Metabolism and autoradiographic evaluation of [¹⁸F]FE@CIT: a Comparison with [¹²³I]β-CIT and [¹²³I]FP-CIT

Abstract 

Purpose

Since the late 1980s, cocaine analogues based on the phenyltropane structure, such as [¹¹C]CFT and [¹²³I]β-CIT have been used for the imaging of the dopamine transporter. FE@CIT (fluoropropyl ester) and FP-CIT (N-fluoropropyl derivative) are further analogues. The aim of this study was to (1) evaluate and compare the metabolic stability of β-CIT, FP-CIT and FE@CIT against carboxyl esterases and (2) evaluate selectivity of [¹⁸F]FE@CIT compared to [¹²³I]β-CIT and [¹²³I]FP-CIT using autoradiography.

Methods

In vitro enzymatic hydrolysis assays were performed using different concentrations of β-CIT, FE@CIT and FP-CIT with constant concentrations of carboxyl esterase. Autoradiography was performed on coronal 20-μm rat brain sections incubated with different radioactivity concentrations of [¹²³I]β-CIT, [¹²³I]FP-CIT or [¹⁸F]FE@CIT and, additionally, with 3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile [serotonin transporter (SERT)] and nisoxetine [norepinephrine transporter (NET)] for blocking experiments.

Results

In vitro assays showed Michaelis-Menten constants of 175 μmol (β-CIT), 183 μmol (FE@CIT) and 521 μmol (FP-CIT). Limiting velocities were 0.1005 μmol/min (β-CIT), 0.1418 μmol/min (FE@CIT) and 0.1308 μmol/min (FP-CIT). This indicates a significantly increased stability of FP-CIT, whereas carboxyl esterase stability of β-CIT and FE@CIT showed no significant difference. Autoradiographic analyses revealed a good correlation between dopamine transporter (DAT)-rich regions and the uptake pattern of FE@CIT. Blocking experiments showed a higher DAT selectivity for [¹⁸F]FE@CIT than for the other two tracers.

Conclusion

We found that (1) the metabolic stability of FE@CIT was comparable to that of β-CIT, whereas FP-CIT showed higher resistance to enzymatic hydrolysis; and (2) the overall uptake pattern of [¹⁸F]FE@CIT on brain slices was comparable to that of [¹²³I]β-CIT and [¹²³I]FPCIT. After blocking of NET and SERT binding, a significantly higher DAT selectivity was observed for [¹⁸F]FE@CIT. Hence, [¹⁸F]FE@CIT may be of interest for further clinical application.

Keywords: FE@CIT, DAT, Metabolism, Autoradiography