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Pembrolizumab in endometrial cancer: Where we stand now

Recently, immunotherapy has shown promising results in solid tumors. To the best of our knowledge, this is the first systematic review of published literature synthesizing all the available data and evaluating both the efficacy and safety of pembrolizumab in endometrial cancer. The present study was performed in accordance with the PRISMA guidelines. Eligible articles were identified by searching the MEDLINE and ClinicalTrials.gov databases, using a predefined combination of the terms ‘endometrial cancer’ and ‘pembrolizumab’. Overall, nine articles incorporating data from 712 patients were eligible. Pembrolizumab was demonstrated to be an effective and safe therapeutic option for the management of advanced/metastatic endometrial cancer. Results of ongoing trials evaluating either pembrolizumab alone or in combination with other antineoplastic regimens are expected to confirm its efficacy in this setting of patients. Pembrolizumab appears to be both durable and robust in endometrial cancer. However, there is an emerging need for novel predictive biomarkers to guide clinical practice.


Introduction
Endometrial cancer (EC) is the most common gynecological malignancy accounting for approximately 61,880 new cases and 12,160 deaths in the United States (1). Metabolic syndrome, diabetes mellitus, Lynch syndrome, hormone therapy and tamoxifen use are the main risk factors for endometrial cancer (2). Most ECs are identified at an early stage (75%), however ~25% of ECs are diagnosed at a late stage. Prognosis for advanced disease remains poor; 5-year survival rates are <50% for patients with lymph node involvement and <20% for patients with peritoneal or distant metastases (3). Indeed, 5-year survival ranges from 91% for FIGO IA disease to as low as 47% for FIGO IIIC and 17% for FIGO IV disease.
Chemotherapy with carboplatin plus paclitaxel has gradually replaced the triple drug combination of cisplatin, doxorubicin and paclitaxel and is currently the standard of care in the metastatic setting (4,5). Hormone treatment with progestins, anti-estrogens and aromatase inhibitors are associated with low response rates (30%) (6,7). Nevertheless, options are limited for patients progressing on first line treatment highlighting the need for novel treatment regimens (8).
The interaction between tumor cells and T cells has been well established. Immune response is mediated by the interaction of the MHC-peptide complex with the T-cell receptor (TCR) and the binding of the co-stimulatory receptor CD28 on T cells with the ligand B7 on antigen-presenting cells (APCs) (9). In order to avoid autoimmune response cytotoxic T lymphocyte antigen-4 (CTLA-4) binds to CD28 and acts as a competitor for B7. Respectively, the programmed death 1 (PD-1) inhibitory receptor is expressed by T cells and binds to two ligands, namely programmed death ligand 1 (PD-L1) and programmed death ligand 2 (PD-L2) within the tumor microenvironment (10). This blockade enables the escape of tumor cells from immunosurveillance by inhibiting the activation of T cells and suppressing the T cell-mediated tumor cytolysis (11,12). Both PD-1 and PD-L1 are targets of immunotherapeutic drugs which block their interaction and promote durable responses in a variety of cancer types (13,14).
High expression of PD-L1 seems to be a predictive biomarker for anti-PD1 therapy, although some PD-L1 negative cancers may also respond to this kind of treatment (15)(16)(17). EC cells may activate the PD-1/PD-L1 signaling pathway since they overexpress PD-L1 receptor in 25 to 100% of cases (18).
Historically, endometrial carcinoma has been classified into two main clinicopathological and molecular types: type I which includes endometrioid adenocarcinomas (80-90%) and type II which includes non-endometrioid subtypes such as serous, clear-cell and undifferentiated carcinomas as well as carcinosarcoma (10-20%) (2). The Cancer Genome Atlas (TCGA) Research Network proposed a different molecular classification of EC that divided EC tumors into four subgroups: i) POLE ultra-mutated, ii) microsatellite instability high (MSI-H)/mismatch repair deficient (dMMR), III) low copy number with low mutation rate, iv) high copy number with high mutation rate (19). According to TCGA Research Network some EC subtypes like POLE mutated cancers and MSI-H/dMMR groups are characterized by an immunogenic tumor microenvironment and express high level of immune checkpoint-associated proteins including PD-L1 (19).
Given the immunogenic profile of these molecular subtypes and the overexpression of PD-L1 in EC cells, monoclonal anti-PD-1 pembrolizumab could prove to be an efficient novel treatment in EC. This is the first systematic review of the literature to synthesize data from recent and ongoing clinical trials regarding the efficacy and safety of pembrolizumab in EC.

