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

Structure-activity relationships in human toll-like receptor 7-active imidazoquinoline analogues

Engagement of toll-like receptors serve to link innate immune responses with adaptive immunity and can be exploited as powerful vaccine adjuvants for eliciting both primary and anamnestic immune responses. TLR7 agonists are highly immunostimulatory without inducing dominant proinflammatory cytokine responses. A structure-activity study was conducted on the TLR7-agonistic imidazoquinolines, starting with 1-(4-amino-2-((ethylamino)methyl)-1H-imidazo[4,5-c]quinolin-1-yl)-2-methylpropan-2-ol as a lead. Modifications of the secondary amine of the C2 ethylaminomethylene side chain are poorly tolerated. The 4-amino group must be retained for activity. Replacement of the imidazole ring of the scaffold with triazole or cyclic urea led to complete loss of activity. A systematic exploration of N(1)-benzyl-C2-alkyl substituents showed a very distinct relationship between alkyl length and TLR7-agonistic potency with the optimal compound bearing a C2-n-butyl group. Transposition of the N(1) and C2 substituents led to the identification of an extremely active TLR7-agonistic compound with an EC(50) value of 8.6 nM. The relative potencies in human TLR7-based primary reporter gene assays were paralleled by interferon-alpha induction activities in whole human blood models.

Pharmacokinetic-pharmacodynamic modeling of alpha interferon response induced by a Toll-like 7 receptor agonist in mice

Recombinant alpha interferon (IFN-alpha) is used in the treatment of hepatitis C virus (HCV)-infected patients but is not optimal in terms of efficacy or tolerability. Toll-like 7 receptor (TLR-7) agonists stimulate the innate immune system to produce, among other cytokines, IFN-alpha and are being evaluated as alternative drugs to treat HCV infection. This paper describes the application of pharmacokinetic-pharmacodynamic (PK-PD) modeling to understanding the behavior of a TLR-7 agonist [9-benzyl-8-hydroxy-2-(2-methoxyethoxy) adenine (BHMA)] in mice, using IFN-alpha as a biomarker. This is the first report of such a PK-PD model, and the conclusions may be of utility in the clinical development of TLR-7 agonists for HCV infection.

Toll-like receptor 7 selective synthetic oligoribonucleotide agonists: synthesis and structure-activity relationship studies

We previously reported a novel class of stabilized immune-modulatory RNA (SIMRA) compounds that activates TLR8 or both TLR7 and TLR8 depending on the nucleotide composition and chemical modifications incorporated. In the present study, to identify TLR7-selective agonists, we designed and synthesized novel SIMRA compounds with varying sequence compositions substituting 7-deaza-G for natural guanosine and studied immune-stimulatory activity in cell-based assays and in vivo in mice. SIMRA compounds activated NF-kappaB in HEK293 cells expressing TLR7 and induced cytokine production in mouse spleen cells and human PBMCs and higher levels of IFN-alpha in human pDCs, which correlated with TLR7 activation. Subcutaneous administration of SIMRA compounds to mice increased serum cytokine levels. TLR knockout mouse studies showed that both TLR7 and MyD88 are required for activity of SIMRA compounds. The presence of a 5'-AA/CN (A > C and N = U/C/7-deaza-G) and/or C/AUU-3' (C > A) trinucleotide at the 5'- and 3'-ends of SIMRA compound along with a 5'-AN(1)N(2)UG1A-3' (N(1) = A/C; N(2) = U/C/7-deaza-G) or UG1AZ(1)G1Z(2)UU (Z(1) = A < C; Z(2) = C < A) motif confers TLR7 selectivity over other sequence compositions. In conclusion, we have designed and synthesized novel SIMRA compounds that selectively act as agonists of TLR7.

Synthesis and immunological characterization of toll-like receptor 7 agonistic conjugates

Activation of toll-like receptors (TLRs) on cells of the innate immune system initiates, amplifies, and directs the antigen-specific acquired immune response. Ligands that stimulate TLRs, therefore, represent potential immune adjuvants. In this study, a potent TLR7 agonist was conjugated to phospholipids, poly(ethylene glycol) (PEG), or phospholipid-PEG via a versatile benzoic acid functional group. Compared to the unmodified TLR7 agonist, each conjugate displayed a distinctive immunological profile in vitro and in vivo. In mouse macrophages and human peripheral blood mononuclear cells, the phospholipid TLR7 agonist conjugate was at least 100-fold more potent than the free TLR7 ligands, while the potency of PEG−phospholipid conjugate was similar to that of the unmodified TLR7 agonist. When administered systemically in mice, the phospholipid and phospholipid−PEG TLR7 conjugates induced prolonged increases in the levels of proinflammatory cytokines in serum, compared to the unmodified TLR7 activator. When the conjugates were used as adjuvants during vaccination, only the phospholipid TLR7 agonist conjugates induced both Th1 and Th2 antigen-specific immune responses. These data show that the immunostimulatory activity of a TLR7 ligand can be amplified and focused by conjugation, thus broadening the potential therapeutic application of these agents.

