**3.1 Introduction**

214 12 Chapters on Nuclear Medicine

impairment, and from patients with AD. Initial studies have shown that patients with AD have significantly more [18F]FDDNP binding in the temporal, parietal, and frontal regions of the brain than the corresponding healthy controls. The non-specificity of [18F]FDDNP, however, appears to have limited its application to date, as the study of this probe has not

Investigations into the possibility of using radiolabeled quinoline and benzamidizole derivatives for PET imaging of tau NFTs have been reported recently by Okamura and colleagues (N. Okamura, et al., 2005). Initial experiments, with [11C]BF-126, [11C]BF-158, and [11C]BF-170, demonstrated that these compounds have a high affinity for tau NFTs. Furthermore, these compounds appear to bind specifically to tau, without extensive nonspecific binding to amyloid plaques. Through the use of *in vit*ro staining of AD brain slices, [11C]BF-158 was shown to be the most promising compound for PET imaging of tau NFTs. These radiopharmaceuticals only interact with tau formations in the brains of AD patients. This could prove beneficial for distinguishing early AD from other types of tauopathies. For example, these radiopharmaceuticals do not bind strongly to the tau structures present in the brain slices of patients with Pick's disease and PSP. Although these radiopharmaceuticals have not yet been translated into human clinical trials, the promising pre-clinical data suggest they possess the appropriate properties to make them realistic

Recently, Kolb and colleagues reported development of PET radiopharmaceuticals with high binding affinity and selectivity for tau tangles (Szardenings, et al., 2011), and three lead compounds were identified: [18F]T794, [18F]T807 and [18F]T808. Initial autoradiographical and rodent microPET studies suggest these compounds have the desired binding affinity for tau and good selectivity for tau over amyloid, to fill the void in clinical tau PET imaging. Translation into the clinic is underway, although human imaging studies with these

The abnormal aggregation of amyloid and tau proteins in AD pathophysiology is accompanied by concomitant decline of neurotransmitter systems, primarily the cholinergic system (Bierer, et al., 1995; Bohnen, et al., 2005; Contestabile, 2011; Francis, et al., 1999; Terry and Buccafusco, 2003). Thus, from a diagnostic perspective, there is interest in being able to image the cholinergic system with PET. To date, efforts have focused upon developing radiolabeled analogs of acetylcholine that are substrates for acetylcholinesterase. Acetylcholinesterase (AChE) is the enzyme responsible for the degradation of acetylcholine, leading to the termination of cholinergic neurotransmission. AChE deficits in *post-mortem* AD brain samples have been observed, suggesting that cholinergic decline is part of the complex neurodegenerative cascade that occurs in AD. Therefore, radiopharmaceuticals suitable for quantifying AChE *in vivo* have potential for tracking the progression of the cholinergic aspect of this cascade in AD patients. The synthetic acetylcholinesterase substrate, 1-[11C]methylpiperidin-4-yl propionate ([11C]PMP) (Shao, et al., 2003; Snyder, et al., 1998), is currently in routine clinical use as a radiopharmaceutical for the study of AChE function in AD patients, and results from such studies have been encouraging (K. A. Frey,

progressed past these initial clinical studies.

radiopharmaceuticals for future testing. **2.4.2.3 [18F]T794, [18F]T807 and [18F]T808** 

compounds have also yet to be reported.

**2.5 Imaging the cholinergic system** 

**2.4.2.2 Quinoline and benzamidizole PET biomarkers** 

Parkinson's disease (PD) is a progressive degenerative neurological disease, characterized by asymmetric onset of resting tremors, rigidity, and bradykinesia in the limbs, leading ultimately to unstable posture. The disease is less common in adults under 60, but not unheard of, and it does become more common with increasing age. Progression of symptoms in PD typically occurs over 10–30 years, but progression can be accelerated in certain individuals, especially those with the so-called Parkinson's-plus syndrome.

The hallmark pathology of Parkinson's disease is loss of dopaminergic neurons in the substantia nigra pars compacta (SNc), leading to striatal dopamine deficiency, and classical symptoms of PD are thought to develop when 80% of striatal dopamine and 50% of the SNc neurons are lost. In addition to dopamine loss, concomitant formation of Lewy bodies also occurs in PD. Lewy bodies are composed primarily of synuclein and appearance of such intraneuronal Lewy body inclusions occurs initially in the lower brainstem and medulla oblongata, followed by midbrain and nigral involvement and, eventually, limbic and association cortical areas. Despite this, Pavese and Brooks indicate that even with the prevalence of Lewy bodies, decline of the dopaminergic system is still the primary factor in PD. Other related Parkinsonian syndromes are known however, and dementia occurs in most of them. For example, Dementia with Lewy bodies (DLB) is a common neurodegenerative dementia that is also associated with the development of α-synuclein positive Lewy body neuronal inclusions in the cortex, substantia nigra and brainstem. Patients with DLB, suffer from progressive cognitive decline including memory loss, visual hallucinations, cognitive circadian fluctuations and sleep disorders. Reflecting the seriousness of these conditions, enormous research has been undertaken to develop

