Combination radiation and immunotherapy in gynecologic malignancies—a comprehensive review

Introduction

Radiation therapy plays a central role in the treatment of most gynecologic malignancies, in both the definitive and palliative setting. Historically, radiation therapy has frequently been given with concurrent systemic therapy to augment the local effects of radiotherapy and improve systemic disease control. As with other disease sites, patients with locally advanced, recurrent, refractory, or metastatic disease continue to have poor clinical outcomes without significant advancement. More recently, cell cycle immune checkpoint inhibition has emerged as a promising systemic treatment in several disease sites, including gynecologic malignancies, for patients with advanced, recurrent, or metastatic disease.

Broadly, the goal of immunotherapy is to harness the ability of the immune system to recognize and eliminate tumor cells (1). Therefore, by modulating the immune system using antibodies against protein receptors such as CTLA-4, PD-1, and PD-L1, the anti-tumor immune response can be amplified, leading to improved systemic response to malignant cells (2). However, tumor responses to immunotherapy monotherapy are low, in the 20–30% range (3-6). This is thought to be due to immune-evasive properties of tumors through selective pressure, tumor evolution, and tumor genetic heterogeneity, resulting in reduced tumor immunogenicity or induction of immune tolerance (anergy) (1).

Radiation can increase antigen quantity, variety, and presentation to increase the immune system’s ability to respond to a tumor through both innate and adaptive immune mechanisms and induction of anti-tumor T-cell responses. Therefore, the therapeutic intent behind combined radioimmunotherapy is to augment the immune response and circumvent immune evasion. The thought is that radiation can increase antigen generation/presentation, T-cell priming, dendritic cell activation, and upregulation of pro-inflammatory cytokines. Meanwhile, immunotherapy can reverse T-cell exhaustion, broaden T-cell receptor recognition, and enhance T-cell activation. Hence, immunotherapy and radiation may have a synergistic effect.

This hypothesis has only just begun to be explored in gynecologic malignancies but is supported by the first reported clinical trials. For example, GOG 9929, a Phase I clinical trial published in 2019 investigating sequential immunotherapy following chemoradiation (CRT) in women with locally-advanced cervical cancer showed an increase in T-cells expressing PD-1 following CRT, and offers a rationale for further investigation into neoadjuvant, concurrent, and adjuvant treatment with immunotherapies such as PD-1/PD-L1 and CTLA-4 inhibitors (7).

The optimal sequencing, dosing, and fractionation of radiation and immunotherapy remains unknown and is the topic of several clinical trials throughout disease sites, including gynecologic malignancies. Herein, we review the current utilization of radiation combined with immune checkpoint inhibitors for treatment of gynecologic malignancies. We will then describe current and potential future investigations into their combined efficacy.

Radiation and immunotherapy sequencing, dose, and fractionation

The optimal sequence, dose, and fractionation schedule for combining radiation and immunotherapies remains unclear and depends on the proposed mechanism of synergy of the two modalities. Pre-clinical data suggests that a close or concurrent sequencing may be beneficial in the case of PD-L1 inhibitors. This setting has been supported by murine models, where immune checkpoint blockade correlates with peak T-cell infiltration of tumors after radiation, leading to superior outcomes with concurrent administration (8-10). Conversely, other data suggest benefits to the neoadjuvant use of CTLA-4 inhibitors, theoretically due to CTLA-4 inactivating intratumoral regulatory T-cells (11). Specific agents being studied in ongoing clinical trials of combined radiation and immunotherapy for gynecologic malignancies are summarized in Table 1.

In addition to optimizing the sequencing of therapies, radiation dose and fractionation may influence therapeutic synergy. Early murine models suggest hypofractionated radiation may be favorable through a resultant increase in antigen presentation, mature CD8 T-cell tumor infiltration, and enhanced effector T-cell reactivity (12,13). This has been attributed to the superior cytoreductive capability of hypofractionated radiation, resulting in reduced tumor burden, which in turn increases reinvigoration of exhausted T-cells and, consequently, radiosensitivity (8,9,14). Other contributors may be increased upregulation of checkpoint molecules and inflammatory cytokines, although it is not yet clear how fractionation schemes influence these factors (15). In the ongoing trials summarized below, sequencing, dose, and fractionation are all reported. Unfortunately, few of them are specifically designed to identify an optimal regimen, and indeed there may not be a one-size-fits-all approach. Future work investigating these parameters in both the pre-clinical and clinical setting will be needed to guide the combined use of radiation treatment with immunotherapy. The ongoing trials investigating combined radiation and chemotherapy, and the existing data and rationale behind them, are reviewed below broken down by disease site, and are summarized in Table 2.

