July 23 (Reuters) - U.S. Food and Drug Administration staff reviewers have raised doubts over the effectiveness of GlaxoSmithKline Plc's drug Nucala in treating a disease that limits airflow to the lungs.

Nucala, already approved for treating severe asthma patients, is being reviewed by the FDA for reducing a sudden worsening of symptoms of chronic obstructive pulmonary disease (COPD).

In documents posted https://www.fda.gov/downloads/AdvisoryCommittees/CommitteesMeetingMaterials/Drugs/Pulmonary-AllergyDrugsAdvisoryCommittee/UCM614138.pdf on the regulator's website on Monday, FDA staff said questions remained as to whether data submitted by GSK provided evidence of the drug's effectiveness.

The FDA staff highlighted the drug's failure to meet a statistical threshold for effectiveness in one of the clinical trials, and uncertainty in defining group of patients that could benefit from the treatment.

An expert panel to the FDA is expected to vote on the efficacy and safety of the treatment on Wednesday. While the FDA is not obliged to follow the advice of its experts, it generally does. (Reporting by Manas Mishra in Bengaluru; Editing by Shailesh Kuber)