Little more than two years ago, Biogen Inc. plunked down about $800 million for gene therapy developer Nightstar Therapeutics plc, which had an X-linked retinitis pigmentosa drug in its pipeline. Top-line results from the phase II/III Xirius study of cotoretigene toliparvovec (BIIB-112), administered by subretinal injection, show the one-time therapy missed its primary endpoint of producing a statistically significant improvement in the proportion of treated study eyes.