Although former U.S. president Jimmy Carter's diagnosis of four spots of melanoma back in August seemed like a fatal one, his brain cancer appears to have disappeared and many, including him, are pointing to pembrolizumab, a recently approved immunotherapy drug, as a key factor in his recovery, according to The Guardian.

"My most recent MRI brain scan did not reveal any signs of the original cancer spots nor any new ones," said Carter. "I will continue to receive regular three-week immunotherapy treatments of pembrolizumab."

Pembrolizumab is sold under the brand name Keytruda in the United States and is one of the first immunotherapy drugs ever created. It is unique due to the fact that it does not kill cancer cells - instead, it boosts the immune system in order to help it kill them more effectively. Keytruda was only approved for melanoma treatment just over a year ago and so far survival rates have been promising - 74 percent of patients given doses every two weeks were alive after one year.

However, in addition to pembrolizumab, Carter was also given radiation treatment, which likely gave his immune system an extra boost to ensure a more effective attack against the cancer cells in his body, according to Quartz.

Despite the promise of pembrolizumab, there are some patients who are not helped by it and doctors believe that this may lie in the fact that different tumors use different tricks and methods to evade the immune system and chemotherapy drugs, according to NBC News. Further research will need to be conducted to uncover what these tricks are and continue to develop cancer drugs to combat them.