Pharmacodynamics of RP 59500 (quinupristin-dalfopristin) administered by intermittent versus continuous infusion against Staphylococcus aureus-infected fibrin-platelet clots in an in vitro infection model

We evaluated the bactericidal activity of RP 59500 (quinupristin-dalfopristin) against fibrin-platelet clots (FPC) infected with two clinical isolates of Staphylococcus aureus, one constitutively erythromycin and methicillin resistant (S. aureus AW7) and one erythromycin and methicillin susceptible (S. aureus 1199), in an in vitro pharmacodynamic infection model. RP 59500 was administered by continuous infusion (peak steady-state concentration of 6 microg/ml) or intermittent infusion (simulated regimens of 7.5 mg/kg of body weight every 6 h (q6h) q8h, and q12h. FPCs were infected with S. aureus to achieve an initial bacterial density of 10(9) CFU/g. Model experiments were run in duplicate over 72 h. Two FPCs were removed from each model at 0, 12, 24, 36, 48, and 72 h, and the bacterial densities (in CFU per gram) were determined and compared to those of growth control experiments. Additional samples were also removed from the model over the 72-h period for pharmacokinetic evaluation. All regimens significantly (P < or = 0.01) decreased bacterial densities in the infected FPCs for both isolates compared to growth controls. This occurred even though MBCs were equal to or greater than the RP 59500 concentrations achieved in the models. There were no significant differences found between the dosing frequencies and levels of killing when examining each isolate separately. However, examination of the residual bacterial densities (CFU per gram at 72 h) and visual inspection of the overall killing effect (killing curve plots over 72 h) clearly demonstrated a more favorable bactericidal activity against 1199 than against the AW7 isolate. This was most apparent when the q8h and the q12h AW7 regimens were compared to all 1199 treatment regimens by measuring the 72-h bacterial densities (P < or = 0.01). Killing (99.9%) was not achieved against the AW7 isolate. However, a 99.9% kill was demonstrated for all dosing regimens against the 1199 isolate. The area under the concentration-time curve from 0 to 24 h was found to be significantly correlated with reduction in bacterial density for the AW7 isolate (r = 0.74, P = 0.04). No resistance was detected during any experiment for either isolate. RP 59500 efficacy against constitutively erythromycin- and methicillin-resistant S. aureus may be improved by increasing organism exposure to RP 59500 as a function of dosing frequency.

In-vitro bactericidal activity of quinupristin/dalfopristin alone and in combination against resistant strains of Enterococcus species and Staphylococcus aureus

In-vitro activities of quinupristin/dalfopristin, a semisynthetic injectable streptogramin, and vancomycin were compared against multidrug-resistant Enterococcus faecium and Enterococcus faecalis, methicillin-susceptible Staphylococcus aureus ATCC 25923 and a methicillin-resistant S. aureus (MRSA). Combinations of quinupristin/dalfopristin and/or vancomycin with ofloxacin or gentamicin were evaluated using the chequerboard technique. The only synergy observed was that between quinupristin/dalfopristin plus vancomycin and quinupristin/dalfopristin plus gentamicin against E. faecium (FIC index < 0.5). Time-kill curves were performed over 24 h with an inoculum of 1 x 10(7) cfu/mL and clinically achievable concentrations of quinupristin/dalfopristin, vancomycin, ofloxacin and gentamicin (6, 30, 5 and 5 mg/L, respectively). In time-kill studies, combinations of quinupristin/dalfopristin plus vancomycin and quinupristin/dalfopristin plus gentamicin were additive, not synergic, against E. faecium and achieved 99.9% killing in 21.2 h and 19.6 h, respectively. None of the combination regimens suppressed the regrowth of E. faecalis. Quinupristin/dalfopristin combined with vancomycin demonstrated consistent synergy against ATCC 25923 and the MRSA, achieving 99.9% killing in 12.1 h and 11.9 h, respectively. Overall, quinupristin/dalfopristin alone demonstrated inhibitory activity against E. faecium, but not against E. faecalis, and bactericidal activity was achieved only with quinupristin/dalfopristin in combination with vancomycin or gentamicin against E. faecium. Quinupristin/dalfopristin plus vancomycin was the most potent and reliable combination against both strains of S. aureus in time-kill studies.

