The Use of a Non-Conventional Long-Lived Gallium Radioisotope (66)Ga Improves Imaging Contrast of EGFR Expression in Malignant Tumours Using DFO-ZEGFR:2377 Affibody Molecule

Epidermal growth factor receptor (EGFR) is overexpressed in many malignancies. EGFR-targeted therapy extends survival of patients with disseminated cancers. Radionuclide molecular imaging of EGFR expression would make EGFR-directed treatment more personalized and therefore more efficient. A previous study demonstrated that affibody molecule [(68)Ga]Ga-DFO-ZEGFR:2377 permits specific positron-emission tomography (PET) imaging of EGFR expression in xenografts at 3 h after injection. We anticipated that imaging at 24 h after injection would provide higher contrast, but this is prevented by the short half-life of (68)Ga (67.6 min). Here, we therefore tested the hypothesis that the use of the non-conventional long-lived positron emitter (66)Ga (T(1/2) = 9.49 h, β(+) = 56.5%) would permit imaging with higher contrast. (66)Ga was produced by the (66)Zn(p,n)(66)Ga nuclear reaction and DFO-ZEGFR:2377 was efficiently labelled with (66)Ga with preserved binding specificity in vitro and in vivo. At 24 h after injection, [(66)Ga]Ga-DFO-ZEGFR:2377 provided 3.9-fold higher tumor-to-blood ratio and 2.3-fold higher tumor-to-liver ratio than [(68)Ga]Ga-DFO-ZEGFR:2377 at 3 h after injection. At the same time point, [(66)Ga]Ga-DFO-ZEGFR:2377 provided 1.8-fold higher tumor-to-blood ratio, 3-fold higher tumor-to-liver ratio, 1.9-fold higher tumor-to-muscle ratio and 2.3-fold higher tumor-to-bone ratio than [(89)Zr]Zr-DFO-ZEGFR:2377. Biodistribution data were confirmed by whole body PET combined with magnetic resonance imaging (PET/MRI). The use of the positron emitter (66)Ga for labelling of DFO-ZEGFR:2377 permits PET imaging of EGFR expression at 24 h after injection and improves imaging contrast.