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

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