Efficacy of Trovafloxacin against Experimental Staphylococcus aureus Endocarditis

Trovafloxacin is a new fluoronaphthyridone chemically and functionally related to members of the fluoroquinolone class of antimicrobial agents. The in vivo efficacy of the drug was compared with that of vancomycin by using the rabbit model of left-sided endocarditis. Rabbits infected with either a nafcillin-susceptible or -resistant test strain were treated with trovafloxacin (13.3 mg/kg of body weight every 12 h) or vancomycin (25 mg/kg of body weight every 8 h) for 4 days. In comparison with untreated controls, both antimicrobial agents effectively cleared bacteremia and significantly reduced bacterial counts in vegetations and tissues of animals infected with either test strain. No resistance to trovafloxacin emerged in test strains during therapy. We conclude that in this model trovafloxacin is as efficacious as vancomycin is and may serve as a viable alternative to vancomycin for use in humans with similar infections.Trovafloxacin (CP-99,219-27) is a fluoronaphthyridone antimicrobial agent related to fluoroquinolones such as ciprofloxacin and norfloxacin (2). It has potent activity against a broad range of bacterial species, including Staphylococcus aureus (4, 5, 15, 17, 22). The antibacterial activity of trovafloxacin versus S. aureus surpasses that of ciprofloxacin, and trovafloxacin may maintain clinically relevant activity against ciprofloxacin-resistant strains (1).The in vivo activity of trovafloxacin against selected pathogens has been assessed by using animal models, and the drug has shown therapeutic efficacy comparable to that of standard forms of therapy (3, 6, 13, 21). However, the efficacy of trovafloxacin in treating serious S. aureus infections has not been determined. In order to address this issue, we compared the therapeutic activities of trovafloxacin and vancomycin by using the rabbit model of left-sided S. aureus endocarditis. This model affords a severe test of antimicrobial activity in a serious systemic infection.     

Efficacy of LY333328 against Experimental Methicillin-Resistant Staphylococcus aureus Endocarditis

The in vivo efficacy of LY333328, a new glycopeptide antibiotic, was compared with that of vancomycin by using the rabbit model of left-sided methicillin-resistant Staphylococcus aureus endocarditis. Animals received LY333328 or vancomycin (25 mg/kg of body weight every 24 or 8 h, respectively) for 4 days. These drugs were equally effective in clearing bacteremia and in reducing bacterial counts in vegetations and tissues. We conclude that in this model, LY333328 was microbiologically effective and may be a therapeutic alternative to vancomycin.     

Comparison of Six Generic Vancomycin Products for Treatment of Methicillin-Resistant Staphylococcus aureus Experimental Endocarditis in Rabbits

Concerns have recently emerged about the potency and the quality of generic vancomycin (VAN) products approved for use in humans, based on experiments in a neutropenic mouse thigh infection model. However, other animal models may be more appropriate to decipher the bactericidal activities of VAN generics in vivo and to predict their efficacy in humans. We aimed to compare the bactericidal activities of six generic VAN products currently used in France (Mylan and Sandoz), Spain (Hospira), Switzerland (Teva), and the United States (Akorn-Strides and American Pharmaceutical Products [APP]) in a rabbit model of aortic valve endocarditis induced by 8 × 10⁷ CFU of methicillin-resistant Staphylococcus aureus (MRSA) strain COL (VAN MIC, 1.5 μg/ml). In vitro, there were no significant differences in the time-kill curve studies performed with the six generic VAN products. Ten rabbits in each group were treated with intravenous (i.v.) VAN, 60 mg/kg of body weight twice a day (b.i.d.) for 4 days. Mean peak serum VAN levels, measured 45 min after the last injection, ranged from 35.5 (APP) to 45.9 μg/ml (Teva). Mean trough serum VAN levels, measured 12 h after the last injection, ranged from 2.3 (Hospira) to 9.2 (APP) μg/ml. All generic VAN products were superior to controls (no treatment) in terms of residual organisms in vegetations (P < 0.02 for each comparison) and in the spleen (P < 0.005 for each comparison). Pairwise comparisons of generic VAN products found no significant differences. In conclusion, a stringent MRSA endocarditis model found no significant differences in the bactericidal activities of six generic VAN products currently used in Europe and America.     

Efficacy of Linezolid in Treatment of Experimental Endocarditis Caused by Methicillin-Resistant Staphylococcus aureus

The efficacies of orally (p.o.) dosed linezolid and intravenously (i.v.) dosed vancomycin against methicillin-resistant Staphylococcus aureus (MRSA) in rabbits with experimental aortic-valve endocarditis were investigated. After endocarditis was established with a recent clinical MRSA isolate, rabbits were dosed for 5 days with linezolid (p.o., three times a day) at either 25, 50, or 75 mg/kg of body weight or vancomycin (i.v., twice a day) at 25 mg/kg. The 25-mg/kg linezolid group had a high mortality rate and bacterial counts in the valve vegetations that were not different from those of the controls. Linezolid dosed p.o. at 50 and 75 mg/kg and i.v. vancomycin produced statistically significant reductions in bacterial counts compared to those of the untreated controls. The reduced bacterial counts and culture-negative valve rates for the animals treated with linezolid at 75 mg/kg were similar to those for the vancomycin-treated animals. Concentrations of linezolid in plasma were determined at several points in the dosing regimen. These results suggest that the efficacy of linezolid in this infection model is related to trough levels in plasma that remain above the MIC for this microorganism. At the ineffective dose of linezolid (25 mg/kg) the concentration at sacrifice was 0.045 times the MIC, whereas the concentrations of linezolid in plasma in the 50- and 75-mg/kg groups were 2 and 5 times the MIC at sacrifice, respectively. The results from this experimental model suggest that the oxazolidinone linezolid may be effective for the treatment of serious staphylococcal infections when resistance to other antimicrobials is present.     