Diagnosis of Dementia Using Nuclear Medicine Imaging Modalities 217

(Fearnley and Lees, 1991; Kish, et al., 1988). Beyond these obvious areas of reduced [18F]DOPA uptake, if voxel analysis of the PET scans is performed, then less obvious reductions in [18F]DOPA uptake can also be detected across the entire brain (Kaasinen, et al.,

In neurodegenerative diseases there are typically characteristic losses of particular types of neurons in the human brain. As outlined above, progressive losses of dopaminergic neurons is the hallmark of Parkinson's disease. Reflecting this, a number of strategies have been developed for *in vivo* imaging of such neuronal losses. One such approach involves targeting the vesicular monoamine transporter type 2 (VMAT2) using radioligands such as (+)-α-[11C]dihydrotetrabenazine ([11C]DTBZ: (2*R*,3*R*,11b*R*)-(1,3,4,6,7,11b-hexahydro-9- [11C]methoxy-10-methoxy)-3-(2-methylpropyl)-2-hydroxy-2H-benzo[a]quinolizine) (K. A. Frey, et al., 2001). The VMAT2 is not specific for any monoamine, but is a common protein capable of transporting dopamine, norepinephrine, serotonin and histamine (Eiden and Weihe, 2011). Despite this non-specificity, the utility of VMAT2 imaging in neurodegenerative disease is still possible due to the compartmentalization of neuronal types in the human brain (K. A. Frey, et al., 2001). For example, dopaminergic nerve terminals predominate in the basal ganglia, and so enable specificity for examining losses of such terminals in PD patients (Figure 3). The VMAT2 is found in presynaptic vesicles, and transports monoamines from the cell cytosol into the storage vesicle, from where they can be

Lee and colleagues conducted a comparison between [11C]DTBZ, [18F]DOPA and [11C]methylphenidate (a radiopharmaceutical targeting the dopamine transporter (DAT)) (C. S. Lee, et al., 2000). Reflecting the upregulation of aromatic amino acid decarboxylase, and concomitant down regulation of the DAT, that occurs to increase dopamine turnover and reduce its reuptake in Parkinson's disease patients, this study found that [18F]DOPA Ki was reduced less than the [11C]DTBZ binding potential in the PD striatum, and [11C]DTBZ binding was reduced when compared to [11C]methylphenidate binding. The authors suggest that [11C]DTBZ PET is the most reliable method for quantifying dopaminergic terminal density although, per Pavese and Brooks (Pavese and Brooks, 2009), this needs to be

validated and the effect of dopaminergic drugs upon [11C]DTBZ uptake determined.

between PD patients and healthy controls were obtained (Figure 4).

**3.4 Measurement of dopamine transporter binding** 

Reflecting the drive to convert short lived carbon-11 labeled radiopharmaceuticals (t1/2 = 20 min) into longer lived fluorine-18 labeled analogs (t1/2 = 110 min) to facilitate distribution to satellite PET centers that do not own a cyclotron, [18F]FP-TBZ ([18F]AV-133) has also been developed to image the VMAT2, and is licensed to Avid Radiopharmaceuticals (H. F. Kung, et al., 2008). In studies by Frey and colleagues, AV-133 PET of normal and PD patients were compared (K. A. Frey, et al., 2008). Findings were similar to [11C]DTBZ, and AV-133 PET provided excellent images of the VMAT2. All PD patients had severe reduction of AV-133 accumulation in the striatum, most severe in the PP contralateral to worst PD symptoms. Similar findings were confirmed in further studies by Okamura and co-workers in 2010 (N. Okamura, et al., 2010), and very clear images showing the differences in AV-133 PET

The presynaptic dopamine transporter (DAT) is found in dendrites and axons of dopaminergic neurons and is responsible for uptake of dopamine. Therefore, measurement

**3.3 Measurement of the Vesicular Monoamine Transporter (VMAT) 2** 

2001; Rakshi, et al., 1999; Whone, et al., 2003).

released into the synapse (Wimalasena, 2011).

numerous radiopharmaceuticals (Figure 3) that can be used to differentiate these conditions and monitor their progression. Progress in this area to date has also been recently reviewed (Pavese and Brooks, 2009; Sioka, et al., 2010).

