Tax inversion, one of the main drivers behind the Pfizer bid, has come under the spotlight in the U.S., with legislation backed by President Barack Obama that could retroactively wipe out the benefits of inversion being debated.
Shire, another U.K. pharma company which is being bought by the U.S.'s AbbVie, partly for tax inversion purposes, secured a £500 million ($844 million) break clause because of concerns over possible execution risks.
AstraZeneca claimed to have "one of the most exciting pipelines" in the pharmaceutical industry, as the U.K. pharma giant reported second-quarter revenue rose by 4 percent.
The company stepped into a glare of publicity this year when its management battled with Pfizer, the biggest pharmaceuticals company in the world, for control of the company. Pfizer is now excluded from making a further approach until November under U.K. law (although it may be able to open talks earlier if AstraZeneca makes the first move).