Abstract
The host-guest interactions of a third-generation fluoroquinone, danofloxacin (DOFL), with the macrocyclic host cucurbit[7]uril (CB7) have been investigated at different pH values (~3.5, 7.5, and 10). The photophysical properties have been positively affected, that is, the fluorescence yield and lifetime increased, as well as the photostability of DOFL improved in the presence of CB7. The antibacterial activity of DOFL is enhanced in the presence of CB7, as tested against four pathogenic bacteria; highest activity has been found towards B. cereus and E. coli, and lower activity towards S. aureus and S. typhi. The antibacterial activity of two additional second-generation fluoroquinones, i.e., norfloxacin and ofloxacin, has also been investigated in the absence as well as the presence of CB7 and compared with that of DOFL. In case of all drugs, the minimum inhibitory concentration (MIC) was reduced 3–5 fold in the presence of CB7. The extended shelf-life (antibacterial activity over time) of the fluoroquinone drugs in the presence of CB7, irrespective of four types of bacteria, can be attributed to the enhanced photostability of their CB7 complexes, which can act as better antibiotics with a longer expiry date than uncomplexed DOFL.
Highlights
Fluoroquinolones (FQs), derived from nalidixic acid, constitute one of the most successful classes of antibiotic drugs in therapeutic applications that are used in the treatment of a variety of bacterial infections[1,2]
Depending on the pH of the solution, danofloxacin exists in three different forms (cationic (I), DOFLH2+; zwitter ion (II), DOFLH, and anionic (III), DOFL−; Fig. 1a); the zwitter ionic form can tautomerize into a putative neutral form in which the carboxylic group is stabilized by forming a six-membered cyclic structure through intramolecular hydrogen bonding[40]
At pH 7.5, DOFL exists as DOFLH, which showed an absorption within the range of 250–425 nm with an intense narrow band at ~275 nm and a weaker broad band at 340 nm[40]
Summary
Fluoroquinolones (FQs), derived from nalidixic acid, constitute one of the most successful classes of antibiotic drugs in therapeutic applications that are used in the treatment of a variety of bacterial infections[1,2]. Host-guest interactions using macrocyclic hosts such as cyclodextrins, cucurbiturils, etc.[7,8] are promising to improve and control the antibacterial activity of drugs. Henriques et al reported that gallic acid shows good antibacterial activity upon complexation with β-cyclodextrin and its derivatives[9]. We establish a supramolecular approach to enhance antibacterial activity and shelf-life of CB7-encapsulated DOFL against two Gram positive (Staphylococcus aureus: S. aures; Bacillus cereus: B. cerus) and two Gram negative (Escherichia coli: E. coli; Salmonella typhi: S. typhi) pathogenic bacteria with 3–5 fold reduced MIC. The extended shelf-life is attributed to an increased photochemical and thermal stability of the drug in the presence of CB7
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