Deficits in iodine-labelled 3-quinuclidinyl benzilate binding in relation to cerebral blood flow in patients with Alzheimer's disease

Abstract

A loss of acetylcholine is one of the most consistent neurochemical findings in Alzheimer's disease (AD) post-mortem, but the debate concerning receptor abnormalities is unresolved. The aim of this investigation was to measure the density of acetylcholine muscarinic receptors in AD patients at various stages in the disease (N = 8) by synthesising a radio-iodinated version of quinuclidinyl benzilate QNB, a potent muscarinic antagonist. Deficits were identified by comparison with a control data set obtained from four elderly volunteers and then compared to the deficit in total functional activity in the same brain regions measured using the cerebral perfusion tracer technetium-99m hexamethylpropylene amine oxime. Iodine-123 (R, R)quinuclidinyl benzilate (QNB) was synthesised using a CuI assisted nucleophilic aromatic exchange reaction. 160 MBq of the radioligand (specific activity 400 Ci/mmol: dose 90 ng/kg) was administered to each subject. Diagnosis of AD was made using the CAMDEX and DSMIIIR criteria with a physical examination, full blood screen, CT and chest X-ray. All subjects were scanned at 21 h post injection on an SME810 emission tomograph. ¹²³I(R, R)QNB activity in the controls was found to be consistent with the known distribution of muscarinic receptors with no activity in the cerebellum and low activity in the thalamus. In the AD patients deficits in ¹²³I-QNB binding which exceeded the corresponding total functional regional perfusion deficit were not found in six of the patiens and were observed only in the two most severely affected patients, both of whom were untestable on the cognitive battery. This adds weight to the evidence that a major reduction in postsynaptic receptor density takes place only at a very late stage of AD.