Mechanism of Action of Inhibition of Allergic Immune Responses by a Novel Antedrug TLR7 Agonist

Abstract

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.

Tolerability in man following inhalation dosing of the selective TLR7 agonist, AZD8848

“…TLR7 agonists suppress Th2 responses [9][10][11][12][13][14][15][16] and as atopic asthma is associated with an enhanced Th2 biological drive, TLR7 agonists could be developed as a new treatment paradigm for this disease. However, the generation of 'influenza-like' symptoms resulting from either systemic TLR7 activation or elevated systemic levels of TLR7-induced mediators has been reported in TLR7 clinical programmes.…”

“…Sensitisation, Ovalbumin (OVA) challenge and dosing of Brown Norway rats along with bronchoalveolar lavage (BAL) methodology is described in Matsui et al's study. 16 For pharmacokinetic studies, following dosing, blood samples were collected into a solution of sodium fluoride (NaF), to inhibit esterase activity, and EDTA followed by extraction with water and methanol and analysed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). For lung sampling, tissue was excised and immersed in a solution of NaF and homogenised, extracted in methanol and analysed by mass spectrometry.…”

“…16 TLR7 agonists have been administered in man for indications such as cancer and hepatitis C. [20][21][22] While positive antiviral effects have been observed, these studies have all reported tolerability challenges, including influenza-like symptoms, which are associated with systemic cytokine induction. To attempt to reduce the systemic exposure of TLR7 agonism, AstraZeneca and Sumitomo Dainippon Pharma have collaborated to develop a series of 8-oxoadenine TLR7 antedrugs which can be administered topically to the lung and are designed to be rapidly metabolised to a less-active form on entry into the circulation.…”

“…23 Although these compounds have short-term lung exposure, this is sufficient to preferentially activate the IFN pathway, 18 leading to the local production of IFNα and modification of adaptive immune responses in animal allergy models. 16 Further refinement of this chemical series culminated in AZD8848, which had characteristics suitable for progression into man. In this report, we document the in vitro and in vivo potency characteristics of AZD8848.…”

“…7 Ten distinct TLRs have been identified 8 and of these stimulation of either TLR7 or TLR9 in mouse disease models modulates many of the end points associated with allergy including suppression of goblet cell hyperplasia, reduction in Th2 cytokines, lung eosinophilia, lung inflammation and antigenspecific IgE. [9][10][11][12][13][14][15][16][17] TLR7 agonists modulate Th2 responses in vitro through the action of interferon α (IFNα) on T cells as well as through the upregulation of the costimulatory ligand DLL-4, 15 18 and via the downregulation of the Th2 transcription factor, GATA-3, by IFNα. 19 In vivo, the importance of the role of type I IFNs, such as INFα, in TLR7-mediated immunomodulation of…”