Shares of Ionis Pharmaceuticals and spinout Akcea Therapeutics kept falling Tuesday morning after a U.S. Food and Drug Administration division declined to sign off on their drug Waylivra.

Ionis on Monday reported a “complete response letter” from the FDA that apparently rejected Waylivra, aimed at Familial chylomicronemia syndrome, or FCS, an ultra-rare and debilitating disease. Ionis did not list the reason for the complete response letter, a shift from an FDA advisory panel in May voting 12-8 to back Waylivra.

Following the recent letter, the companies said they would work with the FDA on a path forward.

“We are extremely disappointed with the FDA’s decision,” said Paula Soteropoulos, CEO of Akcea Therapeutics, in a statement “Our disappointment extends to the patient and physician community who currently do not have a treatment available to them.”

The complete response letter was a setback for Waylivra, but also stirred concerns among investors that another Ionis drug, Tegsedi, will fall short with the FDA.

Ionis’ stock closed at $53.70 a share on Monday, trading at $45.10 at 9:15 a.m. on Tuesday. Akcea closed at $33.12 a share on Monday and Tuesday morning traded at $24.17.