Cervical cancer

Cervical cancer affects an estimated 13,800 women per year in the United States, accounting for 4,290 deaths annually, and is also the most common gynecologic malignancy worldwide (16). The current standard of care for definitive management of locally advanced cervical cancer is concomitant platinum-based CRT (17-21). This treatment paradigm has remained relatively unchanged for two decades; however, overall survival (OS) and progression-free survival (PFS) for patients with the locally advanced, node positive, and metastatic disease remains decidedly poor. More recently, chemotherapy intensification was evaluated with the addition of adjuvant chemotherapy in hopes of improving clinical outcomes for patients with high-risk disease. A Phase III trial showed improved PFS (74% vs. 65%) with “outback” cisplatin/gemcitabine, but had significant increased Grade 3/4 toxicity (22). Chemotherapy intensification continues to be the topic of investigation of OUTBACK (GOG-0274/RTOG 1174) and INTERLACE trials, which are evaluating adjuvant and neoadjuvant platinum-taxane regimens, respectively. In a similar vein, there is also significant interest in investigating immunotherapy combined with radiation to treat cervical cancers to determine if PFS and OS can be improved without excessive toxicity.

A rationale for treatment of cervical cancers with immunotherapy arises from most cervical malignancies developing out of immune tolerance/anergy in response to persistent infection with human papillomavirus (HPV). This virally-driven malignancy results in dysplasia progressing to neoplasia, then carcinoma, cellular immortalization, and immune evasion. The resulting immune downregulation, via mechanisms such as increased PD-1 and PD-L1 expression, is higher in virus-associated cancers such as HPV-associated cancers (23-25). This provides a strong preclinical rationale in HPV-mediated cervical cancers to target modulation of PD-1/PD-L1 in hopes of decreasing immune escape mechanisms.

In June 2018, based on data from KEYNOTE-158, pembrolizumab was approved for patients with recurrent or metastatic cervical cancer with disease progression on or after chemotherapy. In this trial, 98 patients received 200 mg of pembrolizumab every three weeks, and results demonstrated an objective response rate in 12.2% of patients (three complete and nine partial responses) (26). Of note, there were no responses in patients whose tumors did not express PD-L1. Bastilimab ± zalifrelimab is currently under FDA fast track designation as second-line therapy for patients with recurrent, unresectable, and metastatic cervical cancer with progressive disease (NCT03894215). However, as with treatment of other disease site with immunotherapy alone, these existing data suggest low objective response rates (ORR), necessitating either improved patient selection or investigation of other combined therapies.

GOG 9929 is a recent Phase 1 trial examining pelvic CRT followed by sequential ipilimumab in the management of node-positive cervical cancer (7). Patients with FIGO Stage IB2 and IIA disease with positive para-aortic lymph nodes and patients with Stage IIB, IIIB, or IVA disease with positive para-aortic or pelvic lymph nodes were eligible. They were treated with standard pelvic CRT followed by four sequential escalating doses of ipilimumab. Twenty-one patients received at least two cycles of ipilimumab (18 received four cycles and three received two cycles). Only two patients had grade 3 toxicity, with one episode each of self-limited lipase increase and dermatitis. Overall Survival at 12 months was 90% and PFS was 81%. Interestingly, following CRT there was a significant increase in CD4 and CD8 positive T cell expression of PD-1. This suggests that CRT has the potential to expand immune surveillance populations and anti-cancer immune responses (27). Furthermore, the increase in PD-1 expression was maintained following CRT with ipilimumab, suggesting that the CTLA-4 blockade has the potential to generate a durable adaptive anti-tumor response.

Current investigations

Relative to other gynecologic malignancies, cervical cancer is the topic of a large quantity of ongoing clinical trials, and will be helpful in better elucidating questions regarding efficacy, sequencing, dose, and fractionation. These are summarized in Table 2.

In the locally advanced definitive setting, a Phase II University of Virginia study evaluates the addition of pembrolizumab to standard of care chemoradiation (NCT02635360). Pembrolizumab is to be administered in three-week cycles concurrently with definitive chemoradiation and for three months in the adjuvant setting. Similarly, a larger phase III trial (MK-3475-A18/KEYNOTE-A18/ENGOT-cx11) is investigating definitive CRT with or without pembrolizumab per the same protocol as above except for permitting up to 20 cycles, to evaluate the safety and efficacy of combine CRT and pembrolizumab compared to placebo (NCT04221945). A similar study is also underway as part of the CALLA study, a Phase III study using durvalumab plus CRT for locally-advanced cervical cancer (NCT03830866). In this study, the experimental arm consists of durvalumab given in four-week cycles, up to 24 cycles, given concurrently and then adjuvantly with definitive chemoradiation. Similar studies using nivolumab (NCT03298893, NCT03527264), atezolizumab (NCT03612791, NCT03738228), and TSR-042 (NCT03833479) in combination with radiation are also currently underway. Of note, the timing of anti-PD-L1 therapy is being investigated on NRG GY017 with a randomization to concurrent vs neoadjuvant and concurrent CRT + atezolizumab.

