Lisheng Cai

Radioligand Development for PET Imaging of β-Amyloid (Aβ)-Current Status

Two of the main pathological hallmarks of Alzheimers disease (AD) are neuritic plaques and neurofibrillary tangles. Significant evidence supports a critical and probable causative role of βamyloid (Aβ) plaque formation. Since neuroprotective treatments are typically most effective at early stages of injury, the detection and measurement of Aβ load in living brain should be performed at early and perhaps even presymptomatic stages of AD. Two primary targets of molecular imaging research with positron emission tomography (PET) are to develop surrogate markers (radioligands) for assessing disease progression and for monitoring the efficacy of developmental therapeutics. Here, we review the current status of radioligand development for PET imaging of Aβ aggregates. General structure-activity relationships have emerged, including the identification of at least three different ligand binding sites in various Aβ aggregates and recognition of the general structural requirements for ligand binding at each site. Also a few radioligands applicable to imaging Aβ plaques in living human brain with positron emission tomography (PET) have emerged, including [11C]PIB, [11C]SB- 13 and [18F]FDDNP.

Imaging and In Vivo Quantitation of β-Amyloid: An Exemplary Biomarker for Alzheimer’s Disease?

Alzheimers disease (AD) is characterized pathologically by the presence of beta-amyloid plaques in the brain. A substantial body of research indicates that the presence of increased beta-amyloid peptide (Abeta) is neurotoxic and may initiate the further pathology observed in AD, including neurofibrillary tangles, synaptic loss and dysfunction, and neurodegeneration. The use of brain imaging in patients with or at risk for AD has increased our understanding of the pathophysiology of the disease and may potentially aid in diagnosis. The development of new therapeutics that reduce Abeta in the brain has also indicated a potential use for amyloid imaging in monitoring response to treatment. This review explores the utility of amyloid as a biomarker and the use of positron emission tomography and magnetic resonance imaging in the diagnosis and treatment of AD.

Lipophilic Analogs of Thioflavin S as Novel Amyloid-Imaging Agents

Lipophilic analogs of thioflavin S were synthesized and radiolabeled with positron or single photon emitting radionuclides. The binding affinity for Abeta was evaluated using isolated amyloid fibrils from human brain tissue. Binding specificity was assessed using fluorescent tissue staining. In vivo brain uptake was evaluated in mice. Following synthesis, neutral analogs of thioflavin S capable of radiolabeling with (11)C or (125)I, were found to bind isolated human Abeta with affinities in the nanomolar range. Fluorescent tissue staining showed selective binding to Abeta deposits in vitro. Biodistribution of selected compounds displayed high brain permeability at early time points. At later points, the compounds were cleared from the normal brain, indicating low non-specific binding in vivo. These studies indicated that novel amyloid imaging probes can be developed based on thioflavin S that readily entered the brain and selectively bound to Abeta deposits and neurofibrilary tangles. Potential applications of these amyloid binding agents include facilitating drug screening in animal models and use as in vivo markers of early and definitive diagnosis of AD.

Exploration of the labeling of [11C]tubastatin A at the hydroxamic acid site with [11C]carbon monoxide

We aimed to label tubastatin A (1) with carbon-11 (t1/2 = 20.4 min) in the hydroxamic acid site to provide a potential radiotracer for imaging histone deacetylase 6 in vivo with positron emission tomography. Initial attempts at a one-pot Pd-mediated insertion of [(11)C]carbon monoxide between the aryl iodide (2) and hydroxylamine gave low radiochemical yields (<5%) of [(11) C]1. Labeling was achieved in useful radiochemical yields (16.1 ± 5.6%, n = 4) through a two-step process based on Pd-mediated insertion of [(11)C]carbon monoxide between the aryl iodide (2) and p-nitrophenol to give the [(11)C]p-nitrophenyl ester ([(11)C]5), followed by ultrasound-assisted hydroxyaminolysis of the activated ester with excess hydroxylamine in a DMSO/THF mixture in the presence of a strong phosphazene base P1-t-Bu. However, success in labeling the hydroxamic acid group of [(11)C]tubastatin A was not transferable to the labeling of three other model hydroxamic acids.

