An experimental treatment for a rare, children's spine disorder developed by Ionis Pharmaceuticals(IONS) - Get Report achieved the primary endpoint of a late-stage clinical trial, triggering Biogen(BIIB) - Get Report to exercise an option to take over development of the drug and seek marketing approval.

Following an interim analysis, infants with spinal muscular atrophy treated with the Ionis drug nusinersen reported a statistically significant improvement in certain motor symptoms compared to infants who did not receive treatment. This achieved the primary endpoint of the study, which will now be stopped so that all patients can be treated with nusinersen, the companies said Monday.

Nusinersen's safety profile in the study was "acceptable," the companies said.

Spinal muscular atrophy is a rare, genetic disease that causes muscle atrophy and weakness in infants and young children. The severest forms of the disease can lead to paralysis and death.

Nusinersen is the lead drug in Ionis' antisense drug development pipeline. The company will receive $75 million from Biogen for the exercise of the partnership option. If nusinersen is eventually approved, Ionis is eligible for another $150 million in milestone payments plus a mid-teens percentage royalty on sales.

Ionis shares rose 37% to $40.10 in Monday premarket trading, helping the stock partially recover from an investor exodus triggered by the disclosure of blood-clotting safety concerns tied to other antisense drugs in the company's pipeline. Going into Monday, Ionis' stock price was down 53% year to date.

The nusinersen study result is also good news for Biogen and its search for new drugs to restock a depleted pipeline and provide a new growth driver. Biogen shares are up 3% to $299.70 Monday morning.

Spinal muscular atrophy is caused by a defective gene called survival motor neuron 1 (SMN1), which cannot produce enough SMN1 protein to maintain muscle neurons. No drugs are approved to treat SMA, which affects approximately one out of every 10,000 newborns.

Nusinersen is injected into the spine of SMA patients and is designed to produce more functional SMN1 protein. Peak sales of the drug could reach $1 billion or more depending on how widespread the drug is used in patients with severe and more moderate forms of the disease.

The phase III study stopped early for efficacy on Monday enrolled infants with the severe type of SMA. A second phase III study involving children up to 12 year old with later-onset SMA continues.

AveXis (AVXS) is a competitor developing a gene therapy to correct the underlying genetic defect that causes SMA. 

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