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

Small-Molecule Modulators Targeting Toll-like Receptors for Potential Anticancer Therapeutics

Toll-like receptors (TLRs) are key components of the innate immune system and serve as a crucial link between innate and acquired immunity. In addition to immune function, TLRs are involved in other important pathological processes, including tumorigenesis. TLRs have dual regulatory effects on tumor immunity by activating nuclear factor κ-B signaling pathways, which induce tumor immune evasion or enhance the antitumor immune response. Therefore, TLRs have become a popular target for cancer prevention and treatment, and TLR agonists and antagonists offer considerable potential for drug development. The TLR7 agonist imiquimod (1) has been approved by the U.S. Food and Drug Administration as a treatment for malignant skin cancer. Herein, the structure, signaling pathways, and function of the TLR family are summarized, and the structure-activity relationships associated with TLR selective and multitarget modulators and their potential application in tumor therapy are systematically discussed.

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.

Mitochondria-dependent synthetic small-molecule vaccine adjuvants for influenza virus infection

Vaccine adjuvants enhance and prolong pathogen-specific protective immune responses. Recent reports indicate that host factors-such as aging, pregnancy, and genetic polymorphisms-influence efficacies of vaccines adjuvanted with Toll-like receptor (TLR) or known pattern-recognition receptor (PRR) agonists. Although PRR independent adjuvants (e.g., oil-in-water emulsion and saponin) are emerging, these adjuvants induce some local and systemic reactogenicity. Hence, new TLR and PRR-independent adjuvants that provide greater potency alone or in combination without compromising safety are highly desired. Previous cell-based high-throughput screenings yielded a small molecule 81 [N-(4-chloro-2,5-dimethoxyphenyl)-4-ethoxybenzenesulfonamide], which enhanced lipopolysaccharide-induced NF-κB and type I interferon signaling in reporter assays. Here compound 81 activated innate immunity in primary human peripheral blood mononuclear cells and murine bone marrow-derived dendritic cells (BMDCs). The innate immune activation by 81 was independent of TLRs and other PRRs and was significantly reduced in mitochondrial antiviral-signaling protein (MAVS)-deficient BMDCs. Compound 81 activities were mediated by mitochondrial dysfunction as mitophagy inducers and a mitochondria specific antioxidant significantly inhibited cytokine induction by 81. Both compound 81 and a derivative obtained via structure-activity relationship studies, 2F52 [N-benzyl-N-(4-chloro-2,5-dimethoxyphenyl)-4-ethoxybenzenesulfonamide] modestly increased mitochondrial reactive oxygen species and induced the aggregation of MAVS. Neither 81 nor 2F52 injected as adjuvants caused local or systemic toxicity in mice at effective concentrations for vaccination. Furthermore, vaccination with inactivated influenza virus adjuvanted with 2F52 demonstrated protective effects in a murine lethal virus challenge study. As an unconventional and safe adjuvant that does not require known PRRs, compound 2F52 could be a useful addition to vaccines.

Structural evolution of toll-like receptor 7/8 agonists from imidazoquinolines to imidazoles

Several synthetic heterocyclic small molecules like imiquimod, resiquimod, CL097, CL075, bromopirone, tilorone, loxoribine and isatoribine demonstrated TLR7/8 agonistic activity and relatively modest structural changes in such molecules result in major variation in the TLR7 and/or TLR8 activity. A strict dependency of the electronic configuration of the heterocyclic system was also observed to influence the agonistic activity. In the present review, an evolution of imidazole based TLR7/8 agonist from imidazoquinoline based scaffold is delineated along with the elaboration of detailed structure activity relationship (SAR) in each chemotype. The structural and activity details of not only the active compounds but also the related inactive compounds are included to better understand the SAR. TLR7/8 agonists are emerging as promising vaccine adjuvant candidates and the present SAR and structural information will provide a road map towards the identification of more potent and appropriate candidates for further drug discovery.

