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20 p.-15 fig.-1 graph. abst., We disclose here a panel of small-molecule TLR4 agonists (the FP20 series) whose structure is derived from previously developed TLR4 ligands (FP18 series). The new molecules have increased chemical stability and a shorter, more efficient, and scalable synthesis. The FP20 series showed selective activity as TLR4 agonists with a potency similar to FP18. Interestingly, despite the chemical similarity with the FP18 series, FP20 showed a different mechanism of action and immunofluorescence microscopy showed no NF-κB nor p-IRF-3 nuclear translocation but rather MAPK and NLRP3-dependent inflammasome activation. The computational studies related a 3D shape of FP20 series with agonist binding properties inside the MD-2 pocket. FP20 displayed a CMC value lower than 5 μM in water, and small unilamellar vesicle (SUV) formation was observed in the biological activity concentration range. FP20 showed no toxicity in mouse vaccination experiments with OVA antigen and induced IgG production, thus indicating a promising adjuvant activity., The authors acknowledge the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie, project BactiVax (www.bactivax.eu) grant agreement no. 860325; the consortium CINMPIS; the project of excellence CHRONOS, CHRonical multifactorial disorders explored by NOvel integrated Strategies of the Department of Biotechnology and Biosciences; the Agencia Estatal de Investigacion (Spain) for project PID2021-126130OB-I00 (N.G.A.A.), PID2020-113588RB-I00 (S.M.-S.), PRE2018-086249 (A.M.-R), PRE2021-097247 (M.M.-T.); and project FEDER MINECO, the EM-platform at the CIC bioGUNE for support in cryo-EM imaging. J.J.-B. also thanks funding by CIBERES, an initiative of Instituto de Salud Carlos III (ISCIII), Madrid, Spain. Perkin-Elmer Italia is also acknowledged for providing the cell imaging reagents.