(Reuters) – The U.S. Food and Drug Administration on Monday declined to approve a drug from Akcea Therapeutics Inc and Ionis Pharmaceuticals that aims to treat a genetic disease that causes fat accumulation in the blood.

Akcea’s shares plunged 21.5 percent while those of Ionis dropped 14 percent in after-hours trading.

The companies did not cite a reason for the complete response letter from FDA, with the decision coming as a surprise after an independent panel of advisers voted 12-8 in favor of the drug at a meeting held in May.

Regulators had earlier raised safety concerns such as low platelet count associated with the drug, Waylivra, but given the lack of available treatments analysts had expected an approval that would require frequent monitoring of platelet levels.

The drug was designed to treat familial chylomicronemia syndrome, a rare genetic disorder that could cause potentially fatal inflammation of the pancreas.

“We will continue to work with the FDA to confirm the path forward,” Akcea’s CEO Paula Soteropoulos said in a statement.

Cowen analyst Ritu Baral had estimated peak sales of $750 million in 2026.

U.S. regulators are currently evaluating a second drug, Tegsedi, from the companies and which is expected to be a bigger revenue earner.

Tegsedi was approved in Europe in July, but its label included platelet monitoring requirements due to the risk of low platelet count.

(Reporting by Aakash Jagadeesh Babu, Tamara Mathias and Ankit Ajmera in Bengaluru; Editing by Sriraj Kalluvila)