A responsive MRI contrast agent to monitor functional cell status: a feasibility study using dendritic cells.

INTRODUCTION Molecular Imaging of transplanted stem and progenitor cells is essential to understand their therapeutic potential. Stable labeling of cells with either iron oxide particles (USPIO) or lanthanide chelates has proven successful for MRI-based detection of cell deposits and their migration [1, 2]. However, MRI only provides information about the location but not about cellular change of the functional status (e.g. differentiation). This information can be provided by responsive lanthanide chelate-based contrast agents that only generate T1-contrast after enzymatic activation. Lanthanide-based probes responsive to βgalactosidase resulted in an increase of the R1 relaxivity by 20% up to three times the R1 in the “inactive” state [3, 4]. Although applicable for in vivo applications [4], the remaining relaxivity in the inactive state due to the secondary coordination sphere makes the probe too insensitive for in vivo cell tracking. The aim of this study was to develop and test a Gd-chelate based contrast agent for conditional activation by intracellular lipase with zero T1 relaxivity in its inactive state in a proof-of-concept study. METHODS Contrast agent: The Gd-DTPA chelate was linked with two long fatty acid chains (C17H35) through ester bonds (Figure 1). The insoluble Gd-DTPA-FA complex had a relaxivity of zero and the activated complex an in situ relaxivity of 4.7 mM s. Micronization resulted in particle sizes of 0.5 to 1 μm. Cell labelling: Flt3+CD11b+progenitor cells were isolated from mice according to [5]. Undifferentiated cells and fully differentiated dendritic cells were incubated with the Gd-DTPA-FA complex (1 to 20mM) for up to 24 hours, washed four times and either used for in vitro testing or implanted in the brain of Wistar rats (n=4). MRI: T1-weighted MR images were acquired using a Bruker Biospin 7.0 Tesla small animal scanner equipped with an actively shielded gradient sets of 200 mT m using 3D gradient echo sequences (FLASH) with TR=120ms, TE=5ms, 70o pulse, FOV= 3x3x1cm (animal model) and 4.5x4.5x1cm (agar phantoms), the isotropic spatial resolution was 78 μm for phantoms and 50 μm for animal experiments. For rf irradiation and signal detection custom-built coils were used. A 5-cm-diameter transmit-receive coil was used for agar phantoms and a 12-cm-diameter Helmholtz coil arrangement served for rf excitation with a 3.0 cm diameter surface coil for signal detection for animals.