Cancer remains one of the leading causes of death worldwide. While research and treatment options have advanced significantly in recent decades, no cure has yet been found for many types of cancer. However, a new class of drugs known as PARP inhibitors is showing tremendous promise.
What are PARP Inhibitors?
Poly (ADP-ribose) polymerase or PARP is a family of enzymes involved in DNA repair. When DNA damage occurs in cells, PARP enzymes work to detect the breaks and signal other proteins to initiate repair. PARP inhibitors are a class of anticancer drugs that work by blocking the activity of PARP enzymes. By inhibiting PARP’s normal repair function, these drugs allow damaged DNA to persist in cancer cells and eventually force them to die.
PARP Inhibitors were first developed to specifically target cancers driven by defects in the BRCA1 and BRCA2 genes, which are critical for repairing DNA double-strand breaks. Cancers with BRCA mutations, like some breast and ovarian cancers, rely heavily on PARP for survival since they cannot repair breaks via homologous recombination. Blocking PARP leads to a synthetic lethality – the cancer cells are not able to repair DNA damage and die. This selective targeting of BRCA-mutated cancer cells forms the basis of PARP inhibitors’ success.
Applications in Breast and Ovarian Cancer
The initial approvals of PARP inhibitors were for BRCA-mutated breast and ovarian cancers. Clinical trials showed PARP inhibitors delivered significantly improved progression-free survival when compared to standard chemotherapy in these patients.
One such trial involved the PARP inhibitor olaparib. Women with BRCA-mutated recurrent ovarian cancer who received olaparib lived a median of nearly eight months without their cancer worsening compared to just over four months for those on chemotherapy. This drug was the first PARP inhibitor approved by the FDA specifically for BRCA-mutated ovarian cancer.
Similar improvements were seen in BRCA-mutated breast cancer as well. In addition, PARP inhibitors were found to be well-tolerated with mostly mild side effects like nausea, fatigue and anemia. This represented a major step forward in targeted treatment for these genetic subsets of breast and ovarian cancer.
Beyond BRCA Mutations
While originally designed for BRCA cancers, researchers soon realized PARP inhibitors may help treat other tumor types too. Studies found many sporadic or non-BRCA mutated cancers also have defects in homologous recombination and DNA repair pathways that could render them sensitive to PARP inhibition.
Ongoing clinical research is exploring PARP inhibitors in prostate, pancreatic, and other cancers. Early results have been promising. One study of the PARP inhibitor talazoparib in prostate cancer patients found those with defects in DNA repair genes lived significantly longer without disease progression compared to chemotherapy -median of 8.6 months vs 3.6 months.
Similarly, a trial testing olaparib in pancreatic cancer saw impressive anti-tumor activity, especially in patients whose tumors contained certain mutations unrelated to BRCA. Close to 50% of participants responded to the PARP inhibitor compared to just 10% on standard chemotherapy.
These results demonstrate PARP inhibitors can target the underlying DNA repair deficiencies in cancer rather than just specific gene mutations. This wider applicability makes them an increasingly important option.
Improving Treatment Combinations
Researchers are also investigating incorporating PARP inhibitors into combination regimens to make cancer cells even more vulnerable. Combining them with DNA damaging chemotherapy or radiation therapy, for example, may kill more cancer cells by preventing repair of treatment-induced breaks.
Studies of pairing PARP inhibitors with immunotherapy are also ongoing. Immunotherapies like checkpoint inhibitors work best in tumors with high mutation rates, which are more likely to produce new antigens recognized by the immune system. The buildup of unrepaired DNA damage from PARP inhibition could create an immunogenic form of cancer cell death and drive stronger anti-tumor immune responses.
Early results sustaining this hypothesis are emerging. A recent trial combining the PARP inhibitor niraparib with pembrolizumab immunotherapy saw response rates of over 60% in some solid tumor types, far exceeding either approach alone. Larger studies continue to refine optimal combinations.
With their groundbreaking dual mechanisms of BRCA mimicry and wider exploitation of defective DNA repair, PARP inhibitors are revolutionizing cancer treatment. As researchers unravel additional repair pathways and interactions, these drugs may become applicable to an even broader range of cancers. Ongoing investigations of combination strategies also promise to maximize their effectiveness.
Already, several PARP inhibitors have been approved and many are in late-stage trials across many tumor types. With further refinements, they hold immense hope to not only benefit BRCA-associated breast and ovarian cancers but transform care for numerous cancers in the coming decade. PARP inhibitors truly represent the new era of targeted, personalized cancer management based on underlying defects rather than anatomy or histology alone. Their future potential appears limitless.
In summary, PARP inhibitors have emerged as a major breakthrough class of anticancer drugs. By blocking the essential DNA repair function of PARP enzymes, they can selectively target multiple tumor types based on their vulnerabilities in DNA damage response pathways. Ongoing research continues expanding their applications in combination regimens and new cancer subsets. PARP inhibitors signify the revolutionary shift toexploiting intrinsic molecular weaknesses in even complex diseases like cancer.
1. Source: Coherent Market Insights, Public sources, Desk research
2. We have leveraged AI tools to mine information and compile it