In the metastatic setting, Moffit Cancer center is investigating combining SBRT followed by atezolizumab (NCT03614949). In this trial, SBRT is administered to at least 2 sites to a total dose of 24 Gy in 3 fractions and is then followed by atezolizumab given every three weeks. There are other trials not limited to cervical cancer that will be discussed below.

Endometrial cancer

Endometrial cancer is the most common gynecologic malignancy in the United States, affecting an estimated 65,620 women per year and accounting for 12,590 deaths annually (16). Endometrial cancer is typically diagnosed early, with approximately 75% of new diagnoses being Stage I/II, which can often be managed with surgery alone. However, there is a subset of these patients classified as high-intermediate risk (defined by PORTEC-2 as having at least two of the following: outer half myometrium invasion, Grade 3, or Age >60), who are traditionally treated with adjuvant vaginal cuff brachytherapy, and have a non-insignificant rate of 10-year pelvic recurrence and distant metastasis rate of 6% and 10% respectively. Furthermore, patients that are locally-advanced disease have suboptimal outcomes, despite treatment with chemotherapy, with 5-yr OS, FFS, and DMFS rates in PORTEC-3 being 81%, 77%, and 79% respectively (28). The addition of immunotherapy to these existing paradigms might optimally improve on these results.

In endometrial cancers, approximately 40% of tumors genetically express DNA polymerase epsilon (POLE) or microsatellite instability high (MSI-H) phenotypes. These tumor phenotypes are immunogenic and potentially amenable to immunotherapeutic treatment approaches (although patients whose tumors have POLE mutations already have excellent outcomes even with radiation alone) (29-32). In order to examine microsatellite instability, MLH1 methylation and mismatch repair (MMR) protein expression was examined in specimens from GOG 210. In this study, cases with epigenetic and probable mutations in MMR where associated with higher grade and lymphovascular space invasion (33). MMR deficient (dMMR) tumors were associated with a significantly worse PFS, with a HR of 1.37. Further exploratory analysis showed that tumors with probable MMR mutations had a four-fold advantage to adjuvant therapy compared to non-mutant MMR tumors, measured by response rate. These results were supported by another single-institution study, which found dMMR to be associated with advanced stage, tumor size, higher grade, presence of lymphovascular space invasion, and older age (34). Stage III/IV patients with dMMR had significantly decreased recurrence-free survival (47.4% vs. 3.4%) despite receiving similar adjuvant therapies. This relationship was further investigated in high-intermediate risk patients, with MMR defects being associated with worse recurrence rates (28% vs. 11%), with distant recurrences being particularly notable (14.1% vs. 3%) (35). This difference held even when excluding isolated vaginal recurrences (18.8% vs. 4.5%). 5-year RFS was 66% in dMMR tumors compared to 89% in non-mutant MMR tumors.

Immunotherapy for dMMR tumors was first investigated in patients with colon cancer. This led to accelerated FDA approval of pembrolizumab in May 2017 for patients with dMMR or microsatellite instability-high solid tumors who progressed on prior therapy, and provided an opportunity to treat patients with progressive endometrial cancer with that biomarker under such a paradigm (36,37). Beyond the potential targeting of microsatellite instability, the strong influence excess estrogen signaling has on endometrial cancer development, with associated activation of alternative signaling pathways such as MAPK, IGF-1, and cAMP, also provides a possible therapeutic target (38-40).

In 2019, combined pembrolizumab and lenvatinib was approved for patients with metastatic endometrial cancer with disease progression after receiving no more than two systemic therapies. This was based on data from KEYNOTE-146, where patients were treated with oral lenvatinib daily plus pembrolizumab every three weeks. 108 patients were enrolled, with an objective response rate of 38% with 11% complete and 28% partial response rates (41). Again, while not trivial, these response rates are objectively low, and combining immunotherapy with radiation is aimed at improving these disease responses, and is the topic of multiple ongoing trials investigating treatment of endometrial cancer.

