Tacalcitol

Tacalcitol is a vitamin D₃ derivative, which inhibits keratinocyte hyper-proliferation and induces differentiation of these cells. The normalisation of these mechanisms is the basis for the efficacy in the treatment of psoriasis. In biopsies from patients treated with tacalcitol specific indicators for inflammation were improved. Tacalcitol binds to the keratinocyte vitamin D receptor to the same extent as natural active vitamin D₃.

Single or repeated application of tacalcitol ointment in humans results in less than 0.5% of the drug being systemically absorbed through psoriatic skin. Tacalcitol is completely bound to plasma proteins (vitamin D binding protein) The main metabolite is 1α,24,25(OH)₃ vitamin D₃, a metabolite shared with the natural active vitamin, with 5-10 times less vitamin D activity. Tacalcitol and metabolites are excreted mainly in the faeces in rat and dog studies with excretion in urine in man. It cannot therefore be excluded that if there is sufficient systemic absorption accumulation may occur in patients with renal failure.

Tacalcitol is effective in very low concentrations. The no-effect-level following cutaneous application over 12 months in rat studies amounted to only 4 ng/kg daily. Toxicity is focused to the classic vitamin effects of calciferols. Teratogenicity studies in mice and rats showed no teratogenic effects of tacalcitol. The results of mutagenicity studies (Ames test, chromosomal aberration test and micronucleus test), indicate no genotoxic potential.