Radiosynthesis and mouse brain distribution studies of [11C] CP-126,998: a PET ligand for in vivo study of acetylcholinesterase

Nuclear Medicine and Biology 29 (2002) 547–552

www.elsevier.com/locate/nucmedbio

Radiosynthesis and mouse brain distribution studies of [11C] CP-126,998: a PET ligand for in vivo study of acetylcholinesterase John L. Musachioa,*, John E. Fleshera, Ursula A. Scheffela, Paige Rauseoa, John Hiltona, William B. Mathewsa, Hayden T. Raverta, Robert F. Dannalsa,b, J. James Frosta,c a

Johns Hopkins University School of Medicine, Department of Radiology, Division of Nuclear Medicine, Baltimore, MD 21287, USA b Johns Hopkins University School of Public Health, Department of Environmental Health Sciences, Baltimore, MD 21205, USA c Johns Hopkins University School of Medicine, Department of Neurosciences, Baltimore, MD 21287, USA Received 1 November 2001; received in revised form 4 January 2002; accepted 29 January 2002

Abstract The selective, reversible acetylcholinesterase inhibitor 5,7-Dihydro-7-methyl-3- [2-[1-(phenylmethyl]-4-piperidinyl]ethyl]-6H-pyrrolo[3,2-f]-1,2-benzisoxazol3– 6-one (CP-126,998) was labeled with C-11 iodomethane via base-promoted alkylation of the lactam nitrogen. [11C] CP-126,998 was synthesized in good radiochemical yield (13–29% non-decay corrected) and high specific radioactivity (177– 418 GBq/␮mol). In vivo mouse biodistribution studies reveal [11C] CP-126,998 to localize preferentially in striatal tissue, a region known to be rich in acetylcholinesterase. Competitive blocking studies using a variety of acetylcholinesterase inhibitors (diisopropylfluorophosphate, tacrine, CP-118,954) verified the specificity of the PET radiotracer for brain acetylcholinesterase. © 2002 Elsevier Science Inc. All rights reserved. Keywords: C-11; Acetylcholinesterase; PET; Diisopropylfluorophosphate; Alzheimer’s disease

1. Introduction Alzheimer’s disease is associated with degeneration of cholinergic neurons that play a fundamental role in cognitive functions, including memory [31]. Progressive, inexorable decline in cholinergic function and cholinergic markers in the brain of Alzheimer’s disease patients has been observed in numerous studies, and includes for example, a marked reduction in acetylcholine synthesis, choline acetyltransferase activity, acetylcholinesterase activity, and choline uptake [4,10,12,25,32]. Development of tomographic imaging tracers for non-invasive study of the cholinergic system offers the means for longitudinal study of this system in normal aging and in differing stages of Alzheimer’s disease. Development of radiotracers for in vivo study of cerebral acetylcholinesterase (EC 3.1.1.7, AChE) has focused on two approaches. One approach has employed the use of radiolabeled substrates of AChE that are hydrolyzed by the

* Corresponding author. Fax: ⫹1– 410-614 – 0111. E-mail address: [email protected] (J.L. Musachio).

enzyme and are irreversibly trapped. For example, N-[11C]methylpiperidin-4-yl acetate (MP4A or AMP) [16, 24] and N-[11C]methylpiperidin-4-yl propionate (MP4P or PMP) [18,19] have been employed in PET human imaging studies. Such radioligands provide an index of regional AChE activity via kinetic modeling techniques [17,23]. Alternatively, radiolabeled inhibitors of AChE have been developed to image regional density of enzyme molecules. Previous radiolabeled inhibitors of AChE, however, have not proven ideal as in vivo markers of brain AChE. For example, an analog of tacrine, [11C]methyl-tacrine has been reported; but this ligand’s regional distribution in rats and baboon did not parallel that of AChE concentrations [29]. [11C]-physostigmine, a reversible cholinesterase inhibitor, has been utilized to image human AChE [5]. However, this ligand’s high non-specific binding, marginal selectivity for AChE over butyrylcholinesterase (BuChE), and low-yielding radiosynthesis employing C-11 methylisocyanate [6] hinder its overall utility. Several moderate affinity (IC50 ⫽ 8 –10 nM) N-benzyl piperidine inhibitors of AChE have also been prepared [9,13,21,22] but these radiotracers also failed to demonstrate specific binding to AChE in vivo. A series of N-benzylpiperidines with novel isoxazole-