Current investigations

Ongoing trials combining radiation and immune checkpoint inhibitors are summarized in Table 2. The benefit of immunotherapy in the management of patients with high-intermediate risk endometrial cancers will be examined in the Phase I FIERCE trial from the University of Oklahoma (NCT03932409). This trial will investigate the use of neoadjuvant pembrolizumab given seven days prior to the initiation of definitive vaginal brachytherapy, followed by combined chemotherapy and pembrolizumab.

In the setting of medically inoperable Stage I/II endometrial cancer, anti PD-L1 in the form of TSR-042 is being investigated at Washington University (NCT03955978). In this trial, TSR-042, a monoclonal antibody against PD-L1, will be administered both neoadjuvantly every three weeks and concurrently with six fractions of definitive intracavitary brachytherapy for a total of four cycles.

In the advanced and refractory setting, the PRIMMO study (NCT03192059) will examine the use of an immunomodulatory cocktail of vitamin D, aspirin, lansoprazole, cyclophosphamide, and curcumin in combination with pembrolizumab and radiation. The pembrolizumab is given in three-week cycles starting on the first day of radiation, which is given every other day in 8 Gy fractions to a total dose of 24 Gy for patients with uterine and cervical malignancies.

Ovarian cancer

Ovarian cancer is the deadliest gynecologic malignancy in the United States, affecting an estimated 21,750 women per year in the United States and accounting for 13,940 deaths annually (16). Ovarian cancer is often diagnosed at late stages, with poor prognosis despite systemic therapy, and provides an opportunity for the advancement of immunotherapy to improve clinical outcomes. The immunogenicity of ovarian cancer is an important prognostic determinant, with CD8+ T-cell infiltration of tumors being associated with improved OS (2). Furthermore, associated PD-L1 expression inversely correlates with CD8+ T-cell infiltration and, correspondingly, survival (42). This therefore provides rationale for use of immunotherapy to decrease PD-L1 induced immune inhibition.

Single agent immune checkpoint blockade in unselected patients with recurrent ovarian cancer has low to modest efficacy, with response rates ranging from 6% to 15% (2). Use of single-agent pembrolizumab was investigated as part of a Phase 2 study, KEYNOTE-100. PD-L1 expression was determined using a combined positive score (CPS) that examined staining on the tumor and immune cells. When stratified by CPS, ORR was 9% in unselected patients, 14% in patients with a CPS score of 1 or higher, and 25% in patients with a CPS score of 10 or higher (43). Dual immune checkpoint inhibitors have also been investigated. Based on NRG-GY003, adding ipilimumab to nivolumab improved response rate from 12% to 31%, with PFS HR of 0.5 (44). The addition of radiation aims to augment those response rates.

Current investigations

Given that radiotherapy has a limited role in managing early-stage ovarian cancers, existing trials combining radiation and immunotherapy are largely focused on the metastatic setting, and are detailed in Table 2. One such trial from UT Southwestern, which unfortunately closed early due to poor accrual, was investigating combined stereotactic ablative radiotherapy (SABR) given concurrently with avelumab (NCT03312114). Ovarian cancer has also been included in multiple, site agnostic trials evaluating combined radiation and immunotherapy for metastatic disease; however, enrollment of these patients on trial has been limited.

Vulvar cancer

Vulvar cancer is the least common gynecologic malignancy in the United States, affecting an estimated 6,120 women per year in the United States and accounting for 1,350 deaths annually (16). Patients with locally advanced, metastatic, and recurrent disease have poor prognosis, with 5 year OS of 53%, 19%, and 15% respectively (45,46). Treatment for such patients is highly individualized but typically involves some combination of surgery, radiation, and platinum-based chemotherapy. Given the poor prognosis associated with metastatic vulvar cancer, there is significant interest in the development of new clinical treatment paradigms to prevent disease progression, recurrence, or the development of metastases. As with other gynecologic malignancies, many cases arise out of chronic infection/irritation, immune anergy/dysfunction, and/or HPV infection, and there is optimism that adding immunotherapies to current treatment regimens will improve on historic outcomes.

In vulvar cancer, membranous PD-L1 expression is present in about a quarter of vulvar squamous cell carcinoma, and is associated with HPV negativity (47). The increasing degree of PD-L1 was found to be correlated with poor outcomes. It has also been shown to be predictive of tumor stage (48). Additionally, tumors with more activated T-cells, largely present in HPV associated tumors and absent in p53-mutant tumors, are associated with superior survival (49). In combination, these findings provide rationale for a potential benefit for treatment with cell checkpoint inhibitors.

