Synthesis of a fluorine-18-labelled derivative of 6-nitroquipazine, as a radioligand for the In vivo serotonin transporter imaging with PET

Abstract

Considerable efforts have been engaged in the design, synthesis and pharmacological characterization of radioligands for imaging the serotonin transporter, based on its implication in several neuropsychiatric diseases, such as depression, anxiety and schizophrenia. In the 5-halo-6-nitroquipazine series, the fluoro derivative has been designed for positron emission tomography (PET). The corresponding 5-iodo-, 5-bromo- and 5-chloro N-Boc-protected quipazines as labelling precursors, as well as 5-fluoro-6-nitroquipazine as a reference compound have been synthesized. 5-[18F]Fluoro-6-nitroquipazine has been radiolabelled with fluorine-18 (positron-emitting isotope, 109.8 min half-life) by nucleophilic aromatic substitution from the corresponding N-Boc protected 5-bromo- and 5-chloro-precursors using K[18F]F-K222 complex in DMSO by conventional heating (145°C, 2 min) or microwave activation (50 W, 30–45 s), followed by removal of the protective group with TFA. Typically, 15–25 mCi (5.5–9.2 GBq) of 5-[18F]fluoro-6-nitroquipazine (1–2 Ci/μmol or 37–72 GBq/μmol) could be obtained in 70–80 min starting from a 550–650 mCi (20.3–24.0 GBq) aliquot of a cyclotron [18F]F− production batch (2.7–3.8% non decay-corrected yield based on the starting [18F]fluoride). Ex vivo studies (biodistribution in rat), as well as PET imaging (in monkey) demonstrated that 5-[18F]fluoro-6-nitroquipazine ([18F]-1d) readily crossed the blood brain barrier and accumulated in the regions rich in 5-HT transporter (frontal- and posterial cortex, striata). However, the low accumulation of the tracer in the thalamus (rat and monkey) as well as the comparable displacement of the tracer observed with both citalopram, a reference 5-HT re-uptake inhibitor and maprotiline, a norepinephrine re-uptake inhibitor (rat), indicate that 5-[18F]fluoro-6-nitroquipazine ([18F]-1d) does not have the suggested potential for PET imaging of the serotin transporter (SERT).