Billionaire industrialist, donor and political activist David Koch will retire from his roles at Koch Industries and other Koch-affiliated groups due to health reasons, according to an internal memo obtained by CNBC on Tuesday. Koch, 78, and his older brother, Charles , are the key drivers behind a juggernaut political network that has often favored conservative and libertarian politicians and policies, such as tax cuts. His political legacy includes activism in support of free-market initiatives such as eliminating trade barriers between countries and easing regulations on industry.

In the memo to Koch Industries employees, Charles Koch said his brother's health has continued to deteriorate.

"His guidance and loyalty, especially in our most troubled times, has been unwavering," the 82-year-old brother wrote.

David Koch also received well wishes from a top executive in the brothers' political-activist network.

"Due to health reasons, David Koch will be resigning from the board of Americans for Prosperity Foundation," said Mark Holden, co-chairman of the Koch-backed Seminar Network. "We greatly appreciate his vital role on the board and all that he has done to help us build a strong foundation for our future success. We wish him and his family well."

Koch was diagnosed with prostate cancer more than two decades ago. Over the years, he and his family foundation, along with other recipients of Koch philanthropic support, have donated millions to battling cancer. He is a board member of the Prostate Cancer Foundation.

He's also been known as a patron of the arts around the New York City area. He gave $100 million to the New York State Theater at Lincoln Center, now known as the David H. Koch Theater. He also donated $65 million to renovate the plaza at the Metropolitan Museum of Art.

The announcement comes a day after the Koch-backed political network unveiled a multimillion-dollar campaign against President Donald Trump's tariffs. The multiyear campaign will include media buys, activist education, grass-roots mobilization, lobbying and policy analysis.



The network has also recently pivoted from focusing its efforts on supporting candidates to legislative initiatives, including protecting recipients of the Deferred Action Childhood Arrivals program and cutting back on government spending.



The Koch network has recently raised eyebrows for its outreach to Democrats on some issues. Last week, Americans for Prosperity published a digital ad thanking Sen. Heidi Heitkamp, D-N.D., for co-sponsoring a bank deregulation bill that eased regulations on mid-sized banks.

Read Charles Koch's memo to employees here: