Sickle Cell CRISPR Therapy May Offer ‘Functional Cure’

SAN DIEGO, California — Earlier this month, the US Food and Drug Administration (FDA) approved two gene-editing therapies for patients aged 12 years or older with severe sickle cell disease.

One therapy — exagamglogene autotemcel or exa-cel (Casgevy) — is the first to use CRISPR gene-editing technology, and could “provide a one-time functional cure to patients with sickle cell disease,” said Haydar Frangoul, MD, of The Children’s Hospital at TriStar Centennial, Nashville, Tennessee.

Frangoul, who presented a recent interim analysis on the therapy at the American Society of Hematology (ASH) annual meeting, reported that one infusion of exa-cel prompted rapid increases in total hemoglobin levels and almost completely eliminated a common and painful complication of sickle cell disease that can lead to irreversible organ damage, known as vaso-occlusive crisis.

Overall, the gene therapy led to “a rapid, robust, and durable increase in total hemoglobin to normal or near normal levels,” Frangoul said.

Exa-cel, from Vertex Pharmaceuticals and CRISPR Therapeutics, is a single-dose infusion containing a patient’s modified cells. First, a patient’s stem cells are harvested and then genetically modified to produce fetal hemoglobin.

The development of exa-cel was “grounded in human genetics, which show that fetal hemoglobin can substitute for sickle hemoglobin,” Frangoul explained. Patients receive these edited cells, which then help restore normal hemoglobin production.

The analysis showed that a one-time infusion of exa-cel following myeloablative conditioning prevented vaso-occlusive crisis in all but one patient with severe sickle cell disease. The therapy also prevented inpatient hospitalizations for vaso-occlusive crisis in all patients and led to sustained improvements in quality of life.

The results are “really striking,” said Sarah H. O’Brien, MD, of Nationwide Children’s Hospital in Columbus, Ohio, who was not involved in the research. “The majority of our admissions on the hematology service are our patients with sickle cell. They’re uncomfortable, they’re in pain, they’re missing school, and they’re missing their activities,” which makes these interim findings quite “impactful.”

To examine the impact of exa-cel on vaso-occlusive crisis, the phase 3 trial included individuals aged 12 to 35 years with severe sickle cell disease and a history of at least two vaso-occlusive crises per year over the past 2 years.

Participants underwent cell CD34+ stem cell collection. These cells then underwent gene editing using CRISPR technology, explained Frangoul.

At the transplant center, patients received myeloablative conditioning chemotherapy with busulfan for 4 days before receiving an exa-cel infusion.

At the data cutoff in June 2023, 44 patients had been enrolled, of whom 30 were available for efficacy analysis. The mean age at screening was 22.1 years, and almost half (46.7%) were female. Prior to study recruitment, patients had a mean of 3.9 vaso-occlusive crises per year and a mean of 2.7 inpatient hospitalizations per year for severe vaso-occlusive crisis.

All but one patient (96.7%) met the primary endpoint of freedom from severe vaso-occlusive crisis for at least 12 consecutive months. The mean duration of freedom from vaso-occlusive crisis was 22.4 months, ranging from 14.8 months to 45.5 months. Moreover, 28 of the 29 patients who remained crisis-free at 12 months did not have a further vaso-occlusive crisis throughout the rest of the follow-up period.

Frangoul noted that results were similar for both adults and adolescents.

Exa-cel also led to a significant increase in freedom from inpatient hospitalizations, with 100% of patients achieving that goal, as well as early and sustained increases in both total and fetal hemoglobin levels, suggesting a “long-term meaningful benefit” from the therapy.

All 44 patients experienced adverse events related to myeloablative conditioning with busulfan, but only 29.5% had events linked to exa-cel. The most common adverse events overall were nausea (70.5%), stomatitis (63.6%), vomiting (56.8%), and febrile neutropenia (54.5%).

In a separate poster presented at ASH, Akshay Sharma, MBBS, of St. Jude Children’s Research Hospital in Memphis, Tennessee, Frangoul, and colleagues reported that exa-cel also led to better health-related quality of life.

Patients showed “substantial improvements” in measures of quality of life, which included physical, emotional, social, and functional well-being as well as pain at a 6-month follow-up through year 2.

Typical outcomes studied in most trials are “emergency room visits and hospitalizations but what people may not appreciate as much is how much these patients are dealing with pain and discomfort at home,” O’Brien said. These recently reported quality-of-life metrics “are so key and really help us understand the impact” of this new therapy.

O’Brien noted, however, that “patients may be reluctant to undergo” this therapy because of the impact myeloablative conditioning has on fertility. That is why ongoing research on how stem cell transplants can be delivered “without impacting fertility is very important.”

It is “hard to know,” O’Brien explained, whether exa-cel will be a one-time treatment in practice, as many of the patients “already have end-organ damage from their disease.”

To that end, Frangoul noted that patients who complete the current trial can enroll in one that will include 13 years of additional follow-up.

Finally, O’Brien cautioned, gene therapies such as exa-cel “are only going to apply to a small segment of the population” — patients with the most severe form of the disease. That’s why “it’s important that we still prioritize hydroxyurea [and] multidisciplinary care for patients with sickle cell disease,” she said.

The study was sponsored by Vertex Pharmaceuticals in collaboration with CRISPR Therapeutics. Frangoul declared relationships with Editas Medicine, Rocket Pharmaceuticals, Jazz Pharmaceuticals, Vertex Pharmaceuticals, CRISPR Therapeutics, Bluebird Bio, and others. Sharma declared relationships with Vertex Pharmaceuticals, CRISPR Therapeutics, and others. Other authors declare numerous financial relationships.

American Society of Hematology 2023 Annual Meeting (ASH) 2023: Abstract 1052, Abstract 4999. Presented December 11, 2023.