A Synthetic Lipid A Mimetic Modulates Human TLR4 Activity

Abstract

Innate immunity recognition relies on a diverse set of germ line encoded receptors, termed pattern recognition receptors (PRR), which recognize broad classes of molecular structures common to groups of microorganisms. One of the largest and best studied families of PRR are the Toll family of receptors (Toll-like receptors, TLRs) that detect microbial components with high sensitivity and selectivity[1]. Among TLRs, TLR4 selectively responds to bacterial endotoxin (E) (Gram-negative bacterial lipopolysaccharides (LPS) or lipooligosaccharides (LOS)),[2] resulting in the rapid triggering of pro-inflammatory processes necessary for optimal host immune responses to invading Gram-negative bacteria (GNB). TLR4 does not bind directly to endotoxin: LBP,[3] CD14,[4] MD-2[5] are required for efficient extraction and transfer of endotoxin monomers from the GNB outer membrane or aggregates of purified endotoxin to MD-2. The resulting monomeric E·MD-2 complex is the ligand that, depending on the structural properties of E and MD-2, specifies TLR4 activation or antagonism.[6] Although TLR4 plays a key physiologic role in host response to Gram-negative bacterial infection, an excessively potent and/or prolonged TLR4 response can promote life-threatening pathology such as septic shock.[7] TLR4 activation has also been associated with certain autoimmune diseases, non-infectious inflammatory disorders, and neuropathic pain, suggesting a wide range of possible clinical settings for application of TLR4 antagonists.[8] Conversely, agonists of TLR4 can be useful as adjuvants in vaccine development and in cancer immunotherapy [9]. Lipid A[10] (Scheme 1), the hydrophobic part of LPS, is responsible for TLR4-dependent proinflammatory activity.[11] Underacylated lipid A variants, such as tetraacylated lipid IVa[12] and E5564 (Eritoran)[13] are potent LPS antagonists (Scheme 1). The β(1→6) diglucosamine backbone of lipid A can be replaced by an aminoalkyl glucosamine moiety in aminoalkyl glucosaminide 4-phosphates (AGPs)[14] or by other non-carbohydrate structures[15] and the lipid A analogue retains TLR4 agonist or antagonist activity. One or two phosphates are typically present in synthetic lipid A mimics, but these groups could be, in principle, substituted by negatively charged isosteres. A carboxylic acid group replaces the C-1 phosphate in AGP derivatives,[14a] while a sulfate group is present in the monosaccharide lipid A mimic ONO-4007 (Scheme 1) developed by Ono Pharmaceutical Co (Osaka, Japan).[16] This compound showed TLR4 agonist activity inducing TNF-α production in tumour cells, but further clinical development was precluded by the compound's limited water solubility.