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Democrat senators in the U.S. expressed "significant concerns" about how a takeover could affect patients – a point also raised by AstraZeneca chief executive Pascal Soriot this week.
Ian Read, the chief executive of Pfizer who has been questioned twice this week by U.K. politicians, addressed concerns about the potential disruption to drug development caused by a deal by promising to "ring-fence" development of AstraZeneca's most important medicines.
Read MorePfizer CEO: AstraZeneca drugs warning a 'red herring'
An online petition to stop the proposed takeover, on the grounds that it might threaten a ovarian cancer drug AstraZeneca is developing, had nearly 30,000 supporters on Thursday.
The potential deal has been subject to an unprecedented level of scrutiny on both sides of the Atlantic, even though Pfizer has yet to make an offer high enough to engage AstraZeneca management.
Under U.K. takeover rules, the U.S. pharma giant has until May 26 to come back with an offer, which has been sweetened enough to bring AstraZeneca management and shareholders to the table.
The most recent £50 per share offer was dismissed out of hand by the AstraZeneca board. A revised offer could go as high as £55 per share, according to analysts.
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While AstraZeneca has had a difficult few years with patent expiries on some of its most important drugs expected to lead to reduced revenues, it has built a promising pipeline of medicines through a series of acquisitions.
Some of these drugs will come under the microscope at the American Society of Clinical Oncology's annual conference, the most important meeting of cancer doctors globally, later in May. On Wednesday, AstraZeneca released some promising data for a lung cancer treatment currently undergoing clinical trials, ahead of the conference.