Activities of Fosfomycin and Rifampin on Planktonic and Adherent Enterococcus faecalis Strains in an Experimental Foreign-Body Infection Model

Enterococcal implant-associated infections are difficult to treat because antibiotics generally lack activity against enterococcal biofilms. We investigated fosfomycin, rifampin, and their combinations against planktonic and adherent Enterococcus faecalis (ATCC 19433) in vitro and in a foreign-body infection model. The MIC/MBC_log values were 32/>512 μg/ml for fosfomycin, 4/>64 μg/ml for rifampin, 1/2 μg/ml for ampicillin, 2/>256 μg/ml for linezolid, 16/32 μg/ml for gentamicin, 1/>64 μg/ml for vancomycin, and 1/5 μg/ml for daptomycin. In time-kill studies, fosfomycin was bactericidal at 8× and 16× MIC, but regrowth of resistant strains occurred after 24 h. With the exception of gentamicin, no complete inhibition of growth-related heat production was observed with other antimicrobials on early (3 h) or mature (24 h) biofilms. In the animal model, fosfomycin alone or in combination with daptomycin reduced planktonic counts by ≈4 log₁₀ CFU/ml below the levels before treatment. Fosfomycin cleared planktonic bacteria from 74% of cage fluids (i.e., no growth in aspirated fluid) and eradicated biofilm bacteria from 43% of cages (i.e., no growth from removed cages). In combination with gentamicin, fosfomycin cleared 77% and cured 58% of cages; in combination with vancomycin, fosfomycin cleared 33% and cured 18% of cages; in combination with daptomycin, fosfomycin cleared 75% and cured 17% of cages. Rifampin showed no activity on planktonic or adherent E. faecalis, whereas in combination with daptomycin it cured 17% and with fosfomycin it cured 25% of cages. Emergence of fosfomycin resistance was not observed in vivo. In conclusion, fosfomycin showed activity against planktonic and adherent E. faecalis. Its role against enterococcal biofilms should be further investigated, especially in combination with rifampin and/or daptomycin treatment.     

Short-course gentamicin in combination with daptomycin or vancomycin against Staphylococcus aureus in an in vitro pharmacodynamic model with simulated endocardial vegetations

The ability to maximize bactericidal activity while minimizing toxicity is a therapeutic goal in the treatment of infective endocarditis. We evaluated the impact of administering short-course regimens of gentamicin in combination with daptomycin or vancomycin against one methicillin-susceptible (MSSA 1199) and one methicillin-resistant (MRSA 494) Staphylococcus aureus isolate using an in vitro pharmacodynamic model with simulated endocardial vegetations over 96 h. Human therapeutic dosing regimens for daptomycin (6 and 8 mg/kg of body weight), vancomycin, and gentamicin were simulated. Short-course combination regimens involving gentamicin were administered either as a single 5-mg/kg dose or three 1-mg/kg doses for only the first 24 h and compared to the regimens administered for the full 96-h duration. For all experiments, physiologic conditions of albumin, calcium, and pH were simulated. Both regimens of daptomycin achieved 99.9% kill by 32 h and maintained bactericidal activity against both isolates, which was significantly different from vancomycin, which displayed bacteriostatic activity (P < 0.05). The effects of all short-course regimens of gentamicin were equal to those of the full-duration regimens in combination with daptomycin. Adding three doses of gentamicin (1 mg/kg) to daptomycin resulted in enhancement and bactericidal activity at 24 h against both MRSA and MSSA. The addition of a single dose of gentamicin (5 mg/kg) enhanced or improved the activity of daptomycin and resulted in early bactericidal activity at 4 h against both isolates. The addition of three doses of gentamicin (1 mg/kg) did not improve the activity of vancomycin. However, the addition of a single 5-mg/kg dose of gentamicin to vancomycin resulted in early enhancement at 4 h and 99.9% kill at 32 h for MRSA. These results suggest that a single high dose of gentamicin in combination with daptomycin or vancomycin may be of utility to maximize synergistic and bactericidal activity and minimize toxicity. Further investigation is warranted.     

Evaluation of daptomycin pharmacodynamics and resistance at various dosage regimens against Staphylococcus aureus isolates with reduced susceptibilities to daptomycin in an in vitro pharmacodynamic model with simulated endocardial vegetations

