PARP inhibitors are a class of cancer drugs that block an enzyme called poly(ADP-ribose) polymerase, which cells rely on to repair damaged DNA. By shutting down this repair pathway, the drugs force cancer cells with certain pre-existing genetic defects to accumulate so much DNA damage that they die. They were the first cancer therapies designed around a concept called synthetic lethality, and they are now approved for several tumor types, including ovarian, breast, prostate, and pancreatic cancers.
How PARP Inhibitors Work
Your cells constantly sustain small nicks and breaks in their DNA, and a family of enzymes called PARPs detects and helps fix that damage. PARP1 is the most important member of the family for this purpose. When PARP1 finds a single-strand break in the DNA, it attaches chemical tags to nearby proteins, recruiting repair machinery to patch the nick before it becomes something worse.
PARP inhibitors block this process. When the drug binds to PARP1, the enzyme can no longer do its repair job. Worse, the drug physically traps PARP1 on the DNA strand, creating a roadblock. When the cell tries to copy its DNA during division, it runs into that trapped complex, and the single-strand break gets converted into a far more dangerous double-strand break.1PubMed Central. The underlying mechanism for the PARP and BRCA synthetic lethality: clearing up the misunderstandings Healthy cells have a backup system called homologous recombination that can fix double-strand breaks. But cancer cells with mutations in genes like BRCA1 or BRCA2 lack that backup. They cannot repair the double-strand breaks, and the accumulated damage triggers cell death.2PubMed Central. PARP inhibitors: Synthetic lethality in the clinic
This is what makes PARP inhibitors so appealing as cancer drugs: they exploit a weakness that cancer cells have and healthy cells do not. Losing one repair pathway (say, BRCA function) is survivable. Losing PARP-dependent repair is also survivable on its own. But losing both at the same time is lethal. That two-hit principle is synthetic lethality, and it means the drug can be highly selective against tumors with certain DNA repair defects while leaving normal tissue relatively intact.
Which PARP Inhibitors Are Approved
Several PARP inhibitors have received regulatory approval, each with overlapping but distinct indications. They are all taken orally as daily pills.
- Olaparib (Lynparza): The first PARP inhibitor to win approval. It was initially cleared for BRCA-mutated advanced ovarian cancer and has since expanded to include BRCA-mutated breast cancer and maintenance therapy in ovarian and pancreatic cancers.3PubMed. Olaparib: first global approval In the pivotal trial for metastatic breast cancer, olaparib improved progression-free survival compared to standard chemotherapy, though it did not show an overall survival benefit in that setting.4PubMed Central. Olaparib: A Novel Therapy for Metastatic Breast Cancer in Patients With a BRCA1/2 Mutation
- Rucaparib (Rubraca): Approved for advanced ovarian cancer with BRCA mutations and for BRCA-mutated metastatic castration-resistant prostate cancer. In the prostate cancer trial, the confirmed response rate among eligible patients was about 44%.5PubMed Central. FDA Approval Summary: Rucaparib for the Treatment of Patients with Deleterious BRCA-Mutated Metastatic Castrate-Resistant Prostate Cancer
- Niraparib (Zejula): Approved for ovarian cancer maintenance regardless of BRCA mutation status, making it usable in a broader patient population including those with BRCA wild-type tumors.6PubMed Central. A comparative pharmacokinetic study of PARP inhibitors demonstrates favorable properties for niraparib efficacy in preclinical tumor models
- Talazoparib (Talzenna): Approved for BRCA-mutated, HER2-negative metastatic breast cancer. It is considered the most potent PARP trapper among the approved drugs, meaning it locks the PARP-DNA complex especially tightly.
