Original ArticlesRadiosynthesis and autoradiographic evaluation of [11C]NAD-299, a radioligand for visualization of the 5-HT1A receptor
Introduction
The 5-HT1A receptor is a serotonin receptor subtype that has been studied extensively due to its implicated role in several major neuropsychiatric disorders such as anxiety, depression, and schizophrenia (28). [11C]WAY-100635 is the only carbon-11-labeled radioligand that has, so far, been proven useful for in vivo imaging of the 5-HT1A receptor with positron emission tomography (PET) 9, 19, 25, 26, 27. Despite being a selective and potent 5-HT1A receptor antagonist giving a high signal-to-noise ratio, this radioligand has a drawback due to the formation of small amounts of labeled metabolites that pass the blood–brain barrier 10, 21, 22. Moreover, the very low background of unchanged radioligand in reference tissue obstructs reliable use of simplified quantitative methods such as the transient equilibrium analysis for clinical studies 8, 10.
NAD-299 ([R]-3-N,N-dicyclobutylamino-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxamide, Fig. 1) is a 5-HT1A receptor antagonist developed at Astra Arcus AB, Sweden. NAD-299 represents a structurally new chemical class of compounds with 5-HT1A receptor affinity and has high affinity (Ki = 0.6 nM) and selectivity for the 5-HT1A receptor in vitro 7, 17. The lipophilicity of NAD-299 (log P = 2.5 at pH 7.4) should allow for rapid diffusion across the blood–brain barrier and low nonspecific binding (6).
In the present study, we developed a synthesis for the labeling of [11C]NAD-299 with a high radiochemical yield and high specific radioactivity starting with [11C]cyanide as the labeled precursor. The anatomical distribution and specificity of [11C]NAD-299 binding to 5-HT1A receptors was examined by autoradiography on human brain whole-hemisphere cryosections.
Section snippets
General
NAD-299 ([R]-3-N,N-dicyclobutylamino-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxamide hydrogen (2R,3R)-tartrate monohydrate, Fig. 1) and NAD-195 ([R]-3-N,N-dicyclobutylamino-8-fluoro-5-trifluoromethylsulfonyloxy-3,4-dihydro-2H-1-benzopyran) were supplied by Astra Arcus AB (Södertälje, Sweden). The synthesis of NAD-299 and NAD-195 is described elsewhere (7). Tris(dibenzylideneacetone)-dipalladium(0)-chloroform adduct ([Pd2(dba)3] · CHCl3), 1,1′-bis-(diphenylphosphino)-ferrocene (DPPF) and
Chemistry
After 3 min of heating, the incorporation of [11C]cyanide to the nitrile intermediate (II) was higher than 98% using 4 mg of the precursor NAD-195. The subsequent hydrolysis of the nitrile intermediate (II) to the corresponding amide [11C]NAD-299 resulted in 30–50% conversion (Fig. 2). The total radiochemical yield of [11C]NAD-299 based on [11C]CO2 calculated from end of bombardment (EOB) and corrected for decay was 20–40% with a total synthesis time of 40–45 min. Purification of [11C]NAD-299
Chemistry
The key step of the preparation of [11C]NAD-299 is a palladium(0)-catalyzed nucleophilic displacement reaction of the aryl triflate precursor NAD-195 (I) with [11C]cyanide. The incorporation of [11C]cyanide yielded an aryl cyanide intermediate (II). The reaction was catalyzed by [Pd2(dba)3] · CHCl3-DPPF and NMP was used as solvent.
It has been shown that an exchange of the catalyst ligand from conventional triphenylphosphine (PPh3) to DPPF improves the catalytic efficiency of Pd(0) to a great
Conclusion
The 5-HT1A receptor antagonist NAD-299 was labeled with carbon-11 with high yield and high specific radioactivity. Autoradiography on postmortem human brain whole hemisphere cryosections demonstrated specific binding of [11C]NAD-299 to regions known to have a high density of 5-HT1A receptors. NAD-299 represents a structurally new chemical class for binding studies of the 5-HT1A receptor. The high affinity and selectivity combined with its favorable lipophilicity makes [11C]NAD-299 a potential
Acknowledgements
The authors would like to thank Mr. Göran Printz and Mr. Peter Söderholm for assistance with the radionuclide production and Ms. Carita Rask and Mrs. Siv Eriksson for technical assistance. This work was supported by grants from the Swedish Medical Research Council (03560, 09114, and 11640) and the Swedish Natural Science Research Council (K-KU 9973-308), the National Institute of Mental Health USA (NIMH, 41205 and 44814), and Karolinska Institutet.
