Pet dogs have helped researchers show that special bacteria can seemingly fight cancer, causing tumours to shrink.
A modified version of Clostridium novyi bacteria, when injected into solid soft tissue tumours, will eat away at the cancerous cells without harming surrounding healthy tissue, researchers report in the latest Science Translational Medicine.
Using bacteria as treatment
Researchers injected C. novyi spores into 16 pet dogs being treated for naturally occurring tumours. The bacteria caused an anti-tumour response in six of the dogs within three weeks, researchers report.
The bacteria caused complete eradication of the tumour in three of the six dogs, while the other three showed tumour shrinkage of at least 30 percent.
The C. novyi bacteria also worked well in rats implanted with brain tumour cells.
"When we treated those tumours, we found that C. novyi was able to germinate inside the tumour while sparing the normal brain tissue," said co-author Dr. Verena Staedtke, a fellow at the Johns Hopkins Kimmel Cancer Centre in Baltimore.
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The treatment killed tumour cells but spared healthy cells just a few micrometers away. It also prolonged the rats' survival, with treated rodents surviving an average of 33 days after the tumour was implanted, compared with an average of 18 days in rats that did not receive the bacteria.
Treatment for humans
Based on these findings, researchers have begun phase I human trials using the bacteria at multiple sites across the United States.
In one example, a patient at MD Anderson Cancer Centre in Houston received a spore injection directly into an advanced-stage tumour in her shoulder, and experienced significant shrinkage of the tumour in and around the bone, the researchers reported.
"Dog tumours resemble human tumours in many ways," said study lead author Nicholas Roberts, also a fellow at the Kimmel Cancer Centre. "They're treated with many of the same drugs as humans, and they experience the same toxicities. That was the rationale for treating pet dogs in this study."
Bacteria as a cancer fighter
The idea of using bacteria to fight cancer has been around for more than a century, when early cancer researchers found that the presence of certain bacteria appeared to limit tumour development, said senior author Dr. Shibin Zhou, director of experimental therapeutics at the Kimmel Cancer Centre's Ludwig Centre for Cancer Genetics and Therapeutics.
Bacteria that are anaerobic – thriving in oxygen-depleted environments – can serve as an effective means of destroying oxygen-starved cells deep inside solid tumours. Chemotherapy and radiation therapy are less effective against these oxygen-starved cancer cells, Zhou said.
Bacteria may be toxic
Up until now, the problem has been that most bacteria effective against cancer also can do great harm to patients. "Bacteria are very toxic, and those toxins are left behind and can cause problems for the patient," said Greg Adams, director of biological research and therapeutics for Fox Chase Cancer Centre in Philadelphia.
The researchers created a safer version of the C. novyi bacteria by removing a toxin-producing gene.
The dogs and rats treated with C. novyi experienced side effects typical of a bacterial infection – fever, inflammation and discharge from the bacteria-created abscess inside the tumour.
"Those side effects are typically very well-tolerated and managed in this study," Roberts said.
And because C. novyi is anaerobic, it didn't appear able to spread into the oxygen-rich healthy tissues outside the tumour, researchers found.
A new cancer therapy?
If these results pan out in humans – and scientists note that animal research often fails to provide similar results in humans – bacterial treatment of tumours could be a promising new cancer therapy, Adams said. But researchers will probably need to show that the bacterial infection also triggers the immune system to attack the cancer, he said.
"For this to reach the big-time, you need to be able to trigger the immune response with this treatment," he said. Chemotherapy and radiation therapy are effective partly because they draw the immune system into the fight, he noted.
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However, Adams said he doesn't think the therapy would be as useful if a person has multiple tumours, or has tumours in locations where an infection could do more harm than good.
"The thought of building an abscess in the brain is scary for me," he said. "I'm not sure how you would manage that."
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