Preparation of 99mTc-N2S2 conjugates of chrysamine G, potential probes for the beta-amyloid protein of Alzheimer's disease

Chrysamine G is known to bind in vitro to the β‐amyloid protein (Aβ 10–43) and to homogenates of several regions of brain of Alzheimer's patients. The purpose of this study was to develop a 99mTc‐labelled derivative of chrysamine G, in which the structural requirements of β‐amyloid affinity are preserved. Bis‐S‐trityl protected monoamide‐monoaminedithiol (MAMA‐Tr2, a chelating system used to incorporate 99mTc, was coupled with 2‐(chloroacetylamino)‐chrysamine G diethyl ester (4) yielding 2‐(MAMA‐Tr2‐acetylamino)‐chrysamine G diethyl ester (5‐Tr2). To prepare 4, 4,4′‐dinitro‐2‐biphenylamine was treated with chloroacetyl chloride to obtain 2‐(chloroacetylamino)‐4,4′‐dinitrobiphenyl, of which the nitro functions were reduced by catalytic hydrogenation. Diazotation of 2‐(chloroacetylamino)‐4,4′‐diaminobiphenyl, followed by coupling with ethyl salicylate provided 4 in an overall yield of 4.3%. Alkaline hydrolysis of 5‐Tr2 resulted in the monoethyl ester derivative (6‐Tr2) and diacid derivative (7‐Tr2). Detritylation and labelling with 99mTc was performed in the presence of Sn2+ and 99mTcO4− solution at pH 2–3 with heating. RP‐HPLC analysis showed one peak for both the diester derivative (99mTc‐5) and the diacid derivative (99mTc‐7), while two peaks (A and B) were present for the monoethyl ester derivative (99mTc‐6), probably isomers with respect to the ester position. The order of elution from RP‐HPLC reflected the lipophilicity of the 99mTc‐complexes as determined by partitioning between 1‐octanol and phosphate buffer pH 7.4 (log P: 99mTc‐5=2.15, 99mTc‐6A=1.79, 99mTc‐6B=1.93, 99mTc‐7=1.08). Copyright © 1999 John Wiley & Sons, Ltd.