0969-8051/02/$ – see front matter © 2002 Elsevier Science Inc. All rights reserved. PII: S 0 9 6 9 - 8 0 5 1 ( 0 2 ) 0 0 2 9 9 - 8

548

J.L. Musachio et al. / Nuclear Medicine and Biology 29 (2002) 547–552

Fig. 1. Potent, selective benzisoxazole inhibitors of AChE. In vitro binding data from reference [30].

containing tricycles have been reported [30], and these compounds show potent in vitro inhibition of AChE (IC50 ⫽ 0.33–3.6 nM). Moreover, such compounds are highly selective for AChE over BuChE (typically ⬎ 1,000-fold). In particular, the lactam benzisoxazole, CP-118,954 (1, Fig. 1), shows exceptional potency as a specific AChE inhibitor. Methylation of the lactam nitrogen of 1 yields a compound (CP-126,998, 2) with only slightly reduced potency as an AChE inhibitor while still retaining its selectivity for AChE over BuChE. Based on these in vitro assays, we identified 2 as a target for labeling with C-11 methyl iodide with the aim of developing a highly specific, easily prepared PET ligand that is a reversible inhibitor of AChE. Here, we report the radiosynthesis and preliminary in vivo mouse biodistribution studies with [11C] CP-126,998, a promising AChE tracer that we have begun to utilize in human PET imaging trials [2,3].

2. Materials and methods 2.1. Chemistry The precursor for radiolabeling CP-118,954 (5,7-dihydro-7-methyl-3- [2-[1-(phenylmethyl]-4-piperidinyl]ethyl]6H-pyrrolo[3,2-f]-1,2-benzisoxazol-6-one) and authentic CP-126,998 (5,7-dihydro-3- [2-[1-(phenylmethyl]-4-piperidinyl]ethyl]-6H-pyrrolo[3,2-f]-1,2-benzisoxazol-6-one) were obtained from Pfizer Inc. and these compounds were prepared according to the published method [30]. DMF was prepared by sequential distillation under reduced pressure from CaH2 and BaO. All other chemicals and solvents were reagent grade and used as received unless otherwise stated. HPLC analyses and purification were performed with two 590EF HPLC pumps (Waters, Milford, MA), an in-line fixed wavelength (254 nm) ultraviolet (UV) detector, and a flow-through 2-inch NaI(Tl) crystal scintillation detector (model 276; Ortec, OakRidge, TN). All HPLC chromatograms were recorded by a Rainin Dynamax dual channel control/interface module (Rainin Instrument Co., Woburn, MA) connected to a Macintosh computer with appropriate program software (Dynamax, version 1.4). Semi-preparative (7.8 x 300 mm) and analytical radial compression (8 x 100 mm) columns (Waters Nova-Pak C18, 6 ␮M and 4 ␮M, respectively) were used for purification and quality control, respectively, of the radiotracer. A dose calibrator (Capintec

