Pharmacokinetics and Pharmacodynamics of Linezolid in Obese Patients with Cellulitis

Evaluating linezolid dose regimens against methicillin-resistant Staphylococcus aureus based on renal function in populations with different body weight

WHAT IS KNOWN AND OBJECTIVE

Linezolid is an alternative first-line agent for MRSA pneumonia. This study assessed whether dose adjustments of linezolid against methicillin-resistant Staphylococcus aureus (MRSA) infections were needed based on renal function in populations with different body weight.

METHODS

Monte Carlo simulations were conducted to evaluate renal function in relation to the probability of target attainment (PTA) in three population groups with different body weight. Area under the concentration time curve (AUC)/ minimum inhibitory concentration (MIC) ratio and percentage of time above the MIC (%T > MIC) were regarded as pharmacokinetic/pharmacodynamic targets. The PTA and cumulative fractions of response (CFR) were calculated to assess the efficacy. Regarding safety, trough plasma concentration (C_min ) > 8 mg/L was used as target for toxicity.

RESULTS AND DISCUSSION

Using AUC/MIC >100 as the target pharmacodynamic (PD) index, the CFR of linezolid at the standard dose (600 mg every 12 h [q12h]) were 57.01%, 93.22%, and 99.93% in patients with normal renal function, patients with renal dysfunction and low body weight patients with renal dysfunction, respectively. Using 100%T > MIC as the target PD index, all the CFR of three population groups were more than 90% at the standard dose. The percentages of C_min  > 8 mg/L at the standard dose of linezolid were 24.16%, 53.24%, and 90.10% in three population groups on day 7.

WHAT IS NEW AND CONCLUSION

The risk of thrombocytopenia of linezolid was extremely higher in low body weight patients with renal impairment when receiving standard linezolid dose compared with patients with normal renal function. 450 mg q12h and 300 mg q12h might be effective and safe against MRSA infection in patients with renal dysfunction and low body weight patients with renal dysfunction, respectively.

Plasma and synovial fluid concentrations of linezolid in patients with knee osteoarthritis infected with Staphylococcus aureus

Background

Linezolid is a new oxazolidinone antibiotic used for infections caused by methicillin-resistant Staphylococcus and other species.

Case presentation

Two cases of knee osteoarthritis with acute infection were successfully treated using linezolid. The plasma and synovial fluid concentrations of linezolid in two patients [women aged 69 and 73 years (cases 1 and 2)] with knee osteoarthritis infected with Staphylococcus aureus were measured after they were administered 600 mg twice daily by intravenous infusion. The plasma linezolid concentrations during knee surgery in case 1 at day 5 and in case 2 at day 2 were 19.6 and 15.6 μg/mL, respectively. The synovial fluid concentrations of linezolid in samples taken during surgery in case 1 and case 2 were 14.9 and 17.0 μg/mL, respectively; these values corresponded to ratios of synovial fluid/plasma of 76 and 109%. Possible metabolite 2-hydroxylated linezolid potentially mediated by cytochrome P450 2 J2 was not detected in the plasma or synovial fluid samples under the current clinical setting after multiple doses.

Conclusions

These results implied nearly equivalent concentrations of linezolid in plasma and synovial fluid of clinical patients with knee osteoarthritis acutely infected with Staphylococcus aureus.

Antimicrobial Dosing in Specific Populations and Novel Clinical Methodologies: Obesity

Obesity and its related comorbidities can negatively influence the outcomes of certain infectious diseases. Specific dosing recommendations are often lacking in the product label for patients with obesity that leads to unclear guidance in practice. Higher rates of therapeutic failure have been reported with some fixed dose antibiotics and pragmatic approaches to dose modification are limited for orally administered agents. For i.v. antimicrobials dosed on weight, alternate body size descriptors (ABSDs) have been used to reduce the risk of overdosing. These ABSDs are mathematical transformations of height and weight that represent fat-free weight and follow the same principles as body surface area (BSA)-based dosing of cancer chemotherapy. However, ABSDs are rarely studied in pivotal phase III studies and so can risk the underdosing of antimicrobials in patients with obesity when incorrectly applied in the real-world setting. Specific case examples are presented to highlight these risks. Although general principles may be considered by clinicians, a universal approach to dose modification in obesity is unlikely. Studies that can better distinguish human body phenotypes may help reduce our reliance on height and weight to define dosing. Simple and complex technologies exist to quantify individual body composition that could improve upon our current approach. Early evidence suggests that body composition parameters repurposed from medical imaging data may improve upon height and weight as covariates of drug clearance and distribution. Clinical trials that can integrate human body phenotyping may help us identify new approaches to optimal dose selection of antimicrobials in patients with obesity.

A case report of streptococcal toxic shock syndrome caused by Streptococcus mitis in a healthy adult

BACKGROUND

Streptococcal toxic shock syndrome (STSS) is an acute, multisystem and toxin-mediated disease that usually causes shock and multiple organ failure in the early stages of its clinical course. It is associated with a substantial increase in mortality rate. The disease has been associated with invasive group A Streptococcus and is rarely caused by Streptococcus mitis (S. mitis). In healthy adults, S. mitis is closely related to endocarditis but rarely related to STSS.

CASE PRESENTATION

We report a case of STSS caused by S. mitis in a healthy 45-year-old woman. She presented with fever 14 h after surgery and with hypotension 24 h later, and she subsequently suffered from septic shock, low albumin, dysfunction of coagulation, acute kidney dysfunction, respiratory alkalosis and metabolic acidosis, acute respiratory distress syndrome and cellulitis of the incision. The diagnosis was obtained through clinical manifestation and blood culture examination. The patient was treated with aggressive fluid resuscitation, adequate antibiotics for a total of 4 weeks, respiratory support, and surgical debridement and drainage of the incision. She was discharged after her vital signs returned to normal and the incision healed on day 40 after surgery.

