Shire P.L.C. chief executive officer Matthew Emmens will become nonexecutive chairman of Britain's third-largest drugmaker in June, after five years at the helm.

Angus Russell, 51, Shire's chief financial officer since 1999, will succeed Emmens as chief executive.

Emmens, 56, will become chairman, after the retirement of James Cavanaugh, 70, who told the board in the spring that he would retire next June.

"The board ultimately asked me if I'd like to be chairman," said Emmens, who lives in Malvern and opened Shire's U.S. headquarters in Tredyffrin Township, Chester County, in 2004.

"It is a way for me to stay very involved in the company, and do the things I love, which is the strategy, and the mergers and acquisitions, and to keep the company growing."

Analyst Ken Cacciatore at Cowen & Co. said the transition "should be seamless" because "these are the executives who have really transitioned this company," and presided over acquisitions, a patent settlement, and a series of new medicines.

"It's a bit of a different role" for Emmens, "but he's still going to be the strategic visionary of the firm," Cacciatore said. Russell "has the confidence of many on the Street," he added.

Russell, who is British, is currently looking for a permanent residence in the Philadelphia area. He will spend two-thirds of his time in the United States and one-third in Basingstoke, outside London, where Shire is based, a company spokesman said.

Emmens, who is American, now spends about three-quarters of his time in the United States and one-quarter of his time in England.

About 80 percent of Shire's $1.8 billion in sales are in the United States. As the company continues to grow, the suburbs of Philadelphia will "be our primary site in North America," Emmens said. "It is where we have most of the functions of our business and that will not change."

Emmens' discussions with the board took place over the late summer. "I had no particular time in mind" for leaving as CEO, he said. "But when the chairman decided to retire and they wanted me to do that job, I found that very attractive."

Shire, which has more than 600 full-time employees at the Chesterbrook Corporate Center, announced plans in July to expand in Chester County and add 390 jobs over the next four years.

Shire has been growing by acquisitions and partnerships. The company has a pipeline of specialty drugs that target rare diseases and growing sales in its core business, treatment for attention-deficit/hyperactivity disorder. Shire's top-selling ADHD medicine, Adderall XR, generates almost half of the company's revenue.

Shares closed down $3.58, or 5.07 percent, at $67.01 on the Nasdaq.

Contact staff writer Linda Loyd at 215-854-2831 or lloyd@phillynews.com.