Burlington-based Dyax Corp. plunged the most in 11 months after it ended a trial of its leading drug candidate for swelling of the skin caused by blood pressure medicines. The trial was halted because the drug, known as ecallantide, may not be effective, according to a statement from the company. Safety wasn’t a reason for the decision, Dyax said. Dyax said it is assessing options regarding future trials to treat angioedema caused by blood pressure medicines.