Biogen Idec and partner Cardiokine Inc. said they have begun enrolling patients in a Phase III study of lixivaptan, a potential treatment for correcting low sodium serum levels in heart failure patients.

The condition, known as hyponatremia, is common in patients who have been hospitalized with worsening heart failure.

At least a quarter of patients with heart failure have hyponatremia, and recent studies have demonstrated that low sodium levels are an independent predictor of worsening outcomes in this group, the companies said.

In prior clinical studies, lixivaptan-treated patients showed improvement in serum sodium concentrations, decreases in body weight and increases in urine volume, they said.

Clinical trials in patients with acute heart failure typically take about two years to enroll, said Biogen spokeswoman Shannon Altimari. However, the Cambridge, Mass.-based company has not indicated any target date for completing this study, she told BioWorld Today.

The placebo-controlled trial, known as THE BALANCE, will compare lixivaptan to placebo in about 650 patients in the U.S. and Europe. Specifically, the trial will evaluate the safety and effectiveness of lixivaptan vs. placebo in increasing serum sodium in heart failure patients with hyponatremia, according to the companies.

Lixivaptan, an oral vasopressin V2 receptor antagonist, works by causing a decrease in renal water reabsorption (an increase in urine volume) and a decrease in urine osmolality (a measure of urine concentration). The Phase III studies of the drug represent Biogen Idec's lead cardiology program.

Although hyponatremia can occur in isolation, it is most often a complication of another disease such as heart failure, cirrhosis or syndrome of inappropriate anti-diuretic hormone secretion (SIADH). Pivotal studies in SIADH are anticipated to start later this year, Cesare Orlandi, senior chief medical officer at Cardiokine, said in the companies' news release.

Under the terms of the collaboration, Biogen Idec and Cardiokine expect to jointly develop lixivaptan. Biogen Idec will be responsible for its global commercialization and Cardiokine will have an option for co-promotion in the U.S., they said.

Philadelphia-based Cardiokine, a specialty pharmaceutical company, is responsible for running the BALANCE trial.