Although these drugs share the same basic mechanism, they differ in how strongly they trap PARP on DNA and in their side effect profiles. Those differences in trapping potency and off-target activity against other PARP family members help explain why one drug might be chosen over another for a given patient.7PubMed Central. An Overview of PARP Inhibitors for the Treatment of Breast Cancer
Who Is Eligible for Treatment
Not every cancer patient benefits from a PARP inhibitor. The drugs work best when the tumor has a defect in its ability to repair double-strand DNA breaks through homologous recombination. The most straightforward predictor of benefit is a mutation in BRCA1 or BRCA2, whether inherited (germline) or acquired only by the tumor (somatic). A meta-analysis found that response rates to PARP inhibitors were similar whether the BRCA mutation was germline or somatic, at roughly 44% and 56% respectively, with no statistically significant difference.8PubMed Central. Similar response rates and survival with PARP inhibitors for patients with solid tumors harboring somatic versus Germline BRCA mutations: a Meta-analysis and systematic review
Beyond BRCA mutations, oncologists increasingly look at a broader concept called homologous recombination deficiency, or HRD. A tumor can lose its ability to perform homologous recombination through mutations in other genes besides BRCA, or through structural changes scattered across its genome. HRD testing platforms look for these patterns, combining gene sequencing with measurements of genomic “scars” like loss of heterozygosity, telomeric allelic imbalance, and large-scale state transitions.9PubMed Central. Ovarian Cancer Therapy: Homologous Recombination Deficiency as a Predictive Biomarker of Response to PARP Inhibitors The HRD score derived from these tests serves as a biomarker to help identify patients who are likely to respond to PARP inhibitor therapy even without a BRCA mutation.10Journal of the National Cancer Center. Establishing the homologous recombination score threshold in metastatic prostate cancer patients to predict the efficacy of PARP inhibitors
Testing has become standard of care in ovarian cancer and is expanding to prostate and other tumor types. Several commercial platforms now exist, and an active area of research is refining which tests best capture a tumor’s true repair capacity, since some tumors test positive for genomic scars but have actually restored their repair function by the time of testing.11PubMed Central. Unraveling Homologous Recombination Deficiency in Ovarian Cancer: A Review of Currently Available Testing Platforms
How PARP Inhibitors Are Used in Practice
PARP inhibitors serve two broad roles in cancer treatment: as direct therapy to shrink existing tumors, and as maintenance therapy to keep cancer from returning after a response to chemotherapy. The maintenance role is where these drugs have had some of their greatest impact. In ovarian cancer, a patient who responds well to platinum-based chemotherapy may then take a PARP inhibitor daily for months or years to delay recurrence. Real-world data show that even when cancer does eventually return during maintenance treatment, many patients remain sensitive to further therapy. In one study with a median follow-up of about three years, roughly 45% of patients on maintenance experienced recurrence, but more than 90% of those had a treatment-free interval long enough to allow another round of platinum-based therapy.12PubMed Central. Real-world outcomes of PARP inhibitor maintenance in advanced ovarian cancer: a focus on disease patterns and treatment modalities at recurrence
Researchers are also testing PARP inhibitors alongside immune checkpoint inhibitors. The rationale is that when PARP inhibitors cause DNA damage in tumor cells, the dying cells release signals that may help the immune system recognize and attack the cancer. Combining these two drug classes in ovarian cancer is an active area of clinical investigation.13PubMed Central. Application of PARP inhibitors combined with immune checkpoint inhibitors in ovarian cancer Early clinical development of PARP inhibitors actually began with combination chemotherapy, but those trials ran into problems with overlapping bone marrow toxicity, and it was the shift to tolerable single-agent dosing that unlocked the approvals we have today.14PubMed. The DNA Damaging Revolution: PARP Inhibitors and Beyond
Common Side Effects
Because PARP inhibitors affect DNA repair in all dividing cells, not just cancer cells, side effects are expected. The most prominent ones involve the blood. A systematic review and meta-analysis of trials in advanced ovarian cancer found that anemia was the most common blood-related side effect, occurring in about 29% of patients at any severity. PARP inhibitors roughly doubled the risk of anemia compared to controls and increased the risk of low platelet counts by nearly fivefold.15PubMed. Haematological toxicity of PARP inhibitors in advanced ovarian cancer: A systematic review and meta-analysis
These blood count drops tend to show up early. Real-world data from Japanese patients showed that treatment interruptions due to low platelet counts with niraparib occurred mostly within the first eight weeks, while anemia-related interruptions with olaparib clustered between weeks four and twelve.16Scientific Reports. Safety assessments and clinical features of PARP inhibitors from real-world data of Japanese patients with ovarian cancer Regular blood monitoring during the first few months is standard practice, and dose adjustments based on early blood counts have helped reduce the rate of severe toxicity, particularly for niraparib, where individualized starting doses are now recommended based on body weight and platelet count.
Beyond the blood, the most frequent complaints include fatigue, nausea, and changes in taste. Most patients experience these at mild to moderate levels.17PubMed Central. Management of Adverse Events During Rucaparib Treatment for Relapsed Ovarian Cancer: A Review of Published Studies and Practical Guidance Nausea tends to be worst during the first few weeks and often improves, especially when managed proactively with anti-nausea medications. UK clinical guidance for rucaparib recommends that patients take preventive antiemetics during their first days of treatment rather than waiting for nausea to start, a strategy that works well for most patients.18PubMed Central. Practical guidance for the management of side effects during rucaparib therapy in a multidisciplinary UK setting Fatigue can be trickier, since it may overlap with lingering effects of prior chemotherapy and can have multiple contributors including anemia, poor sleep, and nutritional deficiencies.
The Risk of Secondary Blood Cancers
One side effect that has drawn particular attention is a small but real increase in the risk of developing myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML), which are cancers of the bone marrow. A meta-analysis of 18 placebo-controlled trials involving over 7,000 patients found that PARP inhibitors raised the odds of MDS or AML by roughly two and a half times compared to placebo. In absolute terms, the incidence was still low: about 0.73% across the PARP inhibitor groups compared to 0.47% in placebo groups.19The Lancet Haematology. Risk of myelodysplastic syndrome and acute myeloid leukaemia in patients with cancer treated with PARP inhibitors: a systematic review and meta-analysis of randomised controlled trials
A more recent real-world analysis found a somewhat higher incidence of about 1.9% in PARP inhibitor-treated patients compared to 0.1% in an untreated control group.20PubMed Central. Risk of myelodysplastic syndrome and acute myeloid leukemia related to PARP inhibitor maintenance line in real-world ovarian cancer patients These numbers need context. The patients receiving PARP inhibitors have already been through rounds of platinum-based chemotherapy, which itself raises MDS/AML risk. Disentangling the contribution of prior chemotherapy from the PARP inhibitor itself is difficult. Still, the signal is consistent enough that oncologists factor this risk into their treatment discussions, especially for patients considering long-duration maintenance therapy.
