Image source: Merck.

Merck (NYSE:MRK) looks like it will win the race to have the first immunotherapy drug to gain Food and Drug Administration approval to treat newly diagnosed non-small cell lung cancer patients; but don't count rival Bristol-Myers Squibb (NYSE:BMY) out just yet.

In a clinical trial called KEYNOTE-024, Merck said that patients receiving Keytruda fared better than patients who received chemotherapy, the current standard of care. The drug extended both progression-free survival (PFS), which measures when a tumor starts growing again, and overall survival.

Merck didn't put any numbers to those claims. It's waiting for a medical meeting to disclose the extent that Keytruda was able to extend PFS and overall survival. The European Society for Medical Oncology meeting in October would be one logical place to present the data, although I'm sure investors would prefer to see the data before then.

To screen or not to screen

Keytruda and Bristol-Myers Squibb's Opdivo both bind to a protein called PD-1, blocking the interaction between PD-L1 on tumor cells and PD-1 on immune system cells called T lymphocytes, which normally tells the immune system not to attack the tumor cells. In the presence of the drugs, the negative signal is blocked, and the immune system is activated and attacks the tumor.

Both drugs are approved for later-stage lung-cancer patients, but Merck screened its patients for PD-L1 expression, and therefore is only approved for strong expressers of PD-L1. Bristol-Myers Squibb skipped that step, and still saw an effect, so Opdivo is approved for a larger number of patients, and doctors don't have to screen for PD-L1 before using the drug.

In KEYNOTE-024, Merck took the same approach as before, looking at the expression of PD-L1 lung-cancer cells, and only enrolled patients with tumors expressing high levels of PD-L1, defined as a tumor proportion score of 50% or more. Bristol-Myers Squibb is also running a trial in newly diagnosed patients, called CheckMate 026, with data due this year, but its trial allows for all patients to be enrolled if the initial cohort of patients with high PD-L1 sees a benefit from Opdivo.

Combos to come

It may not matter which drug extends overall survival as a monotherapy, or whether Bristol-Myers can capture more patients by not requiring screening, because both companies are running clinical trials testing the drugs in combination with other drugs in newly diagnosed patients. Merck is testing Keytruda in combination with two different types of chemotherapy in newly diagnosed lung-cancer patients in clinical trials called Keynote-189 and Keynote-407.

Likewise, in a clinical trial dubbed Checkmate-227, Bristol-Myers Squibb is testing Opdivo in combination with chemotherapy in newly diagnosed lung-cancer patients. As part of that trial, other patients will get Opdivo in combination with Yervoy, another immunotherapy sold by the company.

If all that competition wasn't enough, Roche is testing its immunotherapy Tecentriq in lung cancer. The PD-L1-blocking drug has already shown good results in bladder cancer, having been approved for that tumor type in May.

Congratulations to Merck for scoring the first punch in round two of this lung-cancer fight,. Investors, however, should keep in mind that this round isn't over, and the battle is a 10-round bout where neither drug is likely to win by KO.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.