Development of a Dual-Isotope Imaging Technique for Simultaneous Quantification and Evaluation of Biodistribution of Antibodies Using "XCam-CdTe".

PURPOSE: The tissue distribution of I- and In-labeled antibodies in a single animal remains challenging to assess simultaneously and sequentially using conventional methods. This study developed a method using "XCam-CdTe" equipped with cadmium telluride double-sided strip detector to simultaneously administer I- and In-labeled antibodies and sequentially quantify their concentrations in tissues. METHODS: Following intravenous administration of I- and In-labeled anti-human interleukin-6 receptor (hIL-6R) antibodies to hIL-6R-Hepa1-6 tumor-bearing syngeneic mice, radioactivity concentrations in blood and tissues including tumors were simultaneously evaluated using XCam-CdTe for up to 72 h. After final imaging, tissues were collected for ex vivo radioactivity measurement using XCam-CdTe. RESULTS: The plasma concentration-time profiles of both labeled antibodies up to 72 h post-dose were consistent between radionuclides and comparable to typical antibodies. The I radioactivity concentration in tumor showed elimination patterns similar to plasma, while the In radioactivity concentration increased up to 72 h post-dose. In the neck region near the thyroid, the I radioactivity concentration increased over time, whereas the In was undetectable except at 1 h post-dose. Reasonable agreement was observed between in vivo and ex vivo measurements for tumors, although In imaging had limited sensitivity for low-activity regions in the neck. CONCLUSIONS: These results suggested that the XCam-CdTe measurement captured the typical pharmacokinetic characteristics of I- and In-labeled antibodies and that both I and In could be assessed simultaneously and over time in the same subject.