UPDATE 1-Amgen CEO says threat from biosimilars not absolute

* Smart biotechs may stem damage from biosimilars

* Sensipar kidney drug could get big boost from study

(Adds details on Amgen, rival drugs, byline)

By Ransdell Pierson

NEW YORK, May 18 (Reuters) - The chief executive of Amgen Inc (AMGN.O: Quote, Profile, Research) forecast on Monday that "smart" biotechnology companies may be able to retain up to half the sales of their drugs once they face competition from cheaper so-called biosimilars.

Kevin Sharer, speaking to investors at Deutsche Bank AG's annual healthcare conference, said he believed that longer- term, some biotechnology companies "can sustain 30 percent to 50 percent of cash flow from products if they're smart competitors."

He said that was considerably better than conventional medicines facing generic competition, meaning pills made of chemicals combined together rather than grown from living cells -- as is the case with biotechnology drugs.

Conventional drugs can lose up to 90 percent of their sales within a year once they face generic competition, in part because many companies often begin selling heavily discounted copycat formulations. By contrast, relatively few companies are now capable of making generic forms of biotech drugs, referred to as biosimilars or biogenerics.

European regulators recently approved biosimilar forms of two Amgen drugs, Epogen for anemia and Neupogen, used to boost infection-fighting white blood cells in patients on chemotherapy. The two drugs are not facing biosimilar competition in the lucrative U.S. market.