A new cancer drug used in a small experimental study in the US has found virtually every participant in the trial go into remission.

Dostarlimab, sold under the brand name Jemperli, is an immunotherapy drug used in the treatment of endometrial cancer in the UK, however this was the first clinical investigation of whether it was also effective against rectal cancer tumours.

It exceeded expectations in a trial held at the Memorial Sloan Kettering Cancer Center in New York.

One year after the trial ended, early results have revealed each of the 18 colorectal cancer patients had gone into remission and doctors were reportedly unable to find signs of the cancer.

“I believe this is the first time this has happened in the history of cancer,” medical oncologist Luis Diaz, and senior author of a new paper reporting the results, told The New York Times.

It’s worth noting the trial is ongoing and the small sample of patients all had tumours with genetic mutations called mismatch repair deficiency (MMRd), seen in a subset of approximately 5–10 percent of rectal cancer patients.

“Our message is: Get tested if you have rectal cancer to see if the tumour is MMRd,” Dr Diaz said.

“No matter what stage the cancer is, we have a trial at Memorial Sloan Kettering that may help you. And MSK has special expertise that really matters.”

Dostarlimab costs about $11,000 ($AU15,200) per 500mg dose in the US.

The findings are reported in The New England Journal of Medicine.