The 'C3aR antagonist' SB290157 is a partial C5aR2 agonist

Innate immune complement activation generates the C3 and C5 protein cleavage products C3a and C5a, defined classically as anaphylatoxins. C3a activates C3a receptors (C3aR), while C5a activates two receptors (C5aR1 and C5aR2) to exert their immunomodulatory activities. The non-peptide compound, SB290157, was originally reported in 2001 as the first C3aR antagonist. In 2005, the first report on non-selective nature of SB290157 was published, where the compound exerted clear agonistic, not antagonistic, activity in variety of cells. Other studies also documented non-selective activities of this drug in vivo. These findings severely hamper data interpretation regarding C3aR when using this compound. Unfortunately, given the dearth of C3aR inhibitors, SB290157 still remains widely used to explore C3aR biology (>70 publications to date). Given these issues, in the present study we aimed to further explore SB290157's pharmacological selectivity by screening the drug against three human anaphylatoxin receptors, C3aR, C5aR1 and C5aR2, using transfected cells. We first confirmed that SB290157 acts as a potent agonist at human C3aR. We also identified that SB290157 exerts partial agonist activity at C5aR2 by mediating β-arrestin recruitment at higher compound doses. Notably, SB290157's activity at C5aR2 was more potent and efficacious than the current 'lead' C5aR2 agonist P32. Notwithstanding this, SB290157 showed inhibitory effect on C3a-mediated signalling in primary human macrophages. Our results therefore provide even more caution against using SB290157 as a research tool to explore C3aR function. Given the reported immunomodulatory and anti-inflammatory activities of C3aR and C5aR2 agonism, any function observed with SB290157 could be due to these off target activities. ### Competing Interest Statement The authors have declared no competing interest.

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