Quinolone carboxylic acid derivatives are synthetic antimicrobial agents. Nalidixic acid and its congener oxolinic acid have been used for treatment of urinary tract infections for years, whereas flumequine has been used successfully in several countries to control intestinal infections in production animals. Many broad-spectrum antimicrobial agents have been produced by modification of the various 4-quinolone ring structures.
Known generically as quinolones or 4-quinolones, these drugs are derived from several closely related ring structures that have certain common features. Examples of the quinolone carboxylic acids and species in which they are approved are presented in Quinolones and Species Approvals in the US Quinolones and Species Approvals in the US . Nalidixic acid, considered a first-generation drug, is the earliest of the quinolones. In general, subsequent generations are based on spectrum; however, this often reflects similar changes in chemical structure. Subsequent drugs contain a fluorine group and, as such, are referred to as fluoroquinolones. Most veterinary drugs and many human drugs, including ciprofloxacin, are considered second generation. Pradofloxacin is an example of a later-generation drug approved for use in cats (US) or dogs and cats (European Union).
Within the diversity of their various ring structures, the quinolones have a number of common functional groups essential for their antimicrobial activity. For example, the quinolone nucleus contains a carboxylic acid group at position 3 and an exocyclic oxygen at position 4 (hence the term 4-quinolones), which are believed to be the active DNA-gyrase binding sites.
Various modifications have produced compounds with differing physical, chemical, pharmacokinetic, and antimicrobial properties. For example, the side chain attached to the nitrogen at position 1 affects potency. Replacement of the ethyl group at this position with a bulkier group (eg, the cyclopropyl group of ciprofloxacin and similar drugs) enhances gram-negative and gram-positive spectra. Addition of a fluorine atom at position 6 profoundly enhances the gram-positive spectrum, whereas the addition of a (heterocyclic nitrogen-containing) piperazyl ring at position 7 enhances bacterial penetration and potency, including toward Pseudomonas aeruginosa. Substitutions on the piperazyl (eg, ofloxacin and its ʟ-isomer, levofloxacin; sparfloxacin) enhance gram-positive penetration, whereas substitutions at position 8 enhance anaerobic activity (eg, sparfloxacin, pradofloxacin, and moxifloxacin). If the substitution is with a methoxy group (rather than a halogen), the risk of phototoxicity is decreased.
The quinolones are amphoteric and, with a few exceptions, generally have poor water solubility at pH 6–8. Although the impact on therapeutic efficacy is not clear, they appear to act as weak bases in that they are much less effective in acidic than in nonacidic urine pH. In concentrated acidic urine, some quinolones form needle-shaped crystals, although this apparently has not been reported with clinical use. 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.
Mode of Action of Quinolones in Animals
The quinolones inhibit bacterial enzyme topoisomerases, including topoisomerase II (otherwise known as DNA gyrase) and topoisomerase IV. Bacterial DNA supercoils and then uncoils during replication. Supercoiling requires transient nicks that are subsequently sealed after DNA polymerase passes. Topoisomerase II allows for single-strand nicks in the DNA that support coiling and uncoiling. Topoisomerase IV supports disentanglement of DNA as chromosomes separate. Inhibition of topoisomerases reduces supercoiling, resulting in disruption of the spatial arrangement of DNA, and decreases DNA repair.
Mammalian topoisomerase enzymes fundamentally differ from bacterial gyrase and are not susceptible to quinolone inhibition. The quinolones are usually bactericidal; susceptible organisms lose viability within 20 minutes following exposure to optimal concentrations of the newer fluoroquinolones. However, lower concentrations may result in only one DNA-DNA gyrase complex binding site being occupied and therefore only nicking a single strand of DNA. High drug concentrations may also lead to decreased efficacy due to dose-dependent inhibition of RNA or protein synthesis. Fluoroquinolones are less susceptible to inducing cell lysis in the absence of RNA and protein synthesis because they are necessary for bacterial autolysin production. Typically, clearing of cytoplasm at the periphery of the affected bacterium is followed by lysis, rendering bacteria recognizable only as bacterial ghosts (ie, empty cell envelopes).
Quinolones are associated with a postantimicrobial effect in a number of bacteria, principally gram-negative (eg, Escherichia coli, Klebsiella pneumoniae, and P aeruginosa). The effect generally lasts 4–8 hours after exposure.
Efficacy of the fluorinated quinolones depends on concentrations in plasma that exceed the minimum inhibitory concentration (MIC) of the infecting organism by 10- to 12-fold. As such, the drugs are concentration dependent. However, efficacy is correlated as well to the magnitude of the area under the inhibitory curve (AUC:MIC ratio), with an optimal AUC:MIC ratio >125. As such, efficacy also takes into account elimination half-life.