12R, Ramsey, NJ) was used for all radioactivity measurements. [11C]Carbon dioxide was produced with either a Scanditronix MC-16F or General Electric PETtrace biomedical cyclotron using the 14N(p,␣) [11]C reaction. For mouse studies, [11C]iodomethane was prepared from [11C]carbon dioxide as described previously [11]. More recently, [11C]CP-126,998 has been synthesized in comparable radiochemical yields but higher specific radioactivities via [11C]iodomethane that has been prepared with the GE PETtrace MeI MicroLab (Milwaukee, WI). This module prepares [11C]iodomethane from [11C]methane. Specific radioactivities are reported for this newer method of radiotracer production. 2.2. Radiosynthesis and purification of [11C] CP-126,998 The free base of the desmethyl precursor was made 10 minutes prior to end-of-bombardment by dissolving 1.2 mg of CP-118,954 maleate salt (2.4 ␮moles) in 0.5–1 mL water. The aqueous solution was made basic by addition of 30 ␮L of 2 M NaOH. The water layer was extracted with diethyl ether (2 x 0.5 mL), and the extracts passed through a 2 cm column of sodium sulfate contained in a Pasteur pipet. The solvent was blown down under a stream of argon and the white film residue was redissolved in 0.2 mL DMF and transferred to a 1 mL v-vial. For preparation of [11C] CP-126,998, [C-11] iodomethane, carried by a stream of nitrogen, was trapped in a solution of the precursor in DMF cooled in a dry ice/ethanol bath. A 2.5 ␮L aliquot of 0.4 M aqueous tetrabutylammonium hydroxide was subsequently added to the radiolabeling reaction, and the reaction was heated in a 80°C water bath for 5 min. The reaction was quenched with 0.2 mL of HPLC mobile phase consisting of 30:70 acetonitrile:water in 0.1 M ammonium formate. The crude reaction was purified by reverse-phase HPLC using the same mobile phase at 7 mL/min. The radioproduct (tR ⫽ 5.5 min, k⬘ ⫽ 3.2), which was separated from the precursor (tR ⫽ 3.5 min, k⬘ ⫽ 1.7), was remotely collected. After concentration to dryness under reduced pressure and heat (80 °C), the radiotracer was reconstituted in sterile 0.9% saline (7.0 mL) and passed through a 0.2 ␮M sterile filter (Acrodisc; Pall Gelman Laboratory, Ann Arbor, MI) into a sterile, pyrogen-free multidose vial. Sterile sodium bicarbonate (3.0 mL, 8.4%) was added to give a final formulation of pH ca. 7.0. An aliquot (0.1 mL) was assayed for radioactivity, and checked by analytical HPLC using a mobile phase of 40/60 acetonitrile/water in 0.1 M ammonium formate at 4 mL/min. A single radioactive peak (tR ⫽ 1.7 min, k⬘ ⫽ 1.4) corresponding to authentic CP-126,998 was observed. Specific radioactivity at end-of-synthesis was calculated by relating radioactivity to the mass associated with the UV absorbance peak of carrier. An average (n ⫽ 4) specific radioactivity at end-of-synthesis of 281 GBq/␮mol (7,600 mCi/␮mol) was obtained. The time for radiosynthesis, HPLC purification, and formulation was approximately 25 min from end-of-

J.L. Musachio et al. / Nuclear Medicine and Biology 29 (2002) 547–552

549

bombardment with an average non-decay corrected radiochemical yield of 21% based on [11C] iodomethane. 2.3. In vivo assays All experimental protocols were approved by the Animal Care and Use Committee of the Johns Hopkins Medical Institutions. Male CD1 mice (Charles River) weighing 21–32 g were used in all experiments. For the kinetic study, each mouse received approximately 11.1 MBq of [11C]CP126,998 (25 nmoles/kg) by injection (0.2 mL in 0.9% saline) into a tail vein. The animals (n ⫽ 3 per time point) were sacrificed by cervical dislocation at various times (5, 15, 30, 45, 60, 90, 120 min) after injection of the radiotracer. The whole brain was rapidly removed and dissected on ice into specific brain regions: cerebellum, hippocampus, striatum, parietal cortex, and thalamus. Each brain region was blotted and weighed, and tissue radioactivity was measured in an automated gamma counter. The percent injected dose per gram of wet tissue (%ID/g) was calculated by comparison of samples to standard dilutions of the initial dose. Blocking studies were performed in the same manner as described above except that 0.2 mL of a freshly made solution containing the blocking dose in saline was administered via intraperitoneal (i.p.) injection 5 min (diisopropylfluorophosphate, tacrine hydrochloride) prior to intravenous (i.v.) injection of the radioligand. Drug-treated (n ⫽ 3– 4) and saline control animals (n ⫽ 3– 4) were sacrificed at 30 min post radiotracer injection. Doses of diisopropylfluorophosphate (DFP) and tacrine were based on previous blocking studies in rodents [26,27]. Tremors and seizures were observed in animals treated with DFP in accord with previous mouse studies [26]. Dose levels of CP-118,954 maleate salt ranged from 0.01 mg/kg to 1 mg/kg and were given i.v. 5 min prior to radiotracer injection. Data were analyzed by one-way analyses of variance (ANOVA) and post-hoc Dunnett’s tests. Differences were considered significant when P ⬍ 0.05. 2.4. Examination of mouse brain radioactivity upon injection of [11C] CP-126,998 Two groups of male CD-1 mice (n ⫽ 2) received 37 MBq of [11C] CP-126,998 in 0.2 mL saline intravenously. Whole brain and heparinized blood were harvested at 5 and 15 min. At each time point pooled brains were homogenized in 0.8 mL of ice-cold acetonitrile. The brain homogenates were centrifuged for 5 min in microcentrifuge. A portion of the supernatant was diluted with water and examined by HPLC using a well-established column switching method [15]. A 4.6 x 250 mm C-18 prodigy HPLC column (Phenomenex, Torrance, CA) running in 40/60 acetonitrile/0.1 M triethylamine/acetate buffer pH 4.1 at 1 mL/min was employed. Similarly, the plasma fraction from centrifuged whole blood was diluted with water and analyzed by HPLC. Control brain and plasma samples were also analyzed by

