Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Merrimack Pharmaceuticals (NASDAQ:MACK), a biopharmaceutical company focused on developing therapies to treat cancer, advanced as much as 24% after announcing positive indications for MM-121 in two mid-stage studies and announcing a collaboration with Actavis (NYSE:AGN).

So what: According to the press release, within the two studies -- a metastatic breast-cancer study in combination with exemestane and a second neoadjuvant setting study in combination with paclitaxel and followed by doxorubicin and cyclophosphamide -- Merrimack's MM-121 delivered impressive clinical benefits in patients identified with select biomarkers. In the first study, the company noted a hazard ratio of 0.32, which would suggest a 68% clinical benefit to patients expressing the ErbB3 biomarker. With confirmation that ErbB3 represents an important biomarker for ovarian and breast cancers, Merrimack can now target its studies at this specific subset of patients.

In addition to its clinical data, Merrimack also announced a nanotechnology collaboration with Actavis. Under the agreement, Merrimack will use its proprietary nanoliposomal technology platform to develop compounds for Actavis to develop and hopefully manufacture around the world.

Now what: It's been a wild ride for Merrimack shares over the past four weeks as MM-121 in late October delivered similar results, but shareholders gave it less than a warm reception. The problem then was that investors were worried MM-121 wouldn't have a large enough market because it was catering to a select biomarker while the remainder of the patient population was unresponsive. With additional mid-stage studies seeming to confirm this, though, it would appear that MM-121 could have a meaningful impact on this smaller, but still substantial, patient subset expressing ErbB3. As always, I'd rather wait for later-stage data before I go jumping onboard any small-cap biopharmaceutical pipeline, but it remains a name to watch nonetheless.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

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