Novel Treatment Options in Metastatic Esophageal Carcinoma: Checkpoint Inhibitors in Combination Therapies

Background: Metastatic esophageal carcinoma (EC) has a poor prognosis and only limited treatment options. While immune checkpoint inhibitors (ICIs) have improved the treatment of a broad spectrum of cancers, patients with EC mostly fail to respond to this treatment. For that reason, it is crucial to understand the immune phenotype of each cancer patient and moreover, to understand how different therapies modulate the cancer microenvironment and sensitize the tumors to the treatment with ICIs. Summary: We have conducted a systematic review of the literature to evaluate the potential of ICI therapy in combination with chemotherapy, radiotherapy, and/or biologic therapy in EC patients. In our review, we have discussed the effects of diverse treatment approaches on the tumor microenvironment of EC. In addition, we have reviewed the current phase II and III clinical trials in EC patients to provide a rationale for immunotherapy application in combination settings with chemotherapy, radiotherapy, and/or biologic therapy. Key Messages: A great effort is already underway in clinical trials evaluating the combinatorial administration of ICIs and other treatment modalities in metastatic EC patients. PD-L1 expression status was shown to be higher in the squamous cell carcinoma (SCC) as compared to adenocarcinoma. Thus, ICIs plus chemotherapy are being discussed as a particularly feasible option for patients with SCC. Radiation was shown to induce the expression of immune checkpoint molecules and to promote the priming and activation of cytotoxic T cells which provides a rationale for ICI administration in a combination with radiotherapy. The combination of ICIs with biologic therapy was shown to be safe; however, the impact on the clinical outcomes of EC patients varied among studies.

© 2022 The Author(s). Published by S. Karger AG, Basel

Introduction

Esophageal cancer (EC) is the seventh most commonly diagnosed malignancy worldwide and the sixth most common cause of cancer-related death [1, 2]. Due to the aggressive nature of EC, the 5-year overall survival (OS) rate remains 30–40% [3, 4]. The poor prognosis of patients with EC is, however, affected by multiple factors, such as the reoccurrence of the disease, the development of metastases, and the treatment complications. There are two subtypes of esophageal carcinoma, esophageal adenocarcinoma (AC) and esophageal squamous cell carcinoma (SCC), which account for most of the cases [5-7].

Esophageal AC usually develops in pre-existing esophageal metaplasia, Barett’s esophagus, in the lower third of the esophagus. As opposed to AC, SCC is found mostly in the upper portion of the esophagus and is linked to tobacco and alcohol use. As compared to AC, SCC has the highest incidence worldwide [8, 9]

Traditional treatment options, such as surgery, chemotherapy, and radiotherapy, are not sufficient in the treatment of advanced and/or metastatic esophageal tumors [10]. Thus, it is crucial to find innovative therapies to improve the prognosis of EC patients [11].

Immunotherapy with immune checkpoint inhibitors (ICIs) has become a plausible treatment option for metastatic diseases [12, 13]. The ICI immunotherapy has become a first-line treatment in various cancer types, such as metastatic melanoma, metastatic nonsmall cell lung carcinoma, and metastatic urothelial carcinoma, and has entered clinical trials in multiple other malignancies [14-16].

ICIs are monoclonal antibodies designed to restore anticancer immune responses by targeting immune inhibitory molecules [17]. To date, the most broadly used ICIs in clinical practice are designed to target either programmed cell death protein 1 (PD-1), programmed cell death protein 1 ligand (PD-L1), or the cytotoxic T lymphocyte antigen 4 (CTLA-4) [18, 19].

The expression of PD-L1 by the tumor microenvironment (TME) results in T cell suppression which promotes local invasion of the tumor and its spread to other sites [3, 20]. PD-1 is a transmembrane protein belonging to the immunoglobulin superfamily that is presented on many immune cells, including T cells, B cells, and dendritic cells (DCs) [21]. Under physiological conditions, PD-1 binds to one of its ligands, PD-L1 or PD-L2, in the presence of inflammatory cytokines. This is a critical step in the mechanisms of immune tolerance protecting against autoimmunity. Tumor cells, however, use the same tools to induce immune tolerance against the tumor itself [22, 23].

