Sept 29 (Reuters) - Amgen Inc said on Thursday it would buy a stake in Arrowhead Pharmaceuticals Inc and team up with the company to develop gene-silencing therapies for heart disease.

Pasadena, California-based Arrowhead will get $35 million up front and will be eligible to receive up to $617 million in milestone payments. It will also get $21.5 million from Amgen through an equity investment.

Arrowhead, which had about 60.43 million shares outstanding as of June 30, has a market value of about $425 million.

The company, along with drug developers such as Alnylam Pharmaceuticals Inc and privately held Quark Pharmaceuticals, focuses on ribonucleic acid interference (RNAi), which aims to silence certain genes to curb the production of disease-causing proteins.

RNAi has long interested drug developers, but has frequently met with clinical failure.

Existing drugs, including monoclonal antibodies, work by trying to inactivate disease-causing proteins by binding to them.

The license and collaboration deal includes two agreements, under which Thousand Oaks, California-based Amgen will take charge of clinical development and commercialization.

Under the first agreement, Amgen will get an exclusive global license to Arrowhead's ARC-LPA program, in which RNAi molecules are designed to reduce elevated levels of a form of cholesterol called Lp(a) that raises the risk of heart disease caused by plaque build-up.

One in five people inherit high levels of Lp(a), which leads to the gradual narrowing of arteries, limiting blood supply to the heart, brain, kidneys and legs, thereby raising the risk of blood clots, heart attack and stroke.

Under the second agreement, Amgen will receive an option to an exclusive global license for a RNAi therapy for an undisclosed cardiovascular target.

Amgen, the world's largest biotechnology company by market value, has a growing suite of heart drugs, both on the market and in development.

These include its potent, expensive new cholesterol fighter Repatha, chronic heart failure treatment Corlanor, and omecamtiv mecarbil, which is in the final stages of development.

Amgen announced last September that it would buy biotechnology company Dezima Pharma BV, adding another cholesterol drug to its pipeline.

(Reporting by Natalie Grover in Bengaluru; Editing by Ted Kerr)