Synthesis of radioiodinated analogs of 2-(4-phenylpiperidino)cyclohexanol (vesamicol) as vesamicol-like agent

Development of tumor-targeting aza-vesamicol derivatives with high affinity for sigma receptors for cancer theranostics

As sigma receptors are highly expressed on various cancer cells, radiolabeled sigma receptor ligands have been developed as imaging and therapeutic probes for cancer. Previously, we synthesized and evaluated a radioiodinated vesamicol derivative, 2-(4-[¹²⁵I](4-iodophenyl)piperidine)cyclohexanol ((+)-[¹²⁵I]pIV), and a radioiodinated aza-vesamicol derivative, trans-2-(4-(3-[¹²⁵I](4-iodophenyl)propyl)piperazin-1-yl)cyclohexan-1-ol ([¹²⁵I]2), as sigma-1 receptor-targeting probes. In order to obtain sigma receptor-targeting probes with superior biodistribution characteristics, we firstly synthesized twelve bromine-containing aza-vesamicol derivatives and evaluated their affinity for sigma receptors. One such derivative exhibited high selectivity for the sigma-1 receptor and another exhibited high affinity for both the sigma-1 and sigma-2 receptors. Thus, their halogen-substituted iodine- and radioiodine-containing compounds were prepared. The ¹²⁵I-labeled compounds exhibited high uptake in tumor and lower uptake in non-target tissues than the two previously developed and evaluated ¹²⁵I-labeled sigma receptor-targeting probes, [¹²⁵I]pIV and [¹²⁵I]2. Therefore, these novel radioiodine-labeled compounds should be promising as sigma receptor-targeting probes.

Syntheses and evaluation of a homologous series of aza-vesamicol as improved radioiodine-labeled probes for sigma-1 receptor imaging

Sigma-1 receptor imaging probes for determining the expression levels are desirable for diagnoses of various diseases and companion diagnoses of therapeutic agents targeting the sigma-1 receptor. In this study, we aimed to develop probes with higher affinity for the sigma-1 receptor. For this purpose, we synthesized and evaluated compounds, namely, vesamicol derivatives, in which alkyl chains of varying chain length were introduced between a piperazine ring and a benzene ring. The binding affinity of the vesamicol derivatives for the sigma-1 receptor tended to increase depending on the length of the alkyl chain between the benzene ring and the piperazine ring. The sigma-1 receptor of 2-(4-(3-phenylpropyl)piperazin-1-yl)cyclohexan-1-ol (5) (Ki = 5.8 nM) exhibited the highest binding affinity; therefore, we introduced radioiodine into the benzene ring in 5. The radioiodine labeled probe [125I]2-(4-(3-(4-iodophenyl)propyl)piperazin-1-yl)cyclohexan-1-ol ([125I]10) showed high accumulation in the sigma-1 receptor expressing DU-145 cells both in vitro and in vivo. Co-injection of [125I]10 with an excess level of a sigma receptor ligand, haloperidol, resulted in a significant decrease in the tumor accumulation in vitro and in vivo, indicating sigma receptor-mediated tumor uptake. These results provide useful information for developing sigma-1 receptor imaging probes.

Development of a novel radiobromine-labeled sigma-1 receptor imaging probe

INTRODUCTION

Sigma-1 receptor is a target for tumor imaging. In a previous study, we synthesized a vesamicol analog, (+)-2-[4-(4-bromophenyl)piperidino]cyclohexanol [(+)-pBrV], with a high affinity for sigma-1 receptor, and synthesized radiobrominated (+)-pBrV. This radiobrominated (+)-pBrV showed high tumor uptake in tumor-bearing mice; however, radioactivity accumulation in normal tissues, such as the liver, was high. We assumed that the accumulation of (+)-pBrV in the non-target tissues was partially derived from its high lipophilicity; therefore, we synthesized and evaluated (+)-4-[1-(2-hydroxycyclohexyl)piperidine-4-yl]-2-bromophenol [(+)-BrV-OH], which is a more hydrophilic compound. Although we aimed to develop a PET tracer using 76Br, in these initial studies, we used 77Br because of its longer half-life.

