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Clinical trials with daptomycin were halted in December 1990 because of treatment failures including two resistant Staphylococcus aureus strains. High protein binding of daptomycin (> 90%) and the lower-than-expected concentrations in serum with the dosage regimen of 3 mg/kg of body weight every 12 h may have contributed to these failures. To evaluate the effect that higher concentrations would have on bactericidal activity measured by time-kill curves, peak and trough concentrations were estimated for dosage regimens of 3, 5, and 10 mg/kg every 12 h. MICs, MBCs, and killing curves for daptomycin and vancomycin were performed by using the estimated concentrations with four S. aureus strains obtained from patients who failed daptomycin therapy for endocarditis. MICs and MBCs of daptomycin demonstrated a greater inoculum effect than those of vancomycin; MICs and MBCs of daptomycin increased three- to fourfold, but those of vancomycin increased only one- to twofold when the inoculum was increased from 5 x 10(5) to 5 x 10(7) CFU/ml. No pH-dependent effect on MICs or MBCs was seen. Strenuous experimental conditions were chosen: high inoculums (5 x 10(7) CFU/ml), extremes of pH (6.4, 7.4, and 8), and stationary and exponentially growing organisms; and all experiments completed in the presence of pooled human serum. Daptomycin exhibited concentration-dependent killing and statistically faster kill rates than vancomycin against stationary- or exponential-growth-phase organisms. A pH-dependent decrease in activity with daptomycin was also demonstrated. Daptomycin and vancomycin produced higher kill rates against exponentially growing organisms. A pH-dependent decrease in activity with daptomycin was also demonstrated. Daptomycin and vancomycin produced higher kill rates against exponentially growing organisms. The results indicate that the use of higher dosage regimens with compounds similar to daptomycin may be capable of overcoming the effects of pH, high inoculum, and protein binding.

The pharmacokinetics and bactericidal killing rates (BR) of daptomycin (D) and vancomycin (V) in 12 intravenous drug abusers (6 treated with daptomycin and 6 treated with vancomycin) were evaluated. Pharmacokinetic parameters were determined from multiple serum samples drawn at steady state over a 12-h dosing interval after intravenous infusions of 3 mg of D per kg of body weight and 1,000 mg of V. The BRs were determined from the 1- and 6-h serum samples by using four isolates of Staphylococcus aureus (three methicillin susceptible and one methicillin resistant) obtained from the patients enrolled in the study. Peak serum daptomycin concentrations were lower and volumes of distribution were higher than reported in healthy volunteers. Although not statistically different, D clearance was 22% higher than reported in healthy volunteers. V pharmacokinetics were similar to those reported in previous studies. Daptomycin's BRs, although comparable to those of V in patients' serum, were significantly decreased compared with those found in broth. This may be related to the high degree of protein binding of D (93% versus 50% for V). Conversely, the BRs of V in serum were significantly greater than those in broth. The BRs of D and V in broth were greater when killing curves were performed with test strains in logarithmic versus stationary-phase growth. The ability to kill organisms in stationary phase may be an important factor in determining the performance of an antibiotic in deep-seated infections such as endocarditis.3+

Initial clinical trials with daptomycin (2 mg/kg per day) were prematurely suspended because of unexplained treatment failures in patients with bacteremia who were treated with daptomycin, despite in vitro data indicating that the gram-positive cocci causing the infection were susceptible to daptomycin. One explanation for these clinical failures may relate to the relatively high degree of daptomycin protein binding (94%). To evaluate the impact of protein on daptomycin activity, a two-chamber in vitro pharmacodynamic model was used to study and compare the interaction between Staphylococcus aureus (clinical isolate) and either daptomycin or vancomycin, each in the presence and absence of physiologic human albumin concentrations. Low-dose (2 mg/kg) daptomycin, high-dose (6 mg/kg) daptomycin, and 10 mg of vancomycin per kg beta-phase elimination serum-concentration-versus-time curves were simulated by using this in vitro pharmacodynamic model. The bacterial kill rates by all three regimens were decreased in the presence of albumin (P less than 0.0002). The average times required for a 99% kill of the initial S. aureus inocula (approximately 5 x 10(7) CFU/ml) without albumin were 0.81 (low-dose daptomycin), 0.33 (high-dose daptomycin), and 6.18 (vancomycin) h. The average times required for a 99% kill of S. aureus with albumin were 7.66 (low-dose daptomycin), 0.95 (high-dose daptomycin), and 10.52 (vancomycin) h. These data demonstrate that, depending on the concentration of daptomycin, the presence of albumin can profoundly diminish the bactericidal activity of daptomycin.

