Bioconjugate Chem. 2007, 18, 1226−1240
1226
Reagents for Astatination of Biomolecules. 2. Conjugation of Anionic Boron Cage Pendant Groups to a Protein Provides a Method for Direct Labeling that is Stable to in Vivo Deastatination D. Scott Wilbur,*,† Ming-Kuan Chyan,† Donald K. Hamlin,† Robert L. Vessella,‡ Timothy J. Wedge,§ and M. Frederick Hawthorne§ Departments of Radiation Oncology and Urology, University of Washington, Seattle, Washington 98195, and Department of Chemistry and Biochemistry, University of California, Los Angeles, California. Received November 3, 2006; Revised Manuscript Received April 25, 2007
Cancer-targeting biomolecules labeled with 211At must be stable to in vivo deastatination, as control of the 211At distribution is critical due to the highly toxic nature of R-particle emission. Unfortunately, no astatinated aryl conjugates have shown in vivo stability toward deastatination when (relatively) rapidly metabolized proteins, such as monoclonal antibody Fab′ fragments, are labeled. As a means of increasing the in vivo stability of 211At-labeled proteins, we have been investigating antibody conjugates of boron cage moieties. In this investigation, protein-reactive derivatives containing a nido-carborane (2), a bis-nido-carborane derivative (Venus Flytrap Complex, 3), and four 2-nonahydro-closo-decaborate(2-) derivatives (4-7) were prepared and conjugated with an antibody Fab′ fragment such that subsequent astatination and in vivo tissue distributions could be obtained. To aid in determination of stability toward in vivo deastatination, the Fab′-borane conjugates were also labeled with 125I, and that material was coinjected with the 211At-labeled Fab′. For comparison, direct labeling of the Fab′ with 125I and 211At was conducted. Direct labeling with Na[125I]I and Chloramine-T gave an 89% radiochemical yield. However, direct labeling of the Fab′ with Na[211At]At and Chloramine-T resulted in a yield of