The fluoroquinolones can have notable antibacterial activity at extraordinarily low concentrations, although efficacy toward some organisms (eg, E coli) is bimodal: some isolates are very susceptible (MIC < 0.01–0.5 mcg/mL), whereas the MIC for a number of other isolates is very high (> 64 mcg/mL). In general, MICs for most susceptible microbes, including E coli, Klebsiella, Proteus, P aeruginosa, and Staphylococcus have increased since the approval of the quinolones in the early 1990s.
Bacterial Resistance to Quinolones in Animals
Chromosomal mutational resistance to the original fluoroquinolones was considered to be low in frequency, and plasmid-mediated resistance nonexistent. However, resistance is increasingly being recognized, indicating that treatment based on results of bacteriologic culture and antimicrobial susceptibility testing is prudent. In general, cross-resistance should be anticipated among the more closely related members of this class.
Resistance mechanisms in gram-negative bacteria more commonly target DNA gyrase; emerging resistance is more often associated with changes in the GyrA compared to the GyrB subunit. In contrast, the primary target of resistance mechanisms in gram-positive organisms tends to be topoisomerase IV, followed by changes in DNA gyrase. Use of the drug selects for resistance.
High-level resistance (3–4 times the breakpoint MIC) generally reflects a second-step mutation that leads to changes in the amino acid sequence of subsequent topoisomerase targets. However, even with this second step of resistance, MICs are often below the resistant breakpoint range on which susceptibility testing is based. With the second increase in MIC, mutations in efflux pump regulators also emerge, causing a marked increase in expression. As a result, high-level, multidrug resistance emerges.
Another mechanism of resistance is the combined effect of increased efflux pumps and decreased porins that act in concert to decrease intracellular concentrations. Virulence of refractory mutants may not diminish.
Because fluoroquinolones have been extensively used in human and veterinary medicine since their development, resistance is continuing to emerge. When resistance emerges to one fluoroquinolone, it is likely to impact all fluoroquinolones. However, resistance to newer drugs (eg, gemifloxacin, trovafloxacin, gatifloxacin, and pradofloxacin) may be slower to emerge because of larger side chains that facilitate binding to either DNA gyrase or topoisomerase IV.
Antimicrobial Spectra of Quinolones in Animals
The fluoroquinolones are active against a wide range of gram-negative organisms and several gram-positive aerobes. These include E coli, Salmonella, Klebsiella, Enterobacter, Proteus, and generally P aeruginosa. The fluoroquinolones are active against intracellular pathogens, including Brucella spp. Quinolones also have substantial activity against Mycoplasma, Rickettsia, and Chlamydia spp. Obligate anaerobes tend to be resistant to most quinolones, as are most enterococci (formerly group D Streptococcus spp, Enterococcus faecalis, and Enterococcus faecium). Nocardia and atypical mycobacteria may also be susceptible.
Ciprofloxacin has the greatest activity against Pseudomonas spp, whereas enrofloxacin has lower MICs for gram-positive bacteria. Although enrofloxacin has evidence of in vitro susceptibility to Pseudomonas spp, resistance develops rapidly in clinical strains, and therefore it is unlikely to be effective. The newer third- and fourth-generation fluorinated quinolones (eg, pradofloxacin) are often characterized by an effective anaerobic spectrum.
A synergistic effect has been demonstrated in vitro between quinolones and beta-lactam antimicrobials, aminoglycosides, clindamycin, and metronidazole.
Among the few quinolones that have been studied to any degree in domestic animals, pharmacokinetic differences can be notable. Because of the physicochemical nature of the group, this is to be expected and some diversity should be anticipated.
Absorption of Quinolones in Animals
Quinolones are commonly administered PO in small animals, although forms of enrofloxacin are available for IV, IM, and SC administration. Absorption into the blood after IM or SC delivery is rapid; after administration PO, blood concentrations usually peak within 1–3 hours. Bioavailability is often >80% for most quinolones in small animals. In ruminating cattle, oral bioavailability of ciprofloxacin is exceedingly poor at 0%–20%. The bioavailability of ciprofloxacin after administration PO in dogs is variable and can be as little as 40%; it is 0%–20% in cats and horses. Enrofloxacin has better bioavailability in horses (60%), which is not affected by feeding. Marbofloxacin oral bioavailability is almost 100% in small animals and is 62% in horses.
The presence of food may delay absorption in monogastric animals, which may impact efficacy. Additionally, the use of antacids that contain divalent cations such as calcium or magnesium decreases fluoroquinolone bioavailability via chelation. Intramuscular bioavailability of the quinolones is nearly 100%; however, it should be noted that IM administration may be irritating to tissues. Oral administration of the injectable solutions has been performed in horses with a 65% bioavailability. However, the injectable solution is irritating to the oral mucosa and oral ulceration may result from its use.
