Application of patient population-derived pharmacokinetic–pharmacodynamic relationships to tigecycline breakpoint determination for staphylococci and streptococci

Use of Monte Carlo simulation and considerations for PK-PD targets to support antibacterial dose selection

Monte Carlo simulation is used to generate data for pharmacokinetic-pharmacodynamic (PK-PD) target attainment analyses to assess antibacterial dosing regimens in early and late stage drug development. Careful consideration of the quality of data for pharmacokinetics, non-clinical PK-PD targets for efficacy, the choice of the bacterial reduction endpoint upon which the PK-PD target is based, variability in the PK-PD target, and effect site exposures ensures optimal dose selection. Relationships between drug exposure and efficacy and/or safety endpoints based on clinical data can also be applied to simulated data to support dose selection. These in silico analyses, conducted throughout drug development, provide the greatest opportunity to de-risk the development of antibacterial agents.

Comparison of Omadacycline and Tigecycline Pharmacokinetics in the Plasma, Epithelial Lining Fluid, and Alveolar Cells of Healthy Adult Subjects

The steady-state concentrations of omadacycline and tigecycline in the plasma, epithelial lining fluid (ELF), and alveolar cells (AC) of 58 healthy adult subjects were obtained. Subjects were administered either omadacycline at 100 mg intravenously (i.v.) every 12 h for two doses followed by 100 mg i.v. every 24 h for three doses or tigecycline at an initial dose of 100 mg i.v. followed by 50 mg i.v. every 12 h for six doses. A bronchoscopy and bronchoalveolar lavage were performed once in each subject following the start of the fifth dose of omadacycline at 0.5, 1, 2, 4, 8, 12, or 24 h and after the start of the seventh dose of tigecycline at 2, 4, 6, or 12 h. The value of the area under the concentration-time curve (AUC) from time zero to 24 h postdosing (AUC_0–24) (based on mean concentrations) in ELF and the ratio of the ELF to total plasma omadacycline concentration based on AUC_0–24 values were 17.23 mg · h/liter and 1.47, respectively. The AUC_0–24 value in AC was 302.46 mg · h/liter, and the ratio of the AC to total plasma omadacycline concentration was 25.8. In comparison, the values of the AUC from time zero to 12 h postdosing (AUC_0–12) based on the mean concentrations of tigecycline in ELF and AC were 3.16 and 38.50 mg · h/liter, respectively. The ratio of the ELF and AC to total plasma concentrations of tigecycline based on AUC_0–12 values were 1.71 and 20.8, respectively. The pharmacokinetic advantages of higher and sustained concentrations of omadacycline compared to those of tigecycline in plasma, ELF, and AC suggest that omadacycline is a promising antibacterial agent for the treatment of lower respiratory tract bacterial infections caused by susceptible pathogens.

Update on ventilator-associated pneumonia

PURPOSE OF REVIEW

To highlight the importance of escalating pathogen resistance in ventilator-associated pneumonia (VAP) along with diagnostic and treatment implications.

RECENT FINDINGS

In a period of rising bacterial resistance, VAP remains an important infection occurring in critically ill patients. Risk factors for multidrug-resistant pathogens depend on both local epidemiology and host factors. New diagnostic techniques and antimicrobials can help with rapid bacterial identification and timely and appropriate treatment while avoiding emergence of bacterial resistance.

SUMMARY

Clinicians should be aware of risk factors for multidrug-resistant pathogens causing VAP and also of particularities of diagnosis and treatment of this important clinical entity.

Comparative activities of vancomycin, tigecycline and rifampin in a rat model of methicillin-resistant Staphylococcus aureus osteomyelitis

OBJECTIVES

Implant-associated methicillin-resistant Staphylococcus aureus (MRSA) infections are challenging to treat. We compared antimicrobial activities in a rat model of chronic osteomyelitis in the context of retention of the foreign body without débridement.

METHODS

MRSA was inoculated into the proximal tibia and a wire implanted. Four weeks after infection, treatment with vancomycin 50 mg/kg every 12 h, tigecycline 14 mg/kg every 12 h, rifampin 25 mg/kg every 12 h, or the combination of vancomycin or tigecycline plus rifampin was administered intraperitoneally for 21 days.