Antibiotic lock therapy (ALT) is an adjunctive procedure to prevent or treat central venous catheter infections, ensuing catheter-related bacteremia, and catheter-related metastatic infections. Daptomycin is a cyclic lipopeptide that is rapidly bactericidal against methicillin-susceptible and -resistant Staphylococcus aureus. The efficacies of daptomycin against central venous catheter biofilms, catheter-related bacteremia, and catheter-related metastatic infections were evaluated by adapting a previously reported central venous catheter biofilm model in rats. Combined daptomycin ALT and systemic dosing resulted in the clearance of an established in vivo S. aureus central venous catheter biofilm after just two daily ALT treatments (30 min with daptomycin at 5 mg/ml) with concurrent systemic daptomycin dosing (40 mg/kg of body weight/day subcutaneously [s.c.]; equivalent exposure of 6 mg/kg/day in people). Daptomycin ALT solutions formulated in either saline or lactated Ringer's solution were equally fast in eradicating established in vivo methicillin-resistant Staphylococcus epidermidis (MRSE) central venous catheter biofilms. However, the lactated Ringer's formulation was superior to that of saline in sustaining the bacterial clearance of treated central venous catheters (83% versus 50%). In MRSE-infected central venous catheter studies, 3 days of daptomycin or vancomycin ALT (18 h at 5 mg/ml) with systemic s.c. dosing (40 mg/kg/day daptomycin or 100 mg/kg/day vancomycin) was equally effective 1 week posttherapy in maintaining cleared central venous catheters (90% [n = 10] versus 100% [n = 8]). These results suggest that daptomycin ALT, along with systemic dosing, could be an effective treatment option for the prevention or eradication of staphylococcal central venous catheter biofilm infections, thereby reducing the occurrence of catheter-related bacteremia or catheter-related metastatic infections.

Twelve volunteers, in two groups of six, received daptomycin at a dose of 1 or 2 mg/kg. In addition, they received in a randomly allocated order amikacin (500 mg), daptomycin-amikacin, and vancomycin (500 mg). Thirty-five clinical isolates, including Staphylococcus aureus, S. epidermidis, Corynebacterium sp. group JK, and Enterococcus faecalis, were tested in vitro for the measure of the serum bactericidal titers and killing rates. The mean peak concentrations of daptomycin in serum 1 h after the administration of 1 and 2 mg/kg were 11 and 20 micrograms/ml, respectively. At 24 h after the administration of 2 mg/kg, the mean level in serum was 1.9 micrograms/ml, which is higher than the MICs for susceptible pathogens. Daptomycin and amikacin provided identical concentrations in serum whether given alone or in combination. Among the six regimens tested, those including daptomycin provided the highest and the most prolonged serum bactericidal titers against S. aureus, S. epidermidis, and E. faecalis. The killing rates measured by the killing curves were correlated with the concentration/MIC and concentration/MBC ratios of daptomycin for all strains tested. Significant killing occurred once the concentration of daptomycin in the serum 4- to 6-fold the MIC or 1- to 1.2-fold the MBC. The combination of daptomycin and amikacin had no effect on either the serum bactericidal titers or the rates of killing. Only vancomycin provided significant killing of the strains of Corynebacterium sp. group JK.

The purpose of this paper is to establish the pharmacokinetics and safety of escalating, once-daily doses of daptomycin, a novel lipopeptide antibiotic active against gram-positive pathogens, including those resistant to methicillin and vancomycin. This phase 1, multiple-dose, double-blind study involved 24 healthy subjects in three dose cohorts (4, 6, and 8 mg/kg of body weight) who were randomized to receive daptomycin or the control at a 3:1 ratio and administered the study medication by a 30-min intravenous infusion every 24 h for 7 to 14 days. Daptomycin pharmacokinetics was assessed by blood and urine sampling. Safety and tolerability were evaluated by monitoring adverse events (AEs) and laboratory parameters. Daptomycin pharmacokinetics was linear through 6 mg/kg, with a slight (∼20%) nonlinearity in the area under the curve and trough concentration at the highest dose studied (8 mg/kg). The pharmacokinetic parameters measured on the median day of the study period, (day 7) were half-life (∼9 h), volume of distribution (∼0.1 liters/kg), systemic clearance (∼8.2 ml/h/kg), and percentage of the drug excreted intact in urine from 0 to 24 h (∼54%). Daptomycin protein binding (mean amount bound, 91.7%) was independent of the drug concentration. No gender effect was observed. All subjects who received daptomycin completed the study. The frequencies and distributions of treatment-emergent AEs were similar for the subjects who received daptomycin and the control subjects. There were no serious AEs and no pattern of dose-related events. The pharmacokinetics of once-daily administration of daptomycin was linear through 6 mg/kg. For all three doses, plasma daptomycin concentrations were consistent and predictable throughout the dosing interval. Daptomycin was well tolerated when it was administered once daily at a dose as high as 8 mg/kg for 14 days.

