8-K: Zentalis Pharmaceuticals Reports Promising Phase 1 Results for Azenosertib Combination in Metastatic Colorectal Cancer

Zentalis Pharmaceuticals, Inc. announced positive Phase 1 dose escalation results for its WEE1 inhibitor azenosertib in combination with encorafenib and cetuximab for previously treated BRAFV600E mutant metastatic colorectal cancer patients, presented at the 2025 ASCO Annual Meeting.

Summary

• A Phase 1 dose escalation study evaluated azenosertib in combination with encorafenib and cetuximab in patients with previously treated BRAFV600E mutant metastatic colorectal cancer (mCRC).
• A total of 44 patients were enrolled and treated across 5 dose cohorts; 34 were BRAF inhibitor-naive, and 10 had been previously treated with a BRAF inhibitor.
• The Maximum Tolerated Dose (MTD) was determined to be azenosertib 300 mg QD, encorafenib 75 mg QD, and cetuximab 500 mg/m2 IV BIW.
• Drug-drug interactions (DDI) were observed between encorafenib and azenosertib, but target exposures for both drugs were achieved.
• In the 34 BRAF inhibitor-naive patients, the overall response rate (ORR) was 35.3%, with 12 confirmed responders, including 2 complete responses (CR) and 10 partial responses (PR).
• The median duration of response (DoR) for BRAF inhibitor-naive patients was 6.0 months.
• The median progression-free survival (PFS) for BRAF inhibitor-naive patients was 5.8 months (95% CI: 3.9-9.2).
• The combination therapy demonstrated an acceptable safety profile at the MTD, with no new safety signals identified.
• The most common all-grades treatment-related adverse events (TRAEs) were asthenia (36%) and decreased appetite (34%).
• The most frequent Grade 3/4 TRAE was asthenia (11% overall, 17% at MTD).
• One patient at a higher dose (azenoseertib 400 mg + encorafenib 75 mg) experienced Grade 4 treatment-related neutropenia and subsequently developed fatal listeria sepsis, which was deemed unrelated to treatment.
• No dose-limiting toxicities (DLTs) were observed at the MTD.
• The primary reason for discontinuation from the study was progressive disease (n=31).

Sentiment

Score: 8

Explanation: The study demonstrated superior efficacy (ORR and PFS) compared to historical data for the current standard of care in BRAF inhibitor-naive mCRC patients, with an acceptable safety profile at the MTD. This is a significant positive for a Phase 1 study, indicating strong potential for the drug combination in a specific patient population.

Positives

• The combination therapy achieved an acceptable safety profile at the MTD with no new safety signals.
• In BRAF inhibitor-naive patients, the overall response rate (ORR) was 35.3%, which is significantly higher than the 20% ORR reported for the encorafenib plus cetuximab (EC) doublet in the BEACON study.
• The median progression-free survival (PFS) of 5.8 months in BRAF inhibitor-naive patients exceeded the 4.2 months reported for the EC doublet in the BEACON study.
• Two complete responses (CR) were observed among the 34 BRAF inhibitor-naive patients, indicating strong anti-tumor activity.
• Target exposures for both azenosertib and encorafenib were successfully achieved despite observed drug-drug interactions.
• Part 1 (dose finding) enrollment has been completed, indicating progress towards further clinical development.

Negatives

• No responses were observed in the 10 patients previously treated with a BRAF inhibitor (4 stable disease, 6 progressive disease), indicating limited efficacy in this pre-treated population.
• Median PFS in BRAF inhibitor-treated patients was very low at 1.8 months (95% CI: 0.3-5.4).
• A high incidence of all-grades treatment-related adverse events (TRAEs) was observed (95% of patients), with asthenia (36%) and decreased appetite (34%) being common.
• Six patients (14%) permanently discontinued all study drugs due to adverse events.
• One patient experienced Grade 4 treatment-related neutropenia and a fatal listeria sepsis (unrelated to treatment) at a higher dose level (azenoseertib 400 mg).
• Drug-drug interactions (DDI) were observed, requiring careful dose management (e.g., azenosertib exposures decreased with increased encorafenib dose, and vice versa).

