Pradofloxacin: a feline fluoroquinolone part 1

• Pradofloxacin: a feline fluoroquinolone – part 2

Pradofloxacin (PRA) is a third generation veterinary fluoroquinolone with enhanced antibacterial activity, indicated for the treatment of skin and soft tissue infections in cats.

This article will discuss data synthesis on chemical structure, pharmacokinetics, antimicrobial activity and mode of action of PRA, while part two will look at indications, contraindications, drug interactions, resistance to PRA and toxicity in cats.

Chemical structure

PRA is distinguished from enrofloxacin by two substituents fixed on the quinolone nucleus at the C₇ and C₈ positions (Figure 1):

• Pyrrolidino-piperidine – a bicyclic amine located at position C₇. Due to the two chiral carbon atoms of the pyrrolidino-piperidine group, four enantiomeric forms can exist. The S,S-pyrrolidino-piperidine enantiomer is chosen because of its high antimicrobial potency¹. The bulky side chain at C₇ diminishes the likelihood of bacterial resistance in wild-type bacterial strains and also increases anti-anaerobic activity².
• A cyano group attached to the C₈ – essential for high activity against Staphylococcus aureus and Escherichia coli¹.

Physical and chemical properties

Lipid solubility (log P) is a factor determinant that enables drugs to cross cell membranes by passive diffusion. The log P octanol/water values amounted to 1.4 for PRA³.

PRA is very stable to heat and acidic hydrolysis⁴. Under alkaline conditions, the 7-hydroxy-8-cyano-fluoroquinoline carboxylic acid is the main degradation product⁴.

Pharmacokinetics

PRA can exist in several crystal forms (polymorphism)^4,5 and changes in the crystal form often change important properties. PRA trihydrate is not converted to other crystal forms in the course of long-lasting storage and it is possible to prepare stable formulations^4,5,6.

Absorption

In cats, after oral therapeutic doses of the suspension, PRA is rapidly absorbed from the duodenum and jejunum, resulting in high peak serum concentration of 2.1µg/ml within one hour^4,6. The bioavailability of PRA oral suspension is close to 60%⁶. The bioavailability is reduced in fed cats compared to fasted animals⁶. Mean peak serum concentrations (C_maxs) are reduced by 53% by food^4,5. PRA plasma concentrations increase linearly with dose^4,6.

Distribution

Plasma protein binding is moderate (29% to 31%)^4,7. The volume of distribution in cats is high (greater than 1l/kg), indicating a widespread distribution and good tissue penetration^4,6.

In phagocytic cells, C_maxs are reported to be six to eight-fold the plasma concentration⁸. In case of obligate intracellular pathogens (such as Chlamydophila and Mycoplasma), high concentrations of PRA within phagocyte cells are essential. In dogs, PRA is also concentrated in the cartilage (Table 1)⁹. Small quantities are distributed to the cerebrospinal fluid⁹.

After oral administration in cats of a single dose of PRA at 5mg/kg, mean maximum concentration (C_max) of PRA in saliva and tear fluid reached 6.3µg/ml and 13.4µg/ml, respectively – exceeding the concentrations in serum several times within the first hour after administration¹⁰. Following the peak, concentrations decreased quickly in tear fluid and, after 10 hours, PRA could not be detected in tear fluid¹⁰.

In saliva, PRA is still detectable after 24 hours¹⁰. Peak concentrations are due to an active transport mechanism of PRA into these fluids10. PRA is probably secreted into saliva and tear fluid by the P-glycoprotein/multidrug resistance protein one transporter¹⁰.

Biotransformation

In cats, PRA is conjugated to glucuronic acid in significant amounts (up to 50%)^4,6. Negligible levels of mono-metabolites and bishydroxylation metabolites are detected^4,6.

Elimination

A total of 10% of PRA administered orally is excreted via the kidneys^4,6. Renal excretion by glomerular filtration is rapid, with approximately 70% of the fraction of PRA recovered in urine within 24 hours^4,6.

Unchanged PRA and glucuronide conjugates are the major excretion products^4,6. The elimination half-life in cats is long, averaging seven hours with the oral suspension, which allows a once-a-day dosage administration^4,6. PRA is cleared from the body at 0.24L/h/kg^4,6.

Pharmacokinetic/pharmacodynamic parameters

For fluoroquinolones, the pharmacokinetic/pharmacodynamic parameters that best correlate with clinical efficacy are the ratios C_max/minimum inhibitory concentration (MIC) greater than 10 and area under the plasma concentration time curve (AUC)_0-24/MIC greater than 125⁹.

