International Journal of Antimicrobial Agents 12 (1999) 107 – 114
Original article
Comparative efficacies of levofloxacin and ciprofloxacin against Streptococcus pneumoniae in a mouse model of experimental septicaemia Cyprian O. Onyeji a, Khanh Q. Bui a,b, Robert C. Owens Jr. d, David P. Nicolau a,b, Richard Quintiliani a,c, Charles H. Nightingale a,c,* a
Department of Pharmacy Research, Hartford Hospital, Hartford, CT 06102, USA b Di6ision of Infectious Diseases, Hartford Hospital, Hartford, CT 06102, USA c Office of Research, Hartford Hospital, Hartford, CT 06102, USA d Department of Clinical Pharmacy Ser6ices, Medical Center of Delaware, Newark, DE 19718, USA Received 1 September 1998; accepted 9 October 1998
Abstract The in vivo efficacies of levofloxacin and ciprofloxacin were compared against three clinical isolates of Streptococcus pneumoniae, using a mouse protection model. Two strains (SP 22 and SP 28) were penicillin-sensitive while one strain (SP 46) was penicillin-resistant. Each strain had identical susceptibility to both drugs. Using mice with renal impairment induced by uranyl nitrate injection, the elimination half-life of each antibiotic was prolonged to approximate human pharmacokinetic profiles of the drugs. The dosing regimen of each drug that yielded serum levels in mice which mimic human therapeutic concentrations of the drugs, were designed. One hour after intraperitoneal inoculation with minimum lethal dose of each strain, either levofloxacin at a dosing regimen of 10.6 mg/kg every 8 h or ciprofloxacin at 9.5 mg/kg every 8 h was subcutaneously administered for a total of six or 15 doses. In treatment, monitored daily for 5 – 8 days, levofloxacin resulted in higher survival compared with ciprofloxacin for the three strains. For example, percent survival following levofloxacin treatment recorded at day 4 postinfection with SP 22, SP 28 and SP 46 were 41, 90 and 30%, respectively, while the corresponding values after ciprofloxacin treatment were 27, 75 and 16%, respectively. However, statistical analysis did not reveal a significant difference (p\ 0.05). The lack of significant difference observed in the efficacies of both drugs reflected the comparability of their 24-h AUC/MIC ratios. It is suggested that, with some strains of S. pneumoniae, the efficacy of levofloxacin may be equivalent to that of ciprofloxacin in the treatment of systemic pneumococcal infections caused by susceptible strains of the organism. © 1999 Published by Elsevier Science B.V. and International Society of Chemotherapy. All rights reserved. Keywords: Comparative efficacy; Levofloxacin; S. pneumoniae
1. Introduction Since their introduction, the early fluoroquinolones, such as ciprofloxacin and ofloxacin, have a proven record in the treatment of gram-negative infections. Although ciprofloxacin has excellent activity against most Enterobacteriaceae and Pseudomonas aeruginosa [1 – 3], it has only modest activity against gram-positive * Correspondent author. Tel.: +1-860-545-2865; Fax: +1-860545-5112. E-mail address:
[email protected] (C.H. Nightingale)
bacteria, especially, Streptococcus pneumoniae. The MIC90 values for ciprofloxacin against S. pneumoniae are in the range of 1–4 mg/l [4] and, occasionally, even higher MICs are encountered [5]. Because of the modest activity of ciprofloxacin against gram-positive bacteria, there has been continued interest in improving the antibacterial activities of the fluoroquinolones against these organisms. Levofloxacin, the optically active L-isomer of the racemate ofloxacin, is twice as active as ofloxacin and ciprofloxacin against gram-positive organisms such as
0924-8579/99/$ - see front matter © 1999 Published by Elsevier Science B.V. and International Society of Chemotherapy. All rights reserved. PII: S 0 9 2 4 - 8 5 7 9 ( 9 8 ) 0 0 0 8 7 - 9
