Synthesis, Radiolabeling, and in Vitro and in Vivo Evaluation of [¹⁸F]ENL30: A Potential PET Radiotracer for the 5-HT₇ Receptor

Abstract

The 5-HT7 receptor (5-HT7R) is involved in a broad range of physiological conditions and disorders. Currently, there is no validated clinical positron emission tomography (PET) tracer available; however, we have recently developed a promising 11C-labeled candidate. In this project, we aimed to further extend our efforts and develop an 18F-labeled derivative, coined [18F]­ENL30. Fluorine-18 has several advantages over carbon-11 especially within the preclinical phase, where a long half-life usually increases evaluation throughput. ENL30 was successfully synthesized in a low albeit sufficient overall yield. Radiolabeling succeeded with a radiochemical yield of approximately 4.5%. Subsequent preclinical PET studies revealed that [18F]­ENL30 binds specifically to the 5-HT7R but suffered from affinity to σ-receptors. Additionally, we identified [18F]­ENL30 to be a P-gp substrate in rats. However, we believe that [18F]­ENL30 may prove to be valuable in higher species that exhibit decreased P-gp dependency. If required, σ-receptor binding could, in such studies, be selectively blocked potentially allowing for selective 5-HT7R imaging.

Benzoxazolone‐arylpiperazinyl scaffold‐based PET ligand for 5‐HT7: Synthesis and biological evaluation

“…R f = 0.7 (30% ethyl acetate in hexane); m.pt. : 44–46°C; IR (KBr) ν max (cm −1 ): 2923, 1751, 1486, 1368, 1242, and 754 (Bard et al, 1993); H‐NMR (CDCl 3 , 400 MHz): δ 7.16–7.03 (3H, m), 6.93 (1H, d, J = 8.4 Hz), 3.80 (2H, t, J = 6.4 Hz), 3.40 (2H, t, J = 6.0 Hz), and 1.95–1.85 (4H, m)(Tampio L'Estrade, Edgar, et al, 2019); C‐NMR (CDCl 3 , 100.6 MHz): δ 153.49, 141.58, 129.82, 122.80, 121.42, 109.07, 107.12, 40.18, 31.62, 28.28, and 25.20.…”

“…R f = 0.3 (5% methanol in dichloromethane); m.p. : 126–128°C; IR (KBr) ν max (cm −1 ): 3356, 2941, 2830, 1771, 1604, 1488, 1366, 1254, 1208, 1169, 1021, and 753 (Bard et al, 1993); H NMR (DMSO‐ d 6 , 400 MHz): δ 7.33–7.30 (2H, m), 7.21 (1H, t, J = 7.6 Hz), 7.13–7.05 (2H, m), 6.48 (1H, d, J = 8.4 Hz), 6.40 (1H, s), 6.33(1H, d, J = 6.0 Hz), 3.83 (2H, t, J = 6.8 Hz), 3.68 (3H, s), 3.06 (4H, s), 2.40 (4H, s), 2.32 (2H, t, J = 7.2 Hz), 1.71 (2H, p, J = 7.0 Hz), and 1.48 (2H, p, J = 7.4 Hz) (Tampio L'Estrade, Edgar, et al, 2019); C NMR (DMSO‐ d 6 , 100.6 MHz): δ 160.21, 153.69, 152.62, 141.84, 130.94, 129.64, 123.96, 122.24, 109.71, 109.29, 108.03, 104.01, 101.42, 57.00, 54.88, 52.67, 48.16, 40.12, 27.05, and 23.21.…”

“…R f = 0.7 (30% ethyl acetate in hexane); m.pt. : 140–141°C; IR (KBr) ν max (cm −1 ): 3218, 1735, 1254, and 739 (Bard et al, 1993); H‐NMR (DMSO‐ d 6 , 400 MHz): δ 11.59 (1H, brs), 7.25 (1H, d, J = 8.0 Hz) and 7.14–7.03 (3H, m) (Tampio L'Estrade, Edgar, et al, 2019); C NMR (DMSO‐ d 6 , 100.6 MHz): δ 154.44, 143.36, 130.34, 123.80, 121.87, 109.79, and 109.53.…”

“…In the frame of its importance, it is believed that targeting the 5‐HT 7 receptor for neuroimaging could be beneficial in defining alterations in the 5‐HT concentration in the synaptic cleft (Thomas et al, 2002). Up to the present time, profuse efforts have been made to develop 5‐HT7 receptor (5‐HT 7R ) positron emission tomography (PET) ligands, comprising SB269970, [ 11 C]DR4446, [ 18 F]2FP3, [ 18 F]ENL30, [ 18 F]ENL10, and many more (Hansen et al, 2019; M. Herth et al, 2014; Tampio L'Estrade, Edgar, et al, 2019; Tampio L'Estrade, Xiong, et al, 2019; Zhang et al, 2002) However, with limitations like nonspecificity, lower brain uptake, and biotransformation associated with these ligands, the development of new ameliorated receptor‐specific high‐affinity ligands is crucial (Matthys et al, 2011; L'Estrade et al, 2020; Zimmer & Billard, 2014).…”

Radiotracers for the Central Serotoninergic System

“…Still in the N -sulfopyrrolidine series, two radioligands were synthesized and tested by Herth et al: [ 18 F]ENL30 [ 247 ] and [ 11 C]CIMBI-701 [ 248 ] ( Figure 20 ). These compounds exhibited mixed labeling of the 5-HT 7 R and σ-receptors and added to a moderate signal in the brain (probably linked to the mechanisms of efflux (P-gp substrate)), thus limiting their use in humans.…”