A dual and conflicting role for imiquimod in inflammation: A TLR7 agonist and a cAMP phosphodiesterase inhibitor

The Toll-like receptor 7 (TLR7) agonist imiquimod is an antitumor and antiviral drug used for the treatment of skin indications such as basal cell carcinoma, squamous cell carcinoma, and genital warts caused by the human papilloma virus. We show that imiquimod has TLR7-independent activity in which it directly inhibits phosphodiesterase (PDE), leading to cAMP increase, PKA-mediated CREB phosphorylation and subsequent CRE-dependent reporter transcription. The activation of the cAMP pathway by imiquimod is synergistically amplified by the β-adrenergic receptor agonist, isoproterenol. PDE inhibition is implied from cAMP measurements and CRE-reporter assays in intact RAW264.7 macrophages and HEK293T cells, and also directly demonstrated in-vitro using macrophages lysate. Moreover, molecular docking simulated the binding of imiquimod in the active site of PDE4B, enabled by the high molecular similarity between imiquimod and the adenine moiety of cAMP. As expected from the known anti-inflammatory role of cAMP inducers in stimulated macrophages, PDE inhibition by imiquimod results in reduced expression of the key pro-inflammatory cytokine TNFα, and enhanced expression of the key anti-inflammatory cytokine IL-10, compared to a different TLR7 agonist, loxoribine, as well as to the TLR4 agonist LPS. To conclude, our results indicate that the widely used inflammatory drug, imiquimod, is not only a TLR7 agonist, but also harbors a novel anti-inflammatory function as a PDE inhibitor. This off-target affects the desired therapeutic inflammatory activity of imiquimod and may be accountable for adverse side effects.