Telavancin in Therapy of Experimental Aortic Valve Endocarditis in Rabbits Due to Daptomycin-Nonsusceptible Methicillin-Resistant Staphylococcus aureus

A number of cases of both methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA) strains that have developed daptomycin resistance (DAP-R) have been reported. Telavancin (TLV) is a lipoglycopeptide agent with a dual mechanism of activity (cell wall synthesis inhibition plus depolarization of the bacterial cell membrane). Five recent daptomycin-susceptible (DAP-S)/DAP-R MRSA isogenic strain pairs were evaluated for in vitro TLV susceptibility. All five DAP-R strains (DAP MICs ranging from 2 to 4 μg/ml) were susceptible to TLV (MICs of ≤0.38 μg/ml). In vitro time-kill analyses also revealed that several TLV concentrations (1-, 2-, and 4-fold MICs) caused rapid killing against the DAP-R strains. Moreover, for 3 of 5 DAP-R strains (REF2145, A215, and B_2.0), supra-MICs of TLV were effective at preventing regrowth at 24 h of incubation. Further, the combination of TLV plus oxacillin (at 0.25× or 0.50× MIC for each agent) increased killing of DAP-R MRSA strains REF2145 and A215 at 24 h (∼2-log and 5-log reductions versus TLV and oxacillin alone, respectively). Finally, using a rabbit model of aortic valve endocarditis caused by DAP-R strain REF2145, TLV therapy produced a mean reduction of >4.5 log₁₀ CFU/g in vegetations, kidneys, and spleen compared to untreated or DAP-treated rabbits. Moreover, TLV-treated rabbits had a significantly higher percentage of sterile tissue cultures (87% in vegetations and 100% in kidney and spleen) than all other treatment groups (P < 0.0001). Together, these results demonstrate that TLV has potent bactericidal activity in vitro and in vivo against DAP-R MRSA isolates.     

Comparative Efficacy of Trovafloxacin in Experimental Endocarditis Caused by Ciprofloxacin-Sensitive, Methicillin-Resistant Staphylococcus aureus

The new fluoroquinolone trovafloxacin was tested against a ciprofloxacin-sensitive, methicillin-resistant Staphylococcus aureus strain in the rabbit model of endocarditis. Trovafloxacin was more effective than vancomycin (CFU/g of vegetation, 2.65 ± 1.87 versus 4.54 ± 2.80 [mean ± standard deviation]; P < 0.05) or ampicillin-sulbactam plus rifampin (4.9 ± 1.1 CFU/g). The addition of ampicillin-sulbactam to trovafloxacin tended to reduce titers further.     

Bactericidal Activity of the Combination of Levofloxacin with Rifampin in Experimental Prosthetic Knee Infection in Rabbits Due to Methicillin-Susceptible Staphylococcus aureus

