(Reuters) -Alnylam Pharmaceuticals said on Monday it would not pursue expanded use of its drug to treat a potentially fatal heart disease in the U.S. after the Food and Drug Administration declined to approve the medication despite a favorable recommendation from its advisers. The regulator, in its so-called complete response letter, said the drug patisiran did not meaningfully improve the condition of patients with heart muscle issues or cardiomyopathy caused by ATTR amyloidosis, characterized by abnormal deposits of transthyretin proteins in organs and tissues. Patisiran, also branded as Onpattro, is already approved in the U.S. to treat nerve damage in adult patients with hereditary ATTR amyloidosis.