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: The shares of Ariad Pharmaceuticals (NASDAQ:ARIA), a biopharmaceutical company focused on developing therapies to treat acute and chronic forms of leukemia and other hard-to-treat cancers, plunged as much as 77% after updating its clinical development pipeline with regard to leukemia drug Iclusig.

So what: Following a meeting with the Food and Drug Administration, Ariad and the FDA are temporarily halting enrollment in new studies of the drug, and reducing the dosage of patients currently in existing trials, due to an elevated number of serious arterial thrombosis events after a two-year follow-up of patients with Philadelphia chromosome-positive leukemias. Today's clinical hold does not affect the standing of Iclusig as it relates to its late 2012 FDA approval for resistant or intolerant chronic myeloid leukemia (CML) and Philadelphia chromosome-positive acute lymphoblastic leukemia, although the FDA will be issuing a drug safety communication later this week on the drug.

Now what: This clearly isn't good news for shareholders or CML patients, but the potential warning signs have been there since Ariad was approved. If you recall, the FDA placed a black-box warning on Iclusig's packaging in December, because the drug had been shown to cause liver toxicity and arterial thrombosis in more than a handful of patients in trials. Rarely does a drug with high toxicity potential really take off, and it certainly makes it much tougher for Iclusig to expand its indications as it's currently trying to do in trials. I'm keeping my bearish stance on the company, even at these levels, until the data, or Ariad's pipeline, proves otherwise.