Radiotherapy-Poly(ADP-ribose) Polymerase Inhibitor Combinations: Progress to Date

PARP1: A comprehensive review of its mechanisms, therapeutic implications and emerging cancer treatments

The Poly (ADP-ribose) polymerase-1 (PARP1) enzyme is involved in several signalling pathways related to homologous repair (HR), base excision repair (BER), and non-homologous end joining (NHEJ). Studies demonstrated that the deregulation of PARP1 function and control mechanisms can lead to cancer emergence. On the other side, PARP1 can be a therapeutic target to maximize cancer treatment. This is done by molecules that can modulate radiation effects, such as DNA repair inhibitors (PARPi). With this approach, tumour cell viability can be undermined by targeting DNA repair mechanisms. Thus, treatment using PARPi represents a new era for cancer therapy, and even new horizons can be attained by coupling these molecules with a nano-delivery system. For this, drug delivery systems such as liposomes encompass all the required features due to its excellent biocompatibility, biodegradability, and low toxicity. This review presents a comprehensive overview of PARP1 biological features and mechanisms, its role in cancer development, therapeutic implications, and emerging cancer treatments by PARPi-mediated therapies. Although there are a vast number of studies regarding PARP1 biological function, some PARP1 mechanisms are not clear yet, and full-length PARP1 structure is missing. Nevertheless, literature reports demonstrate already the high usefulness and vast possibilities offered by combined PARPi cancer therapy.

ATR, CHK1 and WEE1 inhibitors cause homologous recombination repair deficiency to induce synthetic lethality with PARP inhibitors

PURPOSE

PARP inhibitors (PARPi) are effective in homologous recombination repair (HRR) defective (HRD) cancers. To (re)sensitise HRR proficient (HRP) tumours to PARPi combinations with other drugs are being explored. Our aim was to determine the mechanism underpinning the sensitisation to PARPi by inhibitors of cell cycle checkpoint kinases ATR, CHK1 and WEE1.

EXPERIMENTAL DESIGN

A panel of HRD and HRP cells (including matched BRCA1 or 2 mutant and corrected pairs) and ovarian cancer ascites cells were used. Rucaparib (PARPi) induced replication stress (RS) and HRR (immunofluorescence microscopy for γH2AX and RAD51 foci, respectively), cell cycle changes (flow cytometry), activation of ATR, CHK1 and WEE1 (Western Blot for pCHK1S345, pCHK1S296 and pCDK1Y15, respectively) and cytotoxicity (colony formation assay) was determined, followed by investigations of the impact on all of these parameters by inhibitors of ATR (VE-821, 1 µM), CHK1 (PF-477736, 50 nM) and WEE1 (MK-1775, 100 nM).

RESULTS

Rucaparib induced RS (3 to10-fold), S-phase accumulation (2-fold) and ATR, CHK1 and WEE1 activation (up to 3-fold), and VE-821, PF-477736 and MK-1775 inhibited their targets and abrogated these rucaparib-induced cell cycle changes in HRP and HRD cells. Rucaparib activated HRR in HRP cells only and was (60-1,000x) more cytotoxic to HRD cells. VE-821, PF-477736 and MK-1775 blocked HRR and sensitised HRP but not HRD cells and primary ovarian ascites to rucaparib.

CONCLUSIONS

Our data indicate that, rather than acting via abrogation of cell cycle checkpoints, ATR, CHK1 and WEE1 inhibitors cause an HRD phenotype and hence "induced synthetic lethality" with PARPi.

Concurrent Olaparib and Radiation Therapy in Older Patients With Newly Diagnosed Glioblastoma: The Phase 1 Dose-Escalation PARADIGM Trial

PURPOSE

Patients with glioblastoma who are older or have poor performance status (PS) experience particularly poor clinical outcomes. At the time of study initiation, these patients were treated with short-course radiation therapy (40 Gy in 15 fractions). Olaparib is an oral inhibitor of the DNA repair enzyme poly (ADP-ribose) polymerase (PARP) that is well tolerated as a single agent but exacerbates acute radiation toxicity in extracranial sites. Preclinical data predicted that PARP inhibitors would enhance radiosensitivity in glioblastoma without exacerbating adverse effects on the normal brain.

