AstraZeneca (AZN.US) Breast Cancer Drug Etcamah Faces Setback in Phase III Clinical Trial, Potentially Impacting Billions in Sales

According to Zhitong Finance APP, AstraZeneca (AZN.US) announced last Friday that its oral selective estrogen receptor degrader (SERD) Etcamah (camizestrant) combined with palbociclib for first-line treatment of ER-positive, HER2-negative advanced breast cancer failed to meet the primary endpoint in the SERENA-4 Phase III trial. Although there was a numerical improvement in patient progression-free survival (PFS), it did not reach statistical significance.

Bloomberg Intelligence analyst John Murphy stated that this setback in AstraZeneca’s breast cancer drug trial may reduce its 2035 sales by $2.6-3.8 billion. RBC Capital Markets analyst Trung Huynh had previously estimated the breast cancer drug's potential revenue at about $1 billion.

Nevertheless, after Roche's competing experimental drug giredestrant also failed to show benefits in a similar setting, market expectations for success in this indication were not high. For Etcamah, the greater test will come from the results of several ongoing studies targeting a much larger patient population. Some analysts did not include sales from this indication in their forecasts.

It is known that the traditional SERD fulvestrant can only be administered via intramuscular injection, resulting in poor patient compliance. Oral SERDs, taken once daily, could theoretically replace aromatase inhibitors (AI) as the first-line standard endocrine therapy, with significant market potential. However, in reality, oral SERDs combined with CDK4/6 inhibitors have not outperformed AI combined with CDK4/6 inhibitors in the entire first-line population. AstraZeneca's Etcamah did not achieve this, nor did Roche's giredestrant.

What is the problem? It may be that AI is already good enough: in the first-line ER-positive, HER2-negative population, PFS can exceed 28 months, making it very difficult for oral SERDs to significantly improve upon this. It is also possible that oral SERDs do not degrade the ER efficiently enough to provide a differentiated advantage in the overall population. However, oral SERDs are not without opportunities, with the key being patient selection—specifically ESR1 mutation patients.

ESR1 mutation is a major mechanism of endocrine therapy resistance in ER-positive breast cancer. About 30–40% of patients develop ESR1 mutations during AI therapy. Mutant ERs can continue to activate and drive tumor growth even in low-estrogen environments. Etcamah’s SERENA-6 trial is targeting this population: 315 patients have blood ctDNA tested for ESR1 mutation every 2-3 months and are randomized to either switch to Etcamah + CDK4/6 inhibitor or continue on AI + CDK4/6 inhibitor upon detection of a mutation.

The results are compelling: PFS was 16.0 vs 9.2 months, HR 0.44, and the risk of progression or death was reduced by 56%. PFS2 was also significantly improved (25.7 vs 19.1 months, HR 0.63, p=0.00373). Based on this data, the FDA granted accelerated approval on September 8 for Etcamah in combination with a CDK4/6 inhibitor for HR-positive, HER2-negative locally advanced or metastatic breast cancer patients with ESR1 mutation detected during AI + CDK4/6 inhibitor treatment. Multiple countries, including the EU and Japan, have also approved it.

Susan Galbraith, Executive Vice President of Oncology R&D at AstraZeneca, was clear after the SERENA-4 setback: “While we are disappointed with the results from SERENA-4, this further clarifies our focus—maximizing patient benefit based on SERENA-6 and reinforcing the importance of ESR1 gene testing in first-line treatment.”

The failure of SERENA-4 does not mean the end of Etcamah’s story. AstraZeneca is truly betting on adjuvant therapy for early breast cancer. The CAMBRIA-1 and CAMBRIA-2 Phase III trials plan to enroll about 10,000 patients at intermediate or high risk of recurrence to evaluate Etcamah as a monotherapy, in combination with a CDK4/6 inhibitor, and after CDK4/6 inhibitor therapy. CAMBRIA-1 compares five years of extended adjuvant Etcamah therapy with standard endocrine therapy in intermediate/high recurrence risk patients, while CAMBRIA-2 compares seven years of upfront adjuvant Etcamah therapy versus standard endocrine therapy in high/very high recurrence risk patients. Together, these two trials, with about 10,000 cases, represent the largest development program for oral SERD in early breast cancer to date.

AstraZeneca’s reasoning is that while it cannot outperform AI in the overall first-line advanced setting, in early adjuvant therapy, the complete ER antagonism and degradation characteristic of oral SERDs may be more thorough than AI, especially among high-risk patients. If the CAMBRIA trials show positive results, Etcamah’s market potential will far surpass that in the late-stage ESR1 mutation population—this is the core reason for AstraZeneca's expectation of $5 billion peak sales for Etcamah. However, CAMBRIA data will not be available until after 2027. Until then, Etcamah’s growth will rely on the precise ESR1 mutation indication.