Development of an Orthotopic Murine Model of Rectal Cancer in Conjunction With Targeted Short-Course Radiation Therapy

Establishing a standardized murine orthotopic intra-rectal model for the study of colorectal adenocarcinoma

Background

Orthotopic models offer a more accurate representation of colorectal cancer (CRC) compared to subcutaneous models. Despite promising results from the reported intra-rectal models, establishing a standardized method for CRC research remains challenging due to model variability, hindering comprehensive studies on CRC pathogenesis and treatment modalities, such as brachytherapy. This study aimed to establish a standardized workflow for an orthotopic intra-rectal animal model to induce the growth of colorectal adenocarcinoma in male and female mice.

Methods

HT-29 colorectal adenocarcinoma cells were injected into the rectal mucosa of female (n=21) and male (n=26) non-obese diabetic severe combined immunodeficiency (NOD SCID) gamma (NSG) mice. Mice were placed on a 45° wedge elevating their pelvis for better visualization of the anus. Tumor growth and localization were monitored using a 7-T magnetic resonance imaging (MRI) scanner with rapid acquisition with relaxation echo (RARE) sequence at weeks 1, 2, and 3 post-cell instillation. Once tumors reached 5-8 mm in diameter, the mice were euthanized. Histopathology and immunohistochemical analyses confirmed the tumors' morphology, including necrosis, vascularity (CD-31) and apoptosis (cleaved caspase-3).

Results

There was a 92% and 95% tumor growth success rate in male and female mice, respectively. Tumors grew to 5-8 mm in diameter within ~20 days. No significant difference in tumor size was observed between genders. Tumor morphology was consistent across cases. Most tumors exhibited a lack of central blood vessels, accompanied by varying degrees of necrosis and apoptosis, whereas external portions were highly vascularized.

Conclusions

An orthotopic intra-rectal model was successfully developed. This model will be used in future studies to evaluate the efficacy of CRC treatments.

An orthotopic metastatic xenograft model of colorectal cancer

Colorectal cancer (CRC) presents a substantial global health challenge, prompting the necessity for the development and validation of preclinical models to enhance our comprehension and therapeutic interventions. Among the myriad of murine models available for CRC evaluation, orthotopic implantation via intercaecal microinjection stands out as a preferred method for replicating the intricate tumor microenvironment while ensuring uniformity and standardized applicability. In this study, we delineate a methodology addressing the required steps for tumor cell line selection and reporter transduction, animal model preparation, orthotopic tumor implantation, in vivo monitoring of tumor growth and metastasis formation. We comprehensively describe the generation of a xenograft murine model based on the intercaecal implantation of human GFP+/luciferase+ SW620 CRC cells, facilitating the evaluation of responses to pre-clinical human-based therapeutic approaches. The implementation of these standardized protocols promises to augment the reliability and reproducibility of preclinical studies, ultimately advancing our comprehension of CRC pathogenesis and guiding the development of innovative therapeutic strategies.

Orthotopic and metastatic tumour models in preclinical cancer research

Mouse models of disease play a pivotal role at all stages of cancer drug development. Cell-line derived subcutaneous tumour models are predominant in early drug discovery, but there is growing recognition of the importance of the more complex orthotopic and metastatic tumour models for understanding both target biology in the correct tissue context, and the impact of the tumour microenvironment and the immune system in responses to treatment. The aim of this review is to highlight the value that orthotopic and metastatic models bring to the study of tumour biology and drug development while pointing out those models that are most likely to be encountered in the literature. Important developments in orthotopic models, such as the increasing use of early passage patient material (PDXs, organoids) and humanised mouse models are discussed, as these approaches have the potential to increase the predictive value of preclinical studies, and ultimately improve the success rate of anticancer drugs in clinical trials.

Shear wave elastography can stratify rectal cancer response to short-course radiation therapy

Rectal cancer is a deadly disease typically treated using neoadjuvant chemoradiotherapy followed by total mesorectal excision surgery. To reduce the occurrence of mesorectal excision surgery for patients whose tumors regress from the neoadjuvant therapy alone, conventional imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), is used to assess tumor response to neoadjuvant therapy before surgery. In this work, we hypothesize that shear wave elastography offers valuable insights into tumor response to short-course radiation therapy (SCRT)-information that could help distinguish radiation-responsive from radiation-non-responsive tumors and shed light on changes in the tumor microenvironment that may affect radiation response. To test this hypothesis, we performed elastographic imaging on murine rectal tumors (n = 32) on days 6, 10, 12, 16, 18, 20, 23, and 25 post-tumor cell injection. The study revealed that radiation-responsive and non-radiation-responsive tumors had different mechanical properties. Specifically, radiation-non-responsive tumors showed significantly higher shear wave speed SWS (p < 0.01) than radiation-responsive tumors 11 days after SCRT. Furthermore, there was a significant difference in shear wave attenuation (SWA) (p < 0.01) in radiation-non-responsive tumors 16 days after SCRT compared to SWA measured just one day after SCRT. These results demonstrate the potential of shear wave elastography to provide valuable insights into tumor response to SCRT and aid in exploring the underlying biology that drives tumors' responses to radiation.

