Background

[PubMed]

Alzheimer's disease (AD) is a form of dementia with a gradual memory loss and a progressive decline in mental functions overtime (1, 2). It is characterized pathologically by neuronal loss, extracellular senile plaques (aggregates of amyloid-beta peptides consisting of 40 to 42 amino acids) and intracellular neurofibrillary tangles (filaments of microtubule-binding hyper-phosphorylated protein tau) in the brain, especially in the hippocampus and associative regions of the cortex (3, 4). β-amyloid peptides and tau protein are implicated as the main causes of neuronal degeneration and cell death (5, 6).

Early diagnosis of AD is important for treatment consideration and disease management (7). Various β-amyloid imaging agents have been developed for magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT), and positron emission tomography (PET) (8-13). The binding of different derivatives of Congo red, thioflavin, stibene, and aminonaphthalene has been studied in human post-mortem brain tissue and in transgenic mice. Out of these analogues, 2-(1-(6-[(2-[¹⁸F]fluoroethyl)(methyl)amino]-2-naphthyl)ethylidene)malono nitrile ([¹⁸F]FDDNP) was studied in humans, showing more binding in the brains of patients with AD than in those of healthy people (14). However, [¹⁸F]FDDNP showed low signal-to-noise ratios for PET imaging, because it is highly lipophilic. N-Methyl-[¹¹C]-2-(4’-methylaminophenyl)-6-hydroxybenzothiasole, a β-amyloid binding compound based on a series of neutral thioflavin-T derivatives (15), was radiolabeled with the positron-emitting radionuclide ¹¹C ([¹¹C]6-OH-BTA-1 or [¹¹C]PIB). [¹¹C]6-OH-BTA-1 was found to be a promising imaging agent for the senile plaques in the brain (16).

Synthesis

[PubMed]

[¹¹C]6-OH-BTA-1 was readily synthesized by standard ¹¹C-methylation of 2-(4’-aminophenyl)-6-methoxymethoxybenzothiazole with [¹¹C]methyl iodide, followed by hydrolysis (15). The radiochemical yield averaged 12.1% at the end of synthesis based on [¹¹C]methyl iodide, and the specific activity averaged 85 GBq/μmol (2.3 Ci/μmol) at the end of synthesis. Radiochemical and chemical purities were >95% as determined by high-performance liquid chromatography (HPLC).

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

6-OH-BTA-1 has a binding affinity (K_i) of 4.5 nM for aggregated amyloid-beta(1-40) fibrils in competition with [N-methyl-³H]BTA-1 (15). The K_i Value for thioflavin-T was found to be 580 nM. Saturation binding studies with [N-methyl-³H]6-OH-BTA-1 to amyloid-beta(1-40) peptide showed a K_d value of 4.7 nM and a B_max value of 2.7 pmol of 6-OH-BTA-1 per nmol of amyloid-beta(1-40) peptide. 6-OH-BTA-1 showed no inhibition in an array of neurotransmitter receptor and transporter assays. Saturation binding studies with [N-methyl-³H]6-OH-BTA-1 to homogenates of frontal cortex from postmortem advanced AD brain showed a K_D value of 1.4 nM and a B_max value of 1900 pmol per g tissue. There was no specific binding of [N-methyl-³H]6-OH-BTA-1 to homogenates of frontal cortex from age-matched control brain as well as to homogenates of cerebellum from AD brain or control brain. Klunk et al (16) showed that [¹¹C]6-OH-BTA-1 bound specifically to frontal cortex of postmortem AD brain slices, and its binding could be blocked by excess unlabeled BTA-1. There was little binding of [¹¹C]6-OH-BTA-1 to frontal cortex of normal elderly control brain slices.

Animal Studies

Human Studies

[PubMed]

The first human study with [¹¹C]6-OH-BTA-1 in 16 patients with mild AD and 9 healthy people was reported (16). The subjects were given an intravenous injection of 300 MBq (8.1 mCi) of [¹¹C]6-OH-BTA-1. Plasma metabolism of [¹¹C]6-OH-BTA-1 was rapid and similar in the AD patients and normal subjects. The amount of intact [¹¹C]6-OH-BTA-1 was 7.2 ± 3.6% (normal control subjects) and 9.8 ± 3.0% (AD) at 60 min after injection. The dynamic PET data showed that [¹¹C]6-OH-BTA-1 retention is 1.52-1.94-fold greater in brain regions (such as frontal, parietal, temporal, and occipital cortex and the striatum) that are known to contain amyloid plaques in AD patients than in controls. The frontal cortex had the highest tracer uptake. There was a similar retention of [¹¹C]6-OH-BTA-1 in the cerebellum and white matter containing little amyloid deposits in AD and controls. There is an inverse correlation of [¹¹C]6-OH-BTA-1 retention with cerebral FDG metabolism in the cortical regions. However, the differences between AD and controls were greater with [¹¹C]6-OH-BTA-1 than with FDG.

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