A drug whose inventors claim it can cure a rare form of blindness is to be one of the most expensive medicines ever sold at $850,000 (£630,000). Luxturna is injected directly into the eye to address the root cause of visual impairment by replacing a defective gene in the retina. It is the first gene therapy to be approved for use in the US, and was given the go-ahead by the Federal Drug Administration last month. However, the price of the treatment has only just been revealed. Spark Therapeutics, the company behind the treatment, had previously claimed the treatment was worth $1m, citing the cost of a lifetime of blindness in lost earnings and wages for caregivers. But the firm said it had settled for the lower price of $850,000, or $425,000 per eye. It will also offer ways to spread the cost to health insurers, which have expressed concern about their ability to cover the expense.