The combination of levofloxacin and rifampin has been recommended for the treatment of staphylococcal prosthetic infection. In a rabbit model of prosthetic knee infection due to a susceptible clinical strain of Staphylococcus aureus, the combination of levofloxacin and rifampin was bactericidal, significantly reduced bacterial titers in bone compared with levels for rifampin and controls (P < 0.05), sterilized 6 of 12 animals, and prevented the selection of resistant mutants that was observed with rifampin alone, validating clinical recommendations.Because of its pharmacodynamic and pharmacokinetic properties, rifampin is considered a cornerstone of antibiotic treatment of staphylococcal prosthesis joint infections (14, 19). Indeed, rifampin is highly active in vitro against staphylococci, has good diffusion in bone, is active against small-colony variants sequestered into phagocytic cells (16), and retains bactericidal activity against nongrowing staphylococci, such as those that adhere to foreign bodies (11, 19). Rifampin has been shown to be effective in experimental studies (8, 14). However, this drug should not be given alone, due to the frequent development of resistant mutants (19). Thus, the combination of rifampin with fluoroquinolones that share pharmacodynamic and pharmacokinetic properties with rifampin has been shown to be the most effective treatment for such infections (4, 8, 14, 19). Levofloxacin, the l-isomer of the racemate ofloxacin, has a high level of in vitro activity against Staphylococcus aureus (7) and demonstrated good activity in foreign-body experimental models (13, 17) and good penetration in bone samples from patients undergoing arthroplasty (12). Levofloxacin has been recommended for treatment of staphylococcal prosthetic joint infections in adults (20). However, its specific activity in prosthetic joint infections has not been clearly established. We therefore evaluated the efficacy of levofloxacin, alone or in combination with rifampin, for treatment of rabbit experimental prosthetic knee infections due to S. aureus.(This work was presented in part at the 46th Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, CA, 2006.)S. aureus 17848, isolated from a patient with an infected knee prosthesis, was used for all experiments. The MICs of levofloxacin and rifampin were determined by using the Etest method (AB Biodisk, Solna, Sweden) as recommended by the manufacturer. The mutant prevention concentration (MPC), recorded as the lowest antibiotic concentration completely inhibiting bacterial growth after incubation at 37°C for 72 h, was determined for rifampin and levofloxacin, as described previously (2). Time-kill curve studies were used to determine the bactericidal activities of levofloxacin and rifampin alone and in combination. Exponential-phase cultures were diluted in 10 ml of fresh Mueller-Hinton broth to yield a final inoculum of 10⁶ CFU/ml. The concentrations of antibiotic used were equivalent to 1× MIC and 4× MIC for levofloxacin and 2× MIC for rifampin. After 0, 3, 6, and 24 h of incubation in a shaking water bath at 37°C, serial dilutions of 0.1-ml samples were subcultured onto trypticase soy agar plates and incubated at 37°C for 24 h before CFU were counted. Bactericidal effect was defined by a ≥3-log₁₀ decrease of the initial inoculum. Synergy was defined by a decrease of at least 2 log₁₀ CFU/ml between the combination and its most active constituent after 24 h of incubation.New Zealand White rabbits, each weighing between 2.5 and 3 kg, were experimentally infected. Animal experiments were performed in accordance with prevailing regulations in the European Commission (9). This model has been described in detail elsewhere (1, 5). Briefly, a silicone-elastomer implant, commonly used in arthroplasty of the first metatarsophalangeal joint (Silastic HP great toe implant; Swanson Design, Dow-Corning [provided by Ortho Technique, Créteil, France]) was implanted as a tibial prosthetic component under general anesthesia. Immediately after surgery, animals were inoculated with of 10⁷ CFU of S. aureus in a final volume of 0.5 ml in phosphate-buffered saline, injected into the knee close to the prosthesis.Seven days after inoculation (day 7), rabbits were randomly assigned to an untreated control group (n = 10) or a group receiving treatment with levofloxacin alone (n = 12), rifampin alone (n = 11), or levofloxacin combined with rifampin (n = 12). Each regimen was administered for 7 days. Antibiotic dosing regimens were chosen in order to reproduce plasma concentrations in humans. For rifampin, a dosing regimen of 10 mg/kg of body weight twice a day, given intramuscularly, was used, as it produced peak levels of 9.3 ± 0.5 μg/ml in plasma 2 h after injection, which were close to the levels obtained for humans after a 10-mg/kg dose (18). The levofloxacin dosing regimen was determined in pilot studies in order to obtain peak concentrations and areas under the concentration-time curve over 24 h (AUC_0-24 levels) comparable to those achieved for humans with a dose of 750 mg per day, given intravenously (i.v.), i.e., peak levels of 11 to 12 μg/ml and AUC_0-24 levels of 90 μg·h/ml (3, 7). The dosing regimen of levofloxacin tested which best reproduced human levels was 25 mg/kg twice a day, given i.v., which produced peak levels ranging from 12.8 to 14.4 μg/ml and AUC_0-24 levels ranging from 58 to 78 μg·h/ml 15 min after injection.All the animals were killed by i.v. injection of pentobarbital 3 days after the end of therapy (day 17). The upper third (length, 3 cm) of the tibia, including compact bone and marrow, was isolated, split with a bone crusher, weighed, cut into little pieces, frozen in liquid nitrogen, and then crushed in an autopulverizer (Spex 6700 freezer; Mill Industries, Inc., NJ). The pulverized bone was suspended into 10 ml of sterile saline. Serial dilutions were made, and 0.1 ml of the dilution was plated on trypticase soy agar and horse blood agar plates. After 48 h of incubation at 37°C, the number of viable organisms was determined. The lowest detectable bacterial counts according to bone weight were 1.53 to 1.83 log₁₀ CFU/g bone. Mutant-resistant S. aureus isolates were detected in bone samples from six untreated rabbits randomly chosen and in all the treated rabbits. Portions (0.1 ml) of undiluted bone homogenate were plated onto Mueller-Hinton agar containing levofloxacin at a concentration of twofold the MIC or rifampin at a concentration of eightfold the MIC. After 72 h of incubation at 37°C, colonies were detected and MICs of levofloxacin and rifampin were measured.Peak and trough levofloxacin plasma levels were determined for six infected rabbits 10 min (peak) and 12 h (trough) after i.v. injection on the last day of therapy for animals treated with levofloxacin alone. The levofloxacin and rifampin concentrations in bone samples from all treated animals were determined using 1-ml portions of the pulverized bone. Levofloxacin and rifampin concentrations were measured by high-performance liquid chromatography.The MICs of levofloxacin and rifampin were 0.125 and 0.016 μg/ml, respectively, and the MPCs were 1 and 256 μg/ml, respectively. The results for the time-kill studies are presented in Fig. ​Fig.1.1. Rifampin alone at a concentration of 2× MIC was associated with a bacterial regrowth at 24 h after an initial bacterial killing. Levofloxacin alone exhibited a concentration-dependent killing, with a bacteriostatic effect followed by a regrowth after 24 h of incubation at 1× MIC, compared with a rapid bactericidal activity at a concentration of 4× MIC. The combination of rifampin and levofloxacin prevented the regrowth observed with rifampin alone at both concentrations of levofloxacin tested. The combination of rifampin with levofloxacin (1× MIC) was synergistic. In contrast, with a higher concentration of levofloxacin (4× MIC), the bactericidal activity of the combination tended to be reduced compared with that of levofloxacin alone after 3 and 6 h of exposure, without achieving antagonism.Open in a separate windowFIG. 1.Time-kill curve study of Staphylococcus aureus 17848 incubated for 24 h in Mueller-Hinton broth with no antibiotic (Control), levofloxacin at a concentration equal to the MIC (Lev, 1× MIC), levofloxacin at a concentration equal to 4× MIC (Lev, 4× MIC), rifampin at a concentration equal to 2× MIC (Rif, 2× MIC), a combination of levofloxacin at a concentration equal to the MIC and rifampin at a concentration equal to 2× MIC (Lev, 1× MIC + Rif), and combination of levofloxacin at a concentration equal to 4× MIC and rifampin at a concentration equal to 2× MIC (Lev, 4× MIC + Rif).The levofloxacin concentrations in plasma from infected rabbits obtained 15 min and 12 h after the end of the infusion were 17.3 ± 3.3 μg/ml and 0.29 ± 0.13 μg/ml, respectively. In bone samples, 72 h after the end of therapy, the concentrations of levofloxacin were 0.18 ± 0.06 μg/g (range, 0.09 to 0.29 μg/g), whereas the concentrations of rifampin were undetectable in seven animals and ranged from 0.15 to 7.95 μg/g in the remaining 16 rabbits.The results obtained with the different therapeutic regimens tested are shown in Table ​Table1.1. Rifampin alone significantly reduced bacterial titers in bone compared with levels for untreated controls (P < 0.05) and sterilized 5 of 11 animals. As expected, four of the six animals treated with rifampin alone that were not sterilized retained mutant-resistant strains. This result can easily be explained by the high rifampin MPC (256 μg/ml) that could not be achieved by local concentrations of rifampin in bone (<8 μg/g in all cases). Levofloxacin alone was bactericidal, significantly reduced bacterial titers in bone compared with levels for untreated controls (P < 0.05), and sterilized 5 of 12 animals (42%). No mutants resistant to levofloxacin were detected in bone samples from animals that were not sterilized at the time of sacrifice.