CONCLUSIONS

The diagnosis of STSS is often delayed or missed, which leads to a high mortality rate. It is possible to cure patients if the disease can be identified early and treated with aggressive fluid resuscitation, adequate antibiotics and control of the source of infection. Clinicians should consider the disease in the differential diagnosis of septic shock to prevent death.

Pharmacokinetic evaluation of linezolid administered intravenously in obese patients with pneumonia

OBJECTIVES

Altered linezolid pharmacokinetics (PK) in obese individuals has been hypothesized in previous studies. However, specific dosing recommendations for this population are still lacking. The main goal of this study was to evaluate PK/pharmacodynamic (PKPD) target attainment when using a 600 mg intravenous q12h linezolid dose against MRSA in obese patients with pneumonia.

METHODS

Fifteen obese pneumonia patients with a confirmed or suspected MRSA involvement treated with 600 mg of intravenous linezolid q12h were studied for 3 days. Population PK modelling was used to characterize the PK variability and to screen for influential patient characteristics. Monte Carlo simulations were carried out to investigate the PTA and time to target attainment for linezolid dosing against MRSA.

RESULTS

A two-compartment model with linear elimination adequately described the data. Body weight and age both have a significant effect on linezolid clearance. Simulations demonstrate that the probability of attaining PKPD targets is low. Moreover, the PTA decreases with weight, and increases with age. Standard linezolid dosing in obese pneumonia patients with MRSA (MICs of 1-4 mg/L) leads to unacceptably low (near zero to 60%) PTA for patients <65 years old.

CONCLUSIONS

Standard linezolid dosing is likely to provide insufficient target attainment against MRSA in obese patients. Body weight and especially age are important characteristics to be considered when administering linezolid to treat MRSA infections.

Determining therapeutic trough ranges for linezolid

Linezolid (LZD) is an oxazolidinone approved for the treatment of gram-positive infections. Therapeutic drug monitoring is increasingly used to optimize LZD dosing. The therapeutic target for LZD is to achieve an area under the concentration-time curve over 24 h divided by the MIC (AUC/MIC) > 100. In this study, we determined the trough ranges associated with this therapeutic AUC. Concentration-time profiles for 999 virtual patients were simulated using a previously published pharmacokinetic model for LZD. AUC was estimated for each virtual patient using the trapezoidal method. We determined the trough ranges that achieve the therapeutic target of AUC/MIC > 100 at different MIC values of 1, 2 and 4 μg/mL. Trough samples correlated well with LZD AUC (R² = 0.87). For trough concentration of 2–5 μg/mL, 99% had an AUC_0–24 > 100 µg⋅h⋅ml⁻¹, 23% had an AUC_0–24 > 200 µg⋅h⋅ml⁻¹ and none had an AUC_0–24 > 400 µg⋅h⋅ml⁻¹. For trough concentrations of 5–8 µg/ml, 87% of the patients had an AUC_0–24 > 200 µg⋅h⋅ml⁻¹ and none had an AUC_0–24 > 400 µg⋅h⋅ml⁻¹ To achieve the therapeutic target of an AUC/MIC > 100, it is suggested that trough ranges be set at 2–5 µg/ml if the MIC < 2 and 5–8 µg/ml if the MIC = 2; however, at an MIC of 4 µg/ml, it is difficult to achieve an AUC/MIC > 100 without increasing the risk of LZD toxicity.

Clinical Pharmacokinetics and Pharmacodynamics of Oxazolidinones

Oxazolidinones are a class of synthetic antimicrobial agents with potent activity against a wide range of multidrug-resistant Gram-positive pathogens including methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci. Oxazolidinones exhibit their antibacterial effects by inhibiting protein synthesis acting on the ribosomal 50S subunit of the bacteria and thus preventing formation of a functional 70S initiation complex. Currently, two oxazolidinones have been approved by the US Food and Drug Administration: linezolid and more recently tedizolid. Other oxazolidinones are currently under investigation in clinical trials. These antimicrobial agents exhibit a favourable pharmacokinetic profile with an excellent bioavailability and a good tissue and organ penetration. In-vitro susceptibility studies have shown that oxazolidinones are bacteriostatic against enterococci and staphylococci, and bactericidal for the majority of strains of streptococci. In the context of emergence of resistance to glycopeptides, oxazolidinones have become an effective alternative to vancomycin treatment frequently associated with nephrotoxicity. However, oxazolidinones, and linezolid in particular, are associated with significant adverse events, myelosuppression representing the main unfavourable side effect. More recently, tedizolid has been shown to effectively treat acute bacterial skin and skin structure infections. This newer oxazolidinone offers the advantages of once-daily dosing and a better safety profile in healthy volunteer studies (fewer gastrointestinal and haematological side effects). The potential use of tedizolid for other infections that could require longer therapy warrants further studies for positioning this new oxazolidinone in the available antimicrobial armamentarium. Moreover, other oxazolidinones are currently under active investigation.

Evaluation of treatment options for methicillin-resistant Staphylococcus aureus infections in the obese patient

Methicillin-resistant Staphylococcus aureus (MRSA) has emerged as a major cause of infection in both the hospital and community setting. Obesity is a risk factor for infection, and the prevalence of this disease has reached epidemic proportions worldwide. Treatment of infections in this special population is a challenge given the lack of data on the optimal antibiotic choice and dosing strategies, particularly for treatment of MRSA infections. Obesity is associated with various physiological changes that may lead to altered pharmacokinetic parameters. These changes include altered drug biodistribution, elimination, and absorption. This review provides clinicians with a summary of the literature pertaining to the pharmacokinetic and pharmacodynamic considerations when selecting antibiotic therapy for the treatment of MRSA infections in obese patients.

