A clinically relevant mouse model of canine osteosarcoma with spontaneous metastasis.

Background/Aim: Many patients with osteosarcoma (OS) will succumb to distant metastasis, often involving the lungs. Effective therapies for treating lung metastases depend on the availability of a clinically relevant pre-clinical model. Materials and Methods: Mice were surgically implanted with OS tumor fragments. The time course of primary tumor growth and subsequent spread to the lung were determined. Results: Following development of a lytic and proliferative primary bone lesion, tumor metastasized to the lung in the majority of mice. There was no evidence of tumor at three weeks, but 10 out of 11 mice ultimately developed secondary OS in the lung within 12 weeks. Conclusion: Implantation of OS tumor fragments leads to the development of primary bone tumors and secondary lung metastases, recapitulating the clinical behavior of OS. This model offers an advantage over cell suspension injection models by precluding initial seeding of the lung with tumor cells. Osteosarcoma (OS) is the most common primary malignancy of bone in humans and the second most frequent cause of cancer-related death in children (1, 5). OS is also the most common primary bone tumor in dogs, most often affecting middle-aged to older large breeds. The incidence in the canine population is 10-times that of humans, with approximately 10,000 new cases per year in the United States. Furthermore, there are many similarities between human and canine OS including affected sites and gender predilection, association with increased height, and propensity for lung metastasis (8, 13, 14). Most cases of OS involve the metaphysis of long bones, with the distal femur, proximal tibia and proximal humerus being the most common sites in both species (3). The vast majority of OS are histologically high-grade and have a strong propensity to metastasize. Despite improvements in the management of the primary bone disease associated with OS (2), many human and veterinary patients will eventually succumb to distant metastasis, most often involving the lungs. At the time of diagnosis, fewer than 20% of patients have clinically detectable metastases; however, over 80% have micrometastases at the time of diagnosis as shown by distant recurrence of the tumor following resection of the primary mass (6). In order to reduce the morbidity and mortality associated with OS, it is critical to address the issue of lung metastasis. Metastasis is a complex process with numerous steps involved (15). Although a number of canine, human and murine OS cell lines have been characterized in vitro and in vivo (4, 8), a widely accepted orthotopic mouse model of OS with reliable metastasis has not yet been established. One commonly used technique to study OS involves injection of an OS cell suspension into the femur or tibia of the mouse. In pilot studies using the Abrams canine OS cell line, we identified a number of animals that died acutely following intratibial injection of tumor cells. At necropsy, these mice had microscopic evidence of tumor micro-emboli within the pulmonary vasculature. In addition, acute removal of the primary bone lesion, by amputation, did not result in a reduction in the incidence or severity of lung pathology, nor in a change in overall survival. These results indicate that this model did not replicate the complete metastatic cascade in all cases, and so we set out to develop a more clinically relevant model of OS. We selected to continue to use the Abrams cell line for this work because of its aggressive biological behavior and clinical relevance in terms of tissue tropism (8, 11). To avoid potential concerns over embolization of a cell suspension, solid fragments of Abrams tumor were transplanted orthotopically into the medullary canal of the proximal tibia. The temporal patterns of tumor growth at the primary site were evaluated by radiography and histology, and the incidence and severity of subsequent secondary OS development in the lungs was quantified using stereological analysis of histological sections.