Iniparib Doesn’t Seem to Help Treat Metastatic, Triple-Negative Disease

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Iniparib is one of several experimental medicines called PARP inhibitors. Results from a research study suggest that adding iniparib to chemotherapy doesn't help treat metastatic, triple-negative breast cancer.

The research is published in the Journal of Clinical Oncology and was presented at the 2011 annual meeting of the American Society of Clinical Oncology.

Metastatic breast cancer is cancer that has spread to other parts of the body away from the breast, such as the bones or liver.

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.

The PARP (poly ADP-ribose polymerase) enzyme fixes DNA damage in cells, including DNA damage caused by chemotherapy medicines. Scientists developed PARP inhibitors 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.

Results from a phase II trial on iniparib suggested that iniparib combined with chemotherapy might benefit women diagnosed with metastatic, triple-negative breast cancer.

In a more recent, larger phase III study, 519 women diagnosed with metastatic, triple-negative breast cancer had received no more than two other treatment regimens for breast cancer. For more than half the women, the treatment given during the study was their first treatment for metastatic breast cancer.

All the women were treated with the chemotherapy medicines Gemzar (chemical name: gemcitabine) and Paraplatin (chemical name: carboplatin). This combination is sometimes called GC. Half the women were randomly assigned to get iniparib with the chemotherapy regimen; the other half got a placebo with the chemotherapy regimen.

The women in the study who got the placebo instead of iniparib were allowed to start getting iniparib if the cancer showed signs of growing and if they and their doctors wanted to start on iniparib. Researchers call this "crossing over." Eventually, 96% of the women who started getting the placebo decided to cross over and get iniparib.

Gemzar destroys cancer cells by acting as false building blocks in the cells' genes, causing the cells to die as they get ready to divide. Paraplatin weakens or destroys cancer cells by damaging the genetic material in the cells and making it hard for cells to repair any genetic damage. Gemzar, Paraplatin, and iniparib are all given intravenously.

The researchers recorded each woman's response to treatment, how long each woman lived without the cancer growing (called progression-free survival), and the women's overall survival. The women who got iniparib along with the chemotherapy regimen did no better than women who got only the chemotherapy regimen:

  • 34% of women who got iniparib and chemotherapy showed some response to treatment (response rate) compared to 30% of women who got only chemotherapy.
  • Progression-free survival was 5.1 months for women who got iniparib and chemotherapy compared to 4.1 months for women who got only chemotherapy.
  • Overall survival was 11.8 months for women who got iniparib and chemotherapy compared to 11.1 months for women who got only chemotherapy.

The differences in survival weren't statistically significant, which means they could have been due to chance and not because of the difference in treatment.

Although the results suggest that adding iniparib to chemotherapy doesn't help treat metastatic, triple-negative breast cancer, there was a group of women who seemed to benefit. Women who were previously treated with one or two other metastatic cancer treatments before getting iniparib and chemotherapy were more likely to benefit from the addition of iniparib to chemotherapy compared to women who got iniparib and chemotherapy as the first treatment for metastatic cancer.

Though the results were disappointing, doctors are continuing to study PARP inhibitors as potential treatments for breast and other cancers.

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 still 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.

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