Radiosynthesis and microPET evaluation of (S,S)-6-F-[¹¹C]methylreboxetine and (S,S)-3-F-[¹¹C]methylreboxetine as norepinephrine transporter imaging agents

Objectives Attempts to image the norepinephrine transporter (NET) in living systems with PET have so far met with limited success due to the lack of effective radioligands. (S,S)-6-F-methylreboxetine (1) (2-[α-(2-methoxy-6-fluorophenoxy)phenylmethyl]morpholine) was reported to be a potent norepinephrine reuptake inhibitor, with low nanomolar hNET potency (Ki = 2 nM) and high selectivity for hNET over hSERT and hDAT (>1000-fold). We aimed to label this compound and its isomer, (S,S)-3-F-methylreboxetine (2), with carbon-11 for evaluation as radioligands for imaging NET in living brain with PET.

Methods The radiolabeling of [¹¹C]1 and [¹¹C]2 were accomplished through O-methylation of N-Boc phenol precursors with [¹¹C]CH₃I in the presence of 0.1 M Bu₄NOH, followed by removal of N-Boc group under acidic conditions, respectively. The log P7.4 of 1 and 2 were measured between 1-octanol and phosphate buffer. The uptake and distribution of radioactivity in cynomolgus monkey brain was examined in vivo with PET.

Results (S,S)-[¹¹C]1 and (S,S)-[¹¹C]2 were obtained in 44-52% decay-corrected radiochemical yield, with radiochemical purity of >98% and a specific activity of 0.6-1.2 Ci/μmol. 1 and 2 display moderate lipophilicity with log P7.4 of 1.97 and 1.89, respectively. After injection of [¹¹C]1 and [¹¹C]2, radioactivity readily entered brain, with the maximal whole brain uptake of 4.2% I.D. and 2.4% I.D., respectively. Ratios of radioactivity to cerebellum at 90 min for thalamus, occipital cortex, pons, and striatum were 1.4, 1.0, 1.2, 1.2, respectively for [¹¹C]1, and 1.5, 1.05, 1.1, 1.4, respectively for [¹¹C]2.

Conclusions The modest regional differentiation in brain uptake together with the unexpected accumulation of radioactivity in the striatum makes [¹¹C]1 and [¹¹C]2 inferior to existing reboxetine analogue radioligands, such as (S,S)-[¹¹C]MeNER and (S,S)-[¹⁸F]FMeNER-D₂ for the study of brain NETs with PET in vivo.