METHODS

(+)-[77Br]BrV-OH was synthesized using the chloramine-T method with a radiochemical purity of 95%. Lipophilicity and affinity for sigma-1 receptor of (+)-[77Br]BrV-OH were determined, and biodistribution experiments were performed. We also performed an in vivo blocking study by co-injecting excess amounts of the sigma-1 receptor ligand, SA4503, into mice.

RESULTS

The lipophilicity and affinity for sigma-1 receptor of (+)-[77Br]BrV-OH were lower than those of (+)-[77Br]pBrV. (+)-[77Br]BrV-OH also showed high tumor uptake in biodistribution experiments in DU-145 tumor-bearing mice,. Although (+)-[77Br]pBrV was retained in most tissues, (+)-[77Br]BrV-OH was cleared from these tissues. In blocking studies, the co-injection of SA4503 significantly decreased the tumor uptake of (+)-[77Br]BrV-OH.

CONCLUSION

These results indicate that (+)-[76Br]BrV-OH has potential as a PET probe for sigma-1 receptor imaging.

Development and evaluation of a radiobromine-labeled sigma ligand for tumor imaging

INTRODUCTION

Sigma receptors are appropriate targets for tumor imaging because they are highly expressed in a variety of human tumors. Previously, we synthesized a vesamicol analog, (+)-2-[4-(4-iodophenyl)piperidino]cyclohexanol ((+)-pIV), with high affinity for sigma receptors, and prepared radioiodinated (+)-pIV. In this study, to develop a radiobromine-labeled vesamicol analog as a sigma receptor imaging agent for PET, nonradioactive and radiobromine-labeled (+)-2-[4-(4-bromophenyl)piperidino]cyclohexanol ((+)-pBrV) was prepared and evaluated in vitro and in vivo. In these initial studies, (77)Br was used because of its longer half-life.

METHODS

(+)-[(77)Br]pBrV was prepared by a bromodestannylation reaction with radiochemical purity of 98.8% after HPLC purification. The partition coefficient of (+)-[(77)Br]pBrV was measured. In vitro binding characteristics of (+)-pBrV to sigma receptors were assayed. Biodistribution experiments were performed by intravenous administration of a mixed solution of (+)-[(77)Br]pBrV and (+)-[(125)I]pIV into DU-145 tumor-bearing mice.

RESULTS

The lipophilicity of (+)-[(77)Br]pBrV was lower than that of (+)-[(125)I]pIV. As a result of in vitro binding assay to sigma receptors, the affinities of (+)-pBrV to sigma receptors were competitive to those of (+)-pIV. In biodistribution experiments, (+)-[(77)Br]pBrV and (+)-[(125)I]pIV showed high uptake in tumor via sigma receptors. The biodistributions of both radiotracers showed similar patterns. However, the accumulation of radioactivity in liver after injection of (+)-[(77)Br]pBrV was significantly lower compared to that of (+)-[(125)I]pIV.

CONCLUSION

These results indicate that radiobromine-labeled pBrV possesses great potential as a sigma receptor imaging agent for PET.

Development and evaluation of a novel radioiodinated vesamicol analog as a sigma receptor imaging agent

BACKGROUND

Sigma receptors are highly expressed in human tumors and should be appropriate targets for developing tumor imaging agents. Previously, we synthesized a vesamicol analog, (+)-2-[4-(4-iodophenyl)piperidino]cyclohexanol ((+)-pIV), with a high affinity for sigma receptors and prepared radioiodinated (+)-pIV. As a result, (+)-[125I]pIV showed high tumor uptake in biodistribution experiments. However, the accumulation of radioactivity in normal tissues, such as the liver, was high. We supposed that some parts of the accumulation of (+)-pIV in the liver should be because of its high lipophilicity, and prepared and evaluated a more hydrophilic radiolabeled vesamicol analog, (+)-4-[1-(2-hydroxycyclohexyl)piperidine-4-yl]-2-iodophenol ((+)-IV-OH).

METHODS

(+)-[125I]IV-OH was prepared by the chloramine T method from the precursor. The partition coefficient of (+)-[125I]IV-OH was measured. Biodistribution experiments were performed by intravenous administration of a mixed solution of (+)-[125I]IV-OH and (+)-[131I]pIV into DU-145 tumor-bearing mice. Blocking studies were performed by intravenous injection of (+)-[125I]IV-OH mixed with an excess amount of ligand into DU-145 tumor-bearing mice.