A rabbit model for methicillin-resistant Staphylococcus aureus (MRSA) osteomyelitis was used to compare treatment with daptomycin, a new peptolide, and vancomycin. Daptomycin (4 mg/kg) and vancomycin (40 mg/kg) were injected subcutaneously every 12 and 6 h, respectively. After treatment, MRSA was found in bone cultures from 18 of 18 control rabbits, 10 of 17 animals treated with daptomycin, and 11 of 18 animals treated with vancomycin. Drug concentrations were measured in serum, uninfected bone, and infected bone 1 h after daptomycin or vancomycin was injected in a group of rabbits that had been infected for 3 to 4 weeks. Vancomycin was present at the highest concentrations in infected and uninfected bone. The results of this study suggest that daptomycin was similar to vancomycin in the eradication of MRSA from infected bone in an experimental model of osteomyelitis.

Background

Daptomycin is a novel cyclic lipopeptide whose bactericidal activity is not affected by current antibiotic resistance mechanisms displayed by S. aureus clinical isolates. This study reports the therapeutic activity of high-dose daptomycin compared to standard regimens of oxacillin and vancomycin in a difficult-to-treat, rat tissue cage model of experimental therapy of chronic S. aureus foreign body infection.

Methods

The methicillin-susceptible S. aureus (MSSA) strain I20 is a clinical isolate from catheter-related sepsis. MICs, MBCs, and time-kill curves of each antibiotic were evaluated as recommended by NCCLS, including supplementation with physiological levels (50 mg/L) of Ca2+ for daptomycin. Two weeks after local infection of subcutaneously implanted tissue cages with MSSA I20, each animal received (i.p.) twice-daily doses of daptomycin, oxacillin, or vancomycin for 7 days, or was left untreated. The reductions of CFU counts in each treatment group were analysed by ANOVA and Newman-Keuls multiple comparisons procedures.

Results

The MICs and MBCs of daptomycin, oxacillin, or vancomycin for MSSA strain I20 were 0.5 and 1, 0.5 and 1, or 1 and 2 mg/L, respectively. In vitro elimination of strain I20 was more rapid with 8 mg/L of daptomycin compared to oxacillin or vancomycin. Twice-daily administered daptomycin (30 mg/kg), oxacillin (200 mg/kg), or vancomycin (50 mg/kg vancomycin) yielded bactericidal antibiotic levels in infected cage fluids throughout therapy. Before therapy, mean (± SEM) viable counts of strain I20 were 6.68 ± 0.10 log10 CFU/mL of cage fluid (n = 74). After 7 days of therapy, the mean (± SEM) reduction in viable counts of MSSA I20 was 2.62 (± 0.30) log10 CFU/mL in cages (n = 18) of daptomycin-treated rats, exceeding by >2-fold (P < 0.01) the viable count reductions of 0.92 (± 0.23; n = 19) and 0.96 (± 0.24; n = 18) log10 CFU/mL in cages of oxacillin-treated and vancomycin-treated rats, respectively. Viable counts in cage fluids of untreated animals increased by 0.48 (± 0.24; n = 19) log10 CFU/mL.

Conclusion

The improved efficacy of the twice-daily regimen of daptomycin (30 mg/kg) compared to oxacillin (200 mg/kg) or vancomycin (50 mg/kg) may result from optimisation of its pharmacokinetic and bactericidal properties in infected cage fluids.

As methicillin-resistant Staphylococcus aureus (MRSA) becomes more prevalent, vancomycin is becoming increasingly used as a prophylaxis against surgical-site infections for cardiothoracic surgeries. However, vancomycin administration can be challenging, and the pharmacokinetics of alternative antibiotics in this setting are poorly understood. The primary objective of this investigation was to describe the pharmacokinetics of daptomycin in patients undergoing coronary artery bypass graft surgery. We enrolled 15 patients undergoing coronary artery bypass surgery requiring cardiopulmonary bypass. Each subject was administered a single open-label dose of daptomycin (8 mg/kg of body weight) for surgical prophylaxis. Fourteen daptomycin plasma samples were collected. Safety outcomes between subjects who received daptomycin and 15 control subjects who received the standard-of-care antibiotic were compared. The mean maximal concentration of daptomycin (Cmax) was 84.4 ± 27.1 μg/ml; the mean daptomycin concentration during the cardiopulmonary bypass procedure was 33.2 ± 11.4 μg/ml and was 30.9 ± 12.7 μg/ml at sternum closure. Mean daptomycin concentrations at 12, 18, 24, and 48 h were 22.7 ± 9.7, 16.2 ± 8.2, 12.0 ± 4.7, and 3.5 ± 2.3 μg/ml, respectively. Mean daptomycin concentrations were consistently above the MIC at which 90% of the tested isolates are inhibited (MIC90) for S. aureus and S. epidermidis during the cardiopulmonary bypass procedure. Daptomycin was not associated with surgical-site infections or differences in adverse events compared to findings for control subjects. We found that a single dose of daptomycin at 8 mg/kg was well tolerated and achieved adequate plasma concentrations against common pathogens associated with surgical-site infections after cardiothoracic surgery. Daptomycin may be considered an alternative surgical prophylaxis antibiotic for patients undergoing cardiothoracic bypass surgery who are unable to receive vancomycin.

