The biggest U.S.-based drugmaker, Pfizer Inc. (PFE), will stay put thanks to aggressive new Treasury Department rules that succeeded in blocking Pfizer from acquiring rival Allergan (AGN) and moving to Ireland - at least on paper - to reduce its tax bill.

As analysts and investors had expected, the two companies called off the record $160 billion merger early Wednesday "by mutual agreement." Pfizer cited Treasury's new regulations, issued late Monday, to make such "tax inversions" less lucrative.

The merger would have moved Pfizer's address - but not its operations or headquarters - to Ireland, where it would pay far less in corporate taxes.

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Pfizer Inc., based in New York, has agreed to pay Dublin-based Allergan PLC $150 million for reimbursement of expenses.

Now the two drugmakers will focus on other strategies to boost profits and sales quickly, with Pfizer saying it will decide by year's end whether to separate its global established products business, which sells older, mostly off-patent drugs. Pfizer has endured years of relentless pressure from analysts and others to break up the company so growth and profits could accelerate.

"We remain focused on continuing to enhance the value of our innovative and established businesses," Pfizer CEO Ian Read said in a statement.

Allergan's CEO, Brent Saunders, said in a statement that his company is "disappointed that the Pfizer transaction" won't happen, adding that "Allergan is poised to deliver strong, sustainable growth."

Shares of Pfizer edged up 42 cents, or 1.3 percent, to $31.78 in premarket trading, while Allergen shares lost $3.55, or 1.5 percent, to $233.

The companies' decision could deter other tax inversions in the works - exactly the impact the Obama administration is seeking by having Treasury issue 300-plus pages of new regulations that remove or reduce key financial benefits of a tax inversion. Pfizer had expected to save hundreds of millions of dollars in U.S. taxes annually under its planned deal with Allergan.

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Read, Pfizer's CEO for five years, has said the deal was needed because U.S.-based drugmakers are at a major disadvantage to their multinational rivals based in Europe and elsewhere, who face lower corporate tax rates. Other U.S. companies likewise have complained about the top U.S. tax rate - which few ever pay - and the U.S. taxing them on profits made overseas.

With Pfizer's planned inversion looking like the start of a dreaded trend, tax inversions became a hot issue in the presidential race, with some candidates calling companies considering such deals "unpatriotic." And on the heels of the new Treasury rules, President Obama held a news conference Tuesday afternoon, saying the rules are meant to prevent "one of the most insidious tax loopholes out there" and make wealthy corporations shoulder their tax responsibility like working class Americans.

In an inversion, a big company buys a smaller one in another country, usually with a lower tax rate, then moves the combined company's address on paper - but little else - to that country.

Allergan itself is the result of multiple inversions, and despite its Dublin address is operated from offices in Parsippany, New Jersey. That's not far from Pfizer's high-rise headquarters near Manhattan landmarks such as the United Nations complex and the theater district.

The Allergan deal was Pfizer's third and most expensive attempt at an inversion, including its failed hostile attempt to acquire Britain's AstraZeneca PLC in 2014.

Pfizer has a history in this century of doing mega-acquisitions that allow it to cut costs and increase sales to boost profits quickly. That has kept Pfizer among the top global drugmakers but hasn't pleased investors enough, which ultimately led to the ouster of CEO Ian Read's predecessor late in 2010.