process. A photosensitizing agent is topically applied to the patient’s skin, followed by illumination with a light source of the proper wavelength. Aminolevulinic acid (ALA) is used most often in the United States and is converted to protoporphyrin IX by keratinocytes. Keratinocytes lack the final enzyme in the heme synthesis pathway, and the process ends with protoporphyrin IX, which is photosensitizing, particularly in the Soret band (400 to 410 nm). Porphyrins used in combination with blue (415 to 425 nm) or red light (600 to 700 nm) are the most common forms of photodynamic therapy. Smaller absorption peaks are also noted around 585 to 595 nm, which correlates with the pulsed dye laser absorption spectrum. Cancer cells and precancerous cells derived from keratinocytes (actinic keratoses, squamous cell carcinomas, basal cell carcinomas) accumulate porphyrins at higher concentrations than normal keratinocytes and are more susceptible to the phototoxic effects of the treatment. In acne, P. acnes is susceptible to both blue light alone and to porphyrin-induced injury. The sebaceous glands in the skin accumulate significant amounts of porphyrin and are very susceptible to injury by photodynamic therapy (PDT). When used in combination with the pulsed dye laser at 585 to 595 nm, deeper penetration into the sebaceous gland is achieved and a better therapeutic outcome is noted when treating acne. After 3 monthly treatments, acne may stay in remission for up to a year.