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Staphylococcus aureus [1,3,6,7] and S. pneumoniae [1,7– 9]. In animal infection models using S. pneumoniae, oral levofloxacin was found to be more protective than oral ciprofloxacin [7,10]. The greater effectiveness of levofloxacin observed in these earlier studies may be related to the higher oral bioavailability of this drug (100%) as compared with that (78%) of ciprofloxacin [11]. Since both levofloxacin and ciprofloxacin have intravenous formulations, it is possible to clarify the extent to which the demonstrated higher in vivo efficacy of oral levofloxacin can be attributed to its higher bioavailabilty compared with that of ciprofloxacin. There is a desire to find non-b-lactam agents and non-macrolide drugs that can be used for treating pneumococcal infections because there has been a world-wide increase in the prevalence of pneumococci with diminished susceptibility to penicillins, macrolides and even cephalosporins [12 – 14]. Since clinical failures in the treatment of pneumococcal infections have occurred with ciprofloxacin [15,16], it appears worthwhile to test the suitability of levofloxacin as an alternative therapeutic agent. The purpose of the present study was to evaluate and compare the in vivo efficacies of levofloxacin and ciprofloxacin against strains of S. pneumoniae, by using a mouse protection model with experimental septicaemia. Our aim was to determine the survival of mice after intraperitoneal challenge with S. pneumoniae and treatment with the antibiotics at dosing regimens designed to yield serum levels in mice that simulate human therapeutic concentrations of the drugs.
2. Materials and methods
2.1. Bacteria, antibiotics and animals Three clinical isolates of S. pneumoniae were used: two penicillin-sensitive strains designated SP 22 and SP 28, and one penicillin-resistant strain designated SP 46. The isolates were stored at − 70°C in skim milk and were subcultured onto Trypticase blood soy agar (BBL Microbiology systems, Cockeysville, MD) at 37°C for 24 h prior to use. Levofloxacin and ciprofloxacin powders were obtained from Ortho Pharmaceutical Corporation (Raritan, NJ) and Bayer Corporation (West Haven, CT), respectively. The intravenous formulations of levofloxacin (Ortho Pharmaceutical) and that of ciprofloxacin (Bayer) were purchased. For in vitro studies, the powders were dissolved in sterile water while the intravenous preparations were used for in vivo studies. The MICs of the antibiotics for the test isolates were determined in Mueller – Hinton broth (Difco Laboratories, Detroit, MI) supplemented with lysed horse blood, by a standard microtiter dilution method [17].
Swiss albino female mice weighing 20–23 g were obtained from Harlan Sprague–Dawley (Indianapolis, IN).
2.2. Pharmacokinetic studies and dosing regimen determination 2.2.1. Renal impairment, drug administration and sample collection Since mice eliminate drugs much more rapidly than humans [18], preliminary pharmacokinetic studies were performed in uninfected mice to determine the optimal dose of uranyl nitrate necessary to impair renal function so as to simulate in mice the kinetic profiles of levofloxacin and ciprofloxacin observed in humans. Groups of mice (24 mice per group) were given single intraperitoneal administrations of various doses of unranyl nitrate (1, 2.5, 5 and 10 mg/kg). After 72 h, the mice in each group received a single 0.2 ml subcutaneous injection of either ciprofloxacin (20 mg/kg) or levofloxacin (10 mg/kg). Blood was obtained from the mice by intracardiac puncture at 0.25, 0.5, 1, 2, 4, 6, 8 and 12 h, following ciprofloxacin administration. Blood sampling from the animals that received levofloxacin were made at 0.25, 0.5, 1, 2, 4, 8, 12 and 24 h postdosing. A minimum of three mice were used per time point. The blood was centrifuged at 2000×g for 10 min and the serum was transferred into polypropylene tubes and stored at − 70°C until analysed. 