An effective cancer diagnostic and therapeutic contrast agent with suitable properties including high specificity, safety and cost is on high demand. Biocompatible and biodegradable glycogen-based nanoprobe has been already proposed as a promising tumor-targeting contrast agent¹. Glycogen (GG) core was modified by a gadolinium and a fluorescent (cyanin) dye Dy-615 (GG-Gd-DOTA-Dy) for multimodal imaging. In this study, we examined biodistribution and accumulation of the probe in the tumor-bearing rats by MR and fluorescence imaging and we compared the efficiency of GG-Gd-DOTA-Dy with a commercially available contrast agent (gadoterate meglumine – Gd-DOTA-GM).The properties of the probes were assessed by r₁ relaxometry (0.5T, saturation recovery sequence, recycle delay 12s or 5s) and MR imaging (4.7T, T₁-weighted images, RARE sequence, TR=333ms, TE=12ms, turbo factor TF=3, number of acquisition NA=16). Contrast-to-noise ratios (CNR) were calculated for each probe concentration. The toxicity of glycogen-based probes was assessed in vitro on the HepG2 tumor cell line by MTT assay. The tumors were induced in four RNU nude rats by injection of 5 mil. of tumor cells (HepG2 or Huh7) into the subcutaneous area above the hind leg. Three weeks after tumor induction, T1-weighted images (4.7T, RARE sequence, TR=630 ms, TE=14ms, TF=2, NA=8, resolution 0.19x0.19x1.5 mm³) were acquired before and after intravenous administration of GG-Gd-DOTA-Dy (0.006 mmol/kg) and Gd-DOTA-GM (0.006; 0.012; 0.024 mmol/kg). Relative CNR values normalized to muscle tissue were calculated from the manually outlined region of interest containing tumor or kidney. Fluorescence was measured on IVIS Lumina imager (excitation at 605 nm, blue shifted background excitation at 550 nm, emission filter 695-770 nm).Relaxivity and corresponding CNR values of GG-Gd-DOTA-Dy compound were markedly higher compared to Gd-DOTA-GM (Fig. 1). In vitro experiments confirmed non-toxicity of glycogen-based probes on the cells. After intravenous administration in the animals, both GG-Gd-DOTA-Dy and Gd-DOTA-GM were accumulated in the tumor and kidneys (Fig. 2). The signal enhancement in both organs was higher using GG-Gd-DOTA-Dy; even the administration of lower concentration of GG-Gd-DOTA-Dy (0.006 mmol/kg) compared to Gd-DOTA-GM (0.024 mmol/kg) led to a higher CNR increase (Fig. 2E,F). Moreover, the signal enhancement by GG-Gd-DOTA-Dy persisted in the tumor for a long time (up to 24 hours). After injection, fluorescent signal increases in the internal organs (kidneys, liver, gastro-intestinal tract (GIT), bladder) as well as in the tumor tissue (Fig. 3).We confirmed superior MR properties of our novel GG-Gd-DOTA-Dy compound including relaxivity and CNR compared to the commercially available contrast agent gadoterate meglumine. The glycogen-based nanoprobe was effectively accumulated in the tumor tissue confirming its tumor-targeting property and the contrast increase was higher compared to gadoterate meglumine. The contrast enhancement was long-lasting (24 hours) suggesting continuous accumulation into the tumor tissue by EPR (enhanced permeability and retention) effect.In this study, the tumor-targeting properties of a biocompatible and non-toxic glycogen-based nanoprobe were assessed by in vivo multimodal imaging. The novel probe was superior to a commercially available contrast agent.