* Experimental drug significantly cut bad cholesterol levels
* Largest trial yet in new class of cholestrol-lowering drug
* Largest PCSK9 trial so far
* Amgen shares rise 1.4 percent (Adds analyst comments, physician comment, company comment, share price)
By Deena Beasley and Bill Berkrot
LOS ANGELES, Nov 6 (Reuters) - Amgen Inc's experimental cholesterol-lowering drug, AMG145, reduced levels of "bad" cholesterol up to 66 percent in patients already taking statin drugs, according to results from a mid-stage trial that were presented on Tuesday.
The findings were in line with other AMG145 studies showing significant reductions in LDL cholesterol that were unveiled this week at the American Heart Association scientific meeting in Los Angeles, California.
The latest study of 629 patients is the largest trial so far of a drug in a new class of injectable biotech drugs, known as PCSK9 inhibitors, designed to target a protein that prevents the body from removing artery blocking LDL cholesterol from the bloodstream.
The Amgen trial findings were "as good as we could have hoped," UBS analyst Matthew Roden said in a research note.
The Phase 2 trial showed a mean reduction in LDL versus a placebo of 42 percent for patients injected with 70 mg of AMG145 every two weeks, 60 percent in the 105 mg group and 66 percent in the 140 mg group.
When the drug was administered every two weeks, the mean reduction in LDL was 42 percent for the 280 mg group, 50 percent for the 350 mg group and 50 percent in the 420 mg group.
"We had some patients with entry LDLs as low as 85," said Dr. Robert Giugliano, associate professor at Brigham and Women's Hospital in Boston, and the study's lead investigator, "At the end, some had LDLs measured in the teens."
Patients in the trial were taking common statin pills such as Pfizer Inc's Lipitor. Some were taking Merck & Co Inc's cholesterol medicine Zetia.
Current guidelines from the AHA call for LDL levels to be 100, but some cardiologists believe even lower levels would be beneficial.
Analysts have estimated that PCSK9 drugs - also being developed by Regeneron Pharmaceuticals in partnership with Sanofi, Pfizer, and Roche - could eventually generate billions Of dollars in annual sales.
"Though trials vary, we currently assume safety and efficacy is likely similar across various drugs with Regeneron/Sanofi first to market," RBC Capital Markets analyst Adnan Butt said in a research note. "In addition to clinical utility, market size is the next parameter up for debate."
He said the conservative estimate is that 15 percent of high-cholesterol patients need additional treatment, representing two million to three million patients, or an ultimate U.S. market opportunity of at least $10 billion to $15 billion.
The most common side effects seen in the Amgen trial of statin patients were cold-like symptoms, cough and nausea.
Side effects in previous, smaller, trials of the drug included injection-site reactions, headache and muscle pain.
AMG145 has demonstrated "very effective lipid changes," said Dr. Peter Wilson, an endocrinologist at Emory University and the Veterans' Affairs Medical Center in Atlanta. He noted that more work needs to be done to determine the drug's full impact on the liver and on muscles.
Sean Harper, head of research and development at Amgen, said the company plans to begin pivotal-stage trials of AMG145 early next year.
He said the company plans to first file for regulatory approval of AMG145 based on Phase III studies looking at the drug's ability to lower LDL.
Amgen also plans to conduct a broader Phase III trial designed to see whether AMG145 can lower the risk of heart problems. Harper said it was "not unreasonable" to assume that such a trial would take five years to yield results.
"We have seen a very consistent effect of lowering LDL ... which is really important because by reducing LDL by half one would predict that we would be reducing the risk of cardiovascular events by about that same amount," Harper said.
Shares of Amgen, which have risen 37 percent so far this year, were up 88 cents at $87.23 in Nasdaq trading. (Editing by Ken Wills and Marguerita Choy)