One of the most interesting emerging treatments for certain types of cancer aims to starve the tumour to death. The strategy involves destroying or blocking the blood vessels that supply a tumour with oxygen and nutrients. Without its lifeblood, the unwanted growth shrivels up and dies.

One way to do this is with drugs called angiogenesis inhibitors which prevent the formation of new blood vessels that tumours rely on for sustenance. But there is another approach as well — physically blocking the surrounding blood vessels so that blood can no longer flow into the tumour.

Researchers have experimented with a number of blocking mechanisms such as blood clots, gels, balloons, glue, nanoparticles and so on. However, these techniques have never been entirely successful because the blockages can be washed away by the blood flow itself and the materials do not always fill blood vessels entirely, allowing blood to flow round them.

Today, Qian Wang and a couple of pals at Tsinghua University in Beijing propose a different approach. These guys say that it is possible to block blood vessels completely by filling them with liquid metal. And they have tested their idea both in vitro and in vivo on mice and rabbits to get a sense of how well it might work. (All their experiments were approved by their university’s ethics committee.)

The team have experimented with two liquid metals— pure gallium and which melts at around 29 degrees centigrade and a gallium indium alloy with a slightly higher melting point. Both are liquid at body temperature.

Qian and co first tested the cytotoxicity of gallium and indium by allowing cells to grow in its presence and measuring the number that survive after 48 hours. If more than 75 per cent, a substance is deemed safe by China’s national standards.

After 48 hours just over 75 percent of cells in both samples were still alive unlike those grown in the presence of copper which almost all die. Indeed, that corresponds with other studies that suggest that gallium and indium are relatively benign in biomedical situations.

The team then measured how well liquid gallium could spread through a vascular system by injecting it into a pig kidney and into a recently euthanised mouse. X-ray images clearly show how well liquid metal spreads through organs and indeed an entire body.

One potential problem is that the structure of blood vessels in tumours may differ from those in normal tissues. So the team also injected the alloy into a breast cancer tumour grown on the back of a mouse, showing that it could indeed fill the blood vessels in a tumour.

Finally, the team tested how well liquid metal works in cutting off the blood supply around the blood vessels it fills. They did this by injecting the liquid metal into a rabbit’s ear, using the other ear as a control.

They say the tissue around the injected ear began to die after about seven days and after about three weeks, the ear tip had the appearance of “a dry leaf”.

Qian and co are optimistic about their approach. “Body temperature liquid metal yields a promising injectable tumour treatment,” they say. (Incidentally, we reported the same group’s work on injecting liquid metal into a heart earlier this year.)

And the technique allows for other approaches as well. For example, liquid metal is a conductor which raises the possibility of using electric current to heat up and destroy surrounding tissue. The metal can also carry nanoparticles containing drugs which then diffuse into nearby tissue once the metal has settled around the tumour. The possibilities are numerous.

The experiments also reveal a number of potential problems, however. X-rays of the rabbit they injected clearly show that blobs of liquid metal found their way to the animal’s heart and lungs.

This may be the result of injecting the metal into veins rather than arteries since the blood from arteries flows into capillaries, whereas blood from veins flows out of capillaries and around the body. So injections into veins are riskier.

What’s more, their experiments also show blood vessel growth around the blocked arteries, revealing how quickly the body adapts to blockages.

One thing’s for sure, the risks associated with this type of treatment need to be carefully assessed and mitigating strategies developed. For example, the spread of liquid metal around the body could be reduced by slowing the flow of blood during the treatment, changing the melting point of the metal so that it solidifies once in place, pinching the arteries and veins around the tumour while the metal settles, and so on.

These risk will also have to be compared with those associated with other approaches. And more than anything else, of course, researchers will have to work out whether it can really help to kill tumours effectively

That will take considerable time, money and effort. Nevertheless, this is an interesting, innovative approach that must surely be worth further investigation given the huge challenge medics face in tackling the epidemic of cancer in modern society.

Ref: arxiv.org/abs/1408.0989 : Delivery of Liquid Metal to the Target Vessels as Vascular Embolic Agent to Starve Diseased Tissues or Tumors to Death