Hope in Delay: Cancer Trial Slows as Leukemia Patients Live Longer

NEW YORK, NY – December 29, 2025 – In the high-stakes world of cancer drug development, delays are often a source of anxiety for patients and investors alike. But for SELLAS Life Sciences, a recent delay in its pivotal Phase 3 trial for acute myeloid leukemia (AML) is sparking cautious optimism for what could be the best possible reason: patients are living longer than expected.

SELLAS announced today that its REGAL trial, evaluating the cancer vaccine galinpepimut-S (GPS), has not yet reached the number of patient deaths required to trigger its final analysis. The study, which measures overall survival, was projected to hit its target of 80 events by the end of 2025. However, as of December 26, only 72 events had occurred.

While the company and its investigators remain completely blinded to which patients are receiving GPS versus the best available therapy, the slower-than-anticipated event rate suggests that patients in the trial, as a whole, are outliving historical survival benchmarks. This unexpected longevity is fueling hope that the investigational therapy may be providing a significant benefit.

“We appreciate the continued dedication of the patients, families, and investigators participating in the pivotal Phase 3 REGAL trial where survival times, fortunately for patients and caregivers, appear longer than expected,” said Angelos Stergiou, MD, ScD h.c., President and CEO of SELLAS. “While the 80th event has not yet occurred, and we remain fully blinded, every passing month may increase the probability of a successful study.”

The Unmet Need in a Brutal Disease

Acute myeloid leukemia is a fast-growing cancer of the blood and bone marrow that is notoriously difficult to treat. The REGAL trial focuses on a particularly vulnerable group: patients who have already relapsed once and achieved a second complete remission (CR2). For these individuals, especially those unable to undergo a grueling stem cell transplant, the prognosis is grim.

“For patients who are unable to undergo transplant, as in the REGAL study, their treatment usually consists of a combination of hypomethylating agents and/or a BCL-2 inhibitor, with an expected median overall survival of around eight months,” explained Dr. Yair Levy of Texas Oncology Baylor University Medical Center, a member of the trial's steering committee. This stark reality highlights the desperate need for new maintenance therapies that can prolong remission and extend life.

The REGAL study is designed to see if GPS can fill this void. By extending the time until the 80th event, the trial's outcome remains uncertain, but the simple fact that fewer patients have died than predicted offers a glimmer of hope in a disease landscape with few options.

This positive signal is further bolstered by an August 2025 recommendation from the trial’s Independent Data Monitoring Committee (IDMC) to continue the study without modification. Such committees periodically review unblinded data to ensure patient safety and to stop a trial early for futility or overwhelming efficacy. The recommendation to proceed suggests no major safety concerns and that the trial remains on a viable path.

The Science of a Targeted Vaccine

The potential promise of galinpepimut-S lies in its innovative mechanism. It is not a chemotherapy drug but an immunotherapy—a cancer vaccine—that targets a protein called Wilms Tumor 1 (WT1). The National Cancer Institute has ranked WT1 as the number one antigen for cancer immunotherapy due to its high expression across a wide range of blood cancers and solid tumors, yet its limited presence in healthy adult tissues.

GPS is designed to train the body's own immune system, specifically T-cells, to recognize and attack cancer cells that express the WT1 protein. The goal is to create a durable, long-term surveillance system that can hunt down and eliminate residual leukemia cells, thereby preventing or delaying relapse.

This Phase 3 trial did not emerge from a vacuum. It was built on a foundation of highly encouraging Phase 2 data. In a previous study involving the same CR2 patient population, those treated with GPS showed a median overall survival of 21 months. This was a dramatic improvement over the 5.4-month median survival observed in a control group of patients receiving the best standard care, a result that was statistically significant.

Given this strong preliminary evidence, the current delay in the larger, more definitive REGAL trial is seen by many as a potential real-world validation of those earlier findings, albeit one that must await formal unblinding and analysis.

Navigating Biotech's Balancing Act

For a late-stage biopharmaceutical company like SELLAS, the timeline to data readout is a critical factor that weighs heavily on its valuation and strategic planning. The event-driven nature of the REGAL trial, while scientifically robust, introduces an element of unpredictability. The company has committed to announcing when the 80th event occurs, which will officially start the clock on the final data analysis.

Despite the uncertainty, the market has responded positively to the underlying reason for the delay. Shares of SELLAS (NASDAQ: SLS) have seen significant gains in recent months, and analyst ratings remain largely bullish, with price targets suggesting substantial potential upside. This reaction underscores a sophisticated understanding among investors that in an overall survival trial, a delay caused by patients living longer is a fundamentally positive sign.

As the trial continues, all eyes are on that final event count. The wait tests the patience of everyone involved, but it is a wait defined not by setback, but by the profound hope that a new standard of care for one of the deadliest forms of leukemia may be on the horizon. The conclusive data will follow, but for now, the silence from the trial speaks volumes.