TABLE 1.

Effects of 7-day treatment regimens of levofloxacin or rifampin, alone or in combination, in rabbits with experimental prosthetic knee infections due to S. aureus 17848

Lysostaphin Treatment of Experimental Methicillin-Resistant Staphylococcus aureus Aortic Valve Endocarditis

The emergence of clinical isolates of methicillin-resistant Staphylococcus aureus with reduced susceptibility to vancomycin has prompted a search for new and novel therapeutic agents active against S. aureus. Lysostaphin, a peptidase produced by Staphylococcus simulans, specifically cleaves the glycine-glycine bonds unique to the interpeptide cross-bridge of the S. aureus cell wall. The effectiveness of various regimens of dosing with intravenous lysostaphin was compared to that of vancomycin in the rabbit model of aortic valve endocarditis caused by a clinical methicillin-resistant S. aureus isolate. All animals were treated for a total of 3 days. The most active regimen, lysostaphin given three times daily, produced sterile vegetations in 10 of 11 treated rabbits, with a mean reduction in vegetation bacterial counts of 8.5 log₁₀ CFU/g compared to the counts in the untreated controls. In contrast, vancomycin given twice daily sterilized no vegetations and reduced vegetation bacterial counts by only 4.8 log₁₀ CFU/g. Lysostaphin given once daily was less effective, reducing mean vegetation bacterial counts by only 3.6 log₁₀ CFU/g, but the combination of lysostaphin once daily and vancomycin twice daily reduced the mean vegetation bacterial density by 7.5 log₁₀ CFU/g, a result that was significantly better than that for either regimen alone (P < 0.05). Lysostaphin was well tolerated by the rabbits, with no evidence of immunological reactions following up to 9 weeks of intravenous administration. We conclude that lysostaphin given alone or in combination with vancomycin is more effective in the treatment of experimental methicillin-resistant S. aureus aortic valve endocarditis than vancomycin alone.     

Ceftobiprole Is Superior to Vancomycin,Daptomycin, and Linezolid for Treatment of Experimental Endocarditis in Rabbits Caused by Methicillin-Resistant Staphylococcus aureus

