Influence of intestinal microbial metabolites on the abscopal effect after radiation therapy combined with immune checkpoint inhibitors

Background

Most clinical studies failed to elicit a strong antitumor immune response and subsequent systemic tumor regression after radiation therapy (RT), even in combination with the immune checkpoint inhibitors (ICI) anti-CTLA4 or anti-PD1. Mechanistically, type I interferon (IFN-I) activation is essential for the development of such abscopal effects (AE); however, mechanisms driving or limiting IFN-I activation are ill defined. Groundbreaking discoveries have shown that antibiotics (ABx) can affect oncological outcomes and that microbiota-derived metabolites can modulate systemic antitumor immunity. Recent studies have demonstrated that the bacterial metabolites desaminotyrosine (DAT) and indole-3-carboxaldehyde (ICA) can enhance IFN-I activation in models of inflammatory diseases.

Materials and Methods

The subcutaneous bilateral MC38 tumor model is a widely used experimental tool to study the AE in mice. We applied it to explore the influence of broad-spectrum ABx, DAT and ICA on the AE after radioimmunotherapy (RIT). We performed 1x8 Gy of the primary tumor ± anti-CTLA4 or anti-PD1, and ± daily oral application of ABx or metabolites.

Result

Combinatory ABx had neither a significant effect on tumor growth of the irradiated tumor nor on tumor progression of the abscopal tumor after RIT with anti-CTLA4. Furthermore, DAT and ICA did not significantly impact on the AE after RIT with anti-CTLA4 or anti-PD1. Surprisingly, ICA even appears to reduce outcomes after RIT with anti-CTLA4.

Conclusion

We did not find a significant impact of combinatory ABx on the AE. Experimental application of the IFN-I-inducing metabolites DAT or ICA did not boost the AE after combined RIT. Additional studies are important to further investigate whether the intestinal microbiota or specific microbiota-derived metabolites modulate the AE.

Novel Tumor Organoid-Based Mouse Model to Study Image Guided Radiation Therapy of Rectal Cancer After Noninvasive and Precise Endoscopic Implantation

PURPOSE

Preoperative (neoadjuvant) radiation therapy (RT) is an essential part of multimodal rectal cancer therapy. Recently, total neoadjuvant therapy (TNT), which combines simultaneous radiochemotherapy with additional courses of chemotherapy, has emerged as an effective approach. TNT achieves a pathologic complete remission in approximately 30% of resected patients, opening avenues for treatment strategies that avoid radical organ resection. Furthermore, recent studies have demonstrated that anti-programmed cell death protein 1 immunotherapy can induce clinical complete responses in patients with specific genetic alterations. There is significant potential to enhance outcomes through intensifying, personalizing, and de-escalating treatment approaches. However, the heterogeneous response rates to RT or TNT and strategies to sensitize patients without specific genetic changes to immunotherapy remain poorly understood.

METHODS AND MATERIALS

We developed a novel orthotopic mouse model of rectal cancer based on precisely defined endoscopic injections of tumor organoids that reflect tumor heterogeneity. Subsequently, we employed endoscopic- and computed tomography-guided RT and validated rectal tumor growth and response rates to therapy using small-animal magnetic resonance imaging and endoscopic follow-up.

RESULTS

Rectal tumor formation was successfully induced in all mice after 2 organoid injections. Clinically relevant RT regimens with 5 × 5 Gy significantly delayed clinical signs of tumor progression and significantly improved survival. Consistent with human disease, rectal tumor progression correlated with the development of liver and lung metastases. Notably, long-term survivors after RT showed no evidence of tumor recurrence, as demonstrated by in vivo radiologic tumor staging and histopathologic examination.

CONCLUSIONS

Our novel mouse model combines orthotopic tumor growth via noninvasive and precise rectal organoid injection and small-animal RT. This model holds significant promise for investigating the effect of tumor cell-intrinsic aspects, genetic alterations of the host, and exogenous factors (eg, nutrition or microbiota) on RT outcomes. Furthermore, it allows for the exploration of combination therapies involving chemotherapy, immunotherapy, or novel targeted therapies.

