The role of systemic therapy in the treatment of intracranial metastases has traditionally been limited by the blood-brain barrier, and radiation therapy-either with whole-brain treatment or stereotactic radiosurgery-has remained a primary treatment modality. Recent evidence has demonstrated that antigens released in the brain can inform the systemic immune system, and systemic antibodies can traverse into the brain. This has led to a renewed interest in investigating novel immunotherapy agents to treat both systemic and intracranial disease. Currently, several trials of immunotherapy, with or without sequential or concurrent radiation, have been performed in patients with brain metastases to evaluate the safety and efficacy of combined treatment. Combined use of stereotactic radiosurgery and checkpoint inhibitors appears safe and effective in the treatment of various brain metastases. Future studies will evaluate the optimal sequencing of radiosurgery and immunotherapy and assess the radiation doses and fractionations that will provide the best tumor response.

Introduction

Agents that can improve the host immune response against tumors are a significant addition to the current therapeutic armamentarium against cancer. Several phase III trials have demonstrated improved survival with the use of immunotherapeutic agents compared with conventional chemotherapy in the setting of advanced malignancy.[1-3] Since the publication of these trials, anti–cytotoxic T-lymphocyte–associated antigen 4 (CTLA-4) and anti–programmed death 1 (PD-1)/programmed death ligand 1 (PD-L1) antibodies have become a new standard of care in several tumor types; however, the optimal combination and sequencing of these systemic agents with radiation has not been elucidated for any disease site.

In a simplistic model, though one that is useful and widely accepted, radiation is a locoregional therapy and is given to control tumor within the irradiated volume. Radiobiological models developed at the end of the last millennium that implicated DNA damage and its incomplete repair as the primary mechanisms of achieving tumor control led clinicians to not expect any effect on tumors outside of the treatment field; however, animal studies performed early in this millennium demonstrated that the simple addition of a dendritic cell growth factor (fms-like tyrosine kinase 3 ligand) could reduce distant metastases and improve disease-free survival compared with radiation alone.[4] This abscopal effect-the ability of radiation to reduce the tumor burden outside of the treatment field-was described decades ago, but the implication of the immune system in mediating this effect has only recently been described.[4]

While our understanding of the immune system’s role in the response to ionizing radiation continues to evolve, novel opportunities to study how to combine immunotherapy with radiation-induced cell killing are revolutionizing cancer treatment. Accumulating preclinical and clinical data have shown that the combination of radiation with immunotherapy can stimulate the immune response and improve locoregional and distant control, thus improving overall survival. Radiation appears to stimulate the immune system through multiple mechanisms, including increasing the availability of tumor-associated antigens (TAAs), improving antigen presentation and subsequent stimulation of effector T cells, and enhancing infiltration of dendritic cells and T cells into the tumor microenvironment.[5-7]

Tumors in the central nervous system (CNS), behind the blood-brain barrier (BBB), have long been believed to be beyond the reach of the immune system, because the BBB limits trafficking of antigens and immune cells to the CNS. Recent evidence shows intracranial tumor TAAs interact with the peripheral immune system in the cervical lymph nodes. This provides a physiological foundation for testing immunotherapy for intracranial disease. Additionally, preclinical models of glioma using systemically administered anti–CTLA-4 antibodies that improved the ratio of effector cells to regulatory T cells (Tregs) in the CNS resulted in improved survival, demonstrating the role of the immune system in combating intracranial tumors.[8] The limited evidence for immunotherapy to date in the treatment of brain metastases stems from the deliberate exclusion of patients with active brain metastases from many large randomized trials assessing drug efficacy. However, comparable efficacy of immunotherapy agents in the brain and at extracerebral sites has been recently reported.[9] Combining immunotherapeutic agents with stereotactic radiosurgery appears to enhance both local and distant control, and result in better survival. In this review, we highlight preclinical and clinical data to support the rationale for combination of stereotactic radiosurgery with immunotherapy for the treatment of brain metastases; describe some areas of controversy, especially with regard to radiation fractionation and the timing of combination therapy; and discuss ongoing research into multimodality treatment of CNS tumors.

