Exploration of Toll-like Receptor 7 and 8 Agonists as Potential Vaccine Adjuvants

Toll-like receptors (TLRs)-7/-8 are among pathogen recognition receptors (PRRs) present in the endosomal compartment that are activated by viral single-stranded RNA (ssRNA) as well as synthetic small molecules. TLR7/8 agonists hold promise as potential vaccine adjuvants, since they directly activate antigen-presenting cells and enhance T helper 1-driven immune responses. A general introduction to TLRs, with an emphasis on the role of TLR7/8 activation in innate and adaptive immune responses is presented in Chapter 1. Structure-activity relationship (SAR) studies in small molecule TLR8/7-agonistic ligands showed that thiazolo[4,5-c]quinolines display mixed TLR8/7 agonistic activities with the optimal C2-alkyl chain length being butyl (Chapter 2). In an ongoing search toward exploring alternative chemotypes, furo[2,3-c]pyridines with pyridoxal as the aldehyde component in a one-pot multicomponent Groebke-Blackburn-Bienaymé reaction were obtained and found to exhibit TLR8-dependent NF-kB activation and strong adjuvanticity without proinflammatory cytokine induction (Chapter 3). Combinatorial libraries using the Groebke-Blackburn-Bienaymé reaction have also yielded TLR7/8-inactive, but antibacterial imidazo[1,2-a]pyridines (Chapter 4). Based on the previously reported SARs on imidazoquinolines, the syntheses and biological evaluation of novel imidazo[4,5-c]pyridine analogues were undertaken, with modifications at the N4- and C6 positions, which afforded strong Type I IFN inducers in conjunction with attenuated proinflammatory profiles (Chapter 5). With the goal of defining structural requisites governing activity and selectivity at TLR7 and/or TLR8, we undertook scaffold-hopping approach, quantum chemical calculations followed by linear discriminant analyses that permitted the classification of inactive, TLR8-active, and TLR7/8 dual-active compounds, confirming the critical role of partial charges in determining biological activity (Chapter 6). Molecular conjugation of TLR7/8 agonists to hyaluronic acid (HA) was evaluated to enhance selective and targeted delivery of vaccine construct to draining lymph nodes while limiting systemic exposure. The superior adjuvanticity evoking affinity-matured high-avidity immunoglobulins after a single boost was observed with HA conjugate bearing dual TLR7/8 agonist (Chapter 7). Extensive SAR investigations in several TLR7/8 agonistic scaffolds and exploration as vaccine adjuvant candidates have incrementally improved our understanding of how these molecules activate innate and adaptive immune responses and also catalyzed novel approaches to vaccine design and development

Structure-Activity Relationships in Human Toll-like Receptor 2- Specific Monoacyl Lipopeptides

Toll-like receptor 2-agonistic lipopeptides typified by S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-R-cysteinyl-S-serine (PAM2CS) compounds are potential vaccine adjuvants. We had previously determined that at least one acyl group of optimal length (C16) and an appropriately orientated ester carbonyl group is essential for TLR2-agonistic activity. We now show that these structurally simpler analogues display agonistic activities with human, but not murine TLR2. SAR studies on the monoacyl derivatives show that the optimal acyl chain length is C16, and aryl substituents are not tolerated. A variety of alkyl and acyl substituents on the cysteine amine were examined. All N-alkyl derivatives were inactive. In contradistinction, short-chain N-acyl analogues were found to be highly active, with a clear dependence on the chain length. A cysteine N-acetyl analogue was found to be the most potent (EC50: 1 nM), followed by the N-butyryl analogue. The N-acetyl analogue is human TLR2-specific, with its potency comparable to that of PAM2CS

Structure–Activity Relationship Studies of Functionally Selective Kappa Opioid Receptor Agonists that Modulate ERK 1/2 Phosphorylation While Preserving G Protein Over βArrestin2 Signaling Bias

Kappa opioid receptor (KOR) modulation is a promising target for drug discovery efforts due to KOR involvement in pain, depression, and addiction behaviors. We recently reported a new class of triazole KOR agonists that displays significant bias toward G protein signaling over βarrestin2 recruitment; interestingly, these compounds also induce less activation of ERK1/2 map kinases than the balanced agonist, U69,593. We have identified structure–activity relationships around the triazole scaffold that allows for decreasing the bias for G protein signaling over ERK1/2 activation while maintaining the bias for G protein signaling over βarrestin2 recruitment. The development of novel compounds, with different downstream signaling outcomes, independent of G protein/βarrestin2 bias, provides a more diverse pharmacological toolset for use in defining complex KOR signaling and elucidating the significance of KOR-mediated signaling

Structure-Activity Relationships in Human Toll-like Receptor 8-Active 2,3-diamino-furo[2,3-c]pyridines

In our ongoing search toward identifying novel and synthetically simpler candidate vaccine adjuvants, we hypothesized that the imidazo[1,2-a]pyrazines, readily accessible via the Groebke-Blackburn-Bienaymé multicomponent reaction, would possess sufficient structural similarity with TLR7/8-agonistic imidazoquinolines. With pyridoxal as the aldehyde component, furo[2,3- c]pyridines, rather than the expected imidazo[1,2-a]pyridines were obtained, which were characterized by NMR spectroscopy and crystallography. Several analogues were found to activate TLR8-dependent NF-κB signaling. In a focused library of furo[2,3-c]pyridines, a distinct SAR was observed with varying substituents at C2. In human PBMCs, none of the furo[2,3-c]pyridines showed any proinflammatory cytokine induction, but upregulated several chemokine ligand genes. In immunization studies in rabbits, the most active compound showed prominent adjuvantic effects. The complete lack of proinflammatory cytokine induction coupled with strong adjuvantic activity of the novel furo[2,3-c]pyridines render this hitherto unknown chemotype an attractive class of compounds which are expected to be devoid of local or systemic reactogenicity

