AstraZeneca's Durvalumab and Tremelimumab Phase 3 MYSTIC Trial Fails to Meet Primary Endpoint
MYSTIC Phase 3 Results
The MYSTIC trial, sponsored by AstraZeneca (AZN), was a global, randomized Phase 3 clinical trial involving 1,118 patients with advanced or metastatic non-small cell lung cancer (NSCLC) without EGFR or ALK mutations, who were receiving first-line treatment. Imfinzi (durvalumab) targets the programmed death-ligand 1 (PD-L1) on tumor cells, while Imjudo (tremelimumab-actl) blocks cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), inducing complementary immune activation. The trial was conducted in 17 countries and is registered on ClinicalTrials.gov as completed.
Survival Outcomes Demonstrate Limitations
In the 488 patients with PD-L1 tumor cell expression of 25% or greater, the median overall survival (OS) with durvalumab monotherapy was 16.3 months, compared to 12.9 months with chemotherapy; however, the hazard ratio (HR) was 0.76, with a 97.54% confidence interval (CI) of 0.56 to 1.02, and a nominal p-value of 0.036, which did not meet the pre-specified statistical criteria. The combination of durvalumab and tremelimumab also failed to improve overall survival, with 11.9 months compared to 12.9 months, an HR of 0.85, a 98.77% CI of 0.61 to 1.17, and a nominal p-value of 0.202. The progression-free survival (PFS) in the combination arm was also 3.9 months compared to 5.4 months, with an HR of 1.05, failing to demonstrate superiority. This weakens the competitive advantage of the chemotherapy-free, dual immune checkpoint inhibitor strategy in the first-line setting.
Safety and Biomarker Signals
Grade 3 or higher treatment-related adverse events occurred in 14.9% of patients receiving durvalumab monotherapy, 22.9% in the combination arm, and 33.8% in the chemotherapy arm, indicating lower rates in the immune checkpoint inhibitor arms. However, treatment-related deaths occurred in the combination arm, confirming the burden of managing immune-related toxicities associated with the addition of CTLA-4 inhibition. In an exploratory analysis, the overall survival in the combination arm of patients with a blood-based tumor mutational burden (bTMB) of 20 mutations/Mb or greater was 21.9 months, compared to 10.0 months in the chemotherapy arm, with an HR of 0.49; however, this was not a pre-specified confirmatory endpoint and does not change the overall failure of the MYSTIC trial.
Regulatory and Competitive Landscape
Imfinzi was first approved by the FDA on May 1, 2017, and Imjudo was first approved on October 21, 2022, as a combination therapy for hepatocellular carcinoma. On November 10, 2022, Imjudo, Imfinzi, and platinum chemotherapy were approved for the first-line treatment of metastatic NSCLC, based on the POSEIDON 3 trial, which included chemotherapy and demonstrated improved overall survival, rather than the MYSTIC trial. Current competitive standards include Merck & Co. (MRK)'s Keytruda (pembrolizumab, a PD-1 inhibitor)-based regimens, Bristol Myers Squibb (BMY)'s Opdivo (nivolumab, a PD-1 inhibitor) and Yervoy (ipilimumab, a CTLA-4 inhibitor), and Roche's Tecentriq (atezolizumab, a PD-L1 inhibitor). The global NSCLC market is projected to be $20.02 billion in 2025.
The failure of the MYSTIC trial's primary endpoint means that the chemotherapy-free combination of Imfinzi and Imjudo did not establish a basis for directly replacing the Keytruda-based standard of care, even in patients with PD-L1 expression of 25% or greater. In the short term, AstraZeneca (AZN)'s growth in the metastatic NSCLC market will rely on the FDA-approved Phase 3 POSEIDON trial, which combines immune checkpoint inhibitors and chemotherapy, rather than the MYSTIC trial. From a research perspective, the HR of 0.49 in the subgroup of patients with a bTMB of 20 mutations/Mb or greater suggests biomarker selection potential, but confirmatory trials are needed. In the $20.02 billion market projected for 2025, competition with Merck (MRK), Bristol Myers Squibb (BMY), and Roche will depend not only on efficacy but also on CTLA-4 toxicity, ease of administration, and insurance coverage.
Source: ClinicalTrials.gov (api_ct)