June 24, 2009 -- For the second time this month, researchers report success using a novel type of anticancer pill to curb the growth of inherited tumors that often defy standard treatment. Called olaparib, the experimental pill is a member of a new class of drugs called PARP inhibitors that prevent unstable cancer cells from repairing themselves.
Two-thirds of 19 patients with cancer caused by mutations in the BRCA1 or BRCA2 genes responded to treatment with olaparib, and in more than half, tumors shrank or stopped growing, says researcher Johann de Bono, MD, PhD, of the Institute of Cancer Research in Sutton, U.K.
Defects in the BRCA1 and BRCA2 genes place women at sharply increased risk of developing aggressive cancers of the breast and ovaries at a young age. BRCA2 gene mutations also raise a man's risk of prostate cancer.
Olaparib did not help 41 other patients with tumors that were not associated with BRCA mutations, de Bono says.
PARP inhibitors "will very likely change the way we treat patients" whose tumors are caused by BRCA defects, says Daniel Silver, MD, PhD, of the Dana-Farber Cancer Institute in Boston. Silver, who was not involved with the work, co-authored an editorial about the study.
The study appears in the June 25 issue of The New England Journal of Medicine.
PARP Inhibitors for Cancer: How They Work
PARP is short for poly (ADP-ribose) polymerase, an enzyme used by cancer cells to repair DNA damage.
All cells, cancerous and healthy alike, have multiple systems for DNA repair. Even if one pathway is turned off, most cells can survive.
In people with BRCA mutations, one pathway is shut down, much like a table that is missing one leg. The table can still stand on three legs, but it's unstable.
Along comes olaparib, knocking out the PARP pathway. Like a table that only has two legs left, the cancer cell falls over and dies, de Bono says.
Like other targeted therapies -- smart drugs that take aim at the nuts and bolts of tumor growth while leaving healthy tissue relatively unscathed -- PARP inhibitors cause fewer side effects than traditional chemotherapy, he tells WebMD.
"My prostate cancer patient, who had advanced cancer that had spread to the bone, has been taking olaparib for two-and-one-half years. The drug is really active, and the only side effect is indigestion," de Bono says.
Another advantage of olaparib is that it's a pill that only has to be swallowed twice a day, he says.
PARP Inhibitors Targets Other Cancers, Too
PARP inhibitors will likely prove beneficial for other cancers, too, Silver tells WebMD.
In fact, they are already being tested in women with so-called triple-negative breast cancer. Such tumors are hard to treat because they lack receptors for the hormones estrogen and progesterone as well as the protein HER2, which are targeted by current therapies.
The hope is that eventually PARP inhibitors could be used at an early stage to treat or even prevent cancer in high-risk persons, Silver says.
Maker AstraZeneca, which helped to fund the work, plans to continue studying the drug, according to James Carmichael, MD, global medical director for olaparib.