title: [18F]FETO: metabolic considerations.
abstract: OBJECTIVE
  11beta-Hydroxylase is a key enzyme in the biosynthesis of adrenocortical steroid hormones and is a suitable target for the imaging of the adrenal cortex. [(11)C]Metomidate (MTO), [(11)C]etomidate (ETO) and desethyl-[(18)F]fluoroethyl-etomidate (FETO) are potent inhibitors of this enzyme and are used for PET imaging of adrenocortical pathologies. The aims of this study were (1) to evaluate and compare the metabolic stability of MTO, ETO and FETO against esterases and (2) to investigate the metabolic pattern of FETO in vivo.
  
  
  METHODS
  In vitro assays were performed using different concentrations of MTO, ETO and FETO with constant concentrations of carboxylesterase. Human in vivo studies were performed with human blood samples drawn from the cubital vein. After sample clean-up, the serum was analysed by HPLC methods.
  
  
  RESULTS
  In vitro assays showed Michaelis-Menten constants of 115.1 mumol for FETO, 162.0 mumol for MTO and 168.6 mumol for ETO. Limiting velocities were 1.54 mumol/min (FETO), 1.47 mumol/min (MTO) and 1.35 mumol/min (ETO). This implies insignificantly decreased esterase stability of FETO compared with MTO and ETO. In vivo investigations showed a rapid metabolisation of FETO within the first 10 min (2 min: 91.41%+/-6.44%, n=6; 10 min: 23.78%+/-5.54%, n=4) followed by a smooth decrease in FETO from 20 to 90 min (20 min: 11.23%+/-3.79% n=4; 90 min: 3.68%+/-3.65%, n=4). Recovery rate was 61.43%+/-3.19% (n=12).
  
  
  CONCLUSIONS
  In vitro experiments demonstrated that FETO stability against esterases is comparable to that of ETO and MTO. The metabolic profile showed that FETO kinetics in humans are fast.
authors: Department of Nuclear Medicine, Medical University of Vienna, AKH Wien, Waehringer Guertel 18-20, A-1090, Vienna, Austria.
authors: Dagmar E
authors: DE
authors: Ettlinger
authors: Wolfgang
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authors: Wadsak
authors: Leonhard-Key
authors: LK
authors: Mien
authors: Michael
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authors: Machek
authors: Leila
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authors: Wabnegger
authors: Gundula
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authors: Rendl
authors: Georgios
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authors: Karanikas
authors: Helmut
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authors: Viernstein
authors: Kurt
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authors: Kletter
authors: Robert
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authors: Dudczak
authors: Markus
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authors: Mitterhauser