Seven patients treated with omecamtiv mecarbil suffered heart attacks during the study compared to three patients on placebo (2.3 percent vs. 1.3 percent.) Five of the drug-related heart attacks occurred in patients treated at the highest dose.

A further safety analysis showed no relationship between increasing blood levels of omecamtiv mecarbil and increases in troponin, a biomarker for heart-muscle damage. Still, the imbalance in heart attacks observed in the study has prompted Amgen and Cytokinetics to add heart attack as an identified risk for omecamtiv mecarbil, Amgen said.

Cytokinetics CEO Robert Blum, in an interview, said the 1.6 percent incidence of heart attack observed in the study overall was considered reasonably low by study investigators who met Monday in Amsterdam to discuss the study results.

The ATOMIC-AHF study is being presented Tuesday at the annual meeting of the European Society of Cardiology.

"We're impressed and pleased with the study results because the drug behaved consistent with what we've seen in previous studies," Blum said from Amsterdam.

Omecamtiv mecarbil is designed to increase the amount of time the heart is in its contraction phase, or pumping blood to the rest of the body, without causing significant side effects or increased risk of death. Currently approved drugs known as inotropes also boost cardiac output but are associated with an increased risk for heart arrhythmia and deaths.

Amgen and Cytokinetics are developing an intravenous and pill form of omecamtiv mecarbil. The idea is that patients hospitalized with an acute exacerbation of their heart failure could be stabilized quickly with an intravenous form of the drug, then given pills to take when they're sent home. Hospital re-admission of heart failure patients is a significant medical problem, so any drug that could keep patients stable and out of the hospital could see wide use.

More than 5 million people in the U.S. are diagnosed with heart failure and the cost to treat the disease totaled almost $40 billion in 2010, with much of the expense coming from hospitalizations.

-- Reported by Adam Feuerstein in Boston.

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Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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