Conjugation of a novel histidine derivative to biomolecules and labelling with [99mTc(OH2)3(CO)3]+

Abstract

The new histidine derivative 3-{1-[3-(9H-fluoren-9-ylmethoxycarbonylamino)-propyl]-1H-imidazol-4-yl}-2-(3-trimethylsilanyl-ethylcarboxyamino)-propionic acid methyl ester (7) has been prepared via alkylation of the histidine urea derivative (7S)-5,6,7,8-tetrahydro-7-(methoxycarbonyl)-5-oxoimidazo-[1,5-c]-pyrimidine (2) with Fmoc-protected 3-iodopropyl-amine, followed by ring opening with 2-trimethylsilylethanol. After Fmoc cleavage by HNEt₂, the histidine amine derivative was coupled to biotin, to the pentapeptide leucine-enkephalin and to Vitamin B12-b-acid by amide formation, employing TBTU as the coupling reagent. In order to make the histidine accessible for labelling, the teoc protecting group was removed by either NBu₄F (for the biotin conjugate) or by TFA (for the enkephalin and B12 conjugates). Reaction of a 10⁻⁴ M solution of the bioconjugates with [^99mTc(H₂O)₃(CO)₃]⁺ at 50 °C for 30 min led to the formation of one single new peak in the HPLC radiochromatogram in each case, confirming quantitative labelling of the respective biomolecules. To assess the nature of the labelled compounds, the rhenium analogues with Re(CO)₃ were also synthesised and similar retention times confirmed the identity with the ^99mTc labelled conjugates.