Regulation of dendritic cells and macrophages by an anti-apoptotic cell natural antibody that suppresses TLR responses and inhibits inflammatory arthritis

Although natural Abs (NAbs) are present from birth, little is known about what drives their selection and whether they have housekeeping functions. The prototypic T15-NAb, first identified because of its protective role in infection, is representative of a special type of NAb response that specifically recognizes and forms complexes with apoptotic cells and which promotes cell-corpse engulfment by phagocytes. We now show that this T15-NAb IgM-mediated clearance process is dependent on the recruitment of C1q and mannose-binding lectin, which have known immune modulatory activities that also provide "eat me" signals for enhancing phagocytosis. Further investigation revealed that the addition of T15-NAb significantly suppressed in vitro LPS-induced TNF-alpha and IL-6 secretion by the macrophage-like cell line, RAW264.7, as well as TLR3-, TLR4-, TLR7-, and TLR9-induced maturation and secretion of a range of proinflammatory cytokines and chemokines by bone marrow-derived conventional dendritic cells. Significantly, high doses of this B-1 cell produced NAb also suppressed in vivo TLR-induced proinflammatory responses. Although infusions of apoptotic cells also suppressed such in vivo inflammatory responses and this effect was associated with the induction of high levels of IgM antiapoptotic cell Abs, apoptotic cell treatment was not effective at suppressing such TLR responses in B cell-deficient mice. Moreover, infusions of T15-NAb also efficiently inhibited both collagen-induced arthritis and anti-collagen II Ab-mediated arthritis. These studies identify and characterize a previously unknown regulatory circuit by which a NAb product of innate-like B cells aids homeostasis by control of fundamental inflammatory pathways.

Modified adenine (9-benzyl-2-butoxy-8-hydroxyadenine) redirects Th2-mediated murine lung inflammation by triggering TLR7

Substitute adenine (SA)-2, a synthetic heterocycle chemically related to adenine with substitutions in positions 9-, 2-, and 8- (i.e., 9-benzyl-2-butoxy-8-hydroxyadenine), induces in vitro immunodeviation of Th2 cells to a Th0/Th1 phenotype. In this article, we evaluate the in vivo ability of SA-2 to affect Th2-mediated lung inflammation and its safety. TLR triggering and NF-kappaB activation by SA-2 were analyzed on TLR-transfected HEK293 cells and on purified bone marrow dendritic cells. The in vivo effect of SA-2 on experimental airway inflammation was evaluated in both prepriming and prechallenge protocols by analyzing lung inflammation, including tissue eosinophilia and goblet cell hyperplasia, bronchoalveolar lavage fluid cell types, and the functional profile of Ag-specific T cells from draining lymph nodes and spleens. SA-2 induced mRNA expression and production of proinflammatory (IL-6, IL-12, and IL-27) and regulatory (IL-10) cytokines and chemokines (CXCL10) in dendritic cells but down-regulated TGF-beta. Prepriming administration of SA-2 inhibited OVA-specific Abs and Th2-driven lung inflammation, including tissue eosinophilia and goblet cells, with a prevalent Foxp3-independent regulatory mechanism. Prechallenge treatment with SA-2 reduced the lung inflammation through the induction of a prevalent Th1-related mechanism. In this model the activity of SA-2 was route-independent, but adjuvant- and Ag dose-dependent. SA-2-treated mice did not develop any increase of serum antinuclear autoantibodies. In conclusion, critical substitutions in the adenine backbone creates a novel synthetic TLR7 ligand that shows the ability to ameliorate Th2-mediated airway inflammation by a complex mechanism, involving Th1 redirection and cytokine-mediated regulation, which prevents autoreactivity.

