Background of the Early Termination of the KEYNOTE-641 Phase 3 Trial

Merck (MRK) has decided to discontinue the KEYNOTE-641 Phase 3 clinical trial, which evaluated the efficacy of a combination therapy for patients with metastatic castration-resistant prostate cancer (mCRPC). The trial assessed the combination of Keytruda (pembrolizumab), an immune checkpoint inhibitor targeting PD-1, with Xtandi (enzalutamide), an androgen receptor (AR) inhibitor, and androgen deprivation therapy (ADT). However, an interim analysis by the Independent Data Monitoring Committee (DMC) indicated that the trial did not meet the criteria for overall survival (OS) benefit, leading to a recommendation for early termination. The final analysis revealed that the hazard ratio (HR) for overall survival in the combination therapy group was 1.04 (p=0.66), and the HR for radiographic progression-free survival (rPFS) was 0.98 (p=0.41), demonstrating no improvement in efficacy compared to the control group.

Mechanistic Limitations and Unmet Needs of the Combination Therapy

This failure highlights the mechanistic limitations arising from the unique immunological characteristics of prostate cancer. Prostate cancer is generally classified as a 'cold tumor' with limited immune cell infiltration and an immunosuppressive environment, resulting in low response rates to immunotherapy. Although the combination with hormone therapy aimed to achieve synergistic effects, it did not provide significant survival benefits. Furthermore, the incidence of treatment-related adverse events (TRAEs) of grade 3 or higher was 31.2% in the combination group, three times higher than the 10.8% in the control group, increasing toxicity without providing additional benefits. Consequently, the unmet needs of prostate cancer patients who are resistant to hormone therapy remain a significant challenge.

Changes in the Competitive Landscape of the Prostate Cancer Treatment Market

The global mCRPC treatment market is currently valued at approximately $11.6 billion to $21 billion by 2025-2026 and is experiencing significant annual growth. With the failure of the Keytruda combination therapy to enter the market, the dominance of existing standard treatments, including Xtandi from Pfizer (PFE) and Astellas, and Zytiga from Johnson & Johnson, is expected to continue. At the same time, next-generation therapies, such as Pluvicto, a PSMA-targeted radioligand therapy from Novartis, and Lynparza, a PARP inhibitor from Merck and AstraZeneca, are likely to expand their market share. Given the confirmed barriers to entry for immune checkpoint inhibitors, competitors are expected to focus their efforts on developing next-generation targeted therapies.

Strategic Impact on Merck's Oncology Pipeline

This clinical trial termination represents a significant setback for Merck's plan to expand the indications for Keytruda to include prostate cancer. Having previously experienced failures in the KEYNOTE-991 and KEYNOTE-921 trials, a comprehensive revision of Merck's prostate cancer pipeline development strategy is now inevitable. With Keytruda's patent expiration scheduled for 2028, Merck's strategy to diversify indications in order to defend its revenue is facing challenges. In the future, Merck is expected to reallocate resources to develop new modalities, such as antibody-drug conjugates (ADCs) and cancer vaccines, to address the long-term risk of patent expiration, rather than simply combining immune checkpoint inhibitors.