Amneal Pharmaceuticals announced on Monday that the U.S. Food and Drug Administration (FDA) has declined to approve its new Parkinson's disease drug. The FDA cited insufficient safety data regarding the treatment's ability to manage symptoms long-term. This news sent Amneal's shares tumbling over 13% in after-hours trading.

The FDA's Complete Response Letter specifically highlighted concerns about the safety profile of carbidopa, one of the drug's two key ingredients. While the agency acknowledged existing studies supporting the safety of levodopa (the other ingredient), it deemed the available data on carbidopa inadequate. The FDA requested further information addressing these safety concerns, but did not raise any issues regarding the drug's efficacy or manufacturing process.

The FDA's decision presents a setback for Amneal, which has been aiming to expand its portfolio of branded medications and had projected over $500 million in revenue from its specialty drug business by 2027. However, the company stated that this rejection will not affect its 2023 financial projections, as revenue from the new Parkinson's drug was not factored into the forecast.

Amneal's new drug is a novel formulation of carbidopa-levodopa, the current standard treatment for Parkinson's disease. It's designed to reside in a specific area of the small intestine for an extended period, promoting consistent absorption. The company plans to collaborate with the FDA to address the agency's concerns and will schedule a meeting to discuss the next steps.

This rejection comes as a blow to Amneal, which already markets another Parkinson's drug called Rytary. Approved in 2015, Rytary has struggled to gain significant traction, with only about 4% of patients currently using it. Parkinson's disease is a brain disorder characterized by involuntary or uncontrollable movements. It's the second most fatal neurodegenerative disease after Alzheimer's. While there is no cure for Parkinson's, various medications and therapies can help alleviate symptoms.