Background

[PubMed]

Alzheimer’s disease (AD) is a major neurodegenerative disease associated with cognitive impairment and an irreversible decline of mental functions (1, 2). It is characterized by presence in the brain of senile plaques of β-amyloid (Aβ) peptides with intracellular neurofibrillary tangles of filaments that contain the hyperphosphorylated protein tau (3, 4). Accelerated deposition of Aβ deposits seems to be a key risk factor associated with AD, and although the mechanisms of the disease are still not fully understood, reducing the deposition of amyloid plaques seems to benefit patients.

Several radioligands for positron emission tomography (PET) have been developed (5-7) and tested in humans as in vivo diagnostic tools for imaging and measuring the formation and Aβ deposits (5-7) The first successful agent used in human studies was [¹⁸F]FDDNP (8), a malonitrile derivative found to bind to both neurofibrillary tangles and Aβ plaques. The second successful attempt was made with [¹¹C]PIB (9), also known as Pittsburgh Compound B or [¹¹C]6-OH-BTA-1. [¹¹C]PIB showed marked retention in areas of the association cortex known to contain substantial amounts of Aβ deposits. The third PET radioligand tested in humans was [¹¹C]4-N-methylamino-4´-hydroxystilbene, a stilbene derivative commonly named [¹¹C]SB-13.

A series of benzofuran derivatives have recently been synthesized and are being evaluated as Aβ probes with PET. Probes that use radioactive iodine have shown very good binding affinities for Aβ aggregates and good brain penetration (10). Unfortunately, their level of nonspecific binding was found to be very high, which makes them unsuitable for in vivo plaque imaging. Other imaging agents that use benzofuran derivatives labeled with ¹¹C are currently being evaluated. One of them, [¹¹C]5-hydroxy-2-(4-methyaminophenyl)benzofuran , is currently being tested in small animals.

Synthesis

[PubMed]

An experimental procedure for synthesizing 5-hydroxy-2-(4-methyaminophenyl)benzofuran was reported by Ono et al. (11) in 2006. The main step of the procedure involved the formation of the benzofuran backbone, 5-methoxy-2-(4-nitrophenyl)benzofuran, by an intramolecular Wittig reaction between triphenylphosphonium salt and 4-nitrobenzoyl chloride at a 33% yield. The Wittig reagent used in the procedure was prepared from 2-methoxy-5-hydroxybenzyl alcohol and triphenylphosphine hydrobromide at 84% yield.

5-Methoxy-2-(4-aminophenyl)benzofuran was converted to the monomethylamino derivative 5-methoxy-2-(4-methylaminophenyl)benzofuran by reducing the nitro group to an amino group with SnCl₂ and subsequently monomethylating the amino group. 5-Methoxy-2-(4-aminophenyl)benzofuran was also converted to the dimethylamino derivative 5-methoxy-2-(4-dimethylaminophenyl)benzofuran at a 39% yield by a previously described method that uses paraformaldehyde, sodium cyanoborohydride, and acetic acid (11). The removal of the O-methyl groups (using BBr₃) led to the formation of 5-hydroxy-2-(4-aminophenyl)benzofuran, the precursor used in the radiolabelling procedure of -hydroxy-2-(4-methyaminophenyl)benzofuran with ¹¹C-labeled methyl triflate.

The final product, [¹¹C]5-hydroxy-2-(4-methyaminophenyl)benzofuran, was obtained at a 21% yield (decay-corrected to end of bombardment) and with a radiochemical purity >99% . Its specific activity at the end of synthesis was ≈37 GBq/µmol (1 Ci/µmol).

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

In vitro binding assays using AD brain gray matter homogenates showed that [¹¹C]5-hydroxy-2-(4-methyaminophenyl)benzofuran had a high binding affinity to amyloid plaques with an inhibition constant of 0.7 ± 0.2 nM. Measurements of the partition coefficient (P) in 1-octanol/phosphate buffer (pH 7.4) led to a log P value of 2.36 (11), which is considered within the optimal lipophilicity range for crossing the blood-brain barrier.

In vitro fluorescent labeling of AD sections was performed by Ono et al. (11) to investigate the neuropathological staining of senile plaques by 5-hydroxy-2-(4-methyaminophenyl)benzofuran. Results showed staining of neuritic plaques and cerebrovascular amyloids. In addition, [¹¹C]5-hydroxy-2-(4-methyaminophenyl)benzofuran also displayed high binding affinity for neurofibrillary tangles in the AD sections. It was shown in a previous study that a marked increase in the amount of neurofibrillary tangles in the hippocampus and entorhinal cortex occurred in the preclinical AD stage (12).

Overall, the results from neuropathological staining of human AD obtained for sections showed that 5-hydroxy-2-(4-methyaminophenyl)benzofuran could bind to amyloid plaques and neurofibrillary tangles with almost the same pattern of [¹⁸F]FDDNP or X-34 (13).

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

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