 -- Just months after finding out he had metastatic cancer, former President Jimmy Carter announced this weekend that his doctors have said he no longer needs cancer treatment thanks in part to a groundbreaking new kind of medication that trains the immune system to fight cancer tumors.

Carter announced in August that he had melanoma that had spread to his liver and brain. He underwent surgery, radiation therapy and a new kind of cancer treatment called immunotherapy to fight the disease.

Speaking at his church this weekend, Carter announced that his doctors are stopping his immunotherapy treatment called pembrolizumab after they saw no signs of tumors over a period of three months. While he has no evidence of the disease, doctors will monitor Carter closely to see if the cancer reoccurs, a representative for the former president said.

"President Carter said today he did not need any more treatments, which he had August 2015 through February 2016, but will continue scans and resume treatment if necessary," a spokeswoman for the Carter Center told ABC News in an email.

Carter's remarkable outlook is likely due in large part to the drug pembrolizumab, a drug that targets cancer by ramping up the body's immune system, experts said. The treatment was approved for use by the U.S. Food and Drug Administration in 2011.

The drug works as a "checkpoint inhibitor," altering certain pathways in the immune system so that the antibodies can identify and fight any tumors in the body the way they might fight a virus or cold, experts said. The medication is much less toxic than chemotherapy, but it can react in colon, liver or lung inflammation, according to published studies. Researchers are still trying to determine how long the medication can prompt the immune system to keep fighting. This drug is mainly given to melanoma patients.

Dr. Andrew Sloan, director of the Brain Tumor and Neuro-Oncology Center at University Hospitals Case Medical Center in Cleveland, said scientist have only recently understood how “tumors recruit the immune system.”

“Tumors have figured out how to turn off the immune system,” Sloan said in an interview with ABC News when Carter first announced his diagnosis. “They recruit cells that surround them. ... These are not cells that kill the tumor. They protect cells from part of the immune system.”

Drugs like pembrolizumab work by keeping the immune system from turning off. Dr. Leonard Lichtenfeld, deputy chief medical officer for the American Cancer Society, said such therapies, first presented in 2010, were the first new drugs for melanoma since the 1970s.

“Five years ago,” Lictenfeld said, “we would not have much to offer the president.”