RESULTS

MRSA was cultured from all tibias in the control group (median, 6.06 log10 CFU/g bone). Median bacterial counts (log10 CFU/g) at 48 h post-treatment were 6.16 for vancomycin (p = 0.753), 2.29 for vancomycin plus rifampin (p < 0.001), 5.90 for tigecycline (p = 0.270), 0.10 for tigecycline plus rifampin (p < 0.001), and 0.91 for rifampin (p = 0.044) treatment. Three deaths were observed in the tigecycline plus rifampin group. Median bacterial counts (log10 CFU/g) at two weeks post-treatment were 5.65 for vancomycin (p = 0.6), 4.05 for vancomycin plus rifampin (p = 0.105), 5.68 for tigecycline (p = 0.401), 4.05 for tigecycline plus rifampin (p = 0.028), and 5.98 for rifampin (p = 0.297) treatment.

CONCLUSIONS

Tigecycline plus rifampin resulted in a significant bacterial count decrease, an effect more prominent at 48 h than two weeks after treatment completion. Tigecycline was not well tolerated at the dose studied.

Monte Carlo simulation analysis of ceftobiprole, dalbavancin, daptomycin, tigecycline, linezolid and vancomycin pharmacodynamics against intensive care unit-isolated methicillin-resistant Staphylococcus aureus

The aim of the present study was to compare the potential of ceftobiprole, dalbavancin, daptomycin, tigecycline, linezolid and vancomycin to achieve their requisite pharmacokinetic/pharmacodynamic (PK/PD) targets against methicillin-resistant Staphylococcus aureus isolates collected from intensive care unit (ICU) settings. Monte Carlo simulations were carried out to simulate the PK/PD indices of the investigated antimicrobials. The probability of target attainment (PTA) was estimated at minimum inhibitory concentration values ranging from 0.03 to 32 μg/mL to define the PK/PD susceptibility breakpoints. The cumulative fraction of response (CFR) was computed using minimum inhibitory concentration data from the Canadian National Intensive Care Unit study. Analysis of the simulation results suggested the breakpoints of 4 μg/mL for ceftobiprole (500 mg/2 h t.i.d.), 0.25 μg/mL for dalbavancin (1000 mg), 0.12 μg/mL for daptomycin (4 mg/kg q.d. and 6 mg/kg q.d.) and tigecycline (50 mg b.i.d.), and 2 μg/mL for linezolid (600 mg b.i.d.) and vancomycin (1 g b.i.d. and 1.5 g b.i.d.). The estimated CFR were 100, 100, 70.6, 88.8, 96.5, 82.4, 89.4, and 98.3% for ceftobiprole, dalbavancin, daptomycin (4 mg/kg/day), daptomycin (6 mg/kg/day), linezolid, tigecycline, vancomycin (1 g b.i.d.) and vancomycin (1.5 g b.i.d.), respectively. In conclusion, ceftobiprole and dalbavancin have the highest probability of achieving their requisite PK/PD targets against methicillin-resistant Staphylococcus aureus isolated from ICU settings. The susceptibility predictions suggested a reduction of the vancomycin breakpoint to 1 μg/mL.

Pharmacokinetic-pharmacodynamic analysis for efficacy of ceftaroline fosamil in patients with acute bacterial skin and skin structure infections

