The family of Presbyterian pastor and author Tim Keller are asking people to pray for him as he continues to receive treatment for stage 4 pancreatic cancer. 

Michael Keller, his son, shared in a social media post on Monday that his father is struggling with the side effects of an immunotherapy treatment. 

"We ask that you continue to pray for Dad's healing and for the decisions of his doctors and medical team who continue to provide excellent, compassionate care," he wrote.

"Thank you for the outpouring of love and support. We deeply covet your prayers at this time."

Keller, founder of Redeemer Presbyterian Church in New York City, was diagnosed with cancer for the second time in May 2020. 

He previously survived thyroid cancer in 2002. 

Since his 2020 diagnosis, he has been undergoing rounds of chemotherapy and immunotherapy. 

He announced last month that he would be taking part in an immunotherapy trial at the National Cancer Center in Bethesda, Maryland, starting from 1 June.

"This has shown great promise in potentially curing cancer, though it is a rigorous and demanding month-long program (that will need updates up to six months)," he said at the time. 

Keller has regularly updated supporters on his treatment and shared openly about how he is leaning on his faith.

In May, he said that reaching the second anniversary of his diagnosis felt like a "celebration with justification as the chemotherapies have reduced the stage 4 cancer that was found and God has seen it fit to give me more time".