Agonist and antagonist ligands of toll-like receptors 7 and 8: Ingenious tools for therapeutic purposes

The discovery of the TLRs family and more precisely its functions opened a variety of gates to modulate immunological host responses. TLRs 7/8 are located in the endosomal compartment and activate a specific signaling pathway in a MyD88-dependant manner. According to their involvement into various autoimmune, inflammatory and malignant diseases, researchers have designed diverse TLRs 7/8 ligands able to boost or block the inherent signal transduction. These modulators are often small synthetic compounds and most act as agonists and to a much lesser extent as antagonists. Some of them have reached preclinical and clinical trials, and only one has been approved by the FDA and EMA, imiquimod. The key to the success of these modulators probably lies in their combination with other therapies as recently demonstrated. We gather in this review more than 360 scientific publications, reviews and patents, relating the extensive work carried out by researchers on the design of TLRs 7/8 modulators, which are classified firstly by their biological activities (agonist or antagonist) and then by their chemical structures, which total syntheses are not discussed here. This review also reports about 90 clinical cases, thereby showing the biological interest of these modulators in multiple pathologies.

Optimization of 8-oxoadenines with toll-like-receptor 7 and 8 activity

Toll-like receptors 7 and 8 (TLR7/8) agonists are potent immunostimulants that are attracting considerable interest as vaccine adjuvants. We recently reported the synthesis of a new series of 2-O-butyl-8-oxoadenines substituted at the 9-position with various linkers and N-heterocycles, and showed that TLR7/8 selectivity, potency and cytokine induction could be modulated by varying the alkyl linker length and the N-heterocyclic ring. In the present study, we further optimized the oxoadenine scaffold by investigating the effect of different substituents at the 2-position of the oxoadenine on TLR7/8 potency/selectivity, cytokine induction and DC maturation in human PBMCs. The results show that introducing a 1-(S)-methylbutoxy group at the 2-position of the oxoadenine significantly increased potency for TLR7/8 activity, cytokine induction and DC maturation.

Better Adjuvants for Better Vaccines: Progress in Adjuvant Delivery Systems, Modifications, and Adjuvant-Antigen Codelivery

Traditional aluminum adjuvants can trigger strong humoral immunity but weak cellular immunity, limiting their application in some vaccines. Currently, various immunomodulators and delivery carriers are used as adjuvants, and the mechanisms of action of some of these adjuvants are clear. However, customizing targets of adjuvant action (cellular or humoral immunity) and action intensity (enhancement or inhibition) according to different antigens selected is time-consuming. Here, we review the adjuvant effects of some delivery systems and immune stimulants. In addition, to improve the safety, effectiveness, and accessibility of adjuvants, new trends in adjuvant development and their modification strategies are discussed.

Synthesis and immunostimulatory activity of sugar-conjugated TLR7 ligands

Toll-like receptors (TLRs) are a type of pattern recognition receptors (PRRs), which are activated by recognizing pathogen-associated molecular patterns (PAMPs). The activation of TLRs initiates innate immune responses and subsequently leads to adaptive immune responses. TLR agonists are effective immuomodulators in vaccine adjuvants for infectious diseases and cancer immunotherapy. In exploring hydrophilic small molecules of TLR7 ligands using the cell-targeted property of a vaccine adjuvant, we conjugated 1V209, a small TLR7 ligand molecule, with various low or middle molecular weight sugar molecules that work as carriers. The sugar-conjugated 1V209 derivatives showed increased water solubility and higher immunostimulatory activity in both mouse and human cells compared to unmodified 1V209. The improved immunostimulatory potency of sugar-conjugates was attenuated by an inhibitor of endocytic process, cytochalasin D, suggesting that conjugation of sugar moieties may enhance the uptake of TLR7 ligand into the endosomal compartment. Collectively our results support that sugar-conjugated TLR7 ligands are applicable to novel drugs for cancer and vaccine therapy.

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.

Impact of Polymer-TLR-7/8 Agonist (Adjuvant) Morphology on the Potency and Mechanism of CD8 T Cell Induction

Small molecule Toll-like receptor-7 and -8 agonists (TLR-7/8a) can be used as vaccine adjuvants to induce CD8 T cell immunity but require formulations that prevent systemic toxicity and focus adjuvant activity in lymphoid tissues. Here, we covalently attached TLR-7/8a to polymers of varying composition, chain architecture and hydrodynamic behavior (∼300 nm submicrometer particles, ∼10 nm micelles and ∼4 nm flexible random coils) and evaluated how these parameters of polymer-TLR-7/8a conjugates impact adjuvant activity in vivo. Attachment of TLR-7/8a to any of the polymer compositions resulted in a nearly 10-fold reduction in systemic cytokines (toxicity). Moreover, both lymph node cytokine production and the magnitude of CD8 T cells induced against protein antigen increased with increasing polymer-TLR-7/8a hydrodynamic radius, with the submicrometer particle inducing the highest magnitude responses. Notably, CD8 T cell responses induced by polymer-TLR-7/8a were dependent on CCR2+ monocytes and IL-12, whereas responses by a small molecule TLR-7/8a that unexpectedly persisted in vaccine-site draining lymph nodes (T1/2 = 15 h) had less dependence on monocytes and IL-12 but required Type I IFNs. This study shows how modular properties of synthetic adjuvants can be chemically programmed to alter immunity in vivo through distinct immunological mechanisms.

