Mathematical modelling of the synergistic combination of radiotherapy and indoleamine-2,3-dioxygenase (IDO) inhibitory immunotherapy against glioblastoma

Mathematical modeling of the synergistic interplay of radiotherapy and immunotherapy in anti-cancer treatments

Introduction

While radiotherapy has long been recognized for its ability to directly ablate cancer cells through necrosis or apoptosis, radiotherapy-induced abscopal effect suggests that its impact extends beyond local tumor destruction thanks to immune response. Cellular proliferation and necrosis have been extensively studied using mathematical models that simulate tumor growth, such as Gompertz law, and the radiation effects, such as the linear-quadratic model. However, the effectiveness of radiotherapy-induced immune responses may vary among patients due to individual differences in radiation sensitivity and other factors.

Methods

We present a novel macroscopic approach designed to quantitatively analyze the intricate dynamics governing the interactions among the immune system, radiotherapy, and tumor progression. Building upon previous research demonstrating the synergistic effects of radiotherapy and immunotherapy in cancer treatment, we provide a comprehensive mathematical framework for understanding the underlying mechanisms driving these interactions.

Results

Our method leverages macroscopic observations and mathematical modeling to capture the overarching dynamics of this interplay, offering valuable insights for optimizing cancer treatment strategies. One shows that Gompertz law can describe therapy effects with two effective parameters. This result permits quantitative data analyses, which give useful indications for the disease progression and clinical decisions.

Discussion

Through validation against diverse data sets from the literature, we demonstrate the reliability and versatility of our approach in predicting the time evolution of the disease and assessing the potential efficacy of radiotherapy-immunotherapy combinations. This further supports the promising potential of the abscopal effect, suggesting that in select cases, depending on tumor size, it may confer full efficacy to radiotherapy.

Comparable survival in rats with intracranial glioblastoma irradiated with single-fraction conventional radiotherapy or FLASH radiotherapy

Background

Radiotherapy increases survival in patients with glioblastoma. However, the prescribed dose is limited by unwanted side effects on normal tissue. Previous experimental studies have shown that FLASH radiotherapy (FLASH-RT) can reduce these side effects. Still, it is important to establish an equal anti-tumor efficacy comparing FLASH-RT to conventional radiotherapy (CONV-RT).

Methods

Fully immunocompetent Fischer 344 rats with the GFP-positive NS1 intracranial glioblastoma model were irradiated with CONV-RT or FLASH-RT in one fraction of 20 Gy, 25 Gy or 30 Gy. Animals were monitored for survival and acute dermal side effects. The brains were harvested upon euthanasia and tumors were examined post mortem.

Results

Survival was significantly increased in animals irradiated with CONV-RT and FLASH-RT at 20 Gy and 25 Gy compared to control animals. The longest survival was reached in animals irradiated with FLASH-RT and CONV-RT at 25 Gy. Irradiation at 30 Gy did not lead to increased survival, despite smaller tumors. Tumor size correlated inversely with irradiation dose, both in animals treated with CONV-RT and FLASH-RT. Acute dermal side effects were mild, but only a small proportion of the animals were alive for evaluation of those side effects.

Conclusion

The dose response was similar for CONV-RT and FLASH-RT in the present model. Tumor size upon the time of euthanasia correlated inversely with the irradiation dose.

Combined anti-C1-INH and radiotherapy against glioblastoma

BACKGROUND

A more effective immune response against glioblastoma is needed in order to achieve better tumor control. Radiotherapy can induce anti-tumor mediated immune reactions, in addition to its dose response effects. The complement system can function as a bridge between innate and adaptive immune responses. Combining radiotherapy and complement activating therapy is theoretically interesting.

METHODS

Radiotherapy at 8 Gy × 2 was combined with treatment against C1-inhibitor (C1-INH), a potent inhibitor of activation of the classical pathway of the complement system. Anti-C1-INH was delivered as intratumoral injections. Fully immunocompetent Fischer 344 rats with NS1 glioblastoma tumors were treated. Survival was monitored as primary outcome. Models with either intracranial or subcutaneous tumors were evaluated separately.

RESULTS

In the intracranial setting, irradiation could prolong survival, but there was no additional survival gain as a result of anti-C1-INH treatment. In animals with subcutaneous tumors, combined radio-immunotherapy with anti-C1-INH and irradiation at 8 Gy × 2 significantly prolonged survival compared to control animals, whereas irradiation or anti-C1-INH treatment as single therapies did not lead to significantly increased survival compared to control animals.

CONCLUSIONS

Anti-C1-INH treatment could improve the efficacy of irradiation delivered at sub-therapeutic doses and delay tumor growth in the subcutaneous tumor microenvironment. In the intracranial setting, the doses of anti-C1-INH were not enough to achieve any survival effect in the present setting.

Long-term anti-tumor effects following both conventional radiotherapy and FLASH in fully immunocompetent animals with glioblastoma

Radiotherapy can induce an immunological response. One limiting factor is side effects on normal tissue. Using FLASH radiotherapy, side effects could possibly be reduced. The efficacy of FLASH in relation to conventional radiotherapy (CONV-RT) has not been extensively explored in fully immunocompetent animals. Fully immunocompetent Fischer 344 rats were inoculated with NS1 glioblastoma cells subcutaneously or intracranially. Radiotherapy was delivered with FLASH or CONV-RT at 8 Gy × 2 (subcutaneous tumors) and 12.5 Gy × 2 (intracranial tumors). Cured animals were re-challenged in order to explore long-term anti-tumor immunity. Serum analytes and gene expression were explored. The majority of animals with subcutaneous tumors were cured when treated with FLASH or CONV-RT at 8 Gy × 2. Cured animals could reject tumor re-challenge. TIMP-1 in serum was reduced in animals treated with FLASH 8 Gy × 2 compared to control animals. Animals with intracranial tumors survived longer when treated with FLASH or CONV-RT at 12.5 Gy × 2, but cure was not reached. CONV-RT and FLASH were equally effective in fully immunocompetent animals with glioblastoma. Radiotherapy was highly efficient in the subcutaneous setting, leading to cure and long-term immunity in the majority of the animals.

