Drew Pardoll is the Abeloff Professor of Oncology, Medicine, Pathology and Molecular Biology and Genetics at Johns Hopkins University’s School of Medicine. (Marvin Joseph/The Washington Post)

“I like to think of [immunotherapy] like a car. To get from Point A to B with a car, you need three things: You need the gas pedal to go, but you also need the brakes, and it also helps to have a steering wheel. The immune system can steer itself very carefully and in a very focused way. In fact, it’s the genetic signatures in a cancer cell that the immune system can use to distinguish a cancer cell from a normal cell. . . . What we learned on the platform of basic immunology is that the problem was the parking brake was thrown on so you push on the gas and the car still doesn’t go.

“The revolution with checkpoint inhibition was mostly antibodies, but some drugs as well, [that] basically disabled the parking brake. With about 20 percent of all cancers — it’s much higher in melanoma, it’s much lower in pancreatic cancer, but across the board it’s probably about 20 percent of cancers — it’s like the car is sort of facing down the hill. All you have to do is disable one parking brake and the immune system goes after that tumor.

“There are two parking brakes for which there are FDA approvals. There are probably about 25 or 30 parking brakes and probably 25 or 30 accelerators. We think of precision immunotherapy as figuring out, for each of the patients, which of the parking brakes and which of the accelerators to combine. Almost all of the clinical trials, of which there are probably about a thousand that test combinations of immunotherapies, have a checkpoint inhibitor as part of that combination.”

Drew Pardoll , professor of oncology, medicine, pa­thol­ogy and molecular biology and genetics, Johns Hopkins University

This excerpt was from the December 6 Washington Post Live program Chasing Cancer. Video of the discussions can be see at Wapo.st/chasingcancer.