Structure-Activity Relationship (SAR) Study of Rigid Guanidine and Guanidino-mimetic Analogues of the Neuropeptide FF Receptor Antagonist MES304

The Neuropeptide FF (NPFF) receptor system has been implicated in mediating opioid-induced hyperalgesia and tolerance. Understanding the structural features that contribute to high binding affinity of ligands to the NPFF receptors could help in identifying potent and selective NPFF-R probes. We conducted a structure-activity relationship (SAR) study targeting the guanidine functionality of our previously identified lead molecule MES304 in order to probe the guanidine binding site of the NPFF receptors. By applying several rigidifying modifications and substitutions on the guanidine moiety, we concluded that while the guanidine group is optimal for strong binding affinity at both receptor subtypes (NPFF ₁ -R/NPFF ₂ -R), other groups may be tolerated. Compound 17b, in which the guanidine was replaced with a piperidine ring, displayed robust binding affinity at both receptor subtypes (K _i < 300 nM) despite lacking a guanidine group. This indicated that strong binding to the NPFF receptors can be achieved with analogues devoid of guanidine.