The effect of chelator charge on the biodistribution of engineered antibodies labeled with ⁶⁴Cu

Objectives Previous studies in our laboratory showed that, while the sarcophagine-based bifunctional chelator (BFC) SarAr forms extremely stable complexes with ⁶⁴Cu(II), its use with an engineered antibody (ch14.18.ΔC_H2, MW 100 kDa) leads to high renal uptake. The objective of this project was to determine if the high renal uptake was due to the positive charge of the Sar BFC, and to compare the Sar BFCs with a series of polyaminocarboxylate (PAC) BFCs.

Methods The antibody was conjugated with a series of Sar derivatives of decreasing positive charge and three PAC BFCs (Table). The antibody conjugates were labeled with ⁶⁴Cu, injected into mice (n= 3-5), and PET/CT images were obtained at 1, 24 and 48 h post injection (p.i.). At 48 hours p.i. tissues were excised and the percent injected dose per gram (%ID/g) was calculated.

Results The renal uptake of the antibody labeled using SarAr was remarkably high at 40 %ID/g, presumably because the increased local positive charge from the Cu(II) and the ionizable functional groups resulted in increased interaction with the negatively charged basal cells of the glomerulus. Consistent with our hypothesis, reducing the positive charge on the Sar chelator decreased kidney uptake by more than 6-fold, while uptake in most other tissues, including liver, was relatively unchanged (Table). Renal and liver uptake were also low when the antibody was labeling using the PAC BFCs.

Conclusions Decreasing the positive charge of a series of Sar-based BFCs significantly reduced renal uptake of an engineered antibody labelled with ⁶⁴Cu.

Research Support Children’s Hospital Radiology Foundation, the Australian Research Council, Victoria Fellowship (Aus) to BMP.