Just as the prescient comment by Gaston Ramon was relegated to the last footnote of his 1926 paper, 1 so has research on the mechanisms of action of adjuvants, until recently, languished as parenthetical annotations and addenda in the archives of immunology and vaccine development. Ramon defined immunological adjuvants as "substances used in combination with a specific antigen that produced a more robust immune response than the antigen alone." Interestingly enough, he was referring to his empirical findings that the addition of bread crumbs, tapioca, saponin and 'starch oil' to antigenic preparations greatly enhanced antibody responses to diphtheria or tetanus. 2 A year later, the adjuvanticity of aluminum salts (primarily phosphate and hydroxide) was discovered by Glenny and coworkers. 3 In the 83 years that have elapsed, the repertoire of investigational adjuvants has grown to encompass a very wide range of materials, 4 but aluminum salt-based mineral salts (generically, and incorrectly, termed "alum") have remained the only adjuvants currently approved by the FDA. Aluminum salts have enjoyed a good safety record, but they are weak adjuvants for antibody induction and induce a T helper-2 (T H 2)-skewed, rather than a T helper-1 (T H 1) response. 5,6 Furthermore, not only are aluminum salts ineffective at inducing cytotoxic T lymphocyte (CTL) or mucosal IgA antibody responses, but also have a propensity to induce IgE responses, which have been associated with allergic reactions in some subjects. 5,6 Very recent reports implicate the Nalp3 inflammasome, a component of the innate immune response, as the effector limb of alum-associated adjuvanticity. [7][8][9] In 1962, Dresser observed that injection of purified soluble proteins not only failed to stimulate an immune response, but tolerized animals unless a bacterial extract was admixed with the protein immunogen. 10 This led him to redefine adjuvanticity as "a property of a substance which can act as a physiological switch, directing at least some immunologically competent cells to respond by making antibody rather than by becoming immunologically paralyzed by the antigen," 11 confirming Johnson's earlier observations that lipopolysaccharide (LPS) from Gram-negative bacteria exerted potent adjuvant properties, 12 and perhaps paved the way for the subsequent discovery of the wide range of microorganism-derived adjuvants. 13 TLRs are pattern recognition receptors present on diverse cell types that recognize specific molecular patterns present in molecules that are broadly shared by pathogens but distinguishable from host molecules, collectively referred to as pathogen-associated molecular patterns (PAMPs). 14,15 There are 10 TLRs in the human genome;

Toll-like receptor (TLR) 7 and 8 agonists are potential vaccine adjuvants, since they directly activate APCs and enhance Th1-driven immune responses. Previous SAR investigations in several scaffolds of small molecule TLR7/8 activators pointed to the strict dependence of the selectivity for TLR7 vis-à-vis TLR8 on the electronic configurations of the heterocyclic systems, which we sought to examine quantitatively with the goal of developing “heuristics” to define structural requisites governing activity at TLR7 and/or TLR8. We undertook a scaffold-hopping approach, entailing the syntheses and biological evaluations of 13 different chemotypes. Crystal structures of TLR8 in complex with the two most active compounds confirmed important binding interactions playing a key role in ligand occupancy and biological activity. Density functional theory based quantum chemical calculations on these compounds followed by linear discriminant analyses permitted the classification of inactive, TLR8-active, and TLR7/8 dual-active compounds, confirming the critical role of partial charges in determining biological activity.

Toll-like receptor (TLR)-8 agonists activate adaptive immune responses by inducing robust production of T helper 1-polarizing cytokines, suggesting that TLR8-active compounds may be promising candidate adjuvants. We recently reported pure TLR8 agonistic activity in a C2-butyl furo[2,3-c]quinoline. We have obtained the structure of human TLR8 ectodomain co-crystallized with the furoquinoline compound, which indicates ligand-induced reorganization of the binding pocket of TLR8. The loss of a key H-bond between the oxygen atom of the furanyl ring of the agonist and Thr574 in TLR8 suggested that the furan ring was dispensable. We employed a disconnection strategy and examined 3- and 4-substituted aminoquinolines. Focused structure-based ligand design studies led to the identification of 3-pentyl-quinoline-2-amine as a novel, structurally simple, and highly potent human TLR8-specific agonist.

Toll-like receptor (TLR)-8 agonists typified by the 2-alkylthiazolo[4,5-c]quinolin-4-amine (CL075) chemotype are uniquely potent in activating adaptive immune responses by inducing robust production of T helper 1-polarizing cytokines, suggesting that TLR8-active compounds could be promising candidate vaccine adjuvants, especially for neonatal vaccines. Alkylthiazoloquinolines with methyl, ethyl, propyl and butyl groups at C2 displayed comparable TLR8-agonistic potencies; activity diminished precipitously in the C2-pentyl compound, and higher homologues were inactive. The C2-butyl compound was unique in possessing substantial TLR7-agonistic activity. Analogues with branched alkyl groups at C2 displayed poor tolerance of terminal steric bulk. Virtually all modifications at C8 led to abrogation of agonistic activity. Alkylation on the C4-amine was not tolerated, whereas N-acyl analogues with short acyl groups (other than acetyl) retained TLR8 agonistic activity, but were substantially less water-soluble. Immunization in rabbits with a model subunit antigen adjuvanted with the lead C2-butyl thiazoloquinoline showed enhancements of antigen-specific antibody titers.

Toll-like receptor (TLR)-8 agonists activate adaptive immune responses by inducing robust production of T helper 1-polarizing cytokines, suggesting that TLR8-active compounds may be promising candidate adjuvants. We synthesized and evaluated hitherto unexplored furo[2,3-c]quinolines and its regioisomeric furo[3,2-c]quinolines, derived via a tandem, one-pot Sonogashira coupling and intramolecular 5 endo-dig cyclization strategy, in a panel of primary screens. We observed a pure TLR8 agonistic activity profile in select furo[2,3-c]quinolines, with maximal potency conferred by a C2-butyl group (EC50: 1.6 µM); shorter, longer, or substituted homologues, as well as compounds bearing C1 substitutions were inactive, which was rationalized by docking studies using the recently-described crystal structure of human TLR8. The best-in-class compound displayed prominent proinflammatory cytokine induction (including interleukin-12 and interleukin-18), but was bereft of interferon-α inducing properties, confirming its high selectivity for human TLR8.