Emerging therapeutic targets in allergy: IL-4Rα and Stat6

Development of pre-clinical models for evaluating the therapeutic potential of candidate siRNA targeting STAT6

Developing siRNA therapeutics poses technical challenges including appropriate molecular design and testing in suitable pre-clinical models. We previously detailed sequence-selection and modification strategies for siRNA candidates targeting STAT6. Here, we describe methodology that evaluates the suitability of candidate siRNA for respiratory administration. Chemically-modified siRNA exhibited similar inhibitory activity (IC₅₀) against STAT6 in vitro compared to unmodified siRNA and apical exposure testing with Caco-2 cell monolayers showed modification was not associated with cellular toxicity. Use of a modified RNA extraction protocol improved the sensitivity of a PCR-based bio-analytical assay (lower limit of siRNA strand quantification  =  0.01 pg/µl) which was used to demonstrate that lung distribution profiles for both siRNAs were similar following intra-tracheal administration. However, after 6 hours, modified siRNA was detected in lung tissue at concentrations >1000-fold higher than unmodified siRNA. Evaluation in a rat model of allergic inflammation confirmed the persistence of modified siRNA in vivo, which was detectable in broncho-alveolar lavage (BAL) fluid, BAL cells and lung tissue samples, 72 hours after dosing. Based upon the concept of respiratory allergy as a single airway disease, we considered nasal delivery as a route for respiratory targeting, evaluating an intra-nasal exposure model that involved simple dosing followed by fine dissection of the nasal cavity. Notably, endogenous STAT6 expression was invariant throughout the nasal cavities and modified siRNA persisted for at least 3 days after administration. Coupled with our previous findings showing upregulated expression of inflammatory markers in nasal samples from asthmatics, these findings support the potential of intranasal siRNA delivery. In summary, we demonstrate the successful chemical modification of STAT6 targeting siRNA, which enhanced bio-availability without cellular toxicity or reduced efficacy. We have established a robust, sensitive method for determining siRNA bio-distribution in vivo, and developed a nasal model to aid evaluation. Further work is warranted.

Evaluation of nasal epithelium sampling as a tool in the preclinical development of siRNA-based therapeutics for asthma

The development of siRNA-based asthma therapeutics is currently hampered by a paucity of relevant biomarkers and the need to ascertain tissue-specific gene targeting in the context of active disease. Epithelial STAT6 expression is fundamental to asthma pathogenesis in which inflammatory changes are found throughout the respiratory tract. Therefore, to improve preclinical evaluation, we tested the efficacy of STAT6-targeting siRNA within nasal epithelial cells (NEC's) obtained from asthmatic and non-asthmatic donors. STAT6 expression was invariant in both donor groups and amenable to suppression by siRNA treatment. In addition, STAT6 mRNA was also suppressible by apically delivered siRNA treatment in comparative differentiated nasal epithelial cell-line monolayer cultures. Analysis of donor NEC's showed consistent elevation in CCL26 (eotaxin-3) mRNA within the asthmatic group suggesting potential as a relevant biomarker. Furthermore, targeting of STAT6 with siRNA attenuated IL-13-driven CCL26 expression in these cells, pointing to the utility of this approach in preclinical testing. Finally, siRNA-mediated suppression of STAT6 was independent of donor disease phenotype or epithelial cell differentiation status, signifying therapeutic potential.

RNA interference of STAT6 rapidly attenuates ongoing interleukin-13-mediated events in lung epithelial cells

Signal transducer and activator of transcription 6 (STAT6) expression in lung epithelial cells plays a central role in asthma pathogenesis, with its activation driving the development of airway hyper-reactivity and local inflammation. Therefore, inhibition of local STAT6 expression provides a rationale for therapeutic intervention in bronchial asthma. Given the absence of specific inhibitory drugs, we tested the ability of small interfering RNAs (siRNAs) to target STAT6 gene expression through the molecular process of RNA interference (RNAi). At pico-molar concentrations, STAT6-specific siRNAs potently inhibited STAT6 mRNA expression in lung epithelial cells (50% inhibitory concentration range = 134-861 pm) without inducing cellular interferon responses. Detectable STAT6 protein expression was rapidly abolished within 48 hr of treatment (t(1/2) range = or < 12-37 hr) and this was unaffected by pretreatment with STAT6-activating cytokines. Furthermore, STAT6 suppression by RNAi produced downstream functional inhibitory effects in that interleukin (IL)-13- or IL-4-driven eotaxin chemokine family [chemokine (C-C motif) ligand 11 (CCL11), CCL24 and CCL26] mRNA expression was markedly inhibited. Induction of detectable CCL26 protein synthesis was completely ablated by pretreating cells with STAT6-specific siRNA. The therapeutic potential of this approach is further demonstrated by novel findings that cells pre-exposed to IL-13 or IL-4 and subsequently treated with STAT6-targeting siRNA exhibited a rapid and significant attenuation of ongoing CCL26 protein expression, suggesting that chronic asthma-associated lung inflammation will be responsive to this approach.