A sign is seen at a Pfizer manufacturing plant in Newbridge, County Kildare, Ireland, Nov. 12, 2015. Tom Bergin/Reuters

By Pamela Barbaglia and Greg Roumeliotis

(Reuters) -- Pfizer Inc and Allergan Inc chief executives have agreed on the roles they would assume in a combined company, removing one of the last hurdles to the largest ever healthcare merger, people familiar with the matter said.

Pfizer CEO Ian Read will be CEO of the combined company, while Allergan CEO Brent Saunders will have another very senior role, the people said on Friday, without providing more details. A deal announcement is expected in the coming days, one of the people added.

The sources asked not to be identified because the negotiations are confidential. Pfizer and Allergan declined to comment.

A top job at the combined company would position Saunders, 45, to have a claim at the CEO job when Read, 62, steps down. This arrangement also shows that Pfizer's management plans to remain in control, at least in the short term, even if the deal is technically structured so that Allergan buys Pfizer.

The roughly $150 billion merger would see New York-based Pfizer redomicile in Ireland, where Botox-maker Allergan is registered.

The U.S. Treasury Department on Thursday took steps to clamp down on tax-avoiding inversion deals with new rules, though there was scarce evidence it would stop the biggest inversion of them all, between Pfizer and Allergan.

Read, a soft-spoken trained accountant, spent more than 30 years working his way up the Pfizer ranks, taking the reins in 2010 with the sudden departure of CEO Jeffrey Kindler.

He viewed refocusing and reigniting the company's research engine as one of his most important tasks after a decade in which Pfizer had produced no important new medicines from its own labs.

On Read's watch, Pfizer has seen approval of 10 new medicines, including the breast cancer drug Ibrance, which analysts forecast will eventually generate $5 billion a year, and Trumenba, a potentially important meningitis vaccine, both products of Pfizer labs.

In his meteoric career, Saunders has lead Bausch & Lomb, Forest Laboratories and Actavis, which took the Allergan name after acquiring the Botox maker this year, all in the past five years. While his drug discovery credentials are not as strong as Read's, he is highly regarded among his peers as a CEO with top-notch operational expertise.

A protégé of industry veteran Fred Hassan, who fixed Pharmacia and sold it to Pfizer, Saunders is credited with helping turn around struggling Schering-Plough and then leading its integration with Merck & Co.

(Reporting by Pamela Barbaglia in London and Greg Roumeliotis in New York; Additional reporting by Ransdell Pierson and Bill Berkrot in New York; Editing by Bernard Orr)