AstraZeneca chief executive officer David Brennan said Thursday he would retire June 1 in the wake of another difficult quarter for the pharmaceutical manufacturer.

AstraZeneca is based in the United Kingdom and released disappointing first-quarter first-quarter earnings along with Brennan's news before the markets opened in London.

AstraZeneca's U.S. headquarters is in Wilmington and it has a plant in Newark, Del.

The company has struggled lately as it tried to replace revenue from some of its blockbuster drugs that were facing generic competition because their patents are expiring. AstraZeneca's best-selling drugs are Crestor (cardiovascular), Seroquel (anti-psychotic) and Nexium (gastrointestinal).

Total revenue fell 11 percent in the first quarter and the operating profit dropped 37 percent.

The company has had several rounds of layoffs, including to staff in Delaware.

"The pharmaceutical sector is experiencing pressures none of which I've witnessed in my 36 years in the industry," Brennan said during a Thursday conference call. "I remain very confident that AstraZeneca has the capabilities, courage and determination to be successful into the future. If we maintain our focus on meeting the needs of patients and trying to solve unmet patient needs, we can continue to deliver attractive and sustained returns for our shareholders.

"The time for reflection on what has been a long and rewarding career with AstraZeneca will have to wait until I hand over responsibilities. For now, my attention remains and will continue to remain 100 percent focused on delivering our strategy. But I do want to say that I am proud of what we have achieved in my time as chief executive."

AstraZeneca also said that Leif Johansson will succeed Louis Schweitzer as non-executive chairman of the board on June 1 – three months earlier than previously announced – and will become chairman of the nomination and governance committee after Thursday's annual meeting. That means Johansson will guide the selection for Brennan's successor.

Chief financial officer Simon Lowth will act as interim CEO after Brennan departs and until his replacement is named. Julie Brown, a vice president of group finance, will become interim CFO.