# TLR7 Agonist Imiquimod Improves the Therapeutic Antitumor Effect of High–Dose–Rate Brachytherapy

## TL;DR

Combining imiquimod with brachytherapy improves cancer treatment by enhancing the immune response and reducing tumor growth.

## Contribution

The study demonstrates a novel synergistic effect of combining a TLR7 agonist with high-dose-rate brachytherapy in cancer treatment.

## Key findings

- Combining IMQ with HDR brachytherapy significantly inhibited melanoma tumor growth in mice.
- The treatment led to prolonged survival and complete tumor regression in 20% of breast cancer mice.
- CD8+ T cell infiltration was enhanced in tumors, and their depletion negated the therapy's effect.

## Abstract

Brachytherapy (interventional radiotherapy) destroys cancer cells and also affects the tumor microenvironment (TME), including tumor blood vessels and immune cells. Radio-resistance mechanisms, such as increasing immunosuppression and hypoxia, lead to tumor recurrence after radiotherapy. Our study aimed to determine whether adding imiquimod (IMQ) to anticancer therapy would overcome mechanisms of resistance to radiotherapy. IMQ (toll–like receptor 7 (TLR7) agonist) acts as an immunostimulant and a vascular normalizing agent. Our preclinical in vivo study showed that combining a dose of IMQ that normalizes tumor vascularization with high–dose–rate (HDR) BT that acts as an “in situ” vaccination induces a synergistic effect in long–term inhibition of tumor growth. In summary, our research results demonstrate that the changes in the tumor microenvironment by IMQ enhance the effectiveness of radiotherapy and lead to long–term tumor control. Our data indicate the direction for further development of combination therapies in the treatment of cancer patients.

Background/Objectives: Brachytherapy (BT) is a local radiation treatment method for solid tumors. A single 10 Gy high–dose–rate (HDR) BT acts as an “in situ” vaccination. Tumor microenvironment (TME)–dependent radio–resistance mechanisms, such as increasing immunosuppression and hypoxia, lead to tumor recurrence after radiotherapy. Our study aimed to determine whether adding imiquimod (IMQ) to anticancer therapy would overcome TME–mediated mechanisms of radiotherapy resistance. IMQ, a toll–like receptor 7 (TLR7) agonist, acts as an immunostimulant and a vascular normalizing agent. Methods: Mice with well–developed tumors were treated with IMQ at a vascular–normalized dose of 50 μg, followed 5 days later by a single 10 Gy HDR BT. The dose coverage was planned using Discovery RT computed tomography CT scans. Irradiation was performed with a high–dose–rate afterloader equipped with an iridium–192 radioactive source. Results: In mice treated with a combination of IMQ and BT, we observed significant inhibition of melanoma tumor growth. We also noticed an effective therapeutic effect in mice with breast cancer, resulting in significantly prolonged survival and complete tumor regression in 20% of treated mice. In the blood of treated mice, we observed leukopenia with eosinophilia. In tumors, there was enhanced infiltration by cytotoxic CD8+ T lymphocytes. The depletion of CD8+ T cells completely abolished the effect of the combined therapy. Conclusions: The combination of IMQ with HDR brachytherapy induces a synergistic effect, improving the therapeutic antitumor effect of brachytherapy. Our data indicate that it is reasonable to use drugs that prevent changes in the TME in combination with radiotherapy.