Fig. 2. Synthesis of [11C] CP-126,998.

HPLC after spiking with 0.3 MBq of [11C] CP-126,998. Detection and data analysis used a flow-through detector and data capture software (Bioscan, Washington DC).

3. Results 3.1. Chemistry The radiosynthesis of [11C]CP-126,998 proved straightforward. Treatment of CP-118,954 dissolved in 0.2 mL of DMF with [11C]iodomethane and aqueous tetrabutylammonium hydroxide produced the target radioligand in good radiochemical yield (average of 21% non-decay corrected radiochemical yield based on [11C]iodomethane, Fig. 2). Under these radiolabeling conditions, [11C]CP-126,998 of high specific radioactivity (average of 281 GBq/␮mol) and radiochemical purity (⬎98%) was obtained after purification on a semi-preparative reverse phase HPLC column. The formulated radiotracer proved to be sterile and pyrogen-free and was still of high radiochemical purity (⬎98%) 40 minutes after its preparation. 3.2. In vivo biodistribution studies in mice [11C] CP-126,998 readily entered the mouse brain with a brain uptake of 1.59 ⫾ 0.19% at 5 min. The temporal distribution profile of [11C] CP-126,998 in various mouse brain regions is shown in Table 1. Radioactivity uptake was most prominent in the striatum (6.1% ID/g at 5 min)- a region known to be have high AChE activity. Consistent with a reversible radiotracer, regional brain radioactivity uptake declined in all brain regions studied over the 120 min time course of the biodistribution experiment. Comparison of the earliest time-point (5 min) to that at 120 min shows that striatal radioactivity declined by approximately 50%. Radioactivity clearance in other brain regions proved quicker than striatal clearance (Table 1). Three different structural classes of cholinesterase inhibitors were utilized as pharmacologic blocking agents so as to examine the specificity of [11C] CP-126,998 uptake in mouse brain. In these experiments radiotracer accumulation in selected brain regions was measured at 30 min post radiotracer injection and compared to that of saline-treated controls (Fig. 3 and 4). Pretreatment of mice with the irreversible organophosphate diisopropylfluorophosphate (DFP) at 6 mg/kg i.p. resulted in a significant blockade in striatum

550

J.L. Musachio et al. / Nuclear Medicine and Biology 29 (2002) 547–552

Table 1 Temporal brain biodistribution of [11C] CP-126,998 in male CD-1 mice (%ID/g) Region

5 min

15 min

30 min

45 min

60 min

90 min

120 min

Striatum Thalamus Cortex Hippocampus Cerebellum

6.19 ⫾ .63 4.76 ⫾ .30 4.01 ⫾ .31 3.41 ⫾ .30 3.76 ⫾ .27

5.88 ⫾ .44 3.61 ⫾ .51 3.07 ⫾ .13 3.04 ⫾ .05 2.84 ⫾ .20

6.12 ⫾ .31 2.63 ⫾ .21 1.80 ⫾ .13 2.27 ⫾ .14 1.79 ⫾ .08

4.80 ⫾ .30 1.78 ⫾ .09 1.22 ⫾ .03 1.55 ⫾ .05 1.29 ⫾ .06

5.05 ⫾ .30 1.59 ⫾ .07 1.10 ⫾ .05 1.46 ⫾ .08 1.16 ⫾ .06

3.67 ⫾ .03 1.25 ⫾ .06 0.89 ⫾ .06 1.25 ⫾ .18 0.93 ⫾ .03

3.14 ⫾ .07 1.26 ⫾ .08 0.79 ⫾ .02 1.07 ⫾ .05 0.71 ⫾ .01

Data are means ⫾ SEM (n ⫽ 3).

