PANCREATIC cancer is a dreadful disease. Even in rich countries, only about 4% of those diagnosed with it are still alive after five years. In America it is the third-most-common cause of cancer deaths among women, after lung and breast cancer; among men it is fourth, after lung, prostate and colorectal cancer. Dispiritingly, there has been little progress in treating it for more than a quarter of a century.

The reason pancreatic cancer is so deadly is that it metastasises quickly. This spreading of secondary tumours around the body damages other organs and has proved impossible to stop. But a group of researchers led by Claudia Gravekamp of the Albert Einstein College of Medicine, in New York, have an intriguing idea for changing that. As they describe in a paper in the Proceedings of the National Academy of Sciences, they plan to do it by infecting people with radioactive bacteria.

Dr Gravekamp came up with the idea following her discovery that weakened bacteria (specifically, a modified form of Listeria monocytogenes) she was using for other purposes in tumour-afflicted mice, and which were cleared from most of the animal’s body by its immune system over the course of a few days, remained in the tumours. This was thanks to the ability cancer cells evolve to suppress the immune system’s activities within their purview. Without that talent, cancers themselves would find they were under fatal assault from the immune system.

Dr Gravekamp and her colleagues therefore thought that their bacteria might be used to deliver anti-cancer agents specifically to tumours. At first, they considered using drugs. But on the advice of Ekaterina Dadachova, a radiologist and one of the paper’s authors, they eventually plumped for attaching a radioactive isotope of rhenium to their bugs.

Radioactive rhenium is commonly used in conventional radiotherapy. Dr Gravekamp’s hope was that, because the bugs stick around in the cancer, they would provide a novel way to solve radiotherapy’s biggest problem: ensuring that the tumour itself is zapped hard while minimising the amount of radiation hitting surrounding healthy tissue.

And that, more or less, is what happened. The bacteria tolerated their radioactive payloads with little complaint. In mice with pancreatic cancer, a course of several treatments with them killed off around 90% of metastasised tumours and made a notable dent in the original tumour as well. Admittedly, the technique’s aim was not perfect. The animals’ livers and kidneys, in particular, received high doses of radiation—higher, in fact, than those endured by the tumours. Dr Dadachova thinks this is because, as the immune system killed the Listeria, the radioactive remains of the bacteria were shipped to the liver to be broken down and recycled, while anything unusable was collected by the kidneys for excretion.

Fortunately this may not matter, for the high doses of radiation the liver and kidneys received did not seem to do them much damage. The tumours, by contrast, were fatally afflicted. That disparity is probably because of the way in which radiation kills. Its main effect is to break DNA molecules. Enough of these breaks overwhelm a cell’s ability to repair its DNA, causing it to die. The faster a cell is dividing, the more susceptible it is to this sort of damage—and a cancer is nothing more than a mass of cells that are dividing rapidly and uncontrollably.

As far as the researchers could divine, their proposed treatment causes few side-effects. In clinical trials in other contexts, people experimentally infected with a non-radioactive form of the weakened Listeria reported only mild and transient flu-like symptoms. Dr Dadachova’s mice, which got the full-strength, glow-in-the-dark version of the bacteria, seemed similarly unbothered.

Of course, effectiveness in mice is not the same as effectiveness in people. One quibble is that the number of mice given each variant of the treatment was small (just five), though the researchers argue that the strength of the effect they observed is enough to overcome worries about such sample sizes.

Dr Gravekamp and her colleagues are now trying to improve their treatment in mice, and are seeking sponsors for a trial in people. If that works, then three negatives (an infectious bacterium, a radioactive isotope and a nasty cancer) will have come together to produce a positive outcome—a piece of medical algebra that defies conventional mathematics.