Development of the G3 designed ankyrin repeat protein (DARPin) for HER2 imaging

Background: Human epidermal growth factor receptor-2 (HER2) expression predicts response to anti-HER2 therapy in breast and gastric cancer. HER2 status is assessed by tumour biopsy but this may not be representative of the larger tumour mass or other metastatic sites; risking misclassification and selection of suboptimal therapy. The G3 designed ankyrin repeat protein (DARPin) binds HER2 with high affinity at an epitope that does not overlap with trastuzumab and is biologically inert. This research aims to assess the pre-clinical efficacy and safety of G3 DARPin HER2 PET and SPECT imaging. Methods: Hexahistidine (His6), histidine-glutamate (HE)3 and untagged-G3 DARPins were manufactured using a GMP-compatible Pichia pastoris protocol and radiolabelled with 125I or site-specifically with 111In. BALB/c mice were injected with radiolabelled-G3 DARPins and biodistribution was evaluated. The lead construct, (HE)3-G3 was radiolabelled with 111In, 125I or 68Ga for assessment in mice bearing HER2-positive human breast tumour (BT474) xenografts. Mice received (HE)3-G3 at 50-100 times the human equivalent dose to assess acute toxicity. Results: (HE)3-G3 had significantly lower liver uptake than His6-G3 and untagged-G3 in non-tumour bearing mice when radiolabelled with 125I or 111In. In mice bearing HER2-positive tumour xenografts, 111In-(HE)3-G3 was better maintained in tumours and cleared faster from serum than 125I-(HE)3-G3, achieving superior tumour-to-blood ratios of 343.7±161.3 vs. 22.0±11.3 at 24 h, respectively. On microSPECT/CT 111In and 125I labelled (HE)3-G3, imaged HER2-positive tumours at 4 h post-administration but 111In-(HE)3-G3 had less non-specific uptake. 68Ga-(HE)3-G3 can image HER2-positive tumours at up to 2 h post-administration by PET scanning. (HE)3-G3 DARPin was well tolerated by mice treated at 50-100 times the human equivalent dose over 24 h. Conclusion: Radiolabelled (HE)3-G3 is a versatile radioligand with potential to acquire whole-body HER2 scans. (HE)3-G3 will be assessed in a regulatory standard pre-clinical toxicity study, prior to embarking on a first in human trial.