References (30)
- et al.
Synthesis of 11C-labelled benzamide compounds as potential tracers for poly(ADP-ribose) synthetase
Appl. Radiat. Isot.
(1994) - et al.
The production of ultra high activity 11C-labeled hydrogen cyanide, carbon dioxide, carbon monoxide and methane via the 14N(p,α)11C reaction (XV)
Int. J. Appl. Radiat. Isot.
(1975) - et al.
In vivo and in vitro receptor autoradiography of the human brain using an 11C-labelled benzodiazepine analogue
Neurosci. Lett.
(1988) - et al.
Autoradiographic analysis of serotonin 5-HT1A receptor binding in the human brain post mortemEffects on age and alcohol
Brain Res
(1991) - et al.
Autoradiographic localization of 5-HT1A receptors in the post-mortem human brain using [3H]WAY-100635 and [11C]WAY-100635
Brain Res
(1997) - et al.
Serotonin receptors in the human brain. I. Characterization and autoradiographic localization of 5-HT1A recognition sites. Apparent absence of 5-HT1B recognition sites
Brain Res
(1986) - et al.
Antagonism of 5-hydroxytryptamine1A (5-HT1A) receptor-mediated modulation of adenylate cyclase activity by pindolol and propranolol isomers
Biochem. Pharmacol.
(1988) - et al.
Characterisation of the appearance of radioactive metabolites in monkey and human plasma from the 5-HT1A receptor radioligand, [carbonyl-11C]WAY-100635—Explanation of high signal contrast in PET and an aid to biomathematical modelling
Nucl. Med. Biol.
(1998) - et al.
Characterization of the radioactive metabolites of the 5-HT1A receptor radioligand [O-methyl-11C]WAY-100635, in monkey and human plasma by HPLCComparison of the behaviour of an identified radioactive metabolite with parent radioligand in monkey using PET
Nucl. Med. Biol.
(1996) - et al.
Serotonin receptors in the human brain—III. Autoradiographic mapping of the serotonin-1 receptors
Neuroscience
(1987)
Autoradiography with saturation experiments of 11C-Ro 15-1788 binding to human brain sections
J. Neurosci. Methods
First delineation of 5-HT1A receptors in human brain with PET and [11C]WAY-100635
Eur. J. Pharmacol.
Exquisite delineation of 5-HT1A receptors in human brain with PET and [carbonyl-11C]WAY-100635
Eur. J. Pharmacol.
Serotonin receptorsSubtypes, functional responses and therapeutic relevance
Pharmacol. Ther.
Amides from nitriles using basic hydrogenperoxide under phase-transfer catalysed conditions
Synthesis
Cited by (29)
Radiolabeling with [<sup>11</sup>C]HCN for Positron emission tomography
2021, Nuclear Medicine and BiologyCitation Excerpt :In 1999, [11C]NAD-299 (227) was synthesized over two steps in 20–40% DCY in 40–45 min (Fig. 36). The benzonitrile intermediate was synthesized from the aryl triflate with Pd2(dba)3CHCl3 complex with extra dppf ligand in NMP at 80 °C for 3 min [121]. When the conventional Pd(PPh3)4 catalyst was used, the benzonitrile intermediate had a 20% incorporation of [11C]cyanide as opposed to a nearly quantitative yield when Pd2(dba)CHCl3 with dppf was used [121].
Dopamine D<inf>3</inf> receptor antagonists: The quest for a potentially selective PET ligand. Part 3: Radiosynthesis and in vivo studies
2009, Bioorganic and Medicinal Chemistry LettersSynthesis and PET studies of [<sup>11</sup>C-cyano]letrozole (Femara), an aromatase inhibitor drug
2009, Nuclear Medicine and BiologyPET radiopharmaceuticals for neuroreceptor imaging
2006, Nuclear Science and Techniques/HewuliPositron Emission Tomography Agents for Central Nervous System Drug Development Applications
2005, Annual Reports in Medicinal Chemistry