PDE4A and chronic obstructive pulmonary disease: It is reported that PDE3, PDE4 and mixed PDE3/4 inhibitors significantly suppress allergen‐induced contractions in passively sensitized airways in humans.21 Thus, roflumilast, a selective PDE4 inhibitor that reduces exacerbation frequency and significantly improves pulmonary function, is used to treat patients with severe chronic obstructive pulmonary disease (COPD).22