Background

[PubMed]

The gradual development and worsening of dementia is a characteristic feature of Alzheimer’s disease (AD), and currently no cure is available for this ailment. Medications available to treat AD can only modify or delay the onset of this disease, and there are many ongoing preclinical and clinical studies to develop a suitable therapy to treat this condition (1). It is hypothesized (known as the amyloid cascade hypothesis) that accumulation of an amyloid precursor protein derivative, the β-amyloid protein (Aβ), in the brain is responsible for bringing about changes in the biochemical and cellular functions of the brain and leads to the progression and manifestation of AD (2). Therefore, the Aβ is believed to be a good biomarker for the detection of AD. Most earlier studies performed to investigate this disease used human post-mortem brains, and little is known about the early events that promote the development of AD or about how the formation and deposition of the Aβ contributes to the disease process (2). More recently, many noninvasive imaging probes that can be used with the various imaging modalities, have been developed and evaluated for the detection of early stage Aβ plaques; however, due to inadequate permeation across the blood–brain barrier and/or low binding to the Aβ aggregates, these agents were considered unsuitable for visualization of the Aβ plaques in the human brain (2).

In an effort to develop imaging compounds that can be used for the visualization of Aβ plaques, investigators have produced and evaluated radiolabeled compounds that can be used with positron emission tomography (PET) (3). Among these, the ¹¹C-labeled Pittsburg Compound-B ([¹¹C]PIB), which has been shown to bind specifically to Aβ deposits in post-mortem AD brains under in vitro conditions, is most commonly used to diagnose the condition, but the major limitation of using this probe is the short half-life of ¹¹C (t_1/2 = 20 min) and the requirement of an onsite cyclotron for the generation of this radionuclide (2, 3). AS a result, ¹¹C-labeled tracers are not the most suitable for commercialization (4), and [¹¹C]PIB is not approved by the United States Food and Drug Administration (FDA) as a diagnostic imaging agent for the detection of AD. ¹⁸F-Labeled AV-45 ([¹⁸F]AV-45), which has a t_1/2 = 110 min and has been developed, characterized, and evaluated for the detection of Aβ plaques, has been shown to be suitable for the identification of patients with AD, and the binding of this tracer in the brain of AD patients correlated well with the presence of Aβ plaques observed in this organ during post-mortem studies (5). However, [¹⁸F]AV-45 is not approved by the FDA as yet for the detection of AD in patients. Although [¹⁸F]AV-45 appears to be a promising agent for the noninvasive detection of Aβ plaques, it may not be suitable for the visualization of early-onset Aβ deposits because this tracer (and other similar agents) shows high uptake in the white matter of the brain (3, 5). In a continued effort to develop an imaging agent that is superior to those currently available for the detection of Aβ deposits, Hostetler et al. developed 5-[¹⁸F]fluoro-2-(1-methyl-1H-pyrrolo[2,3-b]pyridin-5-yl)-oxazolo[5,4-b]pyridine ([¹⁸F]MK-3328) and showed that it was a promising PET agent for the detection of Aβ plaques because it has a low in vivo uptake in the white matter and the cortical gray matter of the rhesus monkey brain (3).

Synthesis

[PubMed]

The synthesis of MK-3328 has been described by Harrison et al. (6), and its labeling with [¹⁸F]fluoride is detailed by Hostetler et al. (3). The average yield of [¹⁸F]MK-3328 was reported to be 14 ± 13%, with a specific activity of 91.5 ± 51.4 GBq/mmol (2,471 ± 1,389 Ci/mmol) and a radiochemical purity (RP) of >98% (n = 25 synthesis reactions).

For comparison purposes, some other ¹⁸F-labeled compounds were synthesized and studied (3). These radiolabeled compounds are listed in Table 1 below:

Table 1: Comparison of radiolabeled compounds

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

The lipophilicity (Log D) of various Aβ ligands was determined as described by Hostetler et al. (3). The Log D values for the various Aβ binding compounds (Table 2 below) were higher (ranging from 2.91 for MK-3328 to 3.52 for AD-265) than that of PIB (Log D = 2.23).

Table 2: Lipophilicity and in vitro binding characteristics of various Aβ binding compounds

*Single determination.

