Beyond radiation therapy: photodynamic therapy maintains structural integrity of irradiated healthy and metastatically involved vertebrae in a pre-clinical in vivo model

Spinal metastasis commonly occurs in advanced breast cancer. Treatment is often multimodal including radiation therapy (RT), bisphosphonates (BPs), and surgery, yet alternative minimally invasive local treatments are needed. Photodynamic therapy (PDT) has been shown to ablate tumor cells and enhance bone formation secondary to metastatic breast cancer, demonstrating potential as a treatment for spinal metastasis. Combined with previous BP treatment, bone formation was further enhanced by PDT. This study aimed to determine the effects of PDT in combination with previous RT on healthy and metastatically involved vertebrae. Forty-six athymic rats underwent RT (4 Gy on day-7), twenty-three of them were inoculated with MT-1 human breast cancer cells on day 0. Thirteen healthy and ten metastatically involved rats underwent PDT treatment on day 14. All rats were sacrificed on day 21. L2 vertebrae were analyzed using mu CT imaging, mechanical testing, and histological methods. In healthy vertebrae, while modest increases in trabecular structure were found in RT + PDT compared to RT only, mechanical stability was negatively affected. The 4 Gy RT dose was found to ablate all tumor cells and prevent further vertebral metastasis. As such, in metastatically involved rats, no differences in stereological or mechanical properties were detected. RT + PDT and RT-only treatment resulted in greatly improved vertebral structural and mechanical properties versus untreated or PDT-only treatment in metastatically involved rats, due to early tumor destruction in RT-treated groups. Increased amounts of woven bone and osteoid volume were found in PDT-treated vertebrae. Further investigation is needed to understand if structural improvements seen in RT + PDT treatment can translate into longer-term improvements in strength to support the potential of PDT as a viable adjuvant treatment for spinal metastasis postradiation.