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Pharmacology: Pharmacodynamics: Mechanism of action: Moxifloxacin is a 8-methoxy-fluoroquinolone antibiotic with a broad spectrum of activity and bactericidal action. Moxifloxacin has in vitro activity against a wide range of gram-positive and gram-negative organisms, anaerobes, acid-fast bacteria, and atypicals eg. Mycoplasma spp., Chlamydia spp. and Legionella spp.
Moxifloxacin is effective against ß-lactam and macrolide resistant bacteria. Studies in animal models of infection have demonstrated the high in vivo activity.
Mechanism of resistance: Resistance mechanisms that inactivate penicillins, cephalosporins, aminoglycosides, macrolides and tetracyclines do not interfere with the antibacterial activity of moxifloxacin. There is no cross resistance between moxifloxacin and these agents. Plasmid-mediated resistance has not been observed to date. It appears that the C8-methoxy moiety contributes to enhanced activity and lower selection of resistant mutants of gram-positive bacteria compared to the C8-H moiety. The presence of the bulky bicycloamine substituent at the C-7 position prevents active efflux, a mechanism of fluoroquinolone resistance.
In vitro studies have demonstrated that resistance to moxifloxacin develops slowly by multiple step mutations. A very low overall frequency of resistance was demonstrated (10-7 - 10-10). Serial exposure of organisms to sub-MIC concentrations of moxifloxacin showed only a small increase in MIC values.
Cross resistance among quinolones has been observed. However, some gram-positive and anaerobic organisms resistant to other quinolones are susceptible to moxifloxacin.
Effect on the intestinal flora in humans: In two volunteer studies, the following changes in the intestinal flora were seen following oral dosing with moxifloxacin. E. coli, Bacillus spp., Bacteroides vulgatus, Enterococci, and Klebsiella spp. were reduced, as were the anaerobes Bifidobacterium, Eubacterium, and Peptostreptococcus. These changes returned to normal within two weeks. Clostridium difficile toxin was not found.
Moxifloxacin has been shown to be active against most strains of the following microorganisms both in vitro and in clinical infections as described in the "Indications" section. (See Tables 1 to 4.)
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Click on icon to see table/diagram/imageThe frequency of acquired resistance may vary geographically and with time for certain species. Local area information on resistance of organisms is desirable, particularly when treating severe infections. The previously mentioned information is provided as a guide on the probability of an organism being susceptible to moxifloxacin.
Comparison of PK/PD surrogates for intravenous and oral administration of a 400 mg moxifloxacin single dose. In patients requiring hospitalisation AUC/MIC90 parameters greater than 125 and Cmax / MIC90 of 8 - 10 is predictive for clinical cure (Schentag). In outpatients these surrogate parameters are generally smaller, i.e. AUC/MIC90 greater than 30-40 (Dudley and Ambrose).
The following table provides the respective PK/PD surrogates for intravenous and oral administration of 400 mg moxifloxacin calculated from single dose data: (See Table 5.)
Click on icon to see table/diagram/imagePharmacokinetics: Absorption and Bioavailability: Following oral administration moxifloxacin is rapidly and almost completely absorbed. The absolute bioavailability amounts to approximately 91%.
Pharmacokinetics are linear in the range of 50 - 800 mg when given as a single dose and up to 600 mg when given once daily over 10 days. Following a 400 mg oral dose peak concentrations of 3.1 mg/l are reached within 0.5 - 4 h post administration. Peak and trough plasma concentrations at steady-state (400 mg once daily) were 3.2 and 0.6 mg/l, respectively. At steady-state the exposure within the dosing interval is approximately 30% higher than after the first dose.
Distribution: Moxifloxacin is distributed to extravascular spaces rapidly; after a dose of 400 mg an AUC of 35 m∙gh/l is observed. The steady-state volume of distribution (Vss) is approximately 2 l/kg. In vitro and ex vivo experiments showed a protein binding of approximately 40 - 42% independent of the concentration of the drug. Moxifloxacin is mainly bound to serum albumin.
The following peak concentrations (geometric mean) were observed following administration of a single oral dose of 400 mg moxifloxacin: (See Table 6.)
Click on icon to see table/diagram/imageBiotransformation: Moxifloxacin undergoes Phase II biotransformation and is excreted via renal and biliary/faecal pathways as unchanged drug as well as in the form of a sulpho-compound (M1) and a glucuronide (M2). M1 and M2 are the only metabolites relevant in humans, both are microbiologically inactive.
In clinical Phase I and in vitro studies no metabolic pharmacokinetic interactions with other drugs undergoing Phase I biotransformation involving cytochrome P450 enzymes were observed. There is no indication of oxidative metabolism.
Elimination: Moxifloxacin is eliminated from plasma with a mean terminal half life of approximately 12 hours. The mean apparent total body clearance following a 400 mg dose ranges from 179 to 246 ml/min. Renal clearance amounted to about 24 - 53 ml/min suggesting partial tubular reabsorption of the drug from the kidneys.
