In vivo microautoradiography of [3H]1,24(OH)2D3 (tacalcitol) following topical application to normal rats and in vitro metabolism in human keratinocytes

This study was conducted to investigate the mechanism of topical absorption of [3H]1,24(OH)2D3 (1,24-dihydroxyvitamin D3; tacalcitol) by applying an ointment containing 4 micrograms2/g [3H]1,24(OH)2D3 to the skin of rats using an occlusion method. Microautoradiography of the skin at the application site 1 h after topical treatment showed a high concentration of radiolabel in the stratum corneum, the epidermis and around the hair follicles. Radiolabel was also seen in the epidermis and hair follicle areas 8 h and 24 h after application. The radiolabel was distributed to a minor extent to the subcutaneous fat layer. Microautoradiography showed two routes of purcutaneous absorption of 1,24(OH)2D3: through the stratum corneum and epidermis into the microvessels, and through hair follicle areas into the bloodstream. After topical application of an ointment containing 4 micrograms/g or 40 micrograms/g [3H]1,24(OH)2D3 to the shaved neck skin of rats, the absorption rate, estimated by excretion in the urine and faeces, was about 30% of the total applied radioactivity. The main excretion route after topical application was in the faeces. Furthermore, 1,24(OH)2D3 added to human adult keratinocytes was not metabolized into other compounds, and only the unchanged compound was detected. These findings strongly suggest that 1,24(OH)2D3 distributed into the epidermis acts on epidermal keratinocytes. Topical application of 1,24(OH)2D3 appears to be a possible approach to the treatment of psoriasis and other skin diseases through its action on the 1,25(OH)2D3 receptor, which reportedly plays a very important role in the regulation of proliferation and differentiation of keratinocytes.