Alfred E. Mann, the 90-year old billionaire founder of MannKind Corp., has resigned as its executive chairman, according to a company statement. Former Northrop Grumman Corp. Chief Kent Kresa will succeed Mann after serving as a director at the Valencia drugmaker since 2004.

Mann, who founded the company in 1991, poured much of his personal fortune into its operations. He will remain as chairman emeritus and serve as an adviser.

“We don’t really see the situation as a changing of the guard,” said MannKind Chief Executive Matthew Pfeffer, who ascended to his current position in January. “Kent has been with us for years, and is one of the most qualified directors in the country, given his resume here and as a board member with other recognizable companies. Kent is anxious to propel Al’s legacy into the future, and Al will continue to be one of my most trusted advisors.”

Kresa, who was the longtime chairman and chief executive of Northrop Grumman, also was interim chairman of General Motors when it was going through bankruptcy in 2009.

The transition at MannKind comes on the heels of several negative developments for it.

In January, French company Sanofi terminated its marketing and distribution agreement to sell MannKind’s Afrezza, an inhaled-insulin drug that is the company’s only product approved by the Food and Drug Administration.

That news caused MannKind’s stock to plummet by 48 percent on the Nasdaq, closing at 75 cents on Jan. 5. The stock closed at 98 cents on Friday.

Pfeffer, however, said Afrezza would continue to be available to patients despite the end of the Sanofi deal.

Kresa said in the company’s statement that as chairman he would stay true to Mann’s original vision while taking its operations in new directions.

“Al is a true visionary and has created a great company with an impressive lead product and powerful technology to help patients with serious diseases,” said Kresa. “We have much to do to make his dreams a reality, but we have the people, the technology and the know-how to make that happen.”

Ahmed Enany, president and chief executive of Southern California Biomedical Council, a trade organization, described the leadership transition as “a good opportunity for MannKind to explore new growth strategies such as mergers and acquisitions,” adding that Kresa is a person who “could steer the ship in the right direction.”

Enany also noted that Mann, who serves as chairman emeritus of the SoCalBio trade group according to its website, had both philanthropic and business projects in the works, so the move would allow him to put his time into those other pursuits.

Mann was the Business Journal´s Los Angeles Business Person of the Year in 2003, while Kresa received the honor in 2002.

Mann and Kresa were both unavailable for interviews.