Sources and study selection
This systemic review was performed according to PRISMA guidelines (20). The protocol has been submitted to the Institutional Review Board of Alexandra University Hospital and is available upon request. Eligible articles were identified by a search of MEDLINE and Clinicaltrials.gov databases for the period up to 05/28/2021. The search strategy included the following keywords: ((endometrial) AND (neoplasm OR cancer OR carcinoma)) AND (pembrolizumab).
English -language restrictions were applied. Two investigators Aravantinou Fatorou A (AFA) and Andrikopoulou A (AA) working independently searched the literature and extracted data from each of the eligible studies. Clinical trials were considered eligible for our study, while reviews and case reports were excluded. In order to maximize the amount of synthesized information, we systematically examined the reference lists of the articles retrieved to identify potentially eligible studies and/or conference abstracts.
Eligible articles included all studies evaluating the efficacy and safety of pembrolizumab in EC no matter of sample size. For each of these studies, the following data were collected: first author, year of publication, phase of the trial, number of patients enrolled, disease setting (metastatic etc.), overall response rate (ORR) including complete response (CR) rate and partial response (PR) (%), stable disease (SD) rate (%), progression disease (PD) rate (%), median overall survival (mOS) in months, median progression free survival (mPFS) in months, data regarding biomarker analysis and adverse events. From multicohort trials, the number of patients with EC was identified and only data addressing this population were included in our study.
For patients receiving the combination of pembrolizumab with lenvatinib for previously treated advanced/recurrent EC, ORR was 38.4% (63/164) with 6.7% of patients achieving CR (11/164) and 31.7% of PR (52/164) (50,51 there was only one case of suspected treatment-related mortality in a patient with interstitial lung disease (48).

