PDE4A and chronic obstructive pulmonary disease: The anti-inflammatory effects of oral PDE4 inhibitors (e.g., roflumilast, cilomilast, and BAY 19-8004) or inhaled PDE4 inhibitors (CHF6001) have not been clearly established in COPD patients when assessed as the inflammatory cell count in a bronchial biopsy or the levels of inflammatory biomarkers in induced sputum (Gamble et al., 2003; Grootendorst et al., 2003; Profita et al., 2003; Grootendorst et al., 2007; Wells et al., 2015; Singh et al., 2016; Rabe et al., 2018; Singh et al., 2019).