Momenta shares were up 68% at $51.98 before the bell, just shy of the offer price of $52.20.
The deal gives Johnson & Johnson’s Janssen unit access to Momenta’s experimental therapy, nipocalimab, being tested for myasthenia gravis, a neuromuscular disease that causes weakness in muscles.
“Nipocalimab gives Janssen the opportunity to reach significantly more patients by pursuing indications across many autoimmune diseases,” Johnson & Johnson said in a statement.
The drug is being developed to treat diseases where the body’s own antibodies attack or damage proteins and cells.
J&J reiterated its 2020 adjusted earnings per share forecast and said it expected the deal to close in the second half of 2020.