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.     

In vitro and in vivo activities of tigecycline (GAR-936), daptomycin,and comparative antimicrobial agents against glycopeptide-intermediate Staphylococcus aureus and other resistant gram-positive pathogens

Tigecycline (GAR-936) and daptomycin are potent antibacterial compounds in advanced stages of clinical trials. These novel agents target multiply resistant pathogenic bacteria. Daptomycin is principally active against gram-positive bacteria, while tigecycline has broad-spectrum activity. When tested by the standard protocols of the National Committee for Clinical Laboratory Standards in Mueller-Hinton broth II, tigecycline was more active than daptomycin (MICs at which 90% of isolates tested are inhibited, 0.12 to 1 and 0.5 to 16 microg/ml, respectively) against staphylococcal, enterococcal, and streptococcal pathogens. Daptomycin demonstrated a stepwise increase in activity corresponding to an increase in the supplemental concentration of calcium. When tested in base Mueller-Hinton broth supplemented with 50 mg of calcium per liter, daptomycin demonstrated improved activity (MIC(90)s, 0.015 to 4 microg/ml). The activity of daptomycin, however, equaled that of tigecycline against the glycopeptide-intermediate Staphylococcus aureus (GISA) strains only when the test medium was supplemented with excess calcium (75 mg/liter). Tigecycline and daptomycin demonstrated in vivo efficacies against GISA, methicillin-resistant S. aureus, and methicillin-susceptible S. aureus strains in an intraperitoneal systemic murine infection model. These data suggest that tigecycline and daptomycin may offer therapeutic options against clinically relevant resistant pathogens for which current alternatives for treatment are limited.     

In vitro effect of the presence of human albumin or human serum on the bactericidal activity of daptomycin against strains with the main resistance phenotypes in Gram-positives

OBJECTIVES: Bactericidal activity depends on antibiotic-bacteria couples, resistance phenotype and theoretically on protein binding. This work explores the influence of protein binding on the bactericidal activity of two antibiotics, daptomycin versus vancomycin, that exhibit, respectively, different C(max) (56 versus 25.5 mg/L), protein binding (91.7% versus 36.9%) and thus theoretical free-drug fractions (4.7 versus 16.1 mg/L). METHODS: The effect of the presence of physiological concentrations of human albumin (4 g/dL) or human serum (90%) on the bactericidal activity of daptomycin was studied against Gram-positive isolates with troublesome resistance phenotypes [multidrug-resistant Streptococcus pneumoniae (MDRSP), methicillin-resistant Staphylococcus aureus (MRSA), heterogeneous vancomycin-intermediate MRSA (MRSA-hVI) and vancomycin-resistant Enterococcus faecium]. Killing curves (final inocula of approximately 10(7) cfu/mL) were performed using daptomycin and vancomycin concentrations similar to the C(max) obtained in serum. RESULTS: Daptomycin was rapidly bactericidal (> or =3 log(10) initial inocula reduction) against S. pneumoniae and S. aureus, regardless of the strain tested or the presence of albumin or human serum (that slightly delayed bactericidal activity). Against vancomycin-susceptible or -resistant enterococci, daptomycin exhibited rapid bactericidal activity, delayed to 8 and 24 h, respectively, by human albumin. Vancomycin exhibited much slower bactericidal activity against MDRSP and methicillin-susceptible or -resistant S. aureus, but was never bactericidal against MRSA-hVI and vancomycin-susceptible or -resistant E. faecium. CONCLUSIONS: Daptomycin exhibited rapid bactericidal activity against the strains of the three Gram-positive species tested, regardless of resistance phenotype or the presence of physiological concentrations of albumin.     

Antimicrobial activity of daptomycin against multidrug-resistant Gram-positive strains collected worldwide

Daptomycin is a cyclic lipopeptide recently released for clinical use in the treatment of serious Gram-positive infections in hospitalized patients. We evaluated the in vitro activity of daptomycin tested against recently isolated multidrug-resistant Gram-positive clinical strains. A total of 386 isolates were selected from a large collection of strains from more than 70 centers located in Europe, North America, and South America. The strains were tested by reference broth microdilution methods in Mueller-Hinton broth with 50 mg/L Ca++ against daptomycin. Daptomycin was the most potent compound tested against penicillin-resistant Streptococcus pneumoniae with MIC50/90 values at < or =0.12 and 0.25 microg/mL, respectively. Daptomycin was also highly active against vancomycin-resistant enterococci and staphylococcal strains with various resistance patterns. Enterococcus faecium showed higher daptomycin MIC values (MIC90, 4 microg/mL) when compared to E. faecalis (MIC90, 1 microg/mL). In summary, resistance to vancomycin, teicoplanin, quinupristin/dalfopristin, or penicillin among the Gram-positive isolates did not adversely influence daptomycin activity. Daptomycin showed a significant potency and spectrum against Gram-positive species, including multidrug-resistant strains, and may represent a reasonable therapeutic option for infections caused by these important pathogens.     

