S-functionalized cysteine:: Powerful ligands for the labelling of bioactive molecules with triaquatricarbonyltechnetium-99m(1+) ([99mTc(OH2)3(CO)3]+)

S-Alkylated cysteines are used as efficient tridentate N,O,S-donor-atom ligands for the fac-[M(CO)(3)](+) moiety (M = Tc-99m or Re). Reaction of (Et4N)(2)[ReBr3(CO)(3)] (3) with the model S-benzyl-L-cysteine (2) leads to the formation of [Re(2 ')(CO)(3)] (4) as the exclusive product (2 ' = C-terminal anion of 2). The tridentate nature of the alkylated cysteine in 4 was established by X-ray crystallography. Compound 2 reacts with [Tc-99m(OH2)(3)(CO)(3)](+) under mild conditions (10(-4) m, 50 degrees, 30 min) to afford [Tc-99m(2 ')(CO)(3)] (5) and represents, therefore, an efficient chelator for the labelling of biomolecules. L-Cysteine, S-alkylated with a 3-aminopropyl group (-> 7), was conjugated via a peptide coupling sequence with Co alpha-[alpha-(5,6-dimethyl-1H-benzimidazolyl)]-Co beta-cyanocobamic b-acid (6), the b-acid of cyanocob(III)alamin (vitamin B-12) (Scheme 3). More convenient was a one-pot procedure with a derivative of vitamin B-12 comprising a free amine group at the b-position. This amine 15 was treated with NHS (N-hydroxysuccinimide)-activated 1-iodoacetic acid 14 to introduce an I-substituent in vitamin B-12. Subsequent addition of unprotected L-cysteine resulted in nucleophilic displacement of the I-atom by the S-substituent, affording the vitamin B-12 alkylated cysteine fragment 17 (Scheme 4). The procedure was quantitative and did not require purification of intermediates. Both cobalamin-cysteine conjugates could be efficiently labelled with [Tc-99m(OH2)(3)(CO)(3)](+) (1) under conditions identical to those of the model complex 5. Biodistribution studies of the cobalamin conjugates in mice bearing B10-F16 melanoma tumors showed a tumor uptake of 8.1 +/- 0.6% and 4.4 +/- 0.5% injected dose per gram of tumor tissue after 4 h and 24 h, respectively (Table 1).