High Salt Diet Exacerbates Intestinal Radiotoxicity by Promoting Intestinal Epithelial Barrier Dysfunction after RT

PURPOSE/OBJECTIVE(S)

Intestinal side effects have a substantial impact on patients' quality of life and constrain radiation therapy (RT) of abdominal and pelvic tumors. We aim at gaining a better understanding of the pathogenesis of intestinal radiotoxicity and identifying contributing factors. A high salt diet is common among countries of the Global North. The effects of an increased sodium-chloride (NaCl) intake on the development of intestinal side effects have not been investigated.

MATERIALS/METHODS

C57BL/6 wild-type (WT) mice were fed a high salt diet (HSD) or normal salt diet (NSD) and irradiated with 12 Gy total body irradiation (TBI) or 13 Gy abdominal irradiation (ABI). Following TBI the mice received a syngeneic bone marrow transplant to reconstitute hematopoiesis. Readouts after RT included weight monitoring, histopathological analysis and assessment of intestinal epithelial barrier function. To corroborate our findings in vitro, murine organoids of the small intestine were cultivated, treated with varying NaCl concentrations (110-200 mM) and irradiated with 2 or 4 Gy. Organoid survival and growth were determined using multiple methods. Additionally, colony formation assays (CFA) of murine MC38 colon carcinoma cells were performed after stimulation with NaCl. Mechanistically, we analyzed the apoptosis rates of MC38 cells and intestinal epithelial organoids.

RESULTS

Irradiated HSD animals showed increased acute weight loss, reduced weight regeneration, a more severe dysfunction of the intestinal barrier (enhanced FITC-dextran permeability) and increased histopathological damage when compared with NSD animals. Murine intestinal organoids and MC38 cells showed decreased survival after RT in hypernatremic conditions. Interestingly, our preliminary data show that there are no increased rates of apoptotic cells in vitro, suggesting a different mechanism affecting the increased radiosensitivity in hypernatremic conditions.

CONCLUSION

Our data show a NaCl induced intensification of intestinal tissue injury following RT. These findings can potentially pave the way for investigating the effect of high salt diet on intestinal radiotoxicity in clinical trials.

Influence of Bacterial Metabolites on the Systemic Antitumor Immune Response after Radiation Therapy in Combination with Immune Checkpoint Inhibitors

PURPOSE/OBJECTIVE(S)

The gut microbiome influences the pathogenesis of many diseases and numerous medical therapies. In addition, it is well known that cancer patients are often treated with antibiotics during the course of their disease, which can lead to intestinal dysbiosis. Importantly, experimental and clinical studies have already shown that this has implications for tumor therapies and that immune-active bacterial metabolites can play a central mechanistic role. However, little is known about the influence of the bacterial microbiota or its metabolites on the outcome of radiotherapy (RT) in combination with immune checkpoint inhibitors (ICIs).

MATERIALS/METHODS

We used a bilateral tumor model (MC38 tumor cells) to investigate the influence of the intestinal microbiota and selected bacterial metabolites on the abscopal effect after RT in combination with ICIs. In brief, RT (1 × 8 Gy) of the right tumor was performed 7 days after tumor induction, followed by application of α-CTLA4 or α-PD1 for three weeks. Antibiotics (ampicillin, neomycin, metronidazole, and vancomycin) or selected immune-activating metabolites were administered daily by gavage. We used bacterial metabolites that are described in the literature as having strong immune-modulating properties.

RESULTS

In mice, combined treatment with different antibiotics had neither a significant effect on tumor growth of the irradiated tumor nor on tumor progression of the unirradiated tumor after RT in combination with α-CTLA4. In contrast, supplementation of specific bacterial metabolites had different effects depending on the type of ICI and the specific type of bacterial metabolite. Specifically, we identified bacterial metabolites that led to enhanced tumor progression of the unirradiated abscopal tumor after RT combined with ICIs, resulting in reduced survival rates.

