Project Leader

Dr. Jörn Engelmann
Phone: +49 7071 601-704
Fax: +49 7071 601-702
joern.engelmann[at]tuebingen.mpg.de

Current and former Lab members

 
Dr. Aneta Keliris
(PostDoc)
 
Dr. Sven Gottschalk
(PostDoc)
 
Dr. Rajendra Joshi
(PostDoc)
 
Dr. Deepti Jha
(former PhD student & PostDoc)
 
Dr. Ritu Mishra
(former PhD student & PostDoc)
 
Dr. Wu Su
(former PhD student)
 

External Collaborators

 
Dr. Anurag Mishra, Prof. David Parker
(Durham University, UK)
 
Prof. Silvio Aime
(University of Torino, IT)
 
Dr. Wolfgang Koestner, Prof. Martin G. Sauer,
Matthias Hardtke-Wolenski, Dr. Elmar Jaeckel
(Hannover Medical School, DE)
 
Dr. Sandra Ueberberg
(Ruhr-University Bochum, DE)
 
PD Dr. Stephan Schneider
(St. Vinzenz-Hospital Cologne, DE)
 
Prof. Hermann A. Mayer
(University of Tübingen, DE)
 
 
 

 

Novel Contrast Agents for MRI

Localization of one of the novel MR contrast agents (green fluorescence) inside cells.
Magnetic Resonance Imaging (MRI) offers a non-invasive means to map structure and function by sampling the amount, flow and environment of water protons in vivo. Contrast agents increase the intrinsic contrast generated in MR images. They are routinely used to enhance regions, tissues and cells that are magnetically similar but histologically distinct.
 
New classes of 'targeted' and 'smart' MR contrast agents are being developed to report on the physiological status or metabolic activity of biological systems. But commonly used contrast agents are often restricted to the extracellular or vascular space or show only inefficient delivery into cells. The goal is therefore to develop new cell-specific contrast agents which are either internalized or trapped selectively in or on the target cells for structural MRI studies or tracking of cells in vivo by MRI. For this purpose, a small group of chemists and biologists/biochemists is working together e.g. on the
 
 
 
·         Development of novel targeted contrast agents, evaluation of their biochemical properties, and the in vitro and in vivo application for MRI at (ultra)high field magnets.
·         Development of novel vectors for the efficient internalization of the contrast agents into cells.
·         Design and synthesis of molecular sensors for specific accumulation of the probes inside the target cells.
·         Coupling to multifunctional nanomolecular structures (e.g nanoparticles or dendrimers) to enhance the sensitivity of the contrast agent based MRI.
 
Aside of MRI several other techniques, like fluorescence spectroscopy and microscopy, are used to test the new probes in cultured cells and in vivo.
 
These studies are done in collaboration with the High-Field MR and Methodology group of the MRZ and the chemistry group of the Dept. Physiology of Cognitive Processes at the institute as well as external partners e.g. at the University of Tübingen or the Hannover Medical School.


 Our main projects currently are:

 

Intracellular targeting (enzyme and mRNA targeting)

In recent years several, mainly intracellularly targeted CA were developed by our group. Enzymes (model: β-galactosidase) and mRNA (model: mRNA of red fluorescent protein DsRed) were chosen as cellular targets. Although these CA efficiently enhanced the contrast in MR images of labeled cells (even when low micromolar concentrations were applied), a high unspecific background signal in non-targeted cells prevented a successful in vivo application so far. The intracellular localization of the CA (e.g. cytosol vs. endosomes) is not only crucial for a specific interaction and accumulation in the target cells but also for the efficacy of MR contrast enhancement. Even a significantly higher accumulation can be insufficient to increase contrast in MR images when the CA is predominantly located in endosomes or lysosomes.

A new approach to reduce the unspecific background signal in non-targeted cells is followed by using 19F MRI and an enzyme targeted probe where the 19F signal will be only detectable after the action of the enzyme. The synthesized probe was successfully tested in solution and the proof of concept in cells and in vivo in a tumor model expressing β-galactosidase is in progress.

 

New vectors

To circumvent endosomal entrapment we were also looking for new vector molecules (e.g. cell penetrating peptides, CPP) showing optimized cellular distribution. This study was leading to a novel cysteine rich peptide, CyLoP-1, which is able to transport efficiently various cargo molecules (e.g. the bioactive pro-apoptotic SmacN7-peptide) into the cytosol. When coupled to a Gadolinium chelate it is a potent MR CA strongly reducing relaxivity quenching effects due to endosomal entrapment.

