[99mTc(CO)3]+-(HE)3-ZIGF1R:4551, a new Affibody conjugate for visualization of insulin-like growth factor-1 receptor expression in malignant tumours

Radionuclide imaging of insulin-like growth factor type 1 receptor (IGF-1R) expression in tumours might be used for selection of patients who would benefit from IGF-1R-targeted therapy. We have previously shown the feasibility of IGF-1R imaging using the Affibody molecule In-111-DOTA-His(6)-Z(IGF1R:4551). The use of Tc-99m instead of In-111 should improve sensitivity and resolution of imaging, and reduce the dose burden to patients. We hypothesized that inclusion of a HEHEHE tag instead of a His(6) tag in Z(IGF1R:4551) would permit its convenient purification using IMAC, enable labelling with [Tc-99m(CO)(3)](+), and improve its biodistribution. Z(IGF1R:4551) was expressed with a HEHEHE tag in the N terminus. The resulting (HE)(3)-Z(IGF1R:4551) construct was labelled with [Tc-99m(CO)(3)](+). Targeting of IGF-1R-expressing cells using [Tc-99m(CO)(3)](+)-(HE)(3)-Z(IGF1R:4551) was evaluated in vitro and in vivo. (HE)(3)-Z(IGF1R:4551) was stably labelled with Tc-99m with preserved specific binding to IGF-1R-expressing DU-145 prostate cancer cells in vitro. In mice, [Tc-99m(CO)(3)](+)-(HE)(3)-Z(IGF1R:4551) accumulated in IGF-1R-expressing organs (pancreas, stomach, lung and salivary gland). [Tc-99m(CO)(3)](+)-(HE)(3)-Z(IGF1R:4551) demonstrated 3.6-fold lower accumulation in the liver and spleen than In-111-DOTA-Z(IGF1R:4551). In NMRI nu/nu mice with DU-145 prostate cancer xenografts, the tumour uptake was 1.32 +/- 0.11 %ID/g and the tumour-to-blood ratio was 4.4 +/- 0.3 at 8 h after injection. The xenografts were visualized using a gamma camera 6 h after injection. Tc-99m(CO)(3)](+)-(HE)(3)-Z(IGF1R:4551) is a promising candidate for visualization of IGF-1R expression in malignant tumours.