Synthesis and evaluation of two 18F-labeled 6-iodo-2-(4'-N,N-dimethylamino)phenylimidazo[1,2-a]pyridine derivatives as prospective radioligands for beta-amyloid in Alzheimer's disease

Abstract

This study evaluated (18)F-labeled IMPY [6-iodo-2-(4'-N,N-dimethylamino)phenylimidazo[1,2-a]pyridine] derivatives as agents for imaging beta-amyloid plaque with positron emission tomography (PET). The precursor for radiolabeling and reference compounds was synthesized in up to five steps from commercially accessible starting materials. One of the two N-methyl groups of IMPY was substituted with either a 3-fluoropropyl (FPM-IMPY) or a 2-fluoroethyl (FEM-IMPY) group. FPM-IMPY and FEM-IMPY were found to have moderate affinity for Abeta-aggregates with K(i) = 27 +/- 8 and 40 +/- 5 nM, respectively. A "one-pot" method for (18)F-2-fluoroethylation and (18)F-3-fluoropropylation of the precursor was developed. The overall decay-corrected radiochemical yields were 26-51%. In PET experiments with normal mouse, high uptake of activity was obtained in the brain after iv injection of each probe: 6.4% ID/g for [(18)F]FEM-IMPY at 1.2 min, and 5.7% ID/g for [(18)F]FPM-IMPY at 0.8 min. These values were similar to those of [(123)I/(125)I]IMPY (7.2% ID/g at 2 min). Polar and nonpolar radioactive metabolites were observed in both plasma and brain homogenates after injection of [(18)F]FEM or [(18)F]FPM-IMPY. In contrast to the single-exponential washout of [(123)I/(125)I]IMPY, the washouts of brain activity for the two fluorinated analogues were biphasic, with an initial rapid phase over 20 min and a subsequent much slower phase. Residual brain activity at 2 h, which may represent polar metabolites trapped in the brain, was 4.5% ID/g for [(18)F]FEM-IMPY and 2.1% ID/g for [(18)F]FPM-IMPY. Substantial skull uptake of [(18)F]fluoride was also clearly observed. With a view to slow the metabolism of [(18)F]FEM-IMPY, an analogue was prepared with deuteriums substituted for the four ethyl hydrogens. However, D(4)-[(18)F]FEM-IMPY showed the same brain uptake and clearance as the protio analogue. Metabolism of the [(18)F]FEM-IMPY was appreciably slower in rhesus monkey than in mouse. Autoradiography of postmortem brain sections of human Alzheimer's disease patients with [(18)F]FEM-IMPY showed high displaceable uptake in gray matter and low nonspecific binding in the white matter. This study demonstrates that the IMPY derivatives have favorable in vivo brain pharmacokinetics and a moderate affinity for imaging beta-amyloid plaques; however, further improvements are needed to reduce radioactive metabolites, increase binding affinity, and reduce lipophilicity.

Figures

Figure 1

Structures of leading tracers for β-amyloid.

Figure 2

Determination of specific vs nonspecific binding of [18F]FEM-IMPY in postmortem AD brain tissue homogenate. Each data point came from the average of four measurements, and the bar represents the standard deviation (SD).

Figure 3

Autoradiography of [18F]FEM-IMPY and immuno-fluorescent (IF) detection of β-amyloid plaques in postmortem brain tissue sections of medial temporal lobe from an AD patient and an age-matched control: (a) AD tissue + [18F]FEM-IMPY; (b) AD tissue + IF protocol; (c) AD tissue pretreated with FEM-IMPY + [18F]FEM-IMPY; (d) control tissue + [18F]-FEM-IMPY.

Figure 4

Time–activity curve in brain and skull after iv injection of [18F]FEM-IMPY or D4-[18F]FEM-IMPY in normal mice. Each point represents the average of four mice. The standard error of each point for the brain is within 3–12% and that for the skull is 5–25% after 1.2 min. For clarity, the error bar is not shown. D4-skull represents data from D4-[18F]FEM-IMPY.

Figure 5

Time–activity curve in brain and skull after iv injection of [18F]FPM-IMPY in normal mice. Each point represents the average of four mice. The standard error of each point for the brain is within 2–12% and that for the skull is 7–38% after 1.2 min. For clarity, the error bar is not shown.

Figure 6

Time–activity curves of different regions of brain after iv injection of [18F]FEM-IMPY in a rhesus monkey.

Figure 7

Time–activity curves of different radioactive components after iv injection of [18F]FEM-IMPY in the in vivo whole blood of normal mice.

Figure 8

Time–activity curves of different radioactive components after incubation of [18F]FEM-IMPY in the in vitro brain homogenate of normal mice.

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