Synthesis and structure–Activity relationships of 2-Amino-8-hydroxyadenines as orally active interferon inducing agents

Structural Evolution and Translational Potential for Agonists and Antagonists of Endosomal Toll-like Receptors

Toll-like receptors (TLRs) are members of a large family of evolutionarily conserved pattern recognition receptors (PRRs), which serve as key components of the innate immune system by playing a pivotal role in sensing "nonself" ligands. Endosomal TLRs (TLR3, TLR7, TLR8, and TLR9) can recognize pathogen-derived nucleic acid and initiate an innate immune response because they react against both self- and non-self-origin nucleic acid molecules. Accordingly, both receptor agonists and antagonists are potentially useful in disparate clinical contexts and thus are globally sought after. Recent research has revealed that agonists and antagonists share an overlapping binding region. This Perspective highlights rational medicinal chemistry approaches to elucidate the structural attributes of small molecules capable of agonism or antagonism or of elegantly switching between the two. The structural evolution of different chemotypes can provide the framework for the future development of endosomal TLR agonists and antagonists.

Synthetic Toll-like Receptors 7 and 8 Agonists: Structure-Activity Relationship in the Oxoadenine Series

Toll-like receptors 7 and 8 (TLR7/8) are broadly expressed on antigen-presenting cells, making TLR7/8 agonists likely candidates for the development of new vaccine adjuvants. We previously reported the synthesis of a new series of 8-oxoadenines substituted at the 9-position with a 4-piperidinylalkyl moiety and demonstrated that TLR7/8 selectivity and potency could be modulated by varying the length of the alkyl linker. In the present study, we broadened our initial structure-activity relationship study to further evaluate the effects of N-heterocycle ring size, chirality, and substitution on TLR7/8 potency, receptor selectivity, and cytokine (IFNα and TNFα) induction from human peripheral blood mononuclear cells (PBMCs). TLR7/8 activity correlated primarily to linker length and to a lesser extent to ring size, while ring chirality had little effect on TLR7/8 potency or selectivity. Substitution of the heterocyclic ring with an aminoalkyl or hydroxyalkyl group for subsequent conjugation to phospholipids or antigens was well tolerated with the retention of both TLR7/8 activity and cytokine induction from human PBMCs.

Crystal structures of N6-modified-amino acid related nucleobase analogs (II): hybrid adenine-β-alanine and adenine-GABA molecules

H-Bonding networks and anion–π interactions in the crystal structures of N⁶-modified-amino acid adenine analogs are investigated using X-ray crystallography and DFT calculations.

Structure-Activity Relationship Studies of Pyrimido[5,4-b]indoles as Selective Toll-Like Receptor 4 Ligands

Previous high throughput screening studies led to the discovery of two novel, nonlipid-like chemotypes as Toll-like receptor 4 (TLR4) agonists. One of these chemotypes, the pyrimido[5,4-b]indoles, was explored for structure-activity relationship trends relative to production of TLR4 dependent cytokines/chemokines, resulting in a semioptimized lead (compound 1) that provided a starting point for further optimization studies. In this report, compounds belonging to three areas of structural modification were evaluated for biological activity using murine and human TLR4 reporter cells, primary murine bone marrow derived dendritic cells, and human peripheral blood mononuclear cells. The compounds bearing certain aryl groups at the C8 position, such as phenyl (36) and β-naphthyl (39), had potencies significantly greater than compound 1. Compound 36 displayed human TLR4 agonist activity at submicromolar concentrations. The computational analysis suggests that the improved potency of these C8-aryl derivatives may be the result of additional binding interactions at the interface of the TLR4/myeloid differentiation protein-2 (MD-2) complex.

