Preclinical properties of 18F-AV-45: a PET agent for Abeta plaques in the brain

Abstract

beta-amyloid plaques (Abeta plaques) in the brain, containing predominantly fibrillary Abeta peptide aggregates, represent a defining pathologic feature of Alzheimer disease (AD). Imaging agents targeting the Abeta plaques in the living human brain are potentially valuable as biomarkers of pathogenesis processes in AD. (E)-4-(2-(6-(2-(2-(2-(18)F-fluoroethoxy)ethoxy)ethoxy)pyridin-3-yl)vinyl)-N-methyl benzenamine ((18)F-AV-45) is such as an agent currently in phase III clinical studies for PET of Abeta plaques in the brain.

Methods: In vitro binding of (18)F-AV-45 to Abeta plaques in the postmortem AD brain tissue was evaluated by in vitro binding assay and autoradiography. In vivo biodistribution of (18)F-AV-45 in mice and ex vivo autoradiography of AD transgenic mice (APPswe/PSEN1) with Abeta aggregates in the brain were performed. Small-animal PET of a monkey brain after an intravenous injection of (18)F-AV-45 was evaluated.

Results: (18)F-AV-45 displayed a high binding affinity and specificity to Abeta plaques (K(d), 3.72 +/- 0.30 nM). In vitro autoradiography of postmortem human brain sections showed substantial plaque labeling in AD brains and not in the control brains. Initial high brain uptake and rapid washout from the brain of healthy mice and monkey were observed. Metabolites produced in the blood of healthy mice after an intravenous injection were identified. (18)F-AV-45 displayed excellent binding affinity to Abeta plaques in the AD brain by ex vivo autoradiography in transgenic AD model mice. The results lend support that (18)F-AV-45 may be a useful PET agent for detecting Abeta plaques in the living human brain.

FIGURE 1

Inhibition constants (K_i, nM) of various agents against binding of ¹⁸F-AV-45 to Aβ plaques in postmortem AD brain homogenates.

FIGURE 2

Representative saturation binding of ¹⁸F-AV-45 to Aβ plaques in postmortem AD brain homogenates; Scatchard analysis of binding is shown. In this example, K_d was 3.51 nM and B_max was 7,215 fmol/mg of protein. Overall, average values for 4 AD cases were K_d = 3.72 ± 0.30 nM and B_max = 8,811 ± 1,643 fmol/mg of protein.

FIGURE 3

Kinetics of association (A) and dissociation (B) of ¹⁸F-AV-45 with Aβ plaques in postmortem AD brain homogenates. TB = total binding; NSB = nonspecific binding; SB = specific binding.

FIGURE 4

In vitro autoradiograms of frozen human brain sections labeled with ¹⁸F-AV-45. (A and B) Highly intensive labeling of Aβ plaques on brain sections from AD patients. (C) Control subject exhibits no labeling by this tracer.

FIGURE 5

Ex vivo autoradiography of ¹⁸F-AV-45 in 25-mo-old Tg (APPswe/PSEN1) mice. (A) Ex vivo autoradiogram of brain section. (B) Fluorescent image of comparable brain section after thioflavin S staining.

FIGURE 6

Brain uptakes and washout in healthy mice after injection of ¹⁸F-AV-45 and its in vivo metabolites (% ID/g).

FIGURE 7

Kinetics of brain uptake and washout in rhesus monkey after intravenous injection of ¹⁸F-AV-45 (173.9 MBq [4.7 mCi]) are presented. It is evident that uptake in cortex peaked at 7 min, and activity was washed out quickly thereafter. White matter area also displayed good initial uptake, and washout rate was also rapid. It is estimated that brain uptake at peak was about 4.4% of injected dose, which suggested that ¹⁸F-AV-45 penetrated normal blood–brain barrier efficiently.