Shares of BioMarin Pharmaceutical (BMRN -0.50%) were down by more than 35% as of 1:50 p.m. EDT Wednesday after the Food and Drug Administration unexpectedly rejected the company's application for valoctocogene roxaparvovec (valrox) as a gene therapy for severe hemophilia A. The complete response letter (CRL) means the FDA won't approve the treatment with the application in its current form.

BioMarin was taken by surprise, saying that the FDA is now requesting a longer period of monitoring data from the phase 3 clinical trial for valrox after having previously agreed with the company about what data would be necessary for approval. The biologics license application for valrox was submitted based on interim data from the phase 3 trial and long-term results from an earlier, phase 1/2 study. The FDA says it wants BioMarin to complete the phase 3 study and submit two-year follow-up safety and efficacy data on all trial participants to provide more evidence for the durability of the treatment's effects.

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The biotech recently released four-year results from the earlier study, showing that six of the seven participants with the highest dose were free of bleeding episodes in year four after averaging more than 16 bleeds per year before treatment. But the FDA concluded that differences between the two studies limited its ability to rely on the phase 1/2 data to support the durability claims.

The therapy is still likely to be approved in the U.S. eventually, but the FDA's decision will mean a significant delay. Meanwhile, BioMarin is awaiting an approval decision from European health regulators, but the company said in its recent earnings report that the verdict may not come until early 2021.