Immune Checkpoint Inhibitors: A Promising Choice for Endometrial Cancer Patients?

Although around 80% of endometrial cancers are diagnosed at early stages and present with a 5-year survival rate exceeding 95%, patients with advanced and recurrent disease show a poor prognosis and low response rates to standard chemotherapy. In the era of targeted therapy, the great advances in the understanding of programmed death-ligand 1 (PD-L1) upregulation in cancer cells, which is responsible for tumor immune escape, have contributed to the increasing interest in immune checkpoint inhibitors as a promising strategy for the treatment of several refractory solid malignancies, including endometrial cancer. Several clinical trials have investigated the efficacy and safety of immune checkpoint inhibitors in endometrial cancer, which already led to the approval of the anti-programmed cell death protein 1 (anti-PD-1) antibody pembrolizumab as a satisfactory alternative for selected patients with unresectable or metastatic disease. As the future of cancer treatment will probably rely on combination therapy strategies, currently, innovative ongoing trials are exploring the potential role of immune checkpoint inhibitors associated with chemotherapy, radiotherapy, and other targeted therapies. Moreover, further research is warranted to discover new specific biomarkers that can accurately predict the response to immunotherapy.


Introduction
Endometrial cancer (EC) is the most common gynecological cancer in developed countries, with a lifetime risk of 2.8% [1]. The American Cancer Society estimates that, in 2020, about 65,620 new cases will be diagnosed in the United States and around 12,590 women will die from uterine cancers [2]. Around 80% of EC are diagnosed when the disease is limited to the uterus, with a five-year survival rate of over 95% [3]. However, the prognosis is significantly worse in the cases of regional spread or distant disease (68% and 17%, respectively) [4]. According to international guidelines, treatment for patients with early stage EC is based on surgery alone or in combination with pelvic radiation therapy, whereas patients with advanced disease are candidates for systemic chemotherapy, with carboplatin-paclitaxel doublet representing the most effective scheme [5]. Patients with metastatic or recurrent disease have a poor prognosis as they exhibit low response rates to cytotoxic chemotherapy; thus, it is necessary to explore new therapeutic approaches in order to provide them with the best care. In this review, we discuss the scientific rationale behind the use of immune checkpoint inhibitors in advanced and recurrent EC, exploring both published and ongoing clinical trials.

