Synthesis and characterization of radioiodinated (S)-5-iodonicotine: a new ligand for potential imaging of brain nicotinic cholinergic receptors by single photon emission computed tomography

Chemical fate of the nicotinic acetylcholinergic radiotracer [123I]5-IA-85380 in baboon brain and plasma

The fate of the nicotinic acetylcholinergic receptor radiotracer [123I]5-IA-85380 ([123I]5-IA) was studied in baboon by analyzing the chemical composition of brain tissue and plasma after intravenous administration of the tracer. Acetonitrile denaturation and high-performance liquid chromatography (HPLC) analysis showed predominantly unchanged (91-98%) parent tracer in all brain tissues examined, compared to significant metabolism (23% parent) in the plasma at 90 min postinjection, and control tissue recovery of 95-98%. [123I]5-IA was distributed to the thalamus with a standardized uptake value of 9.2 (0.04% dose/g) or a concentration 5.8 times higher than that of the cerebellum. The HPLC behavior of a synthesized sample of one hypothesized metabolite, 5-iodo-3-pyridinol (5-IP), was consistent with plasma radiometabolite fraction. Since only parent radiotracer compound was found in brain tissue, these results add confidence that information derived from single photon emission computed tomography images of 123I activity in the brain after [123I]5-IA administration can be interpreted as distribution of an intact radiotracer.

Nicotinic Agonists, Antagonists, and Modulators From Natural Sources

1. Acetylcholine receptors were initially defined as nicotinic or muscarinic, based on selective activation by two natural products, nicotine and muscarine. Several further nicotinic agonists have been discovered from natural sources, including cytisine, anatoxin, ferruginine, anabaseine, epibatidine, and epiquinamide. These have provided lead structures for the design of a wide range of synthetic agents. 2. Natural sources have also provided competitive nicotinic antagonists, such as the Erythrina alkaloids, the tubocurarines, and methyllycaconitine. Noncompetitive antagonists, such as the histrionicotoxins, various izidines, decahydroquinolines, spiropyrrolizidine oximes, pseudophrynamines, ibogaine, strychnine, cocaine, and sparteine have come from natural sources. Finally, galanthamine, codeine, and ivermectin represent positive modulators of nicotinic function, derived from natural sources. 3. Clearly, research on acetylcholine receptors and functions has been dependent on key natural products and the synthetic agents that they inspired.

Iodination and radiolabeling of alpha-allocryptopine with iodine-125

A procedure for labeling of alpha-allocryptopine with 125I is reported in this article. Alpha-allocryptopine is first iodinated with largely excessive iodine monochloride, and then radiolabeled by isotopic exchange with Na125I in a dichloromethane-water two-phase system. The radiochemical purity and yield of the labeled product are typically as high as 94% and 72%, respectively. Further chromatographic purification gives a highly pure product (radiochemical purity > 99%). The distribution of 125I-labeled alpha-allocryptopine in mice at 2 h after injection shows that the uptake of radioiodine in thyroid is negligibly low.

Studies toward Labeling Cytisine with [11C]Phosgene: Rapid Synthesis of a δ-Lactam Involving a New Chemoselective Lithiation−Annulation Method

With the aim of the radiolabeling of cytisine, a potent agonist of nicotinic receptors, with [(11)C]phosgene, the rapid synthesis of a lactam model of our target has been studied. The key step of the delta-lactam formation is a new chemoselective lithiation-annulation method, under high dilution, of a suitable piperidinylcarbamoyl chloride. This precursor was obtained from (2-hydroxyethyl)piperidine in a linear synthetic sequence involving a Corey-Fuchs olefination of the corresponding aldehyde, followed by a selective reduction, using a diimide equivalent, of an iodoalkyne into a (Z)-iodopropene piperidine. This alkene served as main precursor to study the cyclization according to several procedures using phosgene as the required carbonylating reagent.

[Biomedical imaging in pharmacology with nuclear medical imaging methodologies: positron emission tomography (PET) and single photon emission computed tomography (SPECT)]

The nuclear imaging technologies, positron emission tomography (PET) and single photon emission computed tomography (SPECT), have the power to non-invasively obtain dynamic and real-time information on the in vivo behaviors of radiolabeled molecules not only in humans but also in experimental animals. Thus, PET and SPECT can image molecular interactions of biological processes in vivo directly and reveal biological phenomena that are hidden from view. Furthermore, these imaging procedures also can be repeatedly performed before and after interventions, thereby allowing each subject to be used as its own control. In these studies, the radiolabeled compounds used as imaging probes for non-invasive assays of biochemical processes should have defined in vivo behaviors that can provide valuable information on the physiological and pharmacological processes. This paper describes the principle of the nuclear medical imaging systems, rational design of radiolabeled imaging probes, and the application to in vivo investigation of the change of various neurotransmission systems under disease and drug treatment. The efficient utilization of these nuclear medical imaging technologies will accelerate biomedical studies and drug development.