Why Some Cancers Stop Responding
Resistance to PARP inhibitors is a major clinical challenge. Some tumors that initially respond well eventually find ways to escape the drug’s effects. Researchers have identified several mechanisms behind this, and many of them are frustratingly clever.
The most common route is that the cancer restores the very repair pathway the drug was designed to exploit. Through additional mutations called reversion mutations, a BRCA-deficient tumor can recover enough BRCA function to start repairing double-strand breaks again, neutralizing the synthetic lethality.21Genes & Diseases. Mechanism of PARP inhibitor resistance and potential overcoming strategies One autopsy study of a patient with BRCA2-mutated cancer found ten distinct reversion mutations across ten separate sites of metastatic disease, each independently restoring the protein’s function.22npj Precision Oncology. Convergent evolution of BRCA2 reversion mutations under therapeutic pressure by PARP inhibition and platinum chemotherapy This kind of convergent evolution, where multiple parts of the tumor independently arrive at the same solution, makes resistance especially hard to overcome.
Beyond BRCA reversion, tumors can also develop resistance by altering PARP1 itself so the drug no longer traps it effectively, by activating alternative DNA repair pathways, by stabilizing stalled replication forks to prevent them from collapsing into lethal double-strand breaks, or simply by pumping the drug out of the cell faster than it can accumulate.23PubMed Central. Mechanisms of resistance to PARP inhibitors – an evolving challenge in oncology Because tumors can use several of these strategies simultaneously, overcoming resistance will likely require combination approaches or entirely new drug designs.
Next-Generation PARP Inhibitors
Current PARP inhibitors target both PARP1 and PARP2, along with other members of the 17-protein PARP family. One working theory is that inhibiting PARP2 and other family members does not add much anti-cancer benefit but does contribute to side effects, especially bone marrow suppression. AstraZeneca developed a compound called AZD5305 that is highly selective for PARP1 over PARP2 and other PARPs, based on the hypothesis that this selectivity could maintain anti-tumor activity while reducing toxicity.24PubMed. Discovery of 5-{4-[(7-Ethyl-6-oxo-5,6-dihydro-1,5-naphthyridin-3-yl)methyl]piperazin-1-yl}-N-methylpyridine-2-carboxamide (AZD5305): A PARP1-DNA Trapper with High Selectivity for PARP1 over PARP2 and Other PARPs Early clinical trials of this and similar compounds are ongoing. If the selectivity hypothesis holds up, next-generation PARP inhibitors could allow higher effective doses with fewer blood count problems, potentially improving outcomes for patients who currently need dose reductions.
Other research is targeting downstream resistance mechanisms. Since BRCA reversion is the most common escape route, drugs that block homologous recombination through other means could re-sensitize resistant tumors. Combinations of PARP inhibitors with ATR inhibitors (which target a different checkpoint in the DNA damage response) are also under investigation, with preclinical data suggesting these combinations can create synthetic lethality even in some settings where PARP inhibitors alone lose their effect.25PubMed Central. Mechanism of DNA replication fork breakage and PARP1 hyperactivation during replication catastrophe
PARP Inhibition Beyond Cancer
While approved PARP inhibitors are purely cancer drugs today, the biology of PARP1 extends well beyond tumors. One active area of preclinical research involves stroke and other forms of brain injury. During a stroke, the lack of blood flow triggers massive oxidative stress and DNA damage in brain cells. PARP1 hyperactivates in response, consuming so much of the cell’s energy reserves (NAD+ and ATP) that neurons essentially run out of fuel and die through a process called parthanatos.26PubMed Central. Emerging role of PARP-1 and PARthanatos in ischemic stroke In animal models of stroke, blocking PARP1 either with drugs or by deleting the gene reduces the size of the brain damage and improves neurological recovery.
Similar PARP1-driven energy depletion and cell death have been implicated in heart attack injury, heart failure, and atherosclerosis.27PubMed. Parthanatos: A redox-dependent cell death pathway in cardiovascular disease and myocardial aging The logic is the same: after ischemia-reperfusion injury (when blood flow returns after a blockage), the burst of oxidative damage hyperactivates PARP1, draining cellular energy and causing additional tissue death that goes beyond the initial injury. Whether PARP inhibitors could be used therapeutically in these situations remains to be seen. The doses and treatment durations would be very different from cancer treatment, and the risks of suppressing DNA repair in healthy tissue would need careful evaluation. But the growing understanding of parthanatos as a disease mechanism has made PARP biology relevant to cardiology and neurology in ways that were not appreciated when the first cancer approvals came through.