Abstract
PurposeCurrently, the most commonly used chelator for labelling antibodies with 89Zr for immunoPET is desferrioxamine B (DFO). However, preclinical studies have shown that the limited in vivo stability of the 89Zr-DFO complex results in release of 89Zr, which accumulates in mineral bone. Here we report a novel chelator DFOcyclo*, a preorganized extended DFO derivative that enables octacoordination of the 89Zr radiometal. The aim was to compare the in vitro and in vivo stability of [89Zr]Zr-DFOcyclo*, [89Zr]Zr-DFO* and [89Zr]Zr-DFO.MethodsThe stability of 89Zr-labelled chelators alone and after conjugation to trastuzumab was evaluated in human plasma and PBS, and in the presence of excess EDTA or DFO. The immunoreactive fraction, IC50, and internalization rate of the conjugates were evaluated using HER2-expressing SKOV-3 cells. The in vivo distribution was investigated in mice with subcutaneous HER2+ SKOV-3 or HER2− MDA-MB-231 xenografts by PET/CT imaging and quantitative ex vivo tissue analyses 7 days after injection.Results89Zr-labelled DFO, DFO* and DFOcyclo* were stable in human plasma for up to 7 days. In competition with EDTA, DFO* and DFOcyclo* showed higher stability than DFO. In competition with excess DFO, DFOcyclo*-trastuzumab was significantly more stable than the corresponding DFO and DFO* conjugates (p < 0.001). Cell binding and internalization were similar for the three conjugates. In in vivo studies, HER2+ SKOV-3 tumour-bearing mice showed significantly lower bone uptake (p < 0.001) 168 h after injection with [89Zr]Zr-DFOcyclo*-trastuzumab (femur 1.5 ± 0.3%ID/g, knee 2.1 ± 0.4%ID/g) or [89Zr]Zr-DFO*-trastuzumab (femur 2.0 ± 0.3%ID/g, knee 2.68 ± 0.4%ID/g) than after injection with [89Zr]Zr-DFO-trastuzumab (femur 4.5 ± 0.6%ID/g, knee 7.8 ± 0.6%ID/g). Blood levels, tumour uptake and uptake in other organs were not significantly different at 168 h after injection. HER2− MDA-MB-231 tumour-bearing mice showed significantly lower tumour uptake (p < 0.001) after injection with [89Zr]Zr-DFOcyclo*-trastuzumab (16.2 ± 10.1%ID/g) and [89Zr]Zr-DFO-trastuzumab (19.6 ± 3.2%ID/g) than HER2+ SKOV-3 tumour-bearing mice (72.1 ± 14.6%ID/g and 93.1 ± 20.9%ID/g, respectively), while bone uptake was similar.Conclusion89Zr-labelled DFOcyclo* and DFOcyclo*-trastuzumab showed higher in vitro and in vivo stability than the current commonly used 89Zr-DFO-trastuzumab. DFOcyclo* is a promising candidate to become the new clinically used standard chelator for 89Zr immunoPET.
Highlights
89Zr has shown great potential as a radionuclide for immunoPET imaging with monoclonal antibodies. 89Zr has favourable characteristics: its half-life of 78.4 h matches the kinetics of IgG molecules, it is a residualizing radionuclide that provides images with high target to background ratios [1], and high PET image resolution is obtained because of the relatively low-energy positrons (Emean = 395 keV), which is not affected by its γ-emission at 909 keV
Molecules, it is a residualizing radionuclide that provides images with high target to background ratios [1], and high PET image resolution is obtained because of the relatively low-energy positrons (Emean = 395 keV), which is not affected by its γ-emission at 909 keV. Because of these advantages it is frequently used in preference to other long-lived positron emitters, such as 124I. Despite these favourable characteristics for immunoPET, the use of 89Zr has a major drawback when used in combination with the clinically approved chelator currently used for antibody labelling, desferrioxamine B (DFO)
While open-chain hydroxamic acids, including DFO, are known to exist as a mixture of two E and Z isomers [28,29,30], the Z configuration of the hydroxamate group is imparted by the cyclic structure of PIPO, and is obviously more favourable for Zr4+ coordination and could result in a more stable complex [31]
Summary
Because of these advantages it is frequently used in preference to other long-lived positron emitters, such as 124I. This can result in the release of 89Zr, which may subsequently accumulate in mineral bone [5, 7]. In the absence of an improved, clinically applicable chelator, there is room for more efficient [89Zr]Zr4+ chelators
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