METHODS AND MATERIALS

Phase 1 of the PARADIGM trial was a 3+3 dose-escalation study testing olaparib in combination with radiation therapy (40 Gy 15 fractions) in patients with newly diagnosed glioblastoma who were unsuitable for radical treatment either because they were aged 70 years or older (World Health Organization PS 0-1) or aged 18 to 69 years with PS 2. The primary outcome was the recommended phase 2 dose of olaparib. Secondary endpoints included safety and tolerability, overall survival, and progression-free survival. Effects on cognitive function were assessed via the Mini Mental State Examination.

RESULTS

Of 16 eligible patients (56.25% male; median age, 71.5 years [range, 44-78]; 75% PS 0-1), 1 dose-limiting toxicity was reported (grade 3 agitation). Maximum tolerated dose was not reached and the recommended phase 2 dose was determined as 200 mg twice daily. Median overall survival and progression-free survival were 10.8 months (80% CI, 7.3-11.4) and 5.5 months (80% CI, 3.9-5.9), respectively. Mini Mental State Examination plots indicated that cognitive function was not adversely affected by the olaparib-radiation therapy combination.

CONCLUSIONS

Olaparib can be safely combined with hypofractionated brain radiation therapy and is well tolerated in patients unsuitable for radical chemoradiation. These results enabled initiation of a randomized phase 2 study and support future trials of PARP inhibitors in combination with radiation therapy for patients with brain tumors.

DNA as the main target in radiotherapy-a historical overview from first isolation to anti-tumour immune response

DNA damage is one of the foremost mechanisms of irradiation at the biological level. After the first isolation of DNA by Friedrich Miescher in the 19th century, the structure of DNA was described by Watson and Crick. Several Nobel Prizes have been awarded for DNA-related discoveries. This review aims to describe the historical perspective of DNA in radiation biology. Over the decades, DNA damage has been identified and quantified after irradiation. Depending on the type of sensing, different proteins are involved in sensing DNA damage and repairing the damage, if possible. For double-strand breaks, the main repair mechanisms are non-homologous end joining and homologous recombination. Additional mechanisms are the Fanconi anaemia pathway and base excision repair. Different methods have been developed for the detection of DNA double-strand breaks. Several drugs have been developed that interfere with different DNA repair mechanisms, e.g., PARP inhibitors. These drugs have been established in the standard treatment of different tumour entities and are being applied in several clinical trials in combination with radiotherapy. Over the past decades, it has become apparent that DNA damage mechanisms are also directly linked to the immune response in tumours. For example, cytosolic DNA fragments activate the innate immune system via the cGAS STING pathway.

DNA damage response inhibitors enhance tumour treating fields (TTFields) potency in glioma stem-like cells

BACKGROUND

High-grade gliomas are primary brain cancers with unacceptably low and persistent survival rates of 10-16 months for WHO grade 4 gliomas over the last 40 years, despite surgical resection and DNA-damaging chemo-radiotherapy. More recently, tumour-treating fields therapy (TTFields) has demonstrated modest survival benefit and been clinically approved in several countries. TTFields is thought to mediate anti-cancer activity by primarily disrupting mitosis. However, recent data suggest that TTFields may also attenuate DNA damage repair and replication fork dynamics, providing a potential platform for therapeutic combinations incorporating standard-of-care treatments and targeted DNA damage response inhibitors (DDRi).

METHODS

We have used patient-derived, typically resistant, glioma stem-like cells (GSCs) in combination with the previously validated preclinical Inovitro™ TTFields system together with a number of therapeutic DDRi.

RESULTS

We show that TTFields robustly activates PARP- and ATR-mediated DNA repair (including PARylation and CHK1 phosphorylation, respectively), whilst combining TTFields with PARP1 or ATR inhibitor treatment leads to significantly reduced clonogenic survival. The potency of each of these strategies is further enhanced by radiation treatment, leading to increased amounts of DNA damage with profound delay in DNA damage resolution.