Prevention of autoimmune disease by induction of tolerance to Toll-like receptor 7

Activation of Toll-like receptors (TLR) contributes to the initiation and maintenance of chronic inflammation in autoimmune diseases, yet repeated exposure to a TLR agonist can induce hyporesponsiveness to subsequent TLR stimulation. Here, we used a synthetic TLR7 agonist, 9-benzyl-8-hydroxy-2-(2-methoxyethoxy) adenine (SM360320, 1V136) to study TLR7 induced attenuation of inflammatory responses and its application to autoimmune diseases. Repeated low dose administration of this TLR7 agonist induced hyporesponsiveness or tolerance to TLR2, -7, and -9 activators and limited the course of neural inflammation in an experimental allergic encephalomyelitis model. The hyporesponsiveness did not depend on T or B lymphocytes, but did require bone marrow derived cells. In addition, TLR7 tolerance reduced inflammation in a passive antibody mediated arthritis model. TLR7 tolerance did not cause global immunosuppression, because susceptibility to Listeria monocytogenes infection was not altered. The mechanism of TLR7 tolerance involved the up-regulation of 2 inhibitors of TLR signaling: Interleukin 1 Receptor Associated Kinase (IRAK) M, and Src homology 2 domain-containing inositol polyphosphate phosphatase (SHIP)-1. These findings suggest that induction of TLR7 tolerance might be a new therapeutic approach to subdue inflammation in autoimmune diseases.

An oral TLR7 agonist is a potent adjuvant of DNA vaccination in transgenic mouse tumor models

In vivo electroporation of plasmid DNA (DNA-EP) is an efficient and safe method for vaccines resulting in increased DNA uptake, enhanced protein expression and increased immune responses to the target antigen in a variety of species. To further enhance the efficacy of DNA-EP, we have evaluated the toll-like receptor7 (TLR7) agonist-2, 9, substituted 8-hydroxyadenosine derivative or SM360320--as an adjuvant to vaccines against HER2/neu and CEA in BALB-neuT and CEA transgenic mice (CEA.Tg), respectively. SM360320 induced in vivo secretion of interferon alpha (IFNalpha) and exerted a significant antitumor effect in CEA.Tg mice challenged with a syngenic tumor cell line expressing CEA and an additive effect with a CEA vaccine. Additionally, combination of SM360320 with plasmid encoding the extracellular and transmembrane domain of ratHER2/neu affected the spontaneous tumor progression in BALB-neuT mice treated in an advanced disease setting. The antitumor effect in mice treated with DNA-EP and SM360320 was associated with an anti-CEA and anti-p185(neu) antibody isotype switch from IgG1 to IgG2a. These data demonstrate that SM360320 exerts significant antitumor effects and can act in association with DNA-EP for CEA-positive colon cancer and HER2-positive mammary carcinoma. These observations therefore emphasize the potential of SM360320 as immunological adjuvant for therapeutic DNA vaccines.

Mast cell-dependent anorexia and hypothermia induced by mucosal activation of Toll-like receptor 7

Systemic viral infections produce a highly regulated set of responses in sickness behavior, such as fever, anorexia, and adipsia. Toll-like receptor (TLR)7, activated by viral RNA during infection, potently stimulates the innate and adaptive immune responses that aid in viral clearance. However, the physiological consequences of TLR7 activation have not been thoroughly studied. In these experiments, we used a potent synthetic TLR7 ligand, 9-benzyl-8-hydroxy-2-(2-methoxyethoxy)adenine (SM360320; 1V136), to investigate the consequences of TLR7 activation in genetically defined strains of mice. Administration of the drug by the nasal, intragastric, or intraperitoneal routes caused transient hypophagia, hypodypsia, and hypothermia. Analyses of mutant mouse strains indicated that these effects were dependent on the expression of TLR7, its adaptor protein MyD88, and TNF-alpha, and independent of IL-1beta, IL-6 and cyclo-oxygenase-1 (COX1). Partial roles were also implied for mast cells and COX2. Although plasma TNF-alpha levels were significantly higher after systemic drug delivery, the behavioral effects were maximal when the agent was administered to the mucosa. Tissue and mucosal mast cells are known to express high levels of TLR7 and to rapidly release TNF-alpha upon TLR7 ligation. Mice deficient in tissue mast cells, W/W(v), had significantly less anorexia after TLR7 activation, and this response was restored with mast cell reconstitution. Our results thus suggest that tissue mast cells may play a role in the anorexia induced by mucosal activation of TLR7.

