To determine whether a model developed for amyloid radiotracers can be extended to tau radiotracers, the differences between amyloid and tau radiotracers need to be identified. Problems faced in clinical studies with clinically-applied tau radiotracers are discussed in this section with a detailed explanation of the enantiomeric property of chiral chemical compounds, such as Tohoku University’s THK tau radiotracers.
7.1.1 Issues with Existing Clinically-Applied Tau Radiotracers
The development of a successful tau radiotracer faces new challenges due to its binding target. In general, a good neuroimaging radiotracer needs to cross the BBB and has high binding affinity to
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its target (section 3.3). Tau radiotracers, in addition, need to discriminate PHF-tau from other β-sheet structured aggregates such as Aβ and α-synuclein. Similar to Aβ, tau proteins also have various conformations due to the existence of six (3R and 4R) isoforms, and various post-translational modifications (section 2.1.2). Tau binding sites are present at smaller concentrations compared to Aβ binding sites by 5-20 folds, hence the selectivity of tau over other β-sheet structured aggregates needs to be high to ensure accurate quantification. Moreover, as tau proteins exist intracellularly, tau radiotracers not only need to cross the BBB, they also need to be able to cross the cell membrane.
Existing clinically-applied tau radiotracers showed some limitations. [11C]PBB3 has high binding selectivity to tau over Aβ but it is difficult to synthesise as it is sensitive to photo-isomerization.
Moreover, it is rapidly metabolised in the plasma, leading to limited entry into the brain [Hashimoto et al., 2014]. [18F]T808 showed defluorination, which might affect the quantitative analysis of PET images [Declercq et al., 2016]. THK compounds (Tohoku University, Japan) showed differences in uptake due to enantiomeric properties, which need to be carefully prepared to ensure the synthesis of the targeted enantiomer [Tago et al., 2016]. Some THK compounds, [11C]PBB3 and [18F]T807 (also known as [18F]flortaucipir) showed off-target binding [Harada et al., 2016; Maruyama et al., 2013; Lowe et al., 2016]. Subsequently, [18F]THK5351 was showed to also bind to MAO-B enzymes [Ng et al., 2017]. [11C]Astemizole and [18F]Lansoprazole showed binding to tau proteins but are mostly applied in the treatment of allergies and gastrointestinal disorders respectively [Rojo et al., 2010]. [18F]FDDNP was developed as an amyloid radiotracer but also showed some binding to tau proteins. Moreover, its metabolites also entered the BBB, which makes quantification difficult [Luurtsema et al., 2008].
As the enantiomeric property of a chemical compound affects its binding to the target, the next sub-section describe the different forms of a chemical compound and explain the meaning of enantiomeric property. This is important to ensure correct identification of chemical structures of enantiomeric tau radiotracers, especially THK compounds.
7.1.2 Chirality and Stereoisomers
Isomers are different compounds with the same molecular formula. Constitutional isomers are chemical molecules made up of the same atoms but are bonded together in different ways (Figure 7.1). Stereoisomers are compounds with the same molecular formula but differed from each other in structural configuration. They can be further classified as chiral or achiral, depending on the
161 presence of reflective symmetry element around one or more stereocenters or chiral centers (Figure 7.1). The summary of the different types of chemical compounds is shown in figure 7.1.
A chiral center is single carbon atom surrounded by four different substituents and is typically labelled with an asterisk. Thus, chiral compounds do not have any reflective symmetry elements and exist in pairs as non-superimposable mirror images of each other about the chiral center.
Achiral compounds have an internal point or plane of symmetry, hence they are superimposable on its internal mirror image. Samples containing only a single stereoisomer are considered as enantiomerically pure. However, most processes result in a mixture of stereoisomers, known as a racemic mixture, which consists of two enantiomers in equal amount with zero net optical activity.
Figure 7.1: Classifications of chemical compounds.
Enantiomers exist as a pair of stereoisomers with the same chemical composition but with a non-superimposable mirror-image relationship. Their asymmetry structure around a chiral center resulted in “R” (+, clockwise, right-handedness) or “S” (-, counter-clockwise, left-handedness) handedness. R-enantiomer and S-enantiomer have the same physical properties (e.g. same melting point and solubility) and have exactly the same kinds of intermolecular attractions.
Stereoisomers that are not enantiomers are called diastereomers. These compounds have two or more chiral centers and are non-superimposable and non-mirror images of each other. They can have different physical properties and reactivity. An achiral form of a stereoisomer is called Meso compound. These compounds have an internal point or plane of symmetry and can be
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superimposed on its mirror image. They are optically inactive and have different physical properties and reactivity compared to its equivalent enantiomers and diastereomers.
In cases where only one enantiomer of a pharmaceutical is likely to be therapeutically active or have higher binding affinity to target sites, asymmetric chemical synthesis strategies have been devised for preparing chemical compounds of only one enantiomer. These strategies are: (1) physical separation by temporarily converting the two enantiomers into two diastereomers; (2) physical separation in a chiral chromatographic environment; (3) chemical discrimination in a chiral environment, using enzymes or other chiral platforms as chemical reagents and (4) asymmetric synthesis of one enantiomer in preference to the other.
7.1.3 R-S Enantiomers
Cahn-Ingold-Prelog (CIP) rules, also known as RS-rules were instigated by three chemists: R.S.
Cahn, C. Ingold, and V. Prelog. The RS-rules were used to unambiguously assign the R or S handedness of a molecule. The 3 basic steps to determine the RS configuration are as follows:
Step 1: Order the constituents surrounding the chiral carbon using CIP rules, from 1 (highest priority) to 4 (lowest priority).
CIP rule 1: Isotope substituents with higher atomic mass receives higher priority. (e.g. Br vs. Cl, Br has priority)
CIP rule 2: Molecular substituents with higher molecular mass receives higher priority.
CIP rule 3: Double bonds have higher priority than a single bond and are treated as a chiral carbon bonded to 2 carbons.
Step 2: Rotate the molecule such that substituent, with the least priority, points away from the viewer or is in the back (dashed line, Table 7.1).
Step 3: Draw an arrow from substituent with the highest priority to lowest priority. If the arrow is clockwise, the compound is an R-enantiomer; if the arrow moves counter-clockwise, it is an S-enantiomer.
Table 7.1: Basic chemical bond symbols
Solid line indicates that the bond exists in the plane of the drawing surface.
Dashed line indicates that the bond is extending behind the plane of the drawing surface.
Bold-wedged line indicates that the bond is protruding out from the plane of the drawing surface.
Wavy line indicates that the stereochemistry of the bond is unknown.
Dotted line indicates that the bond is only a partial bond as in a hydrogen bond or a partially formed or broken bond in a transition state.
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