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.

High potency of lipid conjugated TLR7 agonist requires nanoparticulate or liposomal formulation

Conjugation of small molecule agonists of Toll-like receptor 7 (TLR7) to proteins, lipids, or polymers is known to modulate potency, and the physical form or formulation of these conjugates is likely to have a major effect on their immunostimulatory activity. Here, we studied the effect of formulation on potency of a 1,2‑di‑(9Z‑octadecenoyl)‑sn‑glycero‑3‑phosphoethanolamine (DOPE) conjugated TLR7 agonist (DOPE-TLR7a) alongside assessing physical form using Dynamic Light Scattering (DLS), Nanosight Particle Tracking (NTA) analysis and Small Angle X-ray Scattering (SAXS). A very high potency of DOPE-TLR7a conjugate (EC₅₀ around 9 nM) was observed either when prepared by direct dilution from DMSO or when formulated into 400-700 nm large multilamella liposomes containing dimethyldioctadecylammonium bromide salt (DDA) and DOPE. When prepared by dissolution in DMSO followed by dilution in aqueous culture medium, 93 ± 5 nm nanoparticles were formed. Without dilution from solution in DMSO, no nanoparticles were observed and no immunostimulatory activity could be detected without this formulation step. SAXS analysis of the conjugate after DMSO dissolution/water dilution revealed a lamellar order with a layer spacing of 68.7 Å, which correlates with arrangement in groups of 3 bilayers. The addition of another immunostimulatory glycolipid, trehalose‑6,6‑dibehenate (TDB), to DOPE:DDA liposomes gave no further increase in immunostimulatory activity beyond that provided by incorporating DOPE-TLR7a. Given the importance of nanoparticle or liposomal formulation for activity, we conclude that the major mechanism for increased potency when TLR7 agonists are conjugated to macromolecules is through alteration of physical form.

Toll-like Receptor Agonist Conjugation: A Chemical Perspective

Toll-like receptors (TLRs) are vital elements of the mammalian immune system that function by recognizing pathogen-associated molecular patterns (PAMPs), bridging innate and adaptive immunity. They have become a prominent therapeutic target for the treatment of infectious diseases, cancer, and allergies, with many TLR agonists currently in clinical trials or approved as immunostimulants. Numerous studies have shown that conjugation of TLR agonists to other molecules can beneficially influence their potency, toxicity, pharmacokinetics, or function. The functional properties of TLR agonist conjugates, however, are highly dependent on the ligation strategy employed. Here, we review the chemical structural requirements for effective functional TLR agonist conjugation. In addition, we provide similar analysis for those that have yet to be conjugated. Moreover, we discuss applications of covalent TLR agonist conjugation and their implications for clinical use.

Discovery of selective 2,4-diaminoquinazoline toll-like receptor 7 (TLR 7) agonists

The discovery of a novel series of highly potent quinazoline TLR 7/8 agonists is described. The synthesis and structure-activity relationship is presented. Structural requirements and optimization of this series toward TLR 7 selectivity afforded the potent agonist 48. Pharmacokinetic and pharmacodynamic studies highlighted 48 as an orally available endogenous interferon (IFN-α) inducer in mice.

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.

Synergy of purine-scaffold TLR7 agonist with doxorubicin on systemic inhibition of lymphoma in mouse model

Chemo- and radio-therapy suffer from certain well-recognized drawbacks for lymphoma therapy. Passive immunotherapy with monoclonal antibody has improved outcome for patients with CD20⁺ B cell lymphoma, but not for T cell lymphoma. Therefore, novel treatment approaches are clearly required for T cell lymphoma. To date, the combined application targeting TLR7, 8 and 9 has established long-term antitumor immunity. We previously synthesized a purine-scaffold TLR7 agonist named GD5. Here, we report that the intratumoral administration of GD5 combined with doxorubicin (DOX), a conventional chemotherapeutic agent in T cell lymphoma. This combined treatment made mice to produce more cytokines in blood, and generate more potent cytotoxic T lymphocyte response, then result in effective eradication of both local and distant tumors in tumor-bearing mice. Our findings demonstrate the potential for enhancing the efficacy of the current standard DOX therapy through combination with TLR7 agonist GD5 to improve antitumor immune responses and provide durable remissions for T cell lymphoma.

