Sigilon Therapeutics’ first human test of its experimental hemophilia A cell therapy has hit a stumbling block, with the FDA placing a clinical hold on the study after a patient developed a complication.

It’s still not known whether the serious adverse event is related to the therapy, SIG-001. Enrollment in the Phase 1/2 study has stopped, and an inquiry is underway, Cambridge, Massachusetts-based Sigilon disclosed Friday.

Sigilon’s drug candidates are cell therapies encapsulated by biocompatible spheres. These spheres are intended to keep the immune system from seeing the cell therapy inside as foreign, which could then trigger an immune response. Sigilon’s lead disease target is hemophilia A, an inherited blood disorder in which patients lack factor VIII, a clotting protein. It’s currently treated with chronic infusions of the deficient protein. SIG-001 encapsulates cells designed to express high levels of the factor VIII clotting protein that patients lack. This cell therapy is the most advanced product candidate in Sigilon’s drug pipeline.

The clinical trial that was halted is a Phase 1/2 dose-ranging study. The open-label study is designed to enroll up to 18 patients. The first two patients were dosed in the fourth quarter of last year; the administration of the therapy was well tolerated and there were no serious adverse events, the company said in its annual report.

The reported adverse event occurred in the study’s third patient, who received the highest dose of the experimental Sigilon therapy, the company said. That patient developed inhibitors, or antibodies, to factor VIII. Inhibitors are a well-known complication of factor VIII therapy. The problem for Sigilon is that its cell therapy is designed to avoid such immune responses. That concern spooked investors, and the biotech’s stock opened Friday at $6.95 per share, down nearly 25% from Thursday’s closing price.

Sigilon said that the patient who experienced the adverse event is responding well to medical treatment and his condition continues to improve. But the FDA has asked the company to provide more information on what could have potentially contributed to the development of inhibitors, such as family history and immune stimulation from a recent vaccination. The three patients that have been enrolled so far will continue to be followed while Sigilon investigates the adverse event.

“Patient safety is our top priority, and we are encouraged that the patient is recovering,” Rogerio Vivaldi, Sigilon’s president and CEO, said in a prepared statement. “In collaboration with the regulatory agencies and our advisors, we are conducting a thorough investigation of this event to confirm whether there was a causal relationship between the development of inhibitors and SIG-001. We are committed to working with the FDA to resolve the clinical hold.”

Sigilon has competition in the chase for a better hemophilia A therapy. A gene therapy from BioMarin Pharmaceutical was rejected by the FDA last year, as the regulator asked for more data about the treatment’s durability. Partners Pfizer and Sangamo Therapeutics are currently testing their experimental hemophilia A gene therapy in a Phase 3 clinical trial. Roche subsidiary Spark Therapeutics is also in Phase 3 testing with its experimental gene therapy for the disorder.

Photo: Streeter Lecka, Getty Images