People with cancer that starts on the left side of their colon live significantly longer than those with right-side tumors, according to a new study that provides insights into how best to match drugs to patients with advanced disease.

The retrospective analysis, released Wednesday, involved a federally funded clinical trial with more than 1,100 colon-cancer patients. Overall, it found that those with left-side tumors survived for a median of 33.3  months, while those with right-side tumors survived for 19.4 months.

Lead author Alan Venook, a professor of medicine at the University of California at San Francisco, said previous studies had suggested tumor location played a role in patients’ survival, but the effect shown by the new study was greater than he expected.

The data also turned up a striking difference in drug efficacy depending on the location of the tumors. Two therapies were used in the trial, in combination with chemotherapy, to see whether one worked better than the other. The answer was no, based on initial results.

But when the data was evaluated based on a tumor’s location, things changed. Avastin was associated with longer survival times in cases of right-side tumors, while Erbitux was linked to longer survival with left-side malignancies.

Specifically, patients with right-side tumors who were given Avastin survived for 24.2 months compared with 16.7 months for those treated with Erbitux. Patients with left-side tumors treated with Erbitux survived 36 months compared with 31.4 months for those treated with Avastin.

Venook said researchers now are looking for the underlying biological reasons for these findings. He noted that the left and right sides of the colon arise from different embryonic tissues and have different characteristics.

“Are there more bad actors on the right side?” he said, adding that the difference between right- and left-side tumors could be “a surrogate for some bio-marker” that scientists have yet to find.

Richard Kim, an associate professor in the department of gastrointestinal oncology at Moffitt Cancer Center in Tampa, said he wasn’t surprised by the results because earlier studies had pointed to similar, if less definitive, conclusions. But Kim, who wasn’t involved in the latest research, said it was still important because it retrospectively analyzed such a large group of patients. The findings underscore how different the two sides of the colon are and how tumors probably need to be treated differently depending on their location.

Once the molecular reasons for that are determined, he said, physicians can apply the information in their choice of cancer drugs.

The colon, also called the large intestine, is a strong, long muscle critical for the last phase of digestion. Its right side includes the cecum and the ascending colon, while the left side is made up of the descending colon, the sigmoid colon and the rectum.

Venook said that the results might not apply to patients who didn’t have advanced cancer.

The findings will be presented as an abstract at a meeting of the American Society of Clinical Oncology next month.

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