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.

8-Oxo-7,8-dihydroadenine and 8-Oxo-7,8-dihydroadenosine-Chemistry, Structure, and Function in RNA and Their Presence in Natural Products and Potential Drug Derivatives

A description and history of the role that 8-oxo-7,8-dihydroadenine (8-oxoAde) and 8-oxo-7,8-dihydroadenosine (8-oxoA) have in various fields has been compiled. This Review focusses on 1) the formation of this oxidatively generated modification in RNA, its interactions with other biopolymers, and its potential role in the development/progression of disease; 2) the independent synthesis and incorporation of this modified nucleoside into oligonucleotides of RNA to display the progress that has been made in establishing its behavior in biologically relevant systems; 3) reported synthetic routes, which date back to 1890, along with the progress that has been made in the total synthesis of the nucleobase, nucleoside, and their corresponding derivatives; and 4) the isolation, total synthesis, and biological activity of natural products containing these moieties as the backbone. The current state of research regarding this oxidatively generated lesion as well as its importance in the context of RNA, natural products, and potential as drug derivatives is illustrated using all available examples reported to date.

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.

Synthetic Toll-like receptor 7 ligand inhibits porcine reproductive and respiratory syndrome virus infection in primary porcine alveolar macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV), a common viral pathogen, causes huge annual economic losses to the swine industry worldwide. After triggering by specific ligands, the Toll-like receptor 7 (TLR7), a type of pattern-recognition receptor (PRR), induces antiviral cytokines production. Previously, we synthesized an adenine analog, designated SZU101, a TLR7-specific ligand. In this study, we assessed the inhibitory effect of SZU101 on PRRSV infection in vitro. SZU101 significantly suppressed PRRSV infection in primary porcine alveolar macrophages (PAMs) in a dose-dependent manner. Moreover, SZU101-induced inhibition involved NF-κB pathway activation in PAMs to initiate expression of TLR7-mediated cytokines and induce expression of downstream signaling IFN-stimulated genes (ISGs). Chloroquine, a TLR7 inhibitor, and BAY 11-7082, an NF-κB inhibitor, reversed both the SZU101-induced antiviral effect and induction of cytokine genes and ISGs expression. Therefore, SZU101 antiviral effects depend at least in part on TLR7-NF-κB signaling pathway. Additionally, administration of SZU101 enhanced the humoral and cell-mediated immune responses against PRRSV antigens in mice. Given these results, SZU101 holds promise as an antiviral agent and a vaccine adjuvant to prevent PRRSV infection in pigs.

Potential of PEGylated Toll-Like Receptor 7 Ligands for Controlling Inflammation and Functional Changes in Mouse Models of Asthma and Silicosis

Prior investigations show that signaling activation through pattern recognition receptors can directly impact a number of inflammatory lung diseases. While toll-like receptor (TLR) 7 agonists have raised interest for their ability to inhibit allergen-induced pathological changes in experimental asthma conditions, the putative benefit of this treatment is limited by adverse effects. Our aim was to evaluate the therapeutic potential of two PEGylated purine-like compounds, TMX-302 and TMX-306, characterized by TLR7 partial agonistic activity; therefore, the compounds are expected to induce lower local and systemic adverse reactions. In vitro approaches and translation to murine models of obstructive and restrictive lung diseases were explored. In vitro studies with human PBMCs showed that both TMX-302 and TMX-306 marginally affects cytokine production as compared with equivalent concentrations of the TLR7 full agonist, TMX-202. The PEGylated compounds did not induce monocyte-derived DC maturation or B cell proliferation, differently from what observed after stimulation with TMX-202. Impact of PEGylated ligands on lung function and inflammatory changes was studied in animal models of acute lung injury, asthma, and silicosis following Lipopolysaccharide (LPS), allergen (ovalbumin), and silica inhalation, respectively. Subcutaneous injection of TMX-302 prevented LPS- and allergen-induced airway hyper-reactivity (AHR), leukocyte infiltration, and production of pro-inflammatory cytokines in the lung. However, intranasal instillation of TMX-302 led to neutrophil infiltration and failed to prevent allergen-induced AHR, despite inhibiting leukocyte counts in the BAL. Aerosolized TMX-306 given prophylactically, but not therapeutically, inhibited pivotal asthma features. Interventional treatment with intranasal instillation of TMX-306 significantly reduced the pulmonary fibrogranulomatous response and the number of silica particles in lung interstitial space in silicotic mice. These findings highlight the potential of TMX-306, emphasizing its value in drug development for lung diseases, and particularly silicosis.