Background
The human immune system consists of two main complementary subsystems: (1) the innate immune system, which gathers dendritic cells, natural killer (NK) cells, macrophages, neutrophils, eosinophils, basophils, and mast cells and does not depend on a prior stimulation by antigens [6], and (2) the adaptive immune system, which includes B lymphocytes, cluster of differentiation 4 (CD4+) helper T lymphocytes, and cluster of differentiation 8 (CD8+) cytotoxic T lymphocytes and requires a prior antigen exposure mediated by antigen-presenting cells (APCs) for its activation [7]. Immune cells can recognize and eliminate cancer cells through the identification of tumor-specific antigens (TSA) and tumor-associated antigens (TAA) [6]. Specifically, T cells can recognize TAA and TSA peptide small fragments displayed on the surface of antigen-presenting cells (APCs) after being loaded on major histocompatibility complex (MHC) class I and II molecules [8]. Physiologically, the immune system needs to be finely regulated in order to act against pathogens without affecting self-tissues (self-tolerance). Generally, the immune response activation is elicited if two positive signals are present: the first one arises from the interaction between MHC molecules and T-cell receptors (TCR), while the second one imposes that the co-stimulatory receptor cluster of differentiation 28 (CD28), expressed on T cells surface, binds its ligand B7 on APCs [9]. In order to avoid an autoimmune reaction, there are two main inhibitory pathways: the first one depends on the cytotoxic T lymphocyte antigen-4 (CTLA-4), which acts as a competitive inhibitor for the ligand B7 and is mostly expressed within secondary lymphoid organs [10]; the second one is related to the interaction between PD-1 receptor on T cells and PD-L1/2 on the tumor cells surface, and it occurs more frequently within peripheral tumor microenvironment (TME) [11,12]. The PD-1 pathway is an immune checkpoint involved in the regulation of T cell differentiation and apoptosis [13]. PD-1 is a protein receptor expressed by multiple immune cells, including T cells, B cells, NK cells, dendritic cells, and monocytes [14]. The interaction between PD-1 and its ligands, PD-L1 and PD-L2, inhibits cytotoxic T cell activation and differentiation [15]. Particularly, PD-L1 has been shown to be the most important ligand and is upregulated in cancer cells in order to escape the immune surveillance [16]. Indeed, antibodies targeting PD-1 and PD-L1 can block tumor-generated immunosuppression, thus preserving the immune response against cancer [17]. In light of these observations, immune checkpoint inhibitors appeared as a promising choice for the treatment of different solid and refractory malignancies, and in recent years, several studies have been conducted to assess the potential role of specific biomarkers in predicting the response to immunotherapy.
Historically, EC has been classified according to Bokhman's model into two subtypes, based on clinicopathological and molecular features. The type I EC is the most common (60-70% of cases) and includes grades 1-2 endometrioid histology. It is associated with a high expression of estrogen receptors (ERs) and progesterone receptors (PgRs) and a good prognosis (median 5-year survival rates of 85.6%). On the other hand, the type II represents about 30-40% of all EC, typically embracing serous, clear cell or grade 3 endometrioid histology, and is estrogen-independent [18]. It is typically more aggressive than type I and is associated with a poor prognosis, even when diagnosed at early stages [19,20].
In 2013, the Cancer Genome Atlas Research Network, given the new advances on the understanding of EC genomic landscape, introduced a new molecular classification that could change adjuvant treatment for women with an aggressive disease, based on four categories: polymerase ε (POLE)-ultramutated, microsatellite instability-high (MSI-H), copy-number low, and copy-number high [21]. Each of these molecular subclasses present a distinct profile of tumor aggressiveness and thus a different prognosis. The multicenter, randomized phase III trial PORTEC-4a (NCT03469674) is investigating the impact of molecular risk factors in assigning adjuvant treatment for women with stages I-II high-intermediate risk endometrial cancer. POLE-ultramutated and MSI/Mismatch Repair Deficiency (MMRd) tumors are associated with a better prognosis compared to the other subgroups. Specifically, POLE-ultramutated tumors show a better progression free survival (PFS) when compared to other tumors with the same histotype, grade, and stage [22][23][24]. Several authors have reported that MSI-H/MMRd colorectal cancers are significantly associated with a better prognosis following conventional chemotherapy [25,26] and immunotherapy treatment [27]. Indeed, POLE-ultramutated and MSI-H molecular subclasses are characterized by high CD3+/CD8+ tumor-infiltrating lymphocytes and an overexpression of PD-1 and PD-L1 [28][29][30][31][32]. Specifically, mutations that occur in the domain of the POLE gene cause a specific phenotype characterized by a high mutational load, with increased neoantigen expression and an important activation of the patient's immune system [33,34]. Similarly, MSI-H/MMRd cancers are characterized by extremely high numbers of somatic mutations, and several studies have evaluated the relationship between MMR status and the expression of PD-1 and PD-L1 in solid tumors including EC [35,36]. Interestingly, Yamashita et al. reported higher cytotoxic T cells (CD8+) infiltration and PD-1/PD-L1 expression in the MMR-deficient (MMRd) group compared to the MMR-proficient (MMRp) among 149 endometrial cancer patients [37]. The high immunogenicity of these tumors explains the strong rationale behind the use of immunotherapy in these subgroups of cancers. In light of these observations, EC can represent an ideal target for immune checkpoint inhibitors and the use of MMRd/MSI status could be a predictor of the response to PD-1 blockade in endometrial cancer.

Clinical Evidence
Several clinical trials have been published regarding the role of immune checkpoint inhibitors in endometrial cancer (Table 1).

