Poly(ADP-ribose) Polymerase Inhibitors in Cancer Therapy
Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPis) have emerged as a pivotal class of anticancer drugs, particularly in the treatment of cancers with homologous recombination repair (HRR) deficiencies, such as those harboring BRCA1/2 mutations. This review provides a comprehensive overview of the mechanisms of action, clinical applications, adverse effects, resistance mechanisms, and combination therapies of PARPis, highlighting their potential in precision oncology.
Introduction
PARPs are a family of enzymes involved in critical biological processes, including DNA repair, maintenance of genomic stability, and apoptosis. Among the PARP family members, PARP1, PARP2, and PARP3 play distinct yet overlapping roles in DNA damage response (DDR). PARP1, the most prominent member, is essential for the repair of single-strand breaks (SSBs), while PARP2 and PARP3 contribute to DDR and repair processes. In healthy cells, PARPs promptly recognize and repair DNA damage, safeguarding genomic integrity. However, in cancer cells, particularly those with defective DNA repair mechanisms, PARP activity becomes crucial for survival, making it a key target in cancer therapy.
PARP inhibitors (PARPis) disrupt DNA repair mechanisms by inhibiting PARP enzyme activity. Initially developed to enhance the efficacy of chemotherapy, PARPis have shown remarkable efficacy in treating cancers with BRCA1/2 mutations. These mutations lead to the loss of HRR functionality, forcing cancer cells to rely on alternative repair pathways, such as base excision repair (BER). PARPis block the BER pathway, leading to the accumulation of DNA damage and cell death, a phenomenon known as synthetic lethality.
Mechanism of PARPis in Anticancer Treatment
PARPis exert their anticancer effects through several mechanisms, including DNA repair inhibition, synthetic lethality, and replication stress.
DNA Repair Inhibition
PARP1 is a key enzyme in DNA repair. Upon DNA damage, PARP1 binds to the damaged site and uses nicotinamide adenine dinucleotide (NAD+) as a substrate to catalyze the poly(ADP-ribosyl)ation (PARylation) reaction, which is essential for DNA repair. PARPis inhibit this process by competitively binding to the catalytic site of PARP1, preventing NAD+ binding and thereby impeding DNA repair. This leads to the accumulation of DNA damage and ultimately cell death.
PARPis also induce cancer cell death through a mechanism known as PARP trapping. Normally, PARP1 binds to damaged DNA, modifies itself through PARylation, and then detaches from the DNA. Some PARPis prevent this detachment, trapping PARP1 on the DNA and blocking repair. This exacerbates DNA damage, particularly in cells with defective HRR, such as those with BRCA mutations.
Synthetic Lethality
Synthetic lethality refers to a genetic interaction where the combination of defects in two genes leads to cell death, whereas a single gene defect does not affect cell survival. PARPis exploit this effect to selectively kill cancer cells with BRCA1/2 mutations. HRR is the primary mechanism for repairing DNA double-strand breaks (DSBs), with BRCA1 and BRCA2 playing pivotal roles. In normal cells, DSBs are repaired through HRR. However, cancer cells with BRCA1/2 mutations lack effective HRR mechanisms, making them unable to repair DSBs. PARPis block the BER pathway, leading to the accumulation of DSBs and cell death in these HRR-deficient cells.
Replication Stress
Replication stress refers to the slowing or stalling of DNA replication, which can lead to genomic instability and cancer development. PARPs play a crucial role in protecting replication forks, which are essential for maintaining genomic stability. PARPis disrupt this protection, leading to the degradation of nascent DNA strands and cell death, particularly in BRCA1/2-mutated cancer cells.
Clinical Applications of PARPis
PARPis have shown significant efficacy in the treatment of various cancers, including ovarian, breast, prostate, and pancreatic cancers. Several PARPis, such as olaparib, niraparib, and rucaparib, have been approved by the FDA for use as standalone therapies or in combination with chemotherapy.