Daptomycin susceptibility tests: interpretive criteria, quality control, and effect of calcium on in vitro tests

Daptomycin MICs were determined for 844 Gram-positive bacteria in three concentrations of Ca(++) and compared with the MICs of vancomycin and teicoplanin. Daptomycin was twofold to fourfold more active against most species when tested in 50 microg/ml of Ca(++) than in 25 microg/ml. In 50 microg/ml of Ca(++) daptomycin was more active against methicillin-resistant staphylococci and vancomycin-resistant enterococci than teicoplanin or vancomycin; 100% of these isolates were susceptible to < or =2.0 microg/ml of daptomycin. Different lots of Mueller-Hinton agar were variable in Ca(++) content, and daptomycin disk diffusion zone diameters were affected, i.e., zones were 1 to 15 mm smaller on one lot of agar with only 6 microg/ml of Ca(++) compared to another lot with 28 microg/ml. The previously proposed daptomycin interpretive breakpoints performed satisfactorily when MICs were determined in Mueller-Hinton broth with 50 microg/ml of Ca(++) and when the agar gave appropriate zones with quality control strains. To define those control limits, replicate tests with four quality control strains were performed in ten laboratories using broth microdilution tests (with Ca(++) supplemented broth) and disk diffusion tests on Mueller-Hinton agar without cation adjustments.     

Influence of protein binding under controlled conditions on the bactericidal activity of daptomycin in an in vitro pharmacodynamic model

OBJECTIVE: Daptomycin exhibits bactericidal activity against clinically significant Gram-positive bacteria despite being highly bound to human proteins. Evaluations characterizing the effect of protein on daptomycin pharmacodynamics are warranted. METHODS: We utilized an in vitro pharmacodynamic model to simulate daptomycin regimens of 6 mg/kg/day under controlled conditions of pH, calcium and/or protein. Free concentrations were simulated in broth, whereas total concentrations were simulated in broth supplemented with human albumin. Bacterial density was profiled over 48 h for two methicillin-resistant Staphylococcus aureus (MRSA) and two vancomycin-resistant Enterococcus faecium (VREF) clinical isolates. RESULTS: Daptomycin exhibited bactericidal activity against both MRSA isolates, with time to 99.9% killing occurring at 0.5 h and 8 h in broth and in albumin-supplemented broth, respectively. Initial kill was observed against both VREF isolates followed by regrowth. There was no statistical difference (P>0.05) in extent of bacterial kill at 24 or 48 h between the different media. CONCLUSIONS: Although delayed, the extent of kill for daptomycin was unaltered against all isolates in albumin-supplemented broth. Further antimicrobial studies that incorporate protein are warranted to assess the influence of protein in the pharmacodynamic evaluation of antimicrobials.     

Comparative activities of daptomycin and several agents against staphylococcal blood isolates. Glycopeptide tolerance

The activity of daptomycin was evaluated against 702 staphylococcal blood isolates (316 methicillin-susceptible Staphylococcus aureus, 187 methicillin-resistant S. aureus [MRSA], and 199 coagulase-negative staphylococci [CoNS]) collected in 41 Spanish hospitals. Glycopeptide tolerance and the incidence of heterogeneous glycopeptide-intermediate (hGISA) isolates were also examined. Vancomycin MICs determined by the Etest were compared with those obtained by the reference broth microdilution method. Daptomycin exhibited good activity, and only 2 isolates were nonsusceptible to this antibiotic. Resistance to linezolid was observed in 2 MRSA isolates and in 16 CoNS. The cfr gene was detected in 7 of these 18 isolates. Vancomycin and teicoplanin tolerance was 9.6% and 21.9%, respectively, in MRSA isolates. We detected the hGISA phenotype in 5.8% of MRSA isolates. Vancomycin MICs by the Etest were slightly higher than those obtained by broth microdilution. Daptomycin retained activity against isolates that were not susceptible to linezolid, teicoplanin, or quinupristin-dalfopristin.     