Fig. 3. Imaging dopamine terminal function in healthy controls and early Parkinson's disease. (Reprinted with permission from Pavese N and Brooks DJ, Imaging neurodegeneration in Parkinson's disease. Biochim. Biophys. Acta. 2009;1792:722-729)

### **3.2 Measurement of striatal aromatic amino acid decarboxylase activity**

[18F]DOPA is a radiopharmaceutical used for neuroimaging and for evaluation and quantification of presynaptic dopaminergic integrity. For example, analyzing the uptake of [18F]DOPA in the striatal nuclei provides valuable information on both the density of the axonal terminal plexus and the activity of striatal aromatic amino acid decarboxylase (AADC), an enzyme that converts [18F]DOPA to [18F]dopamine (Pavese and Brooks, 2009). Therefore [18F]DOPA uptake in the striatum of patients with PD is dependent upon the number of remaining dopaminergic cells, and can be used to track progression of the disease. It is worth noting, however, that early degeneration can be underestimated due to compensatory upregulation of AADC in remaining terminals (Ribeiro, et al., 2002).

Significant research has been undertaken to investigate the uptake of [18F]DOPA in the putamen region of the brain, and noticeable [18F]DOPA reductions have been shown to correlate with the clinical severity of rigidity and bradykinesia in PD patients (Brooks, et al., 1990; Broussolle, et al., 1999; Vingerhoets, et al., 1997). However, there is no correlation with the degree of tremors, and Pavese and Brooks highlight that this lack of correlation is suggestive of non-nigrostriatal and/or non-dopaminergic origins of the tremors associated with PD (Pavese and Brooks, 2009). In patients with hemiparkinsonism (i.e. Parkinsonian symptoms on one half of the body only), a corresponding reduction in dorsal posterior putamen uptake of [18F]DOPA on the opposite side of the body has been observed (Morrish, et al., 1995). As the disease progresses to become bilateral, so to does the reduction of [18F]DOPA uptake, and losses are detected in the ventral and anterior putamen and dorsal caudate. In the end stages of the disease, reduced [18F]DOPA uptake in the ventral head of the caudate is also apparent. Such findings also correspond well to *post-mortem* data

numerous radiopharmaceuticals (Figure 3) that can be used to differentiate these conditions and monitor their progression. Progress in this area to date has also been recently reviewed

Fig. 3. Imaging dopamine terminal function in healthy controls and early Parkinson's

neurodegeneration in Parkinson's disease. Biochim. Biophys. Acta. 2009;1792:722-729)

compensatory upregulation of AADC in remaining terminals (Ribeiro, et al., 2002).

[18F]DOPA is a radiopharmaceutical used for neuroimaging and for evaluation and quantification of presynaptic dopaminergic integrity. For example, analyzing the uptake of [18F]DOPA in the striatal nuclei provides valuable information on both the density of the axonal terminal plexus and the activity of striatal aromatic amino acid decarboxylase (AADC), an enzyme that converts [18F]DOPA to [18F]dopamine (Pavese and Brooks, 2009). Therefore [18F]DOPA uptake in the striatum of patients with PD is dependent upon the number of remaining dopaminergic cells, and can be used to track progression of the disease. It is worth noting, however, that early degeneration can be underestimated due to

Significant research has been undertaken to investigate the uptake of [18F]DOPA in the putamen region of the brain, and noticeable [18F]DOPA reductions have been shown to correlate with the clinical severity of rigidity and bradykinesia in PD patients (Brooks, et al., 1990; Broussolle, et al., 1999; Vingerhoets, et al., 1997). However, there is no correlation with the degree of tremors, and Pavese and Brooks highlight that this lack of correlation is suggestive of non-nigrostriatal and/or non-dopaminergic origins of the tremors associated with PD (Pavese and Brooks, 2009). In patients with hemiparkinsonism (i.e. Parkinsonian symptoms on one half of the body only), a corresponding reduction in dorsal posterior putamen uptake of [18F]DOPA on the opposite side of the body has been observed (Morrish, et al., 1995). As the disease progresses to become bilateral, so to does the reduction of [18F]DOPA uptake, and losses are detected in the ventral and anterior putamen and dorsal caudate. In the end stages of the disease, reduced [18F]DOPA uptake in the ventral head of the caudate is also apparent. Such findings also correspond well to *post-mortem* data

disease. (Reprinted with permission from Pavese N and Brooks DJ, Imaging

**3.2 Measurement of striatal aromatic amino acid decarboxylase activity** 

(Pavese and Brooks, 2009; Sioka, et al., 2010).

(Fearnley and Lees, 1991; Kish, et al., 1988). Beyond these obvious areas of reduced [18F]DOPA uptake, if voxel analysis of the PET scans is performed, then less obvious reductions in [18F]DOPA uptake can also be detected across the entire brain (Kaasinen, et al., 2001; Rakshi, et al., 1999; Whone, et al., 2003).
