Purpose: To assess the application of succinimidyl iodobenzoate (SIB) iodination method in labeling biotherapeutics to study their pharmacokinetics (PK) and biodistribution.
Method: An IgG molecule (protein-01) and a 40 KDa protein (protein-02) were evaluated. Pharmacokinetics (PK) and biodistribution of the radiolabeled IgG ((125)I-protein-01) in mice compared parameters from SIB and Iodogen protein iodination labeling methods. In addition, PK of radiolabeled 40 KDa protein ((125)I-protein-02) using SIB was compared with non-labeled protein-02 analyzed by ligand binding assay (LBA).
Results: Up to 72 h following a single IP injection to mice, the percentage of "free-label" determined by the soluble counts after TCA precipitation to total radioactivity in serum samples was 2.8-49.4% vs. <1.0% for (125)I-protein-01 iodinated via Iodogen or SIB methods, respectively, suggesting a higher in vivo stability of (125)I-protein-01 labeled via the SIB method. The serum exposure of (125)I-protein-01 was two-fold higher, and correspondingly, the tissue exposure was also higher (averaging 3.6 fold at 168 h) when using SIB protein labeling method than when using the Iodogen method. In addition, following subcutaneous (SC) administration to mice, the serum exposure of (125)I-protein-02 labeled via SIB method was similar to protein-02 measure by LBA.
Conclusion: In this study, iodination of proteins using SIB methodology has overcome the dehalogenation problem in vivo which is inherent in Iodogen method, and PK parameters of a protein iodinated via SIB were comparable to the un-labeled protein measured by LBA. The SIB iodination method is an improved labeling approach for biotherapeutics used in studying PK and biodistribution characteristics.