Now that Amgen ( AMGN)appears very close to acquiring Immunex ( IMNX) for about $17 billion to $18 billion, the toughest task ahead for the biotech companies will be convincing Wall Street that the deal makes sense. And to do this, Amgen has to sell investors on the future growth potential of Enbrel, Immunex's top-selling rheumatoid arthritis drug. If you believe, as Immunex executives claim, that Enbrel's annual sales can grow more than fivefold to $5 billion a year in the next five years, then this deal looks good. Recently, Amgen forecast 20% earnings-per-share growth over the next five years. If that goal has to be set aside in the near term, then gaining access to a super-growth drug such as Enbrel makes earnings dilution more palatable. But if you're someone who sees serious competition from other drugs on the horizon making such rosy growth targets unattainable, then Amgen is spending a whole lot of coin to get not much in return. As rumors of the deal circulated Thursday, Amgen shares fell $4.20, or 6.5%, to $60.19, while Immunex shares rose $2.51, or 10%, to $26.96. Enbrel really is the key to this deal, because the drug is Immunex's chief moneymaker and engine for near- to mid-term growth. Launched just three years ago, Enbrel is expected to rack up sales of $750 million this year, despite well-publicized supply constraints that have limited sales growth. Immunex has taken steps to boost the manufacturing capacity for Enbrel, and the company now forecasts 2002 sales to reach $900 million to $1.3 billion. Immunex is expected to earn 29 cents a share this year and 30 cents in 2002, according to Thomson Financial/First Call. Amgen is the world's largest biotech company, with annual revenue topping $4 billion, powered by two blockbuster drugs: anemia-fighter Epogen and anti-infection agent Neupogen. The company also recently won regulatory approval for Aranesp, an improved version of Epogen that it also believes has billion-dollar sales potential. The company is expected to earn $1.19 a share this year and $1.43 in 2002.