An evaluation of the bactericidal activity of ampicillin/sulbactam, piperacillin/tazobactam, imipenem or nafcillin alone and in combination with vancomycin against methicillin-resistant Staphylococcus aureus (MRSA) in time-kill curves with infected fibrin clots

The activity of piperacillin/tazobactam, ampicillin/sulbactam, imipenem and nafcillin alone and in combination with vancomycin was compared with vancomycin monotherapy against MRSA in test-tube time-kill studies and in infected fibrin clots. Bactericidal activity was achieved with all regimens except nafcillin monotherapy in test tubes but only with imipenem/vancomycin and nafcillin/vancomycin in fibrin clots infected with heterogeneous strains. No regimen was effective against the homogeneous strain. These agents may have potential as alternatives to vancomycin in selected infections.

A pharmacist-initiated program of intravenous to oral antibiotic conversion

A prospective program to convert patients from parenteral to oral antibiotics was evaluated over 12 months to determine its pharmacoeconomic impact on antibiotic acquisition and length of hospital stay. Physicians of patients meeting predetermined clinical criteria for mild and moderate infections were contacted to discuss potential oral alternative therapy. Clinical end points and economic data were followed in 242 patients (200 converted and 42 not converted but meeting criteria). No significant differences were noted between the groups with regard to demographic data, infection diagnosis, clinical outcome, or adverse effects. The average number of days of therapy for patients converted was 1.53 days shorter than that of patients who were not converted to oral therapy (p < 0.003). Cost savings for drug acquisition and length of stay were $15,149.24 and $161,071.88, respectively. The intervention program appeared to provide a cost-effective conversion from parenteral to oral antimicrobial administration without compromising patient care. It is anticipated that expansion of the program to include additional antibiotics will result in even greater cost savings for the institution.

Vancomycin-resistant enterococci

Vancomycin-resistant enterococci (VRE) are a major problem in numerous institutions in the United States. Most VRE are resistant to all available antimicrobial agents, resulting in serious therapeutic dilemmas. The resistance genes are transmitted on transposons, so the potential for dissemination to other species is significant. Risk factors associated with VRE infection and colonization are vancomycin and cephalosporin use, but numerous patient-related factors also contribute. Although resistant strains appear to arise from the patient's endogenous flora, VRE may be spread through direct contact with contaminated environmental surfaces and hands of caregivers. Published guidelines for preventing such spread suggest implementing infection-control practices and vancomycin restrictions. The ideal drug regimen for the treatment of VRE is unknown. Various drug combinations have been studied in the laboratory, but patient treatment data are scarce. There is an urgent need for new antimicrobial agents.

Combination antimicrobial therapy for bacterial infections. Guidelines for the clinician

Therapy with antimicrobial combinations has been used as long as antimicrobials have been available. Combinations of antibiotics are often used to take advantage of different mechanisms of action and/or toxicity profiles. Well established indications for combination antimicrobial therapy include: (a) empirical treatment of life-threatening infections; (b) treatment of polymicrobial infections; (c) prevention of the emergence of bacterial resistance; and (d) for synergism. Disadvantages of combination therapy include: (a) increased expense; (b) increased risk of adverse effects; (c) antagonism; and (d) superinfection. Combination antimicrobial therapy should be considered for the treatment of serious Gram-negative infections caused by Enterobacter cloacae, Pseudomonas aeruginosa and Serratia marcescens, and certain Gram-positive infections caused by Enterococcus spp. and Staphylococcus spp. Selection of agents should be dependent upon local susceptibility patterns, clinical experience, site of infection, potential toxicities and cost.

Bactericidal activities of cefprozil, penicillin, cefaclor, cefixime, and loracarbef against penicillin-susceptible and -resistant Streptococcus pneumoniae in an in vitro pharmacodynamic infection model

