Background and aims The toll-like receptors (TLRs) are critical participants in vertebrate innate immune recognition of pathogen-associated molecular patterns (PAMPs). Diverse ligands act as “danger signals” detected by this component of the innate immune system. TLR7 and 8 are located in the endosomes of specific immune subpopulations, and are activated by single-stranded RNA from viruses or by autologous RNA fragments bound to immune complexes, inducing the generation of cytokines such as interferons (specifically IFN-alpha) and IL-6. Strong genetic evidence supports variants in TLR7 as contributors to development of systemic lupus erythematosus (SLE).

Methods In vitro and in vivo assays were used to guide development of potent and specific small molecule inhibitors.

Results We describe novel and selective small molecule non-oligonucleotide TLR7/8 antagonists for the treatment of SLE. They exhibit potent activity in vitro in TLR-specific reporter systems (IC₅₀ of ˜100 nM) and in primary human blood cells (IC₅₀ of 50–500 nM across various ligands and cytokine readouts), suppressing TLR7 and TLR8 but with no activity against TLR9 or other TLRs tested. Exploration of mechanism of action shows direct interaction of the lead compound with the external domain of TLR8. The compounds are orally available and active in a mouse model of R848 challenge. When tested in long-term dosing in pristane-induced or spontaneous NZB/W disease the compounds slow the advance of autoantibody titers and efficiently suppress development of nephritis and associated proteinuria.

Conclusions We have identified novel small molecule antagonists of human TLR7 and TLR8 with beneficial activity in mouse models of systemic lupus.