A common medication used to treat reflux, heartburn and ulcers could lessen the effectiveness of lung cancer immunotherapy drugs, according to new research.

Researchers at Flinders University in Australia investigated the impact of proton pump inhibitors (PPIs) on patients undergoing treatment for non-small cell lung cancer, the most common type of lung cancer, accounting for 85 per cent of cases. Patients received either chemotherapy or were treated with a combination of chemotherapy and atezolizumab, an immune checkpoint inhibitor drug, designed to boost the immune system into killing cancer cells.

The results published in Nature's British Journal of Cancer showed that PPI use was associated with worse survival in patients with advanced cancer treated with atezolizumab plus chemotherapy, but not in those that received chemotherapy alone, with the study showing PPI use was linked to a significant decrease in the benefit of the immune therapy treatment.

"Stomach issues and reflux are common in cancer patients so the use of antacids and PPIs is common. Nearly 30 per cent of cancer patients use them, and usually for extended periods of time," said lead author Ash Hopkins from the Flinders Health and Medical Research Institute.

"Of concern is that the medication is often overused or used inappropriately, as it is seen to cause little harm, however, our research could indicate a need to change this approach," Hopkins added.

PPIs treat a number of stomach issues by reducing acid production in the wall of the stomach. Recent studies have shown the medication can cause significant gut microbiota changes, which could lead to its impact on cancer immunotherapy.

"Immune checkpoint inhibitor (ICI) drugs help the immune system by switching on T-cells, allowing them to kill or control cancerous tumours but the gut microbiota also plays an important role in regulating our body and its immune function," said Hopkins.

"When this gut microbiota is impacted it can stop the ability of ICIs to activate the immune system, meaning the drugs simply won't work as well to fight off cancer."

While further studies are needed, the researchers noted it could be time for oncologists to reconsider the indiscriminate use of PPIs for their patients.