SYSTEMIC TREATMENTS IMPACT BONE QUALITY IN A RAT MODEL OF MIXED FEMORAL METASTASES

Skeletal metastases impact the bone remodeling process, compromising the mechanical integrity of the bone, and increasing the risk of pathological fracture. Cancer treatments have additionally been shown to influence bone quality in vertebrae with osteolytic metastases. This study aims to quantify the effect of systemic treatments (zoledronic acid (ZA) or docetaxel (DTX)) on bone quality in a preclinical model of mixed femoral metastases. Eleven six-week-old athymic male rats (Hsd: RH-Foxn1rnu, Envigo, USA) were inoculated with luciferase-transfected ACE-1 canine prostate cancer cells via an intracardiac injection after a week of acclimation (day 0). Institutional approval was obtained, and the ARRIVE guidelines were followed. The animals were randomly assigned to the following groups: untreated (n=3), zoledronic acid treated (n=4), and docetaxel treated (n=4). Zoledronic acid (Zometa® Norvartis; 60 μg/kg) or docetaxel (Aventis Pharma; 5 mg/kg) was administered on day 10 post inoculation. In vivo bioluminescence imaging (day 14, day 21) was used to assess tumor burden. All animals were euthanized on day 21. Femora excised bilaterally (n=22) underwent μCT scanning (μCT100, Scanco, Switzerland) and microstructural analysis was conducted on the trabecular bone within the distal femora (Amira). For histological analysis, the right femora (n=11) were stained with hematoxylin and eosin (H&E) to assess bone histoarchitecture. Tumor presence was detected using an anti-wide cytokeratin antibody stain. The left distal femora (n=11) were cut to a 1cm length using a diamond wafering blade on a low-speed saw (Isomet 1000, USA). The samples were stained with BaSO4 and µCT imaged (90kVp, 44µA, 4.9µm) to visualize microdamage location and volume. Damage volume fraction was calculated as the ratio of BaSO4 stain volume (SV) to bone volume (BV). Voxels representative of SV and BV were segmented with constant global thresholds of 10000 HU (~2400 mgHA/cm3) and 3000 HU (~736 mgHA/cm3), respectively. Finally, the samples were loaded to failure under axial compression and force-displacement data recorded. Differences in microstructural parameters, damage volume fraction, and load to failure were compared between treatment groups using one-way ANOVAs; post hoc analyses were performed using Tukey HSD. Docetaxel significantly reduced the mean bone volume (BV/TV, p=0.041) compared to untreated controls. Zoledronic acid effectively diminished tumor-induced osteolysis, consistent with its known capacity to inhibit osteoclast activity resulting in increased bone mineral density (BMD) and BV/TV compared to untreated controls (p=0.022, p=0.025 respectively) and DTX-treated animals (p<.0001 for BMD and BV/TV). For microdamage analysis, both DTX (p=0.006) and ZA (p=0.017) significantly decreased the mean damage volume fraction compared to the untreated group. Compared to the untreated controls, load to failure was significantly increased in ZA treated animals (p=0.024), with DTX treated animals exhibiting a positive trend (p=0.061). This study reinforces the positive role of ZA in preserving bone health. DTX, despite lower BV/TV had less damage volume fraction leading to a trend towards improved load to failure, suggesting better bone quality. Multiple treatment options are available for skeletal metastases, quantifying the impact of such treatments on metastatically affected bone is crucial for comprehending the consequences of cancer therapies and directing treatment delivery.