We examined the bactericidal activities of penicillin, cefprozil, cefixime, cefaclor, and loracarbef against three clinical isolates of Streptococcus pneumoniae which were susceptible, moderately susceptible, and resistant to penicillin. An in vitro two-compartment glass infection model was used to simulate human pharmacokinetics in the presence of bacteria. Also, changes in organism susceptibility and development of resistant subpopulations were evaluated. Simulated pediatric dosage regimens and target peak concentrations in the central compartment were as follows: penicillin V-potassium, 26 mg/kg of body weight every 6 h (q6h) and 14 micrograms/ml; cefaclor, 13.4 mg/kg q8h and 16 micrograms/ml; loracarbef, 15 mg/kg q12h and 19 micrograms/ml; cefprozil, 15 mg/kg q12h and 11 micrograms/ml; and cefixime, 8mg/kg q24h and 4 micrograms/ml. Targeted half-lives of each agent were 1 h for penicillin, cefaclor, and loracarbef; 1.3 h for cefprozil; and 3.5 h for cefixime. Growth controls were performed at two different pump rates, 0.8 and 2.0 ml/min (half-lives = 3.5 and 1 h, respectively). Each isolate demonstrated autolysis at the lower rate which was attributed to a decreased supply of fresh nutrients available to the organisms in the infection compartment. Against the susceptible isolate, the time to 99.9% killing was statistically significant between penicillin V-potassium and both cefaclor and cefixime (P < 0.029). Loracarbef never achieved a 99.9% reduction in the inoculum. At 48 h penicillin, cefprozil, and cefaclor were equivalent in extent of killing. Against the intermediately resistant isolate, cefprozil was superior to all other regimens with respect to rate of killing (P < 0.013) and extent of killing at 24 h (P < 0.0003). At 48 h penicillin, cefprozil, and cefaclor were equivalent in extent of killing. All of the regimens exhibited inferior activity against this penicillin-resistant isolate. A 99.9% kill was never obtained with any of the regimens, nor was there an appreciable decrease in the colony counts. In conclusion, it appears that cefprozil, penicillin, and cefaclor are effective therapies against sensitive and even intermediately sensitive isolates of S. pneumoniae. However, none of the oral therapies appear to be of any benefit against penicillin-resistant isolates. The in vitro model may be an effective tool in evaluating other multiple-dose therapies against this fastidious organism, since the continual supply of fresh medium maintains the viability of S. pneumoniae with minimal stationary-phase autolysis.

Continuous infusion versus intermittent administration of ceftazidime in critically ill patients with suspected gram-negative infections

The pharmacodynamics and pharmacokinetics of ceftazidime administered by continuous infusion and intermittent bolus over a 4-day period were compared. We conducted a prospective, randomized, crossover study of 12 critically ill patients with suspected gram-negative infections. The patients were randomized to receive ceftazidime either as a 2-g intravenous (i.v.) loading dose followed by a 3-g continuous infusion (CI) over 24 h or as 2 g i.v. every 8 h (q8h), each for 2 days. After 2 days, the patients were crossed over and received the opposite regimen. Each regimen also included tobramycin (4 to 7 mg/kg of body weight, given i.v. q24h). Eighteen blood samples were drawn on study days 2 and 4 to evaluate the pharmacokinetics of ceftazidime and its pharmacodynamics against a clinical isolate of Pseudomonas aeruginosa (R288). The patient demographics (means +/- standard deviations) were as follows: age, 57 +/- 12 years; sex, nine males and three females; APACHE II score, 15 +/- 3; diagnosis, 9 of 12 patients with pneumonia. The mean pharmacokinetic parameters for ceftazidime given as an intermittent bolus (IB) (means +/- standard deviations) were as follows: maximum concentration of drug in serum, 124.4 +/- 52.6 micrograms/ml; minimum concentration in serum, 25.0 +/- 17.5 micrograms/ml; elimination constant, 0.268 +/- 0.205 h-1; half-life, 3.48 +/- 1.61 h; and volume of distribution, 18.9 +/- 9.0 liters. The steady-state ceftazidime concentration for CI was 29.7 +/- 17.4 micrograms/ml, which was not significantly different from the targeted concentrations. The range of mean steady-state ceftazidime concentrations for the 12 patients was 10.6 to 62.4 micrograms/ml. Tobramycin peak concentrations ranged between 7 and 20 micrograms/ml. As expected, the area under the curve for the 2-g q8h regimen was larger than that for CI (P = 0.003). For IB and CI, the times that the serum drug concentration was greater than the MIC were 92 and 100%, respectively, for each regimen against the P. aeruginosa clinical isolate. The 24-h bactericidal titers in serum, at which the tobramycin concentrations were < 1.0 microgram/ml in all patients, were the same for CI and IB (1:4). In the presence of tobramycin, the area under the bactericidal titer-time curve (AUBC) was significantly greater for IB than CI (P = 0.001). After tobramycin was removed from the serum, no significant difference existed between the AUBCs for CI and IB. We conclude that CI of ceftazidime utilizing one-half the IB daily dose was equivalent to the IB treatment as judged by pharmacodynamic analysis of critically ill patients with suspected gram-negative infections. No evaluation comparing the clinical efficacies of these two dosage regimens was performed.