The need to investigate novel dosing regimens and combinations is essential in combating poor treatment outcomes for Staphylococcus aureus bacteremia and endocarditis. We evaluated the impact of simulated standard- and high-dose daptomycin in combination with gentamicin or rifampin against daptomycin-susceptible and nonsusceptible matched strains of S. aureus. These strains were collected from the daptomycin bacteremia and endocarditis clinical trial and consisted of three susceptible strains (MIC, 0.25 mg/liter) and four nonsusceptible isolates (MICs, 2 to 4 mg/liter). Daptomycin regimens of 6 and 10 mg/kg of body weight daily alone and in combination with gentamicin at 5 mg/kg daily or rifampin at 300 mg every 8 h were evaluated using an in vitro model with simulated endocardial vegetations over 96 h. Rapid bactericidal activity, identified by time to 99.9% kill, was displayed in all regimens with the daptomycin-susceptible strains. Concentration-dependent activity was noted by more-rapid killing with the 10-mg/kg/day dose. The addition of gentamicin improved activity in the majority of susceptible isolates. Daptomycin 6-mg/kg/day monotherapy displayed bactericidal activity for only one of the nonsusceptible isolates and for only two isolates with increased doses of 10 mg/kg/day. Combination regimens demonstrated improvement with some but not all nonsusceptible isolates. Three isolates developed a reduction in daptomycin susceptibility with 6-mg/kg/day monotherapy, but this was suppressed with both combination therapy and high-dose daptomycin. These results suggest that high-dose daptomycin therapy and combination therapy may be reasonable treatment options for susceptible isolates; however, more investigations are needed to confirm the variability of these regimens with nonsusceptible isolates.     

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].)     

Impact of Different Antimicrobial Therapies on Clinical and Fiscal Outcomes of Patients with Bacteremia Due to Vancomycin-Resistant Enterococci

Vancomycin-resistant enterococci (VRE) are a growing health problem, and uncertainties exist regarding the optimal therapy for bloodstream infection due to VRE. We conducted systematic comparative evaluations of the impact of different antimicrobial therapies on the outcomes of patients with bloodstream infections due to VRE. A retrospective study from January 2008 to October 2010 was conducted at Detroit Medical Center. Unique patients with blood cultures due to VRE were included and reviewed. Three major therapeutic classes were analyzed: daptomycin, linezolid, and β-lactams. Three multivariate models were conducted for each outcome, matching for a propensity score predicting the likelihood of receipt of one of the therapeutic classes. A total of 225 cases of bacteremia due to VRE were included, including 86 (38.2%) cases of VR Enterococcus faecalis and 139 (61.8%) of VR Enterococcus faecium. Bacteremia due to VR E. faecalis was more frequent among subjects treated with β-lactams than among those treated with daptomycin or linezolid. The median dose of daptomycin was 6 mg/kg of body weight (range, 6 to 12 mg/kg). After controlling for propensity score and bacteremia due to VR E. faecalis, differences in mortality were nonsignificant among the treatment groups. Therapy with daptomycin was associated with higher median variable direct cost per day than that for linezolid. This large study revealed the three therapeutic classes (daptomycin, linezolid, and β-lactams) are similarly efficacious in the treatment of bacteremia due to susceptible strains of VRE.     

Impact of high-inoculum Staphylococcus aureus on the activities of nafcillin, vancomycin, linezolid, and daptomycin, alone and in combination with gentamicin, in an in vitro pharmacodynamic model

We evaluated the impact of high (9.5 log10 CFU/g) and moderate (5.5 log10 CFU/g) inocula of methicillin-susceptible and -resistant Staphylococcus aureus (MSSA and MRSA, respectively) on the activities of nafcillin, linezolid, vancomycin, and daptomycin, alone and in combination with gentamicin in an in vitro pharmacodynamic model with simulated endocardial vegetations over 72 h. Human therapeutic dosing regimens for nafcillin, daptomycin, vancomycin, linezolid, and gentamicin were simulated. At a moderate inoculum, nafcillin (MSSA only), vancomycin, and daptomycin demonstrated equivalent and significant (P < 0.01) bactericidal (99.9% kill) activities (decreases of 3.34 +/- 1.1, 3.28 +/- 0.4, and 3.34 +/- 0.8 log10 CFU/g, respectively). Bactericidal activity was demonstrated at 4 h for nafcillin and daptomycin and at 32 h for vancomycin. Linezolid demonstrated bacteriostatic activity over the course of the study period. At a high inoculum, daptomycin exhibited bactericidal activity against both MSSA and MRSA by 24 h (decrease of 5.51 to 6.31 +/- 0.10 log10 CFU/g). Nafcillin (versus MSSA), vancomycin, and linezolid (MSSA and MRSA) did not achieve bactericidal activity throughout the 72-h experiment. The addition of gentamicin increased the rate of 99.9% kill to 8 h for daptomycin (P < 0.01) and 48 h for nafcillin (MSSA only) (P = 0.01). The addition of gentamicin did not improve the activity of vancomycin or linezolid for either isolate for the 72-h period. Overall, high-inoculum Staphylococcus aureus had a significant impact on the activities of nafcillin and vancomycin. In contrast, daptomycin was affected minimally and linezolid was not affected by inoculum.     

