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

The neuropathology of Alzheimer disease (AD) includes cerebral and cerebrov ascular amyloid deposits composed of A beta protein. Extracerebral deposits of A beta, identified immunohistochemically, have also been reported but w ithout evidence for the presence of amyloid fibrils. Serum amyloid P compon ent (SAP) is a normal plasma glycoprotein that binds to the fibrils in all types of amyloid deposit, and radiolabeled SAP is a specific, sensitive, qu antitative 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 det ectable localization of tracer within the brain or elsewhere. Significant a mounts of systemic extracerebral A beta amyloid are thus probably not a fea ture of AD. Also, although plasma-derived SAP is always present in the cere bral and cerebrovascular amyloid lesions of AD, there was insufficient accu mulation of injected labeled SAP to be detected by the present routine meth odology.