Past, Current, and Future Developments of Therapeutic Agents for Treatment of Chronic Hepatitis B Virus Infection

For decades, treatment of hepatitis B virus (HBV) infection has been relying on interferon (IFN)-based therapies and nucleoside/nucleotide analogues (NAs) that selectively target the viral polymerase reverse transcriptase (RT) domain and thereby disrupt HBV viral DNA synthesis. We have summarized here the key steps in the HBV viral life cycle, which could potentially be targeted by novel anti-HBV therapeutics. A wide range of next-generation direct antiviral agents (DAAs) with distinct mechanisms of actions are discussed, including entry inhibitors, transcription inhibitors, nucleoside/nucleotide analogues, inhibitors of viral ribonuclease H (RNase H), modulators of viral capsid assembly, inhibitors of HBV surface antigen (HBsAg) secretion, RNA interference (RNAi) gene silencers, antisense oligonucleotides (ASOs), and natural products. Compounds that exert their antiviral activities mainly through host factors and immunomodulation, such as host targeting agents (HTAs), programmed cell death protein 1 (PD-1)/programmed death ligand 1 (PD-L1) inhibitors, and Toll-like receptor (TLR) agonists, are also discussed. In this Perspective, we hope to provide an overview, albeit by no means being comprehensive, for the recent development of novel therapeutic agents for the treatment of chronic HBV infection, which not only are able to sustainably suppress viral DNA but also aim to achieve functional cure warranted by HBsAg loss and ultimately lead to virus eradication and cure of hepatitis B.

Synthesis and immunostimulatory activity of substituted TLR7 agonists

Fifteen new substituted adenines were synthesized as potential TLR7 agonists. These compounds, along with 9 previously reported compounds, were analyzed for TLR7 activity and for the selective stimulation of B cell proliferation. Several functionalized derivatives exhibit significant activity, suggesting their potential for use as vaccine adjuvants.

New TLR7 agonists with improved humoral and cellular immune responses

Toll-like receptor 7 (TLR7) agonists are of interest as vaccine adjuvants and cancer therapeutics. Therefore, development of new TLR7 agonists that can efficiently promote host immune responses without evoking side effects is of great importance. Here, we describe two new compounds, J4 and F4, which elicit intracellular signaling exclusively via TLR7. Interestingly, both J4 and F4 induced less cytokine secretion (IL-1β, IL-6, IL-10, IL-12p40, TNFα, and IL-12p70) from myeloid dendritic cells (mDCs) and monocytes than CL075 and R848; however, they all generated similar levels of phenotype maturation of antigen presenting cells (APCs), including plasmacytoid DCs. We further found that J4- and F4-induced APC activation was largely dependent on the activation of NF-κB and p38. Lastly, J4 and F4 could efficiently promote B cell proliferation and plasmablast differentiation as well as antigen-specific CD8(+) T cell responses in human in vitro. Therefore, these new TLR7 agonists could be employed to facilitate the development of new therapeutics and vaccine adjuvants against cancers and microbial infections.

Structural requirements for TLR7-selective signaling by 9-(4-piperidinylalkyl)-8-oxoadenine derivatives

We report the synthesis and biological evaluation of a new series of 8-oxoadenines substituted at the 9-position with a 4-piperidinylalkyl moiety. In vitro evaluation of the piperidinyl-substituted oxoadenines 3a-g in human TLR7- or TLR8-transfected HEK293 cells and in human PBMCs indicated that TLR7/8 selectivity/potency and cytokine induction can be modulated by varying the length of the alkyl linker. Oxoadenine 3f containing a 5-carbon linker was found to be the most potent TLR7 agonist and IFNα inducer in the series whereas 3b possessing a 1-carbon linker was the most potent TLR8 agonist.

Facile syntheses of functionalized toll-like receptor 7 agonists

Protein conjugates of toll-like receptor 7 agonists have been shown to elicit powerful immune responses. In order to facilitate our studies in this area our group has developed efficient syntheses for a number of functionalized derivatives that retain immune stimulatory activity.