Influence of Morbid Obesity on the Clinical Pharmacokinetics of Various Anti-Infective Drugs: Reappraisal Using Recent Case Studies-Issues, Dosing Implications, and Considerations

Owing to availability of scanty pharmacokinetic data, dosing decisions in morbid obesity is increasingly challenging in the field of anti-infective drugs. However, in recent years data are emerging that describe the pharmacokinetics of anti-infective drugs in morbidly obese subjects. The objectives of the present work were: (1) to collate the recent reports pertaining to the pharmacokinetics in morbidly obese subjects for several anti-infective drugs and provide an overview of the pharmacokinetic data along with the applicable pharmacodynamics and/or clinical outcome; (2) to perform regression analysis on limited dataset for a few drugs to verify the existence of relationships between Cmax/Ctrough versus steady-state volume of distribution (Vss)/clearance to enable data prediction in morbid obesity subjects; (3) to provide a general discussion on issues and dosing implications. The key findings of this review were: (a) drugs such as vancomycin, ethambutol, and fluconazole, where the VSS is substantially greater in morbidly obese patients, need a dosing strategy with the appropriate body mass descriptors; (b) other drugs such as moxifloxacin, linezolid, doripenem, meropenem, voriconazole, oseltamivir, tigecycline, levofloxacin may not ordinarily need dosing adjustments;

Systematic Therapeutic Drug Monitoring for Linezolid: Variability and Clinical Impact

Linezolid serum trough (C_min) and peak (C_max) levels were determined prospectively in 90 patients. Adequate exposure was defined as a C_min of 2 to 8 mg/liter. Therapy was empirical (73.3%) or targeted (26.7%). Wide interindividual variability in linezolid C_min levels was recorded (0.1 to 25.2 μg/ml). Overall, 65.5% of the patients had out-of-range, 41.1% had subtherapeutic, and 24.4% had supratherapeutic trough levels. We did not find a correlation between abnormal levels and adverse events, in-hospital mortality, or overall poor outcome.

The Surgical Infection Society Revised Guidelines on the Management of Intra-Abdominal Infection

BACKGROUND

Previous evidence-based guidelines on the management of intra-abdominal infection (IAI) were published by the Surgical Infection Society (SIS) in 1992, 2002, and 2010. At the time the most recent guideline was released, the plan was to update the guideline every five years to ensure the timeliness and appropriateness of the recommendations.

METHODS

Based on the previous guidelines, the task force outlined a number of topics related to the treatment of patients with IAI and then developed key questions on these various topics. All questions were approached using general and specific literature searches, focusing on articles and other information published since 2008. These publications and additional materials published before 2008 were reviewed by the task force as a whole or by individual subgroups as to relevance to individual questions. Recommendations were developed by a process of iterative consensus, with all task force members voting to accept or reject each recommendation. Grading was based on the GRADE (Grades of Recommendation Assessment, Development, and Evaluation) system; the quality of the evidence was graded as high, moderate, or weak, and the strength of the recommendation was graded as strong or weak. Review of the document was performed by members of the SIS who were not on the task force. After responses were made to all critiques, the document was approved as an official guideline of the SIS by the Executive Council.

RESULTS

This guideline summarizes the current recommendations developed by the task force on the treatment of patients who have IAI. Evidence-based recommendations have been made regarding risk assessment in individual patients; source control; the timing, selection, and duration of antimicrobial therapy; and suggested approaches to patients who fail initial therapy. Additional recommendations related to the treatment of pediatric patients with IAI have been included.

SUMMARY

The current recommendations of the SIS regarding the treatment of patients with IAI are provided in this guideline.

Infectious Diseases: Pharmacologic Treatment Options for Nosocomial Pneumonia Involving Methicillin-Resistant Staphylococcus aureus

Objective:

To discuss current and potential treatment options for nosocomial pneumonia due to methicillin-resistant Staphylococcus aureus (MRSA).

Data Sources:

A MEDLINE search (1966–January 2007) was conducted to identify English-language literature on pharmacotherapy of nosocomial pneumonia and the bibliographies of pertinent articles. Programs and abstracts from infectious disease meetings were also searched. Search terms included MRSA, nosocomial pneumonia, pulmonary infections, vancomycin, quinupristin/dalfopristin, linezolid, daptomycin, tigecycline, dalbavancin, oritavancin, and ceftobiprole.

Data Selection and Data Extraction:

All articles were critically evaluated and all pertinent information was included in this review.

Data Synthesis:

Vancomycin has been the drug of choice for MRSA infections for many years. Recent data suggest that linezolid may be superior to vancomycin in the treatment of MRSA nosocomial pneumonia. However, there are limitations to the available data. Therefore, prospective, randomized studies are needed before linezolid is recommended as the preferred first-line therapy. Other approved agents for nosocomial MRSA infections, such as quinupristin/dalfopristin and daptomycin, should not be used in the treatment of MRSA pneumonia, as they were inferior in clinical trials. Tigecycline has excellent activity against MRSA in vitro, but should not be routinely used for the treatment of MRSA pneumonia, as clinical data are lacking. In a Phase III clinical trial, an anti-MRSA cephalosporin, ceftobiprole, is being evaluated for effectiveness against nosocomial pneumonia. Investigational glycopeptides may eventually have a role in the treatment of nosocomial pneumonia, but data are currently lacking.

Conclusions:

Vancomycin is still the drug of choice for treatment of MRSA pneumonia, and linezolid should be used as an alternative agent. Linezolid should carry strong consideration for patients with vancomycin-induced nephrotoxicity or a documented lack of response to vancomycin. Tigecycline and investigational agents with activity against MRSA may be future options for nosocomial pneumonia due to MRSA.