In 2011, the anti-CTLA-4 monoclonal antibody was approved as the first-line treatment of metastatic melanoma [24]. CTLA-4 molecule belongs to the immunoglobulin superfamily and can be found mainly in the intracellular vesicles of helper T cells (Th cells) and regulatory T cells (Tregs). Upon T-cell activation, CTLA-4 is upregulated to transmit an inhibitory signal to T cells and to attenuate positive co-stimulation by CD28 [25, 26]. Moreover, CTLA-4 is widely expressed in regulatory T cells and is mandatory for their immunosuppressive function [25, 26].

ICIs disrupt the inhibitory interactions of T cells by blocking checkpoint proteins from binding to their ligands [27]. Although CTLA-4 and the PD-1/PD-L1 axis are the most promising targets of ICI immunotherapy, other molecules have been reported to influence similar molecular pathways. Novel inhibitory molecules, such as lymphocyte activation gene 3 (LAG-3 or CD223), T cell immunoglobulin-3 (TIM-3), or B7 homolog 3 (B7-H3), are currently being extensively studied [28-31].

In EC patients, a number of clinical trials investigated the expression of immune checkpoint molecules in the TME [32]. As a result, several studies, such as KEYNOTE-028 and KEYNOTE-181 also evaluated the efficacy of ICI monotherapy in either esophageal AC or SCC [32].

Although the US Food and Drug Administration (FDA) has granted approval for ICI immunotherapy in various malignancies, including gastric and esophageal cancer, late-stage esophageal cancer still remains one of the most challenging diseases where immunotherapy should be explored both as monotherapy and in combination therapies to increase the proportions of ICI responders [25, 26, 33].

We have conducted a systematic review of literature to evaluate the potential of ICI therapy in combination with chemotherapy, radiotherapy, and/or biologic therapy in EC patients, Figure 1. To provide a focused overview of the systemic treatment options for metastatic/advanced EC, we have not included surgery as a treatment modality.

Fig. 1.

Diverse treatment options for esophageal carcinoma. The tumor microenvironment (TME) of esophageal carcinoma consists of tumor cells and various nontumoral cells, including immune cells. Immune cells can be targeted through a specific blockade of immune checkpoint receptors which leads to a stimulation of anti-tumor cytotoxic functions. Monoclonal antibodies against checkpoint receptors, the immune checkpoint inhibitors (ICIs), alone may not provide clinical benefit in esophageal carcinoma. Therefore, combination therapies are being tested to sensitize the TME to the effects of ICIs. These therapies include chemotherapy, radiotherapy and biologic therapy. Created with BioRender.com (No. MX23PE8S2A).

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Pubmed (Medline), Google Scholar, and Cochrane were searched for English-written and peer-reviewed reports published in indexed international journals until July 2022. The keywords were: esophageal cancer, checkpoint inhibitors, chemotherapy, radiotherapy, biologic therapy. Excluded were articles that did not involve ICIs in combination therapies in EC patients.

Combinatorial Approaches: Chemotherapy and ICIs

Although ICI immunotherapy represented a real breakthrough in the treatment of multiple malignancies, the response rates remain relatively low. A detailed search for specific predictors of immunotherapy response has been initiated and resulted in an establishment of several predictive biomarkers, such as the neoantigen mutational load, the high intratumoral infiltration with CD8+ and CD4+ T cells, and the expression of checkpoint molecules within the TME [34-38]. It has been shown that the differential response rates are largely driven not just by the proportions but also by the phenotypes of TILs [39]. Currently, the pattern of tumor immune infiltration can be quantified in clinical practice by an immunoscore, a valid marker of the TME immune status that is associated with the prognosis and treatment response [38]. Because the TME is a complex structure that displays variable properties among cancer types, and most importantly, among individuals with the same cancer type, it is crucial to investigate the TME patterns in more than just one single time point [40-42]. To achieve immunotherapy response, the TME should contain a large proportion of TILs by the time of ICIs application. Furthermore, the TME should promote the T-cells activation, proliferation and migration [43]. A wider assessment of the immune cell activation by gene expression profiling may also largely contribute to the understanding of the immunotherapy resistance [38].