Daptomycin is a lipopeptide antibiotic with activity against a wide range of gram-positive bacteria. We used the neutropenic murine thigh model to characterize the pharmacodynamics of daptomycin. ICR/Swiss mice were rendered neutropenic with cyclophosphamide; and the thigh muscles of the mice were infected with strains of Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecium. Animals were treated by subcutaneous injection of daptomycin at doses of 0.20 to 400 mg/kg of body weight/day divided into one, two, four, or eight doses over 24 h. Daptomycin exhibited linear pharmacokinetics, with an area under the concentration-time curve (AUC) from time zero to infinity/dose of 9.4 and a half-life of 0.9 to 1.4 h. The level of protein binding was 90%. Free daptomycin exhibited concentration-dependent killing and produced in vivo postantibiotic effects (PAEs) of 4.8 to 10.8 h. Nonlinear regression analysis was used to determine which pharmacokinetic (PK) or pharmacodynamic (PD) parameter was important for efficacy by using free drug concentrations. The peak concentration/MIC (peak/MIC) ratio and 24-h AUC/MIC ratio were the PK and PD parameters that best correlated with in vivo efficacy (R2 = 83 to 87% for peak/MIC and R2 = 86% for the AUC/MIC ratio, whereas R2 = 47 to 50% for the time that the concentration was greater than the MIC) against standard strains of S. aureus and S. pneumoniae. The peak/MIC ratios required for a bacteriostatic effect ranged from 12 to 36 for S. pneumoniae, 59 to 94 for S. aureus, and 0.14 to 0.25 for E. faecium. The AUC/MIC ratios needed for a bacteriostatic effect ranged from 75 to 237 for S. pneumoniae, 388 to 537 for S. aureus, and 0.94 to 1.67 for E. faecium. The free daptomycin concentrations needed to average from one to two times the MIC over 24 h to produce a bacteriostatic effect and two to four times the MIC over 24 h to produce greater than 99% killing. The long PAE and potent bactericidal activity make daptomycin an attractive option for the treatment of infections caused by gram-positive bacteria.

Several single-center studies have suggested that higher doses of vancomycin, aimed at producing trough concentrations of >15 mg/liter, are associated with increased risk of nephrotoxicity. We prospectively assessed the relative incidence of nephrotoxicity in relation to trough concentration in patients with documented methicillin-resistant Staphylococcus aureus (MRSA) infections at seven hospitals throughout South Carolina. Adult patients receiving vancomycin for at least 72 h with at least one vancomycin trough concentration determined under steady-state conditions were prospectively studied. The relationship between vancomycin trough concentrations of >15 mg/ml and the occurrence of nephrotoxicity was assessed using univariate and multivariate analyses, controlling for age, gender, race, dose, length of therapy, use of other nephrotoxins (including contrast media), intensive care unit (ICU) residence, episodes of hypotension, and comorbidities. Nephrotoxicity was defined as an increase in serum creatinine of 0.5 mg/dl or a ≥50% increase from the baseline for two consecutive measurements. MICs of vancomycin for the MRSA isolates were also determined. A total of 288 patients were studied between February 2008 and June 2010, with approximately one-half having initial trough concentrations of ≥15 mg/ml. Nephrotoxicity was observed for 42 patients (29.6%) with trough concentrations >15 mg/ml and for 13 (8.9%) with trough concentrations of ≤15 mg/ml. Multivariate analysis revealed vancomycin trough concentrations of >15 mg/ml and race (black) as risk factors for nephrotoxicity in this population. Vancomycin trough concentrations of >15 mg/ml appear to be associated with a 3-fold increased risk of nephrotoxicity.

We examined sequential methicillin-susceptible Staphylococcus aureus isolates from a patient with mitral valve endocarditis recovered during persistent bacteremia on standard therapy and relapse after treatment with daptomycin. An isolate obtained after 5 days of antimicrobial therapy, but before exposure to daptomycin, showed subtle physiological changes in response to daptomycin, with significant regrowth in the daptomycin killing assay compared to the treatment-naive strain. Once daptomycin was started, the population became more heterogeneous and tested as nonsusceptible. These organisms were examined in a simulated-vegetation in vitro pharmacodynamic model, which confirmed progressive decreases in killing with daptomycin concentrations that simulate those attained in humans with 6-mg/kg of body weight daily dosing. Early surgical intervention or combination therapy or both might have prevented the loss of daptomycin susceptibility.

