I'm all for companies jumping into growing, untapped markets. But investors have to be careful that the companies they own don't become too spastic and lose focus in the process.

That's what worries me about Spectrum Pharmaceuticals (Nasdaq: SPPI) and its announcement today that it's developing a biosimilar version of Roche and Biogen Idec's (Nasdaq: BIIB) Rituxan, which treats types of lymphoma. The knockoff will be developed in a partnership with privately held Viropro, although no terms were given.

Sure, Rituxan fits with Spectrum's oncology focus -- it sells cancer treatments Fusilev and Zevalin -- and with sales of $5.6 billion in 2009, there's a big market for copycat drugs once Rituxan loses patent protection.

But biosimilars -- generic versions of protein-based drugs -- are a whole different can of worms than branded drugs. The EU has a pathway to approval, although getting a biosimilar approved is much more complex than a typical small-molecule drug. In the U.S., a pathway for approval doesn't even exist, although the Food and Drug Administration is working on it as part of last year's health-care reform law.

It's extremely possible that the biosimilars business will turn into a low-margin market if the FDA requires clinical trials to prove similarity and/or makes it so the biosimilars aren't interchangeable with their branded counterpart. If drugmakers have to market their biosimilars to gain prescriptions -- instead of just having pharmacists make a substitution, like they do for small-molecule generics -- the margins could be pretty slim.

That might not be a problem for generic-drug makers such as Teva Pharmaceutical (Nasdaq: TEVA) and Mylan (Nasdaq: MYL), which are used to low-margin products. I can even see the reasoning behind Pfizer (NYSE: PFE) and Merck (NYSE: MRK) jumping into the biosimilar market, since they'll likely be able to make up in volume what they lose in margins.

But will little Spectrum, in a sea of big fish, be able to etch out a large enough corner of a large market to justify jumping in? Unfortunately, we probably won't know for years whether today's move was spastic flailing or a solid shift in the direction of a gold mine.

Interested in keeping track of Spectrum as it moves into biosimilars? Click here to add it to My Watchlist, which will help you keep track of all our Foolish analysis on Specturm.

Pfizer is a Motley Fool Inside Value recommendation. The Fool owns shares of Teva Pharmaceutical Industries. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.