What happened

Shares of Acceleron Pharma (XLRN) are up 41% at 11:19 a.m. EST following a report after the closing bell yesterday that the company's heart drug sotatercept was successful in a phase 2 clinical trial in patients with pulmonary arterial hypertension (PAH), a disease characterized by high blood pressure in the lungs.

So what

After 24 weeks of treatment, sotatercept was able to reduce pulmonary vascular resistance (PVR), the primary endpoint of the study. PVR measures the severity of PAH. The drug also improved secondary endpoints including how far the patients could walk in six minutes (6MWD) as well as two other measures of PAH severity: N-terminal pro b-type natriuretic peptide levels and World Health Organization (WHO) functional class.

Acceleron Pharma didn't put any numbers to the successful endpoints. As drug companies often do, it's saving the data for a medical meeting later this year.

But the company did note that the study was "powered to detect an 18% reduction in the primary endpoint of PVR and a 24-meter improvement in the secondary endpoint of 6MWD," so the drug likely met those goals.

Doctor showing a patient a chest x-ray

Image source: Getty Images.

Now what

The presentation of the actual data will be important in determining the likelihood of success in a phase 3 study. In addition to the magnitude of improvement in PVR and 6MWD, investors should be looking for a dose response with which a higher dose produces better results than a lower dose. The efficacy of sotatercept in the different WHO functional classes -- whether it helps all patients or just the sickest patients, for example -- will also help determine the drug's potential if it makes it to market.