AbbVie will pay around $10 billion to add a potential blockbuster cancer treatment as cheaper versions of the drugmaker's all-time best seller, Humira, cut into sales.

But company leaders cautioned Thursday that a return on the deal they announced for drug developer ImmunoGen will take time to develop.

The company said it will pay $31.26 in cash for each ImmunoGen share. That amounts to a 95% premium on the stock's closing closing price Wednesday of $16.06.

The deal delivers Elahere, an ImmunoGen ovarian cancer treatment that AbbVie says could eventually reap billions of dollars in annual sales and drive long-term revenue growth. But in the near term, AbbVie expects the deal to be dilutive to its earnings until 2027.

U.S. regulators granted Elahere an accelerated approval last year, and the company is seeking a full approval from the Food and Drug Administration as well as expansion in other markets. It also plans to expand Elahere's use.

Elahere brought in $105.2 million in revenue in this year's third quarter, or nearly all of ImmunoGen's total sales.

AbbVie said the acquisition will speed up its entry into the market for treating solid tumors. Company leaders told analysts Thursday that Elahere made up the "most substantial component" of the deal's value.

They expect sales of the treatment to grow, with a significant inflection starting early in the next decade.

AbbVie has been dealing this year with cheaper competition for its top seller, the autoimmune disease treatment Humira. The injectable biologic drug is used to treat rheumatoid arthritis, Crohn's disease and ulcerative colitis.

Humira brought in $21 billion in total revenue last year, or about 36% of AbbVie's total.

Humira also totaled more than $11 billion in revenue through the first nine months of this year. But its U.S. sales have slid about 31%.

AbbVie and ImmunoGen said Thursday that the boards of both companies have approved the transaction. It's expected to close in the middle of 2024. It still needs approval from ImmunoGen shareholders and regulators.

Shares of Waltham, Massachusetts.-based ImmunoGen Inc. soared more than 80% to top $29 in mid-morning trading Thursday. North Chicago, Illinois-based AbbVie Inc. stock was up 2% to $141.26 while broader indexes were mixed.