(Reuters) - Amgen Inc reached a seven-year contract to supply DaVita Inc with nearly all of the anemia drugs needed by the dialysis clinic operator, locking up nearly a third of the U.S. market for its flagship drug Epogen.

Amgen said in a regulatory filing that the contract, which runs through 2018, "provides for discount pricing and rebates."

The deal "removes near-term uncertainty of massive market share erosion from competition on a volume basis," said RBC Capital Markets analyst Michael Yee.

DaVita is the second-largest dialysis company in the United States, topped by Fresenius Medical Care, which has a market share of around 33 percent.

Sales of Epogen, which totaled $2.52 billion in 2010, have waned in recent years due to safety concerns. But Amgen still holds a virtual monopoly on the market for drugs used to boost red blood cell levels in kidney dialysis patients.

That could change in coming years as generic, or "biosimilar," versions of such erythropoiesis-stimulating agents reach the market and competitors such as Roche start selling similar drugs.

Another potential rival is Affymax Inc, which is seeking U.S. regulatory approval for its experimental drug peginesatide as a treatment of anemia in kidney dialysis patients.

A panel of expert advisers to the U.S. Food and Drug Administration is set to review the Affymax drug on December 7, with a final decision from the agency expected by March 27.

Should it win approval, Affymax is expected to price its products below that of Epogen.

"Amgen might have agreed to match or partially match competitors epo pricing on the logic that taking (DaVita) out of the pricing picture would help preserve a far higher price," ISI Group analyst Mark Schoenebaum said in research note.

RBC's Yee expects Amgen's sales of Epogen to fall 16 percent in 2012 and 11 percent in 2013.

The supply deal with DaVita does not involve Amgen's second-generation anemia drug, Aranesp, which generated $2.49 billion in sales last year.

(Reporting by Deena Beasley; Editing by Derek Caney)