PDE4A and chronic obstructive pulmonary disease: The novel inhaled dual phosphodiesterase (PDE)3 and PDE4 inhibitor ensifentrine (CAS 298680-25-8, molecular formula C26H31N5O4, molecular weight 477.6 g/mol) formerly known as RPL554 (Calzetta et al., 2013a, 2013b; Franciosi et al., 2013), is currently under investigation by the Food and Drug Administration (FDA) for the maintenance therapy of chronic obstructive pulmonary disease (COPD) (Verona Pharma).