Update on Combination Strategies of PARP Inhibitors

Abstract

The application of PARP inhibitors has revolutionized cancer treatment and has achieved significant advancements, particularly with regard to tumors with defects in genes involved in homologous recombination repair (HRR) processes, such as BRCA1 and BRCA2. Despite the promising outcomes of PARP inhibitors, certain limitations and challenges still exist, including acquired drug resistance, severe side effects, and limited therapeutic benefits for patients without homologous recombination deficiency (HRD). Various combinations involving PARP inhibitors have been developed to overcome these limitations. Among these, combinations with immune checkpoint inhibitors, antiangiogenic agents, and various small-molecule inhibitors are well-studied strategies that show great potential for optimizing the efficacy of PARP inhibitors, overcoming resistance mechanisms, and expanding target populations. However, the efficiency and overlapping toxicity of these combination strategies for cancers vary among studies, thereby limiting their use. In this review, we describe the mechanisms and limitations of PARP inhibitors to better understand the mechanisms of combination treatments. Furthermore, we have summarized recent studies on the combination of PARP inhibitors with a range of medications and discussed their clinical efficacy. The objective of this review is to enhance the comprehensiveness of information pertaining to this topic.

Key Points

• 1. This comprehensive literature review presents an updated combination therapy of PARP inhibitors with a diverse range of medications.

• 2. The focus of this review encompasses olaparib, rucaparib, talazoparib, niraparib, and veliparib, which have been extensively investigated in clinical trials involving immune checkpoint inhibitors, antiangiogenic drugs, and the DNA damage response pathway. Additionally, this review elucidates the underlying principles and clinical evidence supporting drug combinations while also discussing emerging combinations such as HSP90 inhibitors, BET inhibitors, and novel delivery carriers.

• 3. Furthermore, it addresses current dilemmas encountered in combination treatments and provides insights into future development directions.

Introduction

Poly(ADP-ribose) polymerase (PARP) inhibitors are a group of targeted agents that take advantage of the synthetic lethality ¹ between PARP-mediated single-strand breaks and homologous recombination repair (HRR), which is frequently impaired in tumors carrying BRCA1/2 mutations or other factors affecting the DNA damage response (DDR).^2,3 At present, PARP inhibitors (PARPis) have demonstrated encouraging clinical activity against various solid tumors, particularly ovarian, ⁴ breast, ⁵ prostate, ⁶ and pancreatic cancers.^7,8 However, the development of resistance to PARPis^9,10 and their limited efficacy in tumors without BRCA1/2 mutations pose significant challenges for their clinical application. ¹⁰ Therefore, novel strategies to enhance the anti-cancer effects of PARPis and overcome drug resistance are urgently required. One potential strategy with high prospects involves the combination of PARPis with alternative medications that target different pathways or mechanisms involved in tumor growth, survival, or immune evasion. Here, we summarize the existing evidence and challenges of combining PARPis with other drugs in solid tumors and explore potential future directions and perspectives for this emerging therapeutic modality.

PARP Inhibitors

Mechanisms of PARPis-mediated Cytotoxicity

ADP-ribosyltransferases (ARTs) constitute a class of nuclear enzymes that catalyze the transfer of ADP-ribose to target proteins. ¹¹ ADP-ribosylation plays a crucial role in the transfer of a single ADP-ribose subunit (MARylation) or multiple ADP-ribose subunits (PARylation) from NAD⁺ to a protein in a posttranslational covalent manner.^11,12 This process plays crucial roles in numerous biological processes, including DNA damage, DNA repair, cell proliferation, metabolism, stress response, and differentiation. The current superfamily of ADP-ribosylating enzymes consists of 17 members, including PARP1, PARP2, PARP5A, and PARP5B, which can synthesize poly(ADP-ribose). ¹³ PARP1, a prominent member of the superfamily, accounts for approximately 90% of cellular PARylation. ¹³ It consists of three structural regions: DNA-binding, self-modifying, and catalytic. ¹³

When DNA damage such as single-stranded DNA breaks (SSBs) occurs, the twisted DNA double helix provides a recognition site for the zinc-finger (ZnF) DNA-binding domains of PARP1. Initially, PARP1 is brought to the DNA break site via its N-terminal ZnF domain, which is responsible for binding to DNA. ¹⁴ The interaction between ZnF and the DNA recognition site facilitates the assembly of other functional regions of PARP1 into nucleoproteins. Substrate proteins, including nucleoproteins and PARP1, are then catalyzed by the active PARP. Owing to the presence of two negatively charged phosphate groups in each ADP-ribose subunit, a significant negative charge accumulates at the DNA lesions, reinforcing the enlistment of XRCC1 and additional DDR proteins for mending the impaired DNA. After auto-PARylation, DNA release is facilitated by electrostatic repulsion between the phosphate of PAR and DNA (Figure 1). This dissociation was rapidly facilitated by poly(ADP-ribose) glycohydrolase (PARG) and ADP-ribosylhydrolase 3 (ARH3). Inactivated PARP prepares the next repair task.^2,3,12 Although PARP2 has a distinct domain architecture, it partially overlaps with PARP1. ¹⁵ PARP2 shares many properties with PARP1 but contributes less to DNA damage repair. ¹³ To repair DNA damage and preserving genomic integrity, PARP1/2 is crucial. ¹⁶

Figure 1.

Role of PARP1 in DNA damage repair.

