Synthesis and Biologic Evaluation of a Novel Serotonin 5-HT_1A Receptor Radioligand, ¹⁸F-Labeled Mefway, in Rodents and Imaging by PET in a Nonhuman Primate

Synthesis scheme of mefway (5), tosylate precursor for ¹⁸F-mefway (10), and ¹⁸F-mefway (11). Reaction conditions include (i) BOP reagent, 2 eq Et₃N, CH₃CN, room temperature for 24 h; (ii) LiAlH₄, THF, 0°C–5°C for 30 min followed by 30 min room temperature; (iii) DAST, CH₂Cl₂ room temperature, 24 h; (iv) p-toluenesulfonyl chloride, CH₂Cl₂, Et₃N, room temperaure, 24 h; (v) ¹⁸F-fluoride, Kryptofix 2.2.2./K₂CO₃, CH₃CN, 30 min. (Left inset) HPLC purification of ¹⁸F-mefway using C₁₈ reverse-phase semipreparative column eluted with 60% acetonitrile/0.1% triethylamine at flow rate of 2.5 mL/min. ¹⁸F-Mefway had retention time of 10.5 min and specific activity of 74–111 GBq/μmol. RA = radioactivity; UV = absorbance at 254 nm.

(A) In vitro horizontal brain slices of rat brain show binding of ¹⁸F-mefway (red = highest binding and white = lowest binding). (Left) Rat brain slice, 10 μm thick, shows brain regions. (Center) Same rat brain slice after treatment with 130–148 kBq/mL of ¹⁸F-mefway. (Right) Rat brain slice with nonspecific binding in presence of 10 μmol/L WAY-100635. (B) Horizontal rat brain slices (with dorsal hippocampus) show in vitro binding in competitive study of ¹⁸F-mefway with increasing concentrations of serotonin (5-HT): (a) 1 nmol/L; (b) 10 nmol/L; (c) 100 nmol/L; (d) 1 μmol/L; (e) 10 μmol/L). (C) (Left) Competition curves of WAY-100635 and mefway against ¹⁸F-mefway measured autoradiographically in hippocampus of rat brain slices. Inhibition constant (IC₅₀) of WAY-100635 = 23.2 ± 2.8 nmol/L and of mefway = 25.7 ± 2.4 nmol/L. (Right) Inhibition curves of ¹⁸F-mefway binding by serotonin measured autoradiographically in different brain regions of rat brain slices shown in B. Ctx = cortex; St = striata; Hp = hippocampus; Cb = cerebellum; Col = colliculus.

Distribution of ¹⁸F-mefway in rhesus monkey brain. Coregistered MR images with summed PET images show localization of ¹⁸F-mefway. (First and second rows) Coregistered MR images and PET images, respectively, show binding in various cortical regions, including distinct hot spot (Hs) seen in red near insular cortex. (Third and fourth rows) Coregistered MR images and PET images, respectively, show hippocampus, seen in red in 3 slices, and raphe. St = striata; Hs = hot spot; Cg = cingulate; Oc = occipital cortex; Cb = cerebellum; Th = thalamus; Tc = temporal cortex; Hp = hippocampus; Rp = raphe.

(A) Time–activity curve of ¹⁸F-mefway in various monkey brain regions obtained after intravenous administration of ¹⁸F-mefway. (B). Ratio plot of various brain regions (shown in A) against cerebellum. Hp = hippocampus; Hs = hot spot; Cg = cingulate; Fc = frontal cortex; Tc = temporal cortex; Oc = occipital cortex; St = striata; Rp = raphe; Th = thalamus; Cb = cerebellum.