Role of Stereotactic Radiosurgery in Brain Metastases

Historically, patients with multiple brain metastases were treated with whole-brain radiation therapy (WBRT); however, there has been an increased interest in and acceptance of treating a limited number of brain metastases with stereotactic radiosurgery, since WBRT does not favorably affect survival and negatively affects neurocognitive performance relative to stereotactic radiosurgery. When managed with stereotactic radiosurgery alone, the rate of local control of irradiated intracranial metastases is at least 70% at 1 year, with higher rates of local control for smaller metastases; however, 30% to 50% of patients will develop new distant brain metastases in that same time period.[10] Many patients with multiple metastases now undergo several courses of stereotactic radiosurgery before resorting to WBRT, in order to defer or avoid WBRT-related sequelae.

Lowering the need for retreatment of brain metastases will both decrease costs and reduce the risk of radiation-associated neurocognitive toxicities.

Immunologic Effects of Ionizing Radiation

Established metastases have escaped detection and removal by the immune system through multiple mechanisms, including production of immunosuppressive cytokines, downregulation of surface tumor antigens and major histocompatibility complex (MHC) class I expression, and recruitment of Tregs. While Tregs normally only comprise approximately 4% of CD4+ T cells, their proportion can rise to 20% to 30% of the total CD4+ population in the tumor microenvironment.[11] Impaired host dendritic cell function also contributes to poor immunologic responses to tumor cells, even after exposure to radiation.[12]

A complex interplay governs the interaction of immune cells infiltrating the CNS, metastatic cells within the CNS, and the normal brain parenchyma.[13] Radiation can increase cross-presentation of TAAs by dendritic cells to CD4+ and CD8+ T cells, thereby enhancing the ability of the immune system to target tumor cells.[5] In addition to increasing the availability of TAAs, radiation fosters maturation of antigen-presenting cells and enhances the formation of antigen/MHC complexes.[6,7] Radiation also induces the release of key inflammatory cytokines (including tumor necrosis factor alpha, interferon gamma, and CXCL16) that recruit T cells and other immune cells to cross the BBB to infiltrate the tumor.[14,15]

The immune response also plays an integral role in the local effectiveness of radiation treatment, even at ablative doses. Single-fraction doses of 15 to 25 Gy have been shown in a murine model to generate a CD8+ T-cell–dependent immune response, leading to regression of the treated tumor. Depletion of the CD8+ T cells led to local tumor persistence, increased distant metastases, and decreased survival.[14] Using a murine model, Sharabi et al tested the combination of extracranial radiosurgery with an anti–PD-1 agent, and noticed an increase in the ratio of antigen-specific effector T cells to Tregs, as well as an increase in T-cell tumor infiltration, compared with single-modality treatment.[7]

CNS-Specific Immune Responses

The brain was long considered an “immunologically privileged” site, because of both the protective BBB and the lack of obvious lymphatic drainage, but recent evidence from studies of gliomas and brain metastases has challenged this dogma. Intracerebrally injected radiolabeled antigens migrate through the subarachnoid space and home to the cervical and retropharyngeal lymph nodes.[16] Dendritic cells responsible for antigen presentation to T cells have been found in murine choroid plexus and meninges.[17,18] The integrity of the BBB also appears compromised when cancer develops in the brain, and is further altered with irradiation, allowing for increased permeability and access to lymphocytes.[19]

Just as in extracranial models, radiation upregulates MHC class I expression on glioma cells and increases CD4+ and CD8+ infiltration of tumors.[20] Preclinical evidence has also demonstrated the ability of antibodies to traverse the BBB: in mice with primary CNS tumors, anti–CTLA-4 antibodies administered systemically increased effector T cells, decreased Tregs, and improved survival.[8] In a murine model of glioblastoma multiforme, animals were treated with stereotactic radiosurgery (10 Gy × 1), an anti–PD-1 antibody, a combination of this antibody and stereotactic radiosurgery, or observation only. Combined-modality therapy improved survival compared with both of the single-modality treatments (53 days vs 27 and 28 days). An increased infiltrate of cytotoxic T cells and decreased number of Tregs in brain tissue were observed after combination therapy, compared with the other arms. Long-term survivors in the combined-modality therapy group (mice surviving more than 90 days from initial implantation) rejected a rechallenge of glioma cells, suggesting retained immunity.[21]

The rationale for combining immunotherapy and stereotactic radiosurgery for brain metastases is similar to that for using the combination to treat extracranial metastases. The systemic effect of immunotherapy is enhanced by its combination with stereotactic radiosurgery, with improved local tumor response and long-term control, delayed growth and/or reduction in the size of other unirradiated brain lesions, and prevention of new metastases in the brain and/or elsewhere in the body, via an abscopal effect.