Currently, several clinical trials have yielded positive results for various PARPis, including olaparib, rucaparib, talazoparib, niraparib, and veliparib (ABT-888). By trapping PARP1 and PARP2, PARPis block the repair of DNA single-strand breaks by these enzymes, leading to the cessation of replication forks. On the one hand, PARPis competitively blocks NAD⁺ from binding to PARP-1, inhibiting the polymerization of ADP-ribose and the repair of SSBs.^3,13 On the other hand, PARPis binds to PARP1 like its substrate NAD⁺ and secures the catalytic region, preventing auto-PARylation and capturing PARP-1 at the location of damage. Different PARPis have been shown to vary in their trapping abilities. ¹⁷

Novel insights into the mechanisms of action of PARP inhibitors have been proposed. The synthetic lethality of PARP inhibitors with homologous recombination (HR) defects may be attributed to the impairment of DDR caused by transcription-replication conflicts (TRCs). PARP1 signals impend TRCs to TIMELESS and TIPIN, halting replication to the resolution of the TRC. Inhibition of PARP catalytic activity or dysfunction of TIMELESS and TIPIN functionality leads to TRC-induced DNA damage, which is synthetically lethal, specifically in HR-deficient cancer cells. ¹⁸

The concept of synthetic lethality has been effectively utilized by PARPis, which emerged as the first clinically approved drug in this paradigm. Synthetic lethality is characterized by non-viability resulting from the simultaneous absence of two genes that, when present individually, do not lead to lethality. The HR repair mechanism for double-stranded DNA breaks (DSBs) heavily relies on the essential involvement of the BRCA proteins. In BRCA-deficient tumors, HR is impaired, making tumor cells heavily reliant on PARP-mediated alternative DNA repair pathways. Moreover, PARPis reduces the DNA repair capacity, ultimately resulting in cell death (Figure 1).^2,7

PARP inhibitors have demonstrated varying potencies and anti-tumor activities in the treatment of diverse malignancies. ¹⁹ Olaparib is a pioneering PARP inhibitor that has been approved, followed by Niraparib, Rucaparib, and Talazoparib. PARP inhibitors have shown significant efficacy in the treatment of ovarian cancer, particularly in patients with BRCA mutations. Olaparib has been utilized as a first-line maintenance therapy for advanced ovarian cancer in combination with bevacizumab and metastatic pancreatic cancer. Moreover, it is used as an adjuvant treatment for HER2-negative high-risk early breast cancer harboring gBRCA mutations, HER2-negative metastatic breast cancer harboring gBRCA mutations, and metastatic castration-resistant prostate cancer harboring HRR mutations. ²⁰ Niraparib has exhibited favorable results in advanced ovarian cancer as first-line maintenance therapy and maintenance treatment for recurrent ovarian cancer harboring BRCA mutations. ²⁰ Rucaparib has also been shown to be effective in treating recurrent ovarian cancer and has been used in prostate and pancreatic cancers. ²¹ Talazoparib has been approved for the treatment of HER2-negative advanced or metastatic breast cancers with gBRCA mutation. ¹⁹ Although these PARP inhibitors exhibit similar mechanisms of action, they display variations in pharmacokinetics, pharmacodynamics, safety, and tolerability. ²²

When the DNA single-strand breaks, PARP1 is recruited to the DNA break site via ZnF and facilitates the assembly of other functional regions of PARP1 onto the nucleoprotein. Then activated PARP1 catalyzes PARylation of nuclear proteins and PARP1. The presence of two phosphate groups with negative charges in each ADP-ribose subunit results in a considerable accumulation of negative charge at sites of DNA damage, which facilitates the recruitment of DDR proteins such as XRCC1 for repairing the affected DNA. After auto-PARylation, electrostatic repulsion between the DNA and the PAR phosphate groups allows for dissociation from DNA. Consequently, SSB is repaired. PARPis not only trap PARP1 and PARP2 resulting in halted replication forks but also competitively block NAD+ from binding to PARP-1. The HR plays a crucial role in repairing DSBs correctly. For HRD tumor cells, the use of PARPis can induce cell death by inhibiting the repair process of DNA single-strand breaks. However, it does not significantly impact HRD-negative cells. BRCT (BRCA1 C-Terminal domain), HD (helical domain); WGR (tryptophan-glycine-arginine-rich); ART (ADP-ribosyltransferase domain); PARP1 (poly(ADP-ribose) polymerases 1); ZnF (zinc-finger); XRCC1 (X-ray Repair Cross Complementing group 1), SSB (single-strand DNA breaks); DSB (double-strand DNA breaks); HRR (homologous recombination repair); HRD (homologous recombination deficiency). By Figdraw.

Limitation of PARP Inhibitors

Most cancers that respond well to PARPis have deficiencies in DNA repair, particularly those associated with BRCA1/2 mutations and homologous recombination deficiency (HRD). In tumors without these genetic alterations, the therapeutic benefits of PARPis may be limited. In the PAOLA-1 (PAOLA-1/ENGOT-ov25) trial, it was observed that patients with HRD-negative tumors had a median progression-free survival (PFS) of 16.6 months in the olaparib group and 16.2 months in the group of combination of bevacizumab and placebo. On the other hand, patients with HRD-positive tumors exhibited a significantly longer median PFS of 37.2 months in the olaparib group compared to 17.1 months in the placebo group. ²³ These findings suggest that opinions on the effectiveness of PARPis in HRD-negative patients may differ. Meanwhile, the effectiveness of PARPis treatment among individuals with HRD-negative was found to be inferior compared to those with BRCA-mutant and HRD in phase 3 clinical trials including SOLO1, PRIMA, VELIA, PRIME, and ATHENA-MONO (HR for HRD-negative vs BRCA-mutant in PARP inhibitor arms: 1.92; 95% CI 1.62-2.21; P < .001), ²⁴ corroborating similar findings.

Furthermore, acquired PARP inhibitor resistance is a frequently occurring unmet medical need and is mostly caused by secondary genetic or epigenetic changes that reinstate the proficiency of HRR, which bypasses the effects of PARPis.^25-27 The side effects of PARPis inhibitors may limit patient tolerability and treatment effectiveness.^28,29 Therefore, combination treatment strategies may help overcome these limitations and improve treatment outcomes.

