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Mizuho analyst Difei Yang downgraded FibroGen (FGEN) to Neutral from Buy with a price target of $29, down from $72. The stock in premarket trading is down 29%, or $9.91, to $24.73.

The analyst sees "higher risk and uncertainty" around timelines to regulatory approvals following the clarification of previously disclosed cardiovascular safety analyses on roxadustat. Clinical data previously shared with the scientific and investment community included post-hoc adjustments resulting in more favorable safety data, Yang tells investors in a research note.

While the new data presented using pre-specified criteria still support the benefit/risk profile of roxadustat, the news creates higher risk in the near term, and uncertainty around timelines to potential approval, says the analyst.