Akorn, Inc. has entered into a definitive agreement to acquire Hi-Tech Pharmacal Co., Inc., a specialty pharmaceutical company that manufactures and markets generic, branded, and OTC products, for $640 million in cash. The combined company is expected to have annual revenues of more than $500 million.   Hi-Tech specializes in difficult to manufacture liquid and semi-solid dosage forms. Hi-Tech produces and markets a range of oral solutions and suspensions, as well as topical ointments and creams...