Targeting TLRs with specific ligands

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Several synthetic TLRs ligands are available. Most of them are specific for one TLR and are being tested in different applications (Fig. 2).


Novel therapeutic vaccinations are being developed to activate tumor antigen-specific T cells and prolong their activity in the host. Adjuvants based on dsRNA directly stimulate TLR3, present on myeloid dendritic cells and T cells in humans [28]. Poly I:C was the first dsRNA to be used clinically in leukemia and HIV patients for its ability to stimulate type I interferon [29]. However, it produced toxic side effects in many patients. Modification of the poly I:C structure by the introduction of unpaired uracil and guanine bases resulted in a unique dsRNA, poly I:C12U, which is associated with reduced toxicity in humans [30]. Recently a vaccination protocol was described and tested in patients with ovarian cancer, where the TLR3 agonist was used as an adjuvant with conventional chemotherapy to counter immunosup-pressive effects generated by the tumor and to exploit tumor-derived exosomes as a source of cancerous antigens to generate antigen-specific T cell immunity. This polymer (AmpligenTM) is currently produced for intravenous administration by Bioclones PTY in South Africa. Methods for the production and purification of clinical GMP-grade exosomes have recently been developed, and the Phase I clinical trial is pending to test the potential value of tumor-derived exosomes for immunotherapy [31].

TLR4: Lipid A mimetics

Eritoran (E5564) is a synthetic lipid A analogue (a-D-glucopyranose) that has been shown to antagonize the effects of LPS through its interactions with TLR4 [32, 33]. Clinically, Eritoran is being investigated for the treatment of severe sepsis, septic shock, and other endotoxin-mediated indications. Results from in vitro studies have found Eritoran to be a highly active antagonist of the action of LPS on responsive cells. However, low amounts of Eritoran (300-500 |g) injected into animals display a relatively short pharmacodynamic half-life in blood or plasma that is observable in the absence of clearance or measurable metabolism. In vivo studies

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Figure 2

Schematic representation of the synthetic molecules described in the text to stimulate or inhibit specific Toll-like receptors

Each compound is assigned to the specific TLR it targets and to the therapeutic application which is believed to be applicable or is being tested for. The name compounds are given in the shaded boxes above the respective targets and the class of molecules they belong to is provided above. Refer to the text for detailed explanation.

Figure 2

Schematic representation of the synthetic molecules described in the text to stimulate or inhibit specific Toll-like receptors

Each compound is assigned to the specific TLR it targets and to the therapeutic application which is believed to be applicable or is being tested for. The name compounds are given in the shaded boxes above the respective targets and the class of molecules they belong to is provided above. Refer to the text for detailed explanation.

of low doses administered as short infusions into humans further support this observation by demonstrating that Eritoran is extremely active when co-administered with LPS, but its activity decreases shortly after ending infusion [34]. Higher doses of Eritoran (12-252 mg) infused intravenously over 72 h demonstrated extended pharmacodynamic activity by blocking the effects of LPS in a human model of clinical sepsis [32]. Dosing of Eritoran by continuous infusion or intermittent dosing even at the highest doses has been shown to be safe and effective. A very recent study has shown that long-term infusions of high doses of Eritoran

(2000-3500 |g/h for 72 h) provide ex vivo LPS antagonist activity that persists for at least 72 h after infusion, indicating that in vivo protection against LPS may be maintained after the infusion has been discontinued [35]. It was further demonstrated that Eritoran displayed LPS-antagonist activity using a human endotoxemia model [36]. Combined results from these studies indicate that Eritoran is an effective in vivo antagonist of LPS, and may prove to be of benefit in a variety of endo-toxin-mediated diseases.

A class of synthetic lipid A mimetics, the aminoalkyl glucosaminide 4-phos-phates (AGPs), have been engineered specifically to target human TLR4 and are showing promise as vaccine adjuvants, and as therapeutic agents capable of inducing protection against a wide range of infectious pathogens. It has been demonstrated that intranasal immunization with pathogens mixed with the synthetic adjuvant RC529 in aqueous form induces higher titers of serum and mucosal antibodies against the pathogens in mice [37]. Baldridge et al. recently showed that two AGPs, RC524 and RC529, can induce a protective innate immune response that involves activation of TLR4 as well as stimulating the release of proinflammatory cytokines [38].

Picibanil (OK-432) is a lyophilized penicillin-inactivated preparation of a low-virulence strain of Streptococcus pyogenes. It has been demonstrated that TLR4 signaling is involved in regulating anticancer immunity in mice [39], and that oral cancer patients who lacked expression or displayed reduced expression of TLR4 or MD-2 gene, did not achieve a therapeutic response to Picibanil [40]. Clinical Phase I and Phase II studies have demonstrated therapeutic effects in patients receiving intra-tumoral administration of dendritic cells in combination with Picibanil and a therapeutic effect was obtained in oral cancer patients [41, 42].

