The variable response of plaque psoriasis after a single treatment with topical 5-aminolaevulinic acid photodynamic therapy

We have investigated the clinical response of 22 patients with plaque psoriasis to photodynamic therapy using topical application of 5-aminolaevulinic acid followed by a single exposure to broad-band visible radiation. Light doses in the range 2-16 J/cm2 delivered at dose of 10-40 mW/ cm2 resulted in a variable clinical response. Seven (35%) patients showed clearing of psoriasis at some treated sites. The intensity of protoporphyrin IX fluorescence was recorded before, during and after treatment. Pre-illumination fluorescence intensity varied considerably between sites on the same patient and between patients. Protoporphyrin IX fluorescence recovered and persisted after treatment for up to 14 days and became higher than preillumination levels at 25% of sites. The rate of protoporphyrin IX photo-oxidation during treatment was proportional to both initial fluorescence intensity and incident light dose rate and was almost complete after 16 J/cm2. We have defined the photodynamic dose as the product of time-dependent protoporphyrin IX concentration and light dose and demonstrated that only in those patients who showed clearance of psoriasis was there a relationship between photodynamic dose and clinical response. Discomfort ranged from stinging through to burning, was significant in some patients and tended to be more severe with increasing photodynamic dose but was not predictable. Efficacy may improve by achieving consistent protoporphyrin IX levels or by using multiple treatments.