General Properties - The Merck Veterinary Manual

General Properties

Within the diversity of their various ring structures, the quinolones have a number of common functional groups that are essential for their antimicrobial activity. In addition, various modifications have produced compounds with differing physical, chemical, pharmacokinetic, and antimicrobial properties. For example, substitution at position 6 with a fluorine moiety markedly enhances activity against both gram-negative and gram-positive bacteria, as well as mycoplasmas and chlamydiae. These so-called fluoroquinolones, which are generally the most efficacious within each class, include enrofloxacin, norfloxacin, ciprofloxacin, orbifloxacin, ofloxacin, danofloxacin, flumequine, difloxacin, marbofloxacin, and other newer drugs. In addition, substitution with a piperazine ring at position 7 significantly increases tissue and bacterial penetration with consequent enhancement of activity; substitution with an oxygen atom at position 8 improves activity against gram-positive and anaerobic organisms without affecting the bactericidal profile.

The quinolones are amphoteric and, with a few exceptions, generally exhibit poor water solubility between pH 6 and 8. In concentrated acidic urine, such as may be found in dogs and cats, some quinolones form needle-shaped crystals. Liquid formulations of various quinolones for PO or parenteral administration usually contain freely soluble salts in stable aqueous solutions. Solid formulations (eg, tablets, capsules, or boluses) contain the active ingredient either in its betaine form or, occasionally, as the hydrochloride salt.