The low immunogenicity of the TME which is reflected by the lack of tumor-associated antigens (TAAs) can also represent a great hurdle in certain tumors. In such cases, T cells cannot be activated by the TAAs-primed antigen presenting cells [44]. Moreover, there are multiple pro-tumorigenic immune cells in the TME that are capable of preventing the regular proliferation of the effector T cells. These cells include T regulatory cells (Tregs), tumor-associated macrophages, or myeloid derived suppressor cells (MDSC), and their presence in the TME also restrains the efficacy of ICI immunotherapy [45].

Chemotherapeutic agents are mostly designed to inhibit cancer cell proliferation [46]. However, it has been shown that these agents also display a significant immunomodulatory capacity and by causing a severe lymphodepletion, the patient’s immunological repertoire undergoes a near-to-complete renovation [42]. As a result, conventional cytotoxic chemotherapy was recently shown to provide a synergic effect to ICI immunotherapy by sensitizing the TME. The synergistic activity of different chemotherapeutic agents is reflected at multiple levels. It has been observed that chemotherapeutic agents, such as bleomycin, oxaliplatin, and cyclophosphamide, significantly increase the number of CD8+ TILs in the TME. On the other hand, agents, such as dactinomycin and anthracyclines, can lead to a selective depletion of immunosuppressive cells, such as Tregs or MDSCs [42].

Another factor described upon administration of chemotherapy was the promotion of immunogenic cell death (ICD) in the TME. It is commonly accepted that ICD leads to the secretion of lysosomal adenosine triphosphate (ATP) and causes the presentation of calreticulin and/or heat shock proteins on the surface of the tumor cells [47, 48]. Such effect was reported to significantly increase the presence of TAAs-primed DCs in the TME [49]. Finally, the lymphodepletion induced by the cytotoxic agents restores the function of the immune system by eliminating the exhausted lymphocytes and promoting the development of new and more potent lymphocyte subpopulations [44].

In EC, the PD-L1 expression status is much higher in the SCC as compared to AC, and thus, the administration of ICIs may be more suitable for SCC [50]. The ICIs alone have been applied for the treatment of EC in a number of clinical trials, including Keynote-181, ATTRACTION-3 trial, and MEDI4736 trial [50]. The concomitant administration of ICIs and chemotherapy was evaluated in an open-label, phase III trial CheckMate-648 [51]. In this study, 970 patients were randomized to receive either nivolumab plus chemotherapy, nivolumab plus ipilimumab, or chemotherapy alone. A significant progression-free survival benefit was pronounced in the patients receiving nivolumab plus chemotherapy over patients receiving chemotherapy alone. Of note, the expression of PD-L1 had to be at least 1% or greater [51].

The efficacy of pembrolizumab in combination with chemotherapy as a first-line treatment was evaluated in patients with advanced EC in a randomized, double-blind, placebo-controlled phase III study KEYNOTE-590 [52]. In this study, 749 EC patients were randomly assigned to receive either pembrolizumab plus chemotherapy (n = 373) or placebo plus chemotherapy (n = 376). The PD-L1 expression was assessed in the study participants before the study initiation. The combination of pembrolizumab and chemotherapy was shown to have a good safety profile and the efficacy of pembrolizumab plus chemotherapy was superior to placebo plus chemotherapy. This was reflected mainly in the improvement of progression-free survival in patients with esophageal SCC [53].

To date, there are 30 ongoing clinical trials combining chemotherapy and ICI immunotherapy in EC patients. However, only 14 have reached phase II/III of clinical evaluation. We have provided an overview of all currently ongoing clinical trials in EC patients. The data are summarized in Table 1.

Table 1.

Clinical trials of ICIs in combination therapies for the treatment of esophageal carcinoma

/WebMaterial/ShowPic/1474418Combinatorial Approaches: Radiotherapy and ICIs

For many decades, radiotherapy has been a standard of care therapy in the treatment of multiple malignancies [54]. Radiotherapy kills tumor cells by inducing direct damage to the DNA, such as single-strand breaks, double-strand breaks, or damages to the bases [54]. Radiotherapy can also damage the DNA indirectly via the accumulation of free radicals with mutagenic potential [55]. Both mechanisms, the direct and the indirect, interfere with a number of cellular targets regulating the fate of the cell. Depending on the cancer type and its location, different types of radiation can be applied [54]. Therefore, it has been shown that different doses and qualities of radiation can have either pro-immunogenic or immunosuppressive effects [56]. Radiotherapy induces a cascade of reactions. The secretion of different cytokines upon radiotherapy causes a rapid shift in the proportions, phenotypes, and activation status of tumor infiltrating lymphocytes and thus, radiation significantly modulates the TME [47].

