Pharmacological characterization of a structural hybrid P2X7R antagonist using ATP and LL-37

Antagonists of the P2X7 receptor (P2X7R) have the potential to treat diseases where neuroinflammation is present such as depression, chronic pain and Alzheimer's disease. We recently developed a structural hybrid (C1; 1-((adamantan-1-yl)methyl)-2-cyano-3-(quinolin-5-yl)guanidine) of a purported competitive P2X7R antagonist (C2; 2-cyano-1-((1S)-1-phenylethyl)-3-(quinolin-5-yl)guanidine) and a likely negative allosteric modulator (NAM) of the P2X7R (C3; N-((adamantan-1-yl)methyl)-2-chloro-5-methoxybenzamide). Here we aimed to pharmacologically characterize C1, to gain insights into how select structural components impact antagonist interaction with the P2X7R. A second aim was to examine the role of the peptide LL-37, an apparent activator of the P2X7R, and compare the ability of multiple P2X7R antagonists to block its effects. Compounds 1, 2 and 3 were characterised using washout, Schild and receptor protection studies, all using dye uptake assays in HEK293 cells expressing the P2X7R. LL-37 was examined in the same HEK293 cells and THP-1 monocytes. Compounds 2 and 3 acted as a BzATP-competitive antagonist and NAM of the P2X7R respectively. Compound 1 was a slowly reversible NAM of the P2X7R suggesting the incorporation of an appropriately positioned adamantane promotes binding to the allosteric site of the P2X7R. LL-37 was shown to potentiate the ability of ATP to induce dye uptake at low concentrations (1-3 μg mL^-1) or induce dye uptake alone at higher concentrations (10-20 μg mL^-1). None of the P2X7R antagonists studied were able to block LL-37-induced dye uptake bringing in to question the ability of current P2X7R antagonists to inhibit the inflammatory action of LL-37 in vivo.