Walgreens Chief Executive Stefano Pessina said the company had analyzed the antitrust aspect of the deal but would not speculate on the number of drugstores it might need to divest in order to win regulatory approval. “I have fought tirelessly to promote competition in the health sector and I believe the proposed merger of two of the three largest drug store chains in the country raises serious issues,” Senator Amy Klobuchar, the top Democrat on the Senate Judiciary Committee’s antitrust subcommittee, said in a statement.