Beta lactam agents are the most active drugs for the treatment of streptococci and methicillin-susceptible Staphylococcus aureus endocarditis. However, methicillin-resistant S. aureus (MRSA) is resistant to all beta lactam agents licensed to date, and alternative treatments are limited. Ceftobiprole is a novel broad-spectrum cephalosporin that binds with high affinity to PBP 2a, the penicillin binding protein that mediates the methicillin resistance of staphylococci and is active against MRSA. Ceftobiprole was compared to vancomycin, daptomycin, and linezolid in a rabbit model of MRSA aortic valve endocarditis caused by the homogeneously methicillin-resistant laboratory strain COL. Residual organisms in vegetations were significantly fewer in ceftobiprole-treated rabbits than in any other treatment group (P < 0.05 for each comparison). In addition, the numbers of organisms in spleens and in kidneys were significantly lower in ceftobiprole-treated rabbits than in linezolid- and vancomycin-treated animals (P < 0.05 for each comparison). Anti-MRSA beta lactam agents such as ceftobiprole may represent a significant therapeutic advance over currently available agents for the treatment of MRSA endocarditis.Endocarditis is one of the most difficult infections to treat in humans, with an in-hospital mortality rate of ∼20% even in most recent series, despite significant advances in surgical treatment over the last decades (2, 17). In addition, Staphylococcus aureus, which has emerged as the most common cause of endocarditis worldwide (16), is associated with a high rate of severe complications, such as heart failure and central nervous system emboli. Guidelines for the treatment of streptococci and methicillin-susceptible S. aureus (MSSA) endocarditis include a beta-lactam agent whenever possible (2) because of the bactericidal effect of these drugs and the possibility to use high doses with most agents. However, none of the currently licensed beta-lactam agents are clinically active against methicillin-resistant S. aureus (MRSA), and the alternative agents available for the treatment of MRSA endocarditis are limited. Vancomycin, which remains the first choice in recent guidelines, has a narrow therapeutic index, while recent reports have documented a gradual increase in MICs for MRSA over time (10, 33, 34). Moreover, in vitro studies and clinical data suggest that vancomycin is less active than beta-lactams in the treatment of MSSA bacteremia (35). Daptomycin is an alternative, given its rapid bactericidal activity, but clinical data are limited in left-sided endocarditis (15), and the ideal dose has still to be determined, leaving clinicians “guessing” doses, frequently at even numbers of mg per kg of body weight per day. The emergence of resistance during daptomycin therapy and reports of cross-resistance in non-vancomycin-susceptible S. aureus are also of concern (4, 31, 32). Lastly, linezolid, which has proved its value in the treatment of pneumonia and complicated skin and skin structure infections due to multidrug-resistant Gram-positive organisms (25), has been disappointing for the treatment of bloodstream infections (12) and is considered only with reluctance for the treatment of endocarditis given its lack of bactericidal effect. With the worldwide emergence of community-associated MRSA (3, 8), the limits of our therapeutic armamentarium against MRSA endocarditis are a growing concern.Ceftobiprole (BPR) (formerly BAL9141), is a novel, broad-spectrum, bactericidal cephalosporin with MICs of ≤4 μg/ml for clinical isolates of S. aureus, including MRSA (5, 27). The anti-MRSA activity of ceftobiprole stems from its high affinity for PBP 2a, the penicillin binding protein chiefly responsible for the methicillin-resistant phenotype of staphylococci (1). In addition, ceftobiprole is stable to class A penicillinases produced by S. aureus. To improve solubility, ceftobiprole is administered as its dioxolenylmethyl carbamate prodrug, ceftobiprole medocaril (formerly BAL5788) (1). The purpose of the present study was to test the hypothesis that ceftobiprole would be superior to vancomycin, daptomycin, and linezolid for the treatment of MRSA in the rabbit model of aortic valve endocarditis.     

Effectiveness of Nafcillin, Methicillin, and Cephalothin in Experimental Staphylococcus aureus Endocarditis

Nafcillin, methicillin, and cephalothin (40 mg/kg every 6 h) were all effective in reducing the number of Staphylococcus aureus in vegetations in rabbits with endocarditis. Nafcillin and methicillin reduced the number of S. aureus at a significantly faster rate than did cephalothin. Nafcillin and methicillin also reduced titers of the S. aureus more rapidly than did cephalothin in vitro, both in broth and in rabbit serum.     

Y-688, a New Quinolone Active against Quinolone-Resistant Staphylococcus aureus: Lack of In Vivo Efficacy in Experimental Endocarditis

Y-688 is a new fluoroquinolone with increased activity against ciprofloxacin-resistant staphylococci. The MICs of Y-688 and other quinolones were determined for 58 isolates of ciprofloxacin-resistant and methicillin-resistant Staphylococcus aureus (MRSA). The MICs at which 50% and 90% of bacteria were inhibited were ≥128 and ≥128 mg/liter, respectively, for ciprofloxacin, 16 and 32 mg/liter, respectively, for sparfloxacin, and 0.25 and 1 mg/liter, respectively, for Y-688. This new quinolone was further tested in rats with experimental endocarditis due to either of two isolates of ciprofloxacin-resistant MRSA (namely, P8/128 and CR1). Infected animals were treated for 3 days with ciprofloxacin, vancomycin, or Y-688. Antibiotics were administered through a computerized pump to simulate human-like pharmacokinetics in the serum of rats. The anticipated peak and trough levels of Y-688 were 4 and 1 mg/liter at 0.5 and 12 h, respectively. Treatment with ciprofloxacin was ineffective. Vancomycin significantly decreased vegetation bacterial counts for both organisms (P 0.05). In contrast, Y-688 only marginally decreased vegetation bacterial counts (P 0.05). Moreover, several vegetation that failed Y-688 treatment grew staphylococci for which the MICs of the test antibiotic were increased two to eight times. Y-688 also selected for resistance in vitro, and isolates for which the MICs were increased eight times emerged at a frequency of ca. 10⁻⁸. Thus, in spite of its low MIC for ciprofloxacin-resistant MRSA, Y-688 failed in vivo and its use carried the risk of resistance selection. The fact that ciprofloxacin-resistant staphylococci became rapidly resistant to this potent new drug suggests that the treatment of ciprofloxacin-resistant MRSA with new quinolones might be more problematic than expected.     