RESULTS

The hydrophilicity of (+)-[125I]IV-OH was much higher than that of (+)-[125I]pIV. In biodistribution experiments, (+)-[125I]IV-OH and (+)-[131I]pIV showed high uptake in tumor tissues at 10-min post-injection. Although (+)-[131I]pIV tended to be retained in most tissues, (+)-[125I]IV-OH was cleared from most tissues. In the liver, the radioactivity level of (+)-[125I]IV-OH was significantly lower at all time points compared to those of (+)-[131I]pIV. In the blocking studies, co-injection of an excess amount of sigma ligands resulted in significant decreases of tumor/blood uptake ratios after injection of (+)-[125I]IV-OH.

CONCLUSIONS

The results indicate that radioiodinated (+)-IV-OH holds a potential as a sigma receptor imaging agent.

Syntheses and in vitro evaluation of decalinvesamicol analogues as potential imaging probes for vesicular acetylcholine transporter (VAChT)

A series of vesamicol analogues, o-iodo-trans-decalinvesamicol (OIDV) or o-bromo-trans-decalinvesamicol (OBDV), were synthesized and their affinities to vesicular acetylcholine transporter (VAChT) and σ receptors (σ-1, σ-2) were evaluated by in vitro binding assays using rat cerebral or liver membranes. OIDV and OBDV showed greater binding affinity to VAChT (K(i) = 20.5 ± 5.6 and 13.8 ± 1.2 nM, respectively) than did vesamicol (K(i) = 33.9 ± 18.1 nM) with low affinity to σ receptors. A saturation binding assay in rat cerebral membranes revealed that [(125)I]OIDV had a single high affinity binding site with a K(d) value of 1.73 nM and a B(max) value of 164.4 fmol/mg protein. [(125)I]OIDV revealed little competition with inhibitors, which possessed specific affinity to each σ (σ-1 and σ-2), serotonin (5-HT(1A) and 5-HT(2A)), noradrenaline, and muscarinic acetylcholine receptors. In addition, BBB penetration of [(125)I]OIDV was verified in in vivo. The results of the binding studies indicated that OIDV and OBDV had great potential to be VAChT imaging probes with high affinity and selectivity.

Evaluation of radioiodinated vesamicol analogs for sigma receptor imaging in tumor and radionuclide receptor therapy

It has been reported that sigma receptors are highly expressed in a variety of human tumors. In this study, we selected (+)-2-[4-(4-iodophenyl)piperidino] cyclohexanol [(+)-pIV] as a sigma receptor ligand and evaluated the potential of radioiodinated (+)-pIV for tumor imaging and therapy. (+)-[(125/131)I]pIV was prepared by an iododestannylation reaction under no-carrier-added conditions with radiochemical purity over 99% after HPLC purification. Biodistribution experiments were performed by the intravenous injection of (+)-[(125)I]pIV into mice bearing human prostate tumors (DU-145). Blocking studies were performed by intravenous injection of (+)-[(125)I]pIV mixed with an excess amount of unlabeled sigma ligand into DU-145 tumor-bearing mice. For therapeutic study, (+)-[(131)I]pIV was injected at a dose of 7.4 MBq followed by measurement of the tumor size. In biodistribution experiments, (+)-[(125)I]pIV showed high uptake and long residence in the tumor. High tumor to blood and muscle ratios were achieved because the radioactivity levels of blood and muscle were low. However, the accumulations of radioactivity in non-target tissues, such as liver and kidney, were high. The radioactivity in the non-target tissues slowly decreased over time. Co-injection of (+)-[(125)I]pIV with an excess amount of unlabeled sigma ligand resulted in a significant decrease in the tumor/blood ratio, indicating sigma receptor-mediated tumor uptake. In therapeutic study, tumor growth in mice treated with (+)-[(131)I]pIV was significantly inhibited compared to that of an untreated group. These results indicate that radioiodinated (+)-pIV has a high potential for sigma receptor imaging in tumor and radionuclide receptor therapy.