Distribution of Quinolones in Animals
With few exceptions, the quinolones penetrate all tissues well and quickly due to their high lipid solubility. Tissue concentrations typically exceed plasma concentrations. Particularly high concentrations are found in organs of elimination (kidneys, liver, and bile); however, concentrations found in prostatic fluid, bone, ocular fluid, endometrium, and CSF are also quite notable. Most quinolones also cross the placental barrier. The apparent volume of distribution of most quinolones is large. The degree of plasma-protein binding is extremely variable, from ~10% for norfloxacin to 30% for enrofloxacin in dogs and >90% for nalidixic acid. In horses, plasma-protein binding is relatively stable across the fluoroquinolones, ranging between 21% and 28%. Fluorinated quinolones as a group accumulate in phagocytic WBCs.
Biotransformation of Quinolones in Animals
Some quinolones are eliminated unchanged (eg, ofloxacin), some are partially metabolized (eg, ciprofloxacin and enrofloxacin), and a few are completely degraded. Metabolites are sometimes active; enrofloxacin is de-ethylated to form ciprofloxacin. Characteristically, phase I reactions result in a number of primary metabolites (up to six have been described for some quinolones) that retain some antibacterial action. Conjugation with glucuronic acid then ensues, followed by excretion. In contrast, only ~10% of marbofloxacin is metabolized.
Excretion of Quinolones in Animals
Renal excretion is the major route of elimination for most quinolones. Both glomerular filtration and tubular secretion are involved. Urine concentrations are often high for 24 hours after administration, and crystals may form in concentrated acidic urine. The clinical relevance of this finding is unclear. In renal failure, clearance is impaired, and reductions in dose rates are essential. Biliary excretion of parent drug, as well as conjugates, is an important route of elimination in some cases (eg, ciprofloxacin, marbofloxacin, difloxacin, pefloxacin, and nalidixic acid). Quinolones are excreted in the milk of lactating animals, often at high concentrations that persist for an extended time interval.
Pharmacokinetic Values of Quinolones in Animals
The clearance and volume of distributions of quinolones vary among species, resulting in differences in plasma half-lives. Plasma concentrations attained are usually directly proportional to the dose administered but also vary with volume of distribution and oral bioavailability. Package inserts should be consulted for the peak serum concentration (Cmax) for those drugs approved for use in the target species.
Therapeutic Indications and Dose Rates
Quinolones are indicated for the treatment of local and systemic infections due to susceptible microorganisms, particularly against deep-seated infections and intracellular pathogens. Therapeutic success has been obtained in respiratory, intestinal, urinary, and skin infections, as well as in bacterial prostatitis, meningoencephalitis, osteomyelitis, and arthritis. Because of their lipid solubility and ability to accumulate in phagocytic WBCs, quinolones should be considered for use in infections located in tough-to-penetrate tissues. Therapeutic failure is likely to result with multidrug-resistant organisms; this coupled with their emerging adverse events should cause these drugs to be considered second tier for dogs and cats.
A selection of general dosages for some quinolones is listed in Dosages of Quinolones Dosages of Quinolones . The dose rate and frequency should be adjusted as needed for the individual animal and the MIC of the infecting organisms. Plasma drug concentrations should approximate 10 times the MIC of the infecting microbe. Higher doses are encouraged unless mitigating circumstances preclude the increase; in such instances, unless the MIC is very low, alternative antimicrobials might be considered. In dogs and cats, use ideally is based on bacteriologic culture and antimicrobial susceptibility testing when possible. Extralabel use of fluoroquinolones is prohibited in food-producing animals in the US.
Special Clinical Concerns
Adverse Effects and Toxicity of Quinolones in Animals
Although adverse effects with the older quinolones (nalidixic and oxolinic acids) were relatively common, the newer ones seem to be well tolerated. However, several adverse effects can limit use in selected species.
Retinal degeneration may occur acutely in cats, with the risk greatest for enrofloxacin at doses of 5 mg/kg or higher; because these drugs are concentration dependent, enrofloxacin probably should not be used in cats. The presence of renal disease may increase this risk. Pradofloxacin may be the least retinotoxic, followed by marbofloxacin and orbifloxacin; however, each of these appears to be safe in cats at doses that would be necessary to achieve targeted Cmax:MIC ratios for susceptible organisms.