In parallel, non-peptide delivery systems (e.g. lipid based systems like cholesterol) were tested for mRNA targeted imaging probes. Whilst uptake efficiency could be further increased compared to CPP conjugates the cellular distribution and thus the specificity were not altered. However, the synthesis of peptide nucleic acids, used to target mRNA, could be optimized further.

 

Receptor targeting

Recently we initiated a new project in collaboration with the University of Durham, UK. Here we developed CA responsive for the metabotropic glutamate receptor mGluR5 in the brain. These CA based on highly stable Gadolinium chelates coupled to known antagonists of this receptor. The goal is the non-invasive visualization of the receptors in the brain and potentially a more direct monitoring of neuronal activity by imaging extracellular glutamate fluctuations during activation.

 

Silica-based platforms for MR and multi modality imaging probes

One drawback of MRI is its relatively low sensitivity. A large number of contrast producing moieties is required to obtain a significant change in image contrast. To achieve a higher local accumulation, macromolecular or nanoparticle-based platforms can be used to increase the number of MR reporters per molecule. In collaboration with the University of Tübingen SiO2-based systems (silsesquioxanes, nonporous nanoparticles) functionalized with Gadolinium chelates were developed. The relatively small silsesquioxanes allow the coupling of up to eight chelates by retaining a pharmacokinetic profile of a low molecular weight CA. They undergo a slow degradation process under physiological conditions to mono-chelates [7] readily excreted via the kidneys. In vivo, a rapid accumulation in the gastrointestinal tract after i.v. injection into mice was observed not present when the same concentration of Gadolinium as GdDOTA, a commercially available CA used in the clinics, was injected.

 

 

Silica-based nanoparticles (NPs) are much larger (~ 50 – 100 nm in diameter) and thus will show different biodistribution and pharmacokinetics. However, a large number of imaging reporters can be conjugated per particle and different functionalities (e.g. targeting sensors, vectors for cellular uptake) can be introduced.

 

NPs with a high payload of Gadolinium were synthesized and tested for their ability to enhance MR contrast in vitro [8]. These NPs were further functionalized with fluorophores for optical imaging and a CPP to enhance cellular uptake. Such NPs can be taken up by cells and a initial in vivo study on the biodistribution after i.v. injection showed a fast accumulation particularly in lungs and liver, typical for nanoparticles.

 

β-cell targeting

A different type of NPs (ferromagnetic metallic cobalt with a functionalized carbon coating) is used for in vivo targeting of β-cells in pancreatic islets of Langerhans in mice. A β-cell specific single chain antibody fragment (SCA), developed by our collaborators, was coupled to these NPs to obtain a targeted T2-CA. In combination with the unprecedented spatial resolution which is achievable at our 16.4T animal scanner and optimizing imaging sequences and image processing by our MRI experts, it was possible for the first time to visualize single islets in vivo in a freely breathing mouse after i.v. injection. This study particularly exemplifies the great advantage to have CA developers (chemists, biologists) and MR physicists working close together in one group.

 

Neuroanatomical tracers

Understanding brain connectivity is important to enhance the understanding of brain function and disease. To correlate brain anatomy to the functional outcome and to follow changes e.g. with development, aging or learning, non-invasive longitudinal studies are needed. MRI is providing an excellent measure to perform dynamic investigations of brain connectivity by using MR active neuroanatomical tracers. In a collaboration with former and present members of the Dept. Logothetis we are helping to develop such novel tracer molecules by providing information about the uptake efficacy into and the transport of the probes inside neural cells along their processes. Two classes of tracers, for short term (1 - 3 days) and long term (7 - 14 days) studies, were successfully tested in vitro and in vivo.

 

Molecular in vivo imaging of cellular therapeutics – CeTheProbes

This work is in parts performed in the frame of the joint research project of the MPI for Biological Cybernetics (coordinator), Eberhard-Karls University Tübingen, and Hannover Medical School, funded by the German Federal Ministry of Education and Research (BMBF, FKZ 01EZ0813)

 

Last updated: Monday, 06.02.2012