Synthesis and evaluation of vesamicol analog (-)-o-[11C]methylvesamicol as a PET ligand for vesicular acetylcholine transporter

(-)-O-Methylvesamicol ((-)-OMV) exhibited in vitro a high affinity for vesicular acetylcholine transporter (VAChT) (Ki, 6.7 nM) and a relatively low affinity for sigmal receptor (Ki, 33.7 nM). We prepared (-)-[11C]OMV by a palladium-promoted cross-coupling reaction using [11C]methyl iodide, in a radiochemical yield of 38 +/- 6.9% (n=3), a radiochemical purity of 98 +/- 2.3% (n = 5), and a specific activity of 11 +/- 7.0 TBq/mmol at 30 minutes after EOB (n=5). Then, we evaluated in vivo whether (-)-[11C]OMV has properties as a PET radioligand for mapping VAChT. In rats, the brain uptake of (-)-[11C]OMV was 1.1%ID/g at 5 minutes postinjection, and was retained of a high level for 60 minutes. The brain uptake was significantly inhibited by the co-injection (500 nmol/kg) of cold (-)-OMV (58-66%), (-)-vesamicol (57-65%), and two sigma receptor ligands with modulate affinities for VAChTs: SA4503 (56-71%) and haloperidol (39-64%) in all of the brain regions, including the cerebellum with a low density of VAChTs, but not of sigmal-selective ligand (+)-pentazocine. However, the pretreatment with a large excess amount of (+/-)-pentazocine (50 micromol/kg) reduced the uptake in a different manner in the brain regions: 25% reduction in the striatum with a high density of VAChTs, and a 50-55% reduction in the other regions with a lower density of VAChTs. Ex vivo autoradiography using (-)-[11C]OMV showed a similar regional brain distribution of [3H](-)-vesamicol. In the PET study of the monkey brain, the regional brain distribution pattern of (-)-[11C]OMV was different from that of [11C]SA4503. The uptake of (-)-[11C]OMV was relatively higher in the striatum, was reversible, and an apparent equilibrium state was found at 20-40 minutes. In conclusion, (-)-[11C]OMV exhibited appropriate brain kinetics during the time frame of 11C-labeled tracers and bound mainly to VAChTs; however, the binding to sigmal receptors was not disregarded. Therefore, (-)-[11C]OMV-PET together with help of [11C]SA4503-PET may evaluate VAChTs.

In vitro characterization of radioiodinated (+)-2-[4-(4-iodophenyl) piperidino]cyclohexanol [(+)-pIV] as a sigma-1 receptor ligand

We investigated the binding characteristics of a (+)-enantiomer of radioiodinated 2-[4-(4-iodophenyl)piperidino]cyclohexanol [(+)-[125I]pIV], radioiodinated at the para-position of the 4-phenylpiperidine moiety, to sigma receptors (sigma-1, sigma-2) and to vesicular acetylcholine transporters (VAChT) in membranes of the rat brain and liver. In competitive inhibition studies, (+)-pIV (Ki=1.30 nM) had more than 10 times higher affinity to the sigma-1 (sigma-1) receptor than (+)-pentazocine (Ki=19.9 nM) or haloperidol (Ki=13.5 nM) known as sigma ligands. Also, the binding affinity of (+)-pIV for the sigma-1 receptor (Ki=1.30 nM), was about 16 times higher than the sigma-2 (sigma-2) receptor (Ki=20.4 nM). (+)-pIV (Ki=1260 nM) had a much lower affinity for VAChT than (-)-vesamicol (Ki=13.0 nM) or (-)-pIV (Ki=412 nM). (+)-[125I]pIV had low affinity for the dopamine, serotonin, adrenaline, and acetylcholine receptors. Furthermore, in a saturation binding study, (+)-[125I]pIV exhibited a K) of 6.96 nM with a Bmax of 799 fmol/mg of protein. These results showed that (+)-pIV binds to the sigma-1 receptor with greater affinity than sigma receptor ligands such as (+)-pentazocine or haloperidol, and that radioiodinated (+)-pIV is suitable as radiotracer for sigma-1 receptor studies in vitro.

