Amodiaquine Analogs Are Potent Inhibitors of Interleukin-6 Production Induced by Activation of Toll-Like Receptors Recognizing Pathogen Nucleic Acids

Excessive inflammatory responses to viral infections, known as cytokine storms, are caused by overactivation of endolysosomal Toll-like receptors (TLRs) (TLR3, TLR7, TLR8, and TLR9) and can be lethal, but no specific treatment is available. Some quinoline derivatives with antiviral activity were tried during the recent coronavirus disease 2019 (COVID-19) pandemic, but showed serious toxicity, and their efficacy for treating viral cytokine storms was not established. Here, in order to discover a low-toxicity quinoline derivative as a candidate for controlling virally induced inflammation, we synthesized a series of derivatives of amodiaquine (ADQ), a quinoline approved as an antimalarial, and tested their effects on TLRs-mediated production of inflammatory cytokines and cell viability in vitro. In J774.1 murine macrophages, ADQ inhibited interleukin-6 (IL-6) production induced by TLR3 agonist poly(I:C), TLR7 agonist imiquimod, and TLR9 agonist cytosine-phosphate-guanosine oligodeoxynucleotide (CpG ODN) with IC₅₀ values of 2.43, 3.48, and 0.0359µM, respectively, indicating that ADQ has a high inhibitory selectivity for TLR9 signaling. A structure–activity relationship study revealed that an appropriately substituted amino group on the phenol moiety and a halogen substituent on quinoline are important for potent anti-inflammatory activity and low cytotoxicity. ADQ analogs bearing N-butylethyl, N-3-fluoropiperidinyl, and N-4-fluoropiperidinyl groups in place of the N-diethyl group exhibited more potent activity and lower cytotoxicity than ADQ. ADQ and its analogs appear to inhibit the activity of TLRs recognizing pathogen nucleic acids via alkalinization of endolysosomes. Our results suggest that ADQ analogs are promising candidates as therapeutic agents for cytokine storms mediated by TLRs recognizing pathogen nucleic acid with reduced side effects.