Abstract:Calcipotriol is a novel vitamin D3analogue developed for topical treatment of psoriasis. Calcipotriol is believed to act via regulation of cell proliferation and differentiation. In this respect calcipotriol is as potent as 1α,25(OH)2D3, the physiologically active form of vitamin D3, but its calcaemic activityin vivois 100 to 200 times lower. In the present investigation, the effects of calcipotriol on cell growth regulationin vitroand on calcium metabolismin vivowere compared to those exerted by a number of metabolites and analogues of vitamin D3. Besides 1α,25(OH)2D3, these included the two physiologically occurring metabolites 25(OH)D3and 24,25(OH)2D3, and the two synthetic analogues 1α(OH)D3and 1α,24(OH)2D3. 25(OH)D3and 24,25(OH)2D3were shown to be inactive bothin vitroandin vivo.1α(OH)D3was found to have a low biological activityin vitro,but was highly calcaemicin vivoafter biotransformation to 1α,25(OH)2D3. Calcipotriol, 1α,24(OH)2D3and 1α,25(OH)2D3were all three potent regulators of cell proliferation and differentiationin vitro. In vivo,only calcipotriol showed a greatly reduced calcaemic activity after both oral and intravenous administration. It is concluded that calcipotriol, with a reduced risk of inducing calcaemic side‐effects upon absorption from the skin, possesses a favourable therapeutic profile for topical treatment of hyperproliferative