Ceftaroline is a cephalosporin with broad-spectrum in vitro activity against pathogens commonly associated with acute bacterial skin and skin structure infections (ABSSSI), including methicillin-resistant Staphylococcus aureus. Ceftaroline fosamil, the prodrug of ceftaroline, is approved for the treatment of patients with ABSSSI. Using data from the microbiologically evaluable population from two phase 2 and two phase 3 randomized, multicenter, double-blind studies of patients with ABSSSI, an analysis examining the relationship between drug exposure, as measured by the percentage of time during the dosing interval that free-drug steady-state concentrations remain above the MIC (f%T>MIC), and clinical and microbiological responses was undertaken. The analysis population included 526 patients, of whom 423 had infections associated with S. aureus. Clinical and microbiological success percentages were 94.7 and 94.5%, respectively, among all of the patients and 95.3 and 95.7%, respectively, among those with S. aureus infections. Univariable analysis based on data from all of the patients and those with S. aureus infections demonstrated significant relationships between f%T>MIC and microbiological response (P < 0.001 and P = 0.026, respectively). Multivariable logistic regression analyses demonstrated other patient factors in addition to f%T>MIC to be significant predictors of microbiological response, including age and infection type for all of the patients evaluated and age, infection type, and the presence of diabetes mellitus for patients with S. aureus infections. Results of these analyses confirm that a ceftaroline fosamil dosing regimen of 600 mg every 12 h provides exposures associated with the upper plateau of the pharmacokinetic-pharmacodynamic relationship for efficacy.

Treatment of Gram-positive infections in critically ill patients

Gram-positive bacteria to include methicillin-resistant Staphylococcus aureus (MRSA), methicillin-susceptible Staphylococcus aureus (MSSA), and enterococci, to include vancomycin-resistant enterococci (VRE), display a remarkable array of resistance and virulence factors, which have contributed to their prominent role in infections of the critically ill. Over the last three decades infections with these pathogens has increased as has their overall resistance to available antimicrobial agents. This has led to the development of a number of new antibiotics for the treatment of Gram-positive bacteria. At present, it is important that clinicians recognize the changing resistance patterns and epidemiology of Gram-positive bacteria as these factors may impact patient outcomes. The increasing range of these pathogens, such as the emergence of community-associated MRSA clones, emphasizes that all specialties of physicians treating infections should have a good understanding of the infections caused by Gram-positive bacteria in their area of practice. When initiating empiric antibiotics, it is of vital importance that this therapy be timely and appropriate, as delays in treatment are associated with adverse outcomes. Although vancomycin has traditionally been considered a first-line therapy for serious MRSA infections, multiple concerns with this agent have opened the door for alternative agents demonstrating efficacy in this role. Similarly, the expansion of VRE as a pathogen in the ICU setting has required the development of agents targeting this important pathogen.

Treatment of infections due to resistant Staphylococcus aureus

This chapter reviews data on the treatment of infections caused by drug-resistant Staphylococcus aureus, particularly methicillin-resistant S. aureus (MRSA). This review covers findings reported in the English language medical literature up to January of 2013. Despite the emergence of resistant and multidrug-resistant S. aureus, we have seven effective drugs in clinical use for which little resistance has been observed: vancomycin, quinupristin-dalfopristin, linezolid, tigecycline, telavancin, ceftaroline, and daptomycin. However, vancomycin is less effective for infections with MRSA isolates that have a higher MIC within the susceptible range. Linezolid is probably the drug of choice for the treatment of complicated MRSA skin and soft tissue infections (SSTIs); whether it is drug of choice in pneumonia remains debatable. Daptomycin has shown to be non-inferior to either vancomycin or β-lactams in the treatment of staphylococcal SSTIs, bacteremia, and right-sided endocarditis. Tigecycline was also non-inferior to comparator drugs in the treatment of SSTIs, but there is controversy about whether it is less effective than other therapeutic options in the treatment of more serious infections. Telavancin has been shown to be non-inferior to vancomycin in the treatment of SSTIs and pneumonia, but has greater nephrotoxicity. Ceftaroline is a broad-spectrum cephalosporin with activity against MRSA; it is non-inferior to vancomycin in the treatment of SSTIs. Clindamycin, trimethoprim-sulfamethoxazole, doxycycline, rifampin, moxifloxacin, and minocycline are oral anti-staphylococcal agents that may have utility in the treatment of SSTIs and osteomyelitis, but the clinical data for their efficacy is limited. There are also several drugs with broad-spectrum activity against Gm-positive organisms that have reached the phase II and III stages of clinical testing that will hopefully be approved for clinical use in the upcoming years: oritavancin, dalbavancin, omadacycline, tedizolid, delafloxacin, and JNJ-Q2. Thus, there are currently many effective drugs to treat resistant S. aureus infections and many promising agents in the pipeline. Nevertheless, S. aureus remains a formidable adversary, and despite our deep bullpen of potential therapies, there are still frequent treatment failures and unfortunate clinical outcomes. The following discussion summarizes the clinical challenges presented by MRSA, the clinical experience with our current anti-MRSA antibiotics, and the gaps in our knowledge on how to use these agents to most effectively combat MRSA infections.

