(Reuters) - Regeneron Pharmaceuticals Inc said a combination therapy containing its flagship eye drug, Eylea, was inferior to Eylea alone in a mid-stage trial involving patients with wet age-related macular degeneration (AMD), a leading cause of blindness in the elderly.
Regeneron's shares fell 3.6 percent, and were the biggest drag on the Nasdaq biotech index. Shares of Ophthotech Corp, which is evaluating combinations of its experimental drug with Eylea or rival drugs, also fell.
Eylea sales, which have powered Regeneron's explosive growth since late 2011, have been slowing in recent quarters, raising concerns about the biotechnology company's growth prospects.
The blockbuster injection, which was developed with Germany's Bayer AG to treat macular degeneration and other eye disorders, generated U.S. sales of $831 million in the second quarter.
Macular degeneration accounts for almost 50 percent of all cases of blindness in the developed world. It usually affects people over 50. Wet AMD, which is less common than dry AMD, is generally caused by abnormal blood vessels that leak fluid or blood into the retina.
Patients who received the combination of Eylea and experimental platelet-derived growth factor (PDGF) inhibitor rinucumab were able to see 5.8 more letters on an eye chart test at 12 weeks, compared with a 7.5 letter improvement in those who were given Eylea alone.
The main goal of the study was to show an improvement in visual clarity after three months of treatment. The combination therapy proved to be inferior and was associated with more side effects than the monotherapy.
Data will be evaluated at 28 weeks and 52 weeks, when the study is completed, Regeneron said on Friday.
Eylea belongs to a class of drugs considered the standard of care in AMD as their use makes patients less likely to go blind or move into long-term care.
Ophthotech Corp is evaluating combinations of its PDGF inhibitor, Fovista, with either Eylea, Roche Holding AG's Avastin or Novartis AG's Lucentis.
Positive results from Ophthotech's late-stage trials, expected to read out this year, could see Regeneron lose market share to rival therapies, Leerink's Geoffrey Porges wrote in a client note.
Ophthotech's stock was down about 14 percent at $46.49 in early trading, while Regeneron's shares slipped about 3 percent to $393.43.
Reporting by Natalie Grover in Bengaluru; Editing by Don Sebastian