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Metomidate [(R)-1-(1-phenylethyl)-1 H -imidazole-5-carboxylic acid methyl ester] (MTO, 1, Fig. 1) is a potent and selective inhibitor of the cytochrome P-450 enzyme system in the adrenal cortex. Labelled in the 4-position with radioiodine, (R)-4-[ 131 I]iodometomidate, 2, [ 131 I]IMTO has been evaluated by  in-vitro  studies and also  ex-vivo  in rats. [ 131 I]IMTO was synthesized by oxidative radioiododestannylation using a suitable precursor which was prepared by a new stereoselective synthesis. Optimization of the labelling reaction was performed by systematic variation of the most important reaction parameters. Under optimum reaction conditions, a labelling yield of 95% was obtained.  In-vitro -stability of the tracer was studied over 8 days, indicating slow deiodination (0.27%/h). Displacement studies using [ 131 I]IMTO and rat adrenal membranes revealed the structural requirements for high affinity binding, namely an intact ester group and (R)-configuration of the radioligand. Pharmacokinetic studies in rats showed fast accumulation of [ 131 I]IMTO in the adrenals (approx. 10% ID/g tissue) with an activity plateau for 2 hours. Metabolic degradation was indicated by a steady increase of renal activity up to 4 hours post injection. Based on target to non-target ratios the highest contrast for imaging of the adrenals was observed between 30 and 60 min post injection of [ 131 I]IMTO. We conclude that SPECT using [ 123 I]IMTO will be a promising method for the characterization of adrenal incidentalomas.

[ 123/131 I]Iodometomidate as a radioligand for functional diagnosis of adrenal disease: synthesis, structural requirements and biodistribution