Elsevier

NeuroImage

Volume 46, Issue 3, 1 July 2009, Pages 616-623
NeuroImage

Quantitative analysis of donepezil binding to acetylcholinesterase using positron emission tomography and [5-11C-methoxy]donepezil

https://doi.org/10.1016/j.neuroimage.2009.03.006Get rights and content

Abstract

The aim of this study was to establish kinetic analysis of [5-11C-methoxy]donepezil ([11C]donepezil), which was developed for the in-vivo visualization of donepezil binding to acetylcholinesterase (AChE) using positron emission tomography (PET). Donepezil is an AChE inhibitor that is widely prescribed to ameliorate the cognitive impairment of patients with dementia. Six healthy subjects took part in a dynamic study involving a 60-min PET scan after intravenous injection of [11C]donepezil. The total distribution volume (tDV) of [11C]donepezil was quantified by compartmental kinetic analysis and Logan graphical analysis. A one-tissue compartment model (1TCM) and a two-tissue compartment model (2TCM) were applied in the kinetic analysis. Goodness of fit was assessed with χ2 criterion and Akaike's Information Criterion (AIC). Compared with a 1TCM, goodness of fit was significantly improved by a 2TCM. The tDVs provided by Logan graphical analysis were slightly lower than those provided by a 2TCM. The rank order of the mean tDVs in 10 regions was in line with the AChE activity reported in a previous post-mortem study. Logan graphical analysis generated voxel-wise images of tDV, revealing the overall distribution pattern of AChE in individual brains. Significant correlation was observed between tDVs calculated with and without metabolite correction for plasma time–activity curves, indicating that metabolite correction could be omitted. In conclusion, this method enables quantitative analysis of AChE and direct investigation of the pharmacokinetics of donepezil in the human brain.

Introduction

The cholinergic system is one of the most crucial neurotransmitter systems in the brain, and it has very profound links with the manifestations of dementia (Francis et al., 1999, Perry et al., 1999). The activities of both choline acetyltransferase (ChAT), the enzyme catalyzing acetylcholine synthesis, and acetylcholinesterase (AChE), the enzyme degrading brain acetylcholine, are reported to be decreased in the neocortex and hippocampus of patients with Alzheimer's disease (AD) and Parkinson's disease with dementia (PDD) (Davies and Maloney, 1976, Perry et al., 1985), and this decreased activity correlates with the severity of cognitive impairment (Perry et al., 1978). Significant loss of cholinergic neurons in the nucleus basalis of Meynert has been reported in the brains of patients with both diseases (Whitehouse et al., 1982, Whitehouse et al., 1983). Based on these pathological findings, the use of reversible AChEs was proposed as a means of potentiating cholinergic neurotransmission, with the aim of improving cognitive function.

Currently, several AChE inhibitors (AChEIs) are prescribed to improve the cognitive function of patients with dementia. Donepezil hydrochloride is an AChEI that has been proved to be effective in ameliorating the cognitive impairment of patients with AD (Rogers et al., 1998), and it is widely prescribed for the treatment of this disease.

Various radiotracers have been developed to visualize different types of neurotransmissions in the human brain using positron emission tomography (PET) (Lammertsma et al., 1991, Frey et al., 1996, Parsey et al., 2000, Sakata et al., 2007). These techniques for mapping receptors in the human brain have also provided the opportunity for the development of new drugs, for the evaluation of the therapeutic effects of the drugs, and for better decisions to be made about the appropriate doses of drugs for neurological and psychiatric diseases.

Some radiolabeled AChEIs such as [11C]physostigmine and [11C]methyl-tacrine, a tacrine derivative, have been synthesized for AChE imaging. Unfortunately, the kinetics of [11C]physostigmine proved to be too complex, such that the required model was overspecified (Blomqvist et al., 2001). Amongst the other candidate AChE ligands tested, the distribution of [11C]methyl-tacrine was not correlated with AChE activity in the brain of the baboon (Tavitian et al., 1993), and [6-11C-methoxy]donepezil, which was also synthesized, was devoid of specific binding to AChE in vivo (De Vos et al., 2000).

[5-11C-methoxy]donepezil ([11C]donepezil) was originally developed by Funaki and coworkers (Funaki et al., 2003). The binding of [11C]donepezil to homogenates of rat brain was highest in the brainstem and striatum, and lowest in the cerebellum, and in-vitro autoradiographic studies demonstrated specific binding to AChE. The 50% inhibitory concentration (IC50) value of binding was about 10 nM, which is consistent with the reported value of inhibiting enzyme activity (6 nM). Saturation experiments showed that the maximum binding capacity (Bmax) was 65 fmol/mg tissue, and the dissociation constant (Kd) of [11C]donepezil binding was 39.8 nM in vitro. In-vivo distribution of [11C]donepezil in rat brain was heterogeneous, in accordance with in-vitro binding. The blocking experiment in the earlier study by Funaki et al. showed that the heterogeneous pattern of binding seen in the baseline condition was considerably attenuated by co-injection of a large amount of unlabeled donepezil. In addition, a study by Okamura et al. (2008) revealed reduced binding of [11C]donepezil in the brain of patients with AD, although this study was limited by use of the Logan graphical analysis method without fulfillment of the necessary criteria. However, these reports show that [11C]donepezil holds promise as a potential agent for imaging AChE in vivo using PET.

The aim of this study was to establish a quantitative method of evaluating the binding of [11C]donepezil to AChE in the human brain. We performed [11C]donepezil-PET in six healthy subjects. First, a method for full-compartment analysis was investigated. Then, graphical analysis using the Logan plot (Logan et al., 1990) was applied to the data to visualize the spatial distribution of the binding parameter of [11C]donepezil with AChE. The possibility of omitting metabolite correction for the plasma input function was also investigated as a convenient method of applying [11C]donepezil-PET to various clinical situations.

Section snippets

Subjects

Six elderly normal volunteers (four men and two women, mean age 65.8 ± 5.4 years) were recruited. The subjects had no cognitive impairment, no neurological disorders, and no abnormalities apparent on magnetic resonance images (MRIs) of their brains. None of them was receiving any centrally acting medication. All subjects underwent a PET scan with [11C]donepezil. The Ethics Committee of Tohoku University School of Medicine approved the study protocol, and written informed consent was given by all

pTACs and tTACs

pTACs with and without metabolite correction and time courses for the fraction of unchanged [11C]donepezil in plasma are shown in Fig. 3. At 30 min post-injection, 91.0 ± 3.9% (mean ± SD) of administered [11C]donepezil remained in the intact form. Plasma radioactivity peaked at 30–60 s  post-injection, followed by a rapid decline. The plots in Fig. 4 describe typical tTACs in the parietal cortex, frontal cortex, and putamen. tTACs indicated initial rapid uptake of radioactivity followed by gradual

Discussion

The study contrasted several methods for the kinetic analysis of [11C]donepezil. Initially, compartment analysis of [11C]donepezil in the brain was investigated. Theoretically, four parameters should be estimated in a 1TCM (K1, k2, delay, and blood volume) and six parameters should be estimated in a 2TCM (K1, k2, k3, k4, delay, and blood volume). As an estimation algorithm suffers from instability and dependency on the initial values, it is not feasible to estimate all the parameters

Acknowledgments

This work was supported by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS) and from the Ministry of Health, as well as by a grant for ‘Molecular Imaging’ projects from the Ministry of Education, Culture, Sports, Science and Technology in Japan.

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