2.2.2. Sample analysis An earlier reported high-performance liquid chromatographic method (HPLC) [19] developed for analysis of the quinolones in human serum samples was modified and employed for the assay of the mouse serum drug samples. Sample treatment involved the addition of 50 ml of 10 mg/l solution of internal standard (pipemidic acid, Sigma, St Louis, MO) and 3.5 ml of dichloromethane to a 100-ml sample contained in polypropylene tube. After vortexing and centrifuging the mixture, the dichloromethane layer was back-extracted with 200 ml of 0.1 M NaOH. 20–40 ml of the upper aqueous layer was injected into the HPLC through a Waters autosampler (model 717 plus, Waters Associates, Milford, MA) coupled to a Waters chromatographic pump (model 6000A). Chromatographic separations for both drugs were achieved on a reversedphase 10-mm C18 column (250×4.4 mm, Nucleosil, Allteck Associates, Deerfield, IL). Sample detection was done with a fluorescence detector (model 980, Applied Biosystems, Ramsey, NJ) with the excitation and emission wavelengths set at 278 and 418 nm, respectively. Chromatograms were registered on an integrator (HP 3396 series 11, Hewlett–Packard, Avondale, PA). The mobile phase for ciprofloxacin assay had a pH of 2.5 and consisted of 0.01 M KH2PO4:acetonitrile (87:13,
C.O. Onyeji et al. / International Journal of Antimicrobial Agents 12 (1999) 107–114
v/v) containing 0.005 M tetrabutylammonium hydrogen sulphate (Sigma) and this was delivered at a flow rate of 1.5 ml/min. The intraday coefficients of variation for ciprofloxacin analysis were 1.6 and 1.1% at concentrations of 0.1 and 2 mg/l, respectively, while the interday precisions were 4.6 and 1.5% at the corresponding concentrations, respectively. The assay limit of detection was 0.01 mg/l and the linearity was tested and demonstrated over a range of 0.04 – 4 mg/l. The mobile phase for the analysis of levofloxacin serum samples was different from that used for ciprofloxacin assay only in the ratio of 0.01 M KH2PO4:acetonitrile which was 89:11 (v/v) and the flow rate of 1.5 ml/min was used. The assay linearity was tested and confirmed over a range of 0.02–6 mg/l. The intraday and interday precisions were less than 3% at concentrations of 0.1 and 4 mg/l.
2.2.3. Pharmacokinetic analysis The pharmacokinetic parameters: terminal phase elimination rate constant (b), elimination half-life (T1/ 2b), apparent volume of the central compartment (Vc), apparent steady-state volume of distribution (Vss), area under the serum drug concentration – time curve (AUC), and total-body clearance (ClT), were calculated by using a one-compartment model for ciprofloxacin and a two-compartment model for levofloxacin, with first-order elimination, by nonlinear least-square techniques (PCNONLIN, version 4.2, Statistical Consultants, Lexington, KY). Compartment model selection was based on visual inspection of the fit and use of the correlation between the observed and the calculated concentrations. 2.2.4. Determination of dosing regimens The pharmacokinetic parameters of each drug evaluated from mice with renal impairment induced by a determined optimal dose of uranyl nitrate, were used to calculate a dose required to yield an AUC in the range of 42–53 mg.h/l for levofloxacin and 12 – 19 mg.h/l for ciprofloxacin. These are AUC values obtained following administration of a single therapeutic dose of the antibiotics in healthy humans [11,20,21]. The calculated doses were 14.25 and 32 mg/kg for ciprofloxacin and levofloxacin, respectively. These doses were administered separately to groups of renally impaired mice so as to confirm experimentally the expected AUC values with each single dose. The total daily dose of ciprofloxacin required to yield in mice, the 24-h AUC value obtained for the drug in humans, was calculated as: since ciprofloxacin is normally administered twice daily in humans, the total daily dose required in mice was 14.25 mg/kg 2× (i.e. 28.5 mg/kg). Similarly, the total daily dose required for levofloxacin was 32 mg/kg 1 × , as the drug is normally dosed once daily in
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humans. To ensure that the doses did not produce maximum concentrations that were much higher than the values obtained therapeutically in humans, these total daily doses were fractionated as follows: 28.5/3 mg/kg every 8 h for ciprofloxacin (i.e. 9.5 mg/kg q 8 h), and 32/3 mg/kg every 8 h for levofloxacin (i.e. 10.6 mg/kg q 8 h). These dosing regimens were administered over 24 h to groups of renally impaired mice and the Cmax were determined to confirm PCNONLIN-predicted values.