Pharmacokinetic-pharmacodynamic target attainment analyses to evaluate in vitro susceptibility test interpretive criteria for ceftaroline against Staphylococcus aureus and Streptococcus pneumoniae

To provide support for in vitro susceptibility test interpretive criteria decisions for ceftaroline against Staphylococcus aureus and Streptococcus pneumoniae, as well as dose adjustment recommendations for renal impairment, pharmacokinetic-pharmacodynamic (PK-PD) target attainment was evaluated for simulated patients administered intravenous (i.v.) ceftaroline fosamil at 600 mg twice daily (q12h) and simulated patients with renal impairment administered various dosing regimens. Using a previously developed population PK model, Monte Carlo simulation was used to generate ceftaroline plasma concentration profiles for simulated patients with normal renal function or mild, moderate, or severe renal impairment. Using these profiles, the percentage of time during the dosing interval that free-drug concentrations remained above the MIC (f%T>MIC) for ceftaroline at steady state was calculated. Percentages of simulated patients achieving f %T>MIC targets for S. aureus and S. pneumoniae based on murine infection models were calculated by MIC. At MICs of 2 mg/liter for S. aureus and 1 mg/liter for S. pneumoniae, the percentages of simulated patients with normal renal function and mild renal impairment following administration of ceftaroline fosamil at 600 mg q12h, moderate renal impairment following administration of ceftaroline fosamil at 400 mg q12h, and severe renal impairment following administration of ceftaroline fosamil at 300 mg q12h achieving f %T>MIC targets (≥26 for S. aureus and ≥44 for S. pneumoniae) exceeded 90%. The results of these analyses, which suggested that in vitro susceptibility test interpretive criteria defining susceptible could be as high as MICs of ≤2 and ≤1 mg/liter for ceftaroline against S. aureus and S. pneumoniae, respectively, provide support for current FDA and CLSI criteria, which define susceptible as MICs of 1 and 0.5 mg/liter, respectively. Recommendations for dose adjustments for patients with renal impairment were also supported by the results of these analyses.

Tigecycline displays in vivo bactericidal activity against extended-spectrum-β-lactamase-producing Enterobacteriaceae after 72-hour exposure period

Progressively enhanced activity of a humanized tigecycline (TGC) regimen was noted over 3 days against an extended-spectrum-β-lactamase (ESBL)-producing Escherichia coli isolate and an ESBL-producing Klebsiella pneumoniae isolate. Bacterial density reduction approximated 3 log(10) approaching bactericidal activity at 72 h. This level of activity has not been previously noted for compounds such as tetracyclines, normally considered bacteriostatic antimicrobials. Extended regimen studies in vivo may aid in better delineation of antimicrobial effects, producing improved correlation with clinical outcomes.

New susceptibility breakpoints for first-line antituberculosis drugs based on antimicrobial pharmacokinetic/pharmacodynamic science and population pharmacokinetic variability

