SB265610 is an allosteric, inverse agonist at the human CXCR2 receptor

Background and purpose: In several previous studies, the C-X-C chemokine receptor (CXCR)2 antagonist 1-(2-bromo-phenyl)-3-(7-cyano-3H-benzotriazol-4-yl)-urea (SB265610) has been described as binding competitively with the endogenous agonist. This is in contrast to many other chemokine receptor antagonists, where the mechanism of antagonism has been described as allosteric. Experimental approach: To determine whether it displays a unique mechanism among the chemokine receptor antagonists, the mode of action of SB265610 was investigated at the CXCR2 receptor using radioligand and [S-35]-GTP gamma S binding approaches in addition to chemotaxis of human neutrophils. Key results: In equilibrium saturation binding studies, SB265610 depressed the maximal binding of [I-125]-interleukin-8 ([I-125]-IL-8) without affecting the K-d. In contrast, IL-8 was unable to prevent binding of [H-3]-SB265610. Kinetic binding experiments demonstrated that this was not an artefact of irreversible or slowly reversible binding. In functional experiments, SB265610 caused a rightward shift of the concentration-response curves to IL-8 and growth-related oncogene alpha, but also a reduction in maximal response elicited by each agonist. Fitting these data to an operational allosteric ternary complex model suggested that, once bound, SB265610 completely blocks receptor activation. SB265610 also inhibited basal [S-35]-GTP gamma S binding in this preparation. Conclusions and implications: Taken together, these data suggest that SB265610 behaves as an allosteric inverse agonist at the CXCR2 receptor, binding at a region distinct from the agonist binding site to prevent receptor activation, possibly by interfering with G protein coupling. British Journal of Pharmacology (2009) 158, 328-338; doi: 10.1111/j.1476-5381.2009.00182.x; published online 27 April 2009