Background/Purpose: Toll-like receptor 7, 8 and 9 (TLR7/8/9) are endosomal nucleic acid sensors that when aberrantly activated, induce Type I interferon (IFNα), proinflammatory cytokines, immune cell activation and antibody production. Human genetic analysis of systemic lupus erythematosus (SLE) revealed that TLR7/8/9 pathway is associated with SLE incidence. TLR7 gain-of-function genetic variation has also recently been reported to cause human lupus. TLR7/8/9 antagonism is expected to block IFN production and autoreactive B cell activities- two major disease drivers in lupus. Given that majority of SLE patients receive hydroxychloroquine (HCQ) as a first-line therapy, and HCQ is proposed to inhibit endosomal TLRs, we evaluated TLR7/8/9 pathway activity in fresh whole blood from SLE patients that receive HCQ. We observed that TLR7/8, but not TLR9 pathway, remained active in the presence of HCQ in SLE patient blood samples. We therefore developed a novel TLR7/8 small molecule inhibitor and characterized its activities in vitro and in vivo.

Methods: Fresh whole blood samples from 16 SLE patients that received HCQ, and 10 SLE patients that did not receive HCQ (procured from Sanguine Biosciences), were stimulated ex vivo with TLR7, TLR8 or TLR9 agonists. Blood HCQ concentration, and the production of IFNα and proinflammatory cytokines after stimulation were measured. In vitro potency of HCQ and the novel TLR7/8 inhibitor was evaluated in human whole blood and isolated human primary cells. TLR selectivity was determined in HEK293 cells expressing human TLR2, 3, 4, 5, 7, 8 or 9 (InvivoGen). In vivo potency was evaluated by inhibition of TLR7 agonist stimulated cytokines in C57BL/6 mice. The prophylactic and therapeutic efficacy of the TLR7/8 inhibitor were investigated in the spontaneous and IFNα-accelerated NZB/W F1 murine lupus models.

Results: TLR9 agonist induced IFNα /cytokines, but not TLR7/8 agonists induced activities, were largely suppressed by the presence of HCQ in the SLE blood samples studied (mean blood HCQ concentration= 2.1±1.3 μM). We identified a potent and selective TLR7/8 inhibitor that suppressed TLR7/8 agonist-induced cell activation and cytokine production in human primary B cells, monocytes and dendritic cells, and in blood samples from SLE patients on HCQ.  It also completely blocked RNP-immune complex (IC)- or TLR7 agonist-induced IFNα production in human primary plasmacytoid dendritic cells (pDC). Lastly, the TLR7/8 inhibitor improved survival, proteinuria, renal histopathology and IgG deposit in a spontaneous as well as type I IFN accelerated NZB/W F1 murine SLE model. 

Conclusion: We have identified a novel, potent and selective TLR7/8 inhibitor which blocks the pathogenic TLR7/8 pathway that is insufficiently covered with the typical HCQ exposure in SLE patient blood in vitro. Prophylactic and therapeutic TLR7/8 inhibition are highly efficacious in murine SLE models.  Our results provide therapeutic rationale for combining a TLR7/8 inhibitor with HCQ for the treatment of SLE.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-novel-potent-and-selective-tlr7-8-small-molecule-inhibitor-blocks-tlr7-8-pathway-in-the-presence-of-hcq-and-demonstrates-robust-preclinical-efficacy-in-lupus-models/