Takeda Pharmaceutical Co. Ltd..'s $64 billion bid for London-listed rare-disease specialist Shire PLC bolsters its credentials as Japan's most outward-facing drugs firm, a mold-breaking image that has been more than a decade in the making.

Under CEO Christophe Weber and his predecessor, Yasuchika Hasegawa, Takeda has cut its exposure to Japan, brought in expertise through overseas acquisitions and thrown open its leadership ranks to foreign talent.

Some investors worry that Takeda, with only $4.3 billion in cash on hand for the Shire deal, may be overreaching.