Mechanism of action of inhibition of allergic immune responses by a novel antedrug TLR7 agonist

Triggering innate immune responses through TLRs is expected to be a novel therapeutic strategy for the treatment of allergic diseases. TLR agonists are able to modulate Th2 immune responses through undefined mechanisms. We investigated the mechanism of action of the suppression of Th2 immune responses with a novel antedrug TLR7 agonist. The antedrug is rapidly metabolized by plasma esterases to an acid with reduced activity to limit systemic responses. Topical administration of this compound inhibited features of the allergic airway inflammatory response in rat and murine allergic airways model. Type I IFN played a role in the suppression of Th2 cytokines produced from murine splenocytes. Inhibition of Th2 immune responses with the antedrug TLR7 agonist was shown to be via a type I IFN-dependent mechanism following short-term exposure to the compound, although there might be type I IFN-independent mechanisms following long-term exposure. We have demonstrated that local type I IFN signaling and plasmacytoid dendritic cells, but not Th1 immune responses, are required for in vivo efficacy against murine airway Th2-driven eosinophilia. Furthermore, migration of dendritic cell subsets into the lung was related to efficacy and is dependent on type I IFN signaling. Thus, the mechanism of action at the cytokine and cellular level involved in the suppression of Th2 allergic responses has been characterized, providing a potential new approach to the treatment of allergic disease.

Negative regulation of the type I interferon signaling pathway by synthetic Toll-like receptor 7 ligands

Ten Toll-like receptor (TLR) family members have been reported in humans. Here, the endoplasmatic receptors TLR9, TLR8, TLR7, and TLR3 respond to nucleic acids and derivatives or to small molecules (TLR7 and 8). Another cytoplasmic RNA receptor, retinoic acid inducible gene I (RIG-I), is stimulated by 5' triphosphate double-stranded RNA. We discovered that TLR7 small-molecule agonists inhibit nucleic acid-mediated TLR3, TLR7, TLR9, or RIG-I-dependent interferon-α (IFN-α) immune response. Other cytokines and chemokines stimulated by nucleic acid agonists remained unaffected. The observed blockage of TLR3, TLR7, TLR9, and RIG-I-mediated IFN-α response appears to be driven by a competitive mechanism at the type I IFN pathway. Besides type I IFN, IFN response genes such as IFIT-1, Mx1, OAS1, or IRF7 were affected, which indicates that the key element driving the inhibition is located in the type I IFN pathway. Indeed, the heterotrimeric complex formation of phosphor-signal transducer and activator of transcription factor 1 (STAT1), phosphor-STAT2, and IRF9 (called ISGF3, IFN-stimulated gene factor 3) is inhibited through the TLR7 small-molecule agonists by phosphor-STAT2 blockage. These findings provide novel insights into the use of synthetic TLR7 or TLR7/8 small molecules as ligands for immune activation and suppression.

Small molecule Toll-like receptor 7 agonists localize to the MHC class II loading compartment of human plasmacytoid dendritic cells

TLR7 and TLR8 are intracellular sensors activated by single-stranded RNA species generated during viral infections. Various synthetic small molecules can also activate TLR7 or TLR8 or both through an unknown mechanism. Notably, direct interaction between small molecules and TLR7 or TLR8 has never been shown. To shed light on how small molecule agonists target TLRs, we labeled 2 imidazoquinolines, resiquimod and imiquimod, and one adenine-based compound, SM360320, with 2 different fluorophores [5(6) carboxytetramethylrhodamine and Alexa Fluor 488] and monitored their intracellular localization in human plasmacytoid dendritic cells (pDCs). All fluorescent compounds induced the production of IFN-α, TNF-α, and IL-6 and the up-regulation of CD80 and CD86 by pDCs showing they retained TLR7-stimulating activity. Confocal imaging of pDCs showed that, similar to CpG-B, all compounds concentrated in the MHC class II loading compartment (MIIC), identified as lysosome-associated membrane protein 1+, CD63, and HLA-DR+ endosomes. Treatment of pDCs with bafilomycin A, an antagonist of the vacuolar-type proton ATPase controlling endosomal acidification, prevented the accumulation of small molecule TLR7 agonists, but not of CpG-B, in the MIIC. These results indicate that a pH-driven concentration of small molecule TLR7 agonists in the MIIC is required for pDC activation.

