Rhabdomyosarcomas (RMS) are the most common pediatric soft tissue sarcomas. High-risk and metastatic disease continues to be associated with very poor prognosis. RMS model systems that faithfully recapitulate the human disease and provide rapid, cost-efficient estimates of anti-tumor efficacy of candidate drugs are needed to facilitate drug development and personalized medicine approaches. Here, we present a new zebrafish-based xenotransplant model allowing for rapid and easily accessible drug screening using low numbers of viable tumor cells and relatively small amounts of water-soluble chemicals. Under optimized temperature conditions, embryonal RMS-xenografts were established in zebrafish embryos at 3 hours post fertilization (hpf). In proof-of-principle experiments, chemotherapy drugs with established clinical anti-RMS efficacy (vincristine, dactinomycin) and the MEK inhibitor trametinib were shown to significantly reduce the cross-sectional area of the tumors by 120 hpf. RMS xenograft models in zebrafish embryos henceforth could serve as a valuable addition to cell culture and mammalian models of RMS and represent a rapid and cost-effective solution for pre-clinical candidate drug testing.

Objectives: Clinical studies have shown low toxicity and a favorable safety profile for sirolimus in vascular anomalies. Here, we describe severe adverse events (SAEs) observed during “off-label use”. Methods: We performed a retrospective, multicenter chart review for SAEs during “off-label” sirolimus therapy for vascular anomalies and analyzed these cases by a pre-designed workflow. Results: We identified 17 SAEs in 14 patients diagnosed with generalized lymphatic anomaly (n=4), Gorham-Stout disease (n=2), central conducting lymphatic anomaly (n=1), lymphatic malformation (n=4), tufted angioma (n=1), kaposiform hemangioendothelioma (n=1), and venous malformation in a CLOVES syndrome (n=1). Three patients presented two SAEs. The age at initiation of sirolimus therapy was under the age of 2 years (5x), 2 to 6 years (5x) and older than 12 years (4x). SAEs occurred during the first 3 months of sirolimus therapy (7x), between 3 and 12 months (7x) and after one year of therapy (3x). The most frequent SAE was viral pneumonia (8x) resulting in death due to a metapneumovirus infection in a 3 months old and generalized adenovirus infection in a 28 months old child. Sirolimus blood level at the time of SAEs ranged between 2.7 and 21 ng/L. Five patients were on antibiotic prophylaxis during sirolimus therapy. Conclusions: Most SAEs are observed in the first year of sirolimus therapy; however, SAEs can also occur after a longer treatment period. SAEs are potentially life threatening, especially in early infancy. Presence risk factors, i.e. underlying vascular anomaly or immune status, may contribute to the risk of SAEs.

Purpose: Kaposiform Hemangioendothelioma (KHE) is a rare vascular tumor in children, which can be accompanied by life-threatening thrombocytopenia, referred to as Kasabach-Merritt Phenomenon (KMP). The mTOR inhibitor sirolimus is emerging as targeted therapy in KHE. As the sirolimus effect on KHE occurs only after several weeks we aimed to evaluate if additional transarterial embolization is of benefit for children with KHE and KMP. Methods: 17 patients with KHE and KMP acquired from 11 hospitals in Germany were retrospectively divided into two cohorts. Children being treated with adjunct transarterial embolization and systemic sirolimus, and those being treated with sirolimus without additional embolization. Bleeding rate as defined by WHO was determined for all patients. Response of the primary tumor at 6 and 12 months assessed by Magnetic Resonance Imaging (MRI), time to response of KMP defined as thrombocyte increase >150 x 103/µl, as well as rebound rates of both after cessation of sirolimus were compared. Results: N= 8 patients had undergone additive embolization to systemic sirolimus therapy, sirolimus in this group was started after a mean of 6.5 ± 3 days following embolization. N=9 patients were identified who had received sirolimus without additional embolization. Adjunct embolization induced a more rapid resolution of KMP within a median of 7 days vs 3 months, however tumor response as well as rebound rates were similar between both groups. Conclusion: Additive embolization may be of value for a more rapid rescue of consumptive coagulopathy in children with KHE and KMP compared to systemic sirolimus only.