A cholesterol-cutting drug from Amgen Inc. succeeded in lowering patients’ risk of cardiovascular trouble in a big clinical trial — but the results, announced Friday, may not be good enough to prompt insurers to cover the expensive drug for millions of patients.

Amgen’s treatment, called Repatha, met its goals in a two-year trial on more than 27,000 patients with heart disease who were already taking a maximum dose of statins like Lipitor and yet still had stubbornly high cholesterol. Those who got Amgen’s drug were 15 percent less likely to suffer a bad outcome, defined as a heart attack, stroke, hospitalization for chest pain, placement of a stent, or death.

However, looking at death rates alone, there was no significant difference between the two groups.

Cardiologists said the result is profound, but it will probably be a disappointment to investors, who widely expected Repatha to cut that bundle of cardiovascular problems by 20 percent.

Stock shares of Amgen, based in Thousand Oaks, Calif., fell 6.4 percent to $168.61.

It remains to be seen how the trial results will affect Amgen’s contentious discussion with payers and pharmacy benefit managers, or PBMs, who have balked at the drug’s roughly $14,000-a-year list price.

Before the new data came out — when the drug was known to reduce cholesterol, but when it wasn’t clear it could reduce hospitalizations — insurers and benefit managers rejected more than three-fourths of prescriptions that physicians wrote for Repatha. The question now is whether the new data will persuade them the drug is worth covering.

“We think we’re really at the beginning of a new era in how to further treat cardiovascular patients,’’ said Dr. Jeffrey Kuvin, a cardiologist at Dartmouth-Hitchcock who wasn’t involved in the Amgen study. “The real test now will be how we can actually implement this science into daily practice.’’

Repatha, like a rival drug from Sanofi and Regeneron, is approved to lower LDL, or “bad,’’ cholesterol by blocking a bodily protein called PCSK9. The new class of treatment arrived on the market about a year and a half ago with blockbuster revenue projections; Amgen was expected to ring up annual sales of around $3 billion. But a combination of unconvinced physicians, wary payers, and pugilistic pharmacy benefit managers has limited revenue to but a fraction of that figure.

Amgen says its latest data firmly answers the question of whether dramatically lowering LDL can improve patients’ lives — more than what’s been possible with inexpensive statins. And the company believes Repatha’s performance more than justifies expanded use of the drug.

The company hopes to persuade insurers to cover it in part by offering a sort of money-back guarantee: Amgen will refund the cost of Repatha if patients suffer a heart attack or stroke while taking the drug, said Dr. Joshua Ofman, Amgen’s senior vice president of value, access, and policy.

Repatha’s list price doesn’t account for the customary discounts and rebates offered to payers, which average around 30 percent across the industry, Harper said. In pure dollar terms, he argued, Repatha’s ability to reduce patients’ risk of costly hospitalization more than justifies its price.

But the industry’s gatekeepers may not agree. In a survey of payers conducted by analysts at Leerink, about half of the respondents said they expected to approve more PCSK9 prescriptions if Amgen’s study was a success, but they expected a median improvement of 20 percent on the trial’s primary endpoint, well above the 15 percent Amgen demonstrated.

Express Scripts, the nation’s largest PBM, is now preparing for a spike in Repatha prescriptions. The company has streamlined its process for reviewing scripts, Chief Medical Officer Dr. Steve Miller said, but Amgen’s data — and its offer of a refund program — haven’t changed Express Script’s concerns about Repatha’s price tag.

“These are incremental gains in the right direction, and it’s an improvement for patients,’’ Miller said. “The big question is going to be, at $14,000, is the improvement enough?’’

Damian Garde can be reached at damian.garde@statnews.com.