Shares of Antigenics ( AGEN) were among the worst-performing health-related stocks Friday, plummeting 45% after the biotech company halted its trial for a kidney cancer drug. Antigenics said the trial was halted based on the number of events -- defined as recurrence of disease or death of a patient prior to recurrence -- reported by the study's investigators. An independent review panel found that the number of events that occurred were significantly lower than reported by the investigators. The independent panel determined that the required number of events to conduct an analysis of the drug's primary endpoint, recurrence-free survival, wasn't met. "We believe that further work is necessary to properly interpret these results," the company said. Antigenics said that it plans to analyze the data, which will take about four to six weeks. In the meantime, the company has decided to suspend part two of the kidney cancer trial until it examines data from part one. Antigenics also said it would immediately implement a restructuring plan aimed at reducing cash burn. Shares were trading down $2.28 to $2.83. Idenix Pharmaceuticals ( IDIX) tumbled 27% after the company and its partner, Novartis ( NVS), modified the dosage rate of valopicitabine in its hepatitis C clinical trial. The company lowered the dosage rate from 800 milligrams per day to 200 milligrams or 400 milligrams per day because of dose-related gastrointestinal side effects that were observed in certain patients who were receiving 800 milligrams. "While these modifications will delay the valopicitabine development program, the primary purpose of phase II studies is to identify the optimal dosing regimen with respect to efficacy and safety," Idenix said. Shares were trading down $5.55 to $14.70. Shares of Matria Healthcare ( MATR) rose 4% after Standard & Poor's said it plans to add the health services company to its S&P SmallCap 600 index after the close of trading on March 30. Matria Healthcare will replace Hughes Supply ( HUG), which is being acquired by Home Depot ( HD). Matria shares were trading up $1.54 to $38.09.