¹⁸F-labeled styrylpyridines as PET agents for amyloid plaque imaging

Abstract

Positron emission tomography (PET) imaging of β-amyloid (Aβ) plaques in the brain is a potentially valuable tool for studying the pathophysiology of Alzheimer's disease (AD). It may also be applicable for measuring the effectiveness of therapeutic drugs aimed at lowering Aβ plaques in the brain. We have successfully reported a series of ¹⁸F-labeled fluoropegylated stilbenes for PET imaging studies. Encouraging results clearly demonstrated the usefulness of ¹⁸F-labeled stilbenes as potential Aβ plaque-imaging agents. In the present study, we applied a similar approach to a styrylpyridine backbone structure. Among all derivatives examined, (E)-2-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)-5-(4-dimethylaminostyryl)-pyridine (2) displayed high binding affinity in postmortem AD brain homogenates (K_i=2.5±0.4 nM, with [¹²⁵I]IMPY as radioligand). No-carrier-added [¹⁸F]2 was successfully prepared by [¹⁸F]fluoride displacement of the corresponding tosylate precursor with a high labeling yield (30–40%) and a high radiochemical purity (>99%). Specific activity at the end of synthesis was determined to be 1500–2000 Ci/mmol. The tracer [¹⁸F]2 showed adequate lipophilicity (log P=3.22). In vivo biodistribution of [¹⁸F]2 in normal mice exhibited excellent initial brain penetration and rapid washout (7.77% and 1.03% dose/g in the brain at 2 and 30 min after intravenous injection, respectively) — properties that are highly desirable for Aβ-plaque-specific brain imaging agents. Autoradiography of AD brain sections and homogenate binding with postmortem AD brain tissues confirmed the high binding signal of [¹⁸F]2 due to the presence of Aβ plaques. These preliminary results suggest that novel PET tracers may be potentially useful for the imaging of Aβ plaques in the living human brain.

Introduction

Accumulation of excess senile plaques in the brain is strongly associated with the pathogenesis of Alzheimer's disease (AD) [1], [2], [3], [4]. Recent research has focused on the development of potential imaging agents for the direct imaging of β-amyloid (Aβ) aggregates in the living human brain. These imaging agents could be beneficial for the diagnosis and monitoring of therapeutic effects [5], [6], [7], [8], [9], [10].

Several groups, including the authors, have reported series of potential imaging agents for the in vivo imaging of Aβ plaques with positron emission tomography (PET) or single-photon emission computed tomography. Different tracers such as [¹¹C]PIB [9], [11], [12], [13], [¹¹C] 4-N-methylamino-4′-hydroxy-stilbene (SB-13) [14], [15], [¹⁸F]FDDNP [16], [17] and [¹²³I]IMPY [18], [19] have been tested clinically and have demonstrated the potential utility of the in vivo imaging of Aβ plaque deposition in the brain. Other structurally similar compounds have been reported [20], [21], [22]. Due to the short half-life of ¹¹C (T_1/2=20 min), the supply of ¹¹C agents will likely be limited to facilities with an on-site cyclotron. To broaden the utility of Aβ-plaque-targeting agents, we have then focused our effort on developing ¹⁸F-labeled imaging agents (T_1/2=110 min). Based on the stilbene backbone structure, we have successfully developed two derivatives: MAPO and PEGN3-stilbene (Fig. 1), which were readily labeled with ¹⁸F. To circumvent the problem of high lipophilicity after the addition of a fluoroalkyl group, we have prepared stilbene derivatives with an additional hydroxyl group, such as FMAPO [23] (see Fig. 1). Similarly, we have added polyethylene glycol (PEG) units as tether on the 4-hydroxy group of SB-13 through which the fluorine atom is attached at the end of the PEG chain [fluoropolyethylene glycol (FPEG)] [24]. By varying the chain length of the tethered fluorinated PEG group, we are provided a flexible tool to adjust lipophilicity and to maintain a relatively small size. Both fluorinated tracers FMAPO and FPEGN3-stilbene displayed high binding affinities to Aβ plaques and showed favorable in vivo kinetic properties [23], [24].

In an attempt to further explore ¹⁸F-labeled tracers with a different backbone structure, we extended our fluoropegylation approach from stilbene to styrylpyridine series by replacing one of the benzene rings of stilbene with one pyridine ring (Fig. 1). It has been demonstrated that the p-iodo-derivative of styrylpyridine showed more promising in vivo kinetics than the corresponding stilbene derivatives [25]. As an extension of our effort in developing optimal probes, which will require a careful balancing of size and lipophilicity/hydrophilicity while also preserving binding affinity, we have tested various approaches. We have successfully used the strategy of FPEG as the tether attached to the molecule from stilbenes to styrylpyridines. Alternative styrylpyridine derivatives may provide improved in vivo biological properties of targeting Aβ plaques. They may also lead to different profiles of in vivo kinetics and metabolism, which may enhance the signal between target and nontarget regions. Reported herein are the synthesis and biological evaluation of this series of compounds for targeting amyloid plaques.