The ability of radiation to induce ICD is of major importance [57]. Similarly to chemotherapy-induced ICD, the radiotherapy-induced ICD promotes the release of damage-associated molecular patterns (DAMPs), such as calreticulin, high mobility group box 1 (HMGB1), and extracellular ATP, and allows further downstream of the anti-tumor signaling pathways. Specifically, calreticulin which is normally associated with the endoplasmic reticulum-chaperone proteins attracts the DCs when exposed [58]. In addition, HMGB1 and extracellular ATP play a key role in the regulation of inflammasome and pro-inflammatory cytokines [55, 59, 60]. The effect of radiotherapy and the subsequent production of various stimuli, such as DAMPs and TAAs release, results in a potent type I interferon response. Type I interferons further mediate immunostimulatory processes, including the activation of DCs and the CD8+ T cell priming [56, 61].

Taken together, in most cases, radiotherapy recruits TILs, affects intratumoral cytokine spectrum, and upregulates antigenic expression [62]. However, certain populations of immune cells may not be sensitive to radiation. Those are, for example, Tregs which also produce tumor-growth factor beta (TGF-β), and thus, increase the ability of the tumor cells to escape the immune surveillance [55, 63].

The ambivalent consequences of radiotherapy application highlight the importance of individual radiotherapy type and dose management. Several clinical trials are currently focusing on these aspects of radiotherapy and ICI combinations. Since radiotherapy was shown to generate tumor-specific immune responses, it could also augment the efficacy of ICI immunotherapy [62]. To date, preclinical and clinical studies have aimed to investigate this idea. One of the first reports on the combination therapy was shown in 2005 by Demaria et al. In this study, the efficacy of CTLA-4 blockade in a murine model of mammary carcinoma was significantly augmented by the radiation [64]. In EC patients, chemoradiation was demonstrated to induce tumor-specific cytotoxic T cells in SCCs [65]. In addition, irradiation was found to upregulate the PD-L1 expression in human EC cells [66].

These findings are supported by a study by Kelly et al. where esophageal AC displayed more than a 30% increase in the PD-L1 expression after the application of radiotherapy [67]. These in vivo studies indicate that radiotherapy could sensitize the TME of EC to ICI immunotherapy [65, 67].

Treating patients with radiotherapy prior to the ICI could, therefore, also increase the response rate and improve the OS of EC patients [59]. A number of preclinical studies of murine models have shown a synergistic potential of anti-PD-L1 treatment and radiotherapy leading to an MDSC reduction, CD8+ T cell activation, and an increase in the median survival [68, 69].

The abscopal therapeutic effects of radiotherapy which are observed in nonirradiated zones may also influence the efficacy of ICI. Interestingly, in a patient with widespread oligometastatic lung cancer, a complete and durable response to ipilimumab was achieved after the irradiation of a single liver metastasis [70]. This interesting concept is particularly important since the tumor-specific immune cells that are induced in the irradiated zone may influence the treatment responses in distant sites [71, 72].

Preclinical data on the combination of ICI and radiotherapy in EC have been encouraging [3]. To evaluate whether this strategy could be beneficial in esophageal AC and SCC patients, further studies are, however, needed. To date, most of the clinical trials are still recruiting. However, the anticipation is high, and it is believed that this combination will increase the OS and progression-free survival as compared to the use of ICI alone.

Combinatorial Approaches: Biologic Therapy and ICIs

Targeting specific molecules has become an important part of the treatment of many diseases, including EC [73-75]. Several targeted therapies have shown efficacy or have generated encouraging results in the treatment of patients with advanced or metastatic disease [76, 77]. Therefore, a combination of biologic therapies with ICIs is extensively investigated in many diseases with promising results [75].

