Title: Synthesis, structure-activity relationship studies and biological evaluation of novel heterocyclic structures
Other Titles: Synthese, structuur-activiteitsrelatie studies en biologische evaluatie van nieuwe heterocyclische structuren
Authors: Stella, Alessandro
Issue Date: 23-Jan-2013
Abstract: Heterocyclic structures are of fundamental importance in drug discovery and the vast majority of the marketed drugs contain at least one heterocyclic ring. In this PhD study, the synthesis of compound libraries based on three different heterocyclic cores (benzothiazoles, pyrimidines and thiazolo[4,5-c]pyridazines) has been undertaken. All synthesized compounds were evaluated for their antibacterial and immunosuppressive activity. The antimicrobial properties were assessed in a phenotypic-based screening (by measuring the ability of the compounds to inhibit the growth of selected Gram-positive and Gram-negative bacteria) and in a target-based screening, in which SecA was selected as protein target. SecA, together with SecYEG, forms the so-called “translocon”, a biological nanomachine involved in the translocation of newly synthesized unfolded polypeptidic chains across biological membranes. SecA has been proposed as an optimal target for the development of novel antibiotics. Immunosuppressive compounds are used to prevent graft rejection after organ transplantation, but are alsoso useful for the treatment of several autoimmune diseases. Most of the medications currently available for the induction of immunosuppression display several side effects and for this reason, there is a big interest in the development of new and more selective immunosuppressive agents. In order to evaluate the immunosuppressive activity of the compound libraries, they were screened in the MLR assay, which is a well-known in vitro model for in vivo rejection after organ transplantation. In Chapter II, the synthesis of a 36-membered library of 2-(pyrazol-2-yl)benzothiazoles derivatives is described, using the ring closure between substituted 2-hydrazinobenzothiazoles and ß-ketoesters as key step. The library was evaluated for its antibacterial properties against different Gram-positive and Gram-negative microorganisms, finding that several congeners displayed antimicrobial activity. The SAR study revealed that a phenyl group on the pyrazolone moiety and a chlorine at C-(6) of the benzothiazole scaffold are necessary for strong antibacterial activity. The most potent compound displayed a MIC value of 1.17 μg/mL against S. aureus and P. aeruginosa. The library was also screened in the SecA and MLR assay. None of the tested compounds displayed activity against SecA, whereas only a few benzothiazole congeners are endowed with immunosuppressive activity. In Chapter III, the synthesis of a pyrimidine library is described. The pyrimidine skeleton was constructed via a Biginelli reaction, and subsequently decorated via a BOP-mediated amination reaction as well as via alkylation of the lactam moiety. The pyrimidine derivatives were initially tested against SecA from E. coli and S. aureus. Although similar compounds have been reported as SecA inhibitors, the majority of the pyrimidine derivatives lacked inhibitory activity against SecA and only a few derivatives showed weak inhibitory activity (IC50 > 100 μM). An antibacterial screening campaign against S. aureusrevealed that most pyrimidine congeners are devoid of antibacterial activity and that only a few compounds showed antibacterial activity, with the most potent ones having a MIC value of 16 μg/mL. Finally, the pyrimidine library was screened in the MLR assay. Several derivatives displayed immunosuppressive activity, the most potent compound being 3.36m, with an IC50 value of 1.6 μM. The SAR study indicated that the presence of a thiobenzyl group at C-(2) and a N-containing alkyl or cycloalkyl moiety at C-(4) are essential for immunosuppressive activity. The most potent immunosuppressive pyrimidines lack cellular toxicity. Moreover, in order to find the molecular target responsible for the immunosuppressive activity, the most potent pyrimidine derivative has been assayed for its binding affinity towards 451 kinases. However, as this compound bound only very weakly to a few kinases, it is very unlikely that its immunosuppressive activity is due to kinase inhibition. In Chapter IV, the synthesis of a small library of thiazolo[4,5-c]pyridazine was reported. Since the synthesis of thiazolo[4,5-c]pyridazine has never been disclosed before in literature, a short and straightforward approach for the synthesis of this novel scaffold has been developed. The protocol is based on the reaction between a 3-amino-4-bromopyridazine analogue and alkyl isothiocyanates, in the presence of Cs2CO3 and using CH3CN as solvent. This optimized methodology has been applied for the synthesis of a 20-membered library of thiazolo[4,5-c]pyridazines. All these derivatives were unable to inhibit the growth of S. aureus (at a concentration of 64 μg/mL) and had no effect on SecA activity (at a concentration of 200 μM). In the MLR assay, few thiazolo[4,5-c]pyridazine derivatives showed immunosuppressive activity, with the most potent compound being 4.21b, with an IC50 value of 4.7 μM.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Medicinal Chemistry (Rega Institute)

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