Arguably, one of the most common and consequential laboratory tests performed in the world is Mycobacterium tuberculosis susceptibility testing. M. tuberculosis resistance is defined by growth of > or =1% of a bacillary inoculum on the critical concentration of an antibiotic. The critical concentration was chosen based on inhibition of > or =95% of wild-type isolates. The critical concentration of isoniazid is either 0.2 or 1.0 mg/liter, that of rifampin is 1.0 mg/liter, that of pyrazinamide is 100 mg/liter, that of ethambutol is 5.0 mg/liter, and that of fluoroquinolones is 1.0 mg/liter. However, the relevance of these concentrations to microbiologic and clinical outcomes is unclear. Critical concentrations were identified using the ability to achieve the antibiotic area under the concentration-time curve/MIC ratio associated with > or =90% of maximal kill (EC(90)) of M. tuberculosis in > or =90% of patients. Population pharmacokinetic parameters and their variability encountered in tuberculosis patients were utilized in Monte Carlo simulations to determine the probability that particular daily doses of the drugs would achieve or exceed the EC(90) in the epithelial lining fluid of 10,000 tuberculosis patients. Failure to achieve EC(90) in > or =90% of patients at a particular MIC was defined as drug resistance. The critical concentrations of moxifloxacin and ethambutol remained unchanged, but a critical concentration of 50 mg/liter was identified for pyrazinamide, 0.0312 mg/liter and 0.125 mg/liter were defined for low- and high-level isoniazid resistance, respectively, and 0.0625 mg/liter was defined for rifampin. Thus, current critical concentrations of first-line antituberculosis drugs are overoptimistic and should be set lower. With the proposed breakpoints, the rates of multidrug-resistant tuberculosis could become 4-fold higher than currently assumed.

Ethambutol optimal clinical dose and susceptibility breakpoint identification by use of a novel pharmacokinetic-pharmacodynamic model of disseminated intracellular Mycobacterium avium

Ethambutol, together with a macrolide, is the backbone for treatment of disseminated Mycobacterium avium disease. However, at the standard dose of 15 mg/kg of body weight/day, ethambutol efficacy is limited. In addition, susceptibility breakpoints have consistently failed to predict clinical outcome. We performed dose-effect studies with extracellular M. avium as well as with bacilli within human macrophages. The maximal kill rate (E(max)) for ethambutol against extracellular bacilli was 5.54 log(10) CFU/ml, compared to 0.67 log(10) CFU/ml for intracellular M. avium, after 7 days of exposure. Thus, extracellular assays demonstrated high efficacy. We created a hollow-fiber system model of intracellular M. avium and performed microbial pharmacokinetic-pharmacodynamic studies using pharmacokinetics similar to those of ethambutol for humans. The E(max) in the systems was 0.79 log(10) CFU/ml with 7 days of daily therapy, so the kill rates approximated those encountered in patients treated with ethambutol monotherapy. Ratio of peak concentration to MIC (C(max)/MIC) was linked to microbial kill rate. The C(max)/MIC ratio needed to achieve the 90% effective concentration (EC(90)) in serum was 1.23, with a calculated intramacrophage C(max)/MIC ratio of 13. In 10,000 patient Monte Carlo simulations, doses of 15, 50, and 75 mg/kg achieved the EC(90) in 35.50%, 76.81%, and 86.12% of patients, respectively. Therefore, ethambutol doses of >or=50 mg/kg twice a week would be predicted to be better than current doses of 15 mg/kg for treatment of disseminated M. avium disease. New susceptibility breakpoints and critical concentrations of 1 to 2 mg/liter were identified for the determination of ethambutol-resistant M. avium in Middlebrook broth. Given that the modal MIC of clinical isolates is around 2 mg/liter, most isolates should be considered ethambutol resistant.

Exposure-response analyses of tigecycline tolerability in healthy subjects

Tigecycline exposure (area under the concentration-time curve [AUC((0-infinity))] and maximum serum concentration [C(max)]) and first occurrence of nausea and vomiting were evaluated in 136 healthy subjects after 12.5- to 300-mg single doses. Nausea was more frequent in females (46%, 10/22) compared with males (31%, 11/36) after 100-mg doses. Most nausea (vomiting) events occurred < or =4 h (<6 h) after tigecycline. For doses < or =100 mg, the median duration of nausea and vomiting was approximately 5 h. Based on logistic regression, increased exposure (AUC((0-infinity)) >C(max)) to tigecycline results in an increased rate of nausea (P < or = .0001; = .0022) and vomiting (P < or = .0001; = .0006). At the median AUC((0-infinity)) (C(max)) for the 50-mg dose group, the probability of nausea and vomiting was 0.26 (0.29) and 0.07 (0.11), respectively. Model-predicted rates of nausea and vomiting were comparable with those observed for the tetracycline class of antibiotics, with tolerable rates predicted after 50-mg doses of tigecycline.