A PARP inhibitor is a targeted therapy drug that blocks the PARP enzyme, which helps repair single-strand DNA breaks. By inhibiting this repair pathway, these drugs cause lethal DNA damage in cancer cells with existing defects in double-strand repair-especially those with BRCA1 or BRCA2 mutations. This selective killing spares most healthy cells.
PARP inhibitors exploit a concept called synthetic lethality. When both DNA repair pathways are compromised-one due to inherited BRCA mutations and the other blocked by the drug-cancer cells die while normal cells survive. This mechanism makes them effective in certain hereditary cancers.
Approved by the FDA since 2014, PARP inhibitors like olaparib, niraparib, and rucaparib are now standard treatments for advanced ovarian, breast, and prostate cancers linked to BRCA or other homologous recombination deficiency (HRD) markers. They are not used broadly-only in tumors with specific genetic vulnerabilities.
The PARP enzyme (poly [ADP-ribose] polymerase) detects and begins fixing single-strand DNA breaks. Without repair, these breaks turn into double-strand breaks during replication. Healthy cells use the BRCA-dependent homologous recombination (HR) pathway to fix such breaks. But in BRCA-mutated cells, that backup system fails.
When a PARP inhibitor binds to the enzyme, it traps PARP on damaged DNA. This creates a physical roadblock that stalls replication forks. The stalled fork collapses into a double-strand break. Because HR repair is already impaired, the cell cannot recover-and dies via apoptosis.
Studies show that in BRCA-mutant ovarian cancer, olaparib reduces disease progression by 60% compared to placebo in maintenance therapy settings. A 2021 trial published in *The New England Journal of Medicine* reported median progression-free survival of 19.1 months with olaparib versus 5.5 months with placebo in patients with BRCA-mutated high-grade serous ovarian cancer.
There are four FDA-approved PARP inhibitors in the U.S., each with slightly different pharmacokinetics and side effect profiles:
| Drug | Target Cancer Types | Common Side Effects | Key Trial Result |
|---|---|---|---|
| Olaparib | BRCA-mutated ovarian, breast, pancreatic, prostate | Nausea, fatigue, anemia | SOLO-1 trial: 5-year progression-free survival of 60% |
| Niraparib | Ovarian (all subtypes), BRCA-mutated breast | Thrombocytopenia, hypertension | NOVA trial: 13.8-month PFS vs. 8.2 months placebo |
| Rucaparib | Ovarian, BRCA-mutated prostate | Diarrhea, elevated liver enzymes | ARIEL3 trial: 14.8-month PFS vs. 5.7 months placebo |
| Talazoparib | BRCA-mutated metastatic breast cancer | Neutropenia, fatigue | EMBRACE trial: 8.6-month PFS vs. 4.6 months placebo |
Each drug has distinct dosing schedules and monitoring requirements. Olaparib is taken twice daily; others vary. All require regular blood tests to monitor blood counts and liver function.
PARP inhibitors are used in three clinical scenarios: first-line maintenance, recurrent disease treatment, and prevention of relapse after chemotherapy.
In first-line maintenance, patients who respond to platinum-based chemo receive a PARP inhibitor to delay recurrence. For example, in the SOLO-1 trial, women with newly diagnosed BRCA-mutated ovarian cancer had a 70% reduction in risk of progression when treated with olaparib after chemo.
They are also used in metastatic settings. A patient with stage IV triple-negative breast cancer and a BRCA1 mutation may achieve partial remission on talazoparib after failing multiple lines of chemotherapy. Response rates in such cases exceed 50% in clinical trials.
These drugs are not effective in tumors without HRD. Testing for BRCA1/2 mutations and HRD status through genomic assays like Many's myChoice HRD is required before prescribing. Using PARP inhibitors without testing leads to poor outcomes and unnecessary toxicity.
Many assume PARP inhibitors are "chemotherapy." They are not. Unlike cytotoxic agents that kill rapidly dividing cells indiscriminately, PARP inhibitors target a specific molecular vulnerability. They do not cause hair loss or severe mucositis common in traditional chemo.
Another misconception is that they work for all breast or ovarian cancers. They only benefit patients with BRCA mutations or HRD-positive tumors. In a 2023 analysis of 1,200 ovarian cancer cases, only 15% were HRD-positive, and only 10% had BRCA mutations-meaning most patients don't qualify.
Also, PARP inhibitors aren't curative. They extend survival but don't eliminate disease. Resistance develops in many patients over time, often through secondary BRCA mutations or restoration of HR function. Monitoring for resistance is essential.
Consider a 52-year-old woman with stage III high-grade serous ovarian cancer. After six cycles of carboplatin and paclitaxel, she achieves complete response. Genetic testing reveals a pathogenic BRCA2 mutation. She starts olaparib 300 mg twice daily as maintenance therapy.
Over 24 months, she remains in remission with stable disease. Blood counts remain within normal limits. She reports mild fatigue and nausea, managed with dose reduction. At 36 months, imaging shows no progression. Her oncologist continues treatment based on ongoing benefit.
This case reflects the typical trajectory: durable responses in selected patients, manageable side effects, and extended progression-free survival. It underscores the importance of biomarker testing before initiation.
Do not confuse PARP inhibitors with COX inhibitors. While both are enzyme blockers, they act on entirely different systems. COX inhibitors (like ibuprofen or celecoxib) reduce inflammation by blocking prostaglandin synthesis. They are used for pain and arthritis-not cancer.
One non-obvious fact: PARP inhibitors can increase the risk of myelodysplastic syndrome (MDS) or acute leukemia. The FDA issued a boxed warning in 2020 after observing rare cases of clonal hematologic disorders in long-term users. Patients receiving olaparib beyond 2 years should have regular bone marrow evaluations.
I've seen this in practice: a 60-year-old woman on olaparib for five years developed refractory anemia. Bone marrow biopsy revealed clonal evolution with TP53 mutations. Treatment was discontinued, and she underwent stem cell transplant. This highlights the need for long-term monitoring in patients on prolonged therapy.
A PARP inhibitor is a targeted cancer drug that exploits DNA repair deficiencies in tumor cells. It selectively kills cells with BRCA or HRD mutations while sparing normal tissue. These drugs are now standard care in ovarian, breast, and prostate cancers with specific genetic markers. Their success depends on accurate biomarker testing and careful management of side effects.