Improvement of tumor-to-blood ratio of radioimmunoconjugates by poly(ethyleneimine)-containing chelating agent.

OBJECTIVE

Monoclonal antibody (mAb)-based radioimmunoconjugates (RICs) exhibit marked tumor uptake in cancer imaging and therapy, although their high blood retention has limited the development of RICs. In our previous study, a trifunctional chelating agent with a cationic poly(ethyleneimine) (PEI) structure of tetraethylenepentamine (PEI4), maleimide-DOTA-PEI4 (MDI4), improved the tumor-to-blood ratio of RICs by increasing tumor retention compared with a conventional bifunctional chelating agent. In this study, we developed a novel chelating agent composed of a maleimide moiety, DOTA derivative, and two PEI4 structures as a PEI4-2 unit, maleimide-DOTA-PEI4-2 (MDI4-2), a design for a highly cationized chelating agent to synthesize RICs. The properties of MDI4-2 were compared with MDI4 to evaluate the effect of the PEI4-2 unit on the pharmacokinetics of RICs.

METHODS

Trastuzumab and In were selected as a model mAb and radiometal, respectively. Trastuzumab-based RICs were synthesized using MDI4-2 by two-step radiolabeling, wherein conjugation with mAbs is followed by radiolabeling of chelating agents, to obtain trastuzumab-[In]In-MDI4-2 ([In]In-TMDI4-2). The immunoreactivity and residualizing properties of [In]In-TMDI4-2 were evaluated using human epidermal growth factor receptor 2 (HER2)/neu-expressing SK-OV-3 cells. A biodistribution assay using SK-OV-3 tumor-bearing mice was also performed for [In]In-TMDI4-2 and the results were compared with trastuzumab-[In]In-MDI4 ([In]In-TMDI4).

RESULTS

[In]In-TMDI4-2 was successfully synthesized by two-step radiolabeling at a radiochemical yield of 37.7%. The immunoreactivity of [In]In-TMDI4-2 was determined as 81.7%, suggesting the maintained binding ability through radiolabeling steps. The internalization assay revealed equivalent internalizing properties of [In]In-TMDI4-2 to [In]In-TMDI4. In the biodistribution assay, [In]In-TMDI4-2 exhibited lower blood retention of radioactivity to and comparable tumor uptake with [In]In-TMDI4, resulting in an improved tumor-to-blood ratio. These in vitro and in vivo results indicate that the PEI4-2 unit largely contributed to the decrease in the blood radioactivity of RICs without compromising the tumor uptake.

CONCLUSION

MDI4-2 with the PEI4-2 unit exhibited favorable properties for designing RICs with an improved tumor-to-blood ratio.