Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Various kinds of tumour entities are characterised by an increased activity of transglutaminase 2 (TGase 2), which contributes to an enhanced invasive potential of the tumour cells and their resistance to chemo- and radiotherapy. Therefore, this enzyme represents an interesting target for the development of PET tracers for functional imaging of tumours in vivo [1].
Among the TGase 2 inhibitors described in the literature, N6-acryloyl-lysine-4-arylpiperazides reported by Wityak et al. [2] seem to be most suitable for radiotracer development as these compounds exhibit both strong inhibitory potential and selectivity towards human TGase 2 and show favourable pharmacokinetic properties. Based on this class of compounds, derivatives that allow for labelling with radionuclides such as fluorine-18 and iodine-124 were prepared and their inhibitory potential towards TGase 2 was evaluated.
The N2-acyl-N6-acryloyl-lysine-4-pyridylpiperazides were synthesised in a sequence consisting of N6-acrylation, PyBOP-mediated amide bond formation, Boc deprotection and N2-acylation starting from N2-Boc-lysine. The required pyridylpiperazines were obtained commercially or prepared in a few steps. All final compounds were evaluated in two independent kinetic assays, which detect either the transamidase or hydrolase activity of TGase 2, respectively, with N2-phenylacetyl-N6-acryloyl-lysine-4-(6-methylpyridine-2-yl)piperazide [2] serving as benchmark inhibitor.
Using the outlined synthetic route a series of more than 40 N2-acyl-N6-acryloyl-lysine-4-pyridylpiperazides were prepared in good yields. The kinetic characterisation of the compounds revealed some interesting structure-activity relationships. For example, replacing the 6-methylpyridine-2-yl moiety of the benchmark inhibitor by a 2-nitro-5-pyridyl moiety led to a significantly increased inhibitory effect towards human TGase 2.
Furthermore, substitution of the methyl group by hydrogen or a halogen atom (F, Cl, Br and I) revealed a direct correlation between the van der Waals radius of the substituent and the inhibitory effect towards human TGase 2, with the inhibitor containing iodine being even more potent than the benchmark inhibitor. Covalent docking studies provided information about the binding mode of this inhibitor class for the first time and, thus, allowed for explaining the trends observed in the activity assays and give hints for further improvement of inhibitory potency by specific structural variations.
References:
[1] Pietsch etal. Bioorg. Med. Chern. Lett. 2013, 23, 6528. [2] Wityak etal. ACS Med. Chem. Lett. 2012, 3, 1024.