Gritstone Bio says personalized cancer vaccine falls short in key trial

Gritstone Bio said Monday afternoon that its personalized cancer vaccine didn’t induce more responses than standard of care in a trial for colorectal cancer, raising questions about the future of both the program and the company.

The Phase 2 study had been closely watched as an important test for whether a new field of personalized cancer vaccines could not only treat so-called hot tumors like melanoma that are readily treated with immunotherapy, but also so-called cold tumors that have proven resistant to previous immunotherapies.

To measure whether the vaccine was eliminating cancer cells, Gritstone decided to look at levels of circulating tumor DNA, or ctDNA — genomic fragments that break off from the tumor and circulate in the blood. It’s an untested measure but Gritstone indicated it would be better than the traditional method of looking at whether tumors shrunk, because immunotherapies can sometimes paradoxically make tumors enlarge as immune cells infiltrate the cancer.