Combination Strategy of PARP Inhibitors

Considering the limited application of PARPis in HR-proficient tumors, there is an ongoing comprehensive inquiry into the utilization of PARPis in conjunction with various therapies, such as chemotherapy, radiation, immunotherapy, and novel pharmaceuticals targeting DDR and angiogenesis. Combining PARPis with agents inducing ‘BRCAness’ or those inhibiting HRR represents a potential approach to address acquired and primary resistance to PARPis in tumors with HRR deficiency. The subsequent sections cover preclinical and clinical research on novel combinations of PARPis and their underlying mechanisms.

PARP Inhibitors + Immune Checkpoint Inhibitors

Immune checkpoint inhibitors (ICIs) elicit an anti-tumor immune response by negating inhibitory signals that impede T cell activation. ³⁰ Both anti-CTLA-4 and anti-PD-1/PD-L1 therapies exert their anti-cancer effects by suppressing signals that impede T-cell functionality. ³¹

Administration of PARPis to patients with HR deficiency or other DNA damage repair defects disrupts normal DNA repair fidelity, resulting in increased tumor mutational load and neoantigen burden, thereby enhancing tumor susceptibility to ICIs. ³² The potential of PARPis lies in its capacity to convert chronic low-level DNA damage into an enhanced immune response that selectively targets Th1 cells, thereby fostering the establishment of a tumor microenvironment conducive to favorable outcomes (Figure 2). ³³

Figure 2.

The rationale for combining PARP inhibitors with ICIs.

An accumulation of cytosolic dsDNA induced by PARPis can drive the cGAS-STING-TBK1-IRF3 innate immune pathway, consequently eliciting type I IFN and its related immune responses. The recruitment of cytotoxic T cells to induce cancer cell death is facilitated by CXCL10 and CXCL5. Additionally, PD-L1 expression is increased through activation of the cGAS-STING-TBK1 pathway. CXCL10(C-X-C motif chemokine 10), CCL5 (C-C motif chemokine 5), PD-L1 (programmed cell death 1 ligand 1), DC (Dendritic cells), TBK1 (serine/threonine-protein kinase), IRF3 (interferon regulatory factor 3), STING (stimulator of interferon genes protein), cGAS (Cyclic GMP-AMP synthase). By Figdraw.

Combining PARPis with PD-1 can enhance immune cell infiltration into the tumor microenvironment and augment synergistic antitumor activity. This occurrence has been noted in tumors exhibiting immunogenicity, including breast cancer, colorectal adenocarcinoma, squamous cell lung carcinoma, bladder cancer, and sarcoma, irrespective of BRCA status. ³⁴ Several clinical trials have been conducted to explore the efficacy of combining PARPis with ICIs, which have yielded promising initial findings (Table 1).

Table 1.

Efficacy Data for Selected Clinical Trials Assessing Combination Strategy of PARP Inhibitors and ICIs. ³⁵

NCT number	PARPis	Drugs	Target	Study status	Conditions	Phases	Enrollment
NCT03459846	Olaparib	Durvalumab	PD-L1	Not recruiting	Urinary Bladder Neoplasms	2	154
NCT03404960	Niraparib	Ipilimumab or Nivolumab	CTLA-4/ PD-1	Not recruiting	Pancreatic Adenocarcinoma	1/2	104
NCT03834519	Olaparib	Pembrolizumab	PD-1	Not recruiting	Prostatic Neoplasms	3	793
NCT03775486	Olaparib	Durvalumab	PD-L1	Not recruiting	Non-small Cell Lung Cancer NSCLC	2	401
NCT03167619	Olaparib	Durvalumab	PD-L1	Completed	Triple Negative Breast Cancer	2	45
NCT04034927	Olaparib	Tremelimumab	CTLA-4	Not recruiting	Recurrent Ovarian, Fallopian Tube or Peritoneal Cancer	2	61
NCT02657889	Niraparib	Pembrolizumab	PD-1	Completed	Triple-negative Breast Cancer or Ovarian Cancer	1/2	122
NCT03642132	Talazoparib	Avelumab	PD-L1	Terminated	Ovarian Cancer	3	79
NCT03308942	Niraparib	Pembrolizumab	PD-1	Completed	Neoplasms	2	53
NCT02935634	Rucaparib	Nivolumab	PD-L1	Completed	Advanced Gastric Cancer	2	190
NCT03338790	Rucaparib	Nivolumab	PD-L1	Not recruiting	Prostate Cancer	2	292
NCT02734004	Olaparib	MEDI4736	PD-L1	Not recruiting	Ovarian/Breast/SCLC/Gastric Cancers	1/2	264
NCT03955471	Niraparib	Dostarlimab	PD-1	Terminated	Ovarian Neoplasms	2	41
NCT03565991	Talazoparib	Avelumab	PD-L1	Terminated	Locally Advanced or Metastatic Solid Tumors|Genes, BRCA 1	2	202
NCT03330405	Talazoparib	Avelumab	PD-L1	Terminated	Patients suffering from solid tumors that are either locally advanced(primary or recurrent) or have metastasized	1/2	223
NCT04173507	Talazoparib	Avelumab	PD-L1	Not recruiting	Stage IV or Recurrent Non-Squamous Non-Small Cell Lung Cancer With STK11 Gene Mutation	2	47

The most recent findings from the clinical trial reveal that in a non-randomized controlled trial study of phase 2b (NCT03565991) involving 200 patients with various types of cancer revealed that defection of the BRCA1/2 or ATM gene groups achieved the predetermined goal of a 40% objective response rate (ORR). However, the combination treatment exhibited anti-tumor activity in specific tumor types linked to mutations in the BRCA1/2 genes and uterine leiomyosarcoma patients, but showed limited effectiveness in cancer types unrelated to BRCA. Negligible efficacy was noted for cancer types unrelated to BRCA mutations. ³⁶

