David Koch is retiring from his post at Koch Industries because of deteriorating health, Koch Industries chairman and CEO Charles Koch said in a letter to employees Tuesday morning.

David Koch, 78, is an executive vice president and board member of Koch Industries, a political activist through his involvement with Americans for Prosperity as well as a medical and arts philanthropist, donating hundreds of millions of dollars over the years to hospitals, theaters and museums.

David's brother Charles said in the letter to employees that David's "health has continued to deteriorate. As a result, he is unable to be involved in business and other organizational activities."

"Because of this, David will be retiring from his responsibilities at Koch and other organizations."

In addition to his role at Koch Industries, David Koch also was chairman and CEO of Koch Chemical Technology Group, a Koch Industries subsidiary.

"We are deeply saddened by this, as we miss David's insightful questions and his many contributions to Koch Industries," Charles Koch said in the letter.

A Koch spokeswoman said in an e-mail "no decisions have been made" in filling David Koch's roles.

In Wichita, Koch Industries and its subsidiaries employ more than 3,200 people. The privately held company has 120,000 employees worldwide.

Koch's retirement extends to his roles at dozens of nonprofit organizations, the company confirmed Tuesday. Those organizations include the Lincoln Center for the Performing Arts and the Metropolitan Museum of Arts. He also was a founding member of Americans for Prosperity Foundation, an advocacy group for lower taxes and less government.

"AFP-Kansas Foundation wishes David and his family well," Jeff Glendening, AFP Foundation Kansas state director, said in an e-mail to the Eagle. "His role on the board of the Americans for Prosperity Foundation has been vital in building a firm foundation for our organization's success. We hope to build on that foundation and continue being Kansas' leading grassroots advocate for policies that advance a society of mutual benefit."

Twenty-five years ago, David Koch was diagnosed with and survived prostate cancer and since then has directed some of his philanthropy to cancer research and treatment, including donating $150 million in 2015 to Memorial Sloan Kettering in New York for the David H. Koch Center for Cancer Care, an outpatient medical facility.

"David has always been a fighter and is dealing with this challenge in the same way," Charles Koch said in the letter.

David Koch resides in New York City with his wife, Julia, and children David Jr., Mary and John.