Olaparib (Lynparza) was the first PARPi approved by the FDA in 2014 for the treatment of BRCA-mutated ovarian cancer. It has since been approved for the treatment of metastatic breast cancer, metastatic pancreatic cancer, and metastatic castration-resistant prostate cancer. Niraparib was approved in 2017 for the treatment of recurrent ovarian cancer, and rucaparib was approved in 2020 for the treatment of metastatic prostate cancer with BRCA mutations. Talazoparib, a highly potent PARPi, was approved in 2018 for the treatment of BRCA-mutated breast cancer.
Adverse Effects
PARPis can cause a range of adverse effects, including anemia, fatigue, nausea, vomiting, and neutropenia. These effects are generally mild to moderate, but some patients experience severe reactions. For example, 40% of patients treated with olaparib developed anemia, with 16.1% experiencing grade 3 anemia. Similarly, 68.4% of patients treated with talazoparib developed anemia, with 47.4% experiencing grade 3 anemia.
Resistance Mechanisms to PARPis
Resistance to PARPis can arise through several mechanisms, including drug efflux, PARP1 mutations, restoration of HRR, and replication fork protection.
Drug Efflux
ABCB1 (MDR1) encodes P-glycoprotein, an ATP-dependent efflux pump that actively transports drugs out of cells, reducing intracellular drug concentrations. Some PARPis, such as rucaparib, are substrates for these efflux pumps, leading to reduced efficacy. Inhibitors of these pumps, such as verapamil and elacridar, can reverse this resistance.
PARP1 Mutations
Mutations in PARP1 can lead to resistance by impairing its ability to bind to DNA. Zinc finger (Znf) domains in PARP1 are crucial for DNA damage recognition and repair. Mutations in these domains can prevent PARP1 from binding to DNA, leading to resistance.
Restoration of Homologous Recombination
Restoration of HRR is a major mechanism of PARPi resistance. This can occur through intronic deletions within the BRCA2 gene, which restore the open reading frame (ORF) and produce functional BRCA2 isoforms. Secondary mutations in HR genes, such as BRCA1, RAD51C, and RAD51D, can also restore HRR functionality, leading to resistance.
Replication Fork Protection
Replication fork protection is essential for maintaining genomic stability. Proteins such as PTIP, SMARCAL1, ZRANB3, and FANCD2 protect replication forks from degradation, leading to PARPi resistance.
Combination Therapies with PARPis
Combining PARPis with other therapies has shown promise in enhancing efficacy and overcoming resistance.
Cytotoxic Chemotherapeutic Drugs
Combining PARPis with platinum-based drugs, such as cisplatin or carboplatin, has shown significant efficacy in treating ovarian and breast cancers. This combination increases DNA damage and blocks repair, improving treatment response rates and extending progression-free survival (PFS).
Targeted Therapy
PARPis can be combined with targeted therapies, such as anti-EGFR monoclonal antibodies, PI3K/mTOR inhibitors, and VEGF receptor inhibitors, to enhance efficacy. For example, the combination of olaparib with the PI3K/mTOR inhibitor dactolisib has shown synergistic effects in inhibiting tumor growth.
Immunotherapy
Combining PARPis with immune checkpoint inhibitors (ICIs), such as CTLA-4 and PD-1 blockers, has demonstrated synergistic effects in cancer treatment. PARPis increase the immunogenicity of BRCA1-deficient cancer cells, while ICIs restore T-cell function, enhancing the immune response against tumors.
Challenges in the Development of PARPis
The development of PARPis faces several challenges, including tumor resistance, side effects, and difficulty in identifying reliable biomarkers. Tumor heterogeneity further complicates treatment, as not all patients with BRCA mutations respond to PARPis, while some without these mutations do.
Future Directions
Future research should focus on addressing resistance through combination therapies, advancing personalized treatments, developing next-generation PARPis, and expanding the applications of PARPis to non-BRCA cancers and earlier disease stages. Improving clinical trial designs to incorporate adaptive elements and real-time biomarker monitoring will also be crucial.
Conclusion
PARPis represent a significant advancement in cancer therapy, particularly for BRCA-mutated cancers. They induce cancer cell death by disrupting DNA repair mechanisms, but challenges such as resistance and side effects remain. Future research should focus on overcoming these challenges, developing better PARPis, and combining them with other therapies to improve cancer treatments.
doi.org/10.1097/CM9.0000000000003471
Was this helpful?
0 / 0