C_max/MIC and AUC_0-24/MIC ratios of PRA calculated for the dose of PRA 3mg/kg orally and based on MIC₉₀ values published by Schink et al (2013) for Staphylococcus pseudintermedius, Pasteurella multocida and E coli are indicated in Table 2^9,11.

Antimicrobial activity

PRA has a broad activity spectrum including the following.

Gram-positive bacteria

The MIC₉₀ values (MIC at which 90% of the isolates are inhibited) against S aureus, Staphylococcus felis, S pseudintermedius – the most common causative agent in canine pyoderma – and Streptococcus canis isolated from skin infections (wounds and abscesses) in cats in a US field study (2008 to 2009) are listed in Table 3⁵.

Gram-negative bacteria

In a study, 908 bacterial pathogens from defined infections of dogs and cats were tested for their susceptibility to PRA. Pasteurella multocida, isolated frequently from acute skin and soft tissue infections following a cat or dog fighting bite or scratch, Bordetella bronchiseptica and E coli, exhibit low PRA MIC₉₀ values of less than or equal to 0.25μg/ml¹¹. Solely Proteus species and Pseudomonas aeruginosa had higher MIC₉₀ values of greater than or equal to 4μg/ml¹¹.

PRA is also active against intracellular pathogens, such as Mycoplasma species and Chlamydophila species. PRA has anti-Mycoplasma haemofelis effects similar to those of doxycycline – the treatment of choice – and may be more effective at long-term M haemofelis organism clearance than doxycycline^12,13. Some doxycycline formulations (particularly the hydrate) have been associated with oesophagitis and oesophageal strictures in cats¹³. PRA is also active against Chlamydophila felis, causing upper respiratory tract disease and conjunctivitis in cats^14,15.

Anaerobic bacteria

PRA shows good activity against various anaerobic bacteria, such as Porphyromonas species, Provetella species, Fusobacterium species, Bacteroides species and Clostridium species. Porphyromonas gingivalis is a major aetiological agent in the initiation and progression of severe forms of periodontal disease.

In a total of 630 isolates, 320 of Prevotella species and 310 of Porphyromonas species isolated from subgingival samples from dogs exhibiting clinical periodontal disease in five European countries, overall MIC₉₀ of PRA was 0.25μg/ml¹⁶. No differences in activity were apparent against Porphyromonas and Prevotella isolates¹⁶. In addition, PRA showed activity against metronidazole-resistant isolates¹⁶.

Rapidly growing mycobacteria

PRA has effective in vitro activity against rapidly growing mycobacteria isolates, such as Mycobacterium smegmatis cluster (n=64) and Mycobacterium fortuitum cluster (n=17) collected from feline and canine patients¹⁷.The MIC₉₀ values of PRA are 0.25µg/ml and 0.125µg/ml, respectively¹⁷.

PRA appears to have limited therapeutic potential for most Nocardia infections – the MIC₉₀ of Nocardia nova isolates is 4µg/ml¹⁷. The Committee for Medicinal Products for Veterinary Use agreed the following microbiological breakpoints for PRA^4,6:

• susceptible – less than or equal to 1μg/ml
• resistant – greater than or equal to 2μg/ml

Mode of action

PRA exhibits bactericidal activity. The killing action is of the concentration-dependent type, the rapidity and extent of bacterial killing at the locus of infection are directly proportional to the drug concentration. PRA is a dual-targeting fluoroquinolone¹⁸. The first is topoisomerase type II (DNA gyrase) in Gram-negative bacteria and the second is topoisomerase type IV in Gram-positive bacteria¹⁹. For old fluoroquinolones, DNA gyrase is the principal target²⁰.

PRA is the unique veterinary fluoroquinolone characterised as being a dual-targeting compound with similar affinities for both targets¹⁹. PRA binds different molecular sites within these enzymes, thereby decreasing the cross-resistance between these agents and the older fluoroquinolones¹⁹.

Post-antibiotic effect

PRA can produce significant post-antibiotic effect (PAE), and post-antibiotic sub-MIC (PA-SME) effects on susceptible bacteria, suppressing bacterial growth even after antimicrobial concentrations fall. In vitro, PAE of PRA on S aureus and E coli is two to two-and-a-half hours²¹.

In vivo (in transudate of dogs implanted SC with tissue cages), PA-SME of PRA on S aureus is 17 hours to 18 hours at half MIC. At three-quarters MIC, killing could be observed up to 10 hours, followed by bacteriostatic activity up to 24 hours²¹.

• Pradofloxacin: a feline fluoroquinolone – part 2