The purines: potent and versatile small molecule inhibitors and modulators of key biological targets

The goal of this review is to highlight the wide range of biological activities displayed by purines, with particular emphasis on new purine-based agents which find potential application as chemical-biology tools and/or therapeutic agents. The expanding interest in the biological properties of polyfunctionalized purine derivatives issues, in large part, from the development of rapid high-throughput screening essays for new protein targets, and the corresponding development of efficient synthetic methodology adapted to the construction of highly diverse purine libraries. Purine-based compounds have found new applications as inducers of interferon and lineage-committed cell dedifferentiation, agonists and antagonists of adenosine receptors, ligands of corticotropin-releasing hormone receptors, and as inhibitors of HSP90, Src kinase, p38alpha MAP kinase, sulfotransferases, phosphodiesterases, and Cdks. The scope of application of purines in biology is most certainly far from being exhausted. Testing purine derivatives against the multitude of biological targets for which small molecule probes have not yet been found should thus be a natural reflex.

Synthesis and Biological Evaluation of Novel 9-Substituted-8-Hydroxyadenine Derivatives as Potent Interferon Inducers

Recently we reported the adenine derivatives 3a-d as novel interferon (IFN) inducers. In the present study, we conducted a detailed structure-activity relationship study of analogues of 3a-d with respect to their IFN-inducing activity, mainly focusing on the N(9)-position of the adenine. From this study, we found that introduction of the 3-pyridylmethyl moiety was effective to increase in vitro activity, and compound 9ae was identified as being the most potent IFN inducer. This compound gave a minimum effective concentration (MEC) of 3 nM, which is comparable with that of R-848, a second generation IFN inducer. Compound 9ae also demonstrated potent IFN-inducing activity at a dose of 0.1 mg/kg by oral administration in mice. Furthermore, compound 9ae induced IFN in monkeys in a dose dependent manner, with a potency superior to that of R-848. In addition, 9ae did not cause emesis in ferrets even at a dose of 30 mg/kg. In this study the maximum plasma concentration of 9ae was 1019 ng/mL (ca. 3.1 microM), which was approximately 1000-fold higher than the MEC value. Therefore, with respect to both the efficacy and the safety margin, compound 9ae (SM-276001) is considered to be a promising compound as an orally active IFN inducer.

Activation of anti-hepatitis C virus responses via Toll-like receptor 7

IFN-alpha is used to suppress the replication of hepatitis C virus (HCV) in chronically infected patients with partial success. Here we present evidence showing that a ligand of Toll-like receptor 7 (TLR7) can induce anti-HCV immunity not only by IFN induction, but also through an IFN-independent mechanism. Human hepatocyte line Huh-7 carrying an HCV replicon expressed TLR7, and activation of the receptor induced several antiviral genes including IFN regulatory factor-7. Inhibitors of the enzyme inosine monophosphate dehydrogenase augmented both IFN-dependent and -independent antiviral effect. Prolonged exposure of Huh-7 cells to a TLR7 ligand [SM360320 (9-benzyl-8-hydroxy-2-(2-methoxyethoxy)adenine)], alone or in combination with an inosine monophosphate dehydrogenase inhibitor, reduced HCV levels dose dependently. Immunohistochemical analysis of livers shows that TLR7 is expressed in hepatocytes of normal or HCV-infected people. Because TLR7 agonists can impede HCV infection both via type I IFN and independently of IFN, they may be considered as an alternative treatment of chronic HCV infection, especially in IFN-alpha-resistant patients.

Synthesis and evaluation of 2-substituted 8-hydroxyadenines as potent interferon inducers with improved oral bioavailabilities

In order to create novel compounds which possess potent interferon (IFN) inducing activities with excellent oral bioavailabilities, a series of 8-hydroxyadenines, which have various alkoxy or alkylthio moieties at the adenine C(2)-position, were synthesized and evaluated. The introduction of hydrophobic groups was not considered to be effective for potentiating the IFN-inducing activity, but several compounds having hydrophilic groups were effective. Among the compounds tested, compound 13f induced IFN from the dosage of 0.03 mg/kg, which was approximately 100-fold more potent than that of Imiquimod, and showed an excellent oral bioavailability (F=40%) which was 10-fold improved over 5, a lead compound (F=4%).