Efficacy of Trovafloxacin in Treatment of Experimental Staphylococcal or Streptococcal Endocarditis

The efficacy of trovafloxacin against Staphylococcus aureus and viridans group streptococci was investigated in vitro and in an experimental model of endocarditis. The MICs at which trovafloxacin and ciprofloxacin inhibited 90% of clinical isolates of such bacteria (MIC₉₀s) were (i) 0.03 and 2 mg/liter, respectively, for 30 ciprofloxacin-susceptible S. aureus isolates, (ii) 32 and 128 mg/liter, respectively, for 20 ciprofloxacin-resistant S. aureus isolates, and (iii) 0.25 and 8 mg/liter, respectively, for 28 viridans group streptococci. Rats with aortic vegetations were infected with either of two ciprofloxacin-susceptible but methicillin-resistant S. aureus strains (strains COL and P8), one penicillin-susceptible Streptococcus sanguis strain, or one penicillin-resistant Streptococcus mitis strain. Rats were treated for 3 or 5 days with doses that resulted in kinetics that simulated those achieved in humans with trovafloxacin (200 mg orally once a day), ciprofloxacin (750 mg orally twice a day), vancomycin (1 g intravenously twice a day), or ceftriaxone (2 g intravenously once a day). Against the staphylococci, the activities of both trovafloxacin and ciprofloxacin were equivalent to that of vancomycin, and treatment of endocarditis with these drugs was successful (P < 0.05). However, ciprofloxacin selected for resistant derivatives in vitro and in vivo, whereas trovafloxacin was 10 to 100 times less prone than ciprofloxacin to select for resistance in vitro and did not select for resistance in vivo. Against the two streptococcal isolates, trovafloxacin significantly (P < 0.05) decreased bacterial counts in the vegetations but was less effective than the control drug, ceftriaxone. Thus, a simulated oral dose of trovafloxacin (200 mg per day) was effective against ciprofloxacin-susceptible staphylococci and was less likely than ciprofloxacin to select for resistance. The simulated oral dose of trovafloxacin also had some activity against streptococcal endocarditis, but optimal treatment of infections caused by such organisms might require higher doses of the drug.     

Gatifloxacin Efficacy in Treatment of Experimental Methicillin-Sensitive Staphylococcus aureus-Induced Osteomyelitis in Rabbits

The effectiveness of oral gatifloxacin was compared to that of standard parenteral antibiotic therapy (nafcillin) for the treatment of experimental methicillin-sensitive Staphylococcus aureus-induced osteomyelitis in a rabbit model. Gatifloxacin was as effective as nafcillin in clearing the infection. Therefore, oral gatifloxacin treatment of osteomyelitis may be an effective alternative to intravenous nafcillin treatment.     

Single and Combination Antibiotic Therapy of Staphylococcus aureus Experimental Endocarditis: Emergence of Gentamicin-Resistant Mutants

The efficacy of nafcillin and gentamicin used alone and in combination at doses giving serum concentrations comparable to those achieved in patients was studied in rabbits with experimental Staphylococcus aureus endocarditis. The organism used was a penicillinase-producing, methicillin-susceptible, clinical isolate. The addition of gentamicin to nafcillin significantly increased the rate of killing of organisms in valvular vegetations, compared to the effect of nafcillin alone. Gentamicin alone delayed mortality but was not effective in reducing the bacterial populations of the vegetations. Bacteremia persisted in the animals treated with gentamicin alone, in contrast to the groups treated with nafcillin or the combination. Selection of a subpopulation of aminoglycoside-resistant small-colony variants occurred in animals treated with gentamicin alone. This variant was subsequently employed in the rabbit model and produced endocarditis, metastatic infection, and bacteremia comparable to those caused by the parent strain. Animals with infection produced by the variant died later than animals infected by the parent strain. Nafcillin was equally effective in reducing the population of both parent and variant strains in vitro and in therapy of the infected animals. Population studies showed the variant to be a mutant emerging at a rate of 1.9 x 10(-7). It was shown to differ from the parent strain in coagulase and hemolysin production, colonial morphology, and aminoglycoside susceptibility, but was similar by light and electron microscopy and in phage type, pigmentation of colonies, deoxyribonuclease production, mannitol fermentation, and growth rate.     

Treatment of Experimental Staphylococcus aureus Endocarditis: Comparison of Cephalothin, Cefazolin, and Methicillin

The effectiveness of cefazolin in Staphylococcus aureus endocarditis has been questioned because of in vitro inactivation by staphylococcal beta-lactamase. Cefazolin, although inactivated in vitro by S. aureus beta-lactamase, was as effective as cephalothin in the treatment of left-sided S. aureus endocarditis in rabbits. Cefazolin (20 mg/kg every 6 or 8 h), cephalothin (40 mg/kg every 6 h), and methicillin (40 mg/kg every 6 h), administered intramuscularly, were compared in the treatment of left-sided endocarditis caused in rabbits by a highly penicillin-resistant strain of S. aureus. The three antibiotics were all effective in reducing titers in vegetations. However, at the dose used, methicillin reduced the titers more rapidly than cephalothin or cefazolin. Cefazolin concentrations in serum were about double those achieved with cephalothin or methicillin. However, cefazolin was only half as active as methicillin and one-eighth as active as cephalothin in vitro in a serum assay. The half life in serum of cefazolin, cephalothin, and methicillin were each about 30 min. Serum bactericidal activities of the three antibiotics were very similar.     

