Merck's cancer immunotherapy finally hit a stumbling block in a pivotal study with lung cancer patients that caused the stock to slip 1.5% on Tuesday morning. It's more than a little surprising because Keytruda has been taking huge steps toward becoming the world's top-selling drug since Merck & Co. (NYSE:MRK) revealed game-changing lung cancer trial results in 2016.

During a study with small cell lung cancer that hadn't received any treatment for their disease before, adding Keytruda to standard chemotherapy failed to produce a significant overall survival benefit.

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Merck was quick to point out that it took significantly longer for patients treated with Keytruda in combination with chemotherapy to show signs of their disease worsening than for those given standard chemotherapy on its own. But because overall survival is the gold standard, a label expanding approval to treat newly diagnosed patients doesn't seem likely.

It could be worse for Merck

Third-quarter sales of Keytruda jumped 62% higher than the same period last year to an annualized $12.4 billion, largely because of increasing sales as a first-line treatment for non-small cell lung cancer (NSCLC). Merck's shares haven't been hammered down very far because the recent failure involved small cell lung cancer (SCLC), which makes up around 15% of all new cancer diagnoses.

Lung cancer isn't the most commonly diagnosed malignancy, but it still claims more lives than any other. With a tight grip in the first-line setting for around 85% of lung cancer patients who receive a lung cancer diagnosis of the non-small cell variety, Keytruda's still likely to become the world's top-selling drug within another year or two.