Summit Therapeutics PLC (SMMT) saw its shares plummet 26.04% in intraday trading on Monday following the release of mixed data from a phase 3 trial of its lung cancer drug candidate, ivonescimab. The trial results, while showing some improvements, raised concerns about the drug's effectiveness in different patient populations and its potential for regulatory approval in Western markets.

The company reported that ivonescimab, when combined with chemotherapy, demonstrated a "consistent, clinically meaningful improvement" in progression-free survival for patients with certain types of metastatic non-squamous non-small cell lung cancer. The overall response rate for patients receiving ivonescimab was 45%, compared to 34% for those on chemotherapy alone. Additionally, the median duration of response was 7.6 months for the ivonescimab group, versus 4.2 months for the control group.

However, the results revealed a significant discrepancy between patient populations. Patients from North America and Europe saw their lung cancer return and progress faster than patients from China. This inconsistency could potentially complicate Summit Therapeutics' plans to secure regulatory approval for ivonescimab in the US and Europe. The mixed results, particularly the lack of statistically significant benefit in overall survival compared to chemotherapy plus placebo, appear to have dampened investor enthusiasm, leading to the sharp decline in the company's stock price. Analysts have noted that this discordant result could be a roadblock to potential U.S. approval, casting uncertainty over the drug's future in Western markets.