Evaluation of standard- and high-dose daptomycin versus linezolid against vancomycin-resistant Enterococcus isolates in an in vitro pharmacokinetic/pharmacodynamic model with simulated endocardial vegetations

Daptomycin MICs for enterococci are typically 1- to 2-fold higher than those for Staphylococcus aureus, and there is an imminent need to establish the optimal dose for appropriate treatment of enterococcal infections. We investigated the bactericidal activity of daptomycin at various dose exposures compared to that of linezolid against vancomycin-resistant enterococcus (VRE) in an in vitro pharmacokinetic/pharmacodynamic model utilizing simulated endocardial vegetations over 96 h. Daptomycin at doses of 6, 8, 10, and 12 mg/kg of body weight/day and linezolid at a dose of 600 mg every 12 h were evaluated against two clinical vancomycin-resistant Enterococcus faecium strains (EFm11499 and 09-184D1051), one of which was linezolid resistant (09-184D1051), and one clinical vancomycin-resistant Enterococcus faecalis strain (EFs11496). Daptomycin MICs were 4, 2, and 0.5 μg/ml for EFm11499, 09-184D1051, and EFs11496, respectively. Bactericidal activity, defined as a ≥ 3 log(10) CFU/g reduction from the initial colony count, was demonstrated against all three isolates with all doses of daptomycin; however, bactericidal activity was not sustained with the daptomycin 6- and 8-mg/kg/day regimens. Linezolid was bacteriostatic against EFm11499 and displayed no appreciable activity against 09-184D1051 or EFs11496. Concentration-dependent killing was displayed with more sustained reduction in colony count (3.58 to 6.46 and 5.89 to 6.56 log(10) CFU/g) at 96 h for the simulated regimen of daptomycin at doses of 10 and 12 mg/kg/day, respectively (P ≤ 0.012). No E. faecium mutants with reduced susceptibility were recovered at any dosage regimen; however, the E. faecalis strain developed reduced daptomycin susceptibility with daptomycin at 6, 8, and 10 but not at 12 mg/kg/day. Daptomycin displayed a dose-dependent response against three VRE isolates, with high-dose daptomycin producing sustained bactericidal activity. Further research is warranted.     

Impact of Enterococcus faecalis on the bactericidal activities of arbekacin, daptomycin, linezolid, and tigecycline against methicillin-resistant Staphylococcus aureus in a mixed-pathogen pharmacodynamic model

We inoculated an in vitro pharmacodynamic model simultaneously with clinical isolates of methicillin-resistant Staphylococcus aureus and an enterocin-producing enterococcus (vancomycin-resistant Enterococcus faecalis, ampicillin susceptible) at 7 log10 CFU/ml to examine enterocin effects and antimicrobial activity on staphylococci. The investigated antimicrobial regimens were 100 mg arbekacin every 12 h (q12h), 6 mg daptomycin per kg of body weight/day, 600 mg linezolid q12h, and 100 mg tigecycline q24h alone and in combination (daptomycin, linezolid, and tigecycline) with arbekacin. Simulations were performed in triplicate; bacterial quantification occurred over 48 h, and development of resistance was evaluated throughout. When we evaluated the impact of antimicrobial activity against S. aureus alone, daptomycin demonstrated bactericidal activity (>or=3 log10 CFU/ml kill), whereas arbekacin, linezolid, and tigecycline displayed bacteriostatic activities (<3 log10 CFU/ml kill). In the mixed-pathogen model, early and distinctive stunting of S. aureus growth was noted (1.5 log CFU/ml difference) in the presence of enterocin-producing E. faecalis compared to growth controls run individually (P=0.02). Most noteworthy was that in the presence of enterocin-producing E. faecalis, bactericidal activity was observed with arbekacin and tigecycline and with the addition of arbekacin to linezolid. Antagonism was noted for the combination of tigecycline and arbekacin against S. aureus in the presence of enterocin-producing E. faecalis. Our research demonstrates that the inhibitory effect of E. faecalis contributed significantly to its overall antimicrobial impact on S. aureus. This contribution was enhanced or improved compared to the activity of each antimicrobial alone. Further research is warranted to determine the impact of polymicrobial infections on antimicrobial activity.     

Evaluation of Ceftaroline,Vancomycin, Daptomycin,or Ceftaroline plus Daptomycin against Daptomycin-Nonsusceptible Methicillin-Resistant Staphylococcus aureus in an In Vitro Pharmacokinetic/Pharmacodynamic Model of Simulated Endocardial Vegetations

