Treatment of Skin Inflammation with Benzoxaborole Phosphodiesterase Inhibitors: Selectivity, Cellular Activity, and Effect on Cytokines Associated with Skin Inflammation and Skin Architecture Changes

Psoriasis and atopic dermatitis are skin diseases affecting millions of patients. Here, we characterize benzoxaborole phosphodiesterase (PDE)-4 inhibitors, a new topical class that has demonstrated therapeutic benefit for psoriasis and atopic dermatitis in phase 2 or phase 3 studies. Crisaborole [AN2728, 4-((1-hydroxy-1,3-dihydrobenzo[c][1,2] oxaborol-5-yl) oxy) benzonitrile], compd2 [2ethoxy-6-((1-hydroxy-1,3-dihydrobenzo[c][1,2] oxaborol-5-yl) oxy) nicotinonitrile], compd3 [6-((1-hydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-5-yl) oxy)-2-(2-isopropoxyethoxy) nicotinonitrile], and compd4 [5-chloro-6-((1-hydroxy- 1,3-dihydrobenzo[c][1,2]oxaborol-5-yl) oxy)-2-((4- oxopentyl) oxy) nicotinonitrile] are potent PDE4 inhibitors with similar affinity for PDE4 isoforms and equivalent inhibition on the catalytic domain and the full-length enzyme. These benzoxaboroles are less active on other PDE isozymes. Compd4 binds to the catalytic domain of PDE4B2 with the oxaborole group chelating the catalytic bimetal and overlapping with the phosphate in cAMP during substrate hydrolysis, and the interaction extends into the adenine pocket. In cell culture, benzoxaborole PDE4 inhibitors suppress the release of tumor necrosis factor-alpha, interleukin (IL)-23, IL-17, interferon-gamma, IL-4, IL-5, IL-13, and IL-22, and these cytokines contribute to the pathologic changes in skin structure and barrier functions as well as immune dysregulation in atopic dermatitis and psoriasis. Treatment with compd3 or N-6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate increases cAMP response element binding protein phosphorylation in human monocytes and decreases extracellular signal-regulated kinase phosphorylation in human T cells; these changes lead to reduced cytokine production and are among the mechanisms by which compd3 blocks cytokine release. Topical compd3 penetrates the skin and suppresses phorbol myristate acetate-induced IL-13, IL-22, IL-17F, and IL-23 transcription and calcipotriol-induced thymic stromal lymphopoietin expression in mouse skin. Skin thinning is a major dose-limiting side effect of glucocorticoids. By contrast, repeated application of compd3 did not thin mouse skin. These findings show the potential benefits and safety of benzoxaborole PDE4 inhibitors for the treatment of psoriasis and atopic dermatitis.