The pronounced biological influence of the tumor microenvironment on cancer progression and metastasis has gained increased recognition over the past decade, yet most preclinical antineoplastic drug testing is still reliant on conventional 2D cell culture systems. Although monolayer cultures recapitulate some of the phenotypic traits observed clinically, they are limited in their ability to model the full range of microenvironmental cues, such as ones elicited by 3D cell-cell and cell-extracellular matrix interactions. To address these shortcomings, we established an ex vivo 3D Ewing sarcoma model that closely mimics the morphology, growth kinetics, and protein expression profile of human tumors. We observed that Ewing sarcoma cells cultured in porous 3D electrospun poly(e-caprolactone) scaffolds not only were more resistant to traditional cytotoxic drugs than were cells in 2D monolayer culture but also exhibited remarkable differences in the expression pattern of the insulin-like growth factor-1 receptor/mammalian target of rapamycin pathway. This 3D model of the bone microenvironment may have broad applicability for mechanistic studies of bone sarcomas and exhibits the potential to augment preclinical evaluation of antineoplastic drug candidates for these malignancies.tissue engineering | tumor model | biological therapy | connective tissue D espite the primacy of the cancer cell's dysregulated genotype [e.g., a near universal translocation of the Ewing sarcoma (EWS) breakpoint region 1 gene in EWS cells] as the initial step in malignant transformation, it has become increasingly apparent that the overall tumor phenotype is also dictated by the 3D tumor microenvironment (1-4). Nonetheless, studies of cancer biology and evaluation of antineoplastic drug efficacy remain heavily dependent on conventional 2D cell culture systems despite their limited ability to reflect the 3D tumor architecture, extracellular matrix (ECM), and surrounding cell types that comprise the in vivo tumor milieu.To overcome some of these constraints, 3D in vitro models such as spheroid and gel systems have been extensively studied and, compared with 2D monolayer culture, appear to better mimic the profound effects that the in vivo 3D environment has upon the human tumor phenotype (5-9). For example, malignant cells cultured in 3D exhibit increased chemoresistance (10, 11) and decreased cell proliferation (12), and assume specific phenotypes inducible only under a 3D context, such as angiogenic capability (13-15). Furthermore, striking differences in signaling pathways targeted by proven and experimental therapies have been observed in 3D tumor models (16-18). Accordingly, heightened awareness of the importance of 3D culture for cancer cells has resulted in the increasing use of spheroid culture systems for cancer research. However, these non-adhesion-mediated systems provide poor control over the tumor architecture and cell-cell interactions; as a result of culture conditions that prohibit cellular attachment onto surrounding surfaces, ce...