Clinical Evidence for Combining Immunotherapy With Stereotactic Radiosurgery

Patients with melanoma frequently develop brain metastases. Prior to the introduction of targeted therapies, median survival after a diagnosis of melanoma brain metastases ranged from 4 to 5 months, with post-irradiation local control rates lower than those reported for other cancer histologies. Although melanoma has traditionally been considered resistant to chemotherapy and radiation, several immunotherapy agents have yielded improved survival compared with chemotherapy in the metastatic setting; however, patients with brain metastases were often excluded from these trials. Several single-institution studies have reported on the clinical efficacy and safety of combining anti–CTLA-4 antibodies or anti–PD-1/anti–PD-L1 antibodies with stereotactic radiosurgery, and these will be reviewed in Part 2 of this series.

Optimal Dose and Fractionation

Radiation therapy has been deemed immunosuppressive, perhaps because of the common historical use of conventionally fractionated large radiotherapy portals to deliver partial-brain radiation therapy or WBRT, and simple beam arrangements that included targeting of regional lymphatics and/or irradiation of large numbers of circulating lymphocytes. The use of smaller treatment fields will minimize incidental irradiation of immune cells and reduce previously seen immunosuppressive effects.[22] Additionally, the higher doses of radiation per fraction, reduced number of fractions, and increased radiation dose rate characteristic of stereotactic radiosurgery all may prove beneficial.

In stereotactic radiosurgery, the optimal dose and fractionation of radiation is still under investigation, and the clinical setting of stereotactic radiosurgery applied in spontaneous brain metastasis cannot be mimicked preclinically.[23,24] If the radiation doses are too high in stereotactic radiosurgery, then radiosensitive lymphocytes may be destroyed, hampering the immune response; however, low doses may not activate the immune response. Further, the clinical feasibility of the dose needs to be considered, since low doses may not be effective for tumor cytotoxicity, and high doses (large radiation treatment volumes) may produce unacceptably high toxicities. Recent technological developments permit either single-fraction radiosurgery or the delivery of high focal doses in a fractionated fashion over several days.

Common preclinical experimental models use extracranially implanted tumors in the flank or mammary fat pad, which fail to recapitulate the complex tumor interactions that occur in the CNS. However, useful information can still be gained with preclinical models. In a murine melanoma model of radiation alone to implanted flank tumors, various doses up to 15 Gy were delivered, and it was shown that single-fraction doses between 7.5 and 15 Gy offered the best local tumor control, while doses of 5 Gy had minimal effect on immunostimulation and slowing tumor growth. When evaluating tumor and T-cell responses, a single dose of 7.5 Gy offered an optimal balance of tumor control while maintaining low levels of Tregs. This study also evaluated the effects of fractionating the 15-Gy dose into 2, 3, or 5 fractions, and found that tumor control was maximized and Tregs were minimized when 2 fractions were used. Additionally, both the two- and three-fraction regimens were superior to a single fraction in generating an immune response.[25]

A study of radiation therapy and/or a checkpoint inhibitor-in this case, an anti–CTLA-4 antibody-showed a different result with fractionation.[26] In this study, mice were injected with mammary carcinoma cells at a primary site (for irradiation) and a secondary site outside the radiation field. Mice treated with the anti–CTLA-4 antibody alone had no reduction in tumor volume, and those treated with radiation alone experienced tumor shrinkage in the primary site only. When the antibody was combined with fractionated radiation (8 Gy × 3 or 6 Gy × 5), an improvement in tumor response was seen at both the primary and secondary sites, with a greater improvement at both sites observed in the 8 Gy × 3 cohort. While improved local control at the primary site was also seen with the single-dose (20 Gy × 1) approach, an abscopal effect was not observed. Delayed administration of the anti–CTLA-4 antibody was also associated with reduced therapeutic efficacy.

