Takeda Pharmaceutical Co.¡¯s ambitions to expand in the lucrative U.S. health care market led the drugmaker to begin a painstaking examination of Shire PLC¡¯s assets more than two years prior to striking a $62 billion deal.

In particular, Shire¡¯s neuroscience unit and its gastrointestinal products sparked Takeda¡¯s interest. One hurdle, though, gave them pause: the steep purchase price.

Then the tide turned in Takeda¡¯s favor. Shire¡¯s struggling stock performance after its failed sale to AbbVie Inc. and the acquisition spree that followed ¡ª culminating with the $32 billion takeover of Baxalta Inc. ¡ª frustrated investors and prompted concerns about its strategy. To appease them, Shire in August announced a potential spinoff of its neuroscience unit, potentially valued at $10 billion to $15 billion.