There are limited studies examining immunotherapy for the management of vulvar cancer. One study, the Dual Anti-CTLA-4 and Anti-PD-1 Blockade in Rare Tumors (DART) trial, does include vulvar cancer as an eligible cancer subtype, and is a Phase II trial investigating combined ipilimumab and nivolumab (NCT02834013). Additionally, there is evidence for off-label use of cemiplimab, which had an approximately 50% response rate among patients with advanced and metastatic cutaneous squamous cell carcinoma (50).

Current investigations

The incidence of vulvar malignances makes accrual for radioimmunotherapy trials difficult, but there are ongoing trials, as listed in Table 2. A newly opened Phase II trial based out of Massachusetts General Hospital (NCT04430699) seeks to evaluate combined radiation, cisplatin, and pembrolizumab for unresectable or metastatic vulvar squamous cell carcinoma. In this trial, standard-of-care radiation will be delivered with concurrent pembrolizumab every three weeks and weekly cisplatin. Patients will also receive adjuvant pembrolizumab every three weeks for up to 12 cycles. A separate, broader trial from MD Anderson is investigating tremelimumab and durvalumab combined with SBRT for recurrent or metastatic advanced cervical, vaginal, or vulvar cancer (NCT03452332). In this trial, tremelimumab and durvalumab are given every four weeks, with tremelimumab given up to four cycles and durvalumab up to eight cycles. SBRT is given on days 8, 10, and 12 of cycle 1, to a total dose of 24 Gy in 3 fractions.

General gynecologic malignancies

Current investigations

Beyond current disease-site specific trials, there are also currently trials investigating the role of combined radiation and immunotherapy in gynecologic malignancies as a whole, found in Table 2. One such trial out of Dana Farber Cancer Institute is investigating the use of durvalumab with or without tremelimumab concurrent with 25 Gy EBRT in 5 fractions for recurrent, advanced, and metastatic gynecologic malignancies for which standard curative or palliative measures do not exist or are no longer effective (NCT03277482).

Toxicity

Although there is considerable interest in combination radiation and immunotherapy for gynecologic malignancies to improve clinical outcomes, combined therapy often comes at the cost of increased morbidity and toxicity. Immunotherapy alone has a diverse side effect profile including but not limited to uveitis, hypophysitis, dry mouth, hypothyroidism, pneumonitis, enterocolitis, hepatitis, pancreatitis, auto-immune diabetes, adrenal insufficiency, dermatitis, vitiligo, and arthralgias, although rates of grade 3–5 toxicity are less than 20% (51). There is therefore concern for multimodality combined radiotherapy and immunotherapy having synergistic toxicity profile, although currently available clinical data does not show increased risk of adverse events (27).

In the treatment of gynecologic malignancies, the close anatomic proximity of radiation targets to organs at risk leads to concern for synergistic gastrointestinal toxicity. In a Phase 1 trial using CRT extended-field pelvis without intensity modulation and ipilimumab for node positive cervical cancer in 21 women, there was Grade 1 and 2 diarrhea, but only one case of Grade 3 abdominal pain, nausea, and vomiting as well as an incidence of asymptomatic lipase elevation (7). This toxicity could be further mitigated using either a traditional pelvic field or intensity modulated radiation therapy (IMRT) and image guidance. As data from ongoing clinical trials become available, the toxicity profile of combined radiotherapy and immunotherapy will be better understood and inform future treatment dosing, sequencing, and overall management. In the meantime, as treatment of gynecologic malignancies with multimodal immunotherapy combinations increase, patients should be closely monitored for toxicity.

Conclusions

Combined radiation and immunotherapy is an active area of investigation in modern oncology, and is the topic of multiple ongoing clinical trials for gynecologic malignancies. As outlined above, patients—particularly those with high-risk, locally advanced, recurrent, and metastatic gynecologic malignancies—have the most to potentially gain from combined modality therapy. Newer radioimmunotherapy approaches have the potential to realize meaningful clinical improvements in disease control and patient survival through augmented anti-tumor immune response via increased antigen presentation, phagocytosis, cell death, and immune-mediated tumor surveillance. Although current data is limited, such treatment shows promise as a safe and effective treatment paradigm. The optimal radiation dose, timing, sequencing, and immunotherapy drug(s) and treatment sequencing continue to evolve to best optimize outcomes and associated toxicities.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editors (Dr. Arya Amini and Dr. Tyler Robin) for the series “Synergy in Action: Novel Approaches to Combining Radiation Therapy and Immunotherapy” published in Translational Cancer Research. The article has undergone external peer review.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/tcr-20-3019). The series “Synergy in Action: Novel Approaches to Combining Radiation Therapy and Immunotherapy” was commissioned by the editorial office without any funding or sponsorship. Dr. Glaser reports that he is Co-chair of Elsevier clinical gynecological pathways. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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