Optimal timing of immunotherapy before, during, or after radiation remains a topic of investigation. In theory, immunotherapy delivered prior to stereotactic radiosurgery allows antigen-presenting cells and effector cells to be in circulation when tumor cells are killed by stereotactic radiosurgery. Unfortunately, this sequencing could decrease the antitumor response if circulating lymphocytes have been recruited, only to be destroyed by the subsequent radiation. Radiation use before immunotherapy could both increase circulating TAAs and decrease the integrity of the BBB to improve drug penetration and immune cell infiltration.[27] In practice, optimal sequencing will not only be dependent upon radiation delivery parameters and the mechanism of the immunotherapy used, but also will likely depend on tumor factors such as histology and overall mutational landscape.[28]

KEY POINTS Radiation therapy can enhance the immune response to tumors by increasing tumor antigen availability, improving antigen-presenting efficiency, and creating a microenvironment that enhances T-cell maturation and infiltration.

Radiation for brain tumors increases T-cell infiltration across the blood-brain barrier into this relatively immunologically privileged site.

The optimal dose and fractionation of radiation for tumor control, enhanced immunogenicity, and maintenance of an acceptable level of toxicity are currently under investigation and likely depend on both the systemic therapy and characteristics of the underlying malignancy itself.

It is generally accepted that treatment with checkpoint inhibitors should be started before or during radiation therapy. In preclinical models of colorectal cancer treated with 2 Gy × 5 fractions and a PD-L1 inhibitor, concurrent drug administration (delivered on day 1 or day 5) was most effective. Mice that received the inhibitor 7 days after completion of radiation had no improvement in survival compared with mice treated with radiation alone. The difference in efficacy was attributed to the levels of PD-1 expression after the delivery of radiation: 24 hours after radiation, expression was increased in both CD4+ and CD8+ cells; however, at 7 days after radiation, PD-1 levels on CD4+ cells had returned to baseline and were decreased on CD8+ cells.[29]

Optimal temporal sequencing of radiation seems to also depend on the immune therapy used. Scheduling of anti–CTLA-4 and anti-OX40 (a receptor found on T cells after antigen exposure) antibodies was studied in a series of experiments. The drug was given either 7 days before, 1 day after, or 5 days after a single radiation dose of 20 Gy.[30] When the CTLA-4 antibody was delivered prior to radiation, all tumors were cleared and a median survival time was not reached. With post-irradiation CTLA-4 blockade, only 50% of tumors were cleared and the median survival was 92 days for those receiving the antibody 1 day after radiation and 53 days for mice with a 5-day delay between radiation and treatment with the antibody. Regardless of timing, survival in mice was higher when anti–CTLA-4 therapy was combined with radiation. In contrast, pretreatment and delayed treatment (5-day delay) of anti-OX40 with radiation did not improve outcomes compared with radiation alone: only mice that received anti-OX40 1 day after completion of radiation had improved tumor clearance and a doubling of median survival time. This finding was attributed to the transient upregulation of OX40 seen 12 to 24 hours after antigen exposure.

Similar results were seen with an anti–CTLA-4 antibody, with drug administration 2 days before or on the same day of radiation completion increasing the treatment efficacy, compared with a 2-day delay in administration.[26] Clinically, administration of the CTLA-4 inhibitor ipilimumab during or after stereotactic radiosurgery improved survival compared with its administration pretreatment; this was thought to be due to the radiation priming the immune response with antigen release prior to ipilimumab administration.[31] Clinical data on outcomes for patients with melanoma brain metastases treated with stereotactic radiosurgery and CTLA-4 or PD-1 inhibitors showed that there was an advantage to having the local and systemic modalities delivered within 4 weeks of each other.[32]

Conclusion

There is increasing evidence that the brain is not completely shielded from the systemic immune system, and this has renewed interest in the use of immunotherapy agents in conjunction with radiation to create an additive, and perhaps synergistic, immune response. In addition to radiation enhancing the immune response, T-cell–mediated immune responses are also essential for local and distant control of tumors after radiation therapy. The optimal sequencing of immunotherapy with stereotactic radiosurgery-either before, concurrent with, or after-has not yet been established and likely will depend on the systemic agent used. This clinical issue, the current evidence for combining stereotactic radiosurgery with immunotherapy in the setting of brain metastases, and other areas of ongoing and future research will be discussed in Part 2 of this series.

Financial Disclosure: Dr. Formenti receives grant/research support from Bristol Myers-Squibb, Eisai, Janssen, Merck, Regeneron, and Varian; and honoraria from AstraZeneca, Bristol Myers-Squibb, Dynavax, Eisai, Elekta, GlaxoSmithKline, Merck, Regeneron, and Varian. Dr. Zhang and Dr. Knisely have no significant financial interest in or other relationship with the manufacturer of any product or provider of any service mentioned in this article.

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