Toll-like receptors (TLRs) play an important role in the innate and adaptive immune system, which is the first line of defense against microbes. TLR7 receptors recognize molecular patterns associated with pathogens and damage and trigger a rapid inflammatory immune response. Currently, 10 TLR receptor subtypes are known in humans, and in this work we have focused on TLR7, an endosomal receptor that recognizes viral single-stranded RNA. The immune system activated by TLR7 leads to anti-inflammatory, antiviral and antitumor activity, which is why TLR7 agonists could be useful in the treatment of cancer, asthma and viral infections.
As part of this master's thesis, we synthesized analogs of TLR7 agonists. The compounds were evaluated in biological assays to obtain additional data on the structure-activity relationships of isoxazolo[5,4-d]pyrimidine-based TLR7 agonists.
The six-step synthesis, starting from 4-fluorobenzaldehyde, its conversion to into oxime, imidoyl chloride, isoxazole, isoxazolo[5,4-d]pyrimidin-4(5H)-one and then into aryl chloride, allowed us to attach different substituents at position 4 of the heterobicyclic core and to synthesize six final compounds, potential TLR7 agonists. The solubility of five derivatives in cell media was estimated, followed by the evaluation of TLR7 agonism, TLR8 agonism for certain analogs, and cytotoxicity on the HEK293 cell line for the most potent compound.
All compounds were poorly soluble in the cell medium due to the lipophilic fragments. Compound 12 was the only compound that showed agonism on TLR7 with an EC50 value of 15.9 ± 0.9 µM. Compared to imiquimod, the only approved TLR7 agonist (EC50 = 2.1 µM), the EC50 of compound 12 is higher, indicating its weaker agonist activity. In summary, compound 12 represents a good starting point for further research into novel TLR7 agonists as immunomodulatory agents.
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