New insights into the responder/nonresponder divide in rectal cancer: Damage-induced Type I IFNs dictate treatment efficacy and can be targeted to enhance radiotherapy

Rectal cancer ranks as the second leading cause of cancer-related deaths. Neoadjuvant therapy for rectal cancer patients often results in individuals that respond well to therapy and those that respond poorly, requiring life-altering excision surgery. It is inadequately understood what dictates this responder/nonresponder divide. Our major aim is to identify what factors in the tumor microenvironment drive a fraction of rectal cancer patients to respond to radiotherapy. We also sought to distinguish potential biomarkers that would indicate a positive response to therapy and design combinatorial therapeutics to enhance radiotherapy efficacy. To address this, we developed an orthotopic murine model of rectal cancer treated with short course radiotherapy that recapitulates the bimodal response observed in the clinic. We utilized a robust combination of transcriptomics and protein analysis to identify differences between responding and nonresponding tumors. Our mouse model recapitulates human disease in which a fraction of tumors respond to radiotherapy (responders) while the majority are nonresponsive. We determined that responding tumors had increased damage-induced cell death, and a unique immune-activation signature associated with tumor-associated macrophages, cancer-associated fibroblasts, and CD8+ T cells. This signature was dependent on radiation-induced increases of Type I Interferons (IFNs). We investigated a therapeutic approach targeting the cGAS/STING pathway and demonstrated improved response rate following radiotherapy. These results suggest that modulating the Type I IFN pathway has the potential to improve radiation therapy efficacy in RC.

Radiation Therapy Exacerbates Tumor-Promoting Innervation and Nerve Signaling in Rectal Cancer

PURPOSE

Many solid tumors present with perineural invasion (PNI), and innervation correlates with worsened prognosis. The effects that commonly administered therapies such as radiation therapy (RT) have on PNI status remain unknown. We investigated the contribution of RT on the nervous system and elucidated the implications that increased nerve signaling can have on tumor burden using our previously developed orthotopic murine model of rectal cancer (RC) and our targeted and clinically relevant short-course RT (SCRT) regimen.

METHODS

Medical charts for patients with RC treated at the Wilmot Cancer Institute were obtained and PNI status was analyzed. Human data were accompanied by an orthotopic murine model of RC. Briefly, luciferase-expressing murine colon-38 (MC38-luc) tumor cells were injected orthotopically into the rectal wall of C57BL6 mice. Targeted SCRT (5 gray (Gy) per fraction for 5 consecutive fractions) was administered to the tumor. Intratumoral innervation was determined by immunohistochemistry (IHC), local norepinephrine (NE) concentration was quantified by enzyme-linked immunosorbent assay (ELISA), and β2-adrenergic receptor (B2AR) expression was assessed by flow cytometry. Chronic NE signaling was mirrored by daily isoproterenol treatment, and the effect on tumor burden was determined by overall survival, presence of metastatic lesions, and tumor size. Isoproterenol signaling was inhibited by administration of propranolol.

RESULTS

Human RC patients with PNI have decreased overall survival compared with patients without PNI. In our mouse model, SCRT induced the expression of genes involved in neurogenesis, increased local NE secretion, and upregulated B2AR expression. Treating mice with isoproterenol resulted in decreased overall survival, increased rate of metastasis, and reduced SCRT efficacy. Interestingly, the isoproterenol-induced decrease in SCRT efficacy could be abrogated by blocking the BAR through the use of propranolol, suggesting a direct role of BAR stimulation on impairing SCRT responses.

CONCLUSIONS

Our results indicate that while SCRT is a valuable treatment, it is accompanied by adverse effects on the nervous system that may impede the efficacy of therapy and promote tumor burden. Therefore, we could speculate that therapies aimed at targeting this signaling cascade or impairing nerve growth in combination with SCRT may prove beneficial in future cancer treatment.