Early this month, a release of competitors' data caused Vertex Pharmaceuticals' (NASDAQ:VRTX) stock to plummet. Today, it was news about a collaborator's clinical trial that caused the Rule Breakers stock to fall.

Yesterday evening, Vertex and Merck (NYSE:MRK) announced that they were suspending enrollment in their phase 2 clinical trial for testing the leukemia treatment MK-0457. One patient was observed with a QTc prolongation, a heart condition that results in irregular heartbeat and could result in a heart attack. It's not clear that the drug caused the condition, and patients enrolled in the trial will continue to be dosed, so the drug is far from dead.

Vertex clearly has a lot more riding on this compound than Merck does. Merck is set to pay up to $350 million in milestones if MK-0457 makes it through trials and receives an FDA approval -- not to mention the royalties once the drug is on the market.

MK-0457 is part of Vertex's and Merck's collaboration to target Aurora kinase, a protein that's required for cells to grow and therefore a perfect target for anti-cancer compounds. One compound in the collaboration has been dropped because of a lack of efficacy, but the duo plans to start another, VX-689, in a phase 1 trial early next year.

Vertex's shares have been in steady decline since competitors like Schering-Plough (NYSE:SGP) released data on compounds that will compete with Vertex's hepatitis C virus (HCV) drug telaprevir to become the next HCV blockbuster.

As my Foolish colleague Brian Lawler pointed out, there's still plenty of space in the HVC marketplace for multiple drugs. Combine the two partially unjustified drops in stock price, and you've got a drug developer with a serious markdown just in time for Christmas.

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