Site-Specific Photoaffinity Bioconjugation for the Creation of ⁸⁹Zr-Labeled Radioimmunoconjugates

Purpose: Site-specific approaches to bioconjugation produce well-defined and homogeneous immunoconjugates with potential for superior in vivo behavior compared to analogs synthesized using traditional, stochastic methods. The possibility of incorporating photoaffinity chemistry into a site-specific bioconjugation strategy is particularly enticing, as it could simplify and accelerate the preparation of homogeneous immunoconjugates for the clinic. In this investigation, we report the synthesis, in vitro characterization, and in vivo evaluation of a site-specifically modified, ⁸⁹Zr-labeled radioimmunoconjugate created via the reaction between an mAb and an Fc-binding protein bearing a photoactivatable 4-benzoylphenylalanine residue.

Procedures: A variant of the Fc-binding Z domain of protein A containing a photoactivatable, 4-benzoylphenylalanine residue - Z(35BPA) - was modified with desferrioxamine (DFO), combined with the A33 antigen-targeting mAb huA33, and irradiated with UV light. The resulting immunoconjugate - DFO^Z(35BPA)-huA33 - was purified and characterized via SDS-PAGE, MALDI-ToF mass spectrometry, surface plasmon resonance, and flow cytometry. The radiolabeling of DFO^Z(35BPA)-huA33 was optimized to produce [⁸⁹Zr]Zr-DFO^Z(35BPA)-huA33, and the immunoreactivity of the radioimmunoconjugate was determined with SW1222 human colorectal cancer cells. Finally, the in vivo performance of [⁸⁹Zr]Zr-DFO^Z(35BPA)-huA33 in mice bearing subcutaneous SW1222 xenografts was interrogated via PET imaging and biodistribution experiments and compared to that of a stochastically labeled control radioimmunoconjugate, [⁸⁹Zr]Zr-DFO-huA33.

Results: HuA33 was site-specifically modified with Z(35BPA)-DFO, producing an immunoconjugate with on average 1 DFO/mAb, high in vitro stability, and high affinity for its target. [⁸⁹Zr]Zr-DFO^Z(35BPA)-huA33 was synthesized in 95% radiochemical yield and exhibited a specific activity of 2 mCi/mg and an immunoreactive fraction of ~ 0.85. PET imaging and biodistribution experiments revealed that high concentrations of the radioimmunoconjugate accumulated in tumor tissue (i.e., ~ 40%ID/g at 120 h p.i.) but also that the Z(35BPA)-bearing immunoPET probe produced higher uptake in the liver, spleen, and kidneys than its stochastically modified cousin, [⁸⁹Zr]Zr-DFO-huA33.

Conclusions: Photoaffinity chemistry and an Fc-binding variant of the Z domain were successfully leveraged to create a novel site-specific strategy for the synthesis of radioimmunoconjugates. The probe synthesized using this method - DFO^Z(35BPA)-huA33 - was well-defined and homogeneous, and the resulting radioimmunoconjugate ([⁸⁹Zr]Zr-DFO^Z(35BPA)-huA33) boasted high specific activity, stability, and immunoreactivity. While the site-specifically modified radioimmunoconjugate produced high activity concentrations in tumor tissue, it also yielded higher uptake in healthy organs than a stochastically modified analog, suggesting that optimization of this system is necessary prior to clinical translation.