Mersana Therapeutics Stock Plunges 36% After Phase 1 Trial Data Release

Mersana Therapeutics Stock Plunges 36% After Phase 1 Trial Data Release

Mersana Therapeutics Inc. (NASDAQ: MRSN), a clinical-stage biopharmaceutical company focused on developing antibody-drug conjugates (ADCs) for the treatment of cancer, saw its stock price plummet 36.6% on Friday, closing at $0.8245. This significant drop followed the release of initial data from the Phase 1 dose escalation and backfill cohorts for emiltatug ledadotin (Emi-Le), an investigational ADC targeting B7-H4.

Emi-Le Demonstrates Promising Antitumor Activity but Raises Safety Concerns

The Phase 1 trial data revealed a confirmed objective response rate (ORR) of 23% among evaluable patients with B7-H4 high tumors at intermediate doses (38.1 mg/m2 to 67.4 mg/m2). This ORR was consistent across various tumor types, including triple-negative breast cancer (TNBC), a particularly aggressive form of breast cancer with limited treatment options. Notably, all responding TNBC patients had previously received at least one topoisomerase-1 inhibitor ADC, highlighting Emi-Le’s potential in this heavily pretreated population. For context, the ORR in the ASCENT Phase 3 trial of sacituzumab govitecan, a currently approved ADC for TNBC, was approximately 5% in a similar patient population.

Encouraged by these findings, Mersana has advanced a dose of 67.4 mg/m2 every four weeks into an expansion cohort for TNBC patients who have received one to four prior treatment lines. At higher doses above 76 mg/m2, the confirmed ORR was 22%, with 78% of patients experiencing at least a 30% tumor reduction. However, these higher doses were associated with increased instances of proteinuria, a condition characterized by excess protein in the urine, leading to protocol-mandated dose delays and impacting confirmation of objective responses. Mersana is currently implementing mitigation strategies to address proteinuria and continues to explore higher doses to identify a second dose for the expansion phase of the trial.

FDA Grants Fast Track Designation to XMT-1660 for Advanced Breast Cancer

Concurrent with the data release, Mersana announced that the U.S. Food and Drug Administration (FDA) granted an additional Fast Track designation to XMT-1660 (Emi-Le) for the treatment of advanced or metastatic breast cancer in patients with HER2-low or HER2-negative disease, including TNBC, who have received prior treatment with a topoisomerase-1 inhibitor ADC. This designation underscores the unmet medical need in this patient population and the potential of Emi-Le to address it. The World Health Organization also approved “emiltatug ledadotin” and the abbreviation “Emi-Le” as XMT-1660’s international nonproprietary name.

Market Reaction and Future Outlook

Despite the promising antitumor activity observed in the Phase 1 trial, the market reacted negatively to the data release, primarily due to concerns surrounding the proteinuria observed at higher doses. The significant drop in Mersana’s stock price reflects investor uncertainty regarding the safety profile of Emi-Le and its potential impact on the drug’s development timeline. Moving forward, Mersana’s ability to successfully manage proteinuria and identify a safe and effective dose for the expansion cohort will be crucial for the continued development and potential commercialization of Emi-Le. Further data from the ongoing Phase 1 trial and subsequent clinical trials will be essential in determining the ultimate clinical utility of this promising ADC.

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