Comparison of conventional dosing versus continuous-infusion vancomycin therapy for patients with suspected or documented gram-positive infections

Ten patients were treated with conventional dosing (CD) and continuous-infusion (CI) vancomycin therapy in this prospective, randomized, crossover study. Patients were randomized to receive either CD or CI therapy for 2 consecutive days and then crossed over to receive the opposite regimen for 2 days. CD therapy consisted of 1 g of vancomycin every 12 h. CI therapy consisted of a 500-mg loading dose followed by 2 g infused over 24 h. Ten serum samples were obtained on the second day of each therapy for pharmacokinetic and pharmacodynamic analyses. Two clinical isolates of Staphylococcus aureus, one methicillin sensitive (MSSA 1199) and one methicillin resistant (MRSA 494), were chosen for pharmacodynamic evaluation of both regimens. The patient demographics (means +/- standard deviations [SD]) were as follows: sex, six males, four females; age, 36 +/- 11 years; and serum creatinine, 0.72 +/- 0.18 mg/dl. Mean pharmacokinetic parameters +/- SD for CD therapy were as follows: elimination rate constant, 0.16 +/- 0.07 h-1; half-life, 5.6 +/- 3.5 h; volume of distribution, 33.7 +/- 25 liters, 0.5 +/- 0.2 liters/kg; maximum concentration in serum, 53.4 +/- 19.3 micrograms/ml; and minimum concentration, 8.4 +/- 5.9 micrograms/ml. The steady-state concentration for CI was 20.2 +/- 11.1 micrograms/ml. Overall, both regimens resulted in the MIC being exceeded 100% of the time. The mean CD trough serum bactericidal titer (SBT) was 1:8, and the average CI SBTs were 1:16 for both isolates. Even though there was no statistically significant difference between CD trough and CI SBTs, the CI SBTs remained > 1:8 for 100% of the time versus 60% of the time for CD therapy. During CI therapy, 20 and 40% of the patients maintained SBTs of > 1:32 throughout the dosing interval for MSSA 1199 and MRSA 494, respectively. During CD therapy, however, only 10% of patients maintained SBTs of > 1:32 during the entire dosing interval for both isolates. The mean areas under the bactericidal titer-time curve (AUBC24s) +/- SD for MSSA 1199 were 528 +/- 263 for CD therapy and 547 +/- 390 for CI therapy. The mean AUBC24s +/- SD against MRSA 494 were 531 +/- 247 for CD and 548 +/- 293 for CI therapy. Similar to the AUBC24, the mean area under the concentration-time curve for a 24-h dosing interval divided by the MIC (AUC/MIC24) ratios +/- SD were 550.0 +/- 265.7 for CD and 552.6 +/- 373.4 for CI therapy, respectively. No statistically significant differences were found between any of the pharmacodynamic parameters for CD and CI therapy. In addition, no adverse effects with either CD or CI therapy were observed during the study. We conclude that CI and CD vancomycin therapy demonstrated equivalent pharmacodynamic activities. Although CI therapy was more likely to result in SBTs that remained above 1:8 for the entire regimen, the clinical impact of this result is unknown. Serum drug concentration variability was observed with both treatment regimens but to a lesser extent with CI administration. CI administration of vancomycin should be further evaluated to determine the clinical utility of this method of administration.

Pharmacodynamics of once- or twice-daily levofloxacin versus vancomycin, with or without rifampin, against Staphylococcus aureus in an in vitro model with infected platelet-fibrin clots

We compared the pharmacodynamic activities of levofloxacin versus vancomycin, with or without rifampin, in an in vitro model with infected platelet-fibrin clots simulating vegetations. Infected platelet-fibrin clots were prepared with human cryoprecipitate, human platelets, calcium, thrombin, and approximately 10(9) CFU of organisms (MSSA 1199 and MRSA 494) per g and then were suspended via monofilament line into the in vitro model containing Mueller-Hinton growth medium. Antibiotics were administered by bolus injection into the model to simulate human pharmacokinetics; the regimens simulated included levofloxacin at dosages of 800 mg every 24 h (q24h) and 400 mg q12h, vancomycin at 1 g q12h, and rifampin at 600 mg q24h. Each model was run in duplicate over a 72-h period. Infected platelet-fibrin clots were removed in duplicate from each model, weighed, homogenized, serially diluted with sterile 0.9% saline, and plated on tryptic soy agar plates and plates containing antibiotics at 3, 6, and 12 times the MIC to evaluate the emergence of resistance. Time-kill curves were constructed by plotting the inoculum size versus time. Residual inoculum at 72 h was used to compare regimens. All levofloxacin regimens were significantly better than vancomycin monotherapy against both isolates (P < 0.002). Against MSSA 1199, levofloxacin q24h was significantly better than all other regimens, including levofloxacin q12h (P < 0.002); however, no difference between the levofloxacin monotherapy and combination therapy (with rifampin) regimens against MRSA 494 was seen. Killing activity for levofloxacin appeared to correlate better with the peak/MIC ratio than with the area under the curve/MIC ratio. The addition of rifampin significantly enhanced the activity of vancomycin but had little effect upon the activity of levofloxacin. For MRSA 494, vancomycin plus rifampin resulted in the greatest killing (P < 0.05). Development of resistance was not detected with any regimen. Levofloxacin may be a useful therapeutic alternative in the treatment of staphylococcal endocarditis, and further study with animal models of endocarditis or clinical trials are warranted.