Infective endocarditis (IE) caused by methicillin-resistant Staphylococcus aureus (MRSA) with reduced susceptibility to vancomycin and daptomycin has few adequate therapeutic options. Ceftaroline (CPT) is bactericidal against daptomycin (DAP)-nonsusceptible (DNS) and vancomycin-intermediate MRSA, but supporting data are limited for IE. This study evaluated the activities of ceftaroline, vancomycin, daptomycin, and the combination of ceftaroline plus daptomycin against DNS MRSA in a pharmacokinetic/pharmacodynamic (PK/PD) model of simulated endocardial vegetations (SEVs). Simulations of ceftaroline-fosamil (600 mg) every 8 h (q8h) (maximum concentration of drug in serum [C_max], 21.3 mg/liter; half-life [t_1/2], 2.66 h), daptomycin (10 mg/kg of body weight/day) (C_max, 129.7 mg/liter; t_1/2, 8 h), vancomycin (1 g) q8h (minimum concentration of drug in serum [C_min], 20 mg/liter; t_1/2, 5 h), and ceftaroline plus daptomycin were evaluated against 3 clinical DNS, vancomycin-intermediate MRSA in a two-compartment, in vitro, PK/PD SEV model over 96 h with a starting inoculum of ∼8 log₁₀ CFU/g. Bactericidal activity was defined as a ≥3-log₁₀ CFU/g reduction from the starting inoculum. Therapeutic enhancement of combinations was defined as ≥2-log₁₀ CFU/g reduction over the most active agent alone. MIC values for daptomycin, vancomycin, and ceftaroline were 4 mg/liter, 4 to 8 mg/liter, and 0.5 to 1 mg/liter, respectively, for all strains. At simulated exposures, vancomycin was bacteriostatic, but daptomycin and ceftaroline were bactericidal. By 96 h, ceftaroline monotherapy offered significantly improved killing compared to other agents against one strain. The combination of DAP plus CPT demonstrated therapeutic enhancement, resulting in significantly improved killing versus either agent alone against 2/3 (67%) strains. CPT demonstrated bactericidal activity against DNS, vancomycin-intermediate MRSA at high bacterial densities. Ceftaroline plus daptomycin may offer more rapid and sustained activity against some MRSA in the setting of high-inoculum infections like IE and should also be considered.     

Efficacy of usual and high doses of daptomycin in combination with rifampin versus alternative therapies in experimental foreign-body infection by methicillin-resistant Staphylococcus aureus

The treatment of prosthetic joint infections caused by methicillin-resistant Staphylococcus aureus (MRSA) continues to be a challenge for the clinician. The aim of this study was to evaluate the efficacies of daptomycin at usual and high doses (equivalent to 6 and 10 mg/kg of body weight/day, respectively, in humans) and in combination with rifampin and to compare the activities to those of conventional anti-MRSA therapies. We used MRSA strain HUSA 304, with the following MICs and minimal bactericidal concentrations (MBCs), respectively: daptomycin, 1 μg/ml and 4 μg/ml; vancomycin, 2 μg/ml and 4 μg/ml; linezolid, 2 μg/ml and >32 μg/ml; and rifampin, 0.03 μg/ml and 0.5 μg/ml. In time-kill curves, only daptomycin and its combinations with rifampin achieved a bactericidal effect in log and stationary phases. For in vivo studies, we used a rat foreign-body infection model. Therapy was administered for 7 days with daptomycin at 100 mg/kg/day and 45/mg/kg/day, vancomycin at 50 mg/kg/12 h, rifampin at 25 mg/kg/12 h, and linezolid at 35 mg/kg/12 h, and each antibiotic was also combined with rifampin. Among monotherapies, daptomycin at 100 mg/kg/day and rifampin performed better than vancomycin and linezolid. In combination with rifampin, both dosages of daptomycin were significantly better than all other combinations, but daptomycin at 100 mg/kg/day plus rifampin achieved better cure rates at day 11 (P < 0.05) than daptomycin at 45 mg/kg/day plus rifampin. Resistant strains were found in monotherapies with rifampin and daptomycin at 45 mg/kg/day. In conclusion, daptomycin at high doses was the most effective monotherapy and also improved the efficacy of the combination with rifampin against foreign-body infections by MRSA. Clinical studies should confirm whether this combination may be considered the first-line treatment for foreign-body infections by MRSA in humans.     

In vitro activities of daptomycin, arbekacin, vancomycin, and gentamicin alone and/or in combination against glycopeptide intermediate-resistant Staphylococcus aureus in an infection model

Daptomycin, a lipopeptide antibiotic, has broad activity against gram-positive organisms, similar to vancomycin; however, its mechanism of action differs, resulting in interference with cell membrane transport and a more rapid bactericidal activity. In light of increasing need for alternative treatments against intermediate-resistant Staphylococcus aureus, there is revitalized interest in this antibiotic. We, therefore, evaluated the activity of daptomycin alone or in combination in an in vitro infection model against two glycopeptide intermediate-resistant S. aureus (GISA) isolates. Newly designed regimens of daptomycin at 4 and 6 mg/kg of body weight every 24 h (q24h) were compared to the previous regimen of 3 mg/kg q12h. Daptomycin MICs and minimal bactericidal concentrations (MBCs) (MIC/MBC) for Mu-50, HIP5836 (992), and MRSA-67 were 0.5/1.0, 0.5/1.0, and 0.125/0.5 microgram/ml, respectively. MICs and MBCs of arbekacin for the three strains were 2.0/8.0, 0. 125/0.5, and 0.125/0.25 microgram/ml, respectively. Vancomycin and gentamicin MICs and MBCs for the three strains were 8.0/8.0, 8.0/8.0, and 0.5/1.0 microgram/ml and 128/128, 0.5/1.0, and 0.25/0.5 microgram/ml, respectively. Our experience with daptomycin in an in vitro infection model has shown significant kill against the two GISA strains (Mu-50 and 992) (P < 0.03). We also noted that kill was related to a total dose effect for 992, in which simulated daptomycin in vivo dosages of 6 mg/kg q24h and 3 mg/kg q12h produced similar kill and 4 mg/kg q24h resulted in significant regrowth (P 相似文献   

