Imaging of ROR1 expression in tumors using radiolabeled affibody molecules

Abstract Purpose Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is an attractive molecular target for anti-tumor therapies undergoing late-stage clinical development. Quantitative imaging of ROR1 expression could enable identification of patients likely to respond to such treatments. This study aimed to assess the feasibility of specific in vivo imaging of ROR1 using radiolabeled affibody molecules. Methods The affibody molecule ZROR1:A10, with high affinity for human (4 nM) and murine (2 nM) ROR1, was labeled with indium-111 using a DOTA chelator. In vitro binding specificity, affinity, and cellular processing were evaluated using a panel of ROR1-expressing cell lines. The affibody [ 111 In]In-DOTA-Zcov19s, which does not bind ROR1, served as a nonspecific control. The biodistribution of [ 111 In]In-DOTA-ZROR1:A10 and [ 111 In]In-DOTA-Zcov19s was measured in immunodeficient mice bearing ROR1-positive tumors, with ROR1-negative Ramos xenografts used as an additional specificity control. Results [ 111 In]In-DOTA-ZROR1:A10 bound specifically to ROR1-expressing cells, with significantly higher uptake than [ 111 In]In-DOTA-Zcov19s. The apparent equilibrium dissociation constant for ROR1 binding in vitro was 1–4 nM. Tumor uptake of [ 111 In]In-DOTA-ZROR1:A10 in MDA-MB-468 xenografts was saturable and significantly (p < 0.05) higher than in Ramos tumors, as well as higher than uptake of the control affibody. At 4 h post‑injection of 1 µg (50 µg/kg), tumor‑to‑blood, tumor‑to‑bone, and tumor‑to‑muscle ratios were 14 ± 6, 19 ± 6, and 11 ± 1, respectively. Micro-SPECT/CT imaging clearly visualized ROR1‑positive tumors and discriminated them from ROR1‑negative xenografts. Conclusion The preclinical evaluation shows that visualization of ROR1-expressing tumors using a radiolabeled affibody molecule is feasible.