Doctor Brundle was a scientist at Nezumi Testing laboratories. He was working on something that required animal testing.
According to the notes he left behind, his work relied extensively on testing things on living animals, specially rodents. When his colleague, Dr. Piccinini, showed him his "Pet Project", he wasn't sure what it was, and speculated it was some sort of elaborate, although inappropriate, prank.
Eventually, he was disillusioned with the work that was being done, and feared the research was turning into something dark, beyond the realm of science. He tried to quit while he could when he saw the results of the research.
After he was attacked by a crazed Dr. Piccinini, he tried to escape the facility using the flood tunnels connected to the rat fluid chamber, only to have his path blocked by Sotiris in the warehouse.