Shares in US group Clovis Oncology were trading down nearly 20 percent after regulatory advisors recommended the strongest safety warnings for its experimental lung cancer drug rociletinib, throwing a shadow of doubt over its future.

Rociletinib is up for review as a treatment for patients with EGFR mutation positive metastatic non-small cell lung cancer (NSCLC) who have been previously treated with an EGFR-targeted therapy and who have the EGFR T790M mutation as detected by an FDA approved test.

The US Food and Drug Administration’s Oncologic Drugs Advisory Committee (ODAC) will vote on Tuesday on whether the drug’s benefits outweigh its risks. But in a briefing document posted ahead of the meeting, the FDA said it is pushing for a black-box warning on heart risks linked to the drug.

Agency staff reviewers are recommending “the inclusion of a Boxed Warning for the risk of QTc prolongation leading to Torsades de pointes”, as well as that labelling “describe ECG monitoring of QTc interval at baseline and periodically while receiving treatment with rociletinib”. 

There should also be information under the Warning and Precautions subsections for QTc prolongation, hyperglycaemia, interstitial ling disease/pneumonitis, pancreatitis, and cataracts, the FDA noted.

The move has put a huge question mark over the drug’s future in the US. “It's ugly, but we think the drug could still get approved, though timing is up in the air and its competitive position may be severely damaged," Mizuho Securities analyst Eric Criscuolo wrote in a note, according to Reuters.

The FDA has specifically asked the Committee to consider whether response rates the duration of response for patients treated with rociletinib is better than available therapy for the proposed patient population, and also whether the safety profile is acceptable.

The Agency should reach a decision on the drug’s marketing application by June 28.