In an open-label, two-stage phase II study without randomization or a control group, 35 individuals diagnosed with recurrent endometrial cancer proficient in mismatch repair were enrolled and administered avelumab at a dosage of 800 mg every 2 weeks, along with talazoparib at a daily dose of 1 mg. Nine patients (25.7%) experienced clinical improvement after meeting at least one of the two primary endpoints. Four patients (11.4%) demonstrated objectively confirmed ORR, including four partial responses. Furthermore, eight patients (22.9%) displayed six months of PFS, indicating that tumors harboring combined repair alterations exhibited favorable clinical outcomes when treated with avalumab and talazoparib. ³⁷

The TOPACIO/KEYNOTE-162 trial examined the combination of pembrolizumab and niraparib in patients with triple-negative breast cancer (TNBC) or ovarian cancer (OC) and revealed an overall response rate (ORR) of 18% (90% confidence interval [CI], 11%-29%). Notably, combined treatment with anti-PD-1 antibody and niraparib demonstrated enhanced effectiveness in both the tBRCAwt subgroup (ORR, 19%) and the non-HRD subgroup (ORR, 19%), surpassing that observed with either agent alone as monotherapy. ³⁸

PARP inhibitors + Antiangiogenic Drugs

The combined use of PARPis and antiangiogenic medications exhibits an augmented anti-cancer effect by inducing hypoxia, which results in a reduction in BRCA1 expression through the redistribution of promoter occupancy facilitated by activating and repressing E2Fs. Suppression of BRCA1 and RAD51 expression, pivotal components of HRR, disrupts the equilibrium between HRR and NHEJ, leading to genetic instability within cancer cells.^39-41

Inhibition of VEGFR3 enhances sensitivity towards PARPis in cell lines with wild-type BRCA genes, while reinstating chemosensitivity in resistant cell lines in which a mutation in BRCA2 reverts to its original state. ⁴² Furthermore, treatment with both PARPis and PARP1 knockout impedes the migration response to VEGFs, hinders the development of tubule-like networks, and impedes angiogenesis in vivo. ⁴³

A phase 3 clinical trial (NCT02446600) compared the effectiveness of platinum-based chemotherapy with other treatments, specifically olaparib and olaparib/cediranib, in patients with high-grade serous or endometrioid platinum-sensitive OC. Regrettably, there were no notable variations observed in PFS among the three treatment cohorts comprising chemotherapy, olaparib alone, and olaparib/cediranib combination therapy (median PFS: 10.3 months [95% CI: 8.7 to 11.2], 8.2 months [95% CI: 6.6 to 8.7], and 10.4 months [95% CI: 8.5 to 12.5], respectively). ⁴⁴

A superiority trial (NCT02354131) conducted in 2019 examined the effectiveness of combining niraparib with bevacizumab vs niraparib alone as a single treatment for platinum-sensitive recurrent OC, classified as high-grade serous or endometrioid cancer. The results showed a significant enhancement in PFS, with a median PFS duration of 11 months (95% CI, 8-16 months), compared to 5 months (3-6 months) when niraparib was used alone. These findings were consistent regardless of the HRR status or the interval between treatments without chemotherapy. ⁴⁵

Researchers have conducted investigations to assess the effectiveness of anlotinib in treating recurrent OC that is resistant to platinum-based therapies. Out of the 40 patients included in the study, BRCA1 mutations were only detected in 38.1% (95%CI 38.1%-72.1%) of cases, and their median PFS was determined to be 8.3 months (95%CI 5.86-10.81). However, no conclusive data regarding the median overall survival (OS) could be determined from this study. ⁴⁶

In the AVANOVA2 study, 97 patients diagnosed with platinum-sensitive recurrent OC with measurable disease were enrolled. The patients were administered either a combination of niraparib and bevacizumab or niraparib alone for therapeutic purposes. The median PFS was used to compare the efficacy of combining therapies with that of using a single agent in treating patients. Subgroup analysis revealed notable variations in progression-free survival (PFS) among patients with different HRD and BRCA mutation statuses, particularly in those with wild-type HRD. Regarding safety considerations, there were higher occurrences of grade ≥3 hypertension, deep vein thrombosis, and proteinuria primarily observed in the combination therapy group due to the administration of bevacizumab. ⁴⁵

PARP Inhibitors + PI3K/AKT/mTOR Inhibitors

Aberrant stimulation of the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) pathway occurs in various tumors. This signaling cascade plays a crucial role in regulating tumor cell proliferation, differentiation, and apoptosis. ⁴⁷

Although PARPis has minimal single-agent activity in non-BRCA-mutant tumors, studies have demonstrated that in different breast cancer cell models and xenograft models, there is a notable enhancement in therapeutic effects when PI3K inhibitors are combined with these compounds. ⁴⁷

The tag-developing Phase 1b study enrolled individuals diagnosed with epithelial OC. The findings revealed that among patients with OC, 10 (36%) exhibited a partial response (PR), while 3 (11%) experienced disease progression. The results of this trial suggest that alpelisaparib may lead to unexpected toxicities, specifically in patients with epithelial OC, warranting further investigation. ⁴⁸

