(Sharecast News) - GlaxoSmithKline has received a 'complete response letter' from the United States Food and Drug Administration regarding its application for mepolizumab, it announced on Monday.The FTSE 100 firm said it had submitted for mepolizumab to be used as an add-on treatment to inhaled corticosteroid-based maintenance treatment, for the reduction of exacerbations in patients with chronic obstructive pulmonary disease, guided by blood eosinophil counts."The [response letter] states that more clinical data are required to support an approval," GlaxoSmithKline said in its statement."GSK will work closely with the FDA to determine the appropriate next steps for the supplementary biologics licence application."The pharmaceuticals giant said mepolizumab was approved in 2015 for severe eosinophilic asthma, and was also licenced for EGPA in the US.It described it as "the first-in-class" monoclonal antibody that targeted IL-5."It is believed to work by preventing IL-5 from binding to its receptor on the surface of eosinophils," GSK explained."Inhibiting IL-5 binding in this way reduces blood eosinophils."Mepolizumab had been studied in more than 3000 patients in 16 clinical trials across a number of eosinophilic indications, and was currently being investigated for severe hypereosinophilic syndrome and nasal polyposis, in addition to the supplementary biologics licence application filed for the treatment of patients with chronic obstructive pulmonary disorder.