Preparation of a bromine-76 labelled analogue of epibatidine: a potent ligand for nicotinic acetylcholine receptor studies

Epibatidine analogues have been labelled with I-123 for single photon emission computed tomography and with short half-life positron emitters (C-11 and F-18) for PET. For easier radiopharmacological studies the bromo analogue of epibatidine (norchlorobromoepibatidine or exo-7-azabicyclo-2-(2-bromo-5-pyridyl)-[2.2.1]heptane) was labelled with Br-76, a longer half-life positron emitter, (T1/2 = 16.2h). [76Br]-norchlorobromoepibatidine was prepared by using a Cu+ assisted bromodeiodination exchange from the iodo analogue in reducing conditions at 190 degrees C. The tracer purified by RP-HPLC was obtained in 70% radiochemical yield with a specific radioactivity of 20 GBq/micromol. Radiochemical and chemical purities measured by radio-TLC and HPLC were >98%.

Evaluation of radioiodinated 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine as a ligand for SPECT investigations of brain nicotinic acetylcholine receptors

5-Iodo-3-(2(S)-azetidinylmethoxy)pyridine (5IA), an A-85380 analog iodinated at the 5-position of the pyridine ring, was evaluated as a radiopharmaceutical for investigating brain nicotinic acethylcholine receptors (nAChRs) by single photon emission computed tomography (SPECT). [123/125I]5IA was synthesized by the iododestannylation reaction under no-carrier-added conditions and purified by high-performance liquid chromatography (HPLC) with high radiochemical yield (50%), high radiochemical purity (> 98%), and high specific radioactivity (> 55 GBq/micromol). The binding affinity of 5IA for brain nAChRs was measured in terms of displacement of [3H]cytisine and [125I]5IA from binding sites in rat cortical membranes. The binding data revealed that the affinity of 5IA was the same as that of A-85380 and more than seven fold higher than that of (-)-nicotine, and that 5IA bound selectively to the alpha4beta2 nAChR subtype. Biodistribution studies in rats indicated that the brain uptake of [125I]51A was rapid and profound. Regional cerebral distribution studies in rats demonstrated that the accumulation of [125I]5IA was consistent with the density of high affinity nAChRs with highest uptake observed in the nAChR-rich thalamus, moderate uptake in the cortex and lowest uptake in the cerebellum. Administration of the nAChR agonists (-)-cytisine and (-)-nicotine reduced the uptake of [125I]5IA in all regions studied with most pronounced reduction in the thalamus, and resulted in similar levels of radioactivity throughout the brain. [125I]5IA binding sites were shown to be saturable with unlabeled 5IA. Behavioral studies in mice demonstrated that 5IA did not show signs of behavioral toxicity. Furthermore, SPECT studies with [123I]5IA in the common marmoset demonstrated appropriate brain uptake and regional localization for a high-affinity nAChR imaging radiopharmaceutical. These results suggested that [123I]5IA is a promising radiopharmaceutical for SPECT studies of central nAChRs in human subjects.

5-[I-125/123]lodo-3(2(S)-azetidinylmethoxy)pyridine, a radioiodinated analog of A-85380 for in vivo studies of central nicotinic acetylcholine receptors

The in vivo biodistribution profile of the novel nicotinic acetylcholine receptor (nAChR) radioligand 5-[I-125/123]Iodo-3(2(S)-azetidinylmethoxy)pyridine, [I-125/123]-5-IA, in mouse brain was examined. This radiotracer displayed good brain penetration (3.1% of the injected dose (ID) in whole brain at 15 min post-radioligand injection). Radioligand distribution was consistent with the density of high affinity nAChRs with highest uptake observed in the nAChR-rich thalamus (14.9 %ID/g at 60 min), moderate uptake in cortex (8.5 %ID/g at 60 min), and lowest uptake in the cerebellum (2.4 %ID/g at 60 min). Pretreatment with several different nAChR agonists (A-85380, (-)-nicotine, cytisine) significantly inhibited [I-125]-5-IA binding in all brain regions studied (P < 0.01) demonstrating the high specificity of the radioligand for nAChRs. Blocking doses of the muscarinic antagonist scopolamine and the non-competitive nAChR channel blocker mecamylamine had no significant effect on radioactive uptake supporting the in vitro selectivity of [I-125]-5-IA for the nAChR component of the cholinergic system. [I-125]-5-IA binding sites were shown to be saturable with unlabeled 5-IA. With a relatively low acute toxicity (LD50 > 3 mg/kg via intravenous injection in mice) and high in vivo specificity and selectivity, 5-IA labeled with the imaging radionuclide I-123 may prove useful for single photon emission computed tomography (SPECT) studies of nAChRs in human subjects.

