It was apparently the medicine itself, not a contaminant that caused the horrific reactions to TGN1412:
A preliminary investigation of a UK trial in which six healthy volunteers became critically ill said this was probably due to effects of the drug in humans not predicted by animal studies. But the report stopped short of questioning how the study was carried out.One problem with animal testing for monoclonal antibodies against human proteins is that they are likely to be much more potent in humans than animals since they are specifically engineered for high affinity for a human protein and may have little or no binding to the homologous animal protein. This is unlike more classic small molecules which are likely to have similar potencies in animals and humans.
The investigation into the phase I trial of the monoclonal antibody TGN1412, carried out by the Medicines and Healthcare Products Regulatory Agency (MHRA), the body that approves clinical trials in England, said the trial was run according to the agreed protocol, using the correct dose. It also found no evidence of a manufacturing problem or contamination of the product given to the trial volunteers.