Increasing the Chemical Variety of Small-Molecule-Based TLR4 Modulators: An Overview

Abstract

Toll-Like Receptor 4 (TLR4) is one of the receptors of innate immunity. It is activated by Pathogen- and Damage-Associated Molecular Patterns (PAMPs and DAMPs) and triggers pro-inflammatory responses that belong to the repertoire of innate immune responses, consequently protecting against infectious challenges and boosting adaptive immunity. Mild TLR4 stimulation by non-toxic molecules resembling its natural agonist (lipid A) provided efficient vaccine adjuvants. The non-toxic TLR4 agonist monophosphoryl lipid A (MPLA) has been approved for clinical use. This suggests the development of other TLR4 agonists as adjuvants or drugs for cancer immunotherapy. TLR4 excessive activation by a Gram-negative bacteria lipopolysaccharide (LPS) leads to sepsis, while TLR4 stimulation by DAMPs is a common mechanism in several inflammatory and autoimmune diseases. TLR4 inhibition by small molecules and antibodies could therefore provide access to innovative therapeutics targeting sepsis as well as acute and chronic inflammations. The potential use of TLR4 antagonists as anti-inflammatory drugs with unique selectivity and a new mechanism of action compared to corticosteroids or other non-steroid anti-inflammatory drugs fueled the search for compounds of natural or synthetic origin able to block or inhibit TLR4 activation and signaling. The wide spectrum of clinical settings to which TLR4 inhibitors can be applied include autoimmune diseases (rheumatoid arthritis, inflammatory bowel diseases), vascular inflammation, neuroinflammations, and neurodegenerative diseases. The last advances (from 2017) in TLR4 activation or inhibition by small molecules (molecular weight <2 kDa) are reviewed here. Studies on pre-clinical validation of new chemical entities (drug hits) on cellular or animal models as well as new clinical studies on previously developed TLR4 modulators are reported. Innovative TLR4 modulators discovered by computer-assisted drug design and an artificial intelligence approach are described. Some “old” TLR4 agonists or antagonists such as MPLA or Eritoran are under study for repositioning in different pharmacological contexts. The mechanism of action of the molecules and the level of TLR4 involvement in their biological activity are critically discussed.

LPS Induces Opposing Memory-like Inflammatory Responses in Mouse Bone Marrow Neutrophils

“…LPS as a prominent PAMP is a potent endotoxin that is sensed by the PRR receptor toll-like receptor (TLR) 4 [63,64]. Earlier studies reported that extremely low LPS doses (femtomolar) may trigger the activation of myeloid differentiation factor-2 (MD-2)/TLR4 complex, most probably due to the increased sensitivity driven by CD14 [65][66][67][68]. Activation of TLR4 results in a cascade of signaling events promoting the release of inflammatory mediators mainly by a concordant activation of NFκB, especially p65 [69,70].…”

Immunobiology of Carbohydrates: Implications for Novel Vaccine and Adjuvant Design Against Infectious Diseases

“…Finally, it is worth noting that while MPLA disaccharide is an important scaffold for the phosphate and acyl group, it is not strictly required for eliciting TLR4 signaling. Structural analogues of MPLA in which the sugar backbone is substituted by other chemical groups that can provide scaffold function retain immunological properties ( Romerio and Peri, 2020 ).…”

“…Some molecules like MPLA are key components of adjuvant formulations that are part of licensed vaccine ( O’Hagan et al., 2020 ). However, the sugar component functions mainly as scaffold for the acyl chains which is key to trigger TLR4-mediated signaling ( Cochet and Peri, 2017 ; Romerio and Peri, 2020 ). Several molecules that have shown adjuvant activity in pre-clinical and in some cases clinical studies, such as α-GalCer and TDB, are glycolipids and therefore structure-activity relationship studies will be key to define the relative contribution of sugar and lipid moieties to adjuvant activity ( Pifferi et al., 2021 ).…”

Epigallocatechin Gallate Can Protect Mice From Acute Stress Induced by LPS While Stabilizing Gut Microbes and Serum Metabolites Levels

“…A large amount of experimental data shows that LPS can cause systemic inflammatory response syndrome (SIRS) ( 1 , 2 ). It is an endotoxin that can not only be secreted in the form of outer membrane vesicles (OMVs) but can also be released by forming micelles in the bacterial membrane ( 3 ). LPS is transported to the cytoplasm of immune cells in the form of OMVs, and inflammatory caspases, as the specific receptors of LPS, can activate the production of downstream pro-inflammatory factors ( 4 ).…”