Photochromism has received increasing attention due to the variety of potential applications in molecular electronics, including optoelectronics, optical data storage and molecular switches.[1, 2] All photochromic compounds have the light-induced reversible conversion between two isomers in common. In particular, photochromic GPCR ligands based on dithienylethenes allow a reversible switching of the conformation, electronic properties or the relative orientation of receptor ligands initiated by irradiation with light of an appropriate wavelength. Incorporated into a selective pharmacophore, the kinetics of ligand-induced GPCR activation, which may account for individual drug responses, and the dynamics of receptor dimerization can be investigated very precisely. So far, only a few examples of photochromic enzyme inhibitors,[4, 5] ion channels and receptor ligands have been reported.
This project comprises the development of photoswitchable GPCR ligands based on established GPCR agonists and antagonists. The photophysical properties will be characterized and the receptor binding properties and intrinsic activities will be evaluated.
Research stay abroad
Prof. Neil Branda, Simon Fraser University, Vancouver, Canada, is one of the leading experts in the field of photochromic dithienylethenes. The detailed photophysical characterization and the synthesis of fluorinated photochromic GPCR ligands may be performed during graduate internships in his group; academic contacts to the Branda group have already been established.
Positron emission tomography (PET) has emerged as an effective tool in molecular imaging with high sensitivity. The determination of GPCR densities by PET can be achieved by the use of highly selective radioligands. Moreover, targeting GPCR expression by radiolabeled ligands for radiotherapy provides an important treatment option for cancer patients. The neuropeptide Y1 receptor is expressed in breast carcinomas, while neurotensin receptors (NTS1/NTS2) are reported to be markers of prostate cancer. The development of selective Y1 and NTS1/2 radioligands is needed, since these neuropeptide GPCRs represent promising molecular targets either for PET imaging in vivo or for GPCR-guided radiotherapy.
In this subproject of the research training group, the group of Prof. Olaf Prante aims at the development and (radio)syntheses of selective Y1 and NTS1/2 ligands. We start from the syntheses of suitable precursors for radiolabeling and purified reference compounds, that are subjected to receptor binding studies (in collabora-tion with Prof. Gmeiner and Prof. Buschauer). After the selection of promising ligand candidates, highly-efficient 18F-labelling techniques are applied and PET imaging studies in animal models are performed. This includes the design of non-peptidic 18F-labeled radioligands as well as 18F-labeled glycopepides, metabolically-stabilized peptoids linked to the 68Ga-DOTA for PET, and 177Lu-labeled peptoids for radiotherapy approaches (see graphical abstract). The finding of an optimum balance between GPCR subtype selectivity, receptor affinity and in vivo properties is one of the major aims of this research.
Research stay abroad
The research training in subproject B2 is supported by the group of Dr. Patrick Riss (Wolfson Brain Imaging Centre, University of Cambridge). During a 3-4 month stay in Cambridge, the optimization of 18F-synthetic procedures and collaborative work on small animal PET experiments using new tracers is envisaged.
The use of positron emission tomography (PET) is rapidly expanding worldwide due to the availability of compact medical cyclotrons and automated chemistry for the production of radiopharmaceuticals. Among the positron emitting isotopes, [18F]fluorine now finds high importance as a label for radiotracers that are employed as clinical diagnostics and molecular probes in drug discovery and development. For the design and development of [18F]fluorine-labeled PET tracers, target specifity and selectivity is a major issue because a large number of struc-turally similar GPCRs are expressed in the organism. Thus, novel synthetic methodology giving access to a variety of highly selective GPCR binding chemotypes is of particular importance. The subproject B3 focuses on the development and application of new reactions for the preparation of [18F]fluorine-labeled GPCR selective ligands.
Research stay abroad
Radical generating reactions shall further be investigated in collaboration with Prof. Samir Zard (Ecole Polytechnique, Palaiseau, France).
The goal of this project is to design, generate, analyze and optimize synthetic mimics of GPCRs as molecular tools to explore the molecular and structural basis, as well as the ligand selectivity, of receptor-ligand interactions.
In vitro experiments involving GPCRs are not trivial because these proteins are rather unstable outside their lipid environment. This applies in particular to the structure analysis of GPCRs, as well as complexes with their ligands. Synthetic receptor mimics, on the other hand, are freely soluble in aqueous buffers, facilitating their handling and greatly improving their utility for experiments that require the analytes to be in solution, including NMR spectroscopy. Thus, the synthetic GPCR mimics will serve as molecular tools to assist in elucidating the receptor-bound conformation of GPCR ligands, by structural analysis, through NMR spectroscopy, of the receptor mimics in complex with their ligands. Furthermore, extensive substitution, truncation, deletion and other chemical variants of the synthetic GPCRs will facilitate the identification of receptor hot spot regions for their interaction with the ligand.
Methods directly involved: automated solid-phase peptide synthesis, chemoselective ligation strategies, preparative HPLC, mass spectrometry, LC-MS, binding assays (ELISA, Biacore, fluorescence polarization).
Research stay abroad
During a research visit with Dr. Clemencia Pinilla at the Torrey Pines Institute for Molecular Studies in San Diego (USA), synthetic GPCR mimics will be used as tools to screen for novel ligands using synthetic peptide combinatorial libraries.