Swarm rat CHSs were implanted subcutaneously in NMRI nude mice (n=10). When tumors were measurable (12-16 days post-transplant), mice were imaged by CEST MRI (Dou W et al., Quant Imaging Med Surg, 2019). Proteoglycans, the main component of chondrogenic extracellular matrix, were quantified by GAG CEST contrast. Guanidyl-and APT CEST contrasts were combined to characterize acidic pH, as hypoxia reflect. ☢ These two features, proteoglycans and hypoxia, were assessed in parallel by nuclear imaging with [ 99m Tc]Tc-NTP 15-5 SPECT imaging (Peyrode C et al., Sarcoma, 2011) and [ 18 F]-FMISO PET imaging (Rajendran JG et al., Clin Cancer Res, 2004), respectively. Data were also completed by ex vivo analyses of tumor and muscle proteoglycans (Alcian blue stain and biochemical assay with dimethylmethylene blue) and hypoxia (pimonidazole immunofluorescence). CONCLUSION: The results from CEST MRI, nuclear imaging and ex vivo analyses were in agreement and highlighted a rich proteoglycan extracellular matrix and a heterogeneous hypoxic tumoral microenvironment for Swarm rat CHS xenograft in mice. This study emphasizes the role of multimodal imaging to characterize tumor phenotypes resistant to treatments and allows a better understanding of the relationship between tumor cells and their environment. Grants: "La Ligue contre le Cancer Auvergne-Rhône-Alpes". All imaging experiments were performed at In Vivo Imaging Auvergne (IVIA) facility