Anti-PD-1: Pembrolizumab
In 2015, Le et al. first evaluated the clinical activity of pembrolizumab, an anti-PD-1 immune checkpoint inhibitor, in 41 patients presenting with progressive metastatic colorectal cancer (CRC) with or without MMR deficiency or other MMRd cancers. Pembrolizumab was administered intravenously (IV) at a dose of 10 mg/kg every 14 days in three different cohorts of patients, which included, respectively, MMRd CRC, MMRp CRC, and MMRd noncolorectal cancers, among which there were also 2 cases of EC. Co-primary endpoints were the immune-related objective response rate (ORR) and the 20-week immune-related progression-free survival (PFS). In the MMRd CRC group, the estimated ORR and the 20-week PFS were 40% and 78%, respectively, versus 0% and 11% reported for MMRp CRC patients. In the MMRd cohort including two EC patients, the ORR and PFS were 71% and 67%, respectively [38].
In 2016, Mehnert et al. published the case of a single patient with a rapid and persistent (more than 14 months) clinical response to anti-PD-1 pembrolizumab treatment. The molecular analysis of a pretreatment biopsy revealed a POLE-ultramutated EC, thus suggesting that the identification of a POLE mutation might have a predictive value of treatment response to pembrolizumab. However, these data need to be further investigated to be confirmed [39].
In 2017, Ott et al. published the results of the phase Ib KEYNOTE-028 trial, which enrolled 24 advanced EC patients treated with pembrolizumab 10 mg/kg IV administered every 2 weeks for a maximum of 24 months or until confirmed disease progression, intolerable toxicity, or death. The ORR reported was 13%. Three women showed a partial response, and among them, one had POLE mutations, whereas three patients achieved a stable disease. Six-month PFS and OS were 19.0% and 68.8%, respectively. Thirteen patients (54.2%) experienced treatment-related adverse events (TRAEs), such as fatigue (20.8%), pruritus (16.7%), pyrexia (12.5%), and decreased appetite (12.5%). Grade 3 adverse events (AEs) were reported in 4 patients, including asthenia, back pain, anemia, hyperglycemia, hyponatremia, pyrexia, and diarrhea. No patient experienced grade 4 AEs or any grade immune-mediated AEs, discontinued treatment because of an AE, or experienced treatment-related death. [40]. In a recent phase II trial, Makker et al. tested the anti-PD-1 pembrolizumab at a dosage of 200 mg IV every 3 weeks in combination with lenvatinib, a multikinase-inhibitor of vascular endothelial growth factor (VEGFR), at a dosage of 20 mg daily orally in 108 advanced recurrent EC patients. The primary endpoint was the ORR at 24 weeks, according to the response evaluation criteria in solid tumors (RECIST) criteria 1.1. Forty-two (38.0%) patients had an objective response to lenvatinib plus pembrolizumab. The median PFS and overall survival (OS) were 7.4 months and 16.7 months, respectively. For patients with MSS/MMRp tumors, the ORR was 37.2%, whereas for patients with MSI-H/MMRd tumors, the ORR was 63.6%. The median duration of response (DOR) was not estimable (NE) (95% CI, 7.4 months to NE) for patients with MSS/MMRp tumors and was 21.2 months (95% CI, 7.3 months to NE) for patients with MSI-H/MMRd tumors. The most frequently reported any-grade AEs were hypertension (66 (61%)), fatigue (56 (51.9%)), diarrhea (57 (52.8%)), and hypothyroidism (48 (44.4%)). Grade 3-4 AEs occurred in 75 (69.4%) patients [41].
Results from the Keynote-158 study demonstrated the clinical benefit of pembrolizumab for previously treated unresectable or metastatic MSI-H/MMRd noncolorectal cancer patients. Among 233 patients enrolled, 47 were EC patients. Eligible participants received pembrolizumab 200 mg IV every 21 days for 35 cycles or until documented disease progression, unacceptable toxicity, or patient/investigator decision. The primary endpoint was ORR, defined as the proportion of patients with confirmed complete/partial response per RECIST 1.1. An ORR of 57.1% was reported for EC patients, with a median PFS of 25.7 months. One hundred and fifty-one of 233 patients (64.8%) experienced TRAEs, and among them, 34 (14.6%) had grade 3-5 AEs. The most common toxicities were fatigue (14.6%), pruritus (12.9%), diarrhea (12.0%), and asthenia (10.7%;). Most frequent grade 3 AEs were increased gamma-glutamyltransferase (1.7%) and pneumonitis (1.3%). Three patients had grade 4 AEs: one patient had Guillain-Barre syndrome (a patient with gastric cancer), one had increased alanine aminotransferase (ALT), and another one had decreased neutrophil count and enterocolitis [42].

Anti-PD-1: Nivolumab
In 2016, Santin et al. reported two cases of recurrent/metastatic EC, respectively a mixed clear cell and endometrioid (CC/EAC) POLE-ultramutated EC and a serous MSH6-mutated EC, not responding to surgery, chemotherapy, and radiotherapy and treated with nivolumab, a novel anti-PD-1 antibody, administered IV at a dosage of 3 mg/kg once biweekly. Both tumor samples evaluated in the pretreatment setting showed moderate peri-and intratumoral lymphocytic infiltrate (especially CD8 T cells) and moderate PD-1 and PD-L1 expression. Patients showed a persistent clinical response to nivolumab without severe toxicities, as confirmed by a computed tomography (CT) scan performed, respectively, at 7 and 9 months from the beginning of treatment [43].

