Merck (MRK) Stock Gains After Lung Cancer Trial Delivers a 70% Response Rate

TLDR

• Merck (MRK) stock rose nearly 4% in after-hours trading following the trial results announcement.
• The Phase 3 OptiTROP-Lung05 trial tested sac-TMT combined with Keytruda against Keytruda alone as a first-line treatment for advanced non-small cell lung cancer (NSCLC).
• The combination cut the risk of disease progression or death by 65% and showed a response rate of ~70.2% versus 42% for Keytruda alone.
• Sac-TMT is an antibody-drug conjugate (ADC) targeting TROP2, developed by China’s Kelun-Biotech and licensed by Merck in 2022.
• Results are set to be presented at the 2026 American Society of Clinical Oncology (ASCO) Annual Meeting, and a bid to expand the drug’s label as a first-line NSCLC therapy is under priority review in China.

Merck (MRK) stock climbed nearly 4% in after-hours trading after its partner Kelun-Biotech announced that a new drug combination outperformed Keytruda alone in a late-stage lung cancer trial.

Merck & Co., Inc., MRK

The data comes from the Phase 3 OptiTROP-Lung05 study, which tested the antibody-drug conjugate sacituzumab tirumotecan — known as sac-TMT — combined with Merck’s flagship cancer therapy Keytruda as a first-line treatment for patients with advanced non-small cell lung cancer (NSCLC).

The trial enrolled more than 400 previously untreated patients in China with advanced disease and PD-L1 expression, a biomarker used to predict response to immunotherapy.

The combination cut the risk of disease progression or death by 65% compared to Keytruda alone — a statistically strong result. The response rate also came in at roughly 70.2%, versus 42% for patients who received only Keytruda.

Overall survival data was not mature at the time of the September 2025 analysis cutoff, but Kelun-Biotech noted a favorable trend in that direction.

What Is Sac-TMT?

Sac-TMT is an antibody-drug conjugate — a class of targeted therapy designed to deliver chemotherapy directly to cancer cells by binding to a specific protein called TROP2. The idea is to concentrate the toxic payload at the tumor rather than affecting the whole body.

The drug was discovered by Chinese biotech firm Kelun-Biotech, which is listed in Hong Kong. Merck licensed rights to the therapy outside of Greater China in 2022.

Sac-TMT is already approved in China as a late-line treatment for NSCLC, as well as other tumor types including breast and gastrointestinal cancers.

A bid to expand its label to cover first-line NSCLC — backed by the OptiTROP-Lung05 data — is currently under priority review in China.

Safety and Side Effects

The combination came with a higher rate of serious side effects. About 55% of patients on the combination arm experienced treatment-emergent adverse events graded 3 or above, compared to roughly 31% in the Keytruda-only group.

Common serious side effects included low white blood cell counts and anemia.

Around 4% of patients discontinued sac-TMT and 5% discontinued Keytruda due to side effects, compared to 5% in the Keytruda-only arm.

The companies described the safety profile as consistent with the known profiles of both drugs.

NSCLC is the most common form of lung cancer in the US, accounting for around 87% of all cases, according to the American Cancer Society.

The full results from OptiTROP-Lung05 are set to be presented at the 2026 American Society of Clinical Oncology Annual Meeting.

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