In a phase IIb clinical trial, 17 patients with recurrent breast cancer, including those with TNBC and other subtypes, were selected based on their germline BRCA mutation status. The dose-escalation cohort consisted of four patients, while the dose-expansion cohort included 13 patients. Results from the study indicated that a partial response was observed in three patients, while stable disease was maintained in seven patients. The ORR was determined to be 18%, with a clinical benefit rate of 35% and a disease control rate (DCR) of 53%. Additionally, the median duration of response was found to be 7.4 months, whereas the median PFS stood at 3.6 months. The 6-month PFS rate was 31.2% and the median OS was 11.8 months. Notable grade 3-4 treatment-associated adverse effects were observed in a minimum of 10% of the total patient population, including hyperglycemia (18%) and rashes (12%). This study demonstrated that combining olaparib with alpelisib is well tolerated and safe for use in this patient population who has undergone extensive prior treatments, showing promising response and clinical benefit rates of 18% and 35%, respectively. To better determine the specific patient groups that would derive the maximum advantages from oral olaparib combined with alpelisib, larger prospective randomized studies incorporating biomarker exploration are warranted. ⁴⁹

PARP Inhibitors + Targeting DDR Pathway Drugs

PARPis + ATRis

ATR, a crucial component of the phosphatidylinositol kinase–related protein family, plays a critical role in DDR by sensing replication stress and initiating signaling pathways that regulate the S and G2/M checkpoints. Additionally, it regulates DNA repair mechanisms to promote cell cycle arrest and enhances DNA repair or apoptosis.^50,51

The combination of PARPi-ATRi enhances anti-cancer effects in ATM-deficient cancer cells by promoting genome instability and cell death ⁵² and could be a highly effective approach for addressing OC cases that have developed resistance to both platinum and PARPi treatments. ⁵³ Furthermore, the combination of ATR inhibitors synergistically enhanced the efficacy of PARPis by augmenting SSBs and trapping PARP1 onto DNA in OC BRCA1-mutant cell models and BRCA-mutant TNBC PDX models. ⁵⁴

Several clinical trials are currently assessing the efficacy of four ATR inhibitors in combination with PARPis (Table 2). ⁵¹ Findings from the Phase II VIOLETTE trial (NCT03330847) indicated that there were no notable disparities observed in PFS, the primary endpoint, between the combination of olaparib and ceralasertib and olaparib monotherapy in 273 patients with TNBC, regardless of the tumor mutation status. ⁵⁵ Preclinical data suggest that tumors with loss-of-function mutations in ARID1A (AT-rich interactive domain-containing protein 1A) exhibit increased sensitivity to ATR inhibitors (ATRi). Furthermore, even without ARID1A mutations, the concurrent utilization of ATRis and PARPis demonstrates a synergistic effect, enhancing the sensitivity of gynecological cancer patients. ⁵⁶

Table 2.

Efficacy Data for Selected Clinical Trials Assessing Combination Strategy of PARP Inhibitors and ATR Inhibitors. ³⁵

NCT number	PARPis	Drugs	Target	Study status	Conditions	Phases	Enrollment
NCT04972110	Niraparib/Olaparib	Camonsertib	ATR	Recruiting	Advanced Solid Tumor, Adult	1/2	108
NCT04065269	Olaparib	AZD6738	ATR	Unknown status	Gynaecological Cancers	2	40
NCT04267939	Niraparib	Elimusertib	ATR	Not recruiting	Advanced Solid Tumors (Excluding Prostate Cancer)|Ovarian Cancer	1	14
NCT04497116	Talazoparib	Camonsertib	ATR	Recruiting	Advanced Solid Tumor	1/2	61
NCT03462342	Olaparib	AZD6738	ATR	Recruiting	High Grade Serous Carcinoma	2	451
NCT04170153	Niraparib	M1774	ATR	Recruiting	Metastatic or Locally Advanced Unresectable Solid Tumors	1	86
NCT03787680	Olaparib	AZD6738	ATR	Not recruiting	Prostate Cancer	2	49

The ATARI trial (ENGOT/GYN1/NCRI) is a phase 2 study conducted at multiple centers to assess the effectiveness of ceralasertib as a stand-alone treatment when used in conjunction with olaparib in patients with gynecologic tumors of various histological subtypes. Stratification based on ARID1A status allows the assessment of treatment effectiveness in specific tumor subtypes. ⁵⁷ In a cohort of 29 individuals who experienced relapse of ovarian and endometrial clear cell carcinomas, monotherapy with ceralasertib, an ATR inhibitor, demonstrated an ORR of 14%. The use of ceralasertib as a stand-alone treatment and the combined administration of olaparib exhibited clinical efficacy in treating uterine clear cell tumors. Furthermore, the combination therapy displayed anti-tumor activity in the non-clear cell “basket” cohort encompassing sarcomas, providing compelling evidence for further investigation. ⁵⁸

Additionally, a Phase 2 umbrella trial (NCT04298021) is currently in progress to investigate the potential of combining AZD6738 with Durvalumab or AZD6738 with Olaparib in patients with advanced biliary tract cancer who have experienced first-line chemotherapy failure. The primary aim of this study was to evaluate the combined therapeutic response to olaparib and ceralasertib in the treatment of biliary tract cancer. ⁵⁹

PARPis + WEE1is

WEE1, a protein kinase involved in cell cycle regulation and DNA damage response, is the focus of inhibitors that hinder the phosphorylation of CDK1, which acts as an inhibitor. ⁶⁰ This compromises the capacity to inhibit mitotic commitment upon DNA damage, ultimately leading to DNA replication stress and mitotic catastrophes. ⁶¹ Additionally, excessive activation of the MUS81-SLX4 nuclease by WEE1 inhibitors exacerbates severe DNA damage during the G1/S phase.^62-64 These results established a theoretical foundation for the synergistic use of WEE1 and PARP inhibitors.

