PARP Inhibitors Explained: How DNA Repair Defects Become a Cancer's Achilles Heel

PARP inhibitors represent one of the most elegant examples of precision oncology: drugs that exploit a specific weakness in a tumor's DNA repair machinery to selectively kill cancer cells while sparing normal tissue. Since olaparib received the first approval in BRCA-mutated ovarian cancer in 2014, four PARP inhibitors have been approved across a growing range of tumor types including ovarian, breast, prostate, and pancreatic cancer. The conceptual framework underlying their activity — synthetic lethality — has become a foundational principle in modern cancer drug discovery.

This article explains the biology of DNA repair and why cancer cells with BRCA mutations are vulnerable, how PARP inhibitors mechanistically exploit this vulnerability, what the clinical data show across approved indications, the distinction between BRCA mutations and homologous recombination deficiency (HRD), and the emerging resistance mechanisms and combination strategies that are shaping the next generation of PARP inhibitor development.

DNA Repair: A Brief Primer

Cells are constantly damaged by endogenous processes (reactive oxygen species, replication errors, metabolic byproducts) and exogenous agents (radiation, chemotherapy, carcinogens). To maintain genomic integrity, cells have evolved multiple DNA repair pathways, each specialized for different types of damage. The two most clinically relevant for understanding PARP inhibitors are base excision repair (BER) and homologous recombination (HR).

Base excision repair handles single-strand breaks (SSBs) and small base lesions. A critical enzyme in this pathway is PARP1 (poly(ADP-ribose) polymerase 1), which detects SSBs, binds to the break site, and signals repair machinery by catalyzing the addition of long chains of ADP-ribose units (poly-ADP-ribosylation, or PAR) to itself and surrounding proteins. Once repair is complete, PARP1 releases from DNA. When PARP is inhibited, SSBs go unrepaired and persist until they are encountered by a replication fork during DNA synthesis — at which point the SSB collapses the fork into a far more dangerous lesion: a double-strand break (DSB).

Homologous recombination is the high-fidelity pathway for repairing DSBs. It uses the sister chromatid as a template to precisely reconstruct the damaged sequence, making it essentially error-free. The proteins BRCA1 and BRCA2 are essential mediators of HR — they process the broken DNA ends, recruit repair factors, and facilitate strand invasion into the sister chromatid template. Cells deficient in BRCA1 or BRCA2 cannot perform HR effectively.

Synthetic Lethality: The Core Concept

Synthetic lethality is a genetic concept describing a situation where two gene defects that are individually tolerated become lethal in combination. In the PARP inhibitor context: normal cells have both PARP-mediated SSB repair and HR for DSB repair. PARP inhibition forces reliance on HR to resolve accumulated DSBs. Normal cells, with intact HR (intact BRCA1/2), handle this increased DSB burden and survive. But cancer cells with defective BRCA1/2 — and thus defective HR — cannot repair the DSBs generated by PARP inhibition, leading to genomic catastrophe and cell death.

This creates genuine tumor selectivity: the cell death occurs specifically in cells that lack functional HR, which are the tumor cells. Normal cells — carrying one wild-type copy of BRCA1 or BRCA2 even in carriers — retain sufficient HR activity to survive PARP inhibition. This mechanistic basis for tumor-selective killing is why PARP inhibitors have a relatively favorable therapeutic index compared to traditional cytotoxic chemotherapy.

A second mechanism amplifies this effect: PARP trapping. PARP inhibitors not only block PARP's catalytic activity but physically trap PARP protein on DNA at damage sites, preventing it from dissociating. Trapped PARP-DNA complexes are highly toxic — more so than unrepaired SSBs alone — because they form physical roadblocks to DNA replication machinery. The trapping potency varies among PARP inhibitors: talazoparib and niraparib are more potent trappers than olaparib and rucaparib, which has implications for both efficacy and toxicity profiles.

BRCA1 and BRCA2: Germline vs. Somatic Mutations

BRCA1 and BRCA2 mutations relevant to PARP inhibitor sensitivity can be either germline (inherited, present in every cell of the body) or somatic (acquired, present only in the tumor). Germline BRCA1/2 mutations are the most prevalent hereditary cancer syndrome mutations after Lynch syndrome: approximately 1 in 400 people carry a pathogenic germline BRCA variant. Carriers have dramatically elevated lifetime risks of breast cancer (~70%), ovarian cancer (~40%), and moderate increases in pancreatic, prostate, and other cancers.