After a 400 mg dose, recovery from urine (approximately 19% for unchanged drug, approximately 2.5% for M1, and approximately 14% for M2) and faeces (approximately 25% of unchanged drug, approximately 36% for M1, and no recovery for M2) totalled to approximately 96%.
Concomitant administration of moxifloxacin with ranitidine or probenecid did not alter renal clearance of the parent drug.
Geriatric patients: Pharmacokinetics of moxifloxacin are not affected by age.
Gender: There was a 33% difference in the pharmacokinetics (AUC, Cmax) of moxifloxacin between male and female subjects. Drug absorption was unaffected by gender. These differences in the AUC and Cmax were attributable to the differences in body weight rather than gender. They are not considered as clinically relevant.
Renal impairment: The pharmacokinetic properties of moxifloxacin are not significantly different in patients with renal impairment (Including creatinine clearance < 30 ml/min/1.73m2) and in patients on chronic dialysis i. e. hemodialysis and continuous ambulatory peritoneal dialysis.
Hepatic impairment: On the basis of the pharmacokinetic studies carried out so far in patients with liver failure (Child Pugh A, B), it is not possible to determine whether there are any differences compared with healthy volunteers. There is insufficient experience in the clinical use of moxifloxacin in patients with severe hepatic impairment (Child Pugh C).
Ethnic differences: Possible interethnic differences were examined in Caucasian, Japanese, Black and other ethnic groups. No clinically relevant interethnic differences in pharmacokinetics could be detected.
Children and adolescents: Pharmacokinetics of moxifloxacin were not studied in paediatric patients.
Toxicology: Preclinical safety data: Effects on the haematopoetic system (slight decreases in the number of erythrocytes and platelets) were seen in rats and monkeys. As with other quinolones, hepatotoxicity (elevated liver enzymes and vacuolar degeneration) was seen in rats, monkeys and dogs. In monkeys CNS toxicity (convulsions) occurred. These effects were seen only after treatment with high doses of moxifloxacin or after prolonged treatment.
Moxifloxacin, like other quinolones, was genotoxic in in vitro tests using bacteria or mammalian cells. Since these effects can be explained by an interaction with the gyrase in bacteria and - at higher concentrations - by an interaction with the topoisomerase II in mammalian cells, a threshold concentration for genotoxicity can be assumed. In in vivo tests, no evidence of genotoxicity was found despite the fact that very high moxifloxacin doses were used. Thus, a sufficient margin of safety to the therapeutic dose in man can be provided. Moxifloxacin was non-carcinogenic in an initiation-promotion study in rats.
Many quinolones are photoreactive and can induce phototoxic, photomutagenic and photocarcinogenic effects. In contrast, moxifloxacin was proven to be devoid of phototoxic and photogenotoxic properties when tested in a comprehensive programme of in vitro and in vivo studies. Under the same conditions other quinolones induced effects.
At high concentrations, moxifloxacin is an inhibitor of the rapid component of the delayed rectifier potassium current of the heart and may thus cause prolongations of the QT interval. Toxicological studies performed in dogs using oral doses of ≥ 90 mg/kg leading to plasma concentrations ≥ 16 mg/l caused QT prolongations, but no arrhythmias. Only after very high cumulative intravenous administration of more than 50fold the human dose (> 300 mg/kg), leading to plasma concentrations of ≥ 200 mg/l (more than 40fold the therapeutic level), reversible, non-fatal ventricular arrhythmias were seen.
Quinolones are known to cause lesions in the cartilage of the major diarthrodial joints in immature animals. The lowest oral dose of moxifloxacin causing joint toxicity in juvenile dogs was four times the maximum recommended therapeutic dose of 400 mg (assuming a 50 kg bodyweight) on a mg/kg basis, with plasma concentrations two to three times higher than those at the maximum therapeutic dose.
Toxicity tests in rats and monkeys (repeated dosing up to six months) revealed no indication regarding an oculotoxic risk. In dogs, high oral doses (≥ 60 mg/kg) leading to plasma concentrations ≥ 20 mg/l caused changes in the electroretinogram and in isolated cases an atrophy of the retina.
Reproductive studies performed in rats, rabbits and monkeys indicate that placental transfer of moxifloxacin occurs. Studies in rats (p.o. and i.v.) and monkeys (p.o.) did not show evidence of teratogenicity or impairment of fertility following administration of moxifloxacin. A slightly increased incidence of vertebral and rib malformations was observed in foetuses of rabbits but only at a dose (20 mg/kg i.v.) which was associated with severe maternal toxicity. There was an increase in the incidence of abortions in monkeys and rabbits at human therapeutic plasma concentrations. In rats, decreased foetal weights, an increased prenatal loss, a slightly increased duration of pregnancy and an increased spontaneous activity of some male and female offspring was observed at doses which were 63 times the maximum recommended dose on a mg/kg basis with plasma concentrations in the range of the human therapeutic dose.
