(Reuters) -Alkermes said on Friday it is reviewing its options with advisers after Avadel Pharmaceuticals disclosed it had received an unsolicited takeover proposal from Danish pharmaceutical company Lundbeck.

Shares of Avadel soared over 20% in premarket trading.

The proposal comes weeks after Alkermes agreed to buy Avadel in a deal worth up to $2.1 billion, aiming to enter the sleep medicine market.

Avadel said Lundbeck offered up to $23 per share, including $21 in cash and up to $2 additionally tied to future sales milestones for its sleep disorder drug Lumryz and experimental therapy valiloxybate.

Avadel’s board said the unsolicited offer could reasonably lead to a “Company Superior Proposal” under its agreement with Alkermes, which would allow them to hold talks with Lundbeck.

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