Nov 14 (Reuters) - Members of an experimental class of cholesterol-lowering drugs could get U.S. regulatory approval based on their ability to lower "bad" cholesterol, and may not need to show that they reduce the risk of heart attack and stroke, the Food and Drug Administration said on Thursday.

The statement eased industry concerns that the agency would require more onerous "outcome" studies before approving the drugs, known as PCSK9 inhibitors.

Those concerns emerged earlier this week when two leading U.S. medical organizations recommended that doctors drop their emphasis on specific targets for lowering "bad" LDL cholesterol levels. They also recommended only statin drugs for patients at high risk of heart attack or stroke.

Statins, such as AstraZeneca Plc's Crestor and generic forms of Pfizer Inc's similarly potent Lipitor (atorvastatin), are the most widely used cholesterol treatments and work by blocking the liver's production of LDL cholesterol.

PCSK9 inhibitors, in late-stage testing at Pfizer, Amgen Inc , and a partnership of Sanofi SA and Regeneron Pharmaceuticals Inc, are anitibodies designed to block a protein whose natural function is to maintain the presence of LDL in the bloodstream.

The companies aim to seek regulatory approval of the PCSK9 drugs on the basis of their ability to lower LDL, not on whether they can reduce heart attack and stroke.

The FDA said it is closely monitoring an outcome study of Merck & Co's widely prescribed cholesterol drug Zetia, which is not a statin.

Even though Zetia has been on the market since 2002, Merck has not yet proven that it reduces heart attack risk and stroke. Results from the trial are expected by the second half of next year.

Zetia curbs absorption of cholesterol by the intestines. Zetia and Vytorin, a related drug, have combined annual sales of more than $4 billion.

Wall Street analysts also expect the PCSK9 inhibitors to generate annual sales in the billions of dollars.

The FDA, in an emailed statement, said it will review the PCSK9 inhibitors based on their effects on the entire lipoprotein lipid panel, particularly LDL cholesterol, effects on other markers of potential cardiovascular risk, blood pressure and any evidence of toxicity.

"The compound's overall safety profile will also be factored into this decision," the agency said.