2.3. Drug treatment and efficacy e6aluation The minimum lethal dose (MLD) was determined by intraperitoneal inoculation of groups of mice (10 mice/ group) with 0.5 ml of a suspension of an overnight culture of the organism in Mueller–Hinton broth at serial 10-fold dilutions. The culture of the strain SP 22 was diluted in 6% (w/v) mucin (Sigma) since the isolate is not sufficiently virulent for mice and requires mucin to induce disease. Animals were observed and the minimum infectious dose that caused 100% mortality within 48 h of inoculation was recorded as the MLD of the organism. The in vivo efficacies of ciprofloxacin and levofloxacin were evaluated as follows: Groups of mice (18–20 mice/treatment group) were injected intraperitoneally with a single 10 mg/kg dose of uranyl nitrate to induce renal impairment. After 72 h, each group of the mice was inoculated intraperitoneally with a MLD of the test strain. One hour after the infection, the animals were administered subcutaneously with 0.2-ml volumes containing ciprofloxacin at a dosing regimen of 9.5 mg/kg every 8 h or levofloxacin at 10.6 mg/kg every 8 h for a total of six or 15 doses. The mice were observed and percent survival was recorded daily over 5 or 8 days postinfection. Another set of groups of mice were rendered neutropenic by following a literature method that involved intraperitoneal injection of 150 mg/kg dose of cyclophosphamide (Mead Johnson Pharmaceuticals, Evansville, IN) in a 0.2-ml volume at 4 and 3 days before infection (day 0). These groups of neutropenic mice were induced to be renally impaired, infected with the test isolates and treated with ciprofloxacin or levofloxacin as described for the non-neutropenic infection model. Survivorship was recorded daily over 5 days postinfection. These antibiotic-treatment experiments were done in duplicate and pooled data were used for statistical analysis.
2.4. Statistical analysis The percent survival obtained at different time points following treatment with ciprofloxacin or levofloxacin were compared using the log rank test. A P value of B 0.05 was considered significant.
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3. Results
3.1. In 6itro acti6ity Each of the three isolate of S. pneumoniae used in this study had equal in vitro susceptibilities to levofloxacin and ciprofloxacin (Table 1). The activities of the antibiotics were not affected by the degree of penicillin resistance. Quinolone-susceptibility has previously been found to be unrelated to penicillin-resistance [14].
3.2. Pharmacokinetic studies Table 2 shows the pharmacokinetic parameters obtained for levofloxacin (10 mg/kg) and ciprofloxacin (20 mg/kg) following single-dose subcutaneous administrations of the drugs to groups of mice with renal impairment induced by intraperitoneal administration of different doses of uranyl nitrate. The kinetics of the antibiotics in mice were best described by a one-com-
partment model for ciprofloxacin and a two-compartment model for levofloxacin. A similar compartment model has been used to describe ciprofloxacin kinetics in mice [22]. Both drugs were rapidly absorbed, with maximum serum levels achieved within 15 min. There was a progressive increase in the elimination half-life of both drugs as the administered dose of uranyl nitrate increased. A uranyl nitrate dose of 10 mg/kg was found optimal at it resulted in an increase in the elimination half-life of ciprofloxacin and levofloxacin to 2.19 and 6.61 h, respectively. The value of 6.61 h obtained for levofloxacin is within the range of elimination half-life values reported for the drug in humans whereas, 2.19 h determined for ciprofloxacin is slightly less than the range of 3–5 h observed in humans [6,11,20,21]. A further increase in uranyl nitrate dose in an attempt to extend the elimination half-life of ciprofloxacin beyond 2.19 h was associated with mortality in the animals resulting from uranyl nitrate toxicity. The kinetic parameters obtained following administration of single doses of 14.25 mg/kg of ciprofloxacin
Table 1 Levofloxacin and ciprofloxacin MICs for and minimum lethal doses (MLD) of selected strains of S. pneumoniae S. pneumoniae isolate
MLD (CFU/mouse)
Drug
MICb (mg/l)
SP 22 (penicillin-sensitive)
2×102
SP 28a (penicillin-sensitive)
1×106
SP 46 (penicillin-resistant)
1×104
Ciprofloxacin Levofloxacin Ciprofloxacin Levofloxacin Ciprofloxacin Levofloxacin
2 2 1 1 2 2
a b
Mucin was used to induce infection. Values are based on average of three determinations.
