American pharmaceutical company Johnson & Johnson is set to acquire Massachusetts-based Momenta Pharmaceuticals for $6.5 billion in cash. The acquisition is expected to expand Johnson & Johnson's autoimmune-disease treatment business.

Johnson & Johnson said it had agreed to pay $52.50 for each Momenta share. The price is a 70.4 percent premium over Momenta's $30.81 a share closing price Tuesday. Following the announcement Momenta's share price jumped by close to 70 percent - opening at $52.10 per share Wednesday. It closed higher at $52.12 a share.

After the acquisition Momenta will be integrated into Johnson & Johnson's Janssen Pharmaceutical unit. Momenta's portfolio of products will be transferred to Janssen. Momenta will still operate from its Cambridge headquarters, Johnson & Johnson said.  

The acquisition should enhance Janssen's lead in the autoimmune diseases segment and provide the company with a sustainable source of growth, Johnson & Johnson executive vice president Jennifer Taubert said in a statement. These patients are often underserved and the deal should ensure Johnson & Johnson can meet growing demand, she said.

Johnson & Johnson will take over the international rights of Momenta's nipocalimab anti-FcRn antibody treatment. Johnson & Johnson has plans for Momenta's other assets but needed to conduct due diligence first.

Momenta, founded in 2001, specializes in developing treatments for rare autoantibody-driven diseases. These include rare conditions such as myasthenia gravis, warm autoimmune hemolytic anemia and hemolytic diseases of the fetus. These diseases typically involve a person's own antibodies attacking or damaging cells, tissue and proteins.

Johnson & Johnson expects to complete the deal before the end of the year. Costs involved in further development of Momenta's portfolio will be "incremental" and the cost per share of the transaction will be minimal, the company said.