TOKYO, May 28 (Reuters) - Japan's Asahi Kasei said on Tuesday it offered to acquire Calliditas Therapeutics for about 11.8 billion Swedish crowns ($1.1 billion) as part of a plan to become a global drug company.

The Japanese firm will offer 208 crowns per share of Stockholm-based Calliditas and 416 crowns for each American depositary share in a deal approved by the boards of both companies, Asahi Kasei said in a statement.

The offer represents an 83% premium for Calliditas based on the closing price of 113.6 crowns on Monday.

Asahi Kasei, primarily known for its building materials business, said it wants to grow its healthcare segment globally in the fields of immunology, transplantation, and related diseases.

Calliditas focuses on orphan diseases and patients with unmet medical needs. In recommending that shareholders accept the tender offer, the Calliditas board said in a statement that the company stands to "realise the benefits of being part of a larger platform". ($1 = 10.5681 Swedish crowns) (Reporting by Rocky Swift; Editing by Muralikumar Anantharaman)