Potent and Selective Inhibitors of MTH1 Probe Its Role in Cancer Cell Survival

Recent literature has claimed that inhibition of the enzyme MTH1 can eradicate cancer. MTH1 is one of the "housekeeping" enzymes that are responsible for hydrolyzing damaged nucleotides in cells and thus prevent them from being incorporated into DNA. We have developed orthogonal and chemically distinct tool compounds to those published in the literature to allow us to test the hypothesis that inhibition of MTH1 has wide applicability in the treatment of cancer. Here we present the work that led to the discovery of three structurally different series of MTH1 inhibitors with excellent potency, selectivity, and proven target engagement in cells. None of these compounds elicited the reported cellular phenotype, and additional siRNA and CRISPR experiments further support these observations. Critically, the difference between the responses of our highly selective inhibitors and published tool compounds suggests that the effect reported for the latter may be due to off-target cytotoxic effects. As a result, we conclude that the role of MTH1 in carcinogenesis and utility of its inhibition is yet to be established.

Discovery of 6-Amino-2-{[(1S)-1-methylbutyl]oxy}-9-[5-(1-piperidinyl)pentyl]-7,9-dihydro-8H-purin-8-one (GSK2245035), a Highly Potent and Selective Intranasal Toll-Like Receptor 7 Agonist for the Treatment of Asthma

Induction of IFNα in the upper airways via activation of TLR7 represents a novel immunomodulatory approach to the treatment of allergic asthma. Exploration of 8-oxoadenine derivatives bearing saturated oxygen or nitrogen heterocycles in the N-9 substituent has revealed a remarkable selective enhancement in IFNα inducing potency in the nitrogen series. Further potency enhancement was achieved with the novel (S)-pentyloxy substitution at C-2 leading to the selection of GSK2245035 (32) as an intranasal development candidate. In human cell cultures, compound 32 resulted in suppression of Th2 cytokine responses to allergens, while in vivo intranasal administration at very low doses led to local upregulation of TLR7-mediated cytokines (IP-10). Target engagement was confirmed in humans following single intranasal doses of 32 of ≥20 ng, and reproducible pharmacological response was demonstrated following repeat intranasal dosing at weekly intervals.

Enhancement of the Immunostimulatory Activity of a TLR7 Ligand by Conjugation to Polysaccharides

Toll-like receptors (TLRs) in the innate immune system recognize specific pathogen-associated molecular patterns derived from microbes. Synthetic small molecule TLR7 agonists have been extensively evaluated as topical agents for antiviral and anticancer therapy, and as adjuvants for vaccine. However, safe and reproducible administration of synthetic TLR7 ligands has been difficult to achieve due to undesirable pharmacokinetics and unacceptable side effects. Here, we conjugated a versatile low molecular weight TLR7 ligand to various polysaccharides in order to improve its water solubility, enhance its potency, and maintain low toxicity. The synthetic TLR7 ligand, 2-methoxyethoxy-8-oxo-9-(4-carboxy benzyl)adenine, designated 1V209, was stably conjugated to primary amine functionalized Ficoll or dextran using benzoic acid functional groups. The conjugation ratios using specified equivalents of TLR7 ligand were dose responsive and reproducible. The zeta potential value of the polysaccharides was decreased in inverse proportion to the ratio of conjugated TLR7 ligand. These conjugates were highly water-soluble, stable for at least 6 months at room temperature in aqueous solution, and easy to lyophilize and reconstitute without altering potency. In vitro studies with murine mononuclear leukocytes showed that the TLR7 agonist conjugated to polysaccharides had 10- to 1000-fold higher potencies than the unconjugated TLR7 ligand. In vivo pharmacodynamics studies after injection indicate that the conjugates induced systemic cytokine production. When the conjugates were used as vaccine adjuvants, they enhanced antigen specific humoral and cellular immune responses to a much greater extent than did unconjugated TLR7 ligands. These results indicated that small molecule TLR7 ligands conjugated to polysaccharides have improved immunostimulatory potency and pharmacodynamics. Polysaccharides can be conjugated to a variety of molecules such as antigens, peptides, and TLR ligands. Therefore, such conjugates could represent a versatile platform for the development of vaccines against cancer and infectious diseases.

