Pfizer's Besponsa Fails to Demonstrate Efficacy and is Discontinued Early Due to Toxicity in Phase 3 Trial for First-Line Treatment of Leukemia
Phase 3 Trial Design and Strategy for Expanding into First-Line Treatment
The recently announced Alliance A041501 (NCT03150693) Phase 3 trial involved patients aged 18-39 with newly diagnosed CD22-positive B-cell acute lymphoblastic leukemia (B-ALL). Pfizer's ADC therapy, Besponsa (ingredient: inotuzumab ozogamicin), was combined with standard pediatric chemotherapy (CALGB 10403) in an attempt to expand its use as a first-line treatment. Besponsa targets CD22 receptors and delivers a cytotoxic agent, with the expectation of significantly improving complete remission (CR) and minimal residual disease (MRD) negativity. However, contrary to expectations, its entry into the first-line treatment area was hindered by toxicity issues.
Failure to Demonstrate Efficacy and Early Termination
The Data and Safety Monitoring Board (DSMB) halted patient enrollment at 273, below the target of 341, due to severe toxicities observed during the trial. The analysis revealed that the 3-year event-free survival (EFS) rate in the Besponsa combination group was 69.0%, compared to 66.7% in the control group, which did not demonstrate a statistically significant survival benefit (Hazard Ratio [HR] 0.93, p = 0.65). Furthermore, the 3-year overall survival (OS) rate was 79.4% in the Besponsa combination group and 80.3% in the control group, indicating that the Besponsa group had a lower survival rate. The treatment's negative impact due to toxicity outweighed its efficacy.
Critical Analysis of Overlapping Toxicities
The primary reason for the trial failure was the cumulative hepatotoxicity and bone marrow suppression associated with Besponsa, which overlapped with the toxicities of the existing chemotherapy regimen, leading to an increase in Grade 5 fatal adverse events. In particular, the combination of asparaginase, included in the chemotherapy, and Besponsa's risk of sinusoidal obstruction syndrome (SOS) appeared to cause severe liver damage. This, in turn, led to severe pancytopenia and sepsis, significantly threatening patient safety. This case demonstrates the limitations of precision targeted therapies when combined with other drugs, as the failure to control overlapping toxicities can hinder efficacy.
Market Competition and Challenges for Pfizer
The global acute lymphoblastic leukemia (ALL) treatment market is estimated at $2.3 billion to $5.3 billion, and competition for first-line treatments is fierce. This failure means that Pfizer has lost the opportunity to gain market share from Amgen's Blincyto, which dominates the first-line treatment market. While Besponsa is a potent treatment for relapsed patients, its failure to expand into the first-line treatment group has limited its growth potential. Pfizer will now face the challenge of finding ways to reduce toxicity, such as adjusting the dosage or exploring new combination therapies with Blincyto.
This Phase 3 trial failure signifies that Pfizer's Besponsa will not be able to enter the first-line treatment market in the global ALL market, valued at $2.3 billion to $5.3 billion, and the dominant market position established by its competitor, Amgen's Blincyto, is likely to be further solidified in the medium to long term. From the perspective of academia and researchers, this demonstrates that overcoming cumulative toxicities such as hepatotoxicity and bone marrow suppression that occur when antibody-drug conjugates (ADCs) are combined with pediatric high-intensity chemotherapy will be a key challenge in future drug development. In fact, the case where the 3-year event-free survival (EFS) rate was 69.0% versus 66.7% (HR 0.93, p=0.65), failing to demonstrate superiority in efficacy and leading to early termination, will serve as a catalyst for regulatory agencies to strengthen their safety standards for overly aggressive combination designs. In the short term, this will slow down the expansion of Pfizer's oncology portfolio, and in the medium to long term, it will be necessary to modify the design of clinical protocols for low-dose combinations that can control side effects or triple combinations with other drugs such as Blincyto.
Source: ClinicalTrials.gov (api_ct)