(Sharecast News) - Calliditas Therapeutics surged on Tuesday after Japan's Asahi Kasei offered to buy the Swedish drugs firm for around 11.8bn Swedish kronor.

Asahi Kasei made an offer of SEK208 per share, which is an 83% premium to the closing share price on Monday.

Calliditas said its board unanimously recommended that shareholders accept the offer.

"In the board's opinion, the combination of the offeror and Calliditas will leverage and complement the offeror's product offerings as well as its ability and expertise in rare disease drug development and commercialisation," it said.

"With Asahi Kasei as its new strategic owner, the company aims to realise the benefits of being part of a larger platform and the potential opportunity to accelerate the company's revenue growth trajectory as well as pipeline development."

Calliditas is a biopharma company headquartered in Stockholm focused on identifying, developing, and commercialising novel treatments in orphan indications with significant unmet medical needs.

At 1430 BST, the shares were up 78% at SEK202.60.