Selective inhibitors of vitamin D metabolism--new concepts and perspectives.

BACKGROUND Levels of active vitamin D (VD) are controlled by synthesis via CYP27B1 and self-induced metabolism by CYP24A1. Unbalanced high CYP24A1 expression due to induction by diverse endogenous compounds and xenobiotics, and amplification found in various tumours, might lead to local VD deficiency, thereby causing/reinforcing disorders. MATERIALS AND METHODS Using primary human keratinocytes, CYP24A1 expression was examined at the mRNA level by dot-blot and Northern blot hybridization, and at the enzyme activity level by analysing HPLC profiles from incubations with 3H-labelled VD metabolites. RESULTS We have developed a one-step protocol to screen test compounds for potent inhibition of CYP24A1 along with selectivity over CYP27B1 and adequate metabolic stability. These inhibitors amplified hormone levels and, thereby, its function, indicated by increased CYP24A1 expression. Moreover, they stabilized the expression of a CYP24A1 splice variant, possibly serving as a buffer of VD metabolites. In addition, a low abundant, constitutive 24-hydroxylase, active in the low nanomolar range is described. CONCLUSION Selective CYP24A1 inhibitors could herald a new era for vitamin D research, as well as for therapeutic application. Inhibitors may be used as single entities or in combination with low doses of potent analogs to prevent and treat various defects of growth and differentiation, and neuro-immuno-endocrine disorders.