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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 β-lactam and macrolide resistant bacteria. Studies in animal models of infection have demonstrated the high in vivo activity.
Resistance: Resistance mechanisms which 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/Uses. (See Tables 1 and 2.)
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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 3.)
Click on icon to see table/diagram/imagePharmacokinetics: Absorption and bioavailability: Following oral administration moxifloxacin is absorbed rapidly and almost completely. The absolute bioavailability amounts to approx. 91%". Pharmacokinetics are linear in the range of 50 - 1200 mg single dose and up to 600 mg once daily dosing over 10 days. Steady state is reached within 3 days. Following a 400 mg oral dose peak concentrations of 3.1 mg/l are reached within 0.5 - 4 hours post application. Peak and trough plasma concentrations at steady state (400 mg once daily) were 3.2 and 0.6 mg/l, respectively.
Concomitant administration of moxifloxacin together with food slightly prolongs the time to reach peak concentrations by approximately 2 hours and slightly reduced peak concentrations by approximately 16%. Extent of absorption remained unchanged. As AUC/MIC is most predictive for antimicrobial efficacy of quinolones, this effect is clinically not relevant. Therefore, Moxifloxacin can be administered independently from meals. After a single 400 mg intravenous 1 hour infusion peak concentrations of approximately 4.1 mg/L were reached in the plasma at the end of infusion which corresponds to a mean increase of approx. 26 % relative to the oral application. Exposure to drug in terms of AUC at a value of approximately 39 mg*h/L is only slightly higher compared to the exposure after oral administration (35 mg*h/L) in accordance with the absolute bioavailability of approximately 91%.
Following multiple intravenous dosing (1hour infusion), peak and trough plasma concentrations at steady state (400 mg once daily) were between 4.1 to 5.9 and 0.43 to 0.84 mg/L respectively. At steady-state the exposure to drug within the dosing interval is approximately 30 % higher than after the first dose. In patients mean steady state concentrations of 4.4 mg/l were observed at the end of a 1 hour infusion.
Distribution: Moxifloxacin is distributed very rapidly to extravascular spaces. Exposure to drug in terms of AUC (AUCnorm = 6 kg*h/l) is high with a volume of distribution at steady state (Vss ) of approximately 2 L/kg. In saliva peak concentrations higher than those of plasma may be reached. In in vitro and ex-vivo experiments over a range of 0.02 to 2 mg/l a protein binding of approximately 45 % independent from the concentration of the drug was determined. Moxifloxacin is mainly bound to serum albumin. Due to this low value high free peak concentrations > 10 x MIC are observed.
Moxifloxacin reaches high concentrations in tissues like lung (epithelial fluid, alveolar macrophages, biotic tissue), the sinuses (maxillary and ethmoid sinus, nasal polypi) and inflamed lesions (cantharide blister fluid) where total concentrations exceeding those of the plasma concentrations are reached. High free drug concentrations are measured in interstitial body water (saliva, intramuscular, subcutaneous). In addition, high drug concentrations were detected in abdominal tissues and fluids and female genital tract. (See Tables 4 and 5.)
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Click on icon to see table/diagram/imageThe peak concentrations and site vs. plasma concentration ratios for various target tissues yielded comparable results for both modes of drug administration after a single dose of 400 mg moxifloxacin.
Metabolism: Moxifloxacin undergoes Phase II biotransformation and is excreted via renal and biliary/faecal pathways as unchanged drug as well as in form of a sulfo-compound (M1) and a glucuronide (M2). M1 and M2 are the only metabolites relevant in humans, both are microbiologically inactive. Neither in in vitro nor in clinical Phase I studies metabolic pharmacokinetic interactions with other drugs undergoing Phase I biotransformation involving Cytochrome P-450 enzymes were observed. Independent from the route of administration the metabolites M1 and M2 are found in the plasma at concentrations lower than the parent drug. Preclinical investigations adequately covered both metabolites thus excluding potential implications with respect to safety and tolerability.
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. Concomitant administration of ranitidine and probenecid did not alter renal clearance of the drug (see table as follows). Mass balance of the mother compound and Phase II metabolites of moxifloxacin yielded an almost complete recovery of approximately 96-98% independent from the route of administration with no indication of oxidative metabolism. A detailed overview of the mass balance according to elimination pathways (renal vs non-renal, metabolic vs. non-metabolic) and mode of application is given in the table as follows. Recovery of a 400 mg single dose (arithmetric mean ± standard deviation (SD). (See Table 6.)
Click on icon to see table/diagram/imageGeriatric 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.
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.
Patients with renal impairment: The pharmacokinetics of moxifloxacin are not significantly changed by renal impairment (including creatinine clearance < 30 ml/min/1.73m2) and in patients on chronic dialysis i. e. hemodialysis and continuous ambulatory peritoneal dialysis.
Patients with hepatic impairment: Moxifloxacin plasma concentrations of patients with mild to severe hepatic impairment (Child Pugh A to C) did not reveal clinically relevant differences compared to healthy volunteers or patients with normal hepatic function, respectively. There is no experience in patients with severe hepatic impairment (Child Pugh C).
Toxicology: Preclinical Safety Data: In a local tolerability study performed in dogs, no signs of local intolerability were seen when moxifloxacin was administered intravenously. After intra-arterial injection inflammatory changes involving the peri-arterial soft tissue were observed suggesting that intra-arterial administration of moxifloxacin should be avoided.
Carcinogenicity, Mutagenicity: Moxifloxacin, like other quinolones, was genotoxic 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.
ECG: At high concentrations, moxifloxacin is an inhibitor 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 50 fold the human dose (> 300 mg/kg), leading to plasma concentrations of ≥200 mg/l (more than 30 fold the therapeutic level after intravenous administration), reversible, non-fatal ventricular arrhythmias were seen.
Arthrotoxicity: Quinolones are known to cause lesions in the cartilage of the major diarthodial joints in immature animals. The lowest oral dose of moxifloxacin causing joint toxicity in juvenile dogs was four times maximum recommended therapeutic dose (400 mg/50 kg person) on a mg/kg basis, with plasma concentrations two to three times higher than those at the recommended therapeutic dose.
Reprotoxicity: Reproductive studies performed in rats, rabbits and monkeys indicate that placental transfer of moxifloxacin occurs. Studies in rats (per os and i.v.) and monkeys (per os) did not show evidence of teratogenicity or impairment of fertility following administration of moxifloxacin. Skeletal malformations were observed in rabbits that had been treated with an intravenous dose of 20 mg/kg. This study result is consistent with the known effects of quinolones on skeletal development (see Use in Pregnancy & Lactation). There was an increase in the incidence of abortions in monkeys and rabbits at human therapeutic 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.
