New York, Apr 3 (IANS): Researchers have found a possible explanation for why many cancer drugs that kill tumour cells in mouse models won't work in human trials.

In the study, published in the journal Nature Communications, researchers reported the extensive presence of mouse viruses in patient-derived xenografts (PDX).

PDX models are developed by implanting human tumour tissues in immune-deficient mice, and are commonly used to help test and develop cancer drugs.

"What we found is that when you put a human tumour in a mouse, that tumour is not the same as the tumour that was in the cancer patient," said researcher W. Jim Zheng from the University of Texas.

"The majority of tumours we tested were compromised by mouse viruses," Zheng added.

Using a data-driven approach, the researchers analyzed 184 data sets generated from sequencing PDX samples. Of the 184 samples, 170 showed the presence of mouse viruses.

The infection is associated with significant changes in tumors, and the researcher said that could affect PDX as a drug testing model for humans.

"When scientists are looking for a way to kill a tumour using the PDX model, they assume the tumour in the mouse is the same as cancer patients, but they are not," Zheng said.

"It makes the results of a cancer drug look promising when you think the medication kills the tumour -- but in reality, it will not work in human trial, as the medication kills the virus-compromised tumor in mouse," Zheng added.