Comparison of methodologies for synergism testing of drug combinations against resistant strains of Pseudomonas aeruginosa

The purpose of this study was to determine if synergism was maintained for various combinations of beta-lactams with an aminoglycoside against four clinical strains and one laboratory strain of Pseudomonas aeruginosa which were resistant, according to the MICs, to the beta-lactams and/or aminoglycoside. The results from both the checkerboard and killing curve methodologies were compared. The laboratory strain (ATCC 27853) was manipulated in vitro by serial passage onto agar containing increasing concentrations of each antibiotic to select for resistance. One clinical isolate (R61) was also serially passed to raise the MIC of piperacillin from 128 to 1,024 micrograms/ml. The fractional inhibitory concentration indices for all isolates indicated indifference for all combination therapies, with values ranging from 0.6 to 3. In contrast, killing curve results for all isolates demonstrated synergism with drug concentrations at either one-fourth or one-half the MIC for each organism. The MIC of piperacillin for the laboratory-manipulated clinical isolate R61 was 1,024 micrograms/ml, and synergism was still observed with concentrations of one-half the MIC of piperacillin and amikacin. For clinical isolate R166, which was beta-lactam and tobramycin resistant, synergism continued to be demonstrated with concentrations of tobramycin (1/16 MIC) in combination with piperacillin and cefepime at 1/2 the MIC. The results of this study indicate that against P. aeruginosa, synergism is observed in spite of resistance to beta-lactams and/or aminoglycosides. Synergism appears to be maintained even at very high MICs (piperacillin, 1,024 micrograms/ml; tobramycin, 128 micrograms/ml) with drug concentrations within achievable therapeutic ranges. With current definitions of synergism there was a complete lack of correlation between the results obtained by the checkerboard and killing curve methodologies, with the fractional inhibitory concentration indices showing indifference and killing curves resulting in synergism. The methodologies and definitions of synergism or antagonism are variable and not standardized and should be reevaluated.

Influence of antibiotic and E5 monoclonal immunoglobulin M interactions on endotoxin release from Escherichia coli and Pseudomonas aeruginosa

Recent controversy surrounding the activity of monoclonal antibodies against endotoxin highlights the necessity of identifying all factors associated with increased mortality, one of which is endotoxin concentrations. Antibiotics may induce different patterns of endotoxin release. We compared the release of free endotoxin (in endotoxin units per milliliter) over 6 h and changes in numbers of CFU of exponentially growing Escherichia coli and Pseudomonas aeruginosa (10(6) to 10(7) CFU/ml) cultured in chemically defined endotoxin-free broth combined with pooled human serum and/or 10 micrograms of E5 immunoglobulin M monoclonal antibody per ml. MICs and MBCs were tested in each medium at the same inoculum. The inoculum was exposed to antibiotics at a single fixed multiple of the MIC for each medium (range, two to eight times the MIC). E5 antibody had no effect on MICs, MBCs, bactericidal activity, or endotoxin release. In the presence of 50% serum, amikacin, ceftazidime, imipenem, and ofloxacin each killed equivalent amounts of E. coli over 6 h; however, ceftazidime induced the highest release of endotoxin. Amikacin and ofloxacin produced the most favorable ratio of endotoxin release to amount of bacterial killing. In the presence of 50% serum, ceftazidime and imipenem reduced the P. aeruginosa inoculum to the greatest extent over 6 h. Although its bactericidal activity was diminished, ofloxacin caused the lowest release of free endotoxin. Imipenem and ofloxacin showed similar low ratios of endotoxin release to bacterial killing. In summary, antibiotic class, presence of serum, and type of organism influenced bactericidal activity and endotoxin release.

