Scintigraphy with 123I-serum amyloid P component in Alzheimer disease

The neuropathology of Alzheimer disease (AD) includes cerebral and cerebrovascular amyloid deposits composed of A beta protein. Extracerebral deposits of A beta, identified immunohistochemically, have also been reported but without evidence for the presence of amyloid fibrils. Serum amyloid P component (SAP) is a normal plasma glycoprotein that binds to the fibrils in all types of amyloid deposit, and radiolabeled SAP is a specific, sensitive, quantitative diagnostic tracer for systemic amyloid deposits in vivo. However, we report that in 11 patients with probable or definite AD, conventional whole body planar scintigraphy after injection of I-123-SAP revealed no detectable localization of tracer within the brain or elsewhere. Significant amounts of systemic extracerebral A beta amyloid are thus probably not a feature of AD. Also, although plasma-derived SAP is always present in the cerebral and cerebrovascular amyloid lesions of AD, there was insufficient accumulation of injected labeled SAP to be detected by the present routine methodology.