In vitro activities of daptomycin, vancomycin, linezolid, and quinupristin-dalfopristin against Staphylococci and Enterococci, including vancomycin- intermediate and -resistant strains

The in vitro activity of daptomycin was compared with those of vancomycin, linezolid, and quinupristin-dalfopristin against a variety (n = 203) of gram-positive bacteria, including methicillin-resistant Staphylococcus aureus and S. epidermidis (MRSA and MRSE, respectively), vancomycin-resistant enterococci (VRE), and vancomycin-intermediate S. aureus (VISA). Overall, daptomycin was more active against all organisms tested, except Enterococcus faecium and VISA, against which its activity was similar to that of quinupristin-dalfopristin. In time-kill studies with MRSA, MRSE, VRE, and VISA, daptomycin demonstrated greater bactericidal activity than all other drugs tested, killing > or =3 log CFU/ml by 8 h. Daptomycin may be a potential alternative drug therapy for multidrug-resistant gram-positive organisms and warrants further investigation.     

Synergy of daptomycin with oxacillin and other beta-lactams against methicillin-resistant Staphylococcus aureus

We previously observed marked synergy between daptomycin and both rifampin and ampicillin against vancomycin-resistant enterococci (VRE). Because the synergy between daptomycin and ampicillin was observed for 100% of VRE strains with high-level ampicillin resistance (ampicillin MIC of > or =128 microg/ml), we looked for synergy between daptomycin and other beta-lactams against 18 strains of methicillin-resistant Staphylococcus aureus (MRSA) by employing a time-kill method using Mueller-Hinton broth supplemented to 50 mg of Ca2+/liter. All strains were resistant to oxacillin (16 of 18 strains were resistant at drug concentrations of > or =256 microg/ml), and all strains were susceptible to daptomycin (the MIC at which 90% of the tested isolates were inhibited was 1 microg/ml). Daptomycin was tested at concentrations of 2, 1, 0.5, 0.25, 0.125, and 0.0625 microg/ml alone or in combination with oxacillin at a fixed concentration of 32 microg/ml. Synergy was found for all 18 strains with daptomycin at one-half the MIC in combination with 32 microg of oxacillin/ml, and synergy was found for 11 of 18 strains (61%) with daptomycin at one-fourth the MIC or less in combination with oxacillin. At 24 h, the daptomycin-oxacillin combination with daptomycin at one-half the MIC showed bactericidal activity against all 18 strains, and the combination with one-fourth the daptomycin MIC showed bactericidal activity against 9 of 18 strains. We also used a novel screening method to look for synergy between daptomycin and other beta-lactams. In this approach, daptomycin was incorporated into Ca(2+)-supplemented Mueller-Hinton agar at subinhibitory concentrations, and synergy was screened by comparing test antibiotic Kirby-Bauer disks on agar with and without daptomycin. By this method, daptomycin with ampicillin-sulbactam, ticarcillin-clavulanate, or piperacillin-tazobactam showed synergy comparable to or greater than daptomycin with oxacillin. For seven of the eight strains tested, time-kill studies confirmed synergy between daptomycin and ampicillin-sulbactam with ampicillin in the range of 2 to 8 microg/ml. The combination of daptomycin and beta-lactams may be useful for the treatment of MRSA infection, but further studies are needed to elucidate the mechanisms and to determine the in vivo efficacy of the combination.     

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.     

Comparative In vitro activities of daptomycin and vancomycin against resistant gram-positive pathogens

The in vitro activity of daptomycin against 224 current gram-positive clinical isolates including vancomycin-resistant Enterococcus faecium (VREF), methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant Staphylococcus spp. (MRSS), and penicillin-resistant Streptococcus pneumoniae (PRSP) was evaluated. The MICs at which 90% of isolates are inhibited for daptomycin and vancomycin, respectively, were as follows: MRSA, 1 and 2 microg/ml; MRSS, 1 and 4 microg/ml; PRSP, 1 and 0.5 microg/ml; and VREF, 2 and >64 microg/ml. Daptomycin was bactericidal against 82% of 17 VREF isolates. The antibacterial activity of daptomycin was strongly dependent on the calcium concentration of the medium. Daptomycin was active against all gram-positive cocci tested.     

