Head-to-head comparison of DFO* and DFO chelators: selection of the best candidate for clinical Zr-immuno-PET.

PURPOSE

Almost all radiolabellings of antibodies with Zr currently employ the hexadentate chelator desferrioxamine (DFO). However, DFO can lead to unwanted uptake of Zr in bones due to instability of the resulting metal complex. DFO*-NCS and the squaramide ester of DFO, DFOSq, are novel analogues that gave more stable Zr complexes than DFO in pilot experiments. Here, we directly compare these linker-chelator systems to identify optimal immuno-PET reagents.

METHODS

Cetuximab, trastuzumab and B12 (non-binding control antibody) were labelled with Zr via DFO*-NCS, DFOSq, DFO-NCS or DFOSq. Stability in vitro was compared at 37 °C in serum (7 days), in formulation solution (24 h ± chelator challenges) and in vivo with N87 and A431 tumour-bearing mice. Finally, to demonstrate the practical benefit of more stable complexation for the accurate detection of bone metastases, [Zr]Zr-DFO-NCS and [Zr]Zr-DFO-NCS-labelled trastuzumab and B12 were evaluated in a bone metastasis mouse model where BT-474 breast cancer cells were injected intratibially.

RESULTS

[Zr]Zr-DFO*-NCS-trastuzumab and [Zr]Zr-DFOSq-trastuzumab showed excellent stability in vitro, superior to their [Zr]Zr-DFO counterparts under all conditions. While tumour uptake was similar for all conjugates, bone uptake was lower for DFO conjugates. Lower bone uptake for DFO* conjugates was confirmed using a second xenograft model: A431 combined with cetuximab. Finally, in the intratibial BT-474 bone metastasis model, the DFO* conjugates provided superior detection of tumour-specific signal over the DFO conjugates.

CONCLUSION

DFO*-mAb conjugates provide lower bone uptake than their DFO analogues; thus, DFO* is a superior candidate for preclinical and clinical Zr-immuno-PET.