Synthesis and evaluation of two uncharged 99mTc-labeled derivatives of thioflavin-T as potential tracer agents for fibrillar brain amyloid

Thioflavin-T is a fluorescent dye for in vitro detection of fibrillar amyloid beta, a protein found in the brain of patients suffering from Alzheimer's disease. We synthesized and biologically evaluated two uncharged Tc-99m-labeled derivatives of thioflavin-T. The precursors for labeling were synthesized by coupling an S,S'-bis-triphenylmethyl-N-tert-butoxycarbonyl bis-amino-bis-thiol tetradentate ligand via a propoxy spacer to 2-(4'-aminophenyl)-1,3-benzothiazole at the 6-position or the 2'-position. Deprotection and labeling with Tc-99m were done via a one-pot procedure (15% yield) after which the labeled compound was isolated by high performance liquid chromatography (I-Q. LC in combination with mass spectrometry [MS) was used for identity confirmation of the labeled compounds. Results of electrophoresis and log P determination supported the assumption that the radiolabeled compounds could cross the blood-brain barrier by passive diffusion. However, in normal mice both compounds showed a low brain uptake 2 min post injection. They were mainly excreted through the hepatobiliary system, with some accumulation in the stomach. Sixty minutes after intravenous injection, 37% of the Tc-99m-activity in the blood corresponded to the original compound. In view of the low brain uptake, it is concluded that the studied Tc-99m-labeled derivatives of thioflavin-T are not suitable as tracer agents for in vivo visualization of amyloid in brain.