Identifying & Overcoming Taxane Drug Resistance

Proteomics study reveals a protein that, when suppressed, makes cancers more susceptible to chemotherapy involving taxane drugs.

Taxanes, a group of cancer drugs that includes paclitaxel (Taxol®) and docetaxel (Taxotere®), have become front-line therapy for a variety of metastatic cancers. But as with many chemotherapy agents, resistance can develop, a frequent problem in breast, ovarian, prostate and other cancers. Now, cancer researchers at Children’s Hospital Boston report a protein previously unknown to be involved in taxane resistance and could potentially be targeted with drugs, making a cancer more susceptible to chemotherapy.

The researchers believe that this protein, prohibitin1, could also serve as a biomarker, allowing doctors to predict a patient’s response to chemotherapy with a simple blood test. The study was published online by the Proceedings of the National Academy of Sciences in its online early edition during the week of January 25.

Bruce Zetter, Ph.D., Charles Nowiszewski Professor of Cancer Biology, Vascular Biology Program, Department of General Surgery, Children's Hospital Boston

The study, led by Bruce Zetter, PhD, of Children’s Vascular Biology Program, used proteomics techniques to compare the proteins present in Taxol-susceptible versus Taxol-resistant human tumor cell lines. The researchers found that the resistant cell lines, but not the susceptible cell lines, had prohibitin1 on their surface. When they suppressed prohibitin1 with RNA interference techniques, the tumor cells became more susceptible to Taxol, both in cell culture and in live mice with implanted Taxol-resistant tumors.

Zetter’s lab is still investigating why having prohibitin1 on the cell surface makes a tumor cell resistant to taxanes. But in the meantime, he believes that not only could prohibitin1 be suppressed to overcome taxane resistance, but that it could also be exploited as a means of targeting chemotherapy selectively to resistant cancer cells.

“We are working to target various cancer drugs to taxane-resistant cells by attaching them to compounds that bind to prohibitin,” Zetter explains. One such compound is already known, and works well in animals to target other prohibitin-rich cells, but has yet to be tested in humans.

Suppressing prohibitin1 alone probably isn’t enough to make a cancer fully Taxol-susceptible, but could be combined with other strategies aimed at increasing taxane susceptibility, such as targeting another protein called GST Pi, the researchers say. Other mechanisms of resistance are known, but they so far haven’t been shown to present effective targets for therapy.

Zetter’s lab is also trying to develop prohibitin1 as a biomarker for taxane resistance that physicians could use in the clinic. Since it’s on the surface of the cell, Zetter believes prohibitin1 may circulate in the blood where it could easily be detected. His lab is in talks with several cancer centers to obtain serum samples from patients who did and didn’t respond to Taxol, so that prohibitin1 levels could be measured and compared.

Zetter notes that prohibitin1 could easily have been overlooked, and was found only because the team happened to look specifically at proteins in the cell membrane, rather than simply doing a whole-cell proteomic analysis.

“The interesting finding was that prohibitin was not just another over-expressed protein,” Zetter says. “It was up-regulated primarily on the cell surface. When we looked at the whole cell, the absolute amount of prohibitin wasn’t changed. Instead, prohibitin was moving from the inside of the cell to the cell surface. It had shifted from one location to another, and when it did, the tumor cells became resistant to taxanes. The fact that it moves to the cell surface also makes it easier to direct drugs to it.”

Children’s Hospital Boston has pending and issued international patents on this technology.  Nish Patel, PhD, was the study’s first author. The study was funded by a grant from the National Institutes of Health.

About Children’s Hospital Boston

Founded in 1869 as a 20-bed hospital for children, Children’s Hospital Boston today is one of the nation’s leading pediatric medical centers, the primary pediatric teaching hospital of Harvard Medical School, and the largest provider of health care to Massachusetts children. In addition to 396 pediatric and adolescent inpatient beds and more than 100 outpatient programs, Children’s houses the world’s largest research enterprise based at a pediatric medical center, where its discoveries benefit both children and adults. More than 500 scientists, including eight members of the National Academy of Sciences, 11 members of the Institute of Medicine and 13 members of the Howard Hughes Medical Institute comprise Children’s research community. For more information about the hospital visit: www.childrenshospital.org/newsroom.

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New Study Shows Four-Year Window for Early Detection of Ovarian Cancer

A new study by Howard Hughes Medical Institute researchers shows that most early stage ovarian tumors exist for years at a size that is a thousand times smaller than existing tests can detect reliably.  But the researchers say their findings also point to new opportunities for detecting ovarian cancer—a roughly four-year window during which most tumors are big enough to be seen with a microscope, but have not yet spread.

Tiny Early-Stage Ovarian Tumors Define Early Detection Challenge

Currently available tests detect ovarian cancer when it is about the size of the onion in the photograph. To reduce ovarian cancer mortality by 50 percent, an early detection test would need to be able to reliably detect tumors the size of the peppercorn. (Photo Source:  Patrick O. Brown, Howard Hughes Medical Institute Investigator, Research News Release, July 28, 2009)

Currently available tests detect ovarian cancer when it is about the size of the onion in the photograph. To reduce ovarian cancer mortality by 50 percent, an early detection test would need to be able to reliably detect tumors the size of the peppercorn. (Photo Source: Patrick O. Brown, Howard Hughes Medical Institute Investigator, Research News Release, July 28, 2009)

A new study by Howard Hughes Medical Institute researchers shows that most early stage ovarian tumors exist for years at a size that is a thousand times smaller than existing tests can detect reliably.

