As a Christmas Eve present to itself, Cubist Pharmaceuticals (NASDAQ:CBST) followed through on its October option (structured more like a collaboration deal) to purchase Illumigen. Cubist paid $6 million in October for the option to acquire the development-stage company and its preclinical hepatitis C drug candidate.

The acquisition is costing Cubist $9 million in up-front cash (plus the previous $6 million in October). If everything works out and the Illumigen drug gets approved to treat HCV, then Cubist will only have to pay up to $76 million in clinical trial and regulatory milestones for the drug. And that's cheap for a marketed HCV compound.

Considering that the Illumigen drug isn't expected to start phase 1 testing until next year, it will be another year or two before we see this drug even in phase 2 proof-of-concept studies. No details were released about the Illumigen drug, other than the fact that it is a protein and an "interferon replacement product."

With the HCV treatment space looking like it will move more and more toward treatment with an interferon, plus oral small molecule therapy, it's hard to see how another protein-based drug (which won't be taken orally) will fit in the front-line treatment regimen for the disease. Unless, of course, its efficacy and safety can trump that of Roche's Pegasys and Schering-Plough's Peg-Intron interferon drugs.

That being said, with so many hepatitis C patients of all genotypes not responding well to even the most promising therapies -- like Vertex Pharmaceuticals' (NASDAQ:VRTX) telaprevir, for instance -- there will be room for new HCV drugs, even if their dosing is inconvenient or their efficacy worse than current therapies.

Right now, Cubist's future success or failure is based completely on its approved antibiotic drug, Cubicin. There are many one-drug-wonder specialty pharmas like Cubist looking to acquire new compounds to fill out bare drug pipelines and diversify away from a reliance on one compound.

Sometimes it can be maddening to watch drugmakers spend shareholder cash on these types of acquisitions -- exhibit A: OSI Pharmaceuticals (NASDAQ:OSIP). While there are no visible synergies to Cubist with this acquisition, at least it didn't have to wreck its balance sheet to acquire a cheap drug.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has an A+ disclosure policy.