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 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.