[¹⁸F]FE@SUPPY and [¹⁸F]FE@SUPPY:2 — metabolic considerations

Abstract 

Introduction

Recently, [¹⁸F]FE@SUPPY and [¹⁸F]FE@SUPPY:2 were introduced as the first positron emission tomography (PET) tracers for the adenosine A₃ receptor. Thus, aim of the present study was the metabolic characterization of the two adenosine A₃ receptor PET tracers.

Methods

In vitro carboxylesterase (CES) experiments were conducted using incubation mixtures containing different concentrations of the two substrates, porcine CES and phosphate-buffered saline. Enzymatic reactions were stopped by adding acetonitrile/methanol (10:1) after various time points and analyzed by a high-performance liquid chromatography (HPLC) standard protocol. In vivo experiments were conducted in male wild-type rats; tracers were injected through a tail vein. Rats were sacrificed after various time points (n=3), and blood and brain samples were collected. Sample cleanup was performed by an HPLC standard protocol.

Results

The rate of enzymatic hydrolysis by CES demonstrated Michaelis–Menten constants in a micromolar range (FE@SUPPY, 20.15 μM, and FE@SUPPY:2, 13.11 μM) and limiting velocities of 0.035 and 0.015 μM/min for FE@SUPPY and FE@SUPPY:2, respectively. Degree of metabolism in blood showed the following: 15 min pi 47.7% of [¹⁸F]FE@SUPPY was intact compared to 33.1% of [¹⁸F]FE@SUPPY:2; 30 min pi 30.3% intact [¹⁸F]FE@SUPPY was found compared to 15.6% [¹⁸F]FE@SUPPY:2. In brain, [¹⁸F]FE@SUPPY:2 formed an early hydrophilic metabolite, whereas metabolism of [¹⁸F]FE@SUPPY was not observed before 30 min pi

Conclusion

Knowing that metabolism in rats is several times faster than in human, we conclude that [¹⁸F]FE@SUPPY should be stable for the typical time span of a clinical investigation. As a consequence, from a metabolic point of view, one would tend to decide in favor of [¹⁸F]FE@SUPPY.

Keywords: PET, Adenosine A₃ receptor, Fluorine-18, SUPPY, Metabolism, Carboxylesterase