TLR7 and TLR8: Imidazoquinolines, loxoribine and bropirimine

Synthetic low molecular weight compounds of the imidazoquinoline family, imiquimod (Aldara, R-837, S-26308), resiquimod (R-848), S-27609, and guano-sine analogues such as loxoribine and bropirimine have been shown to activate TLR7. Both imiquimod and resiquimod were unable to induce dendritic cell maturation or TNF-a, IL-12, or IFN-y production in TLR7-deficient mice [43]. It was further demonstrated that imiquimod and resiquimod induce NF-kB activation in HEK293 cells transfected with human or mouse TLR7 [44]. However, resiquimod only activated NF-kB in HEK293 cells transfected with human TLR8 and not in those transfected with mouse TLR8 [45]. Many studies have indicated that the imidazoquinoline family compounds have potent antiviral and antitumor properties in multiple animal models of infection. Imiquimod was also shown to be effective against arbovirus and cytomegalovirus in humans [46]. The activity of imiquimod is mediated predominantly through the induction of cytokines includ ing IFN-y and IL-12. Topical imiquimod therapy is used for the treatment of external genital and perianal warts caused by Papilloma virus infection [17]. The FDA has recently approved imiquimod for the treatment of actinic keratoses, and there is mounting evidence that imiquimod is an effective treatment of certain types of skin cancer [47, 48]. Resiquimod is a more potent analogue of imiquimod, and trials are under way to assess its use in treatment of genital herpes and hepatitis C virus [49].

Loxoribine is a very powerful stimulator of the immune system producing its effects through the enhancement of natural killer (NK) cell activity, B lymphocyte proliferation, and by stimulating the production of interferons [50]. Bropirimine is an orally active immunostimulant that increases endogenous IFN-y and other cytokines, and is used clinically against carcinoma of the bladder and upper urinary tract [51, 52]. Gorden and colleagues at 3M Pharmaceuticals recently described novel synthetic selective agonists for TLR7 (3M-001) and TLR8 (3M-002). The two compounds were used to provide evidence that TLR7 and TLR8 are functionally distinct in human innate immune cells [53]. The data suggest that TLR8 agonists may be effective at driving Th1-like immune responses requiring myeloid dendritic cell activation, whereas TLR7 agonists may be important in driving Ig production. The recent development of TLR7 and TLR8-selective agonists could be used to define the roles of these two receptors in both the innate and adaptive immune response.

TLR9 ligands: Bacterial DNA analogues

Synthetic analogues of bacterial DNA, termed CpG oligodeoxynucleotides (ODN) have shown a great promise in mobilization of protective immunity against pathogens through their general ability to stimulate B cells, NK cells, DC, and mono-cytes/macrophages [54]. Immune activation by CpG ODN depends on the presence of TLR9, as mice genetically deficient in this receptor show no CpG-induced activation of B cells, DC, or NK cells [55]. Transfection of TLR9 in HEK293 cells causes these cells to become CpG-responsive, as cells expressing mouse TLR9 become responsive to the preferred mouse CpG motif, and cells expressing human TLR9 become responsive to the preferred human CpG motif [56, 57].

Studies indicate that CpG DNA can stimulate the innate immune response, improving resistance to infection induced by various pathogens [58]. By slowing the early growth and spread of these pathogens, CpG treatment increases the ability of the host to mount an adaptive immune response. In addition to the immunostimu-latory effects, CpG DNA promotes the initiation of Th1-type immune responses by increasing the production of proinflammatory cytokines and inducing the maturation/activation of professional antigen presenting cells. When CpG ODNs are used as vaccine adjuvants, they promote the immunogenicity of co-administered anti gens. Also, CpG ODNs either administered alone or mixed with allergen are able to reduce susceptibility to allergic diseases.

Multiple Phase I human clinical trials have been designed to explore the safety and immunostimulatory properties of CpG ODNs administered alone, or in combination with vaccines, antibodies or allergens. Several Phase II studies are also underway to evaluate the therapeutic potential of CpG ODNs in the treatment of cancer, allergy and asthma, or as vaccine adjuvants. Studies have investigated the use of CpG ODNs to reduce allergic rhinitis and immunization of allergen mixed with CpG ODN, allergen-CpG ODN conjugates, and CpG ODN alone have proved effective in the reduction of the allergic phenotype in mice [59]. Preliminary results using vaccines containing allergen-CpG ODN conjugates in human patients show that this combination reduces allergic symptoms with relatively few adverse reactions [60]. Clinical trials have used CpG ODNs as vaccine adjuvants coadministered with the Engerix B hepatitis B vaccine and the Fluarix influenza vaccine [61, 62]. Healthy adult volunteers were immunized with the vaccine plus CpG ODNs. The immunized subjects responded favorably to the treatment, showing increased antibody titers and increased release of interferons. All groups of subjects however reported short lived adverse reactions such as injection site reactions and flu-like symptoms.

The administration of CpG ODNs could have therapeutic potential for the treatment of HSV-2, as it was demonstrated that HSV-2 DNA directly stimulates TLR9 [63]. Recent studies in animal models of genital herpes have established that local-vaginal delivery of immunostimulatory CpG ODNs is effective against both primary and recurrent genital herpes infection and disease [64] (see [65] for further review in CpG ODNs applications).

Purified bacterial components

Monophosphoryl lipid A (MPL) is a purified lipopolysaccharide extracted from the cell walls of Salmonella minnesota developed by Corixa Corporation (Seattle, Washington, USA). MPL has been used as a potent adjuvant in allergy vaccines, promoting the development of Th1-type immune response. Antigen presenting cells are stimulated by MPL both in vivo and in vitro leading to the production of IL-12, IL-1, TNF-a and granulocyte-macrophage colony stimulating factor [66]. In humans, MPL activates both human dendritic cells and T cells and promotes the production of IL-10, IL-12 and TNF-a from monocytes and peripheral blood mononuclear cells [67]. A recent review by Francis and Durham identifies cases in which MPL has proven to be safe and effective as a vaccine adjuvant in over 120,000 human doses [68]. In addition to its use as a vaccine adjuvant, therapeutic uses for MPL are being explored in seasonal allergies. It was shown that MPL was effective in preventing seasonal allergies in children and adults when administered over a period of 4 weeks [69, 70].

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