CONCLUSION

To our knowledge, our findings represent the first report of TTFields applied with clinically approved or in-trial DDRi in GSC models and provides a basis for translational studies toward multimodal DDRi/TTFields-based therapeutic strategies for patients with these currently incurable tumours.

PARP inhibitors combined with radiotherapy: are we ready?

PARP was an enzyme found in the nucleus of eukaryotic cells that played a crucial role in repairing damaged DNA. Recently, PARP inhibitors have demonstrated great potential in cancer treatment. Thus, the FDA has approved several small-molecule PARP inhibitors for cancer maintenance therapy. The combination of PARP inhibitors and radiotherapy relies on synthetic lethality, taking advantage of the flaws in DNA repair pathways to target cancer cells specifically. Studies conducted prior to clinical trials have suggested that the combination of PARP inhibitors and radiotherapy can enhance the sensitivity of cancer cells to radiation, intensify DNA damage, and trigger cell death. Combining radiotherapy with PARP inhibitors in clinical trials has enhanced the response rate and progression-free survival of diverse cancer patients. The theoretical foundation of PARP inhibitors combined with radiotherapy is explained in detail in this article, and the latest advances in preclinical and clinical research on these inhibitors for tumor radiotherapy are summarized. The problems in the current field are recognized in our research and potential therapeutic applications for tumors are suggested. Nevertheless, certain obstacles need to be tackled when implementing PARP inhibitors and radiotherapies in clinical settings. Factors to consider when using the combination therapy are the most suitable schedule and amount of medication, identifying advantageous candidates, and the probable adverse effects linked with the combination. The combination of radiotherapy and PARP inhibitors can greatly enhance the effectiveness of cancer treatment.

PARPi treatment enhances radiotherapy-induced ferroptosis and antitumor immune responses via the cGAS signaling pathway in colorectal cancer

In cancer cells, poly (ADP-ribose) polymerase (PARP)-1 and PARP2 initiate and regulate DNA repair pathways to protect against DNA damage and cell death caused by radiotherapy or chemotherapy. Radiotherapy and PARP inhibitors (PARPis) have been combined in clinical trials, but their action mechanisms remain unclear. Here, we show that activated by ionizing radiation (IR) generated dsDNA, cyclic GMP-AMP synthase (cGAS) signaling promoted regulated cell death, specifically ferroptosis, via the activating transcription factor 3 (ATF3)-solute carrier family 7 member 11 axis and the antitumor immune response via the interferon-β-CD8⁺ T cell pathway. Niraparib, a widely used PARPi, augmented cGAS-mediated ferroptosis and immune activation. In colorectal cancer models, cGAS knockdown (KD) compromised IR-induced ferroptosis via downregulation of ATF3 (key ferroptosis regulator) expression. cGAS depletion reversed IR-induced infiltration of CD8⁺ T or CD8⁺GZMB⁺ T cells in the cGAS KD group. Survival analysis of paired tumor samples before and after standard radiotherapy revealed that high expression levels of cGAS, ATF3, and PTGS2 and high density of CD8⁺ T cells resulted in a significantly high disease-free survival rate in patients with rectal cancer. Therefore, PARPi treatment increases the cytoplasmic accumulation of dsDNA caused by IR, triggering the cGAS signaling-mediated tumor control in cancer cell lines and mouse xenograft models.

Medicinal Chemistry Perspective on Targeting Mono-ADP-Ribosylating PARPs with Small Molecules

Major advances have recently defined functions for human mono-ADP-ribosylating PARP enzymes (mono-ARTs), also opening up potential applications for targeting them to treat diseases. Structural biology combined with medicinal chemistry has allowed the design of potent small molecule inhibitors which typically bind to the catalytic domain. Most of these inhibitors are at the early stages, but some have already a suitable profile to be used as chemical tools. One compound targeting PARP7 has even progressed to clinical trials. In this review, we collect inhibitors of mono-ARTs with a typical "H-Y-Φ" motif (Φ = hydrophobic residue) and focus on compounds that have been reported as active against one or a restricted number of enzymes. We discuss them from a medicinal chemistry point of view and include an analysis of the available crystal structures, allowing us to craft a pharmacophore model that lays the foundation for obtaining new potent and more specific inhibitors.