PARP Inhibitors Expand Beyond BRCA-Mutated Cancers: Emerging Evidence Across Solid Tumors and Combination Strategies
核心洞察
PARP inhibitors have become established frontline and maintenance therapies for breast, ovarian, prostate, and pancreatic cancers, with olaparib, rucaparib, niraparib, and talazoparib approved by the FDA.
The OlympiA, SOLO1, PROFOUND, and POLO trials established PARP inhibitors as standard of care in biomarker-selected populations, though consistent overall survival benefits remain elusive.
Emerging evidence suggests PARP inhibitors may benefit additional malignancies, including endometrial, cervical, kidney, colorectal, NSCLC, and SCLC, but efficacy depends heavily on molecular selection.
Poly ADP-ribose polymerase (PARP) inhibitors have revolutionized cancer therapy by leveraging the principle of synthetic lethality to selectively kill tumor cells with defective DNA repair pathways. Initially developed for patients harboring BRCA1 (搜索)/2 mutations, these agents have now become essential components of first- and later-line therapies for breast, prostate, pancreatic, and ovarian cancers, with several agents already approved by the U.S. Food and Drug Administration (搜索) (FDA). However, their therapeutic potential in other malignancies remains less clearly defined, prompting a growing body of research into expanded indications and combination strategies.
The Mechanism of Synthetic Lethality
PARP inhibitors represent one of the earliest and most prominent examples of the synthetic lethality approach in oncology. This concept describes a genetic interaction in which the loss of function of either gene alone does not impair cell viability, whereas their simultaneous inactivation leads to cell death. PARP inhibition initiates synthetic lethality in cells harboring BRCA1 (搜索) or BRCA2 (搜索) mutations by disrupting the repair of single-strand DNA breaks (SSBs) through the base excision repair (BER) pathway.
These inhibitors mainly act on PARP1 (搜索) and PARP2 (搜索), key enzymes responsible for repairing SSBs. Defects in BER can lead to the accumulation of SSBs and their conversion into double-strand breaks (DSBs), resulting in genomic instability—a hallmark of cancer development and progression. PARP-1 also contributes to the activation of ATM (搜索), a key regulator of homologous recombination (HR). Tumors harboring homologous recombination deficiency (HRD), particularly those with mutations in BRCA1 (搜索) and BRCA2 (搜索), show increased susceptibility to PARP inhibition, making HR status a critical biomarker for predicting therapeutic response.
Established Indications Across Four Cancer Types
To date, four PARP inhibitors—olaparib, rucaparib, niraparib, and talazoparib—have received FDA approval for selected indications in breast, ovarian, pancreatic, and prostate cancers. Their clinical use is primarily guided by validated predictive biomarkers, particularly pathogenic BRCA1 (搜索)/2 mutations and, in specific settings, broader homologous recombination repair (HRR) deficiency.
In breast cancer (搜索), the OlympiA trial demonstrated that adjuvant olaparib significantly improved invasive disease-free survival and distant disease-free survival compared with placebo in early-stage HER2-negative disease with germline BRCA1 (搜索)/2 pathogenic variants, establishing olaparib as standard adjuvant treatment for high-risk patients following chemotherapy. In metastatic disease, both the OlympiAD and EMBRACA trials showed that olaparib and talazoparib significantly prolonged progression-free survival (PFS) and increased objective response rates compared with physician's choice chemotherapy. Notably, neither study demonstrated a statistically significant overall survival benefit, suggesting that PARP inhibitors primarily delay disease progression rather than substantially extending survival.
In ovarian cancer (搜索), PARP inhibitors have achieved their greatest clinical impact. The SOLO1 trial established olaparib as the standard first-line maintenance treatment for newly diagnosed advanced BRCA-mutated ovarian cancer, with long-term follow-up demonstrating sustained remission and clinically meaningful overall survival trends extending beyond 7 years. The PAOLA-1 trial showed that adding olaparib to bevacizumab significantly improved PFS, with the greatest benefit in HRD-positive tumors. Niraparib, through the PRIMA/ENGOT-OV26/GOG-3012 trial, demonstrated efficacy across a broader molecular spectrum, including patients with BRCA wild-type tumors.
In prostate cancer (搜索), the phase III PROFOUND trial demonstrated that olaparib significantly improved radiographic progression-free survival, objective response rate, and overall survival compared with androgen receptor signaling inhibitors in patients with HRR-deficient metastatic castration-resistant prostate cancer (mCRPC). The TRITON2 study reported high objective and PSA response rates in BRCA-mutated disease, though subsequent analyses revealed considerably lower activity in tumors carrying non-BRCA alterations such as ATM (搜索), CDK12, and CHEK2.