Evaluation of radioiodinated (-)-o-iodovesamicol as a radiotracer for mapping the vesicular acetylcholine transporter

We evaluated the potencies of radioiodinated (-)-o-iodovesamicol [(-)-oIV] as a selective vesicular acetylcholine transporter (VAChT) mapping agent. (-)-[125I]oIV exhibited significant accumulation (about 2.8% of the injected dose) in rat brain. The regional brain distribution of radioactivity was similar for both (-)-[125I]olV and (-)-[3H]vesamicol. The accumulation of (-)-[125I]oIV in the brain was significant reduced by post-administration of unlabeled vesamicol (0.5 /micromol/kg(-1)) and (-)-oIV (0.5 micromol/kg(-1)). On the other hand, the post-administration of sigma ligands hardly affected the accumulation of (-)-[125I]oIV in the brain. These studies showed that (-)-[125I]oIV, as well as [3H] vesamicol, bound to VAChT with high affinity in the rat brain. Furthermore, (-)-[125I]oIV binding in the ipsilateral cortex to the lesion was significantly reduced by 17.0%, compared with that in the contralateral cortex in a unilateral NBM-lesioned rat. These results suggested that radioiodinated (-)-oIV may potentially be useful for the diagnosis of cholinergic neurodegenerative disorders.

Characterization of radioiodinated (-)-ortho-iodovesamicol binding in rat brain preparations

We investigated the binding characteristics of optical isomers of three iodovesamicol analogs to vesicular acetylcholine transporters (VAChT) and to sigma receptors (sigma-1, sigma-2) in rat brains. In competitive inhibition studies, (-)-enantiomers [(-)-ortho-iodovesamicol ((-)-oIV), (-)-meta-iodovesamicol ((-)-mIV), (-)-vesamicol] displayed a higher affinity for VAChT than (+)-enantiomer [(+)-oIV, (+)-mIV, (+)-vesamicol]. (-)-oIV and (-)-mIV showed the same high affinity for VAChT as (-)-vesamicol. For sigma receptors(sigma-1, sigma-2), (-)-oIV (Ki = 62.2 nM (to sigma-1) and 554 nM(to sigma-2)) showed a lower affinity than (-)-mIV (Ki = 4.5 nM (to sigma-1) and 42.9 nM (to sigma-2)). Furthermore, in a saturation binding study, (-)-[125I]-oIV exhibited a Kd of 17.4 +/- 5.1 nM with a maximum number of binding sites, Bmax, of 559 +/- 51 fmol/ mg of protein. These results showed that (-)-oIV binds to vesicular acetylcholine transporters (VAChT) more selectively than (-)-mIV, previously reported as a VAChT mapping agent, and may be suitable for VAChT imaging studies.

The potential of radioiodinated (-)-m-iodovesamicol for diagnosing cholinergic deficit dementia

We investigated changes in the brain distribution of (-)-[(125)I]-m-iodovesamicol [(-)-[(125)I]mIV] in cholinergic denervation rats produced by a unilateral lesion of the nucleus basalis magnocellularis (NBM). Dual-tracer ex vivo autoradiographic analysis using (-)-[(125)I]mIV and [(99m)Tc]HMPAO was conducted to the effect of regional cerebral perfusion on the brain distribution of (-)-[(125)I]mIV in a unilateral NBM-lesioned rat. (-)-[(125)I]mIV binding in the ipsilateral cortex to the lesion significantly reduced by 10.4 %, compared with that in the contralateral cortex, while (-)-[(125)I]mIV binding in the ipsilateral caudate putamen, hippocampus and thalamus did not change. The rate of reduction in the (-)-[(125)I]mIV binding (10.4 %) was significantly higher than that of [(99m)Tc]HMPAO accumulation (4.0%) in the ipsilateral cortex to the lesion (P < 0.01). These results suggested that radioiodinated (-)-mIV may be useful in the study of dementia characterized by degeneration of the cholinergic neurotransmitter system, such as Alzheimer's disease.

Synthesis and biological characterization of stable and radioiodinated (+/-)-trans-2-hydroxy-3-P[4-(3-iodophenyl)piperidyl]-1,2,3,4-tetrahydronaphthalene (3'-IBVM)

The vesamicol analogue (+/-)-trans-2-Hydroxy-3-[4-(3-iodophenyl)piperidyl]-1,2,3,4-tetrahydronaphthalene (3'-IBVM), a potent ligand for the vesicular acetylcholine transporter (VAChT), was evaluated as a potential radiotracer for studying VAChT density in vivo. In radioligand binding experiments, 3'-IBVM displays subnanomolar affinity for VAChT and 100-fold selectivity for VAChT over sigma1 and sigma2 receptors. Consistent with this profile, radioiodinated (+/-)-3'-IBVM distributed heterogenously in the rat brain following a bolus IV injection, displaying high concentrations in the striatum and moderate to low concentrations in the cortex and cerebellum, respectively. However, co-injection of the radiotracer with the sigma ligand haloperidol resulted in significant reductions of radiotracer levels in all brain regions examined. Therefore, radioiodinated (+/-)-IBVM appears to bind to both VAChT and sigma receptors in vivo.

