Synthesis and nca-Radioiodination of Arylstannyl−Cobalamin Conjugates. Evaluation of Aryliodo−Cobalamin Conjugate Binding to Transcobalamin II and Biodistribution in Mice

Abstract

A new method of preparing radiolabeled cobalamin derivatives has been developed. The method involves the use of cobalamin-tri-n-butylstannyl hippurate conjugates as intermediates to obtain radioiodinated cobalamin-iodohippurate conjugates. The arylstannyl functionality was used as an exchangeable group to obtain high specific activity radioiodinations and to circumvent some deleterious side reactions common to cobalamins under electrophilic iodination conditions. The first step in the synthesis of tri-n--butylstannyl hippurate conjugates was to obtain free carboxylate groups on the cobalamin moiety. This was accomplished by mild acid hydrolysis of the b-, d-, or e-propionamide side chains on the corrin ring, followed by careful separation of the isomeric products. The second step was to couple a linking molecule (diaminododecane) to the carboxylate. The final step was to conjugate p-tri-n-butylstannyl hippurate to the cobalamin-diaminododecane adduct. All three isomeric cobalamin-p-tri-n-butylstannyl hippurate conjugates were prepared, as were the corresponding cobalamin-p-iodohippurate conjugates (HPLC standards). Radioiodination reactions were conducted with N-chlorosuccinimide and Na[*I]I in Me OH using conditions previously developed for arylstannylations. However, unlike the previous reactions, a key factor in obtaining the desired radioiodinated cobalamins was that the reaction be conducted under neutral conditions. Isolated yields of 40-65% were obtained for all three cobalamin isomers. Specific activities of 10-33% theoretical were obtained for the radioiodinated cobalamins. Evaluation of competitive binding of (nonradioactive) cobalamin-iodohippurate conjugates with recombinant human transcobalamin II showed that the e-isomer bound nearly as well as [57Co]cyanocobalamin (50%), whereas the b-isomer had decreased binding (6%) and the d-isomer was significantly decreased in its binding (0.7%). Two biodistributions of the radioiodinated e-isomer were conducted in athymic mice. One biodistribution investigated tissue localization in mice bearing a renal cell carcinoma xenograft, and the other biodistribution investigated tissue localization when the radioiodinated cyanocobalamin was mixed with 1% BSA prior to injection. A comparison of the results of the two biodistributions and a discussion of how they relate to previous [57/60Co]cyanocobalamin biodistributions are provided.

Vitamin-mediated targeting as a potential mechanism to increase drug uptake by tumours

“…Others tissues labeled included the kidney, gastric mucosa, reproductive and some endocrine organs. These studies are supported by the work of Wilbur and co-workers [27] who used radioiodinated arylstannylcobalamin conjugates and showed enhance uptake into renal carcinomas in nude mice when compared with muscle, intestines, spleen and lung. This work was supported by Warnock and colleagues [28] who observed increased uptake of 57 Co-VB 12 in sarcomas in mice when compared with gallium-67 and Thallium-201.…”

Equilibrium and Kinetic Analyses of the Interactions between Vitamin B12 Binding Proteins and Cobalamins by Surface Plasmon Resonance

“…Ligands 1 and 2 were also included in the study, as they are models of cobalamin analogs that have potential therapeutic use. The b-propionamide of Cbl provides a site for conjugation to cellular growth blockers (26,33), imaging agents (24, 34 -36), and chemotherapeutic drugs (37). Most of the K D values determined by the solution competition binding assay are the same order of magnitude for rhTCII, IF, and NIF binding.…”

Vitamin B12 as a Ligand for Technetium and Rhenium Complexes

“…[7][8][9] Both of these strategies require derivatization of B12, and so the introduction of ligands or receptor-binding molecules has been reported. [5,6,9,10] Chelators for radiometals have been conjugated to peripheral acid groups (prepared by controlled amide hydrolysis) and coordinated to the 5'-OH group in the ribose ring of the backloop. Alternatively, a Co III center can be reduced to Co I and a molecule introduced by oxidative alkylation.…”