The findings of the study were published in the journal 'Nature Communications'.

As tumours grow within an organ, they also release cells that enter the bloodstream. These cells can travel to other organs, seeding new tumours called metastases.

MIT engineers have now developed a technique that, for the first time, allows them to measure the generation rate of these circulating tumour cells (CTCs) in mice.

Their approach, which also reveals how long CTCs survive once released into the bloodstream, could help scientists learn more about how different types of cancers spread through the body.

"By exchanging blood between mice while counting CTCs in real-time, we obtained a direct measurement of how quickly CTCs enter the circulation and how long it takes before they're cleared," said Scott Manalis, the David H Koch Professor of Engineering in the departments of Biological Engineering and Mechanical Engineering, a member of the Koch Institute for Integrative Cancer Research, and the senior author of the study.

Using their new system, the researchers were able to study CTCs from pancreatic tumours as well as two types of lung tumours.

Graduate student Alex Miller and Bashar Hamza PhD '20, a Koch Institute visiting scientist, are the lead authors of the paper, which appears today in Nature Communications.

Circulating tumour cells are rare in patients: One millilitre of blood might contain between one and 10 such cells. In recent years, researchers have devised strategies to capture these elusive cells, which can yield a great deal of information about a patient's tumour, and even help doctors track how a tumour is responding to treatment.

"Circulating tumour cells are attractive because you can get them from blood and they provide a window into the tumour. It's a lot easier than biopsying the tumour," Manalis said.

In mice, CTCs are even more difficult to find because mice only have a little more than one millilitre of blood. Being able to study CTCs in mice could help researchers answer many outstanding questions about how rapidly these cells are shed by tumours, how long they survive in circulation, and how efficiently they seed new tumours, Manalis said.

To try to answer some of those questions, Manalis and his students designed a system that lets them remove blood from a mouse with a tumour and flow it into a healthy mouse.

Through a separate tube, blood from the healthy mouse flows back to the tumour-bearing mouse. The system includes two cell counters (one for each mouse) that detect and remove circulating tumour cells from the blood.

Using this setup, the researchers can analyse all of the blood from each mouse in less than an hour. After determining the concentration of CTCs in the bloodstream of the tumour-bearing mouse and of the healthy mouse, they can calculate the rate at which CTCs are generated in the tumour-bearing mouse. They can also calculate the half-life of the cells -- a measure of how long they survive in the bloodstream before being cleared by the body.

Working with members of the Jacks lab in the Koch Institute, the researchers used the system to study mice with three different types of tumours: pancreatic cancer, small cell lung cancer, and non-small cell lung cancer.

They found that the half-life of CTCs was fairly similar between the three types of tumours, with values ranging from 40 seconds to about 250 seconds. However, the generation rates showed much more variability between different tumour types.

Small cell lung tumours, which are known to be aggressively metastatic, could shed more than 100,000 CTCs per hour, while non-small cell lung tumours and pancreatic tumours shed as few as 60 CTCs per hour.

Previous studies that relied on injecting tumour cells from cell lines cultivated in the lab have found that those cells had a half-life of only a few seconds in the bloodstream, but the new results from Manalis' lab suggest that endogenous CTCs actually persist much longer than that.

The researchers also showed that the healthy mice that received CTCs later developed metastases, even after only exchanging a few thousand CTCs.

They found that CTCs from small-cell lung tumours formed metastases in the livers of the recipient healthy mice, just as they did in the mice where the tumours originally formed.

"What we realised was that these CTCs that we're injecting into the healthy recipient mouse start to grow and create metastases that we can detect after a couple of months," Hamza said.

"That was exciting to observe because it validated that our blood-exchange technique can also be used to gently inject a viable CTC sample in its native blood environment without having to enrich it using harsh in vitro techniques," Hamza added.

Using this approach, researchers now hope to study how different drug treatments influence CTC levels. "With this system, we can look at the real-time concentration of CTCs, so we can perform a drug treatment and look at how it is affecting half-life time and generation rate," Miller said.

The researchers also plan to study other types of cancers, including blood cancers such as leukaemias and lymphomas, using this system. The technique could also be used to study the circulation dynamics of other kinds of cells, including immune cells such as neutrophils and natural killer cells.

The research was funded by the Virginia and D.K. Ludwig Fund for Cancer Research, the Cancer Systems Biology Consortium, the National Cancer Institute, the Pew-Stewart Scholars Program for Cancer Research, a Sloan Fellowship in Chemistry, and the National Institutes of Health.