Single dose metyrapone test: 11β-hydroxylase inhibition by metyrapone and reduced metyrapone assayed by radioimmunoassay

To assess the effects of metyrapone and reduced metyrapone on 11 beta-hydroxylase inhibition, the plasma levels of cortisol, 11-deoxycortisol, and the inhibitors were measured by radioimmunoassays in 34 normal subjects 8 h after they received a single oral dose of metyrapone at midnight. The ratio of 11-deoxycortisol to cortisol, as an index of 11 beta-hydroxylase inhibition, was compared to plasma levels of metyrapone and reduced metyrapone. One subject received an infusion of metyrapone ditartrate in order to study the sequential conversion of metyrapone to reduced metyrapone. A new radioimmunoassay was developed for measurement of plasma concentrations of metyrapone and reduced metyrapone. Following intravenous administration of metyrapone, it is rapidly converted to an active metabolite, reduced metyrapone. At 8 h after a dose was given, the average reduced metyrapone level was 1.5 times higher than the average metyrapone level. Following oral administration of the drug, we found a high correlation when plasma levels of metyrapone were compared to reduced metyrapone and when the ratio of 11-deoxycortisol to cortisol was related to metyrapone or to total metyrapone levels. In conclusion, the conversion of metyrapone to reduced metyrapone is such that by 8 hours after a single oral dose, more than one-half of the inhibitory effect on 11 beta-hydroxylase appears to be produced by reduced metyrapone. The inhibitory action of metyrapone and reduced metyrapone on the enzyme system is reflected by their concentration in plasma.