(P ⬍ 0.01), thalamus (P ⬍ 0.05), and hippocampus (P ⬍ 0.05) as compared to controls (Fig. 3). Similarly, preadministration of the reversible cholinesterase inhibitor, tacrine at 10 mg/kg i.p. produced a significant inhibition of [11C] CP-126,998 accumulation in these three brain regions (Fig. 3). Figure 4 shows that increasing i.v. doses of the structurally similar CP-118,954 resulted in dose-dependent inhibition of [11C] CP-126,998 uptake that reached statistical significance in striatum.

that the amount of unchanged [11C] CP-126,998 was 91% and 59% at 5 and 15 min postinjection. A polar plasma radiometabolite (tR ⫽ 1.1 min) that was not retained on a C-18 capture column was detected in both the 5 and 15 minute plasma samples along with another radiometabolite (approximately 1:1 ratio) that eluted (tR ⫽ 4.0 min) prior to the native [11C] CP-126,998.

4. Discussion 3.3. Examination of brain/plasma radioactivity in mice The chemical nature of brain radioactivity following intravenous administration of [11C] CP-126,998 to mice was investigated to determine whether or not radioactive metabolites were contributing to the signal observed in vivo. Extraction of brain radioactivity into acetonitrile proved to be efficient (⬎80%). The major radioactive component (⬎99%) at 5 and 15 min post radiotracer injection possessed an HPLC retention time (4.4 min) identical to that of native radioligand. HPLC analyses of plasma samples revealed

The radiosynthesis of [11C] CP-126,998 involved the N-alkylation of a lactam nitrogen using aqueous tetrabutyl ammonium hydroxide as base in DMF. This is the same radiosynthetic procedure that was reported for N-methylation of another lactam nitrogen (the D2 ligand spiperone [11]). [11C] CP-126,998 was obtained in consistent and good radiochemical yield using the above labeling conditions. Therefore, detailed studies optimizing reaction time, temperature, type and amount of base were not carried out. [11C] CP-126,998 proved to be of high specific activity and

Fig. 3. Effect of AChE inhibitors tacrine and diisopropylfluorophosphate (DFP) on the accumulation of [11C] CP-126,998 in various mouse brain regions 30 min after injection of the radiotracer. Blocking drugs were administered prior to the radioligand (see Material and Methods). Str, striatum; Thal, thalamus; Hip, hippocampus; Ctx, parietal cortex; Cb, cerebellum. Values are the means ⫾ SEM of groups of 3– 4 mice. Nonvisible error bars are within the plot symbol. * P ⬍ 0.05, ** P ⬍ 0.01.

Fig. 4. Effect of increasing doses of CP 118,954 maleate on regional accumulation of [11C] CP-126,998 in mouse brain (30 min). CP 118,954 was administered 5 min prior to the radioligand. Shown are means ⫾ SEM for groups of 3– 4 mice. Abbreviations same as in Fig. 3. Non-visible error bars are within the plot symbol. ** P ⬍ 0.01.

