SYNTHESIS AND NCA-RADIOIODINATION OF ARYLSTANNYL-COBALAMIN CONJUGATES- EVALUATION OF ARYLIODO-COBALAMIN CONJUGATE BINDING TO TRANSCOBALAMIN-II AND BIODISTRIBUTION IN MICE

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 cobala min-iodohippurate conjugates. The arylstannyl functionality was used a s an exchangeable group to obtain high specific activity radioiodinati ons and to circumvent some deleterious side reactions common to cobala mins under electrophilic iodination conditions. The first step in the synthesis of tri-n-butylstannyl hippurate conjugates was to obtain fre e 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 produc ts. The second step was to couple a linking molecule (diaminododecane) to the carboxylate. The final step was to conjugate p-tri-n-butylstan nyl hippurate to the cobalamin-diaminododecane adduct. All three isome ric 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-chlorosucc inimide and Na[I]I in MeOH using conditions previously developed for arylstannylations. However, unlike the previous reactions, a key facto r in obtaining the desired radioiodinated cobalamins was that the reac tion 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. Ev aluation of competitive binding of(nonradioactive) cobalamin-iodohippu rate conjugates with recombinant human transcobalamin II showed that t he e-isomer bound nearly as well as [Co-57]cyanocobalamin (50%), where as the b-isomer had decreased binding (6%) and the d-isomer was signif icantly decreased in its binding (0.7%). Two biodistributions of the r adioiodinated e-isomer were conducted in athymic mice. One biodistribu tion investigated tissue localization in mice bearing a renal cell. ca rcinoma xenograft, and the other biodistribution investigated tissue l ocalization when the radioiodinated cyanocobalamin was mixed with 1% B SA prior to injection. A comparison of the results of the two biodistr ibutions and a discussion of how they relate to previous [Co-57/60]cya nocobalamin biodistributions are provided.