Optimization of a Subcutaneous Epithelioid Hemangioendothelioma Xenograft Model for Evaluation of Preclinical Therapeutics

Epithelioid hemangioendothelioma (EHE) is an ultra-rare vascular sarcoma characterized by an unpredictable clinical trajectory and lack of established standard-of-care treatment regimens. Despite the pressing need for effective therapeutic strategies, the scarcity of clinically relevant preclinical models remains a major barrier to drug development. To address this challenge, we used a patient-derived human EHE cell line to optimize and validate a subcutaneous xenograft model in mice. Optimization of cell-inoculation density revealed that injection of high cell densities was associated with adverse systemic responses and primary tumour growth kinetics unsuitable for longitudinal drug screening. In contrast, inoculation of low, sub-million cell densities generated reproducible xenografts that followed classical sigmoidal growth kinetics, providing a suitable therapeutic window for comparative treatment studies. Subsequent histopathological analysis confirmed that the tumours were highly proliferative and retained key features of EHE pathology, including expression of the EHE-associated markers ERG and CAMTA1. Treatment response validation further demonstrated sensitivity to mTOR inhibition, a characteristic preserved from the original patient-derived sample. Treatment with nab-sirolimus significantly suppressed tumour growth and reduced phosphorylation of ribosomal protein S6, consistent with suppression of downstream mTORC1 signalling. In this study, the optimized CDX model retains key EHE features and provides an extended observation window with sustained growth of untreated controls, supporting longitudinal assessment of antitumour activity and pharmacodynamic responses. This platform provides a practical framework for evaluating and comparing candidate therapies for EHE.