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While clinical testing of the drug is still in its early phases, the Roche treatment is considered one of the most promising in a new class of immunotherapies being developed by global drugmakers.

The drug, called MPDL3280A, significantly shrank tumors in 21 percent of 140 patients with advanced melanoma, lung cancer or kidney cancer, according to data from a scientific abstract of the Phase I study. Of the 29 patients whose cancer responded to the drug, 26 continued to respond - some more than a year after starting treatment - researchers said.

The study will be presented at the American Society of Clinical Oncology meeting in Chicago in early June.

Deutsche Bank yst Tim Race, in a research note, said with this drug Roche "potentially has a greater than $5 billion product with potential for durable benefit across multiple cancers."

Roche's drug is an engineered antibody that targets a protein called PD-L1 on cancerous tumors, a defense mechanism that tumors use to trick the immune system's T cells into remaining inactive. Once the T cells can recognize the cancer, they grow and multiply to more efficiently attack it.

"We have seen really amazing responses in lung cancer, in kidney cancer, in melanoma," Dr Roy Herbst, the study's lead investigator, said in an interview.

Broken down by cancer type, the response rate so far has been 31 percent in melanoma, 22 percent in lung cancer and 13 percent in kidney cancer, the available data showed.

The study has since been expanded to include patients with colon, bladder and head and neck cancers, researchers said.