Merck & Co. became an undisputed leader in cancer immunotherapy thanks to its success with Keytruda, now one of the drug industry’s best-selling medicines. Over the weekend, the pharma used the virtual meeting of Europe’s chief medical oncology society to showcase three experimental drugs that may come next. Chief among them is a new type of immune-boosting drug that blocks a cell receptor protein known as TIGIT. Early study results released by Merck at the European Society for Medical Oncology’s meeting suggest the drug, named vibostolimab, may improve treatment response to Keytruda in patients with metastatic lung cancer. Merck also disclosed data for two other cancer drugs: an immunotherapy developed from the company’s research laboratories in early testing for solid tumors and an oral tablet for kidney cancer that Merck picked up in its 2019 acquisition of Peloton Therapeutics.

Since cancer immunotherapy’s emergence in the middle of the last decade, drugmakers have sought to expand the pool of patients who could potentially benefit by pairing drugs like Keytruda and Bristol Myers Squibb’s Opdivo with other therapies.

Merck’s success with Keytruda has largely come through its chemotherapy combinations, but the drugmaker has also inked billion-dollar licensing deals for first AstraZeneca’s breast and ovarian cancer drug Lynparza and then Eisai’s liver and kidney cancer medicine Lenvima.

Developing the next generation of immunotherapies, however, has been more challenging, as experimental drugs developed by companies like Incyte and Nektar Therapeutics either failed to pan out, or have taken longer to emerge than anticipated.

Merck and Roche, the latter of which sells the Keytruda rival Tecentriq, are now both betting on drugs that block TIGIT, an immune cell protein that tumors seem to be able to co-opt to blunt the body’s response to cancerous growths.

In May, Roche disclosed the first results for its TIGIT-targeting compound, called tiragolumab. While early, the data seemed to show the pairing of tiragolumab and Tecentriq to be more effective than tiragolumab alone.

Merck’s now replicated that result with Keytruda and vibostolimab. In patients with advanced lung cancer who hadn’t previously received immunotherapy, the combination shrank tumors in 29% of the 41 tested patients. Median progression-free survival, a measure of how long before a patient’s disease worsens or death, was 5.4 months.

Results were better in patients with a biomarker known as PD-L1 that’s correlated with response to immunotherapy.

A separate study showed vibostolimab on its own reduced the size of tumors in a handful of patients — an important sign for potential partner therapies when drugs like Keytruda are already known to be effective.

“The fact that we had some monotherapy response means the compound by itself seems to be able to shrink tumors in patients with [immunotherapy] refractory disease,” said Eric Rubin,  head of early-stage cancer drug development at Merck’s research laboratories, in an interview. “That’s encouraging to us.”

Early-stage trials like Merck’s and Roche’s can be deceptive, however, particularly when experimental combinations aren’t compared to a control arm.

Merck is now studying vibostolimab together with Keytruda in a Phase 2 study, and plans to begin a Phase 3 trial in the first half of 2021.

Roche is already recruiting patients into four late-stage studies of tiragolumab and has a fifth planned.