Where I work currently, there was such a failure during the early phase 2 clinical trial period for an investigational drug. The drug had performed remarkably well during animal testing trials. However, when it was used on the intended patient population during limited phase 2 clinical trials, there was a marked increase in adverse events associated with administration of the drug including death. The trial was canceled immediately and the analysis of data was undertaken. Upon further investigation, it appeared that certain nuances in certain IL levels resulted in catastrophic failure of the drug in the at risk population. The ultimate cause for the failure of the study included diminished interleukin levels in the at risk population when compared to healthy controls and discrepancies between animal models and human models.