By Bill Berkrot

NEW ORLEANS, Nov 15 (Reuters) - Amgen Inc's potent new drug Repatha, when added to statin therapy, not only took "bad" LDL cholesterol down to extremely low levels but caused declines in artery-clogging plaque in a majority of high-risk heart patients after 18 months of treatment, according to data from a clinical trial.

Amgen had previously announced the study was a success. But the percentage of patients who experienced a decrease of the substance that is the underlying cause of heart disease and magnitude of plaque regression was revealed at the American Heart Association scientific meeting in New Orleans on Tuesday.

The 968-patient trial compared the effect of monthly injections with Repatha plus a cholesterol-lowering statin with statins alone on the plaques that can break off and cause heart attacks. The plaque measurements were collected with an ultrasound probe placed inside the diseased artery.

Patients in the study had symptomatic heart disease and blockages of 20 percent to 50 percent in the tested artery.

"We saw profound regression," said Dr. Steven Nissen, head of cardiology at Cleveland Clinic who presented the data.

The combination therapy led to a further 60 percent reduction in LDL levels beyond statins alone to an average LDL of a mere 36.6. That translated into a decrease in percent of blockage volume of about 1 percent compared with no change for statins alone.

In all, 64.3 percent on the combination therapy experienced plaque regression compared with 52.7 percent with statin monotherapy.

"We didn't know what would happen to disease progression at LDL cholesterol levels when we go to below about 60," Nissen explained.

For those who began the trial with LDL below 70, the lowest target guideline for high risk patients, 81 percent experienced coronary plaque regression with the addition of Repatha compared with 48 percent for statins alone. Those Repatha patients on average saw LDL levels drop to 24 with a low of about 15.

"That's unbelievable. So when you get down to 24 you've got a really high chance of your plaques melting away," Nissen said.

Repatha and Praluent from Regeneron Pharmaceuticals and Sanofi belong to an expensive new class of drugs known as PCSK9 inhibitors. They carry a list price of more than $14,000 a year before discounts and rebates.

Many heart doctors have been upset by barriers to access to these drugs they encounter from health insurers and pharmacy benefit managers, even for their sickest patients who meet all the criteria mandated by the U.S. Food and Drug Administration.

More than 100,000 prescriptions have been written for Repatha in the United States since its approval, but two-thirds of patients are ultimately denied, Amgen said.

"It's crazy what's going on. They've basically destroyed our capacity to treat our patients with the drugs that we need to treat them with," said Dr. Seth Baum, president of the American Society for Preventive Cardiology.

In the third quarter, Repatha had anemic sales of just $40 million, while Praluent took in $38 million for drugs forecast to be multibillion-dollar products.

It is widely believed that payers will not lift reimbursement restrictions until they see results from huge studies designed to show that the new medicines cut the risk of heart attacks and death in addition to their ability to slash LDL levels. The Repatha outcomes data is expected in early 2017.

But the plaque regression data unveiled on Tuesday is likely a strong indictor that those trials will be positive.

"These findings suggest that the large clinical outcome trials currently underway are likely to show major benefits," said Dr. Stephen Nicholls, one of the study's lead directors.

(Reporting by Bill Berkrot; Editing by Chizu Nomiyama)