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 cancer therapies, fell as much as 23% after reporting results from a mid-stage study involving MM-121 in combination with Paclitaxel for the treatment of platinum-resistant or refractory-advanced ovarian cancer.

So what: According to Merrimack's press release, MM-121, which is being developed in collaboration with Sanofi (NYSE:SNY), did not meet its primary endpoint of improving progression-free survival in a mid-stage trial. The hazard ratio of 1 signifies that there was no statistical difference between the MM-121 with Paclitaxel arm and the control arm with just Paclitaxel. In addition, certain side effects like diarrhea and vomiting were more prevalent in the group taking MM-121. If there was a bright spot, it was that a certain subset of patients with two specific biomarkers did demonstrate a hazard ratio for PFS of 0.37, signifying a 63% improvement over the Paclitaxel control arm, and may encourage new research geared toward patients with these biomarkers.

Now what: The development of MM-121 is still very much in the early stages, but today's news certainly wasn't encouraging. There was no benefit demonstrated via the hazard ratio compared to the control, and the increase in side effects relative to the control arm is to me a bit concerning. Merrimack and Sanofi could certainly still find success with MM-121 as the biomarker subset showed, but I'd rather wait on the sidelines for Merrimack to deliver solid late-stage results than be left pondering what happened on a day like today.

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.

Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.