Shares of rare-disease drug-development company Zogenix (ZGNX) - Get Zogenix, Inc. Report plunged on Tuesday after the company said the U.S. Food and Drug Administration didn't accept its initial marketing application for one of its more promising epilepsy treatment drugs.

In a statement, the pharmaceutical company said its application for FINTEPLA (ZX008, fenfluramine hydrochloride) for the treatment of seizures associated with Dravet syndrome was rejected by the FDA as "not sufficiently complete to permit a substantive review."

The company said it plans to seek immediate guidance, including a so-called Type A meeting with the FDA, to clarify and respond to the issues identified in the RTF letter.

Zogenix shares dropped nearly 25% at the opening bell to $39.02. The shares ended the day Monday at $51.85.

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Dravet syndrome, also known as Severe Myoclonic Epilepsy of Infancy, is a rare form of epilepsy that begins in infancy.