J.L. Musachio et al. / Nuclear Medicine and Biology 29 (2002) 547–552

free from residual precursor that competes with [11C]CP126,998 binding in vivo (see below). Radiolabeled substrates of cholinesterase have met with good success in the in vivo imaging of AChE [18,24]. Such tracers are irreversibly trapped and provide a means to measure AChE enzyme activity. Another approach for an in vivo marker of AChE is to utilize a radiolabeled reversible, inhibitor of AChE as exemplified by the preparation and evaluation of [11C] CP-126,998 as described presently. Such a radioligand can measure enzyme binding site concentration. A report by Brimijoin shows that AChE catalytic activity is well correlated with AChE number [8]. Thus, the two radiotracer design approaches toward in vivo imaging of AChE may yield near equivalent results. A direct comparison between these two approaches, however, has been hampered by the lack of a highly specific reversible AChE inhibitor that labels AChE in vivo. We chose to label CP126,998 with high specific activity carbon-11 (t1/2 ⫽ 20.2 min) owing to the ease of the radiolabeling sequence and the fact that this benzisoxazole possesses potent and remarkably selective inhibitory power for AChE over BuChE. BuChE is found in the brain in rather low concentration in rat relative to AChE [20] while the relative concentrations of these two enzymes in mice is not well established. In human cortex, however, AChE and BuChE concentrations are similar [1,7]. Thus, the use of cholinesterase radiotracers that possess poor selectivity for AChE over BuChE may complicate the interpretation of data generated from such probes. Recently, C-11 labeled substrates that are selective for BuChE have been reported, and these tracers should help delineate the central activity of BuChE relative to AChE in a variety of species [28]. Preliminary mouse regional brain biodistribution studies showed that [11C] CP-126,998 could readily cross the blood-brain barrier (1.59% of injected dose in brain at 5 min). Regional localization of [11C] CP-126,998 was heterogeneous with greatest uptake observed in the striatum, a brain region known to possess high AChE activity in mice [26]. Radioactive uptake in the striatum appeared to be a result of specific binding of [11C] CP-126,998 to AChE. For example, radiotracer uptake was blocked by both an irreversible (DFP) and reversible (tacrine) inhibitor of AChE. Moreover, striatal [11C] CP-126,998 accumulation could be significantly blocked in a dose-dependent manner via pretreatment with CP-118,954, another well-characterized AChE inhibitor of the benzisoxazole structural class [30]. At high specific radioactivity, the target-to-non-target ratio for a receptor-binding radiotracer is proportional to binding site concentration and is also dependent on the radioligand’s binding affinity [14]. The affinity of CP-126,998 for AChE is one of the highest reported for a reversible inhibitor of AChE. Despite this subnanomolar affinity of [11C] CP126,998 for AChE, specific binding in extra-striatal mouse brain regions was much less pronounced. In fact, specific binding of [11C] CP-126,998 in vivo in parietal cortex and cerebellum could not be detected. Such regions in mice are

551

approximately 2- to 10-fold lower in AChE activity [26]. Thus, it may be the low AChE concentration in these brain regions that limits the ability of [11C] CP-126,998 to label these enzyme sites. It should be noted, however, that these rodent blocking studies are qualitative in nature since such studies do not take into effect changes in radiotracer delivery upon injection of near-lethal doses of potent cholinergic drugs. In our initial human PET imaging trials that employed more rigorous kinetic modeling techniques, specific binding of [11C] CP-126,998 to non-striatal regions was detected in a two-scan baseline/blocking protocol [2,3]. The brain radioactivity measured in these mouse biodistribution assays arises from the native radioligand as shown by HPLC analysis of brain extracts. The absence of radiolabeled metabolites that can cross the blood-brain barrier facilitates kinetic modeling approaches to estimate AChE number via PET. [11C] CP-126,998 may prove useful for longitudinal studies of AChE in a variety of neurodegenerative disorders. Moreover, this novel radioligand, may prove useful in the monitoring of cholinergic-based therapies and aid in the development of new drugs targeting AChE.

5. Conclusion The reversible, competitive AChE inhibitor, CP-126,998 is prepared in good radiochemical yield and high specific radioactivity via a facile N-radiomethylation using [11C] iodomethane. Mouse biodistribution studies suggest that [11C] CP-126,998 is a useful radiotracer for mapping AChE in vivo.

Acknowledgments This work was supported by Pfizer Inc., Groton, CT, USA.