CONCLUSION

To the best of our knowledge, we are presenting here for the first-time experimental data on the influence of the intestinal microbiome on the abscopal effect after RT combined with ICIs. Our data clearly show that specific immune-active bacterial metabolites can negatively influence the abscopal effect after combined radioimmunotherapy. This implies the design of further experimental studies aimed at using specific strategies (e.g., antibiotics) to decimate certain gut bacteria and reduce the amount of certain bacterial metabolites to improve response rates after radioimmunotherapy.

Experimental Investigation of Lung Toxicity after Radiation Therapy Combined with Immune Checkpoint Inhibitors

PURPOSE/OBJECTIVE(S)

The combination of radiation therapy (RT) with immune checkpoint inhibitors (ICIs) has resulted in prolonged survival in patients with locally advanced lung cancer (NSCLC). However, RT and ICIs carry the risk of inflammatory and irreversible lung damage and might have synergistic effects on these adverse events. Importantly, only little is known about the risk of enhanced side effects of the lung and which factors modulate such toxicity. In addition, there is a need for sensitive diagnostics for the early detection of lung injury after combined radioimmunotherapy. Recently, experimental x-ray dark-field radiography was able to detect radiation-induced lung injury earlier than conventional radiography in mice.

MATERIALS/METHODS

The right thorax of mice was irradiated with unfractionated RT (1x15 Gy or 1x30 Gy) or fractionated RT (5x9 Gy). In addition, indicated experimental groups received ICIs (a-PD1, a-CTLA4 or combination) for one month. After four months, the mice were analyzed using different methods to quantify lung damage: lung coefficient as surrogate marker for lung fibrosis, histopathological staining of fibrosis, conventional x-ray, and dark-field radiography.

RESULTS

We found enhanced signs of lung fibrosis (lung coefficient and histopathological score) after unfractionated RT in combination with ICIs. Surprisingly, combination of ICIs with RT resulted in opposite effects. Specifically, concomitant combination of ICIs with fractionated RT resulted in reduced lung coefficients and lower histopathological signs of lung fibrosis in mice treated in this way. Importantly, in vivo x-ray dark-field radiographs showed the same trend as the ex vivo assessment of the lungs.

CONCLUSION

To the best of our knowledge, this is the first experimental study to find that the combination of RT with ICIs has an impact on the risk of pulmonary fibrosis. Strikingly, ICIs (a-PD1 + a-CTLA4) combined with unfractionated RT resulted in increased lung damage, while concomitant use of ICIs (a-PD1 + a-CTLA4) with fractionated RT resulted in reduced lung toxicity. In the field of radioimmunotherapy, such studies are of great value for the interpretation of clinical studies and of importance for the design of further clinical trials.

Experimental investigation of skin toxicity after immune checkpoint inhibition in combination with radiation therapy

Immune checkpoint inhibitors (ICIs) have revolutionized cancer therapy. However, structured knowledge to mitigate a patient's specific risk of developing adverse events are limited. Nevertheless, there is an exponential growth of clinical studies combining conventional therapies such as radiation therapy (RT) with ICIs. Cutaneous reactions are amongst the most common adverse events after monotherapy with either ICIs or RT. So far, little is known about inter-individual differences in the risk of developing severe tissue toxicity after the combination of RT with ICIs, and the underlying biological mechanisms are ill defined. We used experimental models of RT-induced skin injury to analyze skin toxicity after simultaneous application of ICIs. We compared different RT regimens such as fractionated or stereotactic RT with varying dose intensity. Strikingly, we found that simultaneous application of RT and ICIs did not significantly aggravate acute skin injury in two different mouse strains. Detailed examination of long-term tissue damage of the skin revealed similar signs of epidermal hyperplasia, dermal fibrosis, and adnexal atrophy. In summary, we here present the first experimental study demonstrating excellent safety profiles of concurrent treatment with RT and ICIs. These findings will help to interpret the development of adverse events of the skin after radioimmunotherapy and guide the design of new clinical trials and clinical decision making in individual cases. This article is protected by copyright. All rights reserved.