In a competition binding assay using brain frontal cortex homogenates (controls) of AD patients ([³H₃]DMAB was used as the binding ligand), the Log D values and IC₅₀ concentrations of various nonradioactive Aβ binding compounds were compared (Table 2) (6). From this assay it was clear that all the compounds except PIB had a low affinity for the homogenates and that AD-265 was the least potent ligand. The absence of Aβ plaques in the homogenates was confirmed with immunostaining. This suggested that the low-affinity compounds (MK-3328, AD-269, and AD-278) showed no specific binding to the control brain homogenates. In addition, none of the compounds were determined to be unsuitable substrates for the P-glycoprotein transporter, and all of them had suitable cell permeability rates (for details, see Hostetler et al. (3).

Hostetler et al. compared the binding of [³H]MK-3328 (5 nM) and [³H]AD-269 (5 nM) to Aβ deposits in human AD donor brain slices with autoradiography of the brain slice exposure to the two labeled compounds (3). From the autoradiographic images it was clear that only [³H]MK-3328 bound to the amyloid deposits in the frontal cortex of the diseased human brain. In a self-blocking experiment, nonradioactive MK-3328 was shown to completely block the binding of [³H]MK-3328 to the Aβ plaques in the AD donor brain slices. In addition, [³H]MK-3328 showed no binding in the cerebellum of an AD patient brain (the cerebellum of the AD patient brain is known to have very few Aβ plaque deposits). Similarly, [³H]MK-3328 did not show any binding to the frontal cortex of a non-AD human donor brain. From this study the investigators concluded that the specific binding of [³H]MK-3328 to the Aβ plaques was probably due to its lower lipophilicity (Log D = 2.91) compared with that of [³H]AD-269 (Log D = 3.42).

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

Supplemental Information

[Disclaimers]

No information is currently available.

References

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Karran E., Mercken M., De Strooper B. The amyloid cascade hypothesis for Alzheimer's disease: an appraisal for the development of therapeutics. Nat Rev Drug Discov. 2011;10(9):698–712. [PubMed: 21852788]

2.

Quigley H., Colloby S.J., O'Brien J.T. PET imaging of brain amyloid in dementia: a review. Int J Geriatr Psychiatry. 2011;26(10):991–9. [PubMed: 21905095]

3.

Hostetler E.D., Sanabria-Bohorquez S., Fan H., Zeng Z., Gammage L., Miller P., O'Malley S., Connolly B., Mulhearn J., Harrison S.T., Wolkenberg S.E., Barrow J.C., Williams D.L. Jr, Hargreaves R.J., Sur C., Cook J.J. [(18)F]Fluoroazabenzoxazoles as potential amyloid plaque PET tracers: synthesis and in vivo evaluation in rhesus monkey. Nucl Med Biol. 2011;38(8):1193–203. [PubMed: 21741254]

4.

Vallabhajosula S. Positron emission tomography radiopharmaceuticals for imaging brain Beta-amyloid. Semin Nucl Med. 2011;41(4):283–99. [PubMed: 21624562]

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Wong D.F., Rosenberg P.B., Zhou Y., Kumar A., Raymont V., Ravert H.T., Dannals R.F., Nandi A., Brasic J.R., Ye W., Hilton J., Lyketsos C., Kung H.F., Joshi A.D., Skovronsky D.M., Pontecorvo M.J. In vivo imaging of amyloid deposition in Alzheimer disease using the radioligand 18F-AV-45 (florbetapir [corrected] F 18). J Nucl Med. 2010;51(6):913–20. [PMC free article: PMC3101877] [PubMed: 20501908]

6.

Harrison S.T., Mulhearn J., Wolkenberg S.E., Miller P.J., O’Malley S.S., Zeng Z., Williams J. D. L., E.D. Hostetler, S. Sanabria-Bohórquez, L. Gammage, H. Fan, C. Sur, J.C. Culberson, R.J. Hargreaves, J.J. Cook, J.D. Hartman, and J.C. Barrow, Synthesis and Evaluation of 5-Fluoro-2-aryloxazolo[5,4-b]pyridines as β-Amyloid PET Ligands and Identification of MK-3328. ACS Medicinal Chemistry Letters. 2011;2(7):498–502. [PMC free article: PMC4018077] [PubMed: 24900338]