Addition of Gentamicin or Rifampin Does Not Enhance the Effectiveness of Daptomycin in Treatment of Experimental Endocarditis Due to Methicillin-Resistant Staphylococcus aureus

This study evaluated the activity of daptomycin combined with either gentamicin or rifampin against three methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates in vitro and one isolate in vivo against a representative strain (MRSA-572). Time-kill experiments showed that daptomycin was bactericidal against these strains at concentrations over the MIC. Daptomycin at sub-MIC concentrations plus gentamicin at 1× and 2× the MIC yielded synergy, while the addition of rifampin at 2 to 4 μg/ml resulted in indifference (two strains) or antagonism (one strain). The in vivo activity of daptomycin (6 mg/kg of body weight once a day) was evaluated ± gentamicin (1 mg/kg intravenously [i.v.] every 8 h [q8h]) or rifampin (300 mg i.v. q8h) in a rabbit model of infective endocarditis by simulating human pharmacokinetics. Daptomycin plus gentamicin (median, 0 [interquartile range, 0 to 2] log₁₀ CFU/g vegetation) was as effective as daptomycin alone (0 [0 to 2] log₁₀ CFU/g vegetation) in reducing the density of bacteria in valve vegetations (P = 0.83), and both were more effective than daptomycin plus rifampin (3 [2 to 3.5] log₁₀ CFU/g vegetation; P < 0.05) for the strain studied. In addition, daptomycin sterilized a ratio of vegetations that was similar to that of daptomycin plus gentamicin (10/15 [67%] versus 9/15 [60%]; P = 0.7), and both regimens did so more than daptomycin plus rifampin (3/15 [20%]; P = 0.01 and P = 0.02, respectively). No statistical difference was noted between daptomycin plus gentamicin and daptomycin alone for MRSA treatment. In the combination arm, all isolates from vegetations remained susceptible to daptomycin, gentamicin, and rifampin. Sixty-one percent of the isolates (8/13) acquired resistance to rifampin during monotherapy. In the daptomycin arm, resistance was detected in only one case, in which the daptomycin MIC rose to 2 μg/ml among the recovered bacteria. In conclusion, the addition of gentamicin or rifampin does not enhance the effectiveness of daptomycin in the treatment of experimental endocarditis due to MRSA.Staphylococcus aureus is a common cause of infective endocarditis (IE), with methicillin-resistant S. aureus (MRSA) strains found in up to one-third of all cases (11, 28). Due to multidrug resistance among many strains, vancomycin is the standard therapy for IE caused by MRSA (1). However, vancomycin therapy has been associated with poor outcomes that may be explained by the drug''s slow bactericidal activity and insufficient diffusion into valve vegetations (5, 10, 23).Daptomycin is a cyclic lipopeptide that is rapidly bactericidal against gram-positive pathogens such as MRSA, including strains that exhibit resistance to vancomycin. It is approved for the treatment of skin and soft tissue infections, S. aureus bacteremia, and right-sided native valve endocarditis (6). However, there is limited information regarding the efficacy of daptomycin in the treatment of left-sided native valve IE caused by MRSA. In a randomized clinical trial (10), none of the patients with left-sided endocarditis treated with daptomycin at 6 mg/kg of body weight/day were cured, and postmarketing registry data (24) revealed a successful clinical outcome in only 9 out of 15 cases (60%). Therefore, given the lack of efficacy data with daptomycin monotherapy in left-sided MRSA endocarditis, the continued evaluation of methods to enhance the activity of daptomycin is warranted. It is unknown whether daptomycin''s activity against MRSA may be improved by combining it with one or more additional antibiotics to produce a potentially additive or synergistic effect. Gentamicin has been shown to augment daptomycin''s activity against strains of MRSA in vitro (4, 20, 35). The combination of daptomycin plus rifampin has demonstrated additive activity against MRSA in vitro (4) and has enhanced activity against MRSA in vivo (4, 32). The aim of this study was to evaluate the in vitro activity of daptomycin combined with gentamicin or rifampin against MRSA and compare treatment with daptomycin alone to treatment with both combinations in experimental MRSA aortic valve endocarditis using a human-adapted pharmacokinetic model.(This work was previously presented at the 47th Interscience Conference on Antimicrobial Agents and Chemotherapy [ICAAC], Chicago, IL, 17 to 20 September 2007 [29a] and at the 48th Annual ICAAC-IDSA Annual Meeting, Washington, DC, 25 to 28 October 2008 [29b].)     

Successful Treatment of Vancomycin-Intermediate Staphylococcus aureus Pacemaker Lead Infective Endocarditis with Telavancin