Human epidermal growth factor receptor 2 (HER2) is overexpressed in gastric or gastroesophageal junction ACs and esophageal SCCs in approximately 15–20% and 5.5–13% of the cases, respectively [78, 79]. HER2 can be targeted by trastuzumab, a recombinant humanized monoclonal antibody directed against the extracellular domain of HER2 that has been approved by the FDA as a first-line treatment of HER-2-positive cancer [73]. According to Bang et al., [80] the median OS rate was 13.8 months in patients with advanced gastric or gastroesophageal junction cancer treated with trastuzumab plus chemotherapy compared with 11.1 months in those treated with chemotherapy alone. However, Safran et al. [81] in their phase III clinical trial demonstrated that the addition of trastuzumab to neoadjuvant chemoradiotherapy (radiation therapy, paclitaxel, and carboplatin) for HER2-overexpressing esophageal cancer was not effective [82]. Janjigian et al. [83] reported that pembrolizumab can be safely combined with trastuzumab and chemotherapy in HER2-positive metastatic gastric, esophageal, or gastroesophageal junction cancer with promising activity. There is an expectation that a randomized phase II clinical trial assessing the efficacy and safety of this treatment will be completed in November 2022 [83]. In a study by Satoh et al., [84] nivolumab was effective and safe as third-line treatment regardless of prior trastuzumab use in patients with advanced gastric and gastroesophageal junction cancer.

Another molecule targeting HER2 is margetuximab, a chimeric IgG monoclonal antibody [85]. Current evidence supports the concept of synergistic antitumor activity with the combination of margetuximab along with pembrolizumab [86]. While recent data from the phase II study suggest the clinical feasibility of combining chemotherapy, trastuzumab and ICIs in esophageal and gastric cancer, the evidence for a chemotherapy-free regimen is also mounting in HER2-positive disease [87].

Ramucirumab is a monoclonal antibody inhibiting a vascular endothelial growth factor receptor-2 (VEGFR2), and thus serves as an anti-angiogenic targeted therapy [88, 89]. Promising clinical activity has been demonstrated in patients with refractory gastric cancer/gastroesophageal junction cancer when treated with dual blockade combination with antiangiogenic agents and ICIs, such as PD-1/PD-L1 inhibitors, in several phase I/II trials [76]. According to Lin et al., ramucirumab plus chemotherapy and PD-1 inhibitors were superior to chemotherapy alone for previously treated cases of advanced esophageal/gastroesophageal junction cancer. Use of PD-1 inhibitors, especially camrelizumab alone, was likely to be the optimal treatment in patients previously treated for advanced esophageal spinocellular cancer [90].

Epidermal growth factor receptor (EGFR) is over-expressed in up to 55 % of gastroesophageal cancers [91, 92]. Cetuximab (CET) is a monoclonal antibody inhibiting EFGR which is used in treatment of various cancers [93, 94]. According to Huang et al. adding CET to multimodal therapy significantly improved response rate and disease control rate for patients with metastatic esophageal cancer. However, CET failed to significantly improve the OS and progression-free survival (PFS) for patients with localized or metastatic esophageal cancer [95]. Moreover, there is no significant evidence in combination of CET and checkpoint inhibitors.

A fibroblast growth factor (FGF) receptor is another promising subject under research in gastric and esophageal cancer [96]. Bemarituzumab is an afucosylated monoclonal antibody against FGFR2b receptor [97]. Catenacci et al. stated that it seems to be well tolerated and that bemarituzumab demonstrated single-agent activity as late-line therapy in patients with advanced-stage gastric and gastroesophageal junction AC. Bemarituzumab is currently being evaluated in combination with chemotherapy in a phase III trial as a frontline therapy for patients with high FGFR2b-overexpressing advanced-stage gastric and gastroesophageal junction AC [98]. However, to date evidence addressing combination of bemarituzumab and ICIs is lacking.

Despite the extensive research, the extensive progress in improving the outcomes of patients with the advanced/metastatic esophageal cancer remains limited [99]. Biologic therapy for upper gastrointestinal tract carcinomas is arguably still in its infancy [100]. The same applies to the combinatorial treatment with the ICIs.

Discussion

The immunotherapy with ICIs has brought a new hope to the treatment of metastatic diseases [12, 101]. The unprecedented clinical responses have changed the treatment landscape of multiple malignancies, including malignant melanoma, metastatic lung cancer, and metastatic kidney cancer, and have placed the ICIs to the frontline of treatment [102].

