A kinetic model for positive allosteric modulator (PAM)-antagonists for the type 1 cannabinoid (CB1) receptor

Abstract

Background and Purpose: The cannabinoid (CB1) receptor is among the most abundant G protein-coupled receptors in brain. Allosteric ligands bind to a different site on receptors than the orthosteric ligand can have effects that are unique to the allosteric ligand and modulate orthosteric ligand activity. We propose a unified mathematical model describing the interaction effects of the allosteric ligand Org27569 and the orthosteric agonist CP55940 on CB1 receptor. Experimental Approach: A ternary complex model was constructed, which incorporated kinetic properties to describe the time course of effects of Org27569 and CP55940 reported in the literature: (i) enhanced receptor binding of CP55940, (ii) reduced internalisation and (iii), time-dependent modulation of cAMP. Underlying mechanisms of time-dependent modulation by Org27569 were evaluated by simulation. Key Results: A hypothetical transitional state of CP55940–CB1–Org27569, which can internalise but cannot inhibit cAMP, was shown to be necessary and was sufficient to describe the allosteric modulation by Org27569, prior to receptors adopting an inactive conformation. The model indicated that the formation of this transitional CP55940–CB1–Org27569 state and final inactive CP55940–CB1–Org27569 state contributes to the enhanced CP55940 binding. The inactive CP55940–CB1–Org27569 cannot internalise or inhibit cAMP, leading to reduced internalisation and cessation of cAMP inhibition. Conclusions and Implications: In conclusion, a kinetic mathematical model for CB1 receptor allosteric modulation was developed. However, a standard ternary complex model was not sufficient to capture the data and a hypothetical transitional state was required to describe the allosteric modulation properties of Org27569.