The in vitro activity of daptomycin against 514 Gram-positive aerobic clinical isolates

The in vitro activity of daptomycin was assessed in comparison with that of vancomycin and penicillin against a wide range of Gram-positive aerobic clinical isolates. MICs were determined by an agar dilution method on Mueller-Hinton agar (NCCLS/EUCAST) and on Isotonic agar adjusted to contain 50 mg/L free calcium (BSAC). Both media were enriched with 5% horse blood for fastidious organisms. Daptomycin MICs for all 172 staphylococci, including methicillin-susceptible and methicillin-resistant Staphylococcus aureus, Staphylococcus epidermidis and Staphylococcus haemolyticus, were 0.03-0.5 mg/L. For 99 of the 100 enterococci (Enterococcus faecalis, n = 50; Enterococcus faecium, n = 50), including 37 vancomycin-resistant isolates, they were 0.25-2 mg/L. For all 108 beta-haemolytic streptococci, including Streptococcus pyogenes and Streptococcus agalactiae, daptomycin MICs were 0.016- 0.25 mg/L; for 101 alpha-haemolytic streptococci, including Streptococcus pneumoniae and 'viridans' streptococci, they were 0.016-2 mg/L. For miscellaneous vancomycin-resistant isolates including Lactobacillus spp., Lactococcus spp., Leuconostoc spp., Pediococcus spp. and isolates of Enterococcus casseliflavus and Enterococcus gallinarum, daptomycin MICs were 0.03-2 mg/L; MICs for the seven isolates of Listeria monocytogenes were 0.25-4 mg/L. There was little difference between the results on Mueller-Hinton agar and on supplemented Isotonic agar The discrepant results occasionally obtained tended to be one dilution higher on supplemented Isotonic agar. Daptomycin was active (MICs < or = 2 mg/L) against all the isolates tested with the exception of one isolate each of E. faecium and L. monocytogenes (MICs = 4 mg/L). Our results indicate that daptomycin MICs are independent of methicillin and vancomycin MICs.     

Activity of daptomycin against susceptible and multidrug-resistant Gram-positive pathogens collected in the SECURE study (Europe) during 2000-2001

Antibiotic resistance was prevalent in Gram-positive pathogens collected from 40 sites in 15 European countries during 2000-2001. Among Staphylococcus aureus, 27.3% of all isolates submitted were resistant to oxacillin and ranged from 0% of isolates from the Netherlands to 36.9% of isolates from Portugal. The overall prevalence of vancomycin-resistant Enterococcus faecium was 25.1%, with Italy submitting the largest percentage of resistant isolates (60.6%). For Streptococcus pneumoniae, 9.4% of all isolates collected were resistant to penicillin with variation by country from 0% in the Netherlands to 20.7% in Portugal. Multidrug resistance (MDR), defined as concurrent resistance to three or more antimicrobials of different chemical classes, was observed in 24.6% of S. aureus, 19.6% of E. faecium and 3.6% of S. pneumoniae. The directed spectrum agents daptomycin, linezolid and quinupristin-dalfopristin were active in vitro against all isolates regardless of their resistance to other agents. Daptomycin and quinupristin-dalfopristin (MIC(90)s 0.5 mg/L) were equally active against oxacillin-resistant S. aureus compared with linezolid (MIC(90) 2 mg/L). The activities of daptomycin, quinupristin-dalfopristin and linezolid were not affected by resistance to vancomycin in E. faecium (MIC(90)s of 4, 2 and 2 mg/L, respectively). Daptomycin was more active against penicillin-resistant S. pneumoniae (MIC(90) 0.25 mg/L) than was quinupristin-dalfopristin (MIC(90) 0.5 mg/L) or linezolid (MIC(90) 2 mg/L). Daptomycin was highly active against clinically important Gram-positive pathogens, including those that were multiply resistant to currently available agents. The results of this study provide a benchmark of the activity of daptomycin against contemporary European isolates and will serve as a baseline to monitor future changes in the susceptibility of these organisms to daptomycin.     

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.     

Calcium-supplemented daptomycin Etest strips for susceptibility testing on Iso-Sensitest agar