Interaction of obesity and infections

There is evidence that certain infections may induce obesity. Obese persons may also have more severe infections and have compromised response to therapies. The objective of this study is to review the available literature identifying infections that potentially contribute to greater body mass index (BMI) and differential responses of overweight and obese persons to infections. A systematic literature review of human studies examining associations between infections and weight gain, differential susceptibility, severity, and response to prevention and treatment of infection according to BMI status (January 1980-July 2014) was conducted. Three hundred and forty-three studies were eligible for inclusion. Evidence indicated that viral infection by human adenovirus Ad36 and antibiotic eradication of Helicobacter pylori were followed by weight gain. People who were overweight or obese had higher susceptibility to developing post-surgical infections, H1N1 influenza and periodontal disease. More severe infections tended to be present in people with a larger BMI. People with a higher BMI had a reduced response to vaccinations and antimicrobial drugs. Higher doses of antibiotics were more effective in obese patients. Infections may influence BMI, and BMI status may influence response to certain infections, as well as to preventive and treatment measures. These observations have potential clinical implications.

Antibiotic dosing in obesity: the search for optimum dosing strategies

Global obesity has nearly doubled and is now a common occurrence in high-income and developing countries. The World Health Organization estimates that more than 1.4 billion adults are obese. Although the prevalence of obesity is increasing over the last decades, pharmacokinetic evaluations are still conducted in individuals with a body weight of approximately 70 kg. Morbid obesity is associated with several pathophysiological changes that can profoundly affect drug distribution and clearance. There are currently no specific dosing recommendations for antibiotics in obese patients, making dosing suggestions primarily based on pharmacokinetic characteristics of the medications and dosing recommendations in other disease states. Understanding of the pharmacokinetic alterations and maximum doses of antibiotics safely used is paramount to appropriate treatment in the obese population.

Linezolid plasma and intrapulmonary concentrations in critically ill obese patients with ventilator-associated pneumonia: intermittent vs continuous administration

PURPOSE

Clinical application of an antibiotic's pharmacokinetic/pharmacodynamic (PK/PD) properties may improve the outcome of severe infections. No data are available on the use of linezolid (LNZ) continuous infusion in critically ill obese patients affected by ventilator-associated pneumonia (VAP).

METHODS

We conducted a prospective randomized controlled trial to compare LNZ concentrations in plasma and epithelial lining fluid (ELF), when administered by intermittent and continuous infusion (II, CI), in obese critically ill patients affected by VAP.

RESULTS

Twenty-two critically ill obese patients were enrolled. At the steady state, in the II group, mean ± SD total and unbound maximum-minimum concentrations (C max/C max,u - C min/Cmin,u) were 10 ± 3.7/6.8 ± 2.6 mg/L and 1.7 ± 1.1/1.2 ± 0.8 mg/L, respectively. In the CI group, the mean ± SD total and unbound plasma concentrations (C ss and C ss,u) were 6.2 ± 2.3 and 4.3 ± 1.6 mg/L, respectively. Within a minimum inhibitory concentration (MIC) range of 1-4 mg/L, the median (IQR) time LNZ plasma concentration persisted above MIC (% T > MIC) was significantly higher in the CI than the II group [100 (100-100) vs 100 (89-100), p = 0.05; 100 (100-100) vs 82 (54.8-98.8), p = 0.009; 100 (74.2-100) vs 33 (30.2-78.5), p = 0.005; respectively]. Pulmonary penetration (%) was higher in the CI group, as confirmed by a Monte Carlo simulation [98.8 (IQR 93.8-104.3) vs 87.1 (IQR 78.7-95.4); p < 0.001].

CONCLUSIONS

In critically ill obese patients affected by VAP, LNZ CI may overcome the limits of standard administration but these advantages are less evident with difficult to treat pathogens (MIC = 4 mg/L). These data support the usefulness of LNZ continuous infusion, combined with therapeutic drug monitoring (TDM), in selected critically ill populations.

Role of linezolid in the treatment of complicated skin and soft tissue infections

Staphylococcus aureus is the most common cause of complicated skin and soft tissue infections (cSSTIs). Antibiotic choices for these infections continue to evolve. History has seen penicillin progress to antistaphylococcal penicillins and cephalosporins, but these drugs are now giving way to drugs that are effective against methicillin-resistant S. aureus (MRSA). While vancomycin has been the gold standard to treat MRSA infections, newer therapeutic options have been developed over the last 5 years. These include quinupristin-dalfopristin, daptomycin, tigecycline and linezolid, which is the focus for this review. Linezolid is efficacious in the treatment of cSSTIs (including diabetic foot infections) caused by Gram-positive organisms (including MRSA), with a well-defined safety profile and straightforward dosing. It is also approved for nosocomial pneumonia, community-acquired pneumonia and uncomplicated skin and skin structure infections. Linezolid has an oral and parenteral formulation, which are equivalent. The oral formulation has the potential to offer economic benefits as compared with other therapies. Currently, there are only a few new antibiotics in development with MRSA activity. The proper use of all antibiotics, including these newer agents, is increasingly important if we are to slow the evolution of microbial resistance.

