Conformational Role of Methyl in the Potency of Cyclohexane-Substituted Squaramide CCR6 Antagonists

CCR6 is a chemokine receptor that mediates the migration of pathogenic inflammatory leukocytes to sites of inflammation in response to its ligand, CCL20. Herein we report the design of a potent CCR6 antagonist capable of inhibiting the chemotactic migration of CCR6⁺ T cells in vitro. Key to this finding was the discovery of a remarkable methyl substituent effect on antagonist potency. A 365-fold improvement in potency was observed for the cis-2-methylcyclohexanamine analogue compared to the unsubstituted cyclohexanamine derivative. Evidence generated through the characterization of conformationally restricted analogues supports the conclusion that the large potency enhancement is the result of the methyl substituent biasing the cyclohexane ring ground state conformation to favor that of the bound ligand and thus decreasing the ligand strain energy.