Development of mirror-image monobodies targeting the oncogenic BCR::ABL1 kinase

Mirror-image proteins, composed of D-amino acids, are an attractive therapeutic modality, as they exhibit high metabolic stability and lack immunogenicity. Development of mirror-image binding proteins is achieved through chemical synthesis of D-target proteins, phage display library selection of L-binders and chemical synthesis of (mirror-image) D binders that consequently bind the physiological L-targets. Monobodies are well-established synthetic (L )binding proteins and their small size (~90 residues) and lack of endogenous cysteine residues make them particularly accessible to chemical synthesis. Here, we developed monobodies with nanomolar binding affinities against the D-SH2 domain of the leukemic tyrosine kinase BCR::ABL1. Two crystal structures of heterochiral monobody-SH2 complexes revealed targeting of the pY binding pocket by an unconventional binding mode. We then prepared potent D-monobodies by either ligating two chemically synthesized D-peptides or by self-assembly without ligation. Their proper folding and stability were determined and high affinity binding to the L-target was shown. D-monobodies were protease-resistant, showed long-term plasma stability, inhibited BCR::ABL1 kinase activity and bound BCR::ABL1 in cells and (to some extent in) cell lysates with high selectivity. Hence, we demonstrate that functional D monobodies can be developed readily. Our work represents an important step towards the possible future therapeutic use of D-monobodies when combined with emerging methods to enable cytoplasmic delivery of monobodies.