Pascal Soriot, chief executive of AstraZeneca, said the U.S. had "almost entirely removed" the effect of tax inversion deals like Pfizer's bid for his company.
Soriot declined to comment on how the moves might affect Pfizer's thinking, but said the board would consider any offer which comes to them.
Sales of the company's medicines rose 5 percent to $6.54 billion in the third quarter, compared to an average $6.41 billion forecast by analysts, as they were boosted by better than expected sales of heartburn and ulcer treatment Nexium.
Despite these results, many remain focused on whether Pfizer, the world's biggest drugs company, will come back with a new bid for AstraZeneca after a £69.5 billion ($118 billion) bid was unsuccessful earlier in the year.
Pfizer can come back with a new bid for AstraZeneca after November 26.
However, new tax inversion rules in the U.S., which have already scuppered AbbVie's $54 billion bid for Shire, may remove the potential tax benefits of the bid.
U.S. President Barack Obama also suggested on Wednesday that tax reform could be on the way, after the Republican Party won control of the Senate.
For AstraZeneca, an investor day on November 18th is expected to outline future plans. The company is increasingly focused on a new kind of cancer treatment known as immuno-oncology, and has bolstered its presence in this area through a number of deals in recent weeks.
Soriot declined to comment on whether he might become the next chief executive of French pharma giant Sanofi Aventis - a job which became vacant when Chris Viehbacher departed last week.
- By CNBC's Catherine Boyle