Toll-like Receptor-8 Agonistic Activities in C2, C4, and C8 Modified Thiazolo[4,5-c]quinolines

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

Covalent Plasmodium falciparum-selective proteasome inhibitors exhibit a low propensity for generating resistance in vitro and synergize with multiple antimalarial agents

Therapeutics with novel modes of action and a low risk of generating resistance are urgently needed to combat drug-resistant Plasmodium falciparum malaria. Here, we report that the peptide vinyl sulfones WLL-vs (WLL) and WLW-vs (WLW), highly selective covalent inhibitors of the P. falciparum proteasome, potently eliminate genetically diverse parasites, including K13-mutant, artemisinin-resistant lines, and are particularly active against ring-stage parasites. Selection studies reveal that parasites do not readily acquire resistance to WLL or WLW and that mutations in the β2, β5 or β6 subunits of the 20S proteasome core particle or in components of the 19S proteasome regulatory particle yield only <five-fold decreases in parasite susceptibility. This result compares favorably against previously published non-covalent inhibitors of the Plasmodium proteasome that can select for resistant parasites with >hundred-fold decreases in susceptibility. We observed no cross-resistance between WLL and WLW. Moreover, most mutations that conferred a modest loss of parasite susceptibility to one inhibitor significantly increased sensitivity to the other. These inhibitors potently synergized multiple chemically diverse classes of antimalarial agents, implicating a shared disruption of proteostasis in their modes of action. These results underscore the potential of targeting the Plasmodium proteasome with covalent small molecule inhibitors as a means of combating multidrug-resistant malaria

Self-assembled adipose-derived mesenchymal stem cells as an extracellular matrix component- and growth factor-enriched filler

The clinical application of mesenchymal stem cells (MSCs) is attracting attention due to their excellent safety, convenient acquisition, multipotency, and trophic activity. The clinical effectiveness of transplanted MSCs is well-known in regenerative and immunomodulatory medicine, but there is a demand for their improved viability and regenerative function after transplantation. In this study, we isolated MSCs from adipose tissue from three human donors and generated uniformly sized MSC spheroids (∼100 µm in diameter) called microblocks (MiBs) for dermal reconstitution. The viability and MSC marker expression of MSCs in MiBs were similar to those of monolayer MSCs. Compared with monolayer MSCs, MiBs produced more extracellular matrix (ECM) components, including type I collagen, fibronectin, and hyaluronic acid, and growth factors such as vascular endothelial growth factor and hepatocyte growth factor. Subcutaneously injected MiBs showed skin volume retaining capacity in mice. These results indicate that MiBs could be applied as regenerative medicine for skin conditions such as atrophic scar by having high ECM and bioactive factor expression

UBR2 of the N-End Rule Pathway Is Required for Chromosome Stability via Histone Ubiquitylation in Spermatocytes and Somatic Cells

The N-end rule pathway is a proteolytic system in which its recognition components (N-recognins) recognize destabilizing N-terminal residues of short-lived proteins as an essential element of specific degrons, called N-degrons. The RING E3 ligases UBR2 and UBR1 are major N-recognins that share size (200 kDa), conserved domains and substrate specificities to N-degrons. Despite the known function of the N-end rule pathway in degradation of cytosolic proteins, the major phenotype of UBR2-deficient male mice is infertility caused by arrest of spermatocytes at meiotic prophase I. UBR2-deficient spermatocytes are impaired in transcriptional silencing of sex chromosome-linked genes and ubiquitylation of histone H2A. In this study we show that the recruitment of UBR2 to meiotic chromosomes spatiotemporally correlates to the induction of chromatin-associated ubiquitylation, which is significantly impaired in UBR2-deficient spermatocytes. UBR2 functions as a scaffold E3 that promotes HR6B/UbcH2-dependent ubiquitylation of H2A and H2B but not H3 and H4, through a mechanism distinct from typical polyubiquitylation. The E3 activity of UBR2 in histone ubiquitylation is allosterically activated by dipeptides bearing destabilizing N-terminal residues. Insufficient monoubiquitylation and polyubiquitylation on UBR2-deficient meiotic chromosomes correlate to defects in double strand break (DSB) repair and other meiotic processes, resulting in pachytene arrest at stage IV and apoptosis. Some of these functions of UBR2 are observed in somatic cells, in which UBR2 is a chromatin-binding protein involved in chromatin-associated ubiquitylation upon DNA damage. UBR2-deficient somatic cells show an array of chromosomal abnormalities, including hyperproliferation, chromosome instability, and hypersensitivity to DNA damage-inducing reagents. UBR2-deficient mice enriched in C57 background die upon birth with defects in lung expansion and neural development. Thus, UBR2, known as the recognition component of a major cellular proteolytic system, is associated with chromatin and controls chromatin dynamics and gene expression in both germ cells and somatic cells