Selectivity, PK, activation

Cytokines like IL-4 are powerful immunomodulators but notoriously difficult to engineer for therapeutic use. They activate multiple receptor complexes on different cell types, making selectivity a challenge. Standard protein engineering, limited to the 20 natural amino acids, cannot independently optimise selectivity, pharmacokinetics, and activation simultaneously.

Multiple ncAAs working together solved this for IL-4: homoserine at ligation junctions maintained structural integrity during chemical synthesis; norleucine replaced oxidation-prone methionines for stability; ornithine at position 116 modulated immune cell selectivity; and PEGylation via an ncAA handle at a nearby position extended half-life. Notably, PEGylation at different sites produced inverse receptor selectivity profiles (Ninomiya et al., 2025).

This multi-ncAA approach, where several non-canonical residues each address a different pharmacological parameter, exemplifies the advantage of an expanded genetic code. Rather than compromising on one property to improve another, each ncAA handles a specific engineering challenge independently.