References [1] J. Atack, E. Perry, J. Bonham, J. Candy, R. Perry, Molecular forms of acetylcholinesterase and butyrylcholinesterase in the aged human central nervous system, J Neurochem 47 (1986) 263–277. [2] B. Bencherif, C. Endres, J. Musachio, A. Villalobos, J. Hilton, U. Scheffel, R. Dannals, S. Williams, J. Frost J, PET imaging of brain acetylcholinesterase using [11C]CP-126,998, a brain selective enzyme inhibitor, Synapse (in press). [3] B. Bencherif, J. Musachio, A. Villalobos, J. Hilton, H. Ravert, W. Mathews, R. Dannals, S. Williams, J. Frost, PET imaging of brain acetylcholinestease using [C-11]CP-126,998, a brain selective enzyme inhibitor, J Nucl Med 40 (1999) 273. [4] L. Bierer, V. Haroutunian, S. Gabriel, P. Knott, L. Carlin, D. Purohit, D. Perl, J. Schmeidler, P. Kanof, K. Davis, Neurochemical corrleates of dementia severity in Alzheimer’s disease: relative importance of the cholinergic deficit, J Neurochem 64 (1995) 749 –760.

552

J.L. Musachio et al. / Nuclear Medicine and Biology 29 (2002) 547–552

[5] G. Blomqvist, B. Tavitian, S. Pappata, C. Crouzel, A. Jobert, I. Doignon, L. Di Giamberardino, Quantitative measurement of cerebral acetylcholinesterase using [11C]physostigmine and positron emission tomography, J Cereb Blood Flow Metab 21 (2001) 114 –131. [6] S. Bonnot Lours, C. Crouzel, C. Prenant, F. Hinnen, Carbon-11 labelling of an inhibitor of acetylcholinesterase: 11C-physostigmine, J Labelled Compd Radiopharm 33 (1993) 277–284. [7] S. Brimijoin, P. Hammond, Butyrylcholinesterase in human brain and acetylcholinesterase in human plasma: trace enzymes measured by two-site immunoassay, J Neurochem 51 (1988) 1227–1231. [8] S. Brimijoin, P. Hammond, Z. Rakonczay, Two-site immunoassay for acetylcholinesterase in brain, nerve, and muscle, J Neurochem 49 (1987) 555–562. [9] C. Brown-Proctor, S. Snyder, P. Sherman, M. Kilbourn, Synthesis and evaluation of 6-[11C]methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2-benzisoxazole as an in vivo radioligand for acetylcholinesterase, Nucl Med Biol 26 (1999) 99 –103. [10] J. Coyle, D. Price, M. DeLong, Alzheimer’s disease: a disorder of cortical cholinergic innervation, Science 219 (1983) 1184 –1190. [11] R. Dannals, H. Ravert, A. Wilson, H.J. Wagner, An improved synthesis of (3-N-[11C]methyl)spiperone, Appl Radiat Isot 37 (1986) 433– 434. [12] P. Davies, Neurotransmitter-related enzymes in senile dementia of the Alzheimer type, Brain Res 171 (1979) 319 –327. [13] F. De Vos, P. Santens, H. Vermeirsch, I. Dewolf, F. Dumont, G. Slegers, R. Dierckx , J. De Reuck, Pharmacological evaluation of [C-11]donepezil as tracer for visualization of acetylcholinesterase by PET, Nucl Med Biol 27 (2000) 745–747. [14] W.C. Eckelman, Sensitivity of new radiopharmaceuticals, Nucl Med Biol 25 (1998) 169 –173. [15] J. Hilton, F. Yokoi, R. Dannals, H. Ravert, Z. Szabo, D. Wong, Column-switching HPLC for the analysis of plasma in PET imaging studies, Nucl Med Biol 27 (2000) 627– 630. [16] M. Iyo, H. Namba, K. Fukushi, H. Shinotoh, S. Hagatsuka, T. Suhara, Y. Sudo, K. Suzuki, T. Irie, Measurement of acetylcholinesterase by positron emission tomography in the brains of health controls and patients with Alzheimer’s disease, Lancet 349 (1997) 1805–1809. [17] R. Koeppe, K. Frey, S. Snyder, P. Meyer, M. Kilbourn, D. Kuhl, Kinetic modeling of N-[11C]methylpiperidin-4-yl propionate: alternatives for analysis of an irreversible positron emission tomography tracer for measurement of acetylcholinesterase activity in human brain, J Cereb Blood Flow 19 (1999) 1150 –1163. [18] D. Kuhl, R. Koeppe, S. Minoshima, S. Snyder, E. Ficaro, N. Foster, K. Frey, M. Kilbourn, In vivo mapping of cerebral acetylcholinesterase activity in aging and Alzheimer’s disease, Neurology 52 (1999) 691– 699. [19] D. Kuhl, S. Minoshima, K. Frey, N. Foster, M. Kilbourn, R. Koeppe, Limited donepezil inhibition of acetylcholinesterase measured with positron emission tomography in living Alzheimer cerebral cortex, Ann Neurol 48 (2000) 391–395.