Activities of Daptomycin and Vancomycin Alone and in Combination with Rifampin and Gentamicin against Biofilm-Forming Methicillin-Resistant Staphylococcus aureus Isolates in an Experimental Model of Endocarditis

The findings of clinical and in vitro research support the theory that infective endocarditis (IE)-causing bacteria form biofilms and that biofilms negatively affect treatment outcomes. The purpose of the present study was to quantify the biofilm formation of methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates obtained from patients with IE and to evaluate the in vitro activities of daptomycin and vancomycin alone and in combination with rifampin (rifampicin) or gentamicin while monitoring the isolates for the development of resistance. A high-inoculum, stationary-phase infection model of IE was used to simulate the pharmacokinetics in humans of daptomycin at 6 mg/kg of body weight/day, vancomycin at 1.25 g every 12 h (q12h) alone and in combination with rifampin at 300 mg every 8 h, and gentamicin at 1.3 mg/kg q12h. Two randomly selected clinical MRSA isolates were obtained from patients with IE; both MRSA isolates quantitatively produced biofilms. The time to bactericidal activity in the presence of daptomycin was isolate dependent but was achieved by 24 h for both MRSA isolates. Vancomycin did not achieve bactericidal activity throughout the experiment. At 24, 48, and 72 h, daptomycin-containing regimens had significantly more activity (greater declines in the mean number of CFU/g) than any of the vancomycin-containing regimens (P = 0.03). Rifampin and gentamicin antagonized or delayed the bactericidal activity of daptomycin (against MRSA B346846 for rifampin and against both isolates for gentamicin) in the first 24 h. Increases in the daptomycin and vancomycin MICs were not observed. We conclude that in an IE model of biofilm-forming MRSA, daptomycin monotherapy has better in vitro activity than daptomycin in combination with rifampin or gentamicin or any vancomycin-containing regimen studied within the first 24 h. Further investigations are needed to understand the initial delay in bactericidal activity observed when gentamicin or rifampin is combined with daptomycin.Biofilm-forming Staphylococcus aureus isolates are frequently found on prosthetic devices and in deep tissue infections (21, 43), and both prosthetic devices and deep tissue infections serve as common sources for bacteremia. Clinical research supports the theory that infective endocarditis (IE)-causing bacteria form biofilms (14, 19) and that S. aureus isolates recovered from the blood of patients with IE tend to produce biofilms at a high inoculum and during the stationary phase of growth (24, 43). These isolates also typically carry accessory gene regulator (agr) groups I and II, which regulate the production of autolysins that promote biofilm formation (21, 43, 44). Despite this information, there are limited data about the biofilm-forming capabilities of S. aureus isolates that cause IE (SAIE).Until recently, effective antimicrobial therapy for methicillin (meticillin)-resistant S. aureus (MRSA) bacteremia and IE was limited to vancomycin. Although vancomycin has commonly been used since the 1980s for the treatment of MRSA infections, including endocarditis, several published studies indicate that it has limited efficacy because of bacterial resistance, bacterial tolerance, and poor tissue penetration (8, 22, 36). Patients with SAIE treated with vancomycin alone may still be bacteremic (as indicated by positive blood cultures) after 7 to 10 days of therapy (25, 26). Guidelines for the treatment of SAIE recommend the use of combination therapy with vancomycin plus gentamicin or rifampin (rifampicin) (2, 3, 41). However, these combinations can be problematic because gentamicin increases the risk of nephrotoxicity and rifampin increases the potential for drug interactions via its induction of cytochrome P450 metabolism. In addition, this recommendation is based on limited clinical data.Daptomycin, a novel lipopetide antimicrobial agent, received FDA approval in May 2006 for the treatment of bacteremia and right-sided SAIE caused by methicillin-susceptible and -resistant strains (12). The FDA indication is for monotherapy against gram-positive pathogens; however, the guidelines recommend combination therapy for SAIE. Data to support the optimal dose of daptomycin required for it to have activity when it is combined with commonly used synergistic agents such as gentamicin and rifampin are lacking. In addition, limited information about the activity of daptomycin in the presence of biofilm-forming S. aureus has been published (23, 32, 35, 37).The purpose of this study was to quantify the biofilm formation of S. aureus isolates obtained from patients with SAIE, to assess the in vitro activities of daptomycin and vancomycin alone and in combination with rifampin or gentamicin, and to evaluate the development of resistance in a high-inoculum, stationary-phase bacterial (biofilm) model of IE.(This work has been presented in part at the 108th General Meeting of the American Society for Microbiology, Boston, MA, 1 to 5 June 2008 [abstr. A-076].)     

