Fig. 5.

Chemical structures of selected ligands for the formyl peptide receptors. Despite their abilities to bind to FPR1 and/or FPR2/ALX, these ligands have quite different structures. Of the ligands shown, t-Boc-FLFLF and CsH are antagonists and others are agonists. Note that the N-formyl group that defines agonistic activities in peptides such as fMLF is replaced with a bulky t-butyloxycarbonyl group that defines antagonistic activities in peptides such as t-Boc-FLFLF. LXA4, Quin-C1, and compound 43 are highly selective agonists for FPR2/ALX, whereas AG-14 and fMLF are selective for FPR1. t-Boc-FLFLF is selective for FPR1 at low concentrations but the selectivity is lost at high micromolar concentrations (e.g., 100 μM).