No abstract

Six cobalamin-biotin conjugates have been prepared. The cobalamin-biotin conjugates were prepared to evaluate the effect that the location of attachment had on the binding with transcobalamin II (TCII), the cobalamin binding protein in plasma, and to evaluate their potential use for in vitro and in vivo applications. This study focused only on the effect of binding with TCII. To decrease the possibility of steric problems in binding of the cobalamin conjugates with TCII, and biotin's binding with streptavidin or avidin, moieties of 11-18 atoms in length were used as linkers. Four biotin conjugates were prepared which were attached to the corrin ring of the cobalamin molecule (on b-, c-, d-, and e-side chains). One conjugate was attached to the 5'-OH of the ribose moiety, and another conjugate was attached at the cobalt metal (in place of the cyanide moiety of cyanocobalamin). Competitive binding studies were conducted where various amounts of the cobalamin-biotin conjugates and their precursor cobalamin derivatives competed with [57Co]cyanocobalamin for binding of recombinant human TCII (rhTCII). Evaluation of cobalamin derivatives which were conjugated at the 5'-OH of ribose or the cobalt metal center indicated that conjugation at either of these positions had little effect on binding with rhTCII. However, conjugates where the attachment was made on the corrin ring substituents had a large variation in binding with rhTCII. Conjugates on the e-propionamide side chain had little effect (relative affinity was equal to or decreased less than a factor of 3) on binding with rhTCII, conjugates of the b-isomer had decreased binding (relative affinity decreased less than a factor of 10), conjugates of the d-propionamide had further decreased binding (relative affinity decreased between 44 and 69 times), and conjugates on the c-acetamide group had poor binding to rhTCII (relative affinity decreased between 295 and 1160 times). The significance of the side chains on the corrin ring in providing specificity and high-affinity binding with rhTCII is discussed.

An investigation has been conducted to assess the in vivo stability of a series of astatinated benzamides and astatinated nido-carborane compounds in mice. It was hypothesized that the higher bond strength of boron-astatine bonds in the nido-carboranes might provide increased stability toward in vivo deastatination. Four tri-n-butylstannylbenzamides were prepared for radiohalogenation and evaluation in vivo. Those compounds were N-propyl-4-(tri-n-butylstannyl)benzamide 1a, N-propyl-3-(tri-n-butylstannyl)benzamide 2a, ethyl 4-tri-n-butylstannylhippurate 3a, and 4-tri-n-butylstannyl-hippuric acid 4a. Seven mono-nido-carboranyl derivatives were prepared for radiohalogenation and in vivo evaluation. Four of the seven mono-carboranyl derivatives (5a, 6a, 7a, 13a) contained a 3-(nido-carboranyl)propionamide functionality, and the remaining compounds (8a, 8g, 10a) contained a 4-(nido-carboranyl)aniline functionality. Two additional derivatives (11a, 12a) were prepared that contained bis-(nido-carboranylmethyl)benzene moieties (also referred to as Venus flytrap complexes (VFCs). All benzamide and nido-carborane compounds underwent facile iodination and radiohalogenation, except a 4-(nido-carboranyl)aniline derivative, 8a. Iodination of 8a resulted in a mixture, of which the desired iodinated product was a minor component. Therefore, radiohalogenation was not attempted. It is believed that the mixture of products is due to the presence of a thiourea bond. Previous studies have shown that thiourea bonds can interfere with halogenation reactions. In vivo comparisons of the compounds were conducted by co-injection of dual labeled (125/131I and 211At) compounds. Tissue distribution data were obtained at 1 and 4 h postinjection of the radiolabeled compounds, as that was sufficient to determine if astatine was being released. Stability of the astatinated compound was assessed by the difference in concentration of radioiodine and astatine in lung and spleen. All of the benzamides were found to undergo rapid deastatination in vivo. The nido-carborane derivatives appeared to be slightly more stable to in vivo deastatination; however, they had long blood residence times. The surprising finding was that the VFC derivatives did not release 211At in vivo, even though they rapidly localized to liver. This finding provides encouragement that stable conjugates of 211At may be attained if appropriate modifications of the VFC can be made to redirect their excretion through the renal system.