6-Mercaptomethy6lpyridine-3-carboxylic acid (MEMNIC): a new reagnet for peptide labeling with Tc-ppm

摘要

As part of our ongoing research into the development of peptide based radiopharmaceuticals, we have explored 2-mercaptopyridines and 2-mercapto-pyrimidines (2MPs) as coligands to control the radiochemical speciation of ~(99m)Tc-babeled hydrazinonicotinamide (HYNIC) derivatized chemotactic peptides. In an attempt to develop an animothiol ligand with greater stability, we synthesized 6-mercaptomethylpyriine-3-carboxylic acid (MEMNIC), assuming a five-member chelate ring complex would be more stable than the four-membered ring possible with 2MPs. Initial experiments suggested that ~(99m)Tc-MEMNIC was stable in vivo and led to our investigation of it as a bifunctional chelator for ~(99m)Tc labeling of peptides. the utility of MEMNIC was tested in a rabbit model of infection. The N-epsilon MEMNIC derivative of FOR-MLFK was prepared using hte N-hydroxysuccinimidyl ester of MEMNIC. For-MLFK-MEMNIC was labeled via ~(99m)Tc-mannitol and its imaging and biodistribution properties compared to Tc-For-MLFK-HYNICa and Tc-For-MLFCys 9labeled via the free thiol of cysteine). Sites of infection were produced in New Zealand white rabbits (n=6/peptide) by injection of a suspension of Eschericahia coli in a positerior thigh. Twenty-four hours after infection, the rabbits were injected intravenously with 0.5 mCi of HPLC purified ~(99m)Tc-peptide and imaged at 2.5 and 18 h p.i. The animals were sacrificed at 18 h and tissue activity determined. At 2.5 and 18 h, the orgen distribution of Tc-For-MLFK-HYNIC and Tc-For-MLFK-MEMNIC were qualitatively similar with the exception of higher renal accumulation of the latter, whereas Tc-For-MLFKCys showed rapid and prolonged accumulation in bowel. ROI analysis renal accumulation of the latter, whereas Tc-For-MLFKCys showed rapid and prolonged accumulation in bowel. ROI analysis gave target/background rations of 3.91+-0.32 and 11.89+2.21 for Yc-For-MLFK-HYNIC, 5.09+-0.66 and 9.88+-1.12 for Tc-For-MLFK-MEMNIC and 2.59+-0.16 and 1.70+-0.37 for Tc-For-MLFCys, at 2.5 and 18 h. Tissue radioactivity measurements (18 h) demonstrated that compared to Tc-For-MLFK-HYNIC, the accumulation of Tc-For-MLFK-MEMNIC was less in all organs except in kidney which was greater. Direct labeling of the thiol group of cysteine led to lower accumulation in all organs compared to Tc-For-MLFK-MEMNIC, except in GI tract which was fivefold higher. While, Tc-For-MLFK-MEMNIC had half the accumulation of Tc-For-MLFK-MEMNIC in infected muscle and pus, the lower accumulation in most organs and normal muscle resulted in good infection localization. these results indicate that MEMNIC can be successfully used to label peptides with ~`(99m)Tc while exhibiting good in vivo stability. The chemistry of MEMNIC with the {M(V)O}~(3+) core characteristic of technetiun and its Grou V congener rhenium was modeled by investigating the reactions of 2-mercaptomethylpyridine with apropriate Re(V)-oxo precursors in the presence of a variety of coligands. With diolate type tridentate donors of the class {X(CH_2CH_2O)_2}~92-) (X=-NR, O, S), the '3+2' compounds [ReO(#eta#~3-(OCH_2CH_2)_2S} {#eta#~1-SCH_2C_5H_4N}] (2) and [ReO{#eta#~3-(OCH_2CH_2)_2NCH_3} {#eta#~2-SCH_2C_5H_4n] (3) were isolated. In contrast, with the more sterically demanding dirmercapto class of triduentate ligands {S(CH_2CH_2S)_2}~(2-), THE '3+1' SPECIES [ReO(#eta#~3-(SCH_2CH_2)_2S} {#eta#~1-SCH_2C_5H_4N}] (1) was isolated.