The FDA has bestowed a green light to a companion diagnostic for Pfizer’s recently approved hemophilia B gene therapy—because, at a cost of $3.5 million per dose, you want to make sure it will work.
Labcorp obtained the agency’s approval for its nAbCyte test that helps determine whether a patient with the bleeding disorder is eligible for treatment with Beqvez (fidanacogene elaparvovec-dzkt), Pfizer’s first FDA-approved gene therapy and the second for the inherited condition after CSL and uniQure’s Hemgenix, which was the world’s most expensive drug when it debuted in late 2022 with the same $3.5 million price tag.
The clinical testing giant’s assay checks for whether a patient already carries specific antibodies that could interfere with the gene therapy’s adeno-associated virus delivery vector. Preexisting immunity can put the patient’s safety at risk as well as blunt the efficacy of the one-time treatment.
While Labcorp and Pfizer estimate that about 6,000 people in the U.S. are living with hemophilia B, which prevents normal blood clotting, the companies project that as many as 60% of the entire U.S. population may already be carrying antibodies against adeno-associated viruses.
AAV-based vectors are also being developed to shepherd gene therapy payloads into the body to treat cystic fibrosis, Duchenne muscular dystrophy and other rare diseases as well as more common conditions such as arthritis, Alzheimer’s and congestive heart failure.
The FDA approved its first gene therapy companion diagnostic test last June, developed through a collaboration between ARUP Laboratories and BioMarin, maker of the hemophilia A treatment Roctavian (valoctocogene roxaparvovec-rvox).
ARUP’s AAV5 DetectCDx immunoassay also aims to capture specific AAV antibodies using an electrochemiluminescence diagnostic. Labcorp’s approach, meanwhile, relies on a cell-based neutralizing antibody assay, targeting AAVRh74var.
“Labcorp is proud to offer the first cell-based, companion diagnostic to receive FDA approval, which represents a pioneering breakthrough in the field of companion diagnostics and will help transform the therapeutic landscape and the lives of patients living with rare, genetically inherited conditions,” Labcorp's chief medical and scientific officer, Brian Caveney, M.D., said in a statement.