Siderocalin fusion proteins enable a new 86Y/90Y theranostic approach

Abstract

The mammalian protein siderocalin binds bacterial siderophores and their iron complexes through cation-π and electrostatic interactions, but also displays high affinity for hydroxypyridinone complexes of trivalent lanthanides and actinides. In order to circumvent synthetic challenges, the use of siderocalin-antibody fusion proteins is explored herein as an alternative targeting approach for precision delivery of trivalent radiometals. We demonstrate the viability of this approach in vivo, using the theranostic pair ⁹⁰Y (β^-, t_1/2 = 64 h)/⁸⁶Y (β⁺, t_1/2 = 14.7 h) in a SKOV-3 xenograft mouse model. Ligand radiolabeling with octadentate hydroxypyridinonate 3,4,3-LI(1,2-HOPO) and subsequent protein binding were achieved at room temperature. The results reported here suggest that the rapid non-covalent binding interaction between siderocalin fusion proteins and the negatively charged Y(iii)-3,4,3-LI(1,2-HOPO) complexes could enable purification-free, cold-kit labeling strategies for the application of therapeutically relevant radiometals in the clinic.

Fig. 1. Targeting fusion proteins explored in this work incorporate the Siderocalin (Scn) mammalian protein, which recognizes negatively charged M(iii)-HOPO complexes (left, non-targeting control), and is fused either to an antibody fragment (Fab, middle) or trastuzumab (IgG, right).

Fig. 2. (A) Coronal PET images (mouse lying prone, head at top) of targeting ⁸⁶Y-HOPO-Scn (left) compared to ⁸⁶Y-HOPO-IgG-Scn (right). (B) Sagittal view (mouse lying prone, head at top) showing tumor uptake. White dotted circle outlines tumor location. The most uptake is seen in the bladder at 5 h P.I. for both constructs.