Comparative in vitro activities of LY191145, a new glycopeptide, and vancomycin against Staphylococcus aureus and Staphylococcus-infected fibrin clots

Bactericidal activities of LY191145, an investigational glycopeptide, and vancomycin against Staphylococcus aureus were evaluated. Only LY191145 at a concentration 16-fold greater than the MIC was able to achieve 99.9% killing against methicillin-susceptible S. aureus (ATCC 25923; 8.0 h). Both agents demonstrated 99.9% killing against methicillin-resistant clinical isolate S. aureus MRSA67 over 24 h at concentrations 4-, 8-, and 16-fold greater than the MIC, but bacteria were killed at a more rapid rate by LY191145 (1.63 versus 5.02 h; P < 0.001). Against strain ATCC 25923- and MRSA67-infected fibrin clots, total reductions by LY191145 and vancomycin over 72 h were not statistically significantly different at a concentration 16 times the MIC (1.12 +/- 0.31 and 1.23 +/- 0.13 and 1.40 +/- 0.17 and 1.36 +/- 0.37 CFU/g; respectively). Increasing the drug concentration to 50 times the MIC did not alter the values significantly, and there was no statistically significant difference between the two agents. Overall, LY191145 exhibited more rapid bactericidal activity than vancomycin against S. aureus, and a concentration 16-fold greater than the MIC appears to be optimal.

Quinupristin/dalfopristin (RP 59500): a new streptogramin antibiotic

OBJECTIVE

To review the current knowledge on RP 59500 (quinupristin/dalfopristin, Synercid), a new streptogramin antibiotic, with respect to its pharmacology, pharmacokinetics, pharmacodynamics, mechanism of resistance, and in vitro inhibitory and bactericidal activity.

DATA SOURCES

A MEDLINE search using keywords RP 59500, pristinamycin, virginiamycin, and streptogramin was performed. Relevant abstracts presented at recent scientific conferences also were consulted.

STUDY SELECTION

Because RP 59500 is a relatively new investigational agent, relevant in vitro and animal studies were selected. All available human studies were included as well.

DATA EXTRACTION

Data from in vitro and in vivo studies were included, with particular emphasis on human studies.

DATA SYNTHESIS

RP 59500 is a new injectable streptogramin antibiotic consisting of a mixture o 2 synergistic pristinamycin compounds. RP 59500 possesses in vitro inhibitory and bactericidal activity against most isolates of gram-positive organisms including vancomycin-resistant Enterococcus faecium, selected gram-negative bacteria, and most anaerobic organisms. Based on preliminary data, the drug appears to be metabolized rapidly and extensively while exhibiting a significant postantibiotic effect. Data from ongoing clinical trials suggests that RP 59500 is well-tolerated except for mild injection site irritations. However, before the role of RP 59500 within the vast armamentarium of antimicrobials can be elucidated, additional studies need to be conducted to document its clinical efficacy.

CONCLUSIONS

Based on in vitro susceptibility testing, in vivo studies, and preliminary clinical data, RP 59500 may be an alternative to the glycopeptides, especially for inherently resistant organisms. Further studies are needed to confirm this agent's in vitro activity and to establish its clinical efficacy.

Pharmacodynamics of ceftazidime administered as continuous infusion or intermittent bolus alone and in combination with single daily-dose amikacin against Pseudomonas aeruginosa in an in vitro infection model

We compared the pharmacodynamics and killing activity of ceftazidime, administered by continuous infusion and intermittent bolus, against Pseudomonas aeruginosa ATCC 27853 and ceftazidime-resistant P. aeruginosa 27853CR with and without a single daily dose of amikacin in an in vitro infection model over a 48-h period. Resistance to ceftazidime was selected for by serial passage of P. aeruginosa onto agar containing increasing concentrations of ceftazidime. Human pharmacokinetics and dosages were simulated as follows: half-life, 2 h; intermittent-bolus ceftazidime, 2 g every 8 h (q8h) and q12h; continuous infusion, 2-g loading dose and maintenance infusions of 5, 10, and 20 micrograms/ml; amikacin, 15 mg/kg q24h. There was no significant difference in time to 99.9% killing between any of the monotherapy regimens or between any combination regimen against ceftazidime-susceptible P. aeruginosa. Continuous infusions of 10 and 20 micrograms/ml killed as effectively as an intermittent bolus of 2 g q12h and q8h, respectively. Continuous infusion of 20 micrograms/ml and an intermittent bolus of 2 g q8h were the only regimens which prevented organism regrowth at 48 h, while a continuous infusion of 5 micrograms/ml resulted in the most regrowth. All of the combination regimens exhibited a synergistic response, with rapid killing of ceftazidime-susceptible P. aeruginosa and no regrowth. Against ceftazidime-resistant P. aeruginosa, none of the ceftazidime monotherapy regimens achieved 99.9% killing. The combination regimens exhibited the same rapid killing of the resistant strain as occurred with the susceptible strain; however, regrowth occurred with all regimens. The combination regimens of continuous infusion of 20 micrograms/ml plus amikacin and intermittent bolus q8h or q12h plus amikacin continued to be synergistic. Overall, continuous infusion monotherapy with ceftazidime at concentrations 4 to 5 and 10 to 15 times the MIC was as effective as an intermittent bolus of 2 g q12h (10 to 15 times the MIC) and q8h (25 to 35 times the MIC), respectively, against ceftazidime-susceptible P. aeruginosa. Combination therapy with amikacin plus ceftazidime, either intermittently q8h or by continuous infusion of 20 micrograms/ml, appeared to be effective and exhibited synergism against ceftazidime-resistant P. aeruginosa.

