Pfizer today said it has ended its planned $160 billion acquisition of Allergan—what would have been the largest-ever merger among drug developers—2 days after the Obama administration issued new rules designed to discourage the deal and other tax-slicing “inversion” mergers.

The termination of the merger came nearly 5 months after Pfizer and Allergan announced their blockbuster deal, which would have created a combined entity called Pfizer plc and maintained Allergan’s Irish legal domicile.

As a result, the combined company would have enjoyed Ireland’s lower corporate taxes than if it maintained Pfizer’s legal domicile in the U.S.; the pharma giant is headquartered in New York.

The companies sought to become a biopharma juggernaut by combining Pfizer’s products and pipeline in six therapeutic areas—cardiovascular and metabolic diseases, immunology and inflammation, neuroscience and pain, oncology, rare diseases, and vaccines—with Allergan’s specialties, which include aesthetics and dermatology, anti-infectives, eye care, gastrointestinal, neuroscience, urology, and women’s health.

“Pfizer approached this transaction from a position of strength and viewed the potential combination as an accelerator of existing strategies,” Pfizer Chairman and CEO Ian Read said in a statement.

Almost immediately, the deal ran into opposition from Republican presidential candidate Donald Trump and Democratic presidential hopefuls Hillary Clinton and U.S. Sen. Bernie Sanders (D-VT).

But the death knell for Pfizer’s Allergan merger came on Monday, when the U.S. Department of the Treasury issued rules that restricted the ability of companies to carry out inversion mergers.

One new rule would change how the Treasury Department decides whether to impose penalties that reduce tax benefits for companies planning inversion deals where a non-U.S. company is 60% or more owned by U.S. shareholders due to acquiring U.S.-based companies within the previous 36 months.

Among companies falling into that category were Allergan, which has grown through a series of mergers and acquisitions that have swollen its market capitalization enough to catapult the company to number three on GEN’s most recent List of Top 25 Biotech Companies, published last year.

Pfizer shareholders would have owned 56% of a combined Pfizer-Allergan company—but that percentage would have surpassed the 60% benchmark if Washington were to disallow Allergan’s previous inversion deals. Among them are the AllerganActavis merger completed for $70 billion last year, after which Actavis adopted the name of its acquisition, as well as Actavis’ $28 billion acquisition of Forest Laboratories (completed 2014) and $8.5 billion purchase of Warner Chilcott.

Pfizer said it agreed to pay Allergan $150 million for reimbursement of merger-related expenses, in connection with the termination—a fraction of Pfizer’s roughly $200 billion market cap.

“We remain focused on continuing to enhance the value of our innovative and established businesses,” Read said.

He said Pfizer plans to decide by year’s end “whether to pursue a potential separation of our innovative and established businesses,” around the same time frame planned before the company announced the Allergan deal.

Such a separation, Read asserted, should prove attractive to investors seeking enhanced share value, citing in part a series of recent product launches that include the pneumococcal pneumonia vaccine Prevnar 13® in adults; the anticoagulant Eliquis® (apixaban); the arthritis treatment Xeljanz® (tofacitinib citrate); and Ibrance® (palbociclib), a treatment for hormone receptor-positive, human epidermal growth factor receptor 2-negative (HER2) breast cancer.

“We believe our late stage pipeline has several attractive commercial opportunities with high potential across several therapeutic areas,” Read added.

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