"Ninety percent of patients have basically no side effects," Dr. Antoni Ribas, a researcher and professor at UCLA's Jonsson Comprehensive Cancer Center who was the lead investigator of a crucial study of Keytruda, told The Associated Press in an interview.
By comparison, most patients getting chemotherapy suffer with nausea, vomiting and hair loss.
In addition, Ribas said, Keytruda and other "immune-therapy" drugs appear likely to work against many more types of cancer than older drugs, and in a much higher percentage of patients.
In a study funded by Merck, which is based in Whitehouse Station, New Jersey, one-third of the 600 patients participating benefited from the drug, with 62 percent of those alive after 18 months.
Chemotherapy drugs have an average survival of about nine months, while some newer cancer drugs on average keep patients alive for 11 to 15 months, noted Ribas, who serves as an adviser to Merck but said he donates all payments to UCLA.
"This is just the start," Ribas said, adding that earlier tries at immune therapy for cancer typically helped only 5 percent to 10 percent of patients.
The average overall survival rate for Keytruda hasn't been determined yet, as most patients in the study are still being followed.
Merck's drug is the first in the class of what's called anti-PD-1 drugs approved in the U.S.
"This drug represents a major step forward," Dr. Louis M. Weiner, a spokesman for the American Association for Cancer Research, said in a statement. "It is an effective immunotherapy (but) not a general immune stimulant. It attacks a specific mechanism employed by some cancers to actively evade immune destruction."
Bristol-Myers Squibb Co. and a partner have a drug similar to Keytruda, called Opdivo, which was approved in Japan in July. They are seeking U.S. approval for it.
Merck said Keytruda will cost about $12,500 per month for many patients — similar to the price of many other new cancer drugs — and on average treatment lasts for just over six months.
The drug did have serious, immune-related side effects in a small number of patients, including hepatitis, colitis, thyroid problems and kidney inflammation. It's administered every three weeks through a slow intravenous drip — until the cancer progresses or the patient has intolerable side effects.
One study participant, 49-year-old Rich Murphy of Marshfield, Massachusetts, said that after about five rounds of treatment with Keytruda in 2012, he stopped taking medicines and was still cancer-free at his latest checkup 2 ½ months ago.
"I owe my life to that drug. There's no question about it," Murphy, a real estate agent, said in an interview.