Figure 4. Schematic diagram of the cellular processing of radiolabeled IgG in the presence and absence of FcRn binding affinity. Both ¹²⁵I- and ¹¹¹In-DOTA-labeled antibodies enter the cell by pinocytosis. In the sorting endosome, FcRn-protected IgG is recycled back to the extracellular space, while non-protected IgG proceeds to lysosomal proteolytic degradation. The radiolabeled catabolite, ¹¹¹In-DOTA-lysine, is residualizing due to its polarity or charge and tends to accumulate within cells. In contrast, radioiodinated protein catabolites are rapidly effluxed from cells.