Former president Jimmy Carter recently announced he’s cancer free, and it was largely thanks to a new immunotherapy drug.

Carter had been diagnosed with cancer in August, but announced just days ago that he was cancer free. According to The Guardian, the 91-year-old was given pembrolizumab, one of the first immunotherapy drugs, which is sold under the brand name Keytruda in the U.S.

Immunotherapy drugs, rather than killing cancer cells, boost the immune system to do it instead. And while the theory behind immunotherapy is not new, only very recently have scientists been able to successfully put it into practice with several outstandingly successful drugs.

Pembrolizumab was one of those drugs, as 74 percent of patients with advanced melanoma (skin cancer) who were given doses every two weeks were still alive 12 months later.

Carter’s doctors had discovered melanoma spots on his liver after discovering a tumor, which then quickly spread to his brain. It is unclear if pembrolizumab was responsible for ridding the cancerous cells in Carter, as the drug is too new to know if its effects are consistent and repeatable.

And although latest MRIs reveal that there are no signs of cancer remaining in Carter’s body, he will continue to get pembrolizumab infusions, as well as radiotherapy and likely some form of chemotherapy as well due to the possibility of future spread.

Thanks to its experimental nature, Keytruda treatment can be very expensive, costing around $150,000 a year, The Washington Post reported.