Oral administration of (11E)-13-oxo-15,16-dinorlabda-8(20),11-dien-19-oic acid strongly reduces photocarcinogenesis in mouse skin exposed to UV-B irradiation.

(11E)-13-Oxo-15,16-dinorlabda-8(20),11-dien-19-oic Acid (1), obtained either from the stem bark of Thuja standishii or readily prepared in larger quantities from the related constituent 2, was found to significantly reduce the formation of papilloma in an in vivo two-stage mouse-skin-carcinogenesis model. Carcinogenesis was initiated by skin exposure to UV-B irradiation and promoted by topical application of 12-O-tetradecanoylphorbol-13-acetate (TPA). Oral administration of 1, starting one week before and ending one week after irradiation, exhibited remarkable effects. First, papilloma formation started two weeks later than in the control group (lacking 1). Second, the average number of skin papilloma after 20 weeks was reduced by ca. 50% in the test group relative to the control.