Allergan to buy experimental psoriasis drugmaker Vitae for $639 million

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Vitae is also testing a potential treatment for atopic dermatitis.

The acquisition would expand the main dermatology business at Allergan, which is best known for its widely used Botox treatment.

The company is paying $21 per share in cash for Vitae, whose stock jumped to $20.89 in premarket trading from $8.10 at Tuesday's close. The previous year high was $18.71 on Dec. 30.

Allergan shares closed at $240.10 on Tuesday.

(Reporting by Caroline Humer; Editing by Lisa Von Ahn)