Serum bactericidal activities of high-dose daptomycin with and without coadministration of gentamicin against isolates of Staphylococcus aureus and Enterococcus species

The purpose of this experiment was to evaluate the pharmacokinetics and serum bactericidal titers (SBTs) of daptomycin alone and in combination with gentamicin against strains of Staphylococcus aureus and enterococci to determine if there might be any benefit to the addition of the aminoglycoside. A multiple-dose, randomized crossover study was performed in 11 healthy volunteers to evaluate the steady-state pharmacokinetic profile of 6 mg/kg of body weight daptomycin once daily with or without 1 mg/kg gentamicin every 8 h. SBTs were determined against clinical isolates of nosocomial (MRSA 494) and community-acquired (CA-MRSA 44) methicillin-resistant S. aureus, vancomycin-susceptible Enterococcus faecalis (VSEF 49452), vancomycin-resistant Enterococcus faecium (VREF 80), and quality control strains of methicillin-susceptible S. aureus (ATCC 29213) and vancomycin-susceptible E. faecalis (ATCC 29212). Enhancement of bactericidal activity was evaluated by calculating and comparing the areas under the bactericidal curve (AUBC) for each dosing regimen against each isolate. The area under the concentration-time curve from 0 to 24 h and clearance for daptomycin alone were 645 +/- 91 microg.h/ml and 9.47 +/- 1.4 mg/h/kg, respectively, compared with 642 +/- 69 microg.h/ml and 9.45 +/- 1.0 mg/h/kg for daptomycin plus gentamicin. Daptomycin alone displayed sustained bactericidal activity against five of the six isolates over the entire 24-h dosing interval; bactericidal activity was maintained for 8 h against VREF 80. Mean AUBCs for daptomycin alone ranged from 935 to 1,263 and 36 to 238 against staphylococcal and enterococcal isolates, respectively, compared with 902 to 972 and 34 to 213 against staphylococci and enterococci when coadministered with gentamicin. The results of this study suggest that the addition of gentamicin does not alter the pharmacokinetic profile or enhance the bactericidal activity of daptomycin against staphylococcal or enterococcal isolates.     

Observed Antagonistic Effect of Linezolid on Daptomycin or Vancomycin Activity against Biofilm-Forming Methicillin-Resistant Staphylococcus aureus in an In Vitro Pharmacodynamic Model

Pharmacodynamic activity in antibiotic combinations of daptomycin, vancomycin, and linezolid was investigated in a 48-h in vitro pharmacodynamic model. Using human-simulated free drug concentrations, activity against clinical biofilm-forming methicillin-resistant Staphylococcus aureus isolates was evaluated. Linezolid antagonized vancomycin activity at 24 and 48 h. Linezolid antagonized daptomycin at 24 and 48 h depending on dose and strain. Adding daptomycin increased vancomycin activity at 48 h (P < 0.03). These results may be strain dependent and require further clinical investigation.     

Antimicrobial susceptibility of daptomycin and comparator agents tested against methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci: trend analysis of a 6-year period in US medical centers (2005-2010)

We evaluated daptomycin activity trends against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE; MIC, ≥8 μg/mL) in a 6-year period (2005–2010) following US regulatory release for clinical use. Consecutive, unique patient strains of clinical significance were collected in 32 US medical centers and susceptibility tested in a central laboratory against daptomycin and various comparator agents by reference broth microdilution methods. A total of 22 858 S. aureus (12 181 [53.3%] MRSA), 4312 Enterococcus faecalis (195 [4.5%] VRE), and 2462 Enterococcus faecium (1867 [75.8%] VRE) were evaluated. Daptomycin susceptibility rates were 99.94%, 99.98%, and 99.68% for S. aureus, E. faecalis, and E. faecium, respectively. Among MRSA (daptomycin MIC_50/90, 0.25/0.5 μg/mL), only 13 (0.11%) daptomycin-non-susceptible (MIC, ≥2 μg/mL) isolates were observed with no MIC creep over the study interval. Daptomycin was very active against vancomycin-resistant E. faecalis (MIC_50/90, 1/2 μg/mL) and E. faecium (MIC_50/90, 2/2 μg/mL). Among VRE, only 4 daptomycin-non-susceptible isolates (all E. faecium) were detected. In conclusion, daptomycin demonstrated sustained activity against an extensive collection of clinical isolates of MRSA and VRE from numerous US medical centers over the last 6 monitored years.     

