Osiris Therapeutics, Inc. (NASDAQ: OSIR), announced today interim one-year results from its groundbreaking clinical trial evaluating Prochymal® ( remestemcel-L) for the treatment of patients experiencing first-time acute myocardial infarction. The trial is the largest study of allogeneic or "off-the-shelf" stem cells ever conducted in heart attack patients. A total of 220 patients were given a single infusion of either Prochymal or placebo through a standard intravenous line within seven days of an acute heart attack. Cardiac MRI assessments were conducted for six months following infarct to evaluate cardiac remodeling. Patients receiving Prochymal had significantly less cardiac hypertrophy, as measured by cardiac MRI, compared to patients receiving placebo (p<0.05). Patients treated with Prochymal also experienced significantly less stress-induced ventricular arrhythmia (p<0.05). Cardiac hypertrophy and ventricular arrhythmia are indicators of pathological remodeling following heart injury and provide insight into the mechanism by which mesenchymal stem cells attenuate heart injury following a myocardial infarction. The mechanistic data is complemented by clinical data showing treatment with Prochymal resulted in a statistically significant reduction in heart failure. In the study, seven patients who were treated with placebo have progressed to heart failure requiring treatment with intravenous diuretics, compared to none of the Prochymal patients (p=0.01). Furthermore, patients receiving placebo tended to require re-hospitalization for cardiac issues sooner than the patients receiving Prochymal (median 27.5 days vs. 85.5 days). “This study is the largest of its kind and provides key insights into the mechanism of action of mesenchymal stem cells in the setting of acute myocardial infarction,” said Lode Debrabandere, Ph.D., Senior Vice President of Therapeutics at Osiris. “These important mechanistic observations are consistent with data obtained from our preclinical models and from the first placebo-controlled human trial with Prochymal published in the Journal of the American College of Cardiology. Given the quality of the data and highly encouraging results observed thus far, we are extending the trial's duration to capture a better understanding of the long-term clinical benefits of MSCs."