Anti-PD-1: Dostarlimab
Preliminary data from the GARNET study were presented at the 2019 Society for Gynecologic Oncology (SGO) annual meeting on women's cancer. One hundred and ten EC patients received dostarlimab 500 mg every 3 weeks. The ORR was 27.7% (50% in MSI-H and 19.1% in MSS). Sixty-eight EC patients (68%) experimented at least one TRAE. Grade ≥ 3 AEs were reported in 13 patients (11.8%), and the most common was increased serum levels of aspartate aminotransferase [44].

Anti-PD-L1: Atezolizumab
Fleming et al. presented the results of a phase Ia clinical trial in which atezolizumab, an anti-PD-L1 antibody, was tested in 15 women with advanced and recurrent EC with microsatellite stability (MSS), MSI-H, or microsatellite unknown status. Patients received atezolizumab 1200 mg or 15 mg/mq IV in monotherapy every 21 days. Two women showed a partial response, while another two achieved a stable disease with an ORR of 13% and a disease control rate (DCR) of 27%. Both responders had a PD-L1 expression above 5% and presented, respectively, MSS and MSI-H status. It was reported a median PFS of 1.7 months and a median OS of 9.6 months. Only two patients experienced severe drug-related AEs (colitis and rash), whereas no G4-5 adverse events were recorded [45].

Anti-PD-L1: Avelumab
Konstantinopoulos et al. published the results of a phase II trial, in which avelumab was tested in patients with MMRd/MMRp-recurrent or -persistent endometrial cancer. Thirty-three patients were enrolled. Avelumab was administered at 10 mg/kg IV every 2 weeks until progression or unacceptable toxicity. No patient with POLE-ultramutated tumor was enrolled in MMRd and MMRp cohorts. Co-primary endpoints were PFS at 6 months (PFS6) and ORR according to RECIST criteria 1.1. The MMRp cohort was closed at the first stage because only one out of 16 patients exhibited both ORR and PFS6 responses. The MMRd cohort achieved the primary endpoint of four objective responses after the accrual of only 17 patients. Of 15 patients who received avelumab, four exhibited an objective response (1 complete response and 3 partial responses; ORR, 26.7%; 95% CI, 7.8% to 55.1%) and 6 (including all four objective responses) PFS6 responses (PFS6, 40.0%; 95% CI, 16.3% to 66.7%). Responses were observed in the absence of PD-L1 expression. Twenty-two patients experienced TRAEs. Particularly, 6 patients reported grade 3 toxicity, including anemia, sinus bradycardia, hypothyroidism, diarrhea, myositis, and rash acneiform [46].

Anti-PD-L1: Durvalumab
Data from the PHAEDRA study was presented at the American Society of Clinical Oncology (ASCO) 2019 and showed the activity of durvalumab as a single agent in MMRd and MMRp cohorts. Seventy-one patients with advanced EC were enrolled: 35 patients with MMRp progressing after 1-3 lines of chemotherapy and 36 patients with MMRd progressing after 0-3 lines of chemotherapy. All patients received durvalumab at dosage of 1500 mg IV every 4 weeks. Among MMRd patients, the ORR was 40% (4 complete responses and 10 partial responses), whereas within the MMRp group, the ORR was 3% (one partial response). Adverse events occurred in 14 patients, including hyperthyroidism, hypothyroidism, pneumonitis, and hepatitis [47].

