Shares of the San Francisco company at last check were down 31% to $23.92.
FibroGen said the new data suggest roxadustat might not be superior to another drug called epoetin-alfa in lowering the risk of cardiovascular events in some patients.
FibroGen said that it had become aware that the primary cardiovascular safety analyses it submitted to the U.S. Food and Drug Administration "included post-hoc changes to the stratification factors."
“While all of the analyses set forth below, including the differences in the stratification factors, were included in the [new-drug application], we promptly decided to clarify this issue with the FDA and communicate with the scientific and investment communities," Chief Executive Enrique Conterno said. "We continue to have confidence in roxadustat’s benefit risk profile.”
Contero also said the analysis doesn't change FibroGen's view that roxadustat is comparable to epoetin-alfa in dialysis-dependent patients.
There is no change in the underlying roxadustat data, or to the efficacy analyses from the Phase 3 program, the company said.
FibroGen said it began a comprehensive internal review to ensure such issues do not occur again.
Mizuho analyst Difei Yang cut FibroGen's rating to neutral from buy with a price target of $29, down from $72.
Yang said that while the new data presented using prespecified criteria still support the benefit/risk profile of roxadustat, the report creates higher risk in the near term and uncertainty about the timelines to potential approval.
H.C. Wainwright analyst Edwin Zhang downgraded FibroGen to neutral from buy.
FibroGen said the FDA has tentatively scheduled a July 15 meeting to review the new-drug application for roxadustat.
Roxadustat has been approved in China, Japan and Chile to treat anemia of chronic kidney disease in both non-dialysis-dependent and dialysis-dependent adults.
Last month, FibroGen posted a wider-than-expected fourth-quarter loss and reported that regulators would be holding an advisory meeting to discuss roxadustat.