How big a drug Antegren will be is obviously a very important question for shareholders of

Biogen Idec

(BIIB) - Get Report

and

Elan Pharmaceuticals

(ELN)

, co-developers of the first-of-its-kind multiple sclerosis drug. But predicting the commercial potential of a new drug before launch is difficult under the best circumstances; for Antegren, it's even more challenging.

That's because for all the attention paid to Antegren over the past year, investors -- heck, even doctors -- still don't know how well the drug performs. And given the fact that Biogen Idec and Elan chose to keep Antegren clinical data

under wraps at an important European multiple sclerosis meeting last week, we're not likely to know anything definitive about Antegren's efficacy until after the drug is approved by U.S. drug regulators, expected around Thanksgiving.

Not that any of this has prevented institutional investors and sell-side analysts from making sales projections for Antegren, using various assumptions to build out financial models. Antegren will likely become the No. 1 multiple sclerosis drug in the market, racking up sales well in excess of $2 billion or higher, many analysts predict. Then, there are hundreds of millions of dollars of additional revenue that may be generated if Antegren is approved for other indications, like Crohn's disease.

With these kinds of expectations, it's no wonder that Biogen Idec is the top-performing profitable biotech stock this year, up 57%. Elan, too, is outperforming its drug company peers, up 197% since January. (In a difficult trading day Thursday, Elan shares fell 59 cents, or 2.8%, to $20.53 while Biogen Idec lost $1.01, or 1.7%, to $56.88.)

That's great performance, but will it last? Much of Antegren's success is priced into both stocks already. So, what happens to Biogen Idec and Elan if the rosy assumptions being used to project Antegren's blockbuster status don't play out as robustly as expected? I'm not suggesting that Antegren will turn into another overhyped flop like

Eli Lilly's

(LLY) - Get Report

sepsis drug Xigris or

Trimeris'

(TRMS)

AIDS drug Fuzeon. But with so much expected of Antegren, there is a risk that even a successful launch may not be successful enough.

Not to necessarily single him out as better or worse than his colleagues, but the Antegren forecasts being made by Sanford Bernstein analyst Geoff Porges illustrate my point about how what we don't yet know about Antegren's efficacy is a risk.

Porges has the highest EPS estimates for Biogen Idec on Wall Street because of his bullish Antegren forecast. Porges sees Biogen Idec earning $1.89 per share in 2005 and $2.36 per share in 2006, compared with Thomson First Call consensus analyst estimates of $1.73 per share and $2.09 per share in 2005 and 2006, respectively. (Porges rates Biogen Idec outperform and his firm doesn't have an investment banking arm.)

He gets to these numbers, in part, by assuming that Antegren will provide a significant clinical benefit to multiple sclerosis patients when used in combination with Biogen Idec's current MS drug Avonex. This implies that there will not be significant cannibalization of Avonex sales ($1.2 billion in 2003) -- a good thing for Biogen Idec because it owns 100% of Avonex, but must split Antegren profits equally with Elan.

Porges further believes Antegren will gain market share as a stand-alone MS treatment, but mainly for patients who don't respond well or fail treatment with current MS drugs, including Avonex,

Serono's

(SRA)

Rebif and

Chiron's

(CHIR) - Get Report

Betaseron.

But when I talked to Porges about his Antegren assumptions, he acknowledged that his Biogen Idec estimates might have to come down significantly if the Antegren clinical data breaks differently: first, if the Antegren-Avonex combination doesn't provide much clinical benefit to MS patients, at least not enough to really justify the cost of treatment, which could be in the $40,000 range; second, if Antegren's efficacy as a stand-alone MS treatment is so strong that doctors are compelled to use the drug at the expense of existing treatments.

Under either of these scenarios, or both, Avonex sales could take a bigger-than-expected hit, which would hurt Biogen Idec's bottom line, given the drug's greater economic leverage.

Again, I'm not making a judgment call on Porges' forecast. He may be right; he may be wrong. And there are other moving parts and issues with Antegren that may impact sales up or down.

The point to understand, however, is that figuring out how big a drug Antegren will become requires more guesswork than is typical in these situations because clinical data on the drug haven't yet been made public. By comparison, when

Genentech

(DNA)

launched its colon cancer drug Avastin in March, the relevant clinical data on the drug already had been presented and critiqued at a major medical meeting months earlier.

So, with Biogen Idec and Elan already up big for the year on hopes for Antegren, investors need to be extra careful and watch for hiccups in the Antegren data or the drug's launch.

Adam Feuerstein writes regularly for RealMoney.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet.com. He also doesn't invest in hedge funds or other private investment partnerships. He invites you to send your feedback to

adam.feuerstein@thestreet.com.