Obesity and infection: two sides of one coin

The prevalence of obesity has exponentially risen worldwide. The etiology of obesity is multifactorial, and genetic inheritance and behavioral/environmental causes are considered the main etiological factors. Moreover, evidence that specific infections might promote the development of obesity has steadily accumulated. Only a few works investigate the impact of obesity on the immune response to infections and the risk of infections in the obese population. The aim of this paper was to review the available data regarding the various aspects of the association between obesity and infections and to highlight the possibility that infectious agents may have an etiological role in obesity, an idea known as "infectobesity". Several microbes have been considered as possible promoter of obesity, but most of the data concern adenovirus-36 that exerts an adipogenic action mainly via a direct effect on adipose tissue leading to weight gain, at least in animal models.Obesity affects the immune response leading to an increased susceptibility to infections. Obese adults and children show an increased incidence of both nosocomial and community-acquired infections. Furthermore, obesity may alter the pharmacokinetics of antimicrobial drugs and impact on vaccine response. However, the various aspects of the association of obesity infections remain poorly studied, and a call to research is necessary to better investigate the problem.In conclusion, obesity impacts millions globally, and greater understanding of its etiology and its effects on immunity, infections, and prevention and management strategies is a key public health concern.

Subtherapeutic linezolid concentrations in a patient with morbid obesity and methicillin-resistant Staphylococcus aureus pneumonia: case report and review of the literature

OBJECTIVE

To report a case of subtherapeutic linezolid concentrations in a patient with morbid obesity.

CASE SUMMARY

A 34-year-old male with morbid obesity (265 kg, body mass index 82 kg/m(2)) was admitted for severe sepsis due to respiratory failure requiring emergent intubation and treatment of community-acquired pneumonia. Admission tracheal aspirate culture revealed methicillin-resistant Staphylococcus aureus (MRSA) for which vancomycin was prescribed. Therapy subsequently was changed to linezolid, because the patient's clinical status worsened, with significant hypoxia (partial pressure of arterial oxygen/fraction of inspired oxygen [PaO2/FiO2] ratio 145), increasing leukocytosis (white blood cell count from 10,800/μL on admission to 15,400/μL on hospital day 6), and persistent fever (38.3 °C). After 48 hours of linezolid monotherapy, the patient remained febrile with continued leukocytosis, worsening hypoxemia, and a persistently positive MRSA culture from a repeat endotracheal aspirate. Linezolid serum concentrations were obtained and vancomycin was reinstituted, after which the patient began to improve (afebrile, improving PaO2/FiO2 ratio, decreasing leukocytosis). On hospital day 12, the patient removed his endotracheal tube, and a sputum sample was obtained for culture. The patient's clinical status subsequently declined, prompting addition of cefepime to his antibiotic regimen. This sputum culture revealed not only MRSA, but also quinolone-resistant Escherichia coli. After completing treatment for both organisms the patient was discharged home.

DISCUSSION

Limited data on linezolid dosing in the morbidly obese population show lower serum drug concentrations than those in nonobese patients, but no clinical failure has been reported when treating MRSA skin and soft tissue infections or MRSA tracheitis. In our patient, low steady-state linezolid serum concentrations (peak 4.13 μg/mL [reference 15-27] and trough 1.27 μg/mL [reference 2-9]) were thought to contribute to his poor clinical response.

CONCLUSIONS

To our knowledge, this is the first report of subtherapeutic linezolid concentrations correlated with decreased clinical effectiveness when during treatment of MRSA pneumonia in a patient with morbid obesity.

Pharmacokinetics of intravenous linezolid in moderately to morbidly obese adults

The pharmacokinetics of linezolid was assessed in 20 adult volunteers with body mass indices (BMI) of 30 to 54.9 kg/m(2) receiving 5 intravenous doses of 600 mg every 12 h. Pharmacokinetic analyses were conducted using compartmental and noncompartmental methods. The mean (±standard deviation) age, height, and weight were 42.2 ± 12.2 years, 64.8 ± 3.5 in, and 109.5 ± 18.2 kg (range, 78.2 to 143.1 kg), respectively. Linezolid pharmacokinetics in this population were best described by a 2-compartment model with nonlinear clearance (original value, 7.6 ± 1.9 liters/h), which could be inhibited to 85.5% ± 12.2% of its original value depending on the concentration in an empirical inhibition compartment, the volume of the central compartment (24.4 ± 9.6 liters), and the intercompartment transfer constants (K(12) and K(21)) of 8.04 ± 6.22 and 7.99 ± 5.46 h(-1), respectively. The areas under the curve for the 12-h dosing interval (AUCτ) were similar between moderately obese and morbidly obese groups: 130.3 ± 60.1 versus 109.2 ± 25.5 μg · h/ml (P = 0.32), and there was no significant relationship between the AUC or clearance and any body size descriptors. A significant positive relationship was observed for the total volume of distribution with total body weight (r(2) = 0.524), adjusted body weight (r(2) = 0.587), lean body weight (r(2) = 0.495), and ideal body weight (r(2) = 0.398), but not with BMI (r(2) = 0.171). Linezolid exposure in these obese participants was similar overall to that of nonobese patients, implying that dosage adjustments based on BMI alone are not required, and standard doses for patients with body weights up to approximately 150 kg should provide AUCτ values similar to those seen in nonobese participants.

Oral bioavailability of linezolid before and after Roux-en-Y gastric bypass surgery: is dose modification necessary in obese subjects?

OBJECTIVES

We characterized the pharmacokinetics of intravenous (iv) and oral linezolid before and after Roux-en-Y gastric bypass surgery (RYGBS).

METHODS

Subjects with a body mass index (BMI)  >35 kg/m(2) received a single iv 600 mg dose of linezolid followed by the same oral dose after a 7 day washout period between doses, before and 3 months after RYGBS. Serum linezolid concentrations were measured by a validated HPLC method with ultraviolet detection. Parametric population pharmacokinetic analysis was used to evaluate bioavailability and the influence of total body weight (TBW) on pharmacokinetic parameters. The area under the serum concentration-time curve extrapolated to infinity (AUC(0-∞)) was compared between subjects before and after RYGBS, and with non-obese controls.