Bactericidal killing activities of cefepime, ceftazidime, cefotaxime, and ceftriaxone against Staphylococcus aureus and beta-lactamase-producing strains of Enterobacter aerogenes and Klebsiella pneumoniae in an in vitro infection model

Cefepime (CP) is a new injectable cephalosporin with a broad spectrum of activity and stability against common chromosomally and plasmid-mediated beta-lactamases. The bactericidal activities of CP, ceftazidime (CZ), cefotaxime (CTX), and ceftriaxone (CAX) against reference and clinical strains of Staphylococcus aureus, an isogenic pair of Enterobacter aerogenes strains (wild type and a CZ-resistant derepressed mutant), and a Klebsiella pneumoniae isolate possessing a TEM-10 beta-lactamase were investigated in a two-compartment pharmacodynamic in vitro infection model which simulates human pharmacokinetics. An inoculum of approximately 10(6) CFU/ml was used in all model experiments. Antibiotics were administered to simulate the following regimens: CP at 2 g every 12 h (q12h), CZ at 2 g q8h, CTX at 2 g q8h, and CAX at 2 g q24h. Human albumin was added during experiments with CAX and staphylococci to simulate protein binding. Samples were removed at multiple time points over a 48-h period to determine the inoculum size for time-kill curves. Development of resistance was detected by inoculating samples obtained at 0, 24, and 48 h onto antibiotic-containing agar plates. The time to 99.9% killing was used to compare drug regimens. Against staphylococci, the time to bacterial eradication was significantly delayed with CAX-albumin. All regimens had similar activities against the wild-type Enterobacter strain; however, regrowth was noted with CZ, CTX, and CAX against the CZ-resistant strain. There were no differences between the CP, CTX, and CAX regimens against K. pneumoniae. Of interest, no regrowth of any organism was noted with CP. These data indicate that CP has activity against S.aureus and CZ-resistant gram-negative bacilli.

Pharmacodynamics of RP 59500 alone and in combination with vancomycin against Staphylococcus aureus in an in vitro-infected fibrin clot model

The bactericidal activity and emergence of resistance to RP 59500 (quinupristin/dalfopristin) when it was administered alone and in combination with vancomycin against fibrin clots that have been infected with methicillin-susceptible Staphylococcus aureus ATCC 25923 or methicillin-resistant S. aureus (MRSA) 67 were evaluated in an in vitro pharmacodynamic infected fibrin clot model. Fibrin clots were infected with S. aureus to achieve an inoculum of approximately 10(9) CFU/g. Antibiotics were administered to simulate pharmacokinetics in humans: RP 59500 (7.5 mg/kg of body weight) every 8 h and vancomycin (15 mg/kg) every 12 h over 72 h. Preliminary test tube time-kill experiments with an inoculum of approximately 10(5) CFU/ml suggested that RP 59500 was more rapid in achieving a 99.9% reduction in the number of CFU per milliliter than vancomycin against ATCC 25923 (6.94 versus 24 h; P = 0.0003) and MRSA 67 (6.77 versus 17.03 h; P = 0.004). At a higher inoculum (approximately 10(8) CFU/ml), 99.9% kill was achieved only with the combination regimen against ATCC 25923 and MRSA 67 (10.9 and 10.5 h, respectively), with total reductions of 6.35 and 6.33 log10 CFU/ml over 24 h, respectively. In the fibrin clot model, RP 59500 was more effective than vancomycin in reducing organism titers over 72 h. In the fibrin clot model, the most optimal therapy was the combination regimen.

Pharmacodynamics of levofloxacin, ofloxacin, and ciprofloxacin, alone and in combination with rifampin, against methicillin-susceptible and -resistant Staphylococcus aureus in an in vitro infection model