Induction of Tolerogenic Dendritic Cells by a PEGylated TLR7 Ligand for Treatment of Type 1 Diabetes

Autoimmune diabetes mellitus (DM) results from the destruction of pancreatic islet cells by activated T lymphocytes, which have been primed by activated dendritic cells (DC). Individualized therapy with ex vivo DC manipulation and reinfusion has been proposed as a treatment for DM, but this treatment is limited by cost, and requires specialized facilities. A means of in situ modulation of the DC phenotype in the host would be more accessible. Here we report a novel innate immune modulator, 1Z1, generated by conjugating a TLR7 ligand to six units of polyethylene glycol (PEG), which skews DC phenotype in vivo. 1Z1 was less potent in inducing cytokine production by DC than the parent ligand in vitro and in vivo. In addition, this drug only modestly increased DC surface expression of activation markers such as MHC class II, CD80, and CD86; however, the expression of negative regulatory molecules, such as programmed death ligand 1 (PD-L1), and interleukin-1 receptor-associated kinase M (IRAK-M) were markedly increased. In vivo transfer of 1Z1 treated DC into prediabetic NOD mice delayed pancreatic insulitis. Daily administration of 1Z1 effectively prevented the clinical onset of hyperglycemia and reduced histologic islet inflammation. Daily treatment with 1Z1 increased PD-L1 expression in the CD11c⁺ population in peri-pancreatic lymph nodes; however, it did not induce an increase in regulatory T cells. Pharmaceutical modulation of DC maturation and function in situ, thus represents an opportunity to treat autoimmune disease.

Toll-like receptor 7 inactive ligands enhanced cytokine induction by conjugation to weak antigens

Toll-like receptors (TLRs) 7/8 are key targets in the design and development of small-molecule drugs serving as anticancer/antiviral agents and vaccine adjuvants. Clinical trials of imiquimod were discontinued owing to its serious adverse side effects. Herein we report the synthesis and biological evaluation of a series of 8-hydroxy-2-(2-methoxyethoxy)adenine derivatives that cannot induce cytokine production and that lack activity toward TLR 7/8. Their ability to triggering remarkable levels of cytokine production were revealed upon their conjugation with antigens that have weak immunogenicity. This discovery demonstrated that TLR 7 can be activated by coupling an antigen to the terminal carboxyl group at N9 of the inactive ligand adenine analogues. These inactive analogues may be well suited as new adjuvants with superior activity after conjugation, effectively decreasing the side effects caused by conventional adjuvants.

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.

Improving both aqueous solubility and anti-cancer activity by assessing progressive lead optimization libraries

Thiazolidinone compounds 1-3 are lead compounds that have cytoselective toxicity toward non-small cell lung cancer (NSCLC) cells and drug-resistant NSCLC cells while showing low toxicity to normal human fibroblasts (NHFB). However, this class of compounds generally has a very low aqueous solubility (∼0.1 μg/ml). In order to improve both solubility and anti-cancer activity, we designed and synthesized two lead-optimization libraries and investigated these libraries using simultaneous high-throughput solubility and cytotoxicity assays. By all-around modifications on R(1), R(2) and R(3) substitutions, consecutive library synthesis, and testing, we improved the aqueous solubility (5-fold improvement in solubility, from 0.1 to 0.5 μg/ml) and anti-cancer activity (10-fold improvement in EC50 from 0.72-0.98 μM to 0.08-0.16 μM) in the new lead thiazolidinone compound 31.

Synthesis and Evaluation of Conjugates of Novel TLR7 Inert Ligands as Self-Adjuvanting Immunopotentiators

During the design and synthesis of a series of 8-hydroxy-2-(2-methoxyethoxy)-adenine derivatives bearing various substituted -RCOOH groups at the 9-position, we identified a TLR7-inert ligand, which does not activate TLR7 signaling pathway. Of interest, the coupling of weakly immunogenic antigens via the -RCOOH group was able to significantly enhance the immunogenicity of the antigens. Herein, an inert ligand, 9-(3-carboxypropyl)-8-hydroxy-2-(2-methoxyethoxy)-adenine (5, GD2), was synthesized and conjugated to 5 different weakly immunogenic antigens (BSA, OVA, MSA, MG7, and thymosin). Compared with the GD2 and the potent agonist UC-1 V150, all conjugates demonstrated potent immunogenicity in vitro and in vivo. All conjugates induced prolonged increases, while UC-1 V150 showed a rapid decline in the levels of proinflammatory cytokines following initial increases. These data indicate that the immunostimulatory activity of TLR7-inert ligands could be amplified and prolonged by conjugation to antigens, thus broadening the potential therapeutic application of these agents.

