A 30-Year Journey: How CHOP Researchers Designed the Gene Therapy Playbook

The FDA has approved a one-time gene therapy for adults with hemophilia B, which was pioneered at CHOP. Lindsey George, MD, (pictured) led the phase 1/2 clinical trial to test its safety and efficacy.

By Lauren Ingeno

As a hematologist during the 1980s, Katherine A. High, MD, faced a dark reality: The blood-clotting factor used to treat hemophilia B, which was revolutionary at the time, had infected many of her patients with human immunodeficiency virus and hepatitis C.

She was determined to find a better treatment.

That quest led her to Children's Hospital of Philadelphia, where researchers, alongside scientists at the University of Pennsylvania, were investigating the potential of in vivo gene therapy — an experimental treatment in which genetic material could be transferred into human cells to cure disease.

"Being at CHOP allowed me to accelerate my research program," Dr. High said. "There was an entire community of scientists working on gene therapy, and the administration was investing in it."

In theory, curing hemophilia B with gene therapy was simple. The inherited disease is caused by a defect in the F9 gene, which leads to a deficiency of a blood-clotting protein called factor IX. Scientists would just need to find a way to deliver a functional F9 gene to patients, which could raise levels of the clotting factor and protect against bleeding.

In reality, the creation of a one-time gene therapy for hemophilia B would require almost 30 years of basic and clinical research.

Katherine A. High, MD, joined CHOP as a clinician-scientist in 1992.

Decades after Dr. High and a team at CHOP first demonstrated that a viral vector could deliver the therapeutic gene and raise levels of factor IX, the U.S. Food and Drug Administration has approved the one-time gene therapy fidanacogene elaparvovec (Beqvez®, now licensed by Pfizer) for the treatment of adults with hemophilia B. The therapy has the potential to be life-changing for patients over 18 who have relied their entire lives on daily therapeutic injections.

For Dr. High and Lindsey George, MD, who led the phase 1/2 clinical trial for the treatment, the FDA approval represents CHOP's role in bringing life-saving gene therapies from the bench to the bedside.

"Dr. High and many at CHOP were fundamental in establishing that first set of rules — the regulatory playbook for gene therapy clinical trials — and those rules, for the most part, remain in place today," said Dr. George, an attending hematologist and director of CHOP's Clinical In Vivo Gene Therapy group (CIGT). "Years ago, hospital leadership said, 'Gene therapy has tremendous potential to impact pediatric health,' and they stayed true to that vision."

Breakthroughs and Setbacks

By the late 1990s, Dr. High and her team demonstrated that they had raised factor IX to curative levels in animal models of hemophilia B.

To deliver genetic material into a cell, you need a delivery vehicle, or vector. Viruses make excellent vehicles, because they are efficient at finding their way into cells, embedding their DNA, and assembling new copies of themselves. To make a viral vector safe, scientists remove its viral genes and infuse it with the therapeutic gene before putting the now-harmless virus into a patient.

When Dr. High arrived at CHOP in 1992, she and colleagues got to work testing various vectors that could deliver the missing clotting factor gene to patients with hemophilia B. Adeno-associated viral (AAV) vectors proved to be a successful in vivo gene therapy approach. With in vivo gene therapy, a clinician injects the treatment that carries the genetic instructions directly into a specific part of the body.

By the late 1990s, Dr. High and her research team at CHOP published two papers, in PNAS and Nature Medicine, demonstrating that they had raised factor IX to curative levels in animal models using an AAV vector that was carrying the gene for human factor IX. A California-based biotech company signed on to manufacture the AAV vector and sponsor the treatment's first human clinical trial.

In the first trial, Dr. High and colleagues tested whether injecting the gene therapy into patients' skeletal muscle would help them to produce measurable levels of factor IX. The treatment proved to be safe, however, the patients were not able to make significant circulating amounts of the protein.

Next, the researchers tried the liver — and the results looked promising: The first patient to receive the injection, a physician himself, initially expressed therapeutic levels of the blood-clotting factor he had lacked since birth.

"He was working at his medical clinic and slammed his hand in a cabinet," Dr. High said. "The whole day, he was running around thinking, 'I need to treat myself,' but it was a busy day, and he didn't have time. By the end of the day, he looked down at his hand and realized nothing had happened. The gene therapy was working."

Six weeks later, the patient's factor IX levels began to drop. While he didn't experience any serious side effects, his body soon was no longer producing the blood-clotting factor that it had been making for weeks.

"I was desperate to figure out what was going on before the factor IX levels disappeared entirely," Dr. High said. "I sought input from liver specialists and immunologists, asking, 'What do you think is going on? And what can we do?'"

As Dr. High and her colleagues eventually showed, the patient's immune system was responding to the viral vector in a way that the researchers had not observed in animal models, since animals are not natural hosts for AAVs. Because many humans have exposure to AAVs through natural infections, the body recognized the vector as an unwanted invader and attacked it soon after it had started to deliver its therapeutic package.

