Design and Synthesis of Polyphenolic Imidazo[4,5-c]quinoline Derivatives to Modulate Toll Like Receptor-7 Agonistic Activity and Adjuvanticity

Abstract

TLR-7/8 agonists are a well-known class of vaccine adjuvants, with a leading example now included in Covaxin, a licensed human COVID-19 vaccine. This thereby provides the opportunity to develop newer, more potent adjuvants based on structure−function studies of these classes of compounds. Imidazoquinoline-based TLR7/8 agonists are the most potent, but when used as a vaccine adjuvant side effects can arise due to diffusion from the injection site into a systemic circulation. In this work, we sought to address this issue through structural modifications in the agonists to enhance their adsorption capacity to the classic adjuvant alum. We selected a potent TLR7-selective agonist, BBIQ (EC 50 = 0.85 μM), and synthesized polyphenolic derivatives to assess their TLR7 agonistic activity and adjuvant potential alone or in combination with alum. Most of the phenolic derivatives were more active than BBIQ and, except for 12b, all were TLR7 specific. Although the synthesized compounds were less active than resiquimod, the immunization data on combination with alum, specifically the IgG1, IgG2b and IgG2c responses, were superior in comparison to BBIQ as well as the reference standard resiquimod. Compound 12b was 5-fold more potent (EC 50 = 0.15 μM in TLR7) than BBIQ and induced double the IgG response to SARS-CoV-2 and hepatitis antigens. Similarly, compound 12c (EC 50 = 0.31 μM in TLR7) was about 3-fold more potent than BBIQ and doubled the IgG levels. Even though compound 12d exhibited low TLR7 activity (EC 50 = 5.13 μM in TLR7), it demonstrated superior adjuvant results, which may be attributed to its enhanced alum adsorption capability as compared with BBIQ and resiquimod. Alum-adsorbed polyphenolic TLR7 agonists thereby represent promising combination adjuvants resulting in a balanced Th1/Th2 immune response.

Modulation of the Adjuvant Potential of Imidazoquinoline-Based TLR7/8 Agonists via Alum Adsorption

“…We recently evaluated the adjuvant potential of m -IMDG ( 12 ) for protein-based vaccines . In the present study, the mice were immunized with Sp and HBs along with m -IMDG ( 12 ) or p -IMDG ( 13 ) or p -IMDP ( 14 ) alone or in combination with alum (Figures and ).…”

“…The SAR investigation on IMDQ revealed that N -acylation of aminomethyl functionality reduced TLR7 and TLR8 agonist activity. Considering that vic -phenolic functionality helps in the adsorption of TLR7/8 agonists to alum, we designed a 7,8-dihydroxyimidazoquinoline derivative (dh- p -AM-BBIQ, 21 ) where the aluminum-coordinating vic -diphenolic functionality was introduced in the quinoline ring, leaving free the aminomethyl group on the N 1-benzyl group to interact with TLR7/8. The synthesis of the desired target compound 21 was achieved from 4-chloro-6,7-dimethoxy-3-nitroquinoline ( 15 ) (Scheme ) following a similar synthetic protocol as described in Scheme , except that the demethylation of intermediate 20 was carried out using BBr 3 in anhydrous dichloromethane at −78 °C (Scheme ).…”

“…The binding of the TLR7/8 agonist with alum reduced its reactogenicity and improved its adjuvanticity. The binding affinity of the TLR agonists toward alum was studied using UV–vis spectroscopy . To determine the % adsorption of the agonists, the extinction coefficients of final compounds 10 – 14 , 20 , and 21 were determined at their characteristic wavelengths (Figure S1).…”