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.

Synthesis and immunoregulatory activities of conjugates of a Toll-like receptor 7 inert ligand

In the synthesis and modification of the analogs of an adenine type of Toll-like receptor (TLR) 7 agonists, we found a special compound, 9-propionyloxy-8-hydroxy-2-(2-methoxyethoxy)-adenine (6). It is a synthesized TLR7 inert ligand, which does not respond to TLR7 itself. However, it can be coupled with protein or peptide antigens via propionyloxy functional group to promote their immunogenicity significantly. The compound was covalently coupled to protein and peptide to get the conjugates. The inductivity of cytokine production by the conjugates was 872.4-fold compared with the unconjugated antigens in vitro by mouse splenocyte. These data show that the immunostimulatory activity of inert TLR7 ligand can be endowed, and the activity of antigens can be amplified by conjugation with various proteins and peptides, thus broadening the potential therapeutic application and reducing the risk of TLR7 agonists' side effects.

Inhibition of keratinocyte proliferation by phospholipid-conjugates of a TLR7 ligand in a Myc-induced hyperplastic actinic keratosis model in the absence of systemic side effects

BACKGROUND

The Toll-like receptor 7 (TLR7) activator imiquimod (IMQ) is safe and effective in treating actinic keratosis; however, an intermittent treatment regimen is necessary because of excessive local reactions.

OBJECTIVES

To evaluate in vitro potency, pharmacodynamics/pharmacokinetics, toxicity and efficacy in vivo of the newly developed TLR7 ligand-phospholipid conjugate, TMX-202, in a gel formulation.

MATERIAL AND METHODS

The effects of TMX-202 were assessed both in vitro on a murine macrophage cell line and in primary bone marrow-derived dendritic cells and in vivo on mice (C57BL/6-wild type, Myd88(-/-) and Tlr7(-/-)).

RESULTS

TMX-202 was more potent than IMQ in vitro using murine and human cells. In contrast, in vivo it showed less systemic pro-inflammatory activity and better safety than IMQ. Moreover, the TMX-202 gel formulation exhibited at least comparable efficacy to Aldara in a mouse model for skin proliferative diseases.

CONCLUSION

TMX-202 is safe and efficacious without causing excessive adverse effects, suggesting that it may be an alternative to Aldara for the treatment of proliferative skin conditions.

A novel allergen-adjuvant conjugate suitable for specific immunotherapy of respiratory allergy

BACKGROUND

Several approaches to find a better adjuvant, focus immunomodulation, and reduce allergenicity are under investigation to improve the efficacy and safety of specific immunotherapy.

OBJECTIVE

We performed an investigation of the in vitro and in vivo effects of a purified allergen chemically conjugated to a novel 8-OH modified adenine as an adjuvant.

METHODS

Purified group 2 major allergen from house dust mite chemically conjugated to 4-(6-amino-9-benzyl-8-hydroxy-9H-purin-2-ylsulfanyl)-butyric acid succinimidyl ester was analyzed by using mass spectrometry. The adduct (nDer p 2-Conj) was assayed for Toll-like receptor activation on transfected HEK293 cells, stimulation of innate cells, and effects on the functional phenotype of specific T-cell lines and clones by means of flow cytometry, real-time PCR, and expression of TH-related transcription factors. Lung cells and sera of nDer p 2-Conj-sensitized C57Bl/6 mice were studied by means of cytology, histology, real-time PCR, and ELISA.

RESULTS

nDer p 2-Conj stimulated IL-12 and IFN-α production from monocytes and plasmacytoid dendritic cells, respectively, retaining the ability to trigger Toll-like receptor 7 exclusively, and expanded human allergen-specific lymphocytes with reduced ability to produce T(H)2-related cytokines and increased IFN-γ levels, as based on GATA-3/T-bet expression. In vivo adduct-sensitized mice exhibited reduced eosinophil infiltration and IL-13 expression in the airways, IFN-γ upregulation together with IgE downregulation, and an increase in allergen-specific IgG(2a) levels in sera. The conjugate exhibited reduced ability to activate human FcεRI(+) cells without inducing T(H)17 cells or autoantibodies.

