INTRODUCTION Cell transplantation has emerged as a promising regenerative medicine therapy for ischemic stroke by replacing death cells and by stimulating endogenous self-repairing mechanisms1. Unfortunately, the therapeutic efficacy is compromised by a substantial loss of grafted cells2. Injectable hydrogels combined with stem cells have been proposed as a promising strategy to improve cell retention at the infarct site and their survival3. For this purpose, we proposed to use a recently developed hyaluronic acid (HA) hydrogel, that exhibits self-healing and injectability properties, as a biomimetic scaffold for delivery of transplanted cells in the brain lesion4. As a combined advanced therapy medicinal product candidate, an in vivo follow-up is required to monitor both the fate of the hydrogel scaffold and cells in a long temporal window. In this context, a radiopaque iodinated-labeled HA hydrogel (HA-I) was designed. Then, the tracking of both hydrogel scaffold and gold-labeled human adipose-derived mesenchymal stem cells (Au-hASC) in a specific and quantitative manner was assessed in vivo by bi-color imaging using spectral photon-counting computed tomography (SPCCT) and synchrotron K-Edge subtraction computed tomography (KES-CT). EXPERIMENTAL METHODS Two modified iodine-labeled hyaluronic acid (HA-I) partners were synthesized by grafting an iodine contrast agent (acetrizoic acid, TBA) on functionalized HA chains in mild conditions. The HA-I hydrogel was prepared by simply mixing in physiological conditions these two modified HA-I partners that can self-crosslink via boronate ester bond formation. The iodine concentration in HA-I hydrogel was quantified by ICP-MS analysis and KES-CT. Cell viability and proliferation of hASC (0.1 106 hASC/ 200 μL HA-I hydrogel at day 0) were evaluated in different cell culture periods. In vivo intracerebral injections of HA-I hydrogel as a control and Au-hASC embedded within HA-I hydrogel (0.5 106 Au-hASC in 10μL HA-I) in healthy rats (n=5) were monitored by magnetic resonance imaging (MRI). Then, on day 4 following the transplantation, bicolor imaging was performed using SPCCT and synchrotron KES-CT. RESULTS AND DISCUSSION An iodine-labeled HA hydrogel was formulated in physiological conditions with an iodine concentration [I]=2.4 mg/mL, suitable for X-Ray imaging techniques (Figure 1A). In vitro cytotoxicity studies demonstrated a high cell viability after 7 days (> 80 % of cell viability). The dynamic rheological behaviour of the HA-I hydrogel showed that the storage moduli (G’) at 25 and 37 °C displayed a plateau in the frequency range 0.01-10 Hz. The G’ value (340 Pa, at 1Hz, 25°C) was suitable for a mini-invasive transplantation of Au-hASC in healthy rat brain (Figure 1B). Then, bicolour imaging using a KES-CT 3D reconstruction revealed cell retention of the biomimetic scaffold (Au-hASC embedded within HA-I hydrogel) (Figure 1C). CONCLUSION A radiopaque iodine-labeled injectable HA hydrogel was successfully developed to deliver Au-hASC in healthy rat brains. Our first results demonstrate the ability to monitor the fate of stem cells on one hand and of cell-embedding hydrogel on the other hand by bicolor imaging using KES-CT. This paves the way for further investigations of this dual-labeled repair kit toward its applicability in cell therapy. One of the perspectives of this work is thus to evaluate its therapeutic effect after intracerebral administration in a rat model of ischemic stroke by coupling multimodal imaging (KES-CT) for monitoring cells and scaffold and advanced multiparametric MRI methods for evaluating tissue repair. REFERENCES 1. Detante O et al., Revue Neurologique, 170 : 779-798, 2014 2. Marquardt L.M et al., Curr Stem Cell Rep, 2: 207-220, 2016 3. Detante O et al., Revue Neurologique, 173 :572-576, 2017 4. Figueiredo T et al., Biomacromolecules, 21 : 230-239, 2020 ACKNOWLEDGM ENTS This project was funded by the French national research agency (ANR) project Breakthru (ANR18-CE19-0003). We thank Lyon’s multimodal imaging platforms Cermep, the European Synchrotron Radiation Facility (ESRF, beamline ID17) and IRMAGE facility for providing imaging platforms.