But the researchers say their findings also point to new opportunities for detecting ovarian cancer—a roughly four-year window during which most tumors are big enough to be seen with a microscope, but have not yet spread.

“Our work provides a picture of the early events in the life of an ovarian tumor, before the patient knows it’s there,” says Howard Hughes Medical Institute researcher Patrick O. Brown. “It shows that there is a long window of opportunity for potentially life-saving early detection of this disease, but that the tumor spreads while it is still much too small to be detected by any of the tests that have been developed or proposed to date.”

According to the American Cancer Society, some 15,000 women in the United States and 140,000 women worldwide die from ovarian cancer each year. The vast majority of these deaths are from cancers of the serous type, which are usually discovered only after the cancer has spread.

“Instead of typically detecting these cancers at a very advanced stage, detecting them at an early stage would be enormous in terms of saving lives,” says Brown, who is at Stanford University School of Medicine. Early detection would enable surgeons to remove a tumor before it spreads, he adds.

The article—co-authored by Chana Palmer of the Canary Foundation, a nonprofit organization focused on early cancer detection—was published July 28, 2009, in the open access journal PLoS Medicine.

“Like almost everything with cancer … the more closely you look at the problem, the harder it looks,” Brown says. “That’s not to say that I don’t believe it’s a solvable problem. It’s just a difficult one.” — Patrick O. Brown, M.D. Ph.D.

Patrick O. Brown, M.D. Ph.D., Howard Hughes Medical Institute Investigator, Stanford University School of Medicine

Patrick O. Brown, M.D. Ph.D., Howard Hughes Medical Institute Investigator, Stanford Univ. School of Medicine

“Like almost everything with cancer … the more closely you look at the problem, the harder it looks,” Brown says. “That’s not to say that I don’t believe it’s a solvable problem. It’s just a difficult one.”

In the quest to develop early detection methods for ovarian cancer, Brown says, science hasn’t had a firm grasp on its target. So he and Palmer took advantage of published data on ovarian tumors to generate a better understanding of how the cancer progresses in its earliest stages.

The team analyzed data on serous-type ovarian tumors that were discovered when apparently healthy women at high genetic [BRCA1 gene mutation] risk for ovarian cancer had their ovaries and fallopian tubes removed prophylactically. Most of the tumors were microscopic in size; they were not detected when the excised tissue was examined with the naked eye.

The analysis uncovered a wealth of unexplored information. Thirty-seven of the early tumors had been precisely measured when they were excised – providing new details about the size of the tumors when they were developing prior to intervention, Brown says. By extrapolating from this “occult” size distribution to the size distribution of larger, clinically evident tumors, the researchers were able to develop a model of how the tumors grew and progressed. “We are essentially trying to build a story for how these tumors progress that fits the data,” Brown explains.

Among the study’s findings:

  • Serous ovarian tumors exist for at least four years before they spread.
  • The typical serous cancer is less than three millimeters across for 90 percent of this “window of opportunity for early detection.”
  • These early tumors are twice as likely to be in the fallopian tubes as in the ovaries.
  • To cut mortality from this cancer in half, an annual early-detection test would need to detect tumors five millimeters in diameter or less – about the size of a black peppercorn and less than a thousandth the size at which these cancers are typically detected today.

Brown’s lab is now looking for ways to take advantage of that window of opportunity to detect the microscopic tumors and intervene before the cancer spreads.

One strategy the laboratory is pursuing is to examine tissues near the ovaries, in the female reproductive tract, for protein or other molecular markers that could signify the presence of cancer. Brown says answering another question might also prove helpful: whether there is any reliable flow of material from the ovaries and fallopian tubes through the uterus and cervix into the vagina—material that might be tested for a specific cancer marker.

Despite science’s broad understanding of cancer at a molecular level, it has been challenging to identify simple molecular markers that signal the presence of early disease. One current blood marker, CA-125, has proven useful in monitoring later-stage ovarian cancer, but it has not been helpful for early detection. So Brown’s lab is also looking for biomarkers that are present only in ovarian tumors and not in healthy cells, instead of relying on tests that look for unusually high levels of a molecule that is part of normal biology (like CA-125).

The researchers are doing extensive sequencing of all messenger RNA molecules (which carry information for the production of specific proteins) in ovarian cancer cells, searching for evidence of proteins in these cells that would never be found in non-cancer cells. These variant molecules could be produced as a result of chromosome rearrangements—when the genome is cut and spliced in unusual ways—in ovarian cancers. “It’s a long shot,” says Brown, “but it’s important enough to try.”

Source: Tiny Early-Stage Ovarian Tumors Define Early Detection Challenge, Research News, Howard Hughes Medical Institute, July 29, 2009 [summarizing Brown PO, Palmer C, 2009 The Preclinical Natural History of Serous Ovarian Cancer: Defining the Target for Early Detection. PLoS Med 6(7): e1000114. doi:10.1371/journal.pmed.1000114].