The drug, romosozumab, which would be sold under the brand name Evenity if approved, is awaiting an approval decision by the Food and Drug Administration.
But the new safety data, which cropped up in an otherwise successful trial, will have to be taken into consideration, delaying any FDA decision.
“The efficacy results from this study comparing Evenity to an active control are robust. At the same time, the newly observed cardiovascular safety signal will have to be assessed as part of the overall benefit/risk profile for Evenity,” Amgen research chief Sean Harper said in a statement.
Evenity is considered to be a potentially important future growth driver for Amgen, and the surprise heart problems and delay in approval will likely not sit well with Amgen investors.
The drug is also being considered for approval in Canada and Japan, the companies said.
ISI Evercore analyst Umer Raffat in a research note called the new data “clearly negative and very surprising.”
Raffat said he was removing all sales of romosozumab from his Amgen forecast models and expected company shares to trade 3 to 4 percent lower on Monday. Amgen shares closed at $156.51 on Nasdaq on Friday.
Analysts on average were forecasting annual sales of the drug to reach about $720 million by 2023, according to Thomson Reuters data.
The medicine met the primary and key secondary goals of a late-stage study. The imbalance in heart-related side effects had not been observed in an earlier Phase III study that had been the basis of the regulatory submission seeking approvals.
In the new trial, romosozumab significantly reduced the incidence of new vertebral fractures through 24 months as well as non-vertebral fractures in postmenopausal women with osteoporosis at high risk for fracture compared with Merck & Co’s Fosomax.
Serious heart problems were reported, however, in 2.5 percent of patients who received the Amgen drug, versus 1.9 percent in the Fosomax group.
Patients in the study received romosozumab for 12 months followed by treatment with Fosomax, known chemically as alendronate, compared with those who received only Fosomax, a current standard of care.
(Reporting by Bill Berkrot; Editing by Sandra Maler and Peter Cooney)