Extrinsic factors associated with the response to immunotherapy in glioblastoma

Glioblastoma (GBM) is a heterogeneous and lethal brain tumor. Despite the success of immune checkpoint inhibitors against various malignancies, GBM remains largely refractory to treatment. The immune microenvironment of GBM is highly immunosuppressive, which poses a major hurdle for the success of immunotherapy. Obviously, except for the GBM cells itself, there are also extrinsic reasons for the lack of efficacy of immunotherapy. Accumulated evidence indicates that factors other than GBM cells determine the efficacy of immunotherapy. In this review, we first described the unique immune microenvironment of the brain, which must be considered when using immunotherapy in patients with GBM. Second, we also described the mechanisms by which different immune and non-immune cells in the GBM microenvironment affect the efficacy of immunotherapy. Furthermore, the impact of standard therapies on the response to immunotherapy was delineated. Finally, we briefly discussed strategies for resolving these problems and improving the efficacy of immunotherapy.

Modeling Radioimmune Response-Current Status and Perspectives

The combination of immune therapy with radiation offers an exciting and promising treatment modality in cancer therapy. It has been hypothesized that radiation induces damage signals within the tumor, making it more detectable for the immune system. In combination with inhibiting immune checkpoints an effective anti-tumor immune response may be established. This inversion from tumor immune evasion raises numerous questions to be solved to support an effective clinical implementation: These include the optimum immune drug and radiation dose time courses, the amount of damage and associated doses required to stimulate an immune response, and the impact of lymphocyte status and dynamics. Biophysical modeling can offer unique insights, providing quantitative information addressing these factors and highlighting mechanisms of action. In this work we review the existing modeling approaches of combined ‘radioimmune’ response, as well as associated fields of study. We propose modeling attempts that appear relevant for an effective and predictive model. We emphasize the importance of the time course of drug and dose delivery in view to the time course of the triggered biological processes. Special attention is also paid to the dose distribution to circulating blood lymphocytes and the effect this has on immune competence.

Increased effect of two-fraction radiotherapy in conjunction with IDO1 inhibition in experimental glioblastoma

Objectives

The aim of the study was to investigate therapeutic efficacy of single- or two-fraction radiotherapy in conjunction with IDO1-inhibition in a syngeneic rat glioblastoma model. IDO is known to cause immunosuppression through breakdown of tryptophan in the tumor microenvironment.

Methods

Gene expression analyses of IDO in glioblastoma were performed with data from publicly available datasets. Fractionation studies were done on animals to evaluate tumor size, immune cell infiltration of tumors and serum profile on day 18 after tumor inoculation. Survival analyses were done with animals carrying intracranial glioblastomas comparing two-fraction radiotherapy+IDO1-inhibition to controls. IDO inhibition was achieved by administration of 1-methyl tryptophan (1-MT), and radiotherapy (RT) was delivered in doses of 8Gy.

Results

The expression of IDO1 was increased on gene level in glioblastoma stem cells. Tumor size was significantly reduced in animals treated with 1-MT+RTx 2 (both long and short intervals, i.e. 7 and 4 days between the treatments) as compared to control animals, animals treated with only 1-MT or animals treated with 1-MT+RTx1. Serum levels of IL-1A were significantly altered in all treated animals as compared to control animals. Survival was significantly increased in the animals treated with 1-MT+RTx2 (7-day interval) compared to control animals.

Conclusions

Addition of two-fraction RT to IDO1 inhibition with 1-MT significantly reduced tumor size in animals with glioblastoma. Survival was significantly increased in animals treated with two-fractioned RT+1-MT as compared to untreated controls increased significantly.

Advances in knowledge

The currently used combination of only two fractions of radiotherapy and immune therapy is a promising area of research, increasing efficacy compared to single fraction irradiation, while potentially lowering radiation side effects compared to radiation in current clinical practice.

Assessing the interactions between radiotherapy and antitumour immunity

Immunotherapy, specifically the introduction of immune checkpoint inhibitors, has transformed the treatment of cancer, enabling long-term tumour control even in individuals with advanced-stage disease. Unfortunately, only a small subset of patients show a response to currently available immunotherapies. Despite a growing consensus that combining immune checkpoint inhibitors with radiotherapy can increase response rates, this approach might be limited by the development of persistent radiation-induced immunosuppression. The ultimate goal of combining immunotherapy with radiotherapy is to induce a shift from an ineffective, pre-existing immune response to a long-lasting, therapy-induced immune response at all sites of disease. To achieve this goal and enable the adaptation and monitoring of individualized treatment approaches, assessment of the dynamic changes in the immune system at the patient level is essential. In this Review, we summarize the available clinical data, including forthcoming methods to assess the immune response to radiotherapy at the patient level, ranging from serum biomarkers to imaging techniques that enable investigation of immune cell dynamics in patients. Furthermore, we discuss modelling approaches that have been developed to predict the interaction of immunotherapy with radiotherapy, and highlight how they could be combined with biomarkers of antitumour immunity to optimize radiotherapy regimens and maximize their synergy with immunotherapy.