The proposed scientific research consists of a multicenter, randomized, double-blind, placebo-controlled, parallel-group study evaluating the efficacy and safety of clarithromycin as add-on therapy in adult subjects with severe persistent asthma. Adult asthmatic patients, who remain inasequately controlled despite GINA step 4 therapy, will be thoroughly examined for comorbidities associated with asthma. Moreover, at screening, the fractional concentration of exhaled nitric oxide (FENO) will be determined to separate eosinophilic asthma, which is more sensitive to corticosteroids and anti-IgE, from neutrophilic asthma, which is corticosteroid resistant.