Reagents for astatination of biomolecules. 4. Comparison of maleimido-closo-decaborate(2-) and meta-[²¹¹At]astatobenzoate conjugates for labeling anti-CD45 antibodies with [²¹¹At]astatine

An investigation was conducted to compare the in vivo tissue distribution of a rat antimurine CD45 monoclonal antibody (30F11) and an irrelevant mAbs (CA12.10C12) labeled with ²¹¹At using two different labeling methods. In the investigation, the mAbs were also labeled with ¹²⁵I to assess the in vivo stability of the labeling methods toward deastatination. One labeling method employed N-hydroxysuccinimidyl meta-[²¹¹At] astatobenzoate, [²¹¹At]1c, and N-hydroxysuccinimidyl meta-[ ¹²⁵I]iodobenzoate, [¹²⁵I]1b, in conjugation reactions to obtain the radiolabeled mAbs. The other labeling method involved conjugation of a maleimido-closo-decaborate(2-) derivative, 2, with sulfhydryl groups on the mAbs, followed by labeling of the mAb-2 conjugates using Na[²¹¹At]At or Na[¹²⁵I]I and chloramine-T. Concentrations of the ²¹¹At/¹²⁵I pair of radiolabeled mAbs in selected tissues were examined in BALB/c mice at 1, 4, and 24 h post injection (pi). The co-injected anti-CD45 mAb, 30F11, labeled with [¹²⁵I]1b and [ ²¹¹At]1c targeted the CD45-bearing cells in the spleen with the percent injected dose (%ID) of ¹²⁵I in that tissue being 13.31 ± 0.78; 17.43 ± 2.56; 5.23 ± 0.50; and ²¹¹At being 6.56 ± 0.40; 10.14 ± 1.49; 7.52 ± 0.79 at 1, 4, and 24 h pi (respectively). However, better targeting (or retention) of the ¹²⁵I and ²¹¹At was obtained for 30F11 conjugated with the closo-decaborate(2-), 2. The %ID in the spleen of ¹²⁵I (i.e., [ ¹²⁵I]30F11-2) being 21.15 ± 1.33; 22.22 ± 1.95; 12.41 ± 0.75; and ²¹¹At (i.e., [²¹¹At]30F11-2) being 22.78 ± 1.29; 25.05 ± 2.35; 17.30 ± 1.20 at 1, 4, and 24 h pi (respectively). In contrast, the irrelevant mAb, CA12.10C12, labeled with ¹²⁵I or ²¹¹At by either method had less than 0.8% ID in the spleen at any time point, except for [²¹¹At]CA12.10C12-1c, which had 1.62 ± 0.14%ID and 1.21 ± 0.08%ID at 1 and 4 h pi. The higher spleen concentrations in that conjugate appear to be due to in vivo deastatination. Differences in ¹²⁵I and ²¹¹At concentrations in lung, neck, and stomach indicate that the meta-[ ²¹¹At]benzoyl conjugates underwent deastatination, whereas the ²¹¹At-labeled closo-decaborate(2-) conjugates were very stable to in vivo deastatination. In summary, using the closo-decaborate(2-) ²¹¹At labeling approach resulted in higher concentrations of ²¹¹At in target tissue (spleen) and higher stability to in vivo deastatination in this model. These findings, along with the simpler and higher-yielding ²¹¹At-labeling method, provide the basis for using the closo-decaborate(2-) labeling reagent, 2, in our continued studies of the application of ²¹¹At-labeled mAbs for conditioning in hematopoietic cell transplantation.