Johnson & Johnson has today agreed to pay about $6.5 billion in cash to buy Momenta Pharmaceuticals in a deal aimed at expanding its portfolio of autoimmune disease treatments. 

The deal gives J&J's Janssen unit access to Momenta's experimental therapy, nipocalimab.

This is being tested for myasthenia gravis, a neuromuscular disease that causes weakness in muscles, and other diseases where the immune system attacks the body. 

J&J hopes to get the drug approved to treat several conditions and eventually bring in blockbuster sales, as the company bulks up its pharmaceuticals unit, its largest business. 

Shares of Massachusetts-based Momenta were up 69.3% at $52.15 before the bell, just a hair's breadth away from the offer price of $52.50. 

"Janssen will have the potential to introduce multiple launches, many as first-in-class indications with potential for significant peak year sales, some of which could exceed $1 billion," Johnson & Johnson said in a statement. 

In the past two years, the healthcare conglomerate has unloaded some divisions that manufacture medical devices for diabetes care and has also been aiming to bolster its presence in fast-growing markets such as cancer. 

Nipocalimab is being developed to treat diseases where the body's own antibodies attack or damage proteins and cells. 

The deal comes just days after France's Sanofi struck a $3.7 billion deal to buy Principia Biopharma for its pipeline of autoimmune disease treatments. 

Johnson & Johnson said an estimated 2.5% of the global population have some type of autoantibody-driven disease, many of which are rare diseases. 

Its drug Stelara targets an autoimmune disease called psoriatic arthritis. 

J&J, which is also one of the drugmakers racing to develop a vaccine for the coronavirus, has recently signed deals with the US and Britain for future doses of the vaccine.