Discussion
Generally, pembrolizumab administered either as a single agent or in combination with other compounds is a safe and well-tolerated drug for multiple solid tumors (57,58). Molecular characterization of the tumor is a prerequisite for guiding     therapeutic selection of pembrolizumab in EC patients. All the ongoing clinical trials with pembrolizumab as a single agent or in combination with other treatment regimens are presented in Table II. As already discussed, KEYNOTE-146 Phase Ib/II basket trial evaluated the combination of lenvatinib plus pembrolizumab in patients with advanced EC, regardless of PD-L1 or MSI status (51). Lenvatinib is an oral multikinase inhibitor of the vascular endothelial growth factor receptors (VEGFR)1-3, fibroblast growth factor receptors (FGFR)1-4, platelet-derived growth factor receptor-α (PDGFR-α), RET, and KIT (59)(60)(61). On September 17, 2019 the FDA granted accelerated approval to pembrolizumab plus lenvatinib for the treatment of patients with advanced non-MSI-H/dMMR EC who exhibited disease progression to prior systemic therapy and are not candidates for curative surgery or radiation.
Two phase III, randomized, clinical trials (NCT03517449, NCT03884101) were designed to compare this combination treatment with taxane-based chemotherapy in patients with advanced EC. Final data of KEYNOTE-775 are anticipated (54). LEEP-001 (NCT03884101) Phase III trial aims to compare the efficacy of pembrolizumab plus lenvatinib to standard chemotherapy (carboplatin plus paclitaxel) for the first-line treatment of advanced (Stage III/IV) or recurrent EC (62,63).
Adjuvant Radiation therapy (RT) is the treatment of choice for patients with early-stage EC and high risk of recurrence (64). Irradiated tumor cells secrete various factors that stimulate the activation of T cells and thus RT and immunotherapy may have a synergistic effect (65,66). A randomized Phase III study (NCT04214067) explores the addition of pembrolizumab to standard RT in patients with newly diagnosed early stage dMMR EC of high/intermediate risk (67).
PD-L1 tumor expression presents certain limitations as a predictive biomarker (68). Firstly, a standard cut off level is not yet established. Moreover, the interaction between tumor and immune cells is a dynamic process and therefore evaluating PD-L1 at a single time point may not directly corelate to response to anti-PD1 treatment (69). In addition, PD-L1 positive EC tumors showed only a modest response (13%) to anti-PD1 treatment compared to MSI-H/dMMR tumors (55.9%). Thus, it could not be safely speculated that patients with EC and high expression of PDL-1 will benefit more from pembrolizumab.
DNA mismatch repair (MMR) is a key DNA repair system that identifies erroneous bases during DNA replication and recombination. Microsatellites are short DNA sequences in which a motif of one to six bases is repeated (70). Potential errors to this region are repaired by DNA MMR genes. Both mismatch repair deficiency and high microsatellite instability have been proposed as predictive biomarkers of response to immune checkpoint inhibition. dMMR and MSI-H tumors comprise approximately the 30% of ECs (17-33%) (71). Thus, the use of MMR or/and MSI testing could be of importance in identifying patients with EC who will benefit from pembrolizumab.
DNA polymerase epsilon is a member of the DNA polymerase family that catalyzes the synthesis of DNA molecules and is essential for ensuring the fidelity of DNA replication (72). Mutations in POLE genes have been associated with high tumor mutational burden, elevated number of tumor-infiltrating and peritumoral lymphocytes, higher expression of PD-1 and PD-L1 (73) and consequently higher response rates to immunotherapy (72,74). POLE-mutated EC occurs in approximately 10% of EC cases (19). Ultra-mutated POLE ECs represent the 6.4% of low-grade and 17.4% of high-grade endometrioid tumors. Evaluation of POLE mutations may serve as an attractive predictive biomarker of susceptibility to immunotherapy (75).
Tumor mutational burden (TMB) is defined as the total number of somatic mutations per coding area. High TMB of at least 10 mutations per megabase was identified as a biomarker of favorable prognosis in KEYNOTE-158 study (76). ORR was achieved in 30 (29%; 95% CI: 21-39) of 102 patients in the TMB-high group versus 43 (6%; 95% CI: 5-8) of 688 participants in the non-TMB-high group, while duration of response was also longer in the first subgroup (not reached versus 33.1 months) (76). Overall, TMB could also serve as a biomarker of response to immune checkpoint treatment. All these potential biomarkers should be evaluated in large clinical trials before any conclusion is drawn.
To the best of our knowledge, this is the first systematic review evaluating the clinical role of pembrolizumab in EC patients. However, our study is characterized by certain limitations which should be highlighted. Eligible clinical trials were single arm, non-randomized, early Phase trials in the majority of cases (47)(48)(49)(50)(51)(52)(53)55).

Conclusion
Targeting PD-1 with pembrolizumab in pretreated patients with advanced or recurrent EC represents a promising therapeutic approach and has thus received two FDA approvals as a single agent and in combination with lenvatinib. Given the profound benefit of dMMR/MSI-H patients from immunotherapy, all patients should be tested for predictive biomarkers including MMR deficiency, MSI or PD-L1 expression to guide treatment selection. Ongoing randomized, Phase III clinical trials are warranted to confirm the clinical benefit of pembrolizumab in EC patients. critically revised the manuscript and gave the final approval. ML, MG and MAD critically revised the manuscript and gave the final approval. Data authentication is not applicable. All authors read and approved the final manuscript.

Ethics approval and consent to participate
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Patient consent for publication
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Competing interests
FZ has received honoraria for lectures and has served in an advisory role for Astra-Zeneca, Daiichi, Eli-Lilly, Merck, Novartis, Pfizer and Roche. ML has received honoraria from Roche, Astra Zeneca, Astellas, MSD, Janssen, BMS and IPSEN. MG has received honoraria from Janssen, Genesis Pharm, Amgen, Karyopharm and Takeda. MAD has received honoraria and has served on advisory boards from Jannsen, Genesis Pharm, Amgen, Karyopharm and Takeda, outside the submitted work. AAF, AA, VEG and OF declare that they have no competing interests.