Main Ongoing Trials
Immune checkpoint inhibitors are under evaluation in several phase I, II, and III clinical trials, alone or in combination with other drugs (Table 2). NCT03572478 is a phase Ib/IIa study on the combination of nivolumab with the poly (ADP-ribose) polymerase (PARP) inhibitor rucaparib in metastatic or recurrent EC patients. In phase Ib, patients will receive rucaparib plus nivolumab in 4-week cycles, while in phase IIa, patients will be randomized to receive nivolumab alone, rucaparib alone, or rucaparib in combination with nivolumab. The primary endpoint is to evaluate the dose-limiting toxicities (DLT) rate for the combination of rucaparib and nivolumab [48]. Nivolumab is also being studied in a phase II clinical trial (NCT02982486) in association with ipilimumab in patients with nonresectable/metastatic MMRd EC. Patients will receive nivolumab 240 mg IV every 2 weeks plus ipilimumab 1 mg/mq IV every 6 weeks. The primary endpoint is ORR according to RECIST 1.1 [49]. NCT03241745 is a single-center, non-randomized, open-label, phase II study with the aim to assess the safety of nivolumab in patients with MSI/MMRd hypermutated uterine cancers, including endometrial carcinoma. Patients will receive nivolumab 480 mg IV once every 4 weeks until disease progression or unacceptable toxicity. The primary endpoint is PFS [50].
Moreover, pembrolizumab is being investigated in phase I, II, and III clinical trials. NCT02549209 is a single-arm, open-label, multi-center phase II study in patients with newly diagnosed stage III/IV or recurrent EC that aims to assess the association of pembrolizumab with carboplatin/paclitaxel therapy. Patients with no prior therapy will receive pembrolizumab 200 mg plus carboplatin AUC 6 and paclitaxel 175 mg/mq every 21 days, whereas those with prior external beam radiation therapy (EBRT) and/or platinum-based chemotherapy must initiate paclitaxel and carboplatin at a reduced dose (paclitaxel 135 mg/mq plus carboplatin AUC 5). The primary endpoint is the ORR [51].    The TOPIC study (NCT03276013) is non-randomized, single-arm, multi-center, phase II study of pembrolizumab in combination with doxorubicin in patients with recurrent/metastatic EC. Patients will receive doxorubicin 60 mg/kg IV every 3 weeks for up to 9 cycles in combination with pembrolizumab (MK-3475) 200 mg IV every 3 weeks. The primary endpoint is the PFS6 [52]. NCT03517449 (KEYNOTE-775) is a multi-center, open-label, randomized, phase III trial investigating the efficacy and safety of combined lenvatinib and pembrolizumab compared to treatment with doxorubicin or paclitaxel in patients with advanced EC. Women in the experimental arm will receive pembrolizumab 200 mg IV on day 1 of each 21-day cycle plus lenvatinib 20 mg orally once daily during each 21-day cycle for up to 35 cycles. Patients in the comparator arm will receive doxorubicin 60 mg/mq IV on day 1 of each 21-day cycle for up to a maximum cumulative dose of 500 mg/mq or paclitaxel 80 mg/mq IV on a 28-day cycle (3 weeks of paclitaxel once a week and 1 week without paclitaxel). Co-primary endpoints are PFS and OS [53].
Two phase III clinical trials are evaluating the efficacy of pembrolizumab combined with other drugs for advanced or recurrent EC patients. In the randomized, placebo-controlled NCT03914612 study, patients will receive pembrolizumab every 21 days in association with carboplatin and paclitaxel for up to 6 cycles and then pembrolizumab every 3 weeks for up to 29 cycles as maintenance in the absence of disease progression or unacceptable toxicity [54]. The randomized, open-label NCT03884101 trial aims to compare the efficacy of pembrolizumab 200 mg IV on day 1 of each 21-day cycle in association with lenvatinib 20 mg orally once daily versus standard chemotherapy [55]. In both clinical trials, the primary endpoint is PFS.
MITO END-3 (NCT03503786) is a randomized, phase II clinical trial which compares carboplatin/paclitaxel to carboplatin/paclitaxel plus avelumab in advanced or recurrent EC. Patients in the experimental arm will receive carboplatin AUC 5, paclitaxel 175 mg/mq, and avelumab 10 mg/kg every 21 days for 6-8 cycles plus avelumab 10 mg/kg as maintenance every 14 days until disease progression or unacceptable toxicity. The primary endpoint is PFS [56].
Furthermore, durvalumab is currently under evaluation in several clinical trials. NCT04269200 is a randomized, multicenter, double-blind, placebo-controlled, phase III study which aims to assess the efficacy and safety of durvalumab in combination with platinum-based chemotherapy (paclitaxel plus carboplatin) followed by maintenance durvalumab with or without olaparib for patients with newly diagnosed advanced or recurrent EC. In cohort A, patients will receive carboplatin/paclitaxel plus durvalumab placebo followed by maintenance durvalumab placebo and olaparib placebo; in cohort B, patients will undergo platinum-based chemotherapy and durvalumab followed by maintenance durvalumab and olaparib placebo; and in cohort C, they will receive platinum-based chemotherapy and durvalumab followed by maintenance durvalumab and olaparib. PFS is the primary endpoint [57].
Finally, the EndoBARR trial (NCT03694262) is an open label, non-randomized multicenter, phase II trial evaluating the efficacy and safety of combined rucaparib, atezolizumab, and bevacizumab in recurrent and progressive EC. Atezolizumab will be administered at dosage of 1200 mg IV (every 3 weeks) together with bevacizumab 15 mg/mq (every 3 weeks) and rucaparib 600 mg twice daily. The primary endpoint is ORR [58].