The covalent modification and regulation of TLR8 in HEK-293 cells stimulated with imidazoquinoline agonists

The mammalian TLRs (Toll-like receptors) mediate the rapid initial immune response to pathogens through recognition of pathogen-associated molecular patterns. The pathogen pattern to which TLR8 responds is ssRNA (single-stranded RNA) commonly associated with ssRNA viruses. TLR8 also responds to small, purine-like molecules including the imidazoquinoline IRMs (immune-response modifiers). The IRMs include molecules that selectively activate TLR7, selectively activate TLR8 or non-selectively activate both TLR7 and TLR8. Using HEK-293 cells (human embryonic kidney cells) stably expressing an NF-kappaB (nuclear factor kappaB)/luciferase promoter-reporter system as a model system, we have examined the regulation of TLR8 using the non-selective TLR7/8 agonist, 3M-003. Using conservative tyrosine to phenylalanine site-directed mutation, we show that of the 13 tyrosine residues resident in the cytosolic domain of TLR8, only three appear to be critical to TLR8 signalling. Two of these, Tyr898 and Tyr904, reside in the Box 1 motif and the third, Tyr1048, lies in a YXXM putative p85-binding motif. TLR8 is tyrosine-phosphorylated following 3M-003 treatment and TLR8 signalling is inhibited by tyrosine kinase inhibitors. Treatment with 3M-003 results in the association of the p85 regulatory subunit of PI3K (phosphoinositide 3-kinase) with TLR8 and this association is inhibited by tyrosine to phenylalanine mutation of either the YXXM or Box 1 motifs. As a further consequence of activation by 3M-003, TLR8 is modified to yield both higher and lower molecular mass species. These species include a monoubiquitinated form as deduced from ubiquitin peptide sequencing by HPLC/MS/MS (tandem MS).

Experimental and theoretical study on the structure and electronic spectra of imiquimod and its synthetic intermediates

Crystal structure of the imiquimod has been determined by single crystal X-ray analysis, imiquimod crystallizes in orthorhombic space group P2(1)2(1)2(1) and the molecules are linked along the c axis by the strong N-H ... N hydrogen bonds. A density functional theory (DFT) study on the electronic properties of imiquimod and its synthetic intermediates has been performed at B3LYP/6-31G* level, while taking solvent effects into account. Both the single configuration interaction (CIS) method and the time-dependent DFT (TDDFT) approaches have been used to calculate the electronic absorption spectra, and there is a good agreement between the calculated and experimental UV-visible absorption spectra. The fluorescence emission spectra of these three compounds in solution have also been measured, the relatively low fluorescence intensity is attributed to a chlorine-modulated heavy atom effect that enhances intersystem crossing between excited singlet and triplet states, and the relatively high fluorescence intensity of imiquimod results from an extended pi-conjugated system which enhances S(1)-->S(0) radiant transition.

Synthesis and immunostimulatory activity of 8-substituted amino 9-benzyladenines as potent Toll-like receptor 7 agonists

Several 9-benzyl adenine derivatives bearing various substituted amines at the 8-position have been prepared and evaluated for interferon induction in peripheral blood mononuclear cells (PBMC) from healthy human donors. The 8-bromoadenine derivative 5 was used as a versatile intermediate for all substitutions. The most active 8-substituted amino compound was found to be the 8-morpholinoethylamino derivative 19 which had an EC(50) in the submicromolar range.

Synthesis of 2-alkoxy-8-hydroxyadenylpeptides: towards synthetic epitope-based vaccines

The preparation of three different 2-alkoxy-8-hydroxyadenylpeptide conjugates has been accomplished by solid-phase synthesis combined with 'on-resin' Cu(I) catalyzed Huisgen cycloaddition. The immunogenicity of the compounds has been evaluated in IL-12 production and antigen presentation assays.

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.

Solid-phase synthesis of 7-substituted 3H-imidazo[2,1-i]purines

A method for solid-supported synthesis of N,N-disubstituted (3H-imidazo[2,1-i]purin-7-yl)methyl amines has been developed. The key features of this library synthesis are: (i) immobilization of commercially available N6-benzoyl-5'-O-(4,4'-dimethoxytrityl)-2'-deoxyadenosine 3'-(2-cyanoethyl N,N-diisopropylphosphoramidite) by phosphitylation to a hydroxyl-functionalized support, (ii) quantitative conversion of the deprotected adenine base to 3H-imidazo[2,1-i]purine-7-carbaldehyde with bromomalonaldehyde in DMF in the presence of formic acid and 2,6-lutidine, (iii) reductive amination of the formyl group followed by N-alkylation or N-acylation, and (iv) release from the support by acidolytic cleavage of the N-glycosidic bond. Steps (ii) and (iii) have been optimized in some detail by using (adenin-9-yl)acetic acid anchored to a Phe-Wang resin as a model compound.