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Hemgenix, CSL’s one-time gene therapy for patients with hemophilia B, is experiencing a “temporary global stockout” that will cause treatment delays for “some” patients in countries with established commercial access, the company’s senior vice president of medical affairs Deborah Long, M.D., said in a community letter dated March 17. The last available hemophilia gene therapy is temporarily unavailable, according to CSL Behring. Hemgenix, CSL’s one-time gene therapy for patients with hemophilia B, is experiencing a “temporary global stockout” that will cause treatment delays for “some” patients in countries with established commercial access, the company’s senior vice president of medical affairs Deborah Long, M.D., said in a community letter dated March 17. Long attributed the situation to the “complexity of manufacturing gene therapies” and CSL’s “commitment to adhering to the highest regulatory and quality standards.” “We are working with regulatory authorities on strategies to ensure stable ongoing supply for Hemgenix while preserving our high-quality standards,” the executive wrote, emphasizing that the issue is not related to the safety or effectiveness of the product. CSL remains “fully committed to delivering this innovative, one-time gene therapy to the hemophilia B community,” Long added in the letter. The global stockout is specifically due to a “preliminary test result that is currently under investigation,” Long clarified in a separate statement to Fierce. “Gene therapy manufacturing is inherently complex,” she explained. “Hemgenix is produced in small batches which undergo multi-step, repeated testing to meet stringent industry standards for release as well as CSL’s rigorous standards.”The company cannot currently provide a timeline for when supply will be “back to normal," and determining exactly how many people with hemophilia B will be impacted is “going to depend on the length of the stockout,” Long said. The Australian drugmaker licensed Hemgenix from uniQure in 2020 in a deal that still left uniQure responsible for manufacturing the gene therapy. In 2024, uniQure sold off its Massachusetts gene therapy production plant to contract manufacturer Genezen, an agreement that also transferred the Hemgenix manufacturing and supply activities to Genezen, uniQure explained in its recent annual report. Two years before that, in 2022, CSL communicated that it intended to transfer Hemgenix manufacturing to a third-party contractor “in the coming years,” uniQure said in the report. Either way, uniQure pointed out in the manufacturing risks section of its report that additional business risks could stem from Genezen’s ability to manufacture Hemgenix “in accordance with regulatory requirements and to meet CSL Behring’s supply requirements for commercial product.”“Gene therapies are complex, expensive and difficult to manufacture,” uniQure said in the report. “Genezen or any third-party manufacturer that we engage could experience capacity, production or technology transfer challenges that could result in delays in our development or commercialization schedules or otherwise adversely affect our business.”  UniQure did not immediately respond to Fierce Pharma’s requests for comment. Hemgenix represents the world’s lone hemophilia gene therapy after Pfizer and BioMarin both bowed out of the commercial market within the last year, with Pfizer revoking its hemophilia B therapy Beqvez last February and BioMarin pulling the plug on its Roctavian for hemophilia A more recently. Pfizer, after not dosing a single commercial patient during its drug’s year on the market, attributed its Beqvez withdrawal to “limited interest toward hemophilia gene therapies,” a notion that has received pushback from some experts in the field. Although CSL hasn’t wavered in its commitment to its product, it hasn’t been immune to the same manufacturing, patient access and eligibility hurdles that prevented Pfizer and BioMarin from attracting broader uptake.  Since its 2022 approval, Hemgenix has attracted more than 75 commercial patients across eight countries, CSL said in December. While this level of uptake may be marginal for other drug classes, “we need to think about gene therapy in a completely different way” than for any other therapy, CSL’s head of U.S. Diego Sacristan told Fierce Pharma in a December interview, noting that “the system has a lot of hurdles.” The company hopes to reach a total eligible patient population of 800 people in the U.S., a process that might be slow given the “one-at-a-time type of work” the gene therapy presents, Sacristan said. Hemgenix is also approved in a handful of other countries, where CSL is introducing its medicine through a “phased approach,” he added. The one-time treatment carries a $3.5 million price tag and collected $57 million in sales over the latter half of last year, CSL reported last month. Editor's note: This story was updated with an additional statement from CSL's Deborah Long, M.D.