Experimental Iniparib Shows Promise in Treating Advanced-Stage Disease

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Iniparib is one of several new, experimental medicines called PARP inhibitors. A very small, very early study found that about 33% of women diagnosed with locally advanced or metastatic breast cancer that wasn't responding to other treatments had some response to a combination of iniparib and Camptosar (chemical name: irinotecan), a chemotherapy medicine.

Locally advanced breast cancer is breast cancer that has spread outside the breast to other tissue in the breast area. Metastatic breast cancer is cancer that has spread to other parts of the body away from the breast, such as the bones or liver. These results were presented at the 2010 San Antonio Breast Cancer Symposium (SABCS).

The PARP (poly ADP-ribose polymerase) enzyme fixes DNA damage in cells, including DNA damage caused by chemotherapy medicines. Scientists developed PARP inhibitors such as iniparib based on the idea that a medicine that interferes with or inhibits the PARP enzyme might make it harder for cancer cells to fix damaged DNA. This would make the cancer more susceptible to chemotherapy and make it harder for cancer to become resistant to chemotherapy.

Camptosar kills cancer cells by interfering with the cells' genes during reproduction. This chemotherapy medicine isn't commonly used to treat breast cancer right now, but this study suggests that Camptosar might be a good treatment option for advanced-stage breast cancer when combined with a PARP inhibitor.

Thirty-four women diagnosed with locally advanced or metastatic breast cancer that wasn't responding to standard advanced-stage treatments participated in this study. About 65% of the women were diagnosed with triple-negative breast cancer. Triple-negative breast cancer is:

  • estrogen-receptor-negative
  • progesterone-receptor-negative
  • HER2-negative

Overall, about 15% to 20% of breast cancers are triple-negative. Triple-negative cancers are usually more aggressive, harder to treat, and more likely to come back (recur) than cancers that are hormone-receptor-positive and/or HER2-positive. Hormonal therapy and the targeted therapies Herceptin (chemical name: trastuzumab) and Tykerb (chemical name: lapatinib) usually don't work on triple-negative breast cancer.

All the women in the study got Camptosar at increasing doses -- a low dose, then an intermediate dose, then a high dose. Not all of the women had moved to the intermediate or high dose when these results were reported.

All the women also got iniparib. The treatments were repeated in 3-week cycles.

Nine of the women got only the low or intermediate dose of Camptosar. None of the women who got the low dose and only one woman who got the intermediate dose had the tumor shrink during treatment.

Twenty-five women got the high dose of Camptosar; 45.5% of them got some benefit from treatment (clinical benefit rate):

  • 1 woman (4.5%) had a complete response, which means no active cancer cells could be found
  • 6 women (27.3%) had a partial response, which means the cancer was weakened
  • 3 women (13.6%) had stable disease for at least six treatment cycles (18 weeks); this means the cancer didn't grow during treatment

The 45.5% clinical benefit rate with the combination of high-dose Camptosar and iniparib is much better than the 23% clinical benefit rate found in other studies evaluating using Camptosar alone to treat women diagnosed with advanced-stage breast cancer that wasn't responding to other treatments.

More than half (56%) of the women who got the highest dose of Camptosar had serious (grade 3 or grade 4) side effects. The side effects were those commonly seen with chemotherapy treatment:

  • 26.5% of the women had grade 3 or 4 low white blood cell counts (neutropenia, which increases the risk of serious infection)
  • 8.8% of the women had grade 3 or 4 diarrhea

Though the side effects were serious, the researchers felt they were manageable.

Similar to this study, other small, early studies have shown that some advanced-stage breast cancers -- including triple-negative breast cancers -- that have stopped responding to standard treatments will respond to one of several PARP inhibitors combined with chemotherapy.

If you're being treated for advanced-stage breast cancer, you and your doctor may be considering a number of options, especially if the cancer is triple-negative and/or has stopped responding to standard treatments. Treatment with an experimental regimen that includes a PARP inhibitor such as iniparib may be an option if you're willing to participate in a clinical trial. Ask your doctor if there are any clinical trials that might be a good fit for you and your unique situation. Visit the Breastcancer.org Clinical Trials pages for more information.

Stay tuned to Breastcancer.org Research News to learn about the latest information on new ways to more effectively treat breast cancer.

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