Former U.S. President Jimmy Carter attends the 2008 Democratic National Convention in Denver, Colorado August 25, 2008. Eric Thayer/Reuters Former US President Jimmy Carter announced on Sunday that his latest brain scan showed no sign of cancer, a few months after revealing that he had been diagnosed with melanoma that had spread from his liver to his brain.

Carter was being treated with a cancer drug called Keytruda that uses the immune system to fight off cancerous cells.

Keytruda, made by pharmaceutical company Merck, was originally approved by the FDA in September 2014 to treat melanoma, a deadly form of skin cancer that can also show up in other organs of the body, as it did in Carter's case.

In someone with melanoma, certain proteins called PD-1 stop the immune system from doing its job and fighting the cancerous cells. Keytruda works by getting in the way of those proteins, allowing the immune system to access the cancer cells. Then, with the help of radiation therapy, which works to shrink tumors by killing cancer cells, it can knock the cancer out.

The drug is delivered intravenously every three weeks, costing about $12,500 a month.

And the drug isn't just being used in cases like Carter's. Keytruda, which got approved to treat a form of lung cancer in October, is also being explored to treat a number of other cancers, including head and neck, breast, and bladder cancers and Hodgkin lymphoma.

It's also not the first cancer immunotherapy drug. Scientists have been seriously exploring using the immune system to battle cancerous cells for decades as an alternative to chemotherapy, radiation therapy, and surgery. But it's taken a long time for the treatments to be effective in humans.

On Monday, Merck also announced that it has initiated two final phase trials using Keytruda in patients with multiple myeloma, a type of blood cancer that affects a type of white blood cell called plasma cells.

In November, the FDA approved three multiple myeloma drugs, including another cancer-immunotherapy drug called Empliciti.