In pancreatic cancer (搜索), the landmark phase III POLO trial established maintenance olaparib as the first targeted therapy to significantly prolong PFS in patients with metastatic germline BRCA1 (搜索)/2-mutated pancreatic cancer whose disease had not progressed following first-line platinum-based chemotherapy.
Expanding Beyond Established Indications
Increasing evidence shows that PARP inhibitors can also be effective in a broader range of cancers, particularly when tumors exhibit homologous recombination deficiency or related molecular vulnerabilities. This expanding role highlights the importance of biomarker-driven therapy.
In endometrial cancer (搜索), the phase IIb UTOLA trial demonstrated that maintenance olaparib did not significantly improve PFS or overall survival in an unselected population, but exploratory analyses suggested clinically meaningful benefit in molecularly selected subgroups, including tumors with p53 abnormalities. A network meta-analysis demonstrated that combining PARP inhibitors with immune checkpoint inhibitors significantly improved PFS in patients with p53-abnormal, mismatch repair-proficient endometrial cancer.
In cervical cancer (搜索), initial studies demonstrated that veliparib can be safely combined with topotecan or platinum-based chemotherapy, with tumors showing low PARP-1 expression experiencing longer progression-free and overall survival. Preclinical studies showed that combining niraparib with radiotherapy significantly enhanced tumor growth inhibition by suppressing DNA repair pathways and increasing apoptosis.
In non-small cell lung cancer (搜索) (NSCLC), maintenance olaparib demonstrated encouraging improvements in PFS in platinum-sensitive disease, though the primary endpoint was not statistically significant. Combining niraparib with pembrolizumab produced encouraging response rates, particularly among patients with high PD-L1 expression.
In small cell lung cancer (搜索) (SCLC), veliparib has been the most extensively investigated agent, demonstrating modest improvements in PFS and objective response rates when combined with temozolomide or platinum-based chemotherapy. Several studies consistently identified SLFN11 (搜索) expression as a promising predictive biomarker, with SLFN11-positive tumors deriving greater clinical benefit.
Combination Strategies and Resistance
Beyond approved monotherapy, considerable efforts have focused on combination strategies designed to enhance efficacy and overcome acquired resistance. The combination of olaparib with the immune checkpoint inhibitor durvalumab demonstrated encouraging antitumor activity in the MEDIOLA study, while niraparib combined with pembrolizumab produced improved responses in biomarker-selected triple-negative breast cancer (搜索).
In prostate cancer (搜索), talazoparib combined with enzalutamide significantly improved radiographic PFS and overall survival in the TALAPRO-2 trial, with the greatest benefit among patients with HRR-deficient tumors. In pancreatic cancer (搜索), the phase II POLAR trial reported encouraging clinical activity for pembrolizumab plus olaparib in patients with HRD-positive disease.
Despite these advances, approximately 40% of BRCA-deficient breast cancers exhibit primary resistance or eventually acquire resistance through restoration of homologous recombination repair, BRCA reversion mutations, loss of 53BP1, replication fork stabilization, reduced PARP trapping, and increased drug efflux. This has driven interest in next-generation PARP inhibitors, such as the highly selective PARP1 (搜索) inhibitor saruparib (AZD5305), which has shown superior antitumor activity with reduced hematologic toxicity in preclinical models.
The Broader DNA Damage Response Landscape
The clinical success of PARP inhibitors established the DNA damage response (DDR) as a major therapeutic target in oncology and validated the concept of synthetic lethality. With resistance to first-generation therapies increasingly observed and foundational patents approaching expiry, the field is expanding beyond PARP inhibition toward new therapeutic strategies targeting ATR, ATM (搜索), WEE1 (搜索), DNA-PK (搜索), Chk1 (搜索), and polymerase theta (搜索).
Artificial intelligence and machine learning, often combined with CRISPR-based functional genomic screening, are increasingly used in DDR drug discovery to identify DNA repair vulnerabilities, predict synthetic lethal interactions, and develop biomarker strategies for patient stratification. DDR inhibitors are also being evaluated in combination with radiopharmaceutical and radioligand therapies, as well as immune checkpoint inhibitors, to enhance tumor immunogenicity and convert "cold" tumors into more immunologically active "hot" tumors.
The expanding role of PARP inhibitors across multiple cancer types underscores the importance of biomarker-driven therapy and supports the growing use of these agents beyond their original indications, while highlighting the need for prospective validation of predictive biomarkers and optimization of combination strategies.