Table 2 Pharmacokinetic parameters in serum after administration of a single subcutaneous dose of ciprofloxacin or levofloxacin to mice with renal impairment induced with varying intraperitoneal doses of uranyl nitratea Antibiotic
Levofloxacin (10 mg/ kg)
Levofloxacin (32 mg/ kg) Ciprofloxacin (20 mg/ kg)
Ciprofloxacin (14.25 mg/kg)
a
Dose of uranyl nitrate (mg/kg)
T1/2b (h)
Cmax (mg/l)
Vss (l/kg)
Vc (l/kg)
ClT (l/h/kg)
AUC (mg.h/l)
1
3.45
3.02
6.24
2.96
3.23
3.1
2.5 5 10
4.60 6.40 6.61
3.89 4.80 5.37
6.30 5.52 4.57
1.48 1.10 1.83
1.83 0.88 0.78
5.5 11.4 12.7
10
6.69
22.60
4.45
1.75
0.73
44.0
1.42
6.24
3.20
3.20
1.56
12.8
5 10
1.70 2.19
7.25 6.63
2.76 3.01
2.76 3.01
1.13 0.95
17.8 21.1
10
2.10
6.44
2.21
2.21
0.81
17.5
2.5
T1/2b, Vss, Vc, ClT, and AUC are as described in the text. Vc =Vss for ciprofloxacin since the drug confers a one-compartment model.
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Fig. 1. PCNONLIN-predicted 24-h mouse serum levels of ciprofloxacin (9.5 mg/kg every 8 h) and levofloxacin (10.6 mg/kg every 8 h) using one- and two-compartment models, respectively.
and 32 mg/kg of levofloxacin are shown in Table 2. Fig. 1 depicts PCNONLIN-predicted mouse serum concentration profiles of ciprofloxacin at a dosing regimen of 9.5 mg/kg every 8 h and also that of levofloxacin at a dosage of 10.6 mg/kg every 8 h. For ciprofloxacin, the predicted Cmax values were 4.3 mg/l after first dose and 4.6 mg/l after second dose, while the experimentally determined values were 4.4 and 4.5 mg/l, respectively. On the other hand, Cmax of 6.2 and 7.4 mg/l after first and second doses, respectively, were experimentally determined for levofloxacin. These are also comparable to the predicted values. These Cmax values are clinically achievable for both drugs [11,20].
3.3. In 6i6o efficacy studies The MLDs of the isolates are presented in Table 1 and these indicate that the strains selected have varying degrees of virulence. Table 3 shows the percent survival
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of non-neutropenic infected mice following treatment with the determined dosing regimes of ciprofloxacin or levofloxacin. There was a 100% mortality between 36 and 48 h postinfection in the control groups that received no antibiotic treatment. From day 3 postinfection, levofloxacin appeared more effective than ciprofloxacin against the three strains. However, statistical analysis of the data showed that the observed efficacy of levofloxacin was not significantly higher (P\0.05) than that of ciprofloxacin at any of the time points. Increasing the duration of treatment from a total of six to 15 doses resulted in significantly increasing the survivorship of the infected mice in both ciprofloxacin- and levofloxacin-treated groups but, the efficacy of levofloxacin was still not significantly higher than that of ciprofloxacin (Fig. 2). The 24-h AUC/MIC ratios for levofloxacin were 22, 44 and 22 for the isolates SP 22, SP 28 and SP 46, respectively, while the corresponding values for the isolates with ciprofloxacin were 17.5, 35 and 17.5, respectively. Results of the evaluation of the efficacies of the antibiotics using a neutropenic infection model are indicated in Table 4. The comparative effectiveness of both drugs against the tested isolates followed the same trend as was observed in the non-neutropenic infection model.