However, a large proportion of patients, including those with EC, fail to display substantial responses to ICI immunotherapy. The TME dictates the response to immunotherapy and determines its efficacy [13]. The mechanisms of primary and acquired immunotherapy resistance are poorly understood and to date, extensive research is being focused on deciphering the factors that modulate the TME sensitivity to ICI immunotherapy.

In a number of studies, ICI immunotherapy has been shown less effective as a monotherapy compared to combination therapies. Chemotherapy, radiotherapy, and biologic therapy were shown to change the immune signatures, impact the TME, and thus, the treatment-induced changes could be indicative of successful ICI immunotherapy.

Because metastatic EC has a very poor prognosis with limited treatment options, finding new therapeutic avenues is very challenging. In this review, we have focused on the promising treatment options for EC by searching for ICI immunotherapy in combinatorial settings with chemotherapy, radiotherapy, and biologic therapy.

In EC, PD-L1 expression status was shown to be higher in the SCC as compared to AC. For that reason, ICIs plus chemotherapy, or even dual checkpoint inhibition, are being discussed as a particularly feasible option for patients with SCC [103].

Studies correlating the efficacy of the chemotherapy and ICI combination with the chemotherapy alone, demonstrated an overall improvement in the progression-free survival in patients with esophageal SCC in the combinatorial setting group. Similar results were obtained in another study, where the expression of PD-L1 in EC patients had to be at least 1% or greater.

Radiation was shown to increase the PD-L1 expression in human EC cells, providing a rationale for ICI administration in a combination with radiotherapy. The radiotherapy-induced changes of the TME of EC were shown in multiple studies and mostly indicated the ability of radiotherapy to induce the expression of immune checkpoint molecules. Other studies also highlight the ability of radiation to promote the priming and activation of cytotoxic T cells [104]. Currently, clinical trials combining radiotherapy and ICI immunotherapy in EC patients are ongoing.

Since biologic therapies offer a more tailored treatment option, their combination with ICIs may hold potential benefit [105]. In EC, several studies evaluating the efficacy of ICIs in combination with biologic therapy have been initiated. The combination of ICIs with trastuzumab was shown to be safe, however, the impact on the patients’ outcome varied among studies. Both margetuximab and ramucirumab seemed to provide at least a certain benefit when combined with anti-PD-1 inhibitors, whereas bemarituzumab and CET are yet to be investigated in combination therapies in EC patients.

Conclusion

Taken together, great effort is already underway in clinical trials evaluating the combinatorial administration of ICIs and other treatment modalities in metastatic EC patients. It is yet to be established, whether ICIs should be given in an adjuvant or neodajuvant setting. In EC patients, neoadjuvant therapy is an essential part of multi-modality treatments, and several studies indicated that the effectiveness of complete surgical resection may be enhanced by incorporating an early intervention with ICIs [106-108]. On the other hand, adjuvant administration of ICIs was also shown to significantly improve the disease-free survival [109, 110].

To determine which agents sensitize, the TME to immunotherapy and pose the new hope of bringing substantial and meaningful benefits for EC patients, further preclinical and clinical trials urgently needed. Furthermore, combinatorial therapeutic schedules need to be designed to shed light on possible immunotherapeutic options for EC patients in either adjuvant or neoadjuvant setting.

Acknowledgments

The authors would like to thank all the patients participating in the above-mentioned studies and the technical staff for their assistance.

Conflict of Interest Statement

The authors Capucine Casari, Rene Novysedlak, Jiri Vachtenheim Jr, Robert Lischke, and Zuzana Strizova declare no conflict of interest.

Funding sources

The study was supported by (a) the Ministry of Health, Czech Republic – Conceptual Development of Research Organization, University Hospital Motol, Prague, Czech Republic (No.6028); (b) the Cooperatio Program, Research Area SURG; (c) Loreal- UNESCO For Women in Science.

Author Contributions

Capucine Casari performed the data acquisition and analysis, wrote and revised the manuscript. Rene Novysedlak performed the data acquisition and analysis, wrote and revised the manuscript. Jiri Vachtenheim Jr contributed to the manuscript writing and editing. Robert Lischke contributed to the manuscript editing. Zuzana Strizova supervised the data acquisition and analysis, wrote, and revised the manuscript. All of the authors contributed to the manuscript writing and reviewed the final version of the manuscript.

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