99mTc-MAMA-chrysamine G, a probe for beta-amyloid protein of Alzheimer's disease.

Chrysamine G (CG), an analogue of Congo red, is known to bind in vitro to the beta-amyloid protein (Abeta 10-43) and to homogenates of several regions of the brain of Alzheimer's disease (AD) patients. We synthesised a conjugate of 2-(acetamido)-CG with a bis-S-trityl protected monoamide-monoaminedithiol (MAMA-Tr(2)) tetraligand, which was efficiently deprotected and labelled with a 75% yield with technetium-99m, to obtain (99m)Tc-MAMA-CG. In mice, (99m)Tc-MAMA-CG was cleared mainly by the hepatobiliary system, resulting in a fast blood clearance. Brain uptake of (99m)Tc-MAMA-CG was low. Co-injection with the blood pool tracer iodine-125 human serum albumin ((125)I-HSA) demonstrated a brain/blood activity ratio for (99m)Tc-MAMA-CG that was significantly higher than that for (125)I-HSA (t test for dependent samples, P<0.02), indicating the ability of (99m)Tc-MAMA-CG to cross the blood-brain barrier. In vitro autoradiography demonstrated pronounced binding of (99m)Tc-MAMA-CG to beta-amyloid deposits in autopsy sections of the parietal and occipital cortex of an AD patient as compared with controls. Adding 10 microM Congo red during incubation displaced the binding of (99m)Tc-MAMA-CG. Congo red staining and autoradiography identified the same lesions. (99m)Tc-MAMA-CG seems to bind selectively to beta-amyloid deposition in human brain parenchyma and blood vessels in vitro and thus might be a lead compound for further development of a useful tracer agent for the in vivo diagnosis of Alzheimer's disease.