The inflammatory response represents a defense mechanism that is activated when exposed to harmful factors. The innate and adaptive immune system plays a crucial role in this process, as it is also activated via Toll-like receptors (TLR). To date, 10 types of TLR have been identified. It is known that they recognize small molecules or microbial components as their ligands. The focus of our thesis was on TLR7, which is activated by single-stranded RNA. The activation triggers a cascade of responses leading to anti-inflammatory, antiviral and antitumor effects. Thus, TLR7 agonists could play a crucial role in the treatment of viral infections, cancers and allergic asthma.
Our research was based on the structure of the known TLR7 agonist (3-(4-fluorophenyl)-4-(3-methylpiperidin-1-yl)-6-(trifluoromethyl)isoxazolo[5,4-d]pyrimidine). Five analogs with an isoxazolo[5,4-d]pyrimidine ring and two analogs with a quinazoline ring were prepared with the optimized synthetic and isolation procedures. All seven final compounds were characterized and biologically tested. Agonist activity was tested on the HEK293 cell line, expressing human TLR7. All active compounds were then tested for their cytotoxicity.
Based on the results from biological evaluation, we found out that four out of the seven final compounds showed agonist activity, while three of them were not active. Compounds with an isoxazolo[5,4-d]pyrimidine scaffold substituted at position 4 by a six-membered ring, with a smaller group at position 3, were found to be the most optimal to obtain agonist activity. All four active compounds were not cytotoxic at any concentration. Out of all final compounds, compound 8 proved to be the most promising, as it has the lowest EC50 value (EC50 = 11 μM). Therefore, it represents a good starting point for further design and synthesis of TLR7 agonists.
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