By Bill Berkrot

March 13 (Reuters) - Data that should help unlock the sales potential of a potent new cholesterol medicine will be unveiled at the American College of Cardiology meeting this week as the future of the only rival drug rests with the courts in an ongoing patent dispute.

Highly anticipated results from a 27,500-patient trial of Amgen Inc's Repatha will be presented on Friday at the meeting in Washington, informing doctors and investors of just how much the expensive injectable drug cut the risk of heart attack, stroke and death in people with heart disease already taking maximum doses of cholesterol-lowering statins, such as Lipitor.

Insurers and pharmacy benefit managers have been rejecting some 75 percent of prescriptions written for the new medicines while awaiting proof of their clinical value. They prefer to keep patients on cheap, generic statins already shown to prevent heart attacks.

Repatha and a competing drug, Praluent from Regeneron Pharmaceuticals Inc and Sanofi SA, have list prices of over $14,000 a year, before discounts and rebates.

They belong to a class of medicines called PCSK9 inhibitors that were approved on their ability to dramatically lower "bad" LDL cholesterol. But with a raging debate over high U.S. prescription drug prices, there has been pressure for hard data to justify the cost of medicines that may have to be taken for life.

The high rejection rate has severely constrained sales of the potential multibillion-dollar products, which were just $58 million for Repatha and $41 million for Praluent in the fourth quarter.

Rejections have frustrated cardiologists trying to get these new drugs for patients whose cholesterol remains dangerously high despite taking the most potent statins, and for those unable to tolerate them.

"What I care about is there are patients for whom the PCSK9 is absolutely vital," said Dr. Leslie Cho, head of preventive cardiology and cardiac rehabilitation at the Cleveland Clinic.

Industry analysts said they hope to see at least a 20 percent reduction of major adverse heart events from Repatha, and that anything less might be viewed as disappointing to investors.

Dr. Jorge Plutzky, director of the vascular disease prevention program at Brigham and Women's Hospital in Boston, said a risk reduction greater than 20 percent would be very hard for insurers to ignore. But, he added, "any statistically significant reduction in cardiovascular events would be important to me."

Amgen shares rose 3 percent last month, when it announced the Repatha trial had succeeded without providing details.

AWAITING APPEAL DECISION

Regeneron expects to have results from Praluent's large cardiovascular outcomes trial by year end. Whether Praluent will still be available depends on an appeal of a U.S. court ruling that found it infringed Repatha patents.

Praluent was allowed to remain on the market pending the appeal decision, which could take into account the benefit to society of competition that lowers drug prices.

New prescriptions have been higher for Repatha since the January court ruling. In the week ended March 3, there were 1,288 written for Repatha and 892 for Praluent, according to data reported by analysts.

But interviews with physicians found they were not taking Praluent's uncertain future into account when selecting which to prescribe.

Those decisions were driven by preferences of individual patients' health insurer or PBM, they said. Insurers negotiate discounts with each drugmaker and then offer the cheaper one to patients with a lower co-payment.

Express Scripts Holding Co, the nation's largest PBM, declined to speculate on how much clinical benefit must be demonstrated before making it available to more patients.

(Reporting by Bill Berkrot in New York; Editing by Lisa Shumaker)