4. Discussion The determination of survivorship in mouse experimental septicaemia remains a primary model for evaluation of efficacy of antimicrobial agents [23]. Literature reports suggest that the therapeutic activities of quinolones in experimental infections are related to their pharmacokinetic behavior [7,22,24,25]. Thus, in comparative evaluation studies of efficacies of levofloxacin and ciprofloxacin in mouse infection models, the difference in the efficacies of both drugs may become obscured because the elimination half-life of both drugs are comparable in the animal [26,27], unlike
Table 3 Survivorship of non-neutropenic mice following infection with minimum lethal doses of isolates of S. pneumoniae and treatment with multiple doses of ciprofloxacin or levofloxacin for 2 days Isolate
Drug treatmenta
Number of survivors (percent survival) time (days) postinfection 1
SP 22 SP 46 SP 28
Ciprofloxacin (n =37) Levofloxacin (n=37) Ciprofloxacin (n= 37) Levofloxacin (n= 37) Ciprofloxacin (n= 40) Levofloxacin (n= 40)
36 36 32 33 40 40
2 (97) (97) (87) (89) (100) (100)
29 31 26 29 39 40
3 (78) (84) (70) (78) (97.5) (100)
23 29 16 24 36 39
4 (62) (78) (43) (65) (90) (97.5)
10 15 6 11 30 36
4 (27) (41) (16) (30) (75) (90)
6 9 2 4 25 30
(16) (24) (5) (11) (62.5) (75)
a Ciprofloxacin dosing regimen, 9.5 mg/kg every 8 h (a total of six doses); levofloxacin dosing regimen, 10.6 mg/kg every 8 h (a total of six doses). Rationale for the different dosing regimens is described in the text.
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Fig. 2. Percent Survival of mice following infection with a minimum lethal dose of a penicillin-resistant clinical isolate of S. pneumoniae designated SP 46 and treatment with ciprofloxacin (9.5 mg/kg every 8 h) or levofloxacin (10.6 mg/kg every 8 h) for a total of 5 days. Forty mice were used for each treatment group. Determination of the treatment schedules is described in the text. Table 4 Survivorship of neutropenic mice following infection with minimum lethal doses of isolates of S. pneumoniae and treatment with multiple doses of ciprofloxacin or levofloxacin for 2 days Isolate
Drug treatmenta
Number of survivors (percent survival) time (days) postinfection 1
SP 22 SP 46
Ciprofloxacin (n= 40) Levofloxacin (n= 40) Ciprofloxacin (n= 40) Levofloxacin (n= 40)
39 39 38 39
2 (98) (98) (95) (98)
37 39 37 38
3 (93) (98) (93) (95)
20 21 29 34
4 (50) (53) (73) (85)
9 13 6 9
5 (23) (33) (15) (23)
6 9 2 7
(16) (24) (5) (18)
a Ciprofloxacin dosing regimen, 9.5 mg/kg every 8 h (a total of six doses); levofloxacin dosing regimen, 10.6 mg/kg every 8 h (a total of six doses).
