Having rejected two bids since January, Pfizer made its next move on Monday—a $99 billion stock and cash offer to acquire AstraZeneca, to create a single U.K.-based company that would become the world’s largest pharmaceutical company.

The proposed purchase also would create the largest foreign takeover of a U.K. firm. It is the latest in a growing number of major pharmaceutical acquisitions. For Pfizer it also would surpass its $90 billion takeover of Warner-Lambert Pharmaceuticals 14 years ago, which included gaining Lipitor, and the purchase of Wyeth Laboratories in 2009 for $68 billion.

This latest bid, formally made over the weekend, follows a complicated series of asset swaps estimated at $74 billion between Novartis and GlaxoSmithKline, plus Valeant Pharmaceuticals’ offer to acquire Allergan, the maker of Botox wrinkle treatment.  

Most of those deals and the new Pfizer offer for AstraZeneca share a commonality of large drug makers seeking new areas of growth as onetime blockbuster therapeutics lose patent protection. One of the biggest is Crestor, the blockbuster cholesterol-fighting drug from AstraZeneca. Crestor will lose its patent exclusivity in the next two years, paving the way for cheaper generic versions, while Pfizer’s painkiller drug, Celebrex, will face generic competition by the end of this year.  

AstraZeneca alone stands to lose patent protection on $12.3 billion in sales from existing products by 2022, according to ISI Group analyst Mark Schoenebaum. 

“Society wants products faster, they want more products and they want value,” said Ian Read, Pfizer’s chairman and CEO, during a media conference call. “Industry is responding to society’s request for increased efficiencies and productivity. We believe patients all over the globe would benefit from our shared commitment to R&D, which is critical to the future success of the pharmaceutical industry, in the form of potential new therapies that help to fight some of the world’s most feared diseases, such as cancer.”

One of AstraZeneca’s major appeals is its oncology drugs, a portfolio that is attractive to Pfizer, which is looking to restock its product pipeline. In January, AstraZeneca announced an agreement with Immunocore to develop new treatments that use immune cells in addition to its own immune-based drugs now being tested in multiple cancers. These are immunotherapies that boost the body’s immune system to fight tumors.

Both companies have complementary cardiovascular and inflammation drugs and ongoing research that, if combined with oncology, would “provide more value for patients,” enabling scientists to pick the winners to proceed and build value for the combined company, said Michael Dolsten, Pfizer R&D chief.

Paul Soriot, AstraZeneca CEO, has not commented on the latest Pfizer offer but did speak negatively about big deals at a recent earnings call.

“Large mergers and acquisitions sometimes work, but often they are very disruptive, so we are better focusing on what we do well and partnering where it make sense,” said Soriot, a former operating officer at Roche.

In reviving the company’s pipeline of new drugs, he has cut costs at AstraZeneca and laid off thousands of employees in an effort to replenish the company’s drug pipeline, doing a variety of small deals in cancer, diabetes and heart and inflammatory diseases. The company’s stock has risen 33% over the last year, outpacing the broader maker.

The proposed Pfizer-AstraZeneca deal—70% in stock for each AstraZeneca share and 30% in cash—is similar to an earlier bid and has led some stock analysts to predict that the final deal will exceed $100 billion. Under British takeover rules, Pfizer has until five pm British time on May 26 to announce a firm intention to make an offer or take its offer off the table. Pfizer, which will retain its New York headquarters, has—according to press reports—tens of billions of dollars accumulated through foreign subsidiaries to pay for the potential acquisition. 

“I doubt Pfizer will want to go completely hostile,” said Dan Mahoney, a fund manager at Polar Capital in an interview with the London Telegraph.

He reportedly raised his stake in AstraZeneca in February.