Synthesis of [11C]astemizole as a lead candidate radioligand for imaging brain neurofibrillary tangles

Introduction: Neurofibrillary tangles and amyloid plaques are the two hallmarks of Alzheimers Disease (AD) [1]. Since neurofibrillary tangles are positively correlated with the cognitive decline [2], the PET imaging of tangles is a major unfulfilled objective for the possible diagnosis and monitoring of AD. A fundamental reason for this omission is that few lead compounds have been identified with potential to meet this purpose. Recently, however, astemizole has been shown to bind with brain neurofibrillary tangles in in vitro assays and has been proposed as a candidate for labelling with a positron-emitter and PET imaging of tangles in brain in vivo [3]. Here we report the synthesis of [C]astemizole for future study as a lead candidate PET radioligand.

Low Retention of [S-methyl-11C]MeS-IMPY to β-amyloid Plaques in Patients with Alzheimers Disease

Alzheimers disease (AD) is characterized pathologically by the accumulation of β-amyloid plaques in brain. We developed a novel positron emission tomographic (PET) radioligand, [11C]MeS-IMPY ([S-methyl-11C]N,N-dimethyl- 4-(6-(methylthio)imidazo[1,2-a]pyridine-2-yl)aniline). The objective of this study was to determine if [11C]MeS-IMPY can detect β-amyloid plaques in patients with AD. Ten subjects (4 patients with AD and 6 healthy controls) had PET scans of 90 min as well as serial sampling of arterial plasma. Total distribution volume was calculated from radioactivity in brain and concentrations of radioligand in arterial plasma. After injection of [11C]MeS-IMPY in both groups, peak brain uptake was high (∼ 450% SUV), peaked early at ∼ 2 min and washed out quickly. The time-course of brain radioactivity in all regions was virtually identical in AD patients and healthy controls. Although distribution volume of neocortical regions was ∼ 26% higher in AD patients than in controls, the differences were statistically insignificant (P = 0.143). In addition, distribution volume in all regions and in both groups increased during the entire scan, which is consistent with the accumulation of radiometabolite(s) in brain. The rapid washout of [11C]MeS-IMPY from brain may have been caused by inadequate binding affinity, and the small differences between patients and healthy subjects were likely caused, in part, by the accumulation of radiometabolite(s) in brain. We conclude that [11C]MeS-IMPY is able to image β-amyloid in brains of patients with AD, but that chemical analogs with higher affinity and non-problematic radiometabolites will be more useful imaging agents.

Quinuclidine and DABCO Enhance the Radiofluorinations of 5-Substituted 2-Halo-pyridines

Positron emission tomography (PET) is an important molecular imaging technique for medical diagnosis, biomedical research and drug development. PET tracers for molecular imaging contain β+-emitting radionuclides, such as carbon-11 (t1/2 = 20.4 min) or fluorine-18 (t1/2 = 109.8 min). The [18F]2-fluoro-pyridyl moiety features in a few prominent PET radiotracers, not least because this moiety is usually resistant to unwanted radiodefluorination in vivo. Various methods have been developed for labeling these radiotracers from cyclotron-produced no-carrier-added [18F]fluoride ion, mainly based on substitution of a leaving group, such as halide (Cl or Br), or preferably a better leaving group, such as nitro or trimethylammonium. However, precursors with a good leaving group are sometimes more challenging or lengthy to prepare. Methods for enhancing the reactivity of more readily accessible 2-halopyridyl precursors are therefore desirable, especially for early radiotracer screening programs that may require the quick labeling of several homologous radiotracer candidates. In this work, we explored a wide range of additives for beneficial effect on nucleophilic substitution by [18F]fluoride ion in 5-subsituted 2-halopyridines (halo = Cl or Br). The nucleophilic cyclic tertiary amines, quinuclidine and DABCO, proved effective for increasing yields to practically useful levels (> 15%). Quinuclidine and DABCO likely promote radiofluorination through reversible formation of quaternary ammonium intermediates.