Synthesis and Biological Evaluation of 3-Amidoquinuclidine Quaternary Ammonium Compounds as New Soft Antibacterial Agents

Abstract

Quaternary ammonium compounds (QACs) are among the most effective antimicrobial agents that have been used for more than a century. However, due to the growing trend of bacterial resistance and high toxicity of QACs, research in this field remains a pressing matter. Recent studies of the structure–activity relationship suggest that the introduction of the amide functional group into QAC structures results in soft variants that retain their antimicrobial properties while opening the possibility of fine-tuned activity regulation. Here, we report the synthesis and structure-function study of three structurally distinct series of naturally derived soft QACs. The obtained 3-amidoquinuclidine QACs showed a broad range of antibacterial activities related to the hydrophobic-hydrophilic balance of the QAC structures. All three series yielded candidates with minimal inhibitory concentrations (MIC) in the single-digit μM range. Time-resolved growth analysis revealed subtle differences in the antibacterial activity of the selected candidates. The versatile MIC values were recorded in different nutrient media, suggesting that the media composition may have a dramatic impact on the antibacterial potential. The new QACs were found to have excellent potential to suppress bacterial biofilm formation while exhibiting low ability to induce bacterial resistance. In addition, the selected candidates were found to be less toxic than commercially available QACs and proved to be potential substrates for protease degradation. These data suggest that 3-amidoquinuclidine QACs could be considered as novel antimicrobial agents that pose a low threat to ecosystems and human health.

Further Study of the Polar Group’s Influence on the Antibacterial Activity of the 3-Substituted Quinuclidine Salts with Long Alkyl Chains

“…Interestingly, the QNH 2 -C 16 derivative had excellent activity against L. monocytogenes ATCC 7644, similar to what we observed with 3-acetamidoquinuclidine QACs with a C16 chain [ 27 ]. The eradication of L. monocytogenes by conventional sanitization and disinfection methods has become increasingly difficult in recent years, as resistance to QACs has become more common [ 31 , 32 , 33 ].…”

“…To investigate the influence of the polarity of the substituent on the antibacterial properties of QACs, we decided to compare two structural analogs that differed only by the substituent on the 3C-atom. 3-Amidoquinuclidine QACs were previously synthesized as described in [ 27 ], while 3-aminoquinuclidine QACs were obtained from commercial 3-aminoquinuclidine dihydrochloride as the starting material. After removal of the dihydrochloride, quaternization was carried out by the Menshutkin reaction using alkyl bromide reagents with the appropriate number of C-atoms (C12, C14, C16) ( Scheme 1 ).…”

“…The structures exhibited different topological polar surface areas, with the amide-substituted salts having higher polarity due to the carbonyl oxygen at the 3C position of the quinuclidine head. In our previous study with pyridinium-4-adoxime, we similarly compared the structure–activity relationship of the two quaternary salts, namely the commercially available cetylpyridinium chloride (CPC) and its analog, which differed only by the substituent on the QACs backbone [ 27 ]. We found that the derivative with oxime functionalization at the backbone had reduced antibacterial activity compared with the otherwise active CPC without the oxime group, suggesting that the addition of polar groups at the quaternary backbone actually reduced the biological potential.…”

“…The antibacterial activities of the 3-aminoquinuclidine precursor (QNH 2 ) and the corresponding QACs (QNH 2 -C 12 , QNH 2 -C 14 , and QNH 2 -C 16 ) were investigated by determining the minimum inhibitory concentrations (MICs) against six Gram-positive strains, namely, Staphylococcus aureus (ATCC25923, methicillin-resistant MRSA, and ATCC 33591), Bacillus cereus (ATCC 14579), Listeria monocytogenes (ATCC 7644), and Enterococcus faecalis (ATCC 29212), and three Gram-negative strains, Escherichia coli (ATCC 25922), Salmonella enterica (food isolate), and Pseudomonas aeruginosa (ATCC 27853). The influence of the polar group on the antibacterial properties of the QACs was resolved by comparing the obtained MIC values with the previously synthesized structurally similar quaternary salts of 3-acetamidoquinuclidine (QAc) [ 27 ] and two reference quaternary salts, namely benzalkonium bromide (benzyldimethyldodecylammonium bromide, BAB) and cetylpyridinium chloride (CPC).…”

“…We have previously shown that naturally occurring bicyclic quinuclidine bearing a bridgehead nitrogen can be quaternized, resulting in potent QACs with a broad spectrum of antimicrobial activity with minimum inhibitory concentrations (MICs) in the range of nM units [ 26 ]. The introduction of an amide group into such a structure leads to soft QACs that are potentially degradable by trypsin but less active than originally thought [ 27 ]. In an attempt to further investigate the structure–activity relationship in the development of soft QACs, we synthesized three 3-aminoquinuclidine salts to compare with 3-acetamidoquinuclidine QACs, which did not exhibit antibacterial activity despite retaining the ubiquitous structural features, i.e., long alkyl chains at the quaternary center.…”