FDA probing Regeneron and Sanofi’s investigational drug alirocumab

French drug major Sanofi (Euronext: SAN) has revealed in a regulatory filing that the US Food and Drug Administration has directed the company, and US partner Regeneron (Nasdaq: REGN), to determine whether any neurocognitive adverse events occurred in any trials of the global development program for their experimental PCSK9 inhibitor alirocumab, particularly in longer-term studies.

Regulators also asked the companies to assess the feasibility of incorporating new tests for the potential cognitive effects “into at least a subset of patients” in their late-stage trials of the investigational cholesterol-lowerer. Regeneron, whose shares plunged as much as 10% on the news last Friday, noted the FDA’s request in a February 13 Securities and Exchange Commission filing, according to Reuters. The stock closed 3.1% lower at $328.11.

It is unclear whether the information the FDA requested will derail early approval of PCSK9 drugs in high-risk patients, who may be younger and less prone to the side effects on the brain, Andrew Berens and Sam Fazeli, analysts with Bloomberg Industries, wrote in a report on Friday. They had estimated limited approval of a drug in the class in 18 months.