While gastric cancer cells and xenograft models have shown limited responses to PARPis alone, their efficacy was significantly enhanced when combined with the dual inhibitor AZD1775, which targets WEE1/PLK1. ⁶⁵ Additionally, this combination exhibited efficacy in small-cell lung cancer(SCLC) cell lines with PALB2 mutations and a prolonged response time to platinum-based therapy, leading to an enhanced and long-lasting reaction. ⁶⁶

Receiving PARP and WEE1 inhibitors in a sequential manner resulted in remarkable improvements in both the extent and length of the response among patients with OC with diverse genetic backgrounds and histology, surpassing the efficacy observed with either agent alone. This therapeutic approach has demonstrated favorable tolerability profiles, establishing its viability as a promising clinical strategy. ⁶⁷ Phase I/II clinical trials are currently evaluating the efficacy of a combination therapy involving AZD1775, a WEE1 inhibitor, and olaparib for the treatment of OC, breast cancer, and small-cell lung cancer. ⁶⁶

PARPis + CHK1is

Activation of checkpoint kinase I (CHK1) occurs in response to DNA replication stress and damage and functions as a component of the intra-S and G2/M checkpoint control pathways. This activation occurs downstream of ATR. The inhibition of CHK1 results in mitotic catastrophe and cell death, contributing to its antitumor effect.^68,69 The concurrent use of CHK1 inhibitors and PARPis has demonstrated a synergistic effect in mammary, gastric, SCLC, high-grade serous OC cell lines, and PDX models by increasing the number of single- and double-stranded DNA breaks. Furthermore, this combination enhanced radiotherapy-induced DNA breaks in pancreatic cancer cells harboring p53 mutations.^70,71

Based on preclinical rationale and evidence, a study was initiated to evaluate the potential of combining Prexasertib, a CHK1 inhibitor, with Olaparib, a PARP inhibitor. Encouraging partial responses were observed in patients with BRCA1-mutant HGSOC who had previously shown resistance to PARPis. ⁷² Furthermore, a single-arm clinical trial conducted in patients with HGSOC carrying BRCA1 or BRCA2 mutations treated with Prexasertib and Olaparib demonstrated partial responses in patients with PARP inhibitor resistance. ⁷³ Synergistic effects of WEE1 inhibition and PARP blockade have been observed in biliary tract cancer as well as in TNBCs harboring Cyclin E or BRCA1 alterations in cell lines and PDX models.^63,74,75 The anti-cancer efficacy of WEE1 inhibitors is potentiated by modulating the DDR pathway, specifically by inducing improper cleavage of stalled replication forks caused by the entrapment of PARP enzymes by AZD1775.^66,76

PARPis + DNA-PKcsis

DNA-dependent protein kinase (DNA-PK) consists of Ku-70, Ku-80, and the catalytic subunit DNA-PKcs. This enzyme belongs to the family of phosphoinositide 3 lipid kinases (PI3K)-related protein kinases (PIKK).

Research suggests that genetic deficiencies in DNA-PKcs enhance the efficiency of ionizing radiation (IR) and DSB-inducing agents. Studies have also demonstrated that the depletion of ataxia-telangiectasia mutated (ATM) kinase ⁷⁷ or the deficiency of MutS homologue 3 (MSH3) sensitizes DNA-PK inhibitors. ⁷⁸ Jacqueline and colleagues. proposed that combining a DNA-PK inhibitor (AZD7648) with the PARP inhibitor olaparib exhibits antiproliferative efficacy, resulting in genome instability and apoptosis in ATM-knockout cells. ⁷⁹ Similarly, Anastasia et al ⁸⁰ demonstrated that the DNA-PKcs inhibitor AZD7648 promoted the effectiveness of Olaparib in BRCA-deficient high-grade serous epithelial OC models (OC-PDX). In hepatocellular carcinoma (HCC), there is excessive activation of both the HR and NHEJ pathways compared to the neighboring normal tissues. Interestingly, the simultaneous inhibition of PARP1 and DNA-PKcs led to the remarkable synergistic suppression of HCC cell survival. These findings have been validated in vivo. Mechanistically, PARPis hinders the removal of nucleosomes from DNA damage sites by preventing ALC1 recruitment to DSB sites, which in turn inhibits the recruitment of RPA2 and RAD51 for HR repair. ⁸¹ In head and neck squamous cell carcinoma(HNSCC), combination treatment with the DNA-PKcs inhibitors NU7441, Olaparib and irradiation exerted significantly greater antiproliferative effects than any single agent alone. This combination induced increased apoptosis and G2/M cell cycle arrest. Similar results have been observed in animal models. ⁸²

PARP Inhibitors + Toxicity Chemotherapy

Toxic chemotherapy has traditionally served as the cornerstone of treatment for most solid tumors; however, its notable systemic toxicity and emergence of resistance to anti-cancer chemotherapeutics present significant challenges. For example, cisplatin forms DNA adducts that disrupt DNA replication and repair in cancer cells, ultimately inducing apoptosis. However, the development of cisplatin resistance in certain cancer cells restricts their therapeutic efficacy. The rationale behind combining a PARP inhibitor with a cisplatin regimen to achieve synergistic effects is supported by an expanding body of research establishing an association between cisplatin and PARPis across various malignancies.

In a phase 2 trial (NCT02595905) investigating the efficacy of a treatment for metastatic TNBC and breast cancer linked to the BRCA gene, veliparib and cisplatin were administered. Participants were assigned to two treatment groups: one receiving cisplatin chemotherapy combined with PARP inhibitor veliparib, and the other receiving cisplatin chemotherapy along with a placebo. Patients harboring wild-type germline BRCA1/2 were further categorized into two groups based on their phenotype, namely, BRCA-like and non-BRCA-like, using a well-established biomarker panel. The study findings revealed that among individuals with BRCA-like breast cancer, those who received veliparib treatment experienced a longer median PFS of 5.9 months compared to 4.2 months in the placebo group (HR = 0.57; 95% CI 0.37-0.88; P = .011). Moreover, these patients demonstrated numerical improvements in the median OS and ORR compared to those in the placebo group. Notably, the addition of veliparib to cisplatin had a positive impact on PFS in patients with BRCA-like metastatic TNBC, while no such effect was observed in individuals with non-BRCA metastatic breast cancer; however, it is important to note that these differences did not reach statistical significance. ⁸³

