Photodynamic therapy for acne vulgaris: a pilot study of the dose-response and mechanism of action.

Acne vulgaris does not always respond to conventional therapy. Photodynamic therapy (PDT) has been proposed as a treatment option. The aim of this study was to determine the optimal light dose for effective PDT treatment of acne and to investigate whether PDT reduces sebum excretion and the amount of Propionibacterium acnes. Fifteen patients (9 men, 6 women, age range 16-44 years, mean age 25 years) with mild to severe acne were enrolled in an open, unblinded study. Aminolaevulinic acid cream (20% in Unguentum Merck) was applied on two circular areas 3 h before PDT. The areas of investigation were irradiated with red light (635 nm) from a Waldman PDT 1200 lamp. Ten patients with facial acne were treated with a light dose of 50 J/cm(2) on the right cheek and 30 J/cm(2) on the left cheek. Five patients with acne on their back were treated either with 50 J/cm(2) or with 70 J/cm(2). Clinical follow-up was performed for at least 10 weeks. In the patients with facial acne, sebum excretion was determined before PDT and at every follow-up visit. The amount of P. acnes was measured in a skin surface biopsy using a cyano-acrylate polymer to extract the content of the sebaceous follicles. In 9 patients with facial acne the improvement of lesions was the same for the two light doses. According to the patients' own assessment, 8 improved after PDT (p=0.02). No difference was found between the two doses in patients with acne on the back. Hyperpigmentation was more common at higher doses of light, and pain was experienced more often by the patients when higher doses were used. No significant reduction in P. acnes or sebum excretion was found at any time after PDT. It is concluded that PDT could be an alternative treatment of acne lesions. The lowest possible light dose should be used for minimal side-effects. Other mechanisms of action for PDT than eradication of P. acnes and sebosuppression should be considered.