Preparation of Tc-99m-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 beta-amyloid protein (A beta 10-43) and to homogenates of several regions of brain of Alzheimer's patien ts. The purpose of this study was to develop a Tc-99m-labelled derivative o f chrysamine G, in which the structural requirements for beta-amyloid affin ity are preserved, Bis-S-trityl protected monoamide-monoaminedithiol (MAMA-Tr-2), a chelating system used to incorporate Tc-99m, was coupled with 2-(chloroacetylamino)-c hrysamine G diethyl ester (4) yielding 2-(MAMA-Tr-2-acetylamino)-chrysamine G diethyl ester (5-Tr-2) To prepare 4, 4,4'-dinitro-2-biphenylamine was tr eated with chloroacetyl chloride to obtain 2-(chloroacetylamino)-4,4'-dinit robiphenyl, of which the nitro functions were reduced by catalytic hydrogen ation. 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- Tr-2) and diacid derivative (7-Tr-2) Detritylation acid labelling with Tc-9 9m was performed in the presence of Sn2+ and (TcO4-)-Tc-99m solution at pH 2-3 with heating. RP-HPLC analysis showed one peak for both the diester der ivative (Tc-99m-5) and the diacid derivative (Tc-99m-7), while two peaks (A and B) were present for the monoethyl eater derivative (Tc-99m-6), probabl y isomers with respect to the ester position. The order of elution from RP- HPLC reflected the lipophilicity of the Tc-99m-complexes as determined by p artitioning between l-octanol and phosphate buffer pH 7.4 (log P: Tc-99m-5 = 2.15, Tc-99m-6(A) = 1.79, Tc-99m-6(B) = 1.93, Tc-99m-7 = 1.08).