However while Spinraza had a very strong launch in the first half of 2017, its US sales faltered in Q3 to $197.6 million, barely above the $194.8 million achieve in the previous quarter.

Spinal muscular atrophy (SMA) therapy Spinraza has been a key earner for Biogen in recent years, but with sales now heading into reverse, the company is looking to extend its franchise.