According to new research from the University of Michigan Rogel Cancer Center, high levels of ammonia in tumours cause a decrease in T cells and immunotherapy resistance in colorectal cancer experimental models.

The researchers discovered, ammonia inhibits the growth and function of T cells, which are essential for anti-tumour immunity. The findings were published in Cell Metabolism.

"We identified the mechanism of how ammonia dysregulates T cell function and showed that reducing ammonia levels using FDA-approved drugs for hyperammonemia can reduce tumour size in several different models including metastatic colorectal cancer," says Hannah Bell, PhD, a postdoctoral fellow in cancer biology and author on this paper.

"Use of this drug also synergises with immunotherapy. If you treat the experimental model with immunotherapy when you also treat them with this ammonia-reducing agent, you're able to sensitise the tumours to treatment."

"Most colorectal cancers are insensitive to immune therapies," adds Yatrik Shah, PhD, Horace W. Davenport Collegiate Professor of Physiology and first author of the study. "We've found that one of the mechanisms that lead to this resistance is likely the high level of ammonia that accumulates in the microenvironment."

Bell explains that while there are many factors that contribute to immunotherapy resistance, there are few therapeutic interventions that can reactivate therapy sensitivity.

"Our study shows that this is a relatively safe and FDA-approved method that could work alongside immunotherapy to make treatment more effective for patients. This new method provides a potentially direct avenue to treat tumours and reactivate the immune system."

How does ammonia accumulate in colorectal tumours in the first place? Ammonia levels are regulated by a balance of production and cellular detoxification.

Most ammonia is generated by the microbiota, but Bell and Shah's work suggests that increased production of ammonia is not what results in accumulation. "Our work demonstrates that tumours have lost the ability to detoxify ammonia leading to build-up," said Shah.

Further, the accumulation of ammonia is likely not isolated to just colorectal tumours. Shah says this discovery may open doors in explaining resistance to other cancer types as well.

"Only about 20-30% of all cancer patients are sensitive to immunotherapy. 70% of patients don't derive any benefit from it," Shah said. "Now, we have a mechanism that could explain this resistance in tumours beyond colon cancer."