OBJECTIVE: Iso-Sensitest agar (ISA), which is recommended by the BSAC for routine susceptibility testing of staphylococci and enterococci, contains insufficient calcium for testing daptomycin. Isotonic agar supplemented with 50 mg/L calcium has been advocated, but is not routinely available in many laboratories. We evaluated a daptomycin Etest that incorporates a constant level of calcium throughout the daptomycin gradient, designed to give an appropriate concentration around the strip during testing, as an alternative for susceptibility testing on ISA. METHODS: Ninety-one isolates of Staphylococcus aureus (45 methicillin-susceptible, 46 methicillin-resistant) and 90 enterococci (47 Enterococcus faecalis, 43 Enterococcus faecium) were tested. Daptomycin Etest MICs were determined on ISA, whereas agar dilution MICs were determined in parallel on Isotonic agar supplemented with calcium to 50 mg/L as a control. RESULTS: The agar dilution and Etest MIC ranges of daptomycin for S. aureus were 0.25-1 mg/L (mode 0.5 mg/L), and 0.125-2 mg/L (mode 0.25 mg/L), respectively. The corresponding MIC values for enterococci were 0.25-4 mg/L (mode, 1 mg/L) and 0.125-4 mg/L (mode, 2 mg/L). For staphylococci, 86% of the Etest MIC results were within one dilution of the agar dilution values, and for enterococci, 90% of the Etest MIC results met these criteria. When results from the two methods were not identical, there was a tendency for the Etest MIC values to be lower than the agar dilution values. CONCLUSIONS: This study shows that calcium-supplemented daptomycin Etests on ISA are an accurate and convenient alternative to calcium-supplemented Isotonic agar.     

In vitro activity of daptomycin against 297 staphylococcal isolates

The in vitro activity of daptomycin against 297 clinical isolates of Staphylococcus aureus and S. epidermidis sensu strictu was compared with the activities of cephalothin, dicloxacillin, tobramycin, and vancomycin. Minimal inhibitory concentrations (MICs) were determined by an agar dilution method. Cephalothin and dicloxacillin showed the highest activity against S. aureus on a weight-for-weight basis, all isolates being inhibited by 0.5 mg/l or less of either agent. Cephalothin was somewhat more active against S. epidermidis than was dicloxacillin. Daptomycin and vancomycin exhibited high and similar activity against both S. aureus and S. epidermidis (MIC90% = 1 and 2 mg/l, respectively). Tobramycin was highly active against S. aureus, but the activity against S. epidermidis was greatly variable (MIC range less than or equal to 0.03 - greater than or equal to 16 mg/l). The activity of daptomycin was markedly influenced by the test medium; the MICs were generally 32 times higher when the isolates were tested on Iso-Sensitest agar than on Mueller-Hinton agar. Supplementation of Iso-Sensitest agar with increasing concentrations of calcium potentiated the activity of daptomycin substantially, the results obtained on Iso-Sensitest agar supplemented with 20 mg Ca2+/l being similar to those obtained on Mueller-Hinton agar.     

Antimicrobial activity of daptomycin tested against clinical strains of indicated species isolated in North American medical centers (2003)

Daptomycin is a cyclic lipopeptide approved for use by the US Food and Drug Administration (FDA) for the treatment of complicated skin and skin structure infections caused by Staphylococcus aureus, groups A and B beta-hemolytic streptococci, and vancomycin-susceptible Enterococcus faecalis. We evaluated the daptomycin spectrum against these pathogens by testing 2759 clinical strains consecutively collected in more than 30 hospitals located across the United States and Canada. The isolates were susceptibility tested against daptomycin and many comparators by the reference broth microdilution method. Daptomycin was very active against indicated species with the highest MIC results being 1, 2, and 0.5 microg/mL for S. aureus, E. faecalis, and beta-hemolytic streptococci, respectively. All isolates tested were considered susceptible to daptomycin, according to Clinical and Laboratory Standards Institute and FDA breakpoints, and daptomycin was the most potent (lowest MIC90) among selected antimicrobials commonly used to treat Gram-positive infections. Resistance to oxacillin or vancomycin did not influence daptomycin activity against S. aureus or E. faecalis.     

In-vitro activity of vancomycin, teicoplanin, daptomycin, ramoplanin, MDL 62873 and other agents against staphylococci, enterococci and Clostridium difficile.

The minimum inhibitory concentrations (MICs) of different antibiotics were determined by a broth microdilution method for staphylococci, enterococci and Clostridium difficile. The antimicrobial agents tested were vancomycin, teicoplanin, daptomycin, ramoplanin, MDL 62873, rifampicin and piperacillin, the latter limited to enterococci. In terms of MIC90S, daptomycin (0.89 mg/l). MDL 62873 (0.99 mg/l), and teicoplanin (1.50 mg/l) were found to be highly active against methicillin-resistant Staphylococcus aureus (MRSA). Daptomycin (MIC90 0.48 mg/l), MDL 62873 (0.95 mg/l) and ramoplanin (1.45 mg/l) were the most active drugs against methicillin-resistant S. epidermidis (MRSE). Teicoplanin (MIC90 0.45 mg/l) was the most active agent against enterococci, followed by MDL 62873 (0.65 mg/l) and daptomycin (1.60 mg/l). MDL 62873 gave the lowest MIC90 (0.17 mg/l) for C. difficile. Teicoplanin (MIC90 0.42 mg/l), daptomycin (0.87 mg/l) and ramoplanin (0.98 mg/l) were also very active. Our results indicate that teicoplanin, daptomycin, ramoplanin and MDL 62873, a teicoplanin derivative, are potentially effective alternative antibiotics for treatment of infections caused by staphylococci, enterococci and C. difficile.     