Reduction of vesicular acetylcholine transporter in beta-amyloid protein-infused rats with memory impairment

The aim of this study was to investigate spatial memory and quantitative acetylcholine transporter autoradiography using a high-sensitivity imaging plate system in rats treated with beta-amyloid protein, a model of Alzheimer's disease. An eight-arm radial maze was used to evaluate spatial memory. The performance of the eight-arm radial maze task was impaired in beta-amyloid protein-treated rats. In the parietal cortex, [3H]-vesamicol binding to the vesicular acetylcholine transporter was significantly lower in beta-amyloid protein-treated rats than in vehicle-treated rats, and was significantly correlated with the mean number of correct selections in the maze task of the first 5 days in the post-operative state. These results indicate that the reduction in [3H]-vesamicol binding to vesicular acetylcholine transporter is related to memory impairment induced by beta-amyloid protein. Beta-amyloid protein-infused rats with spatial memory impairment may be useful for the development of new radiolabelled vesamicol analogues for the objective evaluation of Alzheimer's disease.

N-(3-Iodophenyl)trozamicol (IPHT) and related inhibitors of vesicular acetylcholine transport: synthesis and preliminary biological characterization

Four isomeric N-(halophenyl)trozamicol analogues (6a-d) were synthesized and evaluated as potential vesicular acetylcholine transporter (VAChT) ligands. Of the four compounds, N-(3-bromophenyl) trozamicol (6b) and N-(3-iodophenyl)trozamicol (6d) displayed the highest affinity for the VAChT in vitro, whereas the para-substituted compound 6c showed the lowest affinity for this transporter. Tissue distribution studies of N-(3-[125I]iodophenyl)trozamicol ([125I]6d, [125I)IPHT) suggest that the central distribution of the latter is consistent with cholinergic innervation. However, only moderate target-to-background ratios were obtained, suggesting little improvement over the N-(halobenzyl)trozamicols described previously.

In vitro characterization of radioiodinated (-)-m-iodovesamicol in rat cerebral membranes

We investigated the binding characteristics of 125I-(-)-m-iodovesamicol (125I-(-)-mIV), radioiodinated at the meta position of the 4-phenylpiperidine moiety, in cerebral membranes of the rat brain. The receptor binding affinity of (-)-mIV (Ki = 37 nM) was comparable to that of (-)-vesamicol (Ki = 30 nM). The stereoselectivity of (-)-mIV and (-)-vesamicol for the vesamicol receptor was very high [both (-)-mIV and (-)-vesamicol binding more than 20-fold more active than their (+)isomers], and 125I-(-)-mIV had low affinity for the sigma, dopamine, serotonin, adrenaline and acetylcholine receptors. Furthermore, in a saturation binding study using cerebral membrane preparations, (-)-mIV exhibited a Kd of 18.2 nM with maximum number of binding site Bmax of 660 fmol/mg of protein. These results showed that the characteristics of binding between (-)-mIV and (-)-vesamicol to cholinergic binding sites in the rat brain were similar.

In vivo characterization of radioiodinated 2-(4-phenylpiperidino)cyclohexanol (vesamicol) analogs: potential radioligand for mapping presynaptic cholinergic neurons

Iodovesamicol analogs, radioiodinated at the ortho (1), meta(2) and para(3) positions of the 4-phenylpiperidine moiety, were evaluated as potential presynaptic cholinergic neuron mapping agents. Significant accumulation of m-[125I]iodovesamicol (mIV(2)) (about 3% of the injected dose) was noted in the rat brain with prolonged retention times. The accumulation of mIV(2) in the rat brain was decreased by 67% by 5 min pre-injection of dl-vesamicol(1 mumol/kg). Pre-injection of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperdine[(+)-3-PPP](0.75 mumol/kg) did not markedly decrease the levels of radiotracers (oIV(1) and mIV(2)) in the rat brain. These results suggest that radioidinated m-iodovesamicol (mIV(2)) is suitable for use in presynaptic cholinergic neuron mapping.