Evaluation of the Novel Combination of High-Dose Daptomycin plus Trimethoprim-Sulfamethoxazole against Daptomycin-Nonsusceptible Methicillin-Resistant Staphylococcus aureus Using an In Vitro Pharmacokinetic/Pharmacodynamic Model of Simulated Endocardial Vegetations

Daptomycin-nonsusceptible (DNS) Staphylococcus aureus is found in difficult-to-treat infections, and the optimal therapy is unknown. We investigated the activity of high-dose (HD) daptomycin plus trimethoprim-sulfamethoxazole de-escalated to HD daptomycin or trimethoprim-sulfamethoxazole against 4 clinical DNS methicillin-resistant S. aureus (MRSA) isolates in an in vitro pharmacokinetic/pharmacodynamic model of simulated endocardial vegetations (10⁹ CFU/g). Simulated regimens included HD daptomycin at 10 mg/kg/day for 14 days, trimethoprim-sulfamethoxazole at 160/800 mg every 12 h for 14 days, HD daptomycin plus trimethoprim-sulfamethoxazole for 14 days, and the combination for 7 days de-escalated to HD daptomycin for 7 days and de-escalated to trimethoprim-sulfamethoxazole for 7 days. Differences in CFU/g (at 168 and 336 h) were evaluated by analysis of variance (ANOVA) with a Tukey''s post hoc test. Daptomycin MICs were 4 μg/ml (SA H9749-1, vancomycin-intermediate Staphylococcus aureus; R6212, heteroresistant vancomycin-intermediate Staphylococcus aureus) and 2 μg/ml (R5599 and R5563). Trimethoprim-sulfamethoxazole MICs were ≤0.06/1.19 μg/ml. HD daptomycin plus trimethoprim-sulfamethoxazole displayed rapid bactericidal activity against SA H9749-1 (at 7 h) and R6212 (at 6 h) and bactericidal activity against R5599 (at 72 h) and R5563 (at 36 h). A ≥8 log₁₀ CFU/g decrease was observed with HD daptomycin plus trimethoprim-sulfamethoxazole against all strains (at 48 to 144 h), which was maintained with de-escalation to HD daptomycin or trimethoprim-sulfamethoxazole at 336 h. The combination for 14 days and the combination for 7 days de-escalated to HD daptomycin or trimethoprim-sulfamethoxazole was significantly better than daptomycin monotherapy (P < 0.05) and trimethoprim-sulfamethoxazole monotherapy (P < 0.05) at 168 and 336 h. Combination therapy followed by de-escalation offers a novel bactericidal therapeutic alternative for high-inoculum, serious DNS MRSA infections.     

Efficacy of daptomycin in experimental endocarditis due to methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus is becoming increasingly prevalent as both a nosocomial and a community-acquired pathogen. Daptomycin, a lipopeptide antibiotic now in phase III clinical trials, is rapidly bactericidal in vitro against a range of gram-positive organisms, including methicillin-resistant S. aureus (MRSA). In this study, we compared the efficacy of daptomycin with that of vancomycin, each with or without rifampin, in a model of experimental aortic valve endocarditis due to MRSA. The infecting strain (MRSA strain 32) was susceptible to daptomycin (MIC = 1 micro g/ml), vancomycin (MIC = 0.5 micro g/ml), and rifampin (MIC = 0.5 micro g/ml). Daptomycin was administered at 25 or 40 mg/kg q24h (q24h) by subcutaneous injection in an attempt to simulate human doses of 4 and 6 mg/kg q24h, respectively. Vancomycin was given at 150 mg/kg q24h by continuous intravenous infusion. Rifampin was given at 25 mg/kg by intramuscular injection q24h. Treatment was started 6 h postinoculation and continued for 4.5 days. Outcome was assessed by counting the residual viable bacteria in vegetations. The mean peak daptomycin levels in serum at 2 h after subcutaneous administration of 25 and 40 mg/kg were 64 and 91 micro g/ml, respectively. Daptomycin was undetectable in serum at 24 h. The total exposure was comparable to that achieved clinically in humans receiving the drug. Bacterial counts (mean log(10) number of CFU per gram +/- the standard deviation) in untreated controls reached 10.6 +/- 0.8. In treated rats, bacterial counts were as follows: vancomycin, 7.1 +/- 2.5; daptomycin at 25 mg/kg, 5.5 +/- 1.7; daptomycin at 40 mg/kg, 4.2 +/- 1.5. The difference between daptomycin at 40 mg/kg and vancomycin at 150 mg/kg was statistically significant (P = 0.004). In the study of combination therapy, vegetation bacterial counts were as follows: daptomycin at 40 mg/kg, 4.6 +/- 1.6; rifampin, 3.6 +/- 1.3; vancomycin plus rifampin, 3.3 +/- 1.1; daptomycin plus rifampin, 2.9 +/- 0.8. The difference between daptomycin and daptomycin plus rifampin was statistically significant (P = 0.006). These results support the continued evaluation of daptomycin for serious MRSA infections, including infective endocarditis.     

