The U.S. Food and Drug Administration on Tuesday declined to approve a higher-dose version of Regeneron Pharmaceuticals' blockbuster treatment Eylea, the company said, marking a blow in the drugmaker's plans to defend the eye drug against a fast-gaining rival.

The company was seeking approval for the drug to treat wet age-related macular degeneration (AMD), the leading cause of blindness among the elderly, and two eye diseases that are common in people with diabetes.

The FDA cited an ongoing review of inspection findings at a third-party filler as the reason for the decline.

The complete response letter did not identify any issues with the drug's clinical efficacy or safety, trial design, labeling or drug substance manufacturing, and no additional clinical data or trials have been requested.

Eylea's sales have been hit by Roche's rival treatment Vabysmo, approved last year, for wet AMD.

(Reporting by Raghav Mahobe and Khushi Mandowara in Bengaluru; Editing by Krishna Chandra Eluri and Maju Samuel)