Helicogenic Amino Acid Increases Passive Permeability and Oral Bioavailability of Macrocycles by Preorganizing γ‑Turn

Macrocyclic
peptides (MCPs) are promising drug candidates but often
exhibit poor passive membrane permeability due to their polar backbone.
This study demonstrates that incorporation of α-aminoisobutyric
acid (Aib), a helicogenic residue, can enhance MCP permeability in
a scaffold-dependent manner by enforcing γ-turns and promoting
saddle-shaped conformations that stabilize intramolecular hydrogen
bonds (IMHBs). Systematic Pro → Aib substitutions across several
representative 6–8-residue scaffolds improved lipophilicity,
reduced polar surface area, and minimized conformational rearrangements
between aqueous and membrane environments, as confirmed by NMR, hydrogen–deuterium
exchange mass spectrometry, and molecular dynamics simulations. However,
Aib incorporation did not increase permeability in all scaffolds,
particularly those that were already highly preorganized. Permeability
improvements were most pronounced in flexible scaffolds, with up to
8-fold gains and increased oral bioavailability in mice, primarily
through enhanced absorption. Mechanistically, Aib and aromatic residues
contribute additively to hydrophobic patch formation, promoting membrane
affinity. These findings establish Aib as a valuable design element
for optimizing passive permeability and oral exposure of macrocyclic
peptides.