Emerging infections caused by vancomycin-intermediate Staphylococcus aureus (VISA) isolates are more likely to be associated with treatment failures than infections caused by other types of S. aureus. We present a case of pacemaker lead infective endocarditis caused by a non-daptomycin-susceptible strain of VISA. After 8 weeks of parenteral telavancin therapy, the patient achieved microbiological and clinical cure.This case involves a 57-year-old woman with rheumatoid arthritis, paroxysmal atrial fibrillation, asthma, and congenital complete atrioventricular heart block which had required a pacemaker for the previous 20 years. She presented to an outside hospital with a 2-week history of fevers, chills, and tachycardia. On physical exam, she was found to have small red macular lesions in her toes and fingers. She was transferred to our hospital because blood cultures were persistently positive for methicillin-resistant Staphylococcus aureus (MRSA) with an elevated MIC of vancomycin of 2 μg/ml by the broth microdilution method. Intravenous vancomycin was replaced with daptomycin at 8 mg/kg intravenously (i.v.) daily. She remained bacteremic for the next 5 days. A transesophageal echocardiogram (TEE) revealed multiple small, highly mobile, linear echogenic densities attached to the pacemaker leads in the right atrium, consistent with small vegetations. The first S. aureus isolate cultured from the blood at our hospital was resistant to cefazolin, clindamycin, erythromycin, oxacillin, and penicillin by the disk diffusion method and susceptible to doxycycline, linezolid, trimethoprim-sulfamethoxazole, and vancomycin. By broth microdilution, MICs for this S. aureus isolate were 2 μg/ml of vancomycin, 0.5 μg/ml of daptomycin, and 4 μg/ml of linezolid. On day 6 of hospitalization, the pacemaker was explanted and replaced with a temporary transvenous pacemaker because of her complete heart block. MRSA was also isolated from the pacemaker pocket. On day 10, blood cultures became sterile for the first time since the patient had been on intravenous daptomycin at 8 mg/kg i.v. daily. On day 15, however, the patient developed ventricular tachycardia, and blood cultures were again positive for MRSA. At this point, daptomycin was increased to 10 mg/kg i.v. daily. On day 17, because of back pain and the recurrence of bacteremia, a computed tomography of the chest, abdomen, and pelvis was performed to search for other potential sources of bacteremia. The exam revealed a large epidural abscess at the level of the L4-L5 spine and two retained foreign bodies (part of the pacemaker wire, approximately 1 cm and 2 cm in length) lodged in her pulmonary arteries. On day 18, the MICs for the MRSA isolate from the blood sample drawn on hospital day 15 were available, and they were 4 μg/ml of vancomycin (VISA) and 2.0 μg/ml of daptomycin (non-daptomycin-susceptible S. aureus [DNSSA]); the linezolid MIC remained at 4 μg/ml by the broth microdilution method and the telavancin MIC was 0.25 μg/ml by Etest. Daptomycin was discontinued and telavancin was initiated at 10 mg/kg i.v. daily. On day 19, blood cultures were negative, and the patient underwent incision and drainage and laminectomy of the L4-L5 vertebrae. Gram stain of the operating room cultures was positive for Gram-positive cocci in clusters, but cultures were sterile. The blood samples drawn on the same day were sterile again, and all blood samples drawn afterwards remained sterile. A repeat TEE on day 24 (day 6 of telavancin) showed new multiple smaller echogenic densities consistent with vegetations along the superior cava vein and on the temporary wire. On day 33, at 15 days after initiation of telavancin, since all blood samples drawn after day 19 remained sterile, new epicardial ventricular leads were implanted and the temporary, presumably infected, pacemaker wire was removed. The patient continued to improve clinically and completed an 8-week course of telavancin at a skilled nursing care/rehabilitation facility. Her renal function (creatinine level) was monitored at least twice a week, and it remained normal throughout the course of therapy. Because of the potentially infected piece of pacemaker wire lodged in the pulmonary artery, she was treated with doxycycline at 100 mg orally twice a day as chronic suppressive therapy. At follow-up 2 months after completion of therapy, she complained only of chronic back pain and was able to walk short distances again. Surveillance blood samples drawn after discontinuation of telavancin and 2 weeks prior to the follow-up were sterile. A summary of the clinical course and MICs is presented in Table ​Table11.

TABLE 1.

Summary of days of bacteremia, blood culture results, antibiotics MICs, and therapy

Efficacy of Telavancin in a rabbit model of aortic valve endocarditis due to methicillin-resistant Staphylococcus aureus or vancomycin-intermediate Staphylococcus aureus

The activities of telavancin and vancomycin were compared in vitro and in the rabbit model of aortic valve endocarditis against a methicillin-resistant Staphylococcus aureus strain, COL, and a vancomycin-intermediate S. aureus (VISA) strain, HIP 5836. Telavancin was bactericidal in time-kill studies at a concentration of 5 microg/ml against both COL and HIP5836. Vancomycin was bacteriostatic at 5 microg/ml and bactericidal at 10 microg/ml against COL and was bacteriostatic at 10 microg/ml against VISA strain HIP 5836. Compared to untreated controls, a twice-daily regimen of 30 mg/kg of telavancin reduced mean aortic valve vegetation titers of the COL strain by 4.7 log(10) CFU/g after 4 days of therapy and sterilized 6/11 vegetations compared to 3.4 log(10) CFU/g with 3/10 vegetations sterilized for a regimen of twice-daily vancomycin, 30 mg/kg; these differences were not statistically significant. Telavancin was significantly more effective than vancomycin in the VISA model, producing a 5.5 log(10) CFU/g reduction versus no reduction in CFU with vancomycin. In experiments comparing 2-day regimens of telavancin at 30 mg/kg and 50 mg/kg twice daily, organisms were rapidly eliminated from vegetations, but the effect was not different between the two doses. These results suggest that telavancin may be an effective treatment for endocarditis and other serious staphylococcal infections accompanied by bacteremia, including infections caused by staphylococci not susceptible to vancomycin.     

Lysostaphin Treatment of Experimental Aortic Valve Endocarditis Caused by a Staphylococcus aureus Isolate with Reduced Susceptibility to Vancomycin

The rabbit model of endocarditis was used to test the effectiveness of vancomycin and two different lysostaphin dosing regimens for the treatment of infections caused by a Staphylococcus aureus strain with reduced susceptibility to vancomycin (glycopeptide-intermediate susceptible S. aureus [GISA]). Vancomycin was ineffective, with no evidence of sterilization of aortic valve vegetations. However, rates of sterilization of aortic valve vegetations were significantly better for animals treated with either a single dose of lysostaphin (43%) or lysostaphin given twice daily for 3 days (83%) than for animals treated with vancomycin. Rabbits given a single dose of lysostaphin followed by a 3-day drug-free period had mean reductions in aortic valve vegetation bacterial counts of 7.27 and 6.63 log10 CFU/g compared with those for untreated control rabbits and the vancomycin-treated group, respectively. We conclude that lysostaphin is an effective alternative for the treatment of experimental aortic valve endocarditis caused by a clinical VISA strain.