Toxicities
Understanding the spectrum of the unique toxicities associated with immune checkpoint inhibition is crucial, as they may be acute and life-threatening. The precise pathophysiology underlying immune-related AEs is unknown, although it appears related to the role that immune checkpoints play in maintaining immunologic homeostasis. As CTLA-4 and PD-1 agents act differently, their use causes different immune-related AEs. Generally, the PD-1 blockade seems to be better tolerated and has a lower AE frequency than the CTLA-4 blockade. Moreover, CTLA-4-related AEs increase with increasing dose whereas PD-1-related AEs seem not to be dose-related [59]. Overall, AEs affect over two-thirds of patients. Severe immune-related AEs occur in 10% to 20% of cases treated with CTLA-4 inhibitors, whereas their frequency is lower (1% to 2%) in patients treated with PD-1 inhibitors [60].
Immune-related AEs usually develop within the first few weeks to months after treatment initiation, and there is currently no known predisposing risk factor. Furthermore, most studies indicate that prolonged treatment does not result in an increased incidence of immune-related AEs [61]. A current area of investigation is whether the degree of side effects is correlated with treatment response, though durable responses have been seen in the absence of severe reactions. Although any organ system can be affected, immune-related AEs most commonly involve gastrointestinal tract, endocrine glands, skin, and liver. Less often, the central nervous system and cardiovascular, pulmonary, musculoskeletal, and hematologic systems are involved [62]. The management of all potential immune-related adverse events requires a multidisciplinary approach.

Conclusions and Future Perspectives
Over the last years, immunotherapy has meaningfully changed the paradigm of care of several solid tumors and, currently, it represents an active area of research aiming to identify which types of cancer could benefit most from it. Recently, the great advances in the understanding of cancer genome have contributed to the increasing interest in immune checkpoint inhibitors as a promising alternative strategy for refractory cancers [63,64]. Based on the results from several clinical trials, in 2017, the Food and Drug Administration (FDA) approved pembrolizumab for patients with unresectable or metastatic MSI-H or MMRd solid tumors, including endometrial cancer. Moreover, data coming from Keynote 146 have led to the approval of the combination of pembrolizumab with lenvatinib in patients with highly pretreated, advanced non-MSI-H, or MMRd endometrial carcinoma, not amenable to surgery or radiation. Results from several clinical trials have shown that MMRd EC patients respond better to immune checkpoint inhibitors than MMR-p. However, there was no stratification of MSI-H EC tumors according to their sporadic or inherited Lynch syndrome (LS) origins. Since MSI-H tumors display an increased immunogenicity compared with MSS tumors, some authors have investigated differences in immune cell populations between sporadic and LS MSI-H endometrial cancer, showing that these two groups are distinct immunological entities with different types of immune cells in stroma and tumor, respectively [65,66]. In conclusion, given the evident benefit found in MSI-H/MMRd cancer patients treated with immune checkpoint inhibitors, all EC patients should be tested to define the MMR status at the time of diagnosis, including the immune-infiltrate and the expression of PD-1 and PD-L1, in order to establish their future eligibility for immunotherapy treatment [67,68]. Indeed, as the future of cancer treatment will rely on combination therapy strategies, several ongoing studies are evaluating the efficacy of immune checkpoint inhibitors associated with other immunotherapeutic agents, chemotherapy, radiotherapy, and targeted therapies in endometrial cancer. Interestingly, clinical evidence has suggested a strong correlation between PARP inhibition and PD-1/PD-L1 upregulation in different types of cancer. Therefore, the combined use of PARP inhibitors and anti-PD-L1 agents could show synergistic effects [69]. Further research is warranted to find new combination therapy strategies and to discover new specific biomarkers that can accurately predict the response to immunotherapy.

Conflicts of Interest:
The authors declare no conflict of interest.