Development of ligands for in vivo imaging of cerebral nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) mediate a variety of brain functions. Findings from postmortem studies and clinical investigations have implicated them in the pathophysiology and treatment of Alzheimer's and Parkinson's diseases and other CNS disorders (e.g. Tourette's syndrome, epilepsy, nicotine dependence). Therefore, it ultimately might be useful to image nAChRs noninvasively for diagnosis, for studies on how changes in nAChRs might contribute to cerebral disorders, for development of therapies targeted at nAChRs, and to monitor the effects of such treatments. To date, only (S)-(-)-nicotine, radiolabeled with 11C, has been used for external imaging of nAChRs in human subjects. Since this radiotracer presents drawbacks, new ligands, with more favorable properties, have been synthesized and tested. Three general classes of compounds, namely, nicotine and its analogs, epibatidine and related compounds, and 3-pyridyl ether compounds, including A-85380, have been evaluated. Analogs of A-85380 appear to be the most promising candidates because of their low toxicity and high selectivity for the alpha4beta2 subtype of nAChRs.

Synthesis of analogues of (-)-cytisine for in vivo studies of nicotinic receptors using positron emission tomography

[formula: see text] 9-Substituted analogues of (-)-cytisine were synthesized in high yields via palladium-mediated couplings of either 9-(-)-bromocytisine and organostannanes or 9-(-)-trimethylstannylcytisine and fluorobromobenzene. The protection of the amine with a nitroso group and the use of PdCl2(PPh3)2 to carry out the Stille reaction allowed the rapid synthesis of 9-(4'-[18F]fluorophenyl)cytisine (18F: t1/2 = 109.7 min), a new promising radioligand (radiochemical yield: 10% from [18F]KF, 150 min, four steps) for positron emission tomography studies of alpha 4 beta 2 nicotinic receptors.

Synthesis of an I-123 analog of A-85380 and preliminary SPECT imaging of nicotinic receptors in baboon

A radiosynthetic method to prepare the nicotinic acetylcholine receptor radioligand (S)-5-[123I]iodo-3-(2-azetidinylmethoxy)pyridine, 5-IA, has been developed. The two-step sequence produced [123I]-5-IA in high radiochemical yield (52%), high radiochemical purity (98%), and high specific radioactivities (> 8,500 mCi/mumol). Preliminary single photon emission computed tomography studies with [123I]-5-IA in baboon demonstrated the appropriate regional localization for a high-affinity nicotinic radioprobe (thalamus > frontal cortex > cerebellum). Pretreatment with cytisine blocked [123I]-5-IA uptake in all brain regions (78-59% reduction), demonstrating the specificity of the radiotracer.

2-, 5-, and 6-Halo-3-(2(S)-azetidinylmethoxy)pyridines: synthesis, affinity for nicotinic acetylcholine receptors, and molecular modeling

3-(2(S)-Azetidinylmethoxy)pyridine (A-85380) has been identified recently as a ligand with high affinity for nicotinic acetylcholine receptors (nAChRs). Here we report the synthesis and in vitro nAChR binding of a series of 10 pyridine-modified analogues of A-85380. The novel compounds feature a halogen substituent at position 2, 5, or 6 of the 3-pyridyl fragment. Those with the substituents at position 5 or 6, as well as the 2-fluoro analogue, possess subnanomolar affinity for nAChRs in membranes from rat brain. For these ligands, Ki values range from 11 to 210 pM, as measured by competition with (+/-)-[3H]epibatidine. In contrast, 2-chloro, 2-bromo, and 2-iodo analogues exhibit substantially lower affinity. AM1 quantum chemical calculations demonstrate that the bulky substituents at position 2 cause notable changes in the molecular geometry. The high-affinity members of the series and (+)-epibatidine display a tight fit superposition of low-energy stable conformers. The new ligands with high affinity for nAChRs may be of interest as pharmacological probes, potential medications, and candidates for developing radiohalogenated tracers to study nAChRs.