Pfizer Terminates Phase III Sickle Cell Study Due to Slow Patient Recruitment

Pfizer's oncology business

Pictured: Exterior view of Pfizer's office in Belgium/iStock, Alexandros Michailidis

Pfizer has discontinued one of its two Phase III studies evaluating the anti-P-selecting antibody inclacumab in sickle cell disease, according to an update on the webpage.

According to the online database, Pfizer terminated the late-stage study “due to poor accrual and associated recruitment challenges.” The trial was launched in December 2021. However, at the time of Monday’s update, the study had only enrolled 78 patients out of its target size of 280 participants.

A Pfizer spokesperson confirmed to Fierce Biotech that the company ended the study “due to slow recruitment,” while adding that the pharma is still eyeing an approval for inclacumab in sickle cell disease by 2026.

Originally developed by Global Blood Therapeutics (GBT), Inclacumab is a fully human monoclonal antibody that works by selectively blocking the P-selectin protein, which is known to play a role in cell adhesion and is a clinically validated target for reducing pain associated with vaso-occlusive crises (VOC) in patients with sickle cell disease (SCD).

Inclacumab in June 2022 secured the FDA’s Orphan Drug and Rare Pediatric Disease designations for SCD treatment due to its best-in-class potential as well as its quarterly dosing regimen instead of monthly infusions.

In August 2022, Pfizer moved to acquire GBT in a $5.4 billion deal for the biotech’s portfolio of SCD therapies, including the FDA-approved Oxbryta (voxelotor), an oral hemoglobin S polymerization inhibitor indicated for patients four years of age and older. Inclacumab was also part of the acquisition.

Following the buyout, Pfizer took over the Phase III THRIVE program, which comprised two studies—including the subsequently terminated trial—designed to evaluate the efficacy and safety of inclacumab at reducing VOCs in SCD patients. The other trial remains active and has completed recruitment at 240 patients. According to its page, a readout is expected later this year.

The termination of Pfizer’s late-stage study follows back-to-back landmark approvals in the SCD space. In December 2023, the FDA on the same day gave its nod to CRISPR Therapeutics and Vertex Pharmaceuticals’ CRISPR-based Casgevy (exagamglogene autotemcel) and bluebird bio’s lentiviral delivery-based Lyfgenia (lovotibeglogene autotemcel).

Casgevy and Lyfgenia are cell-based gene therapies. Both products are made from the patients’ own blood stem cells, which are modified, and are given back as a one-time, single-dose infusion. Casgevy has a U.S. list price of $2.2 million per patient, while bluebird set the wholesale acquisition cost of Lyfgenia in the U.S. at $3.1 million.

In January 2024, the Centers for Medicare and Medicaid Services announced that it would seek outcomes-based agreements with the developers of these SCD gene therapies to help curb healthcare costs.

Tristan Manalac is an independent science writer based in Metro Manila, Philippines. Reach out to him on LinkedIn or email him at or

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