So what's next?
Pfizer could, under the U.K. code, come back to AstraZeneca in six months, or in three months if AstraZeneca extends an invitation. During this time, AstraZeneca will be under pressure from investors to deliver on its promises that its pipeline can deliver the same value—or more—than Pfizer's bid offered. The first glimpse of whether that will be true may come this weekend at the American Society of Clinical Oncology, or ASCO, conference in Chicago, where cancer research results will be discussed.
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It's also possible, notes ISI Group analyst Mark Schoenebaum, that Pfizer could make another bid between three and six months from Monday's deadline, as the U.K. takeover panel "almost always makes an exception" to the 6-month rule, he said in an email to clients Tuesday after a teleconference with the panel.
So Pfizer either waits out the U.K.-imposed stay on discussions and comes back again later this year, or it takes its considerable buying power elsewhere.
One analyst suggested New York-neighbor Bristol-Myers Squibb, with a market value of about $81 billion.