RESULTS

Five (four male) obese subjects were studied with a mean (SD) age of 51.4 (5.01) years, TBW of 124 (10.6) kg and initial BMI of 44.9 (7.52) kg/m(2). The bioavailability was a mean (95% CI) of 1.14 (0.816-1.47) before and 1.14 (1.01-1.26) after RYGBS. The mean (SD) AUC(0-∞) with oral linezolid before RYGBS was 41.6 (20.9) mg h/L compared with 98.9 (24.7) mg h/L after RYGBS (P<0.001). This increase in AUC(0-∞) corresponded with a 25.3% reduction in the TBW after RYGBS, as the TBW was a significant covariate of clearance. The probability of pharmacodynamic target attainment with standard doses of linezolid is lower in obese versus non-obese individuals.

CONCLUSIONS

The bioavailability of linezolid is not impaired by RYGBS. The serum exposure of linezolid is more than 50% lower in obese compared with non-obese subjects, suggesting that dose modification may be needed.

The impact of obesity on the immune response to infection

There is strong evidence indicating that excess adiposity negatively impacts immune function and host defence in obese individuals. This is a review of research findings concerning the impact of obesity on the immune response to infection, including a discussion of possible mechanisms. Obesity is characterised by a state of low-grade, chronic inflammation in addition to disturbed levels of circulating nutrients and metabolic hormones. The impact of these metabolic abnormalities on obesity-related comorbidities has undergone intense scrutiny over the past decade. However, relatively little is known of how the immune system and host defence are influenced by the pro-inflammatory and excess energy milieu of the obese. Epidemiological data suggest obese human subjects are at greater risk for nosocomial infections, especially following surgery. Additionally, the significance of altered immunity in obese human subjects is emphasised by recent studies reporting obesity to be an independent risk factor for increased morbidity and mortality following infection with the 2009 pandemic influenza A (H1N1) virus. Rodent models offer important insight into how metabolic abnormalities associated with excess body weight can impair immunity. However, more research is necessary to understand the specific aspects of immunity that are impaired and what factors are contributing to reduced immunocompetence in the obese. Additionally, special consideration of how infection in this at-risk population is managed is required, given that this population may not respond optimally to antimicrobial drugs and vaccination. Obesity impacts millions globally, and greater understanding of its associated physiological disturbances is a key public health concern.

Immediate hematological toxicity of linezolid in healthy volunteers with different body weight: a phase I clinical trial

Linezolid is an important therapeutic option for infections from multi-drug resistant Gram-positive pathogens. However, prolonged linezolid treatment (>14 days) is considered to increase the risk of hematological adverse events. We aimed to evaluate the hematological safety profile of an i.v. single dose of linezolid in healthy volunteers of different body weight. We conducted a phase I clinical trial involving 20 healthy male Chinese volunteers that received an i.v. single dose of linezolid (600 mg). The study participants were assigned to two groups: low-weight (LW) group: 50 kg <body weight ≤55 kg and high-weight (HW) group: ≥80 kg. A significant decrease in the hemoglobin (Hb) levels and red blood cell (RBC) count was observed at the end of administration of the study drug in both groups. White blood cell (WBC) count was simultaneously decreased in the HW group. In the LW group, Hb levels and RBC count were also significantly decreased at 5, 7 and 24 h. In the HW group, both values were significantly decreased at 5 h. At 48 h all values were normal. The observed decreases were numerically higher in the LW group compared with the HW group. Yet, no statistical significance was noted. No difference was observed in the platelet count in both the groups. Our findings suggest that linezolid-associated hematological toxicity may also occur shortly after the i.v. administration of the drug in both LW and HW healthy volunteers. Early initiated continuous monitoring of hematological values and linezolid dosage adjustment for body weight are recommended.

Linezolid pharmacokinetics and pharmacodynamics in clinical treatment

Linezolid has been widely used in the treatment of Gram-positive infections for more than a decade. It is unique amongst antibiotics active against most multiply-resistant Gram-positive bacteria in that there is an oral preparation with 100% bioavailability and an extensive volume of distribution. This review examines pharmacokinetic data relating to linezolid use in different patient groups (obesity, enteral feeding, renal failure, neonates, and paediatrics) and in different clinical conditions (sepsis syndrome, skin and soft tissue infection, diabetic foot infection, pneumonia, bone and joint infection, infection of the central nervous system, eye infection, and neutropenic sepsis).

Pharmacotherapy in the critically ill obese patient

Despite the growing epidemic of obesity in the United States, dosing medications in such patients remains poorly studied and understood. Most recommendations are based on small independent studies, case reports, and expert opinion. Applying manufacturer kinetics and dosing recommendations in the obese patient may result in toxicity or treatment failure, leading to increased morbidity, mortality, and hospital length of stay.

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

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

Pharmacological issues of linezolid: an updated critical review

Linezolid is the first oxazolidinone agent introduced into clinical practice for use against Gram-positive bacteria that are resistant to beta-lactams and glycopeptides, including methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). An optimal antibacterial effect is achieved when plasma drug concentrations are above the minimum inhibitory concentration (MIC) [T>MIC] for the entire length of treatment and the ratio between the area under the plasma concentration-time curve (AUC) and the MIC (AUC/MIC) is greater than 100, as is most commonly obtained with administration of the standard dosage of linezolid 600 mg twice daily. A wide tissue distribution, including the CNS and respiratory tract, nearly linear pharmacokinetics and good tolerability are additional characteristics of linezolid. However, variability in the drug pharmacokinetics associated with clinical conditions (e.g. sepsis, burn injuries, end-stage renal disease, cystic fibrosis), haemodialysis and/or young age may lower the T>MIC and the AUC/MIC ratio, thus impairing both antibacterial activity and prevention of mutants. In most cases, changes in the dosage or in the schedule of administration (e.g. an additional [third] daily dose) may improve the effectiveness of linezolid. It is worth noting that linezolid could affect its own metabolism as a result of protein synthesis inhibition in mitochondria, and this could lead to high plasma concentrations and an increased risk of non-negligible toxicities. The latter may be reported during long-term administration of linezolid or in the presence of some pathological conditions (e.g. renal disease or kidney transplantation) associated with high plasma drug concentrations. Therefore, treatment optimization should be considered a requirement for more effective and tolerable use of the drug, particularly in special populations.

