SAN DIEGO — An experimental sickle cell disease drug being developed by Pfizer has shown enough potential to advance further into testing, highlighting the expanding pipeline of therapies for the condition behind the two gene therapies approved Friday by U.S. regulators.

Pfizer’s results, presented Saturday at the annual meeting of the American Society of Hematology, come from a portion of a broader Phase 2/3 study that’s designed to select a drug dose.

The oral drug, dubbed GBT-601, is a successor to the company’s marketed medicine Oxbryta, which Pfizer acquired via its 2022 deal to acquire Global Blood Therapeutics. They both work by stabilizing the oxygen-transporting protein hemoglobin, keeping red blood cells from turning into the hard sickle shapes that get stuck and clog blood vessels.

While the newly approved gene therapies, Casgevy and Lyfgenia, offer the potential for dramatic benefit, oral drugs are still greatly needed for sickle cell because of the disease’s “wide variability,” said Kim Smith-Whitley, an adviser of scientific and clinical affairs at Pfizer and a former director of Children’s Hospital of Philadelphia’s sickle cell center.

Sickle cell was for years an understudied field of medicine and, until 2019, the main treatment option for most patients was a repurposed cancer drug. Bone marrow transplants can be curative, but are only available for a minority of patients and carry additional risks.

For some people with sickle cell, Casgevy and Lyfgenia could eliminate the disease’s worst consequences. But they may not be the right choice for many, and will initially be only available at a select few specialized treatment centers. They’re also very expensive: Vertex set Casgevy’s price at $2.2 million, while Bluebird went higher at $3.1 million for Lyfgenia. Insurers are expected to cover them in some fashion, but it’s unclear how widely.

“One therapy doesn’t exclude the other,” said Elliott Vichinsky, director of the University of California, San Francisco’s Pediatric Sickle Cell Anemia Center. “The more therapy types, the better. There are just going to be issues of how you allocate these and where you put money into therapies.”

Pfizer’s data on GBT-601 come from 35 adults with sickle cell enrolled into “Part A” of the study; 17 were given a 100 milligram dose and the rest a 150 milligram dose. In both groups, treatment led to increases in hemoglobin levels of about three grams per deciliter after 12 weeks.

The trial is not designed to formally assess the frequency of the excruciating pain episodes, known as vaso-occlusive crises, that are a defining feature of sickle cell. However, researchers at Pfizer analyzed biomarkers involved in hemolysis, or the death of red blood cells, and found signs of improvement. In particular, they looked at “adherent” blood cells in nine study participants and found treatment led to a decline, suggesting the potential for lower risk of blockage.

Six of the 35 trial participants had at least one serious adverse event after starting treatment, including three who experienced sickle cell anemia with crisis. One participant died due to a cerebrovascular event that investigators judged to be unrelated to treatment.

“If industry, government and scientists are committed to completing these trials, and dedicated to this community to develop new therapies long term, it’s an exciting time,” said Smith-Whitley, of Pfizer. “To see this much interest and potential in therapies that could mature in the next decade, I think that, hopefully, it will translate into not only increased survival, but a better quality of life for those with sickle cell.”