Development of a Dual-Isotope Imaging Technique for Simultaneous Quantification and Evaluation of Biodistribution of...

The tissue distribution of ¹²⁵I- and ¹¹¹In-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 ¹²⁵I- and ¹¹¹In-labeled antibodies and sequentially quantify their concentrations in tissues.

Following intravenous administration of ¹²⁵I- and ¹¹¹In-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.

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 ¹²⁵I radioactivity concentration in tumor showed elimination patterns similar to plasma, while the ¹¹¹In radioactivity concentration increased up to 72 h post-dose. In the neck region near the thyroid, the ¹²⁵I radioactivity concentration increased over time, whereas the ¹¹¹In was undetectable except at 1 h post-dose. Reasonable agreement was observed between in vivo and ex vivo measurements for tumors, although ¹¹¹In imaging had limited sensitivity for low-activity regions in the neck.

These results suggested that the XCam-CdTe measurement captured the typical pharmacokinetic characteristics of ¹²⁵I- and ¹¹¹In-labeled antibodies and that both ¹²⁵I and ¹¹¹In could be assessed simultaneously and over time in the same subject.