Effect of obesity on the pharmacokinetics of drugs in humans

The prevalence of obesity has dramatically increased in recent years and now includes a significant proportion of the world's children, adolescents and adults. Obesity is linked to a number of co-morbidities, the most prominent being type 2 diabetes mellitus. While many agents are available to treat these conditions, the current knowledge regarding their disposition in the obese remains limited. Over the years, both direct and indirect methodologies have been utilized to assess body composition. Commonly used direct measures include underwater weighing, skinfold measurement, bioelectrical impedance analysis and dual-energy x-ray absorptiometry. Unfortunately, these methods are not readily available to the majority of clinicians. As a result, a number of indirect measures to assess body composition have been developed. Indirect measures rely on patient attributes such as height, bodyweight and sex. These size metrics are often utilized clinically and include body mass index (BMI), body surface area (BSA), ideal bodyweight (IBW), percent IBW, adjusted bodyweight, lean bodyweight (LBW) and predicted normal weight (PNWT). An understanding of how the volume of distribution (V(d)) of a drug changes in the obese is critical, as this parameter determines loading-dose selection. The V(d) of a drug is dependent upon its physiochemical properties, the degree of plasma protein binding and tissue blood flow. Obesity does not appear to have an impact on drug binding to albumin; however, data regarding alpha(1)-acid glycoprotein binding have been contradictory. A reduction in tissue blood flow and alterations in cardiac structure and function have been noted in obese individuals. At the present time, a universal size descriptor to describe the V(d) of all drugs in obese and lean individuals does not exist. Drug clearance (CL) is the primary determinant to consider when designing a maintenance dose regimen. CL is largely controlled by hepatic and renal physiology. In the obese, increases in cytochrome P450 2E1 activity and phase II conjugation activity have been observed. The effects of obesity on renal tubular secretion, tubular reabsorption, and glomerular filtration have not been fully elucidated. As with the V(d), a single, well validated size metric to characterize drug CL in the obese does not currently exist. Therefore, clinicians should apply a weight-normalized maintenance dose, using a size descriptor that corrects for differences in absolute CL between obese and non-obese individuals. The elimination half-life (t((1/2))) of a drug depends on both the V(d) and CL. Since the V(d) and CL are biologically independent entities, changes in the t((1/2)) of a drug in obese individuals can reflect changes in the V(d), the CL, or both. This review also examines recent publications that investigated the disposition of several classes of drugs in the obese--antibacterials, anticoagulants, antidiabetics, anticancer agents and neuromuscular blockers. In conclusion, pharmacokinetic data in obese patients do not exist for the majority of drugs. In situations where such information is available, clinicians should design treatment regimens that account for any significant differences in the CL and V(d) in the obese.

The importance of tissue penetration in achieving successful antimicrobial treatment of nosocomial pneumonia and complicated skin and soft-tissue infections caused by methicillin-resistant Staphylococcus aureus: vancomycin and linezolid

BACKGROUND

The rising prevalence of nosocomial methicillin-resistant Staphylococcus aureus (MRSA) and the recent emergence of community-associated MRSA are major clinical, public health, and economic challenges. MRSA is a leading cause of nosocomial pneumonia and complicated skin and soft-tissue infections (cSSTI). Vancomycin and linezolid are two commonly used antimicrobial agents with activity against Gram-positive pathogens, particularly MRSA, that are used to treat both nosocomial pneumonia and cSSTI. Recently, the therapeutic efficacy of vancomycin in the treatment of hospitalized patients with MRSA infections has been questioned due to the emergence of MRSA strains with reduced susceptibility to vancomycin together with concerns related to inadequate dosing and poor tissue penetration of the drug.

SCOPE

A literature review was conducted to investigate the pharmacokinetics and pulmonary and tissue penetration of vancomycin and linezolid. Using MEDLINE and EMBASE, the most relevant articles in English published over the past 25 years (up to October 2008) were identified and summarized. Studies in human volunteers and adult patients that measured concentrations of antibiotic in serum, epithelial lining fluid (ELF), and tissue were selected for further review.

FINDINGS

For both drugs, pharmacokinetic studies were conducted in diverse patient populations and employed varying techniques to measure tissue concentrations. Vancomycin concentrations in ELF ranged from 5 to 25% of simultaneous plasma levels, while concentrations in whole homogenized lung tissue were slightly higher (24-41%). Distribution of vancomycin into soft tissue was variable. For linezolid, overall mean concentrations in ELF and in soft tissue were generally similar or higher than simultaneous plasma levels, although variability in tissue penetration across studies in healthy volunteers and patients was seen.

LIMITATIONS

The studies included in this review vary greatly in their designs and patient populations; this, together with methodologic difficulties, limits the interpretation of the data.

