Understanding CXCR2 antagonism with a dynamic allosteric ternary complex model.

The CXC chemokine receptor 2 (CXCR2) antagonist SB265610 displays different patterns of antagonism using in vitro binding and cell-based assays. In addition, CXCR2 antagonists, although likely sharing a similar allosteric binding mechanism, display different patterns in the same cell-based assays. Furthermore, clinical studies with CXCR2 antagonists had mixed success in demonstrating target modulation and efficacy, despite favorable exposures based on published binding affinities. Herein, we aimed to understand the mechanism leading to these apparent inconsistencies with a dynamic allosteric ternary complex model. The model was applied in analyzing both in vitro data and clinical neutrophil counts data of CXCR2 antagonists. We extended previous hypotheses into a unified hypothesis, which postulates that, although allosteric binding of a CXCR2 antagonist is not affected by the endogenous agonist, the antagonism is surmountable as the antagonist loses its potency with increased concentrations of endogenous agonist because of the hyperbolic relationship between agonist-occupied receptor and biological response (which is possibly a result of receptor reserve). Antagonists with slow binding kinetics are apparently insurmountable, but only under unsteady-state conditions. Dynamic allosteric ternary complex model following this hypothesis can describe both in vitro and clinical data of CXCR2 antagonists. The inconsistent patterns of CXCR2 antagonism are interpreted as potential receptor reserve in cell-based assays with unsteady-state binding for some compounds. Because the binding process likely reaches quasi steady state in clinical trials, the lack of pharmacology effect for some antagonists is due to suboptimal potency rather than fast binding kinetics. This model may be applicable to other receptors to help predict clinical responses of allosteric antagonists. SIGNIFICANCE STATEMENT: Known CXC chemokine receptor 2 (CXCR2) antagonists are allosteric and do not compete with endogenous agonists. However, this antagonism is surmountable in some assays, but not others, and for some antagonists, but not others. This study proposes a unified hypothesis to explain observed inconsistent antagonism patterns and apply a mechanistic model to link in vitro findings with clinical outcomes. This study improves our understanding of the pharmacology of CXCR2 antagonists and facilitates the future discovery of antagonists with similar mechanisms for CXCR2 or other G protein-coupled receptors.