Halozyme Therapeutics saw its stock collapse 53.7% in after-hours trading as regulators in the USA turned down an immune deficiency product from Baxter that uses technology developed by the former.

Baxter announced that the US Food and Drug Administration has issued a complete response letter for HyQ, a combination product for the treatment of patients with primary immunodeficiencies that includes Baxter's immune globulin infusion and Halozyme's recombinant human hyaluronidase enzyme (rHuPH20). The agency has requested additional preclinical data to support the biologics license application (BLA) for HyQ and expressed concern about the possible effects of non-neutralising antibodies on reproduction, development and fertility.

Baxter says it plans to file an amendment to the BLA for HyQ following additional discussions with FDA and individuals enrolled in ongoing studies in the USA will now be treated with Baxter's immune globulin therapy without recombinant human hyaluronidase.

Trial with ViroPharma put on hold

More bad news for Halozyme came as the FDA halted a trial of partner ViroPharma’s hereditary angioedema drug Cinryze (C1 esterase inhibitor [human]) combined with  rHuPH20. The study is on hold until the aforementioned data is produced and like Baxter, ViroPharma is preparing to begin a trial that will evaluate two different doses of Cinryze as a stand-alone therapy.

The news is no great surprise given that in April, the FDA asked for more information on HyQ but this latest share slide is due to the extent of the additional data required and the specific safety fears.

Halozyme noted that "elevated anti-rHuPH20 antibody titres were detected in the registration trial [for HyQ], but have not been associated with any adverse events". Also the FDA's moves do not affect its other candidates, such as subcutaneous versions of Roche's Herceptin (trastuzumab) and MabThera(rituxumab), or its already-approved fluid absorption drug Hylenex recombinant (hyaluronidase human injection).

Chief executive Gregory Frost said that "with receipt of the CRL, we can directly address the questions raised [and] we look forward to collaborating with the agency to address their concerns". While Halozyme stock fell, Baxter and ViroPharma were little changed.