CONCLUSIONS

In the absence of clinical data correlating ELF concentrations and clinical outcome, the clinical significance of differences in pulmonary penetration of vancomycin and linezolid is unknown. Higher vancomycin serum concentrations may be necessary to achieve appropriate lung concentrations to optimize treatment outcomes. Linezolid demonstrates adequate penetration into lung and other soft issues with sustained concentrations above the minimum inhibitory concentrations for susceptible pathogens, including MRSA, for the majority of the dosing interval. Examination of the pharmacokinetic data adds insights not provided by the clinical trial data and together provides clinicians with a more comprehensive basis for selecting appropriate antimicrobial therapy for the treatment of serious MRSA infections.

Linezolid for the treatment of drug-resistant infections

Multidrug-resistant pathogens have become increasingly common in contemporary healthcare. Specific to Gram-positive pathogens, methicillin-resistant Staphylococcus aureus (MRSA) is of particular concern, as it has been associated with increased hospital length of stay, higher healthcare expenditures and poorer outcomes. To date, linezolid is the first and only oxazolidinone approved by the US FDA for the treatment of infections caused by Gram-positive pathogens, including MRSA. This article will serve as a comprehensive review of linezolid, including an overview of the current market and its in vitro activity, with an in-depth review of its pharmacokinetic and pharmacodynamic profile. Emphasis will be placed on clinical data for the drug, both on- and off-label. The article will conclude with a brief overview of linezolid's pharmacoeconomic implications and safety profile, followed by a commentary and 5-year prospective analysis remarking on the future of the antimicrobial field as it relates to MRSA.

Linezolid versus glycopeptide or beta-lactam for treatment of Gram-positive bacterial infections: meta-analysis of randomised controlled trials

Linezolid has been approved for the treatment of patients with infections caused by Gram-positive cocci that are resistant to traditionally used antibiotics, including glycopeptides. This oxazolidinone antibiotic has been reported to have excellent pharmacokinetics and effectiveness. We did a meta-analysis of randomised controlled trials (RCTs) to clarify whether linezolid is superior to glycopeptides or beta-lactams for the treatment of Gram-positive infections. 12 RCTs, involving 6093 patients, were included. Overall, with respect to treatment success, linezolid was more effective than glycopeptides or beta-lactams (odds ratio [OR] 1.41 [95% CI 1.11-1.81]). Mortality was similar between the groups (OR 0.97 [0.79-1.19]). Linezolid was more effective than comparators in patients with skin and soft-tissue infections (OR 1.67 [1.31-2.12]) and bacteraemia (OR 2.07 [1.13-3.78]). However, there was no difference in treatment success for patients with pneumonia (OR 1.03 [0.75-1.42]). Treatment with linezolid was not associated with more adverse effects in general (OR 1.40 [0.95-2.06]); however, thrombocytopenia was recorded more commonly in patients receiving linezolid (OR 11.72 [3.66-37.57]). Although linezolid is more effective than its comparators for the empirical treatment of selected patients, several points, such as the use of less potent antistaphylococcal beta-lactams, the same all-cause mortality, and the higher probability of thrombocytopenia, should be taken into account and may limit the use of linezolid to specific patient populations or infections that are difficult to treat with other antibiotics.

Staphylococcus epidermidis is cleared from biomaterial implants but persists in peri-implant tissue in mice despite rifampicin/vancomycin treatment

Infections associated with implanted biomedical devices (BAI) are predominantly caused by Staphylococcus epidermidis. We previously observed in murine experimental BAI that S. epidermidis persists in peri-implant tissue rather than on the implanted biomaterial itself (Boelens et al., J Infect Dis 2000;181:1337-1349; Broekhuizen et al., Infect Immun 2007;75:1129-1136). To investigate the efficacy of rifampicin/vancomycin to clear S. epidermidis from implants and peri-implant tissues, mice with two implants were challenged with 10(7) cfu S. epidermidis per implant and received daily injections of rifampicin (25 mg/kg) and vancomycin (50 mg/kg). On the day of termination, implants and peri-implant tissue were collected and processed for culture and histology. After 1 and 8 days, implants of control mice were culture positive in 14/18 and 5/16 cases, respectively, and tissue biopsies were all culture positive. In the antibiotic-treated mice, bacteria were recovered from only 1/18 and 1/16 implants after 1 and 8 days, respectively, whereas the tissues were culture positive in 14/18 and 7/16 biopsies, respectively. In microscopy, bacteria were seen in the tissue at a distance of several cell layers from the tissue-implant interface, colocalized with host cells. Thus, although a regimen of rifampicin/vancomycin sterilized the implants, S. epidermidis persisted in peri-implant tissue, which might be an as yet unrecognized reservoir in the pathogenesis of BAI.

Antimicrobial dosing considerations in obese adult patients

As obesity continues to increase in prevalence throughout the world, it becomes important to explore the effects that obesity has on antimicrobial disposition. Physiologic changes in obesity can alter both the volume of distribution and clearance of many commonly used antimicrobials. These changes often present challenges such as estimation of creatinine clearance to predict drug clearance. Although these physiologic changes are increasingly being characterized, few studies assessing alterations in tissue drug distribution and the effects of obesity on antimicrobial pharmacokinetics have been published. The available data are most plentiful for antibiotics that historically have included clinical therapeutic drug monitoring. These data suggest that dosing of vancomycin and aminoglycosides be based on total body weight and adjusted body weight, respectively. Obese patients may require larger doses of beta-lactams to achieve similar concentrations as those of patients who are not obese. Fluoroquinolone pharmacokinetics are variably altered by obesity, which prevents a uniform approach. Data on the pharmacokinetics of drugs that have activity against gram-positive organisms-quinupristin-dalfopristin, linezolid, and daptomycin-reveal that they are altered in the presence of obesity, but more data are needed to solidify dosing recommendations. Limited data are available on nonantibacterials. An understanding of the physiologic changes in obesity and the available literature on specific antibiotics is valuable in providing a framework for rational selection of dosages in this increasingly common population of obese patients.