Daptomycin is effective in treatment of experimental endocarditis due to methicillin-resistant and glycopeptide-intermediate Staphylococcus aureus

Daptomycin is a lipopeptide antibiotic with potent in vitro activity against gram-positive cocci, including Staphylococcus aureus. This study evaluated the in vitro and in vivo efficacies of daptomycin against two clinical isolates: methicillin-resistant S. aureus (MRSA) 277 (vancomycin MIC, 2 microg/ml) and glycopeptide-intermediate S. aureus (GISA) ATCC 700788 (vancomycin MIC, 8 microg/ml). Time-kill experiments demonstrated that daptomycin was bactericidal in vitro against these two strains. The in vivo activity of daptomycin (6 mg/kg of body weight every 24 h) was evaluated by using a rabbit model of infective endocarditis and was compared with the activities of a high-dose (HD) vancomycin regimen (1 g intravenously every 6 h), the recommended dose (RD) of vancomycin regimen (1 g intravenously every 12 h) for 48 h, and no treatment (as a control). Daptomycin was significantly more effective than the vancomycin RD in reducing the density of bacteria in the vegetations for the MRSA strains (0 [interquartile range, 0 to 1.5] versus 2 [interquartile range, 0 to 5.6] log CFU/g vegetation; P = 0.02) and GISA strains (2 [interquartile range, 0 to 2] versus 6.6 [interquartile range, 2.0 to 6.9] log CFU/g vegetation; P < 0.01) studied. In addition, daptomycin sterilized more MRSA vegetations than the vancomycin RD (13/18 [72%] versus 7/20 [35%]; P = 0.02) and sterilized more GISA vegetations than either vancomycin regimen (12/19 [63%] versus 4/20 [20%]; P < 0.01). No statistically significant difference between the vancomycin HD and the vancomycin RD for MRSA treatment was noted. These results support the use of daptomycin for the treatment of aortic valve endocarditis caused by GISA and MRSA.     

Daptomycin dose-effect relationship against resistant gram-positive organisms

Daptomycin exhibits in vitro bactericidal activity against clinically significant gram-positive bacteria. We employed pharmacodynamic modeling to determine a once-daily dosing regimen of daptomycin that correlates to pharmacodynamic endpoints for different resistant gram-positive clinical strains. An in vitro pharmacodynamic model with an initial inoculum of 6 log(10) CFU/ml was used to simulate daptomycin regimens ranging in dose from 0 to 9 mg/kg of body weight/day, with corresponding exposures reflecting free-daptomycin concentrations in serum. Bacterial density was profiled over 48 h for two methicillin-resistant Staphylococcus aureus (MRSA-67 and -R515), two glycopeptide intermediate-resistant S. aureus (GISA-992 and -147398), and two vancomycin-resistant Enterococcus faecium (VREF-12366 and -SF12047) strains. A sigmoid dose-response model was used to estimate the effective dose required to achieve 50% (ED(50)) and 80% (ED(80)) bacterial density reduction at 48 h. Daptomycin MICs for study isolates ranged from 0.125 to 4 micro g/ml. Model fitting resulted in an r(2) of >0.80 for all tested isolates. Control growths at 48 h ranged from 7.3 to 8.5 log(10) CFU/ml. Sigmoid relationships were not superimposable between categorical resistant species: ED(50) and ED(80) values were 1.9 and 3.1, 4.2 and 5.6, and 5.4 and 6.8 mg/kg for MRSA, GISA, and VREF isolates, respectively. Doses required to achieve ED(50) and ED(80) values correlated with MIC differences between tested organisms. Corresponding area under the concentration-time curve from 0 to 24 h/MIC exposure ratios demonstrated a wide range of ED(80) values among the tested isolates. Doses ranging between 3 and 7 mg/kg produced significant bactericidal activity (ED(80)) against these multidrug-resistant S. aureus and E. faecium isolates.