Reagents for Astatination of Biomolecules. 6. An Intact Antibody Conjugated with a Maleimido-closo-Decaborate(2-) Reagent via Sulfhydryl Groups Had Considerably Higher Kidney Concentrations than the Same Antibody Conjugated with an Isothiocyanato-closo-Decaborate(2-) Reagent via Lysine Amines

We are investigating the use of an ²¹¹At-labeled anti-CD45 monoclonal antibody (mAb) as a replacement of total body irradiation in conditioning regimens designed to decrease the toxicity of hematopoietic cell transplantation (HCT). As part of that investigation, dose-escalation studies were conducted in dogs using ²¹¹At-labeled anticanine CD45 mAb, CA12.10C12, conjugated with a maleimido-closo-decaborate­(2-) derivative, 4. Unacceptable renal toxicity was noted in the dogs receiving doses in the 0.27–0.62 mCi/kg range. This result was not anticipated, as no toxicity had been noted in prior biodistribution and toxicity studies conducted in mice. Studies were conducted to understand the cause of the renal toxicity and to find a way to circumvent it. A dog biodistribution study was conducted with ¹²³I-labeled CA12.10C12 that had been conjugated with 4. The biodistribution data showed that 10-fold higher kidney concentrations were obtained with the maleimido-conjugate than had been obtained in a previous biodistribution study with ¹²³I-labeled CA12.10C12 conjugated with an amine-reactive phenylisothiocyanato-CHX-A″ derivative. The difference in kidney concentrations observed in dogs for the two conjugation approaches led to an investigation of the reagents. SE-HPLC analyses showed that the purity of the CA12.10C12 conjugated via reduced disulfides was lower than that obtained with amine-reactive conjugation reagents, and nonreducing SDS-PAGE analyses indicated protein fragments were present in the disulfide reduced conjugate. Although we had previously prepared closo-decaborate­(2-) derivatives with amine-reactive functional groups (e.g., 6 and 8), a new, easily synthesized, amine-reactive (phenylisothiocyanate) derivative, 10, was prepared for use in the current studies. A biodistribution was conducted with coadministered ¹²⁵I- and ²¹¹At-labeled CA12.10C10 conjugated with 10. In that study, lower kidney concentrations were obtained for both radionuclides than had been obtained in the earlier study of the same antibody conjugated with 4 after reduction of disulfide bonds.