Somatic BRCA1/2 mutations — arising de novo in tumor cells — are also common and confer the same HR deficiency and PARP inhibitor sensitivity as germline mutations. In ovarian cancer, approximately 15% of patients carry germline BRCA mutations and an additional 7% have somatic BRCA mutations — together accounting for ~22% of all ovarian cancers with BRCA-driven HR deficiency. For therapeutic purposes, the source of the mutation (germline vs. somatic) matters less than whether the tumor has lost functional BRCA1/2 protein.

Homologous Recombination Deficiency (HRD): Beyond BRCA

BRCA1 and BRCA2 are the most common, but not the only, causes of HR deficiency. Mutations in other HR pathway genes — PALB2, RAD51C, RAD51D, BRIP1, CDK12, and others — also impair HR to varying degrees. The concept of HRD-positive tumor encompasses all causes of HR impairment, not just BRCA mutations. Genomic HRD scores — which quantify genome-wide scarring from chronic HR deficiency (using measures of loss of heterozygosity, telomeric allelic imbalance, and large-scale state transitions) — have been developed to identify HRD-positive tumors regardless of which gene is mutated.

The clinical relevance of non-BRCA HRD is highest in ovarian cancer, where approximately 50% of all high-grade serous ovarian cancers are HRD-positive — including BRCA-mutated tumors and BRCA-wild-type HRD-positive tumors (sometimes called "BRCAness"). PARP inhibitors niraparib and olaparib-bevacizumab have received FDA approval for first-line maintenance in HRD-positive ovarian cancer regardless of BRCA status, based on significant improvements in progression-free survival. The PFS benefit is most pronounced in BRCA-mutated tumors but is also significant in BRCA-wild-type/HRD-positive tumors.

FDA-Approved PARP Inhibitors and Their Indications

Toxicities and Clinical Management

PARP inhibitors share a common toxicity profile. Hematologic toxicities are the most clinically significant: anemia occurs in 20–40% of patients (Grade 3+ in 10–25%), and thrombocytopenia and neutropenia are also common, particularly with niraparib and talazoparib — the highest-potency PARP trappers. Regular blood count monitoring is required, and dose modifications for cytopenias are frequently necessary. Prophylactic growth factors are not routinely recommended but may be used for Grade 3+ neutropenia.

Gastrointestinal toxicities — nausea, fatigue, vomiting, and decreased appetite — are common, particularly early in treatment, and often improve over the first 4–8 weeks. Taking PARP inhibitors with food reduces GI side effects. Myelodysplastic syndrome/acute myeloid leukemia (MDS/AML) is a rare but serious long-term risk, occurring in approximately 1–2% of patients with prolonged exposure, particularly those who have received prior platinum-based chemotherapy. The mechanism involves secondary mutations in hematopoietic stem cells accumulated from PARP inhibition in normal bone marrow cells.

Resistance Mechanisms and Future Directions

Despite remarkable initial efficacy, most patients eventually develop resistance to PARP inhibitors. The most common mechanism is reversion mutations — secondary mutations in BRCA1 or BRCA2 that restore the open reading frame of the mutated gene, allowing partial restoration of HR function. These reversion mutations are detectable in ctDNA and are associated with particularly poor prognosis, as they also confer cross-resistance to platinum chemotherapy. Other resistance mechanisms include upregulation of alternative DNA repair pathways, loss of 53BP1 or other HR-suppressing factors, and PARP protein downregulation.

Combination strategies to overcome or prevent resistance include PARP inhibitors with CDK inhibitors (to maintain replication stress), with immunotherapy (exploiting the immunogenic cell death caused by PARP inhibition), with WEE1 inhibitors (to further destabilize the G2/M checkpoint), and with ATR or ATM inhibitors (to block additional components of the DNA damage response). Several of these combinations are in Phase 2–3 trials, with early data suggesting enhanced activity beyond BRCA-mutated populations.