Southfield, Mich.—Alan Kalter, chairman-CEO of advertising agency Doner, announced plans to leave the company after 43 years. The news was announced in a memo sent to employees on Thursday. Kalter said he will be selling the agency to Tim Blett, president; David DeMuth, chief operating officer; and Rob Strasberg, chief creative officer. Financial terms were undisclosed. In the memo, the remaining executives said they will be working on an agency rebranding, a new Web site and more aggressive new business development.