The blood-retinal barrier is composed of capillary endothelial cell tight junctions, retinal pigment epithelial cell tight junctions, and a variety of transporters including ABCG2. Retinal damage occurs due to changes in the ABCG2 transporter leading to accumulation of photoreactive fluoroquinolones in the retina. Once the retina is exposed to light, accumulation of these photoreactive fluoroquinolones leads to the generation of reactive oxygen species. Reactive oxygen species such as hydroxyl radical, singlet oxygen, superoxide, and hydrogen peroxide then attack cellular lipid membranes and cause tissue damage, retinal degeneration, and blindness.
In cats, ABCG2 has four feline-specific amino acid changes that lead to a functional defect in this transport protein compared to human ABCG2, resulting in retinal damage and degeneration in cats that is not observed in other species. However, pharmacological inhibition of ABCG2 in other species may result in retinal degeneration when fluoroquinolones are concurrently administered. Enrofloxacin doses >5 mg/kg/day have been associated with retinal degeneration in cats.
Quinolones tend to be neurotoxic, and convulsions can occur at high doses due to gamma-aminobutyric acid (GABA) receptor antagonism. Rapid IV administration of high doses of enrofloxacin in horses results in transient neurologic clinical signs that include excitability and seizure-like activity. Vomiting and diarrhea may develop with fluoroquinolones. Both ciprofloxacin and moxifloxacin have been associated with potentially fatal antimicrobial-induced colitis. Dermal reactions and photosensitization have been described in people, but the occurrence seems low. Hemolytic anemia has also occurred.
Administering large doses of quinolones for any length of time during pregnancy may result in embryonic loss and maternal toxicity. Because high prolonged dosages in growing dogs and foals have produced cartilaginous erosions leading to permanent lameness, excessive use of quinolones should be avoided in immature animals. The mechanism for cartilage damage is incompletely elucidated but may be via the chelation of magnesium in cartilage, which, due to the poor vascular supply to cartilage, is not readily replaced. Magnesium chelation leads to decreased cell-matrix interaction in the chondrocytes, leading to radical damage, apoptosis, and tissue damage. Pradofloxacin is labeled for use in dogs in some countries but is not approved for dogs in the US due to its adverse effects of bone marrow suppression and arrhythmogenic potential.
In 2008, the FDA added a black-box warning for seven fluoroquinolones that increased the risk of tendinitis and tendon rupture.
An emerging toxicity associated with fluoroquinolones is mitotoxicity (ie, damage to mitochondrial topoisomerase or other mitochondrial structures). Mitochondrial effects may not emerge until some time after fluoroquinolone treatment is instituted. Although the entirety of the clinical impact of this toxicity is not known, nor is its relevance to veterinary medicine, adverse events ranging from neurologic to musculoskeletal to cardiovascular may ultimately be attributed to this effect.
Interactions With Quinolones in Animals
The fluorinated quinolones may be involved in a number of drug interactions. Antacids or other drugs containing multivalent cations and sucralfate appear to interfere with the GI absorption of the quinolones. Nitrofurantoin impairs the efficacy of quinolones if used concurrently for urinary tract infections. Quinolones inhibit the biotransformation of methylxanthines, with theophylline being the most clinically relevant but also including caffeine and theobromine. This inhibition leads to increased serum concentrations of methylxanthines that can result in CNS and cardiac toxicity. This is a class effect, with the risk varying among the fluoroquinolones in people. A similar ranking of risk is not available for veterinary medicine. In people, cyclosporine concentrations may also be increased via concurrent administration with fluoroquinolones, leading to prolonged and potentially toxic plasma concentrations.
Effects of Quinolones on Laboratory Tests in Animals
Quinolones may increase activities of AST, ALT, and ALP as well as BUN concentration. Urine glucose test results may be altered, and urinalysis may reveal needle-shaped crystals.
Drug Withdrawal and Milk Discard Times of Quinolones in Animals
Although prolonged tissue residues for most fluoroquinolones are not anticipated, withdrawal times are not available for most of the quinolones because they are not approved for use in food-producing animals in most countries. In the US, fluoroquinolones are prohibited from extralabel drug use (ELDU) in all food-producing animal species. This includes deviations from the approved dose, treatment duration, frequency, indication, or administration route on the product label; the use of a product in an unapproved species or animal production class; and the use of the product for the purpose of disease prevention. Withdrawal times can vary between products, and therefore it is imperative to follow the label meat and milk withdrawal times for the particular product used.
For More Information
Cushnie, T.P.T., O’Driscoll, N.H. & Lamb, A.J. Morphological and ultrastructural changes in bacterial cells as an indicator of antibacterial mechanism of action. Cell. Mol. Life Sci. 73, 4471–4492 (2016). https://doi.org/10.1007/s00018-016-2302-2