We investigated the emergence and spread of community-associated strains of methicillin-resistant Staphylococcus aureus (CA-MRSA) in central and northern Wisconsin by determining the temporal and clonal relationships and geographic expansion among 581 of 956 clinical isolates of MRSA collected between 1989 and 1999. Based on EcoRI plasmid profiles (PP), two types, PP-11 and PP-13, were highly stable over time and were consistently associated with multidrug-sensitive strains recovered from outpatients treated at Native American community clinics. Pulsed-field gel electrophoresis (PFGE) yielded six major clonal groups (MCGs) and 19 minor clonal groups. The six MCGs represented 82% of the isolates. All strains with either PP-11 or -13 were present in MCG-2. Eighty-nine percent of the isolates in MCG-2 originated from Native American clinics, and 90% belonged to two PFGE types (19 and 20), the types associated with an outbreak of MRSA in a Native American community in 1992. MCG-2 isolates were multidrug sensitive, harbored type IVa staphylococcal cassette chromosome mec, and were very closely related by PFGE to the Midwestern CA-MRSA strain MW2. MCG-2 strains were mostly obtained from skin infections and affected patients with a mean age of 24 (±18.0) years. MCG-2 strains spread to four additional Native American communities and 20 other communities. Our findings suggest that CA-MRSA in Wisconsin likely originated in Native American communities in the early 1990s and since has become widespread throughout the state. Two early CA-MRSA strains (WI-33 and WI-34) in Wisconsin represent progenitors of the MW2 strain, based on their almost indistinguishable genotypic characteristics.

A 72-year-old man, receiving 8 mg daptomycin/kg body weight/day for methicillin-resistant Staphylococcus aureus (MRSA) bacteremia, was diagnosed with MRSA/vancomycin-resistant Enterococcus faecium (VRE) cholecystitis (daptomycin MIC values, 1 and 2 mg/liter, respectively). After the fifth drug dose, the bile concentration of daptomycin was 66 mg/liter 5 min after drug administration, with the biliary concentration/MIC values higher than 30 for both bacterial strains. Therefore, daptomycin achieved therapeutic levels in bile, hence suggesting a role for the drug in the treatment of MRSA/VRE cholecystitis.

AIMS--To compare the safety and efficacy of two loading doses of diamorphine in 27 ventilated newborn infants in a randomised double blind trial. METHODS--Fifty or 200 mcg/kg were infused intravenously over 30 minutes, followed by a 15 mcg/kg/hour continuous infusion. Serial measurements were made of physiology, behaviour, and stress hormones. RESULTS--Both loading doses produced small but significant falls in blood pressure. The 200 mcg/kg dose produced greater respiratory depression, and two infants deteriorated clinically, requiring resuscitation. Loading reduced respiratory effort in most of the infants, but had little effect on behavioural activity. Stress hormone concentrations were reduced at six hours in both dosage groups; differences between loading doses were not significant. Morphine, morphine-3-glucuronide, and morphine-6-glucuronide were detected in the plasma of all patients. No significant differences in concentrations between loading doses were found. CONCLUSIONS--Diamorphine reduces the stress response in ventilated newborn infants. A high loading dose confers no benefit, and may produce undesirable physiological effects. A 50 mcg/kg loading dose seems to be safe and effective.

Daptomycin is the first antibacterial agent of the cyclic lipopeptides with in vitro bactericidal activity against gram-positive organisms, including vancomycin-resistant enterococci, methicillin-resistant staphylococci, and glycopeptide-resistant Staphylococcus aureus. The pharmacokinetics of daptomycin were determined in 29 adult oncology patients with neutropenic fever. Serial blood samples were drawn at 0, 0.5, 1, 2, 4, 8, 12, and 24 h after the initial intravenous infusion of 6 mg/kg of body weight daptomycin. Daptomycin total and free plasma concentrations were determined by high-pressure liquid chromatography. Concentration-time data were analyzed by noncompartmental methods. The results (presented as means ± standard deviations and ranges, unless indicated otherwise) were as follows: the maximum concentration of drug in plasma (Cmax) was 49.04 ± 12.42 μg/ml (range, 21.54 to 75.20 μg/ml), the 24-h plasma concentration was 6.48 ± 5.31 μg/ml (range, 1.48 to 29.26 μg/ml), the area under the concentration-time curve (AUC) from time zero to infinity was 521.37 ± 523.53 μg·h/ml (range, 164.64 to 3155.11 μg·h/ml), the volume of distribution at steady state was 0.18 ± 0.05 liters/kg (range, 0.13 to 0.36 liters/kg), the clearance was 15.04 ± 6.09 ml/h/kg (range, 1.90 to 34.76 ml/h/kg), the half-life was 11.34 ± 14.15 h (range, 5.17 to 83.92 h), the mean residence time was 15.67 ± 20.66 h (range, 7.00 to 121.73 h), and the median time to Cmax was 0.6 h (range, 0.5 to 2.5 h). The fraction unbound in the plasma was 0.06 ± 0.02. All patients achieved Cmax/MIC and AUC from time zero to 24 h (AUC0-24)/MIC ratios for a bacteriostatic effect against Streptococcus pneumoniae. Twenty-seven patients (93%) achieved a Cmax/MIC ratio for a bacteriostatic effect against S. aureus, and 28 patients (97%) achieved an AUC0-24/MIC ratio for a bacteriostatic effect against S. aureus. Free plasma daptomycin concentrations were above the MIC for 50 to 100% of the dosing interval in 100% of patients for S. pneumoniae and 90% of patients for S. aureus. The median time to defervescence was 3 days from the start of daptomycin therapy. In summary, a 6-mg/kg intravenous infusion of daptomycin every 24 h was effective and well tolerated in neutropenic cancer patients.

