title: Synthesis and characterization of a cell-permeable bimodal contrast agent targeting β-galactosidase.
abstract: Noninvasive monitoring of intracellular targets such as enzymes, receptors, or mRNA by means of magnetic resonance imaging (MRI) is increasingly gaining relevance in various research areas. A vital prerequisite for their visualization is the development of cell-permeable imaging probes, which can specifically interact with the target that characterizes the cellular or molecular process of interest. Here, we describe a dual-labeled probe, Gd-DOTA-k(FR)-Gal-CPP, designed to report the presence of intracellular β-galactosidase (β-gal) enzyme by MRI. This conjugate consists of a galactose based core serving as cleavable spacer, incorporated between the cell-penetrating peptide D-Tat(49-57) and reporter moieties (Gd-DOTA, fluorescein (FR)). We employed a facile building block approach to obtain our bimodal probe, Gd-DOTA-k(FR)-Gal-CPP. This strategy involved the preparation of the building blocks and their subsequent assembly using Fmoc-mediated solid phase synthesis, followed by the complexation of ligand 14 with GdCl(3). Gd-DOTA-k(FR)-Gal-CPP showed a considerably higher relaxivity enhancement (16.8±0.6 mM(-1)s(-1), 123 MHz, ∼21°C) relative to the commercial Gd-DOTA (4.0±0.12 mM(-1)s(-1), 123MHz, ∼21 °C). The activation of Gd-DOTA-k(FR)-Gal-CPP was based on a cellular retention strategy that required enzymatic cleavage of the delivery vector from galactose moiety following the cell internalization to achieve a prolonged accumulation of the reporter components (Gd-DOTA/FR) in the β-gal expressing cells. Cellular uptake of Gd-DOTA-k(FR)-Gal-CPP in β-gal expressing C6/LacZ and enzyme deficient parental C6 rat glioma cells was confirmed by fluorescence spectroscopy, MR imaging and ICP-AES measurements. All methods showed higher accumulation of measured reporters in C6/LacZ cells compared to enzyme deficient parental C6 cells. Fluorescence microscopy of cells labeled with Gd-DOTA-k(FR)-Gal-CPP indicated a predominantly vesicular localization of the green fluorescent conjugate around cell nuclei. This cellular distribution was most likely responsible for the observed non-specific background signal in the enzyme deficient C6 cells. Even though the specific accumulation of our bimodal probe has to be further improved, it could be already used for cell imaging by MRI and optical modalities.
authors: Max Planck Institute for Biological Cybernetics, Tüebingen, Germany. aneta.brud@tuebingen.mpg.de
authors: Aneta
authors: A
authors: Keliris
authors: Thomas
authors: T
authors: Ziegler
authors: Ritu
authors: R
authors: Mishra
authors: Rolf
authors: R
authors: Pohmann
authors: Martin G
authors: MG
authors: Sauer
authors: Kamil
authors: K
authors: Ugurbil
authors: Jörn
authors: J
authors: Engelmann
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D002463:Cell Membrane Permeability; D003287:Contrast Media / Q000737:chemistry / Q000493:pharmacokinetics*; D019793:Fluorescein; D005690:Galactose; D005910:Glioma / Q000175:diagnosis / Q000473:pathology; D006571:Heterocyclic Compounds; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008856:Microscopy, Fluorescence; D009942:Organometallic Compounds; D010446:Peptide Fragments; D051381:Rats; D001616:beta-Galactosidase / Q000187:drug effects*; D054322:tat Gene Products, Human Immunodeficiency Virus
chemical_list: D003287:Contrast Media; D006571:Heterocyclic Compounds; D009942:Organometallic Compounds; D010446:Peptide Fragments; D054322:tat Gene Products, Human Immunodeficiency Virus; C490722:tat peptide (49-57), Human immunodeficiency virus 1; C050823:gadolinium 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetate; D001616:beta-Galactosidase; D019793:Fluorescein; D005690:Galactose