Synthesis and in vitro evaluation of metyrapone derivatives as potential inhibitors of 11β-hydroxylase activity

Receptor imaging offers a biochemical approach to the measurement of regional organ function by using ligands labelled with gamma- or positron-emitting radionuclides. Metyrapone is known to inhibit 11β-hydroxylation of deoxycorticosterone (DOC) in the adrenal cortex by binding to the mitochondrial P-450 enzyme system. The introduction of a radionuclide into metyrapone requires several synthetic intermediates. The halogenation of ring B produced 4-prime-bromo-metyrapone as a precursor for labelling with 131I(123I). Radiolabelling was performed in the melt using carrier-free 131I(123I) iodide for the isotopic exchange reaction. The evaluation of enzyme binding is based on the formation of 14C-corticosterone from 14C-DOC, using a crude preparation of adrenocortical mitochondria. From kinetic studies, the Michaelis-Menten constant was determined as 12.5 μmol/L. The inhibitory effect of metyrapone and 4-prime-bromo-metyrapone was compared. Both substances are competitive inhibitors of the 11β-hydroxylase system, with inhibition constants of Ksub(i)=0.39+-0.14μmol/L for metyrapone and Ksub(i)=0.86+-0.14μmol/L for 4-prime-bromo-metyrapone. (author)