Limited treatment options are available for implant-associated infections caused by methicillin (meticillin)-resistant Staphylococcus aureus (MRSA). We compared the activity of daptomycin (alone and with rifampin [rifampicin]) with the activities of other antimicrobial regimens against MRSA ATCC 43300 in the guinea pig foreign-body infection model. The daptomycin MIC and the minimum bactericidal concentration in logarithmic phase and stationary growth phase of MRSA were 0.625, 0.625, and 20 μg/ml, respectively. In time-kill studies, daptomycin showed rapid and concentration-dependent killing of MRSA in stationary growth phase. At concentrations above 20 μg/ml, daptomycin reduced the counts by >3 log10 CFU/ml in 2 to 4 h. In sterile cage fluid, daptomycin peak concentrations of 23.1, 46.3, and 53.7 μg/ml were reached 4 to 6 h after the administration of single intraperitoneal doses of 20, 30, and 40 mg/kg of body weight, respectively. In treatment studies, daptomycin alone reduced the planktonic MRSA counts by 0.3 log10 CFU/ml, whereas in combination with rifampin, a reduction in the counts of >6 log10 CFU/ml was observed. Vancomycin and daptomycin (at both doses) were unable to cure any cage-associated infection when they were given as monotherapy, whereas rifampin alone cured the infections in 33% of the cages. In combination with rifampin, daptomycin showed cure rates of 25% (at 20 mg/kg) and 67% (at 30 mg/kg), vancomycin showed a cure rate of 8%, linezolid showed a cure rate of 0%, and levofloxacin showed a cure rate of 58%. In addition, daptomycin at a high dose (30 mg/kg) completely prevented the emergence of rifampin resistance in planktonic and adherent MRSA cells. Daptomycin at a high dose, corresponding to 6 mg/kg in humans, in combination with rifampin showed the highest activity against planktonic and adherent MRSA. Daptomycin plus rifampin is a promising treatment option for implant-associated MRSA infections.

A possible explanation for the difficulties encountered in curing deep fibrin-embedded infections is that antibiotic diffusion inside the infected fibrin matrix is not homogeneous and is insufficient to neutralize the pathogen. To evaluate this conjecture, the differential pharmacodynamics of daptomycin in fibrin clots infected with methicillin-susceptible and -resistant Staphylococcus aureus and Staphylococcus epidermidis was estimated. Daptomycin (20 or 50 mg/kg of body weight) was infused over 30 min. Fibrin clots and blood samples were evaluated from 0.5 to 42 h after the injections. The half-lives of daptomycin in serum and fibrin clot were close to identical after the two doses and averaged 5.4 and 22 h, respectively. The mean areas under the concentration-time curves from 0 to 42 h (AUC0-infinity) for daptomycin concentrations in serum and infected clots were 575 +/- 36.7 and 215 +/- 6.2 micrograms/g/h after administration of 20 mg/kg and 1,089 +/- 39.9 and 326 +/- 16.8 micrograms/g/h after administration of 50 mg/kg. A concentration gradient from the periphery to the core of the clots was observed in many clots up to 18 h after treatment. Mean peak concentrations in the core of the clots reached 60% of the peripheral values (P < 0.05) and were delayed for at least 3 h compared with the peripheral peak concentrations. AUC0-42 h of daptomycin concentration in the periphery and the core of clots were significantly different (P < 0.01). Survival of microorganisms was better in the core than in the periphery, with as much as a 3 log10 CFU/g difference between the center and the surface of the clot. Bacterial examination by transmission electron microscopy also showed noticeable differences in ultrastructural changes between those in the periphery and those in the core of the clots. In conclusion, the pharmacokinetics of daptomycin are significantly different at the periphery and within the core of fibrin clots, which may have led to the higher bacterial survival in the core of clots. Limited diffusion of daptomycin in fibrin, an essential component of the vegetation in bacterial endocarditis, could explain at least in part some of the treatment failures.