The pharmacodynamic properties of levofloxacin (an optically active isomer of ofloxacin), ofloxacin, and ciprofloxacin, alone and in combination with rifampin, were evaluated over 24 to 48 h against clinical isolates of methicillin-susceptible and -resistant Staphylococcus aureus (MSSA 1199 and MRSA 494, respectively) in an in vitro infection model. The incidence of the emergence of resistance among the test strains was also determined. The fluoroquinolones were administered to simulate dosage regimens of 200 mg, 400 mg given intravenously (i.v.) every 12 h (q12h), and 400 and 800 mg given i.v. q24h. Rifampin was dosed at 600 mg i.v. q24h. Although the MICs and MBCs of the quinolones were similar (< or = 0.49 microgram/ml), levofloxacin was the most potent agent in time-kill studies on the basis of the time required to achieve a 99.9% reduction in the number of log10 CFU per milliliter (e.g., with the regimen of levofloxacin [400 mg q24h, 6.5 h] versus ofloxacin [12.5 h], P < 0.024, and levofloxacin versus ciprofloxacin [6.5 versus 9.0 h], P < 0.0017) against MSSA 1199. The killing activity of levofloxacin was similar to that of ofloxacin against MRSA 494 (time to achieve a 99.9% reduction in the number of log10 CFU per milliliter, 11.1 versus 13.8 h, respectively). Levofloxacin and ofloxacin dosed once daily demonstrated greater bactericidal activity than when they were dosed twice daily against MSSA 1199. Resistance to levofloxacin or ofloxacin was not observed with any dosage regimen. Furthermore, resistance to ofloxacin was not detected when the half-life was reduced from 6 to 3 h. Regrowth and stable resistance (65-fold increase in the MIC for MSSA 1199; 16-fold increase in the MIC for MRSA 494) were noted within 24 h of exposure to ciprofloxacin at 200 mg q12h. Combination therapy with rifampin prevented the emergence of resistance to ciprofloxacin. Neither DNA gyrase alteration nor an energy-dependent efflux process mediated by the norA gene appeared to be responsible for the resistance observed. Our data suggest that with levofloxacin there is a more rapid onset of bactericidal activity than with ofloxacin or ciprofloxacin against MSSA 1199 and that the activity of levofloxacin is similar to that of ofloxacin but better than that of ciprofloxacin against MRSA 494. Resistance was noted only after exposure to the low dose of ciprofloxacin. Resistance to ofloxacin did not develop even when the pharmacokinetics of the drug were set to equal those of ciprofloxacin, suggesting that ofloxacin differs from ciprofloxacin irrespective of time of exposure. The resistance to ciprofloxacin that developed in our vitro model may be mediated by the cfx-ofx locus, which has been shown to be associated with low-level fluoroquinolone resistance. Overall, levofloxacin demonstrated potent bactericidal activity against S. aureus, without the emergence of resistance in our infection model. Quinolones dosed once daily were more effective than equivalent dosages administered twice daily. The addition of rifampin was not synergistic but prevented the emergence of ciprofloxacin resistance.

Bactericidal activities of teicoplanin, vancomycin, and gentamicin alone and in combination against Staphylococcus aureus in an in vitro pharmacodynamic model of endocarditis

We adapted an in vitro pharmacodynamic model of infection to incorporate simulated endocardial vegetations. The bactericidal activities of teicoplanin, vancomycin, gentamicin, and various combinations of these drugs were studied against a strain of methicillin-susceptible Staphylococcus aureus obtained from a patient being treated for endocarditis at Detroit Receiving Hospital. Bacteria were grown overnight, concentrated, and added to a mixture of cryoprecipitate (80%) and thrombin (10%) to achieve approximately 5 x 10(9) CFU/g. Fibrin clots (8 to 10) were suspended into the model, removed at 24, 48, and 72 h in duplicate, weighed, and homogenized in 1.25% trypsin. Control experiments were conducted to characterize the growth kinetics. The following antibiotics were administered to simulate the pharmacokinetics of the drugs in humans: teicoplanin at 3 and 15 mg/kg of body weight, vancomycin at 15 mg/kg, and gentamicin at 1 mg/kg. Fibrin clot samples used to detect resistance were plated on antibiotic-containing tryptic soy agar plates. For the teicoplanin and vancomycin regimens, protein binding to cryoprecipitate, thrombin, and fibrin clot was determined to be 32, 43, and 50% and 26, 28, and 29%, respectively. In comparison with no treatment, vancomycin or teicoplanin at 15 mg/kg or either of these regimens combined with gentamicin significantly reduced bacterial counts (P < 0.0001). Monotherapy with teicoplanin at 3 mg/kg or gentamicin resulted in no killing activity. Combination treatment with teicoplanin at 3 mg/kg and gentamicin resulted in the killing of approximately 2 log10 CFU/g by 72 h and the development of resistance to gentamicin. The results obtained with the in vitro model of endocarditis are similar to the results reported by several investigators with the rabbit model of infective endocarditis. This unique infection model is useful for designing initial drug dosage regimens and may be predictive of drug efficacy against infective endocarditis.