in humans. It was, therefore, reasoned that the difference in the in vivo efficacies of both drugs would be made more apparent in an experimental mouse infection model by simulating the different kinetics observed in humans for the two drugs. Antibiotic efficacy studies in animals involving induction of renal impairment designed to simulate human kinetic drug profiles in the animals have been reported [28,29]. Out of a variety of methods available for the induction of renal impairment, uranyl nitrate administration was chosen in this study because it has the advantage of technical simplicity and sustained diminution of renal function [30]. As
is evident in Table 2, the levofloxacin kinetic profile was more responsive to renal impairment than that of ciprofloxacin. This is not surprising because, renal excretion in humans is known to account for more than 70% of total-body clearance of levofloxacin while that for ciprofloxacin is 25–50% [8]. The 10-mg/kg dose of uranyl nitrate used in the infection model in this study did not contribute to the mortality of the infection. The results of this study demonstrate a trend towards a relatively higher in vivo efficacy of levofloxacin compared with that of ciprofloxacin against the clinical isolates of S. pneumoniae. However, statistical treat-
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ment of our data revealed a nonsignificant difference (P \ 0.05). In earlier animal model studies [7,10] with S. pneumoniae strains of equal susceptibilities to ciprofloxacin and levofloxacin, a significant difference was observed between the effectiveness of both antibiotics after oral administration. This is an indication that the higher oral bioavailabilty of levofloxacin compared to that of ciprofloxacin [6,11] has a marked impact on their comparative in vivo efficacies. It has been established that the 24-h AUC/MIC ratio is the parameter that best correlates with the efficacy of fluoroquinolones [31,32]. Also, the peak/MIC ratios correlate better with fluoroquinolones efficacy when the ratios are \10 [25]. Our results agree with these assertions. For example, with the strains SP 22 and SP 46, the 24-h AUC/MIC ratio is 22 for levofloxacin, and 17.5 for ciprofloxacin The minimal difference between the values of this pharmacokinetic/pharmacodynamic parameter for both drugs reflect the narrow difference observed in the efficacies of the drugs. The peak/MIC ratios are B10 for both drugs with the three isolates. A significant difference in the efficacies of both drugs may be observed against other strains of the organism since the MIC90 of levofloxacin is 2 mg/l while that for ciprofloxacin is in the range of 1 – 4 mg/l with even higher MICs occasionally encountered [4,5]. Since the duration of antimicrobial therapy can influence treatment outcome, it was thought worthwhile in this study to prolong the treatment duration from 2 to 5 days with a view to finding whether the apparent absence of significant difference in the efficacy of both drugs could be related to the duration of therapy. Fig. 2 shows that after infecting the animals with SP 46 and treating with both drugs for 5 days, the difference in the survival rates obtained with both drugs was still not significant. The efficacies of both drugs were also evaluated in a neutropenic infection model so as to delineate the intrinsic potencies of the antibiotics from the microbicidal activities of neutrophils that tend to augment drug efficacy [33,34]. The survival profiles were not different from those obtained with the non-neutropenic infection model, indicating that neutropenia had no demonstrable effect on the efficacies of both drugs. This observation is important because several reports indicate that, in neutropenia, higher doses of antibiotics are needed in order to obtain the same antibacterial effect as in the non-neutropenic infection models [27,35]. The lack of effect of neutropenia as observed in this study may be explained in part by the diminishing influence of neutrophils on antimicrobial efficacy at high total drug dosages [36]. The doses administered in this study may have been high enough to dampen the influence of neutrophils. Since the infected animals received antibiotic treatments in multiple dosing regimens, any impact of post-antibiotic leukocyte enhancement effect [37] which may be lacking in the neutropenic model, would also have been less pronounced.
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Published non-comparative clinical trials indicate that microbiological eradication rates observed for levofloxacin against S. pneumoniae in respiratory tract infections were 81–97% [6,38–40], while older quinolones including ciprofloxacin usually provide lower eradication rates [4,8]. There is no report of a controlled comparative clinical evaluation of the efficacy of ciprofloxacin and levofloxacin against S. pneumoniae. It is pertinent to note that, the low survival profiles of the infected animals after treatment with both quinolones, as observed in this study, do not reflect the clinical success rates for the drugs in humans. This is expected since the treatment duration and severity of infection in this model and in human infections are not the same. Our study suggests that, with some strains of S. pneumoniae, levofloxacin activity may be comparable to that of ciprofloxacin in the treatment of pneumococcal infections caused by susceptible strains of the organism.
Acknowledgements We thank Ortho Pharmaceuticals (Raritan, NJ, USA) for funding this study.
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