In central and northern Wisconsin methicillin-resistant Staphylococcus aureus (MRSA) was first detected in 1989. Over the next 10-year period, 581 MRSA isolates were collected, 17.2% of which came from patients who were treated at five Native American clinics. These isolates were typed by SmaI-macrorestricted pulsed-field gel electrophoresis (PFGE). The PFGE patterns clustered the isolates into six major clonal groups (MCGs), i.e., MCGs 1 to 6, and 19 minor clonal groups (mCGs). The 25 clonal groups were represented by 109 unique PFGE types. Sixty-five percent of the MCG-2 isolates were recovered from patients who were treated at Native American clinics. Ninety-four percent of the MCG-2 isolates harbored the staphylococcal cassette chromosome mec (SCCmec) IVa. These isolates also had PFGE profiles that were clonally related to the midwestern community-associated MRSA (CA-MRSA) strain, MW2. The representative isolates from MCG-2 had the multilocus sequence type allelic profile 1-1-1-1-1-1-1 and contained pvl genes. They were also susceptible to various antibiotics, a finding consistent with the CA-MRSA phenotype. SCCmec IV was also present in other mCGs. Unlike MCG-2, isolates from the remaining five MCGs harbored SCCmec II and were resistant to multiple antibiotics, suggesting their nosocomial origin. The 19 mCGs were represented by diverse SCCmec types and three putative new variants referred to as SCCmec Ib, IIa, and IIb.

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 ≥128 μg/ml), we looked for synergy between daptomycin and other β-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 ≥256 μg/ml), and all strains were susceptible to daptomycin (the MIC at which 90% of the tested isolates were inhibited was 1 μg/ml). Daptomycin was tested at concentrations of 2, 1, 0.5, 0.25, 0.125, and 0.0625 μg/ml alone or in combination with oxacillin at a fixed concentration of 32 μg/ml. Synergy was found for all 18 strains with daptomycin at one-half the MIC in combination with 32 μg 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 β-lactams. In this approach, daptomycin was incorporated into Ca2+-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 μg/ml. The combination of daptomycin and β-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.

Purpose

The primary purpose of this trial was to define and describe the toxicities of oral valproic acid (VPA) at doses required to maintain trough concentrations of 100–150 mcg/ml or 150–200 mcg/ml in children with refractory solid or CNS tumors. Secondary objectives included assessment of free and total VPA pharmacokinetics and histone acetylation in peripheral blood mononuclear cells (PBMC) at steady state.

Patients and Methods

Oral VPA, initially administered twice daily and subsequently three times daily, was continued without interruption to maintain trough concentrations of 100–150 mcg/ml. First-dose and steady state pharmacokinetics were studied. Histone H3 and H4 acetylation in PBMCs was evaluated using an ELISA technique.

Results

Twenty six children, 16 of whom were evaluable for toxicity, were enrolled. Dose-limiting somnolence and intra-tumoral hemorrhage were associated with VPA troughs of 100–150 mcg/ml. Therefore, the final cohort of six children received VPA to maintain troughs of 75–100 mcg/ml and did not experience any dose-limiting toxicity. First-dose and steady state VPA pharmacokinetic parameters were similar to values previously reported in children with seizures. Increased PBMC histone acetylation was documented in 50% of patients studied. One confirmed partial response (glioblastoma multiforme) and one minor response (brainstem glioma) were observed.

Conclusions

VPA administered three times daily to maintain trough concentrations of 75–100 mcg/ml was well tolerated in children with refractory solid or CNS tumors. Histone hyper-acetylation in PBMCs was observed in half of the patients at steady state. Future trials combining VPA with chemotherapy and/or radiation therapy should be considered, especially for CNS tumors.

Despite the use of daptomycin alone at high doses (greater than 6 mg/kg of body weight/day) against difficult-to-treat infections, clinical failures and resistance appeared. Recently, the combination daptomycin-cloxacillin showed enhanced efficacy in clearing bacteremia caused by methicillin-resistant Staphylococcus aureus (MRSA). The aim of this study was to evaluate the efficacy of daptomycin at usual and high doses (equivalent to 6 and 10 mg/kg/day in humans, respectively) in combination with cloxacillin in a rat tissue cage infection model by MRSA and to compare its efficacy to that of daptomycin-rifampin. We used MRSA strain ATCC BAA-39. In the log- and stationary-phase kill curves, daptomycin-cloxacillin improved the bactericidal activity of daptomycin, especially in log phase. For in vivo studies, therapy was administered intraperitoneally for 7 days with daptomycin at 100 mg/kg/day and 45/mg/kg/day (daptomycin 100 and daptomycin 45), daptomycin 100-cloxacillin at 200 mg/kg/12 h, daptomycin 45-cloxacillin, and daptomycin 100-rifampin at 25 mg/kg/12 h. Daptomycin-rifampin was the best therapy (P < 0.05). Daptomycin 45 was the least effective treatment and did not protect against the emergence of resistant strains. There were no differences between the two dosages of daptomycin plus cloxacillin in any situation, and both protected against resistance. The overall effect of the addition of cloxacillin to daptomycin was a significantly greater cure rate (against adhered bacteria) than that for daptomycin alone. In conclusion, daptomycin-cloxacillin enhanced modestly the in vivo efficacy of daptomycin alone against foreign-body infection by MRSA and was less effective than daptomycin plus rifampin. The benefits of adding cloxacillin to daptomycin should be especially evaluated against infections by rifampin-resistant MRSA and for protection against the emergence of daptomycin nonsusceptibility.

