– PMX53 (Ac-Phe-cyclo(Orn-Pro-D-Cha-Trp-Arg)) is a potent C5a receptor (CD88) antagonist with IC50 of 20 nM, also is an agonist for Mas-related gene 2 (MrgX2) in human mast cells; PMX-53 (10 nM) inhibited C5a-induced Ca(2+) mobilization in HMC-1 cells, but at higher concentrations (>30 nM) it caused degranulation in LAD2 mast cells, CD34(+) cell-derived mast cells, and RBL-2H3 cells stably expressing MrgX2; inhibits zymosan-, carrageenan-, LPS- and antigen-induced hypernociception in rats.