The protective effect of indomethacin on photocarcinogenesis in hairless mice

Orally administered indomethacin at 10–600 μg per mouse per day has been shown to inhibit UV radiation-induced erythema dose responsively. At the higher doses tested (200–600 μg) there was evidence of drug toxicity. Indomethacin administered orally at 20 μg per mouse daily during photocarcinogenesis induction both increased the probability of remaining tumour free and reduced the average tumour multiplicity. When indomethacin was administered only during the UV irradiation period (initiation), a reduction in tumour multiplicity and in the progression of tumours to malignant squamous cell carcinomas was observed; when administered only during the post-irradiation promotion period, there was a significant increase in the probability of remaining tumour free. Thus both tumour initiation and promotion by UV radiation appear to be indomethacin-sensitive, possibly affected by different mechanisms.