The efficacies of daptomycin, teicoplanin, and vancomycin were compared in the therapy of experimental Staphylococcus aureus endocarditis. Rabbits infected with either of two methicillin-susceptible strains (SA-12871 or its moderately teicoplanin-resistant derivative SA-12873) or a methicillin-resistant S. aureus strain (MRSA-494) were treated with daptomycin, 8 mg/kg of body weight, every 8 h; teicoplanin, 12.5 mg/kg (low-dose teicoplanin [teicoplanin-LD], excluding MRSA-494) or 40 mg/kg (high-dose teicoplanin [teicoplanin-HD]) every 12 h; or vancomycin, 17.5 mg/kg every 6 h, for 4 days. Compared with no treatment daptomycin, teicoplamin-HD, and vancomycin significantly reduced bacterial counts of all test strains in vegetations and renal and splenic tissues (P less than 0.001). Teicoplanin-LD was equally effective against SA-12871 but failed against SA-12873, with three of six animals still being bacteremic at the end of therapy. For SA-12871, daptomycin was as effective as teicoplanin-HD and was superior to teicoplanin-LD and vancomycin (P = 0.02) in lowering vegetation bacterial counts. There were no differences between daptomycin, teicoplanin-HD, or vancomycin in the reduction of bacterial counts in tissues for any of the test strains. In rabbits infected with SA-12871, vegetations from 33% of teicoplanin-LD-treated, 6% of teicoplanin-HD-treated, and 13% of daptomycin-treated animals yielded organisms for which there were up to eightfold increases in the MICs. Resistance may have contributed to early death in one daptomycin-treated animal. No increases in the MICs for the test strain were detected in animals infected with SA-12873 or MRSA-494. We conclude that in this model and against these strains of S. aureus, daptomycin and teicoplanin-HD are as efficacious as vancomycin, but diminished susceptibility to both can develop during therapy.

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 log10 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% (ED50) and 80% (ED80) bacterial density reduction at 48 h. Daptomycin MICs for study isolates ranged from 0.125 to 4 μg/ml. Model fitting resulted in an r2 of >0.80 for all tested isolates. Control growths at 48 h ranged from 7.3 to 8.5 log10 CFU/ml. Sigmoid relationships were not superimposable between categorical resistant species: ED50 and ED80 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 ED50 and ED80 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 ED80 values among the tested isolates. Doses ranging between 3 and 7 mg/kg produced significant bactericidal activity (ED80) against these multidrug-resistant S. aureus and E. faecium isolates.

Daptomycin, a novel lipopeptide, is bactericidal against a broad range of gram-positive strains, including methicillin- (MRSA) and vancomycin-resistant Staphylococcus aureus. Daptomycin is approved at 4 mg/kg of body weight given intravenously once daily for the treatment of complicated skin and skin structure infections and at 6 mg/kg for the treatment of S. aureus bloodstream infections (bacteremia), including right-sided endocarditis caused by methicillin-susceptible S. aureus and MRSA. The present study was designed to evaluate the multiple-dose pharmacokinetics and safety of daptomycin at doses of 6 to 12 mg/kg in healthy volunteers. Three cohorts of 12 subjects each were given daptomycin (10 mg/kg) or placebo once daily for 14 days, daptomycin (12 mg/kg) or placebo once daily for 14 days, or daptomycin (6 or 8 mg/kg) once daily for 4 days. Daptomycin produced dose-proportional increases in the area under the plasma concentration-time curve and in trough daptomycin levels and nearly dose-proportional increases in peak daptomycin concentrations. Other pharmacokinetic parameters measured on day 1 and at steady state were independent of the dose, including the half-life (approximately 8 h), weight-normalized plasma clearance (9 to 10 ml/h/kg), and volume of distribution (approximately 100 ml/kg). Plasma protein binding was 90% to 93% and was independent of the daptomycin concentration. Daptomycin did not produce electrocardiographic abnormalities or electrophysiological evidence of muscle or nerve toxicity. Daptomycin was well tolerated in subjects dosed with up to 12 mg/kg intravenously for 14 days. Doses of daptomycin higher than 6 mg/kg once daily may be considered in further studies to evaluate the safety and efficacy of daptomycin in difficult-to-treat infections.