The agreement is set to draw political ire in a U.S. presidential election year for Pfizer’s decision to use the transaction to redomicile in Ireland, where Allergan is registered, in a so-called “inversion,” thereby reducing its corporate tax rate. It will also reignite debate in the pharmaceutical industry over the role of research and development, with Allergan Chief Executive Brent Saunders, a prolific dealmaker and a skeptic of in-house drug discovery, joining the combined company in a position to influence its strategy.