It would be fairly easy to determine whether the antibodies bind to commonly used drugs that are derivatives of cocaine (this is basically what ELISA testing is

It would be fairly easy to determine whether the antibodies bind to commonly used drugs that are derivatives of cocaine (this is basically what ELISA testing is for, and computer simulations would give you a general idea of whether it would bind or not). I would assume that this sort of testing would have been done prior to it being tested on humans, or at the very least would be done before it enters into common usage.