[20] T. Lassiter, S. Barone, S. Padilla, Ontogenetic differences in the regional and cellular acetylcholinesterase and butyrylcholinesterase activity in the rat brain, Developmental Brain Res 105 (1998) 109 – 123. [21] S. Lee, Y. Choe, H. Sugimoto, S. Kim, S. Hwang, K. Lee, Y. Choi, J. Lee, B. Kim, Synthesis and biological evaluation of 1-(4-[F-18] fluorobenzyl)-4-[(5,6-dimethoxy-1-oxoindan-1-yl) methyl]piperidine for in vivo studies of acetylcholinesterase, Nucl Med Biol 27 (2000) 741–744. [22] G. Mullholland, H. Suigimoto, D. Jewett, M. Kilbourn, Simple preparation of a novel C-11 acetylcholinesterase inhibitor. J Nucl Med 30 (1989) 822. [23] H. Namba, M. Iyo, K. Fukushi, H. Shinotoh, S. Nagatsuka, T. Suhara, Y. Sudo, K. Suzuki, T. Irie, Human cerebral acetylcholinesterase activity measured with positron emission tomography: procedure, normal values and effect of age, Eur J Nucl Med 26 (1999) 135–143. [24] H. Namba, M. Iyo, H. Shinotoh, S. Nagatsuka, K. Fukushi, T. Irie, Preserved acetylcholinesterase activity in aged cerebral cortex, Lancet 351 (1998) 881– 882. [25] E. Perry, B. Tomlinson, G. Blessed, K. Bergman, P. Gibson, R. Perry, Correlation of cholinergic abnormalities with senile plaques and mental scores in senile dementia, Br Med J 2 (1978) 1457–1459. [26] A. Smolen, T. Smolen, J. Wehner, A. Collins, Genetically determined differences in acute responses to diisopropylfluorophosphate, Pharmacol Biochem Behav 22 (1985) 623– 630. [27] S. Snyder, P. Sherman, D. Kuhl, M. Kilbourn, Evaluation of N-[11C]methyl-4-piperidinyl propionate as a radiotracer for acetylcholinesterase activity in vivo, J Labelled Compd Radiopharm 37 (1995) 234 –236. [28] S. Snyder, N. Gunupudi, P. Sherman, E. Butch, M. Skaddan, M. Kilbourn, R. Koeppe, D. Kuhl, Radiolabeled cholinesterase substrates: in vitro methods for determining structure-activity relationships and identification of a positron emission tomography radiopharmaceutical for in vivo measurement of butyrylcholinesterase activity, J Cereb Blood Flow Metab 21 (2001) 132–143. [29] B. Tavitian, S. Pappata, S. Bonnott-Lours, C. Prenant, A. Jobert, C. Crouzel, L. Di Giamberadino, Positron emssion tomography study of [11C]methyl-tetrahydroaminoacridine (methyl-tacrine) in baboon brain, Eur J Pharmacol 236 (1993) 229 –238. [30] A. Villalobos, T. Butler, D. Chapin, Y. Chen, S. DeMattos, J. Ives, S. Jones, D. Liston, A. Nagel, D. Nason, J. Nielsen, A. Ramirez, I. Shalaby, W. Frost White, 5,7-dihydro-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-6H-pyrrolo[3,2-f}-1,2-benzisoxazol-6-one: a potent and centrally-selective inhibitor of acetylcholinesterase with an improved margin of safety, J Med Chem 38 (1995) 2802–2808. [31] P. Whitehouse, The cholinergic deficit in Alzheimer’s disease, J Clin Psych 59 (Suppl. 13) (1998) 19 –22. [32] P. Whitehouse, D. Price, A. Clark, J. Coyle, M. DeLong, Alzheimer’s disease: evidence for selective loss of cholinergic neurons in the nucleus basalis, Ann Neurol 10 (1981) 122–126.