COLUMBIA, Md. (AP) — Osiris Therapeutics Inc. said Friday it discovered a flaw in a trial of its stem cell candidate Prochymal, and decided to end enrollment of patients early. The trial tested Prochymal as a treatment for Crohn's disease, a digestive tract disorder. The late stage trial compared the effectiveness of Prochymal against a placebo, and Osiris said the design flaw made the placebo appear more effective. As a result, the company said, Prochymal was unlikely to appear significantly more effective. Osiris stopped enrollment with 210 patients in the trial, out of a planned 270. In premarket trading, Osiris shares plunged $7.40, or 40.4 percent, to $10.90, near their lowest price of the last year. The stock finished at $18.30 Thursday. Prochymal is a formulation of adult stem cells and is Osiris' lead drug candidate. Osiris is developing the product through a partnership with Genzyme Corp., and is also testing Prochymal as a treatment for graft vs. host disease and type 1 diabetes. Osiris said there were no safety issues with the drug. The company did not immediately return calls seeking comment.