A debilitating side effect of a widely used but harshly potent treatment for colon cancer could be eliminated if a promising new laboratory discovery bears fruit.
The pre-clinical finding, published in the Nov. 5, 2010, issue of the journal Science, relates to the drug CPT-11, or Irinotecan, a chemotherapeutic agent used against colon cancer and other solid malignancies. It is believed to be the first successful targeting of an enzyme in symbiotic bacteria found in the digestive system.
While it has proven a valuable tool for attacking tumors, CPT-11 can also cause severe diarrhea, which limits the dosage that patients can tolerate, curbing the drug's potential effectiveness.
Now, a team of researchers led by Matthew R. Redinbo, Ph.D., from the University of North Carolina at Chapel Hill, has discovered it is possible to target and block the enzyme, beta glucuronidase, which is thought to play a major role in the gastric side effects.
First, the scientists had to overcome a major hurdle: the culprit enzymes are found in microbes in the gut that play a major role in human health, so eliminating the anticancer drug's toxicity without making things worse for patients was a real challenge, Redinbo said.
"We need to retain our intestinal bacteria - they help us digest food, make critical vitamins and protect us from infection," said Redinbo, professor and chair of the chemistry department in the UNC College of Arts and Sciences and a member of the UNC Lineberger Comprehensive Cancer Center. "This targeted approach stops the one bacterial protein thought to cause the drug's devastating side effect, but without damaging the beneficial microbes or the intestines."
The trouble with CPT-11 begins part way through the body's process of excreting the drug, presumably after it has already killed tumor cells and been rendered inert. When it reaches the intestines, beta glucuronidase enzymes in the gut bacteria snip a sugar off the inactivated cancer drug, essentially reactivating it. The revived drug then causes tissue damage, which triggers the diarrhea seen as a side effect in the vast majority of patients who receive it, with up to 30 percent suffering severe diarrhea.
"In a manner of speaking, we cured the bacteria's sweet tooth without damaging the microbes or intestines and, in the process, the toxic side effect was alleviated," Redinbo said.
Instead of changing the makeup of the drug itself, Redinbo decided to take a new tack. Working with researchers at North Carolina Central University in Durham and the Albert Einstein College of Medicine in New York, the UNC team looked for chemical compounds that would latch on to and block the action of the beta glucuronidase enzymes. From a database of more than 10,000 compounds, they narrowed the field to four prime candidates, then tested them in the laboratory on several types of bacterial cells.
The result: all four compounds worked - the enzymes remained dormant and the cells were unscathed. Scientists also tested one of the inhibitors in mice. Again, it proved successful - animals treated with CPT-11 that also received the compound had significantly less diarrhea than those that only received the cancer drug.
"With further development, this approach could improve anticancer drug efficacy and tolerance," Redinbo said. "In general, though, this also shows that specific bacterial proteins can be selectively targeted without killing these health-promoting microbial symbiotes."
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