PARP Inhibitors + New Hormonal Agent (NHA)

The combination of PARP inhibitors and NAH has ushered in a new era as a first-line treatment option.^84,85 Notable published studies on the use of a PARPi plus NHA as a first-line treatment for mCRPC include PROpel, ⁸⁶ TALAPRO-2, ⁸⁷ and MAGNITUDE. ⁸⁸ A systematic review encompassing these pivotal phase III studies and meta-analyses was conducted to compare radiographic progression-free survival (rPFS), OS, and treatment-related adverse events between patients with mCRPC receiving first-line PARPi plus ARPi and those receiving placebo/ARPI. ⁸⁹

In the all-comer population, pooled results for the common main endpoint of rPFS showed a 35% rPFS improvement (HR 0.65, 95% CI 0.56-0.76, P < .001), with the degree of the effect varying depending on the subgroup by HRR status. The BRCA1/2 mutant group (HR 0.32, 95% CI 0.17-0.61, P < .001) showed the largest advantage with rPFS, followed by the HRRm cohort (HR 0.55, 95% CI 0.39-0.77, P < .001) and the non-HRRm cohort (HR 0.74, 95% CI 0.61-0.90, P = .003). ⁸⁹

The combination of ARPI-PARPi has exhibited favorable clinical outcomes in BRCA1/2 mutated subgroup. Positive outcomes were also observed in the HRRm patient cohort, although varying responses were observed based on a particular genetic mutation. Furthermore, the clinical benefits in the all-population and non-HRRm populations must be balanced by the toxicity associated with combination therapy. Notably, prostate cancer is the only tumor in which the combination of a PARP inhibitor with an NHA has demonstrated positive clinical results, even in patients without HR gene mutations.

PARP Inhibitors + Others

Heat shock protein 90 (HSP90), an essential molecular chaperone, is involved in multiple cellular processes such as ensuring the stability, development, and optimal performance of various substrates. Among these substrates, BRCA1/2 and RAD51 play pivotal roles in HRR. Inhibition of HSP90 can disrupt the HRR, rendering cancer cells more susceptible to PARPis. ⁹⁰

Preclinical in vivo studies have been conducted using the HSP90 inhibitor onalespib in combination with olaparib. Additionally, a phase 1 clinical trial evaluated the safety and tolerability of this combination treatment, as well as its steady-state pharmacokinetics and initial effectiveness. Co-administration of these agents has demonstrated effectiveness and early indication of anti-tumor activity. ⁹¹

Furthermore, synergistic anti-cancer effects were observed with combined PARP and BET inhibition in SCLC cells with MYCs amplification and in SCLC cells without MYCs mutation. Mechanistic investigations have elucidated that the combinatorial efficacy is associated with a reduced HR double-strand break repair process as well as downregulation of various players involved in DDR, such as CHEK2, PTEN, NBN, and FANCC, through BET inhibition (Table 3). ⁹²

Table 3.

Efficacy Data for Selected Clinical Trials Assessing Combination Strategy of PARP Inhibitors and BETis. ³⁵

NCT number	PARPis	Drugs	Target	Study status	Conditions	Phases	Enrollment
NCT05327010	Talazoparib	ZEN003694	BET	Recruiting	Malignant Solid Neoplasm	2	88
NCT05071937	Talazoparib	ZEN003694	BET	Recruiting	Ovarian Cancer/Peritoneal Cancer/Fallopian Tube Cancer	2	33
NCT05252390	Olaparib	NUV-868	BET	Recruiting	Advanced Solid Tumor	1/2	657

Histone deacetylase(HDAC) inhibitors, a prominent class of epigenetic modulators, regulate transcription and tumor progression by modulating histone acetylation. ⁹³ In prostate cancer cells, HDAC inhibitors exhibit significant synergistic effects with PARPis by suppressing HR repair and specifically disrupting the UHRF1/BRCA1 complex. ⁹⁴

PARP Inhibitors + Radiotherapy

Radiotherapy is an essential component in the treatment of malignant tumors as it specifically targets the DNA of cancerous cells. The anti-tumor effect of radiotherapy is achieved through its precise action on the DNA of malignant cells. Although radiotherapy alone has limited efficacy in esophageal squamous cell carcinoma (ESCC), studies using ESCC cell lines and xenograft models have demonstrated significant synergistic benefits when combined with radiotherapy AZD2281. ⁹⁵ This combination effectively suppressed the proliferation and induced apoptosis of ESCC cell lines, exhibiting remarkable effects under chronic hypoxic conditions. These effects may be attributed to the inhibition of HRR and down-regulation of Rad51 recombinase protein expression. ⁹⁵

Additionally, Park et al ⁹⁶ demonstrated the potentiating effects of PARP inhibition after exposure to externally induced DNA damage, such as ionizing radiation, in ARID1A-deficient tumor cells. Similar effects were observed in lung cancer and triple-negative MDA-MB-231 human breast carcinoma xenograft models. ⁹⁷ However, the optimal ranges of radiotherapy dosage and treatment sequence remain unclear. Moreover, the quest for a clinically significant biomarker capable of predicting the effectiveness of combining radiotherapy with PARPis remains unresolved.

