In vitro residualization and binding of figitumumab radioimmunoconjugates

Objectives Residualizing synthons for mAb radioiodination have been shown to increase radionuclide accumulation in tumours. Figitumumab (F) developed by Pfizer is a human mAb directed against the IGF-1 receptor (IGF-1R). F is known to be internalized and undergo lysosomal degradation. Biodistribution studies of F radioiodinated with standard synthons identified a need for improved tumour retention. A series of residualizing synthons were conjugated to F to evaluate their impact on radioiodine retention in vitro.

Methods F was radioiodinated with 125-I by iododestannylation of SIB (non-residualizing control), SIPMB (negative-charged residualizing), or SGMIB (positive-charged residualizing) synthons followed by NHS-conjugation. ¹¹¹In-DPTA labeling was performed by standard methods. Internalization/residualization and saturation binding assays were established to evaluate cellular radionuclide retention and binding affinity of F-conjugates using IGF-1R overexpressing A431 cells.

Results Internalization studies demonstrated that ¹¹¹In-DTPA-F and ¹²⁵I-SIPMB-F provided the highest degree of residualization. At 24hrs post- internalization, cellular retention measured 54.6±1.2% and 43.5±0.6% of the initial intracellular activity. The balance of internalized activity was effluxed into the extracellular media. ¹²⁵I-SGMIB-F offered little residualizing benefit (15.4±0.8%) compared to control ¹²⁵I-SIB-F (7.5±1.1%). Kd values for all compounds ranged from 1.5 to 11.7 nM, similar to the reported binding affinity of native F.

Conclusions ¹¹¹In-DTPA- and ¹²⁵I-SIPMB-F had greater residualizing properties compared to SGMIB and SIB iodinated conjugates. Negatively charged residualizing synthons for iodine radiolabeling promote better intracellular retention of activity, comparable to radiometals, than those with a positive charge when applied to F radioiodination.