Background/Aims: CID16020046 blocks the effect of the lipid lysophosphatidylinositol (LPI) at its receptor, GPR55. CID16020046 and another antagonist, ML193, have been used to investigate GPR55-mediated effects of LPI on cells, tissues, and in vivo. Here we describe the structure-activity relationship of CID16020046. Methods: Yeast or human cells were engineered to express GPR55 or control receptors. Cells were pretreated with a test agent before agonist challenge. Functional responses were quantified by yeast gene-reporter or calcium imaging. Results: Three substituents around the central pyrazololactam core of CID16020046 are each tolerant to substitution without abolishing GPR55 activity. Analogues of CID16020046 with potency at GPR55 ranging >1,000-fold are described, including several lacking activity up to the top concentration tested. One analogue, compound 1 (GSK875734A), has approximately 50-fold greater potency than CID16020046 in an inverse agonist assay. CID16020046, ML193 and 2 further antagonists (ML191 and ML192) all block the effect of a surrogate agonist at human GPR55. ML193, CID16020046 and several other examples of the pyrazololactam chemotype were also shown to antagonise rat GPR55. Conclusion: These data confirm the utility of CID16020046 and ML193 as tools to investigate the physiological role of GPR55, and offer starting points for GPR55 antagonists with optimised pharmacokinetic or other properties.

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