(P 295) ORTHOTOPIC XENOGRAFTS OF PAIRED SARCOMA PRIMARY TUMOUR, LOCAL RELAPSE AND METASTATIC SITES REVEALS PHENOTYPIC HETEROGENEITY IN A MINIMAL RESIDUAL DISEASE MODEL

Objective: Soft-tissue sarcoma as a disease is extremely heterogenous, with over 70 distinct histological subtypes. However, there are not only significant differences between the different subtypes, but even within a single patient and a single tumour. We aimed to evaluate the functional differences of macroscopically different regions of a paired primary tumour and its metastatic regions, using a minimal-residual disease mouse model (Fischer, S. et al. - PMID: 37282823).

Methods: A single patient was selected, that suffered from a high-grade malignant peripheral nerve sheath tumour (MPNST). During the disease course, the patient relapsed with a local recurrence followed by a lung metastasis and kidney metastasis. Tissue samples were collected from macroscopically different regions of the primary tumour (4 regions), the local recurrence (4 regions), the lung metastasis (1 region) and the kidney metastasis (1 region) (figure 1). To create mouse models, we used the minimal residual disease model (MRD-PDX) that was developed by our lab. All 10 tumour regions were implanted in four mice per group to study the primary tumour take ratio and growth rate. Tumour volume measurements were performed using MRI images and 3DSlicer. When the tumours reached a volume of 250 - 500mm³, the hindlimb containing the primary tumour was surgically resected to achieve minimal residual disease. The animals were then followed until a year after the initial implantation and were sacrificed if humane endpoints were reached earlier. The tumours with the highest take-ratios were reimplanted to create a second generation of mouse models, to test for differences in response to chemotherapy. Once the tumour volume reached 50mm³, the animals were treated with docetaxel 20mg/kg once a week during 2 weeks and tumour volume was measured as previously described. The animals were sacrificed at two weeks after start of treatment and all organs were collected for pathological examination. All tissue samples were (immuno)histologically assessed by a dedicated sarcoma pathologist.

Results: Of 10 different tumour regions, a take-rate of 100% was seen in two of the primary tumour regions and both metastases. The metastatic potential was highest in the mouse models derived from the patient metastases, with the shortest minimal residual disease status. However, one of the primary tumour regions did show a high metastatic potential, with rapid development of metastases in the lungs and kidneys. This was similar to the patient, indicating the predictive value of the model. Interestingly, histological examination of the mouse metastases of this region revealed a high degree of dedifferentiation and epitheloid-like growth pattern, that looked similar to the patients metastases. Furthermore, the take-rate of the primary tumours was significantly lower than the metastases and there was a clear difference between the primary tumour regions (figure 2). When treated with the same chemotherapy, there was a clear difference in response to treatment between both metastases (figure 3). The experiment is currently still ongoing, so more results will be available in the upcoming weeks.

Conclusion: Heterogeneity within a single tumour and its metastases are maintained under in vivo conditions and the MRD-PDX is a trustworthy instrument to model a patients disease course. We showed that intratumoural heterogeneity affects model development significantly, since models from single biopsies may impact outcome and interpretation. Only one of the primary tumour regions showed a high propensity to metastasise, showing that intratumoural heterogeneity plays a significant role in the disease course of the patient. Furthermore, the differences in treatment response between the two metastases indicate that, even without being treated systemically, a patients metastases can develop a different response profile throughout the disease course.