Synthesis and evaluation of new radioligands [11C]A-833834 and [11C]A-752274 for positron-emission tomography of α7-nicotinic acetylcholine receptors
Introduction
Nicotinic acetylcholine receptors (nAChR) are members of the ligand-gated cation channel superfamily and involved in many physiological functions [1], [2]. Neuronal central nAChR populations are mostly comprised of the homomeric α7-nAChR and heteromeric α4β2-nAChR [3], [4]. These acetylcholine receptor subtypes are involved in great variety of neuronal processes and of interest as targets for therapeutic intervention in a number of conditions and disorders [5], [6].
The α7-nAChR subtype is highly expressed in the hippocampus and cortex, the brain regions that are involved in cognition and memory [7], [8]. Recent research suggests that this receptor subtype is associated with several neurodegenerative and psychiatric disorders [9], [10], [11], [12], [13]. Post-mortem studies of human brains have demonstrated reduced expression of α7-nAChR in schizophrenics versus control brains [14], [15]. Successful experiments with the α7-nAChR agonist DMXB that improves the auditory gating deficit in animal models of schizophrenia [16], [17] have led to the first clinical study [18].
Reduced expression of α7-nAChR protein has also been observed in the hippocampus of patients suffering from Alzheimer's disease (AD) [11], [13], [19]. The neuroprotective effects of nicotine against β-amyloid toxicity can be blocked by selective α7-nAChR antagonists [20], and inhibition of α7-nAChR mRNA and protein expression aggravate the toxicity of β-amyloids in vitro [21]. These findings suggest a therapeutic role for α7-nAChR agonists in the treatment of AD [20], [21].
Recent progress in the pharmacology of α7-nAChR agonists has stimulated the interest of pharmaceutical companies in these compounds as potential medications for treatment of schizophrenia and AD and several new drug candidates that are now undergoing clinical trials [22].
Successful imaging of α7-nAChR with positron-emission tomography (PET) would open an opportunity to evaluate novel nicotinic drugs and, perhaps, lead to an understanding of the relationship of these receptors to brain function and the progression of various diseases. While PET imaging of α4β2-nAChR in human subjects is a current reality (see for review [23], [24], [25]), a lack of PET radioligands for α7-nAChR in human subjects impedes progress in the neuroscience of this receptor subtype. A number of α7-nAChR PET radioligands have been developed, but a quality radiotracer remains to be synthesized [26], [27], [28] (see also recent reviews [25], [29]).
Abbott Laboratories has recently developed two α7-nAChR ligands (A-833834 and A-752274) with very high binding affinities (Fig. 1) [30], [31]. This paper describes our endeavor to explore radiolabeled [11C]A-833834 and [11C]A-752274 as potential PET tracers for imaging the α7-nAChR.
Section snippets
General
1H NMR spectra were recorded on a GE QE-300 (300 MHz) spectrometer, and chemical shifts are reported in parts per million (ppm) downfield of tetramethylsilane (TMS) as internal standard. Mass spectra were acquired on a JEOL JMS-SX-102 spectrometer. Unless otherwise specified, all reagents and solvents were obtained from commercial sources and used without further purification. Preparative high performance liquid chromatography (HPLC) for cold chemistry was performed on an automated system
Chemistry
The nor-methyl precursors 3 and 7 for radiolabeling [11C]A-833834 and [11C]A-752274 were synthesized as shown in Scheme 1, Scheme 2. Since both precursors are highly polar compounds, their purification with silica-gel flash chromatography was expected to be problematic. Therefore, the final purification of free base 3 and 7 was performed with preparative reverse-phase HPLC.
Radiochemistry
The carbon-11 radiosynthesis of 5-(6-(5-[11C]methylhexahydropyrrolo[3,4-c]pyrrol-2(1H)-yl)pyridazin-3-yl)-1H-indole ([11
Conclusion
Radiolabeling of α7-nAChR ligands A-833834 and A-752274 with [11C] have been achieved with high radiochemical yields and specific radioactivities by 11C-methylation of corresponding nor-methyl precursors that were synthesized in this study.
Regional brain distribution studies demonstrated that [11C]A-833834 enters the mouse brain but its specific binding in the α7-nAChR-rich brain regions was low due to insufficient binding affinity.
The very high binding affinity radioligand [11C]A-752274
Acknowledgments
We are grateful to Robert C. Smoot for radiochemistry assistance, and David J. Clough and Karen Edmonds for PET scanner operation and Paige Finley for help with animal experiments. The authors thank Mrs. Judy Buchanan for her kind editorial assistance. This research was supported in part by the Division of Nuclear Medicine of Johns Hopkins University School of Medicine and by NIH Grants DA020777 and MH079017 (AGH).
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