Most drugs are metabolised by the liver. Even something as routine as Paracetamol/Acetaminophen can be liver toxic – while 2 x 500 mg four times a day is safe, if you take all 20 at once there is a good chance you get liver failure.
So as a rule of thumb many medications are liver toxic at high doses – in effect the liver can not process them fast enough and toxic products build up. A number of promising drugs across many areas have failed in Phase 2/3 trials when patient have got liver failure from them.