Novel Delivery Carriers

To overcome drug resistance and side effects associated with PARPis, various nanoscale drug carriers have been developed, including liposomes, nanoparticles (NPs), nanomicelles, and dendrimers. To modify the delivery approach of the agent, Mensah et al developed liposomal NPs using an electrostatic layer-by-layer (LbL) technique, incorporating a hyaluronic acid layer at the end that selectively targets the CD44 receptor of HGSOC cells. The advanced structure of these NPs demonstrates superior activity in inhibiting tumor metastasis, prolonging survival, and reducing systemic toxicity. ⁹⁸ A nanoparticle system for delivering olaparib was administered intraperitoneally (i.p.) therapy for disseminated OC treatment. Daily administration of Nano-Olaparib effectively suppressed tumor growth and mitigated the variability observed in treatment response compared to daily oral Olaparib administration. ⁹⁹ Furthermore, an epidermal growth factor receptor (EGFR)-targeting self-assembling amphiphilic peptide nanoparticle system called GENP was designed to co-deliver gemcitabine and PARPis olaparib to treat BRCA-mutant pancreatic cancer. This vehicle extends the half-life of both drugs while achieving optimal accumulation in tumors, thereby enhancing antitumor efficacy and reducing side effects. ¹⁰⁰ Additionally, nanoparticles not only carry single agents, but also provide a potent platform for combination strategies. PI-103, a prodrug DNA-PK inhibitor, self-assembles into a phospholipid bilayer encapsulating olaparib resulted in significantly greater suppression of proliferation compared to treatment with nano-olaparib particles or nano-PI-103 particles alone. ¹⁰¹

Compared to traditional drugs, novel drug delivery systems not only effectively transport drugs to tumor sites but also reduce adverse effects by protecting PAPR inhibitors from degradation and prolonging the half-life of drugs. However, there remains a significant gap between the experimental research and clinical practice.

PARP Inhibitors + Antibody-Drug Conjugate

Antibody-drug conjugates (ADCs) not only address the limitations of chemotherapy, such as nonspecific tumor targeting, dose-dependent cytotoxicity, and narrow therapeutic window, but also overcome the drawbacks of traditional targeted therapies, including insufficient cytotoxic potency and resistance. ¹⁰² ADCs typically consist of a monoclonal antibody (mAb) covalently linked to a cytotoxic payload via chemical linker. ¹⁰³ Krista et al assessed the efficacy of AZD8205, a novel ADC that specifically targets the B7-H4 protein, which is overexpressed in certain cancers. Preclinical evaluations demonstrated the potential of AZD8205 as both monotherapy and in combination with AZD5305, a PARP1 inhibitor, for the treatment of B7-H4-positive cancers. Further investigations and clinical trials are necessary to validate these preclinical findings and determine the translational prospects of this therapeutic approach. ¹⁰⁴

Conclusion and Outlook

Current evidence suggests that combination strategies hold great promise for enhancing the effectiveness of PARPis and broadening their scope of application. However, certain limitations and areas of concern exist regarding the use of PARPis in combination with other drugs for cancer treatment.

Target Population

Currently, biomarkers for the application of PARPis as single agents are limited to germline BRCA and HRR gene mutations, following the guidelines of the National Comprehensive Cancer Network (NCCN). ¹⁰⁵ However, these biomarkers encompass only a small subset of cancers carrying BRCA mutations. Future studies should focus on exploring the possible indications of PARPis in other tumor types and developing predictive biomarkers tailored to specific patient populations.

Drug Resistance

Given the escalating use of PARPis, it is foreseeable that the incidence of PARP inhibitor-resistant patients will continue to increase. However, further clinical evidence is required to validate whether combination strategies can effectively prolong or hinder the development of resistance to PARPis.

Optimal Combinatorial Strategies

The optimal conditions for combinatorial use, including dosage and administration sequence, have not been investigated extensively. Furthermore, understanding the mechanisms underlying drug interactions is imperative for mitigating adverse effects and enhancing synergistic effects.

Safety of Combination

Not all combinations effectively mitigate the toxicity and side effects of treatment; therefore, careful consideration should be given to continuous monitoring and management during and after drug administration. This necessitates the reliance on meticulously designed high-quality clinical trials to assess the safety profile of the regimen.

Appendix.

Abbreviations

HR

homologous repair

PARP

Poly(ADP-ribose) polymerase

HRR

homologous recombination repair

DDR

DNA damage response

PARPis

PARP inhibitors

ARTs

ADP-ribosyltransferases

SSBs

single-strand DNA breaks

ZnF

zinc-finger

PARG

poly(ADP-ribose) glycohydrolase

ARH3

ADP-ribosylhydrolase 3

TRCs

transcription-replication conflicts

DSBs

double-stranded DNA breaks

HRD

homologous recombination deficiency

ICIs

Immune checkpoint inhibitors

ORR

objective response rate

OC

ovarian cancer

TNBC

triple-negative breast cancer

OS

overall survival

PI3K

phosphoinositide 3-kinase

AKT

protein kinase B

mTOR

mammalian target of rapamycin

DCR

disease control rat

PR

partial response

SCLC

small cell lung cancer

CHK1

checkpoint kinase I

DNA-PK

DNA-dependent protein kinase

IR

ionizing radiation

ATM

ataxia-telangiectasia mutated

MSH3

MutS homologue 3

HCC

hepatocellular carcinoma

HNSCC

neck squamous cell carcinoma

mCRPC

castration-resistant prostate cancer

rPFS

progression-free survival

HDAC

Histone deacetylase

ESCC

esophageal squamous cell carcinoma

NPs

nanoparticles

LbL

layer-by-layer

EGFR

epidermal growth factor receptor

ADCs

Antibody-drug conjugates

mAb

monoclonal antibody

NCCN

National Comprehensive Cancer Network

Ethical Statement

Ethical Approval

This manuscript is a review article and does not involve a research protocol requiring approval by the relevant institutional review board or ethics committee.

ORCID iD

Xiaodong Cheng https://orcid.org/0000-0002-6073-7261

Data Availability Statement

These data were derived from the following resources available in the public domain: ClinicalTrials.gov [https://clinicaltrials.gov/].