In vitro synergism between daptomycin and fosfomycin against Enterococcus faecalis isolates with high-level gentamicin resistance.

Daptomycin and fosfomycin are two agents which inhibit different steps in peptidoglycan synthesis. We studied the in vitro activities of these drugs, alone and in combination, by time-kill techniques against 21 clinical isolates of Enterococcus (Streptococcus) faecalis demonstrating high-level resistance to gentamicin. Combinations of fosfomycin and daptomycin exhibited synergistic bactericidal activity (100-fold decrease in CFU per milliliter at 24 h compared with daptomycin alone) against all strains (mean +/- standard deviation of increment in killing = 2.7 +/- 0.7 log10 CFU/ml). In a subgroup of strains against which daptomycin (5 micrograms/ml) alone was bactericidal (greater than 3 log10 killing), synergistic activity was demonstrable only when the concentration of daptomycin was lowered to 0.25 to 0.5 microgram/ml. A 50% dilution of human serum diminished the bactericidal activity of daptomycin alone at 24 h but did not affect killing observed with the daptomycin-fosfomycin combination. The inhibition of peptidoglycan synthesis by the combination was greater than the inhibition observed with either drug alone. The combination of daptomycin and fosfomycin exhibited consistent synergistic bactericidal activity against strains of E. faecalis possessing high-level resistance to gentamicin. This synergism may be the result of sequential inhibition of early steps in peptidoglycan synthesis.     

Novel daptomycin combinations against daptomycin-nonsusceptible methicillin-resistant Staphylococcus aureus in an in vitro model of simulated endocardial vegetations

Reduced susceptibility to daptomycin has been reported in patients with infections due to methicillin-resistant Staphylococcus aureus (MRSA). Although infections with daptomycin-nonsusceptible (DNS) MRSA are infrequent, optimal therapy of these strains has not been determined. We investigated the killing effects of novel antibiotic combinations with daptomycin (DAP) against two clinical DNS MRSA isolates (SA-684 and R6003) in a 72-h in vitro pharmacokinetic/pharmacodynamic (PK/PD) model with simulated endocardial vegetations (SEV). Simulated regimens included DAP at 6 mg/kg every 24 h (q24h) alone or in combination with trimethoprim-sulfamethoxazole (TMP/SMX) at 160/800 mg q12h, linezolid (LIN) at 600 mg q12h, cefepime (CEF) at 2 g q12h, and nafcillin (NAF) at 4 g q4h. Bactericidal activity was defined as a ≥3-log(10) CFU/g kill. Differences in CFU/g were evaluated between 4 and 72 h by analysis of variance with the Bonferroni post hoc test. DAP MICs were 4 and 2 mg/liter for SA-684 and R6003, respectively. In the PK/PD model, DAP alone was slowly bactericidal, achieving a 3-log(10) kill at 24 and 50 h for SA-684 and R6003, respectively. Against SA-684, DAP plus TMP/SMX, CEF, LIN, or NAF was bactericidal at 4, 4, 8, and 8 h, respectively, and maintained this activity for the 72-h study duration. DAP plus TMP/SMX or CEF exhibited superior killing than DAP alone against SA-684 between 4 and 72 h, and overall this was significant (P < 0.05). Against R6003, DAP plus TMP/SMX was bactericidal (8 h) and superior to DAP alone between 8 and 72 h (P < 0.001). The unique combination of DAP plus TMP/SMX was the most effective and rapidly bactericidal regimen against the two isolates tested and may provide a clinical option to treat DNS S. aureus infections.     

In vitro bactericidal activity of daptomycin against staphylococci

MICs and minimum bactericidal concentrations (MBCs) of daptomycin, vancomycin, linezolid and quinupristin-dalfopristin (Q-D) were determined for 108 staphylococcal isolates. All strains were susceptible (MICs) to daptomycin (< or =2.0 mg/L) and Q-D (< or =1.0 mg/L). All but three isolates were susceptible to vancomycin (< or =4.0 mg/L) and all but one methicillin-resistant Staphylococcus aureus strain were susceptible to linezolid (< or =4.0 mg/L). Q-D had the lowest geometric mean MIC (0.29 mg/L) and daptomycin had the lowest geometric mean MBC (0.57 mg/L). Time-kill tests were performed on 25 isolates. Bactericidal activity (>99.9% kill) was observed with daptomycin at 2 mg/L and at 2 x MBC for 92% of strains tested. In comparison, the bactericidal rates for the other drugs at breakpoint concentrations and at 2 x MBC were 72% and 70% for vancomycin, 46% and 60% for Q-D, and 7% and 14% for linezolid. Of the four drugs tested, daptomycin was bactericidal against the most strains and had the most rapid cidal activity.