Determination of the Negative Allosteric Binding Site of Cannabidiol at the CB1 Receptor: A Combined Computational and Site-Directed Mutagenesis Study

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Cannabinoid receptor 1 (CB1R) has been extensively studied as a potential therapeutic target for various conditions, including pain management, obesity, emesis, and metabolic syndrome. Unlike orthosteric agonists such as Δ⁹-tetrahydrocannabinol (THC), cannabidiol (CBD) has been identified as a negative allosteric modulator (NAM) of CB1R, among its other pharmacological targets. Previous computational and structural studies have proposed various binding sites for CB1R NAMs. An X-ray crystal structure revealed a binding site for the NAM, ORG27569, at an extrahelical location within the inner leaflet of the membrane. In contrast, multiple computational studies have previously proposed several potential allosteric binding sites for CBD within the CB1R structure. Given that a prior structural study suggested CBD might occupy the same site as ORG27569, we conducted a comprehensive investigation of potential CBD binding sites using molecular docking, molecular dynamics (MD) simulations, metadynamics (MTD) simulations, binding free-energy calculations, and in vitro mutagenesis experiments. Molecular docking, MD, and MTD simulations results, along with binding free-energy calculations, suggest that CBD may potentially bind to either the same extrahelical site as ORG27569 or a previously unidentified intracellular site located near TMHs 2, 6, and 7 and helix 8. This intracellular site is consistent with allosteric binding sites observed in other G protein-coupled receptors (GPCRs). To establish the most favorable allosteric site for CBD, we conducted site-directed mutagenesis of key residues at each site. Mutations at S401^8.47ΔA and D403^8.49ΔA augmented the binding of [³H]-SR141716A, suggesting these residues play critical roles in CBD binding. As a result, the combined computational and mutagenesis results identified a binding site for CBD between TMHs 2, 6, and 7 and helix 8, involving residues Y153^2.40, I156^2.43, M337^6.29, L341^6.33, S401^8.47, and D403^8.49. These findings provide valuable insights into how CBD binds to CB1R, thereby informing the rational design of new, selective, and potent NAMs. Moreover, the elucidation of this previously unexplored allosteric site might explain the polypharmacology of CBD due to structural conservation among Class A GPCRs.