Monoamine oxidase (MAO) is an enzyme involved in the oxidation of both endogenous and exogenous amines in the human body. MAO is found in two isoforms – MAO-A and MAO-B, both of which are located in the membranes of mitochondria, but differ mainly in the substrates they oxidize. MAO-A is involved in the degradation of the neurotransmitter serotonin, making it an important target in the treatment or inhibition of neurodegenerative diseases, depression and attention deficit disorders. Recent research has also confirmed its involvement in some cancers. MAO-A is an important factor in the spread of cancer cells via the so-called perineural invasion, an important process in the metastasis of prostate cancer. MAO-A inhibitors have effectively inhibited this process in mouse models of prostate cancer. An important new and promising group of active substances are heterobifunctional degraders – PROTAC molecules, which exploit the ubiquitin-proteasome system to counteract the effect of a protein via its degradation.
In this thesis, we synthesized and biochemically evaluated five ligands – potential degraders that target MAO-A. The ligand for the E3 von Hippel-Lindau ligase was attached to harmine, the ligand for the target MAO-A, via different linkers – alkyl chains, ethylene glycole chains and piperidine or piperedine-4-methanol. The linker was attached to harmine via an O-alkylation, i.e., Williamson synthesis of ethers, by the S2N nucleophilic substitution mechanism, followed by alkaline hydrolysis or hydrolysis of the protected carboxyl group, and in the last step an amide bond formation using the coupling reagent HATU. Following purification and characterization, five compounds were subjected to biochemical testing to verify how efficiently and selectively they inhibit MAO-A. All compounds selectively inhibited MAO-A over MAO-B, and the most potent inhibitor was 34 (IC50 = 34.0 ± 2.7 nM). Arguably, linear alkyl chain is favorable for the potent inhibition of MAO-A, whereas the use of bulkier piperidine spacers and ethylene glycol analogues with approximately the same distance between the harmine and the ligand for the von Hippel-Lindau ligase is unfavorable. The observations can be related to the structural characteristics of the MAO-A active site, which is relatively hydrophobic and also rather narrow at its entrance. In further evaluation steps, selected ligands should be tested for their intracellular MAO-A degradation in a suitable neurodegenerative or oncological cellular disease model.
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