Ionis Pharmaceuticals Inc said its rare metabolic disorder drug for severe hypertriglyceridemia, characterized by elevated levels of a type of fat in the blood, met the main goal in a late stage study.

The drug, volanesorsen, brought about a statistically significant 71.2 percent mean reduction in triglycerides after 13 weeks of treatment, compared with 0.9 percent in those who got the placebo, the company said.

However, Ionis said that a patient who was on the drug experienced a serious side-effect. The episode of serum sickness occurred two weeks after the last study dose and was resolved by itself. The case was unlikely caused by volanesorsen, the company added.

Patients in the trial had two rare metabolic disorders: familial chylomicronemia syndrome and familial partial lipodystrophy.

Familial chylomicronemia syndrome is a rare genetic disorder characterized by extremely high levels of triglycerides and the risk of recurrent, potentially fatal pancreatitis.

Familial partial lipodystrophy is a rare, underdiagnosed metabolic disorder characterized by an inability of the body to store fat in normal locations.

The data from the study will form part of the marketing applications for volanesorsen in the United States, Canada and Europe next year, CEO Paula Soteropoulos said.

There are three other late-stage studies currently testing the drug, the company said.

Ionis's stock was up 1.4 percent at $50 in premarket trading. Up to Friday's close, the company's stock had fallen about 20 percent this year.