Dr. High and her team knew they would have to return to the drawing board to define the immune response and figure out how to prevent it from happening again. But by that time, the biotech company investing in the gene therapy had pulled its funding from the project.

To continue the quest she had begun a decade earlier, Dr. High needed a new plan.

A New Path Forward

The spotlight was on Dr. High in the early 2000s, as her team began to test their AAV gene therapy in human patients.

Dr. High approached CHOP's then-CEO Steven Altschuler, MD, with a bold request: Would the hospital consider investing its own funding to establish a facility that could manufacture clinical-grade AAV vector for gene therapy research?

One week later, Dr. Altschuler gave her the green light — with one condition: The resources could not only go toward hemophilia; they would also have to fund gene therapy clinical trials for other diseases.

"It's still one of the most important and surprising events to happen throughout my entire career," Dr. High said.

That day marked the start of what would become the Raymond G. Perelman Center for Cellular and Molecular Therapeutics (CCMT). Dr. High wasted no time competing for (and earning) a National Institutes of Health (NIH) grant to support the production facility, hiring scientific staff, investing in manufacturing equipment, and establishing a partnership with Jean Bennett, MD, PhD, the Penn scientist who directed clinical trials for what would later become voretigene neparvovec-rzyl (LUXTURNA® , Spark Therapeutics) — the first FDA-approved gene therapy for a genetic disease.

Today, the CCMT manufactures a wide range of viral and non-viral vectors for gene therapy trials. Supporting it are three core units: the Human Pluripotent Stem Cell Core, Clinical Vector Core, and Research Vector Core.

In 2005, the CHOP team was armed with the resources needed to manufacture and test AAV vectors in-house, but the researchers still faced a major hurdle: There were no approved gene therapies, and thus no clear roadmap to FDA approval.

Working with the FDA and the NIH, alongside CHOP physician-scientists, Dr. High and colleagues developed an Investigational New Drug Application and clinical protocol for gene therapies that is still used today.

Measuring Success

Lindsey George, MD, (center) is an attending hematologist and director of CHOPs Clinical In Vivo Gene Therapy group (CIGT).

The research team worked diligently to perfect the hemophilia B gene therapy with Dr. George, who had joined CHOP as a Hematology/Oncology fellow. Dr. George trained in the lab of Rodney M. Camire, PhD, the CHOP endowed chair in Pediatric Hematology who was working alongside Dr. High on her first-in-human gene therapy studies.

They were preparing to launch a clinical trial to test the safety and efficacy of an AAV vector that contained a newly-identified, naturally-occurring gene variant that could lower the risk of an immune reaction to the hemophilia B therapy.

"While I was early in my career, the translational and clinical aspects of AAV gene therapy were novel, and there really was not well-developed expertise," Dr. George said. "It was an exciting time to be part of the work demonstrating the first early signs of clinically meaningful success."

Philadelphia-based biotech company Spark Therapeutics sponsored the phase 1/2 trial, led by Dr. George as a principal investigator. Co-founded by Dr. High in 2013, Spark's mission was to build on the foundational research conducted over a 10-year period by CHOP's CCMT to accelerate the timeline for bringing new gene therapies to market.

Beginning in 2015, 10 adult patients with hemophilia B received a single dose of the investigational gene therapy. The FDA granted the therapy orphan product designation, and the product received a breakthrough therapy designation in 2016.

In a landmark paper published in the New England Journal of Medicine in 2017, Dr. High and Dr. George described the success of the trial: All 10 patients who received the gene therapy sustained curative levels of factor IX, and nine out of 10 had no bleeding events after receiving the treatment.

Pfizer took over phase 3 of the trial, and in April 2024, the FDA granted approval of the hemophilia B gene therapy.

"It all started at CHOP — developing the vector, describing the immune response, building the regulatory infrastructure to do the research, and creating a research core to execute these trials," Dr. George said.

CHOP continues to lead the way in gene therapy development, clinical trials, and administration. Thirty-two active in vivo gene therapy clinical trials are ongoing at the hospital, and in 2021, CHOP launched the CIGT, which is helping to transform preclinical discoveries into approved therapies. To date, the hospital's Clinical Vector Core has treated more than 1,000 patients, supported more than 175 clinical trials, and led to the development of four licensed gene therapy products.

For Dr. High, though the culmination of three decades of scientific research is, of course, personally rewarding, the patient stories are what bring her the most joy.

"I think about one of the patients we treated with the new product — he was a nurse from Mississippi. Early 20s. Two little kids," she said. "The therapy made a tremendous difference for him. That was eight years ago, and he's still doing well."

How does she feel now that the FDA has finally approved the gene therapy she pioneered decades ago?

"Nothing will make me happier than seeing the curve in the levels of the very first person we treated," Dr. High said, before politely mentioning she needed to get to her next meeting.

There are, after all, many more patients to treat.