CONCLUSIONS

The codelivery of an allergen with a modified adenine as a conjugate inducing modulatory cytokines from innate cells redirects in vitro and in vivo pathogenic TH2 responses without eliciting harmful effects.

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.

Treatment of autoimmune inflammation by a TLR7 ligand regulating the innate immune system

The Toll-like receptors (TLR) have been advocated as attractive therapeutic targets because TLR signaling plays dual roles in initiating adaptive immune responses and perpetuating inflammation. Paradoxically, repeated stimulation of bone marrow mononuclear cells with a synthetic TLR7 ligand 9-benzyl-8-hydroxy-2-(2-methoxyethoxy) adenine (called 1V136) leads to subsequent TLR hyporesponsiveness. Further studies on the mechanism of action of this pharmacologic agent demonstrated that the TLR7 ligand treatment depressed dendritic cell activation, but did not directly affect T cell function. To verify this mechanism, we utilized experimental allergic encephalitis (EAE) as an in vivo T cell dependent autoimmune model. Drug treated SJL/J mice immunized with proteolipid protein (PLP)_139–151 peptide had attenuated disease severity, reduced accumulation of mononuclear cells in the central nervous system (CNS), and limited demyelination, without any apparent systemic toxicity. Splenic T cells from treated mice produced less cytokines upon antigenic rechallenge. In the spinal cords of 1V136-treated EAE mice, the expression of chemoattractants was also reduced, suggesting innate immune cell hyposensitization in the CNS. Indeed, systemic 1V136 did penetrate the CNS. These experiments indicated that repeated doses of a TLR7 ligand may desensitize dendritic cells in lymphoid organs, leading to diminished T cell responses. This treatment strategy might be a new modality to treat T cell mediated autoimmune diseases.

Antiviral applications of Toll-like receptor agonists

In the past, antiviral research has focused mainly on viral targets. As the search for effective and differentiated antiviral therapies continues, cellular targets are becoming more common, bringing with them a variety of challenges and concerns. Toll-like receptors (TLRs) provide a unique mechanism to induce an antiviral state in the host. In this review we introduce TLRs as targets for the pharmaceutical industry, including how they signal and thereby induce an antiviral state through the production of type I interferons. We examine how TLRs are being therapeutically targeted and discuss several clinically precedented agents for which efficacy and safety data are available. We describe some of the chemistries that have been applied to both small molecule and large molecule leads to tune agonist potency, and offer a differentiated safety profile through targeting certain compartments such as the gut or the lung, thereby limiting systemic drug exposure and affecting systemic cytokine levels. The application of low-dose agonists of TLRs as vaccine adjuvants or immunoprotective agents is also presented. Some of the challenges presented by this approach are then discussed, including viral evasion strategies and mechanism-linked inflammatory cytokine induction.

Synthesis and characterization of PEGylated toll like receptor 7 ligands

Toll-like receptor 7 (TLR7) is located in the endosomal compartment of immune cells. Signaling through TLR7, mediated by the adaptor protein MyD88, stimulates the innate immune system and shapes adaptive immune responses. Previously, we characterized TLR7 ligands conjugated to protein, lipid, or poly(ethylene glycol) (PEG). Among the TLR7 ligand conjugates, the addition of PEG chains reduced the agonistic potency. PEGs are safe in humans and widely used for improvement of pharmacokinetics in existing biologics and some low molecular weight compounds. PEGylation could be a feasible method to alter the pharmacokinetics and pharmacodynamics of TLR7 ligands. In this study, we systematically studied the influence of PEG chain length on the in vitro and in vivo properties of potent TLR7 ligands. PEGylation increased solubility of the TLR7 ligands and modulated protein binding. Adding a 6-10 length PEG to the TLR7 ligand reduced its potency toward induction of interleukin (IL)-6 by murine macrophages in vitro and IL-6 and tumor necrosis factor (TNF) in vivo. However, PEGylation with 18 or longer chain restored, and even enhanced, the agonistic activity of the drug. In human peripheral blood mononuclear cells, similar effects of PEGylation were observed for secretion of proinflammatory cytokines, IL-6, IL-12, TNF-α, IL-1β, and type 1 interferon, as well as for B cell proliferation. In summary, these studies demonstrate that conjugation of PEG chains to a synthetic TLR ligand can impact its potency for cytokine induction depending on the size of the PEG moiety. Thus, PEGylation may be a feasible approach to regulate the pharmacological properties of TLR7 ligands.