Asthma patients have elevated levels of IL-4 and IL-13 in their airways, the result of T helper cell type 2 (Th2) activation, which causes mucus production, airway hyperresponsiveness (AHR), inflammatory cell recruitment, and immunoglobulin class switching to IgE. Signal transducer and activator of transcription 6 (Stat6) transmits IL-4 and IL-13 signals by binding to phosphotyrosine residues the receptors via its Src homology 2 (SH2) domain.  Subsequent phosphorylation on Tyr641 (pStat6), dimerization, and translocation to the nucleus leads to transcription of genes resulting in asthma. Elevated pStat6 levels have been found in the bronchial epithelium of asthma patients and Stat6 knockout mice do not develop AHR or asthma-associated lung pathology.

Our laboratory is developing small-molecule, cell-permeable phosphopeptide mimetic prodrugs that target the SH2 domain of Stat6 and inhibit phosphorylation of Tyr641 in vitro and in vivo.  Our lead inhibitor, PM-242H, when administered intranasally, inhibited the development of AHR, Th2 cell recruitment to the lung, eosinophilia, and goblet cell metaplasia in an Aspergillus-induced allergic lung disease model.  Furthermore, in established allergic lung disease, AHR was reversed accompanied by reduced goblet cell metaplasia, suggesting that inhibition of Stat6 phosphorylation is a potential ideal treatment modality for asthma.  In this paper we will discuss the optimization of PM-242H.  Peptides are screened for affinity to STAT6 protein and prodrugs are assayed for pStat6 inhibition in in Beas-2B human airway cells.  We have developed inhibitors with ~100 nM IC₅₀ values in cellular assays and are selective for our target over Stat1, Stat3, Stat5, PI3K, and Src. Below 5 µM the new prodrugs do not inhibit proliferation thus ruling out cytotoxicity as the cause of pStat6 inhibition.