Jan 23 (Reuters) - A cholesterol fighter developed by Amgen Inc from a new class of medicines succeeded in significantly lowering levels of "bad" LDL cholesterol in a late stage trial in patients unable to tolerate statin drugs, the company said on Thursday.

The results marked the third successful Phase III test of the drug evolocumab reported by Amgen in recent months - this one in a patient population among the most in need of alternative therapies.

Evolocumab belongs to a class of medicine called PCSK9 inhibitors that work by blocking a protein that reduces the liver's ability to remove LDL cholesterol from the blood.

Based on dramatic LDL lowering demonstrated by Amgen and other companies in earlier studies, PCSK9 inhibitors could be the most important new heart drugs to come along in several years with multibillion-dollar sales potential.

The injected medicines are likely to be used in high risk heart patients unable to lower their LDL levels sufficiently with high doses of widely used statin drugs, such as Pfizer's Lipitor, and in those unable to tolerate statins.

Amgen will provide details of its latest 307-patient Phase III study, called GAUSS-2, which compared evolocumab to Merck & Co's Zetia (ezetimibe), at an upcoming medical meeting.

However, the world's largest biotech company said the percentage of cholesterol lowering was consistent with those observed in a similar, smaller Phase II trial. In that study, evolocumab led to 51 percent reduction in LDL levels and 61 percent when combined with Zetia versus a 15 percent reduction seen with Zetia alone.

Regeneron Pharmaceuticals Inc, in partnership with Sanofi, is also in Phase III testing of a rival drug. Pfizer is developing its own PCSK9 inhibitor.