Risks

• Potential for significant treatment-related adverse events, including Grade 3/4 events such as asthenia, which may impact patient tolerability and adherence.
• Observed drug-drug interactions between azenosertib and encorafenib necessitate careful dose management and monitoring, which could complicate treatment regimens.
• The lack of efficacy in patients previously treated with BRAF inhibitors suggests a limited patient population for this specific combination, potentially narrowing its market applicability.
• Risk of serious infections (e.g., listeria sepsis) in immunocompromised patients, although the specific fatal event in the study was deemed unrelated to treatment, it highlights the vulnerability of this patient population.

Future Outlook

The completion of Part 1 (dose finding) of the study suggests that Zentalis Pharmaceuticals will proceed with further clinical development of the azenosertib + encorafenib + cetuximab combination, likely in a Phase 2 or expansion cohort, at the determined Maximum Tolerated Dose (MTD) to further evaluate its efficacy and safety.

Management Comments

• "This study is sponsored by Zentalis Pharmaceuticals, Inc. and conducted in collaboration with Pfizer, Inc."
• "Zentalis Pharmaceuticals would like to extend our gratitude and thanks to the patients, families, and treatment teams associated with this study."

Industry Context

Colorectal cancer is a significant global health burden, ranking as the second leading cause of cancer-related deaths worldwide. BRAF mutations, particularly V600E, are found in 10% to 15% of CRC cases and are associated with a poor prognosis. The current standard of care in the US for previously treated BRAFV600E-mutant mCRC includes encorafenib plus cetuximab (EC), based on the BEACON study. Zentalis' study aims to improve upon this established doublet therapy by adding azenosertib, a WEE1 inhibitor, which targets critical cell cycle checkpoints to enhance anti-tumor activity.

Comparison to Industry Standards

• The BEACON study, which established encorafenib plus cetuximab (EC) as an approved treatment for previously treated BRAFV600E-mutant mCRC, reported an Overall Response Rate (ORR) of 20% and a median Progression-Free Survival (PFS) of 4.2 months in the second-line/third-line patient population.
• Zentalis' combination of azenosertib + encorafenib + cetuximab achieved an ORR of 35.3% and a median PFS of 5.8 months in BRAF inhibitor-naive patients, demonstrating superior efficacy compared to the historical data from the EC doublet.
• This suggests that the triplet therapy could potentially offer a more effective treatment option for BRAF inhibitor-naive patients with mCRC compared to the current standard of care.

Stakeholder Impact

• Shareholders: The positive clinical trial results are likely to increase investor confidence and could lead to a positive impact on the company's stock price due to the potential for a new, more effective treatment.
• Patients: Patients with BRAFV600E mutant metastatic colorectal cancer, particularly those who are BRAF inhibitor-naive, may benefit from a potentially more effective treatment option with improved response rates and progression-free survival.
• Collaborators (Pfizer, Inc.): The successful clinical development strengthens the collaboration with Pfizer, Inc. and enhances the potential for future commercialization and market penetration of the drug combination.

Next Steps

• Further clinical development of the azenosertib + encorafenib + cetuximab combination, likely in a Phase 2 or expansion cohort, at the determined MTD to confirm and expand on these promising results.

Key Dates

Recommendation

Keywords

Zentalis Pharmaceuticals, Azenosertib, WEE1 inhibitor, Encorafenib, Cetuximab, BRAFV600E mutant, Metastatic Colorectal Cancer, mCRC, ASCO, Clinical Trial, Phase 1, Oncology, Cancer Therapy, Targeted Therapy, Drug-Drug Interaction, Progression-Free Survival, Overall Response Rate, Maximum Tolerated Dose, Adverse Events