Therapeutic drug monitoring of vancomycin in a morbidly obese patient

Weight-based cefuroxime dosing provides comparable orthopedic target tissue concentrations between weight groups - a microdialysis porcine study

Antibiotic prophylaxis is a key element in prevention of surgical site infections. For the majority of orthopedic procedures, antibiotic administration follows fixed dosing regimens irrespective of weight. However, this may result in insufficient antibiotic target tissue concentrations and higher risk of surgical site infections in obese individuals. The aim of this study was to investigate the effect of weight-based cefuroxime dosing on plasma and target tissue concentrations. Eighteen female pigs were allocated into three groups differentiated by weight: 53-57 kg, 73-77 kg, and 93-97 kg. Microdialysis catheters were placed for continuous sampling in bone, muscle, and subcutaneous tissue during an 8h sampling interval. Blood samples were collected as reference. Cefuroxime was administered intravenously as a bolus according to weight (20 mg/kg). The primary endpoint was the time above the cefuroxime minimal inhibitory concentration for Staphylococcus aureus (T > MIC (4 μg/mL)). Comparable target tissue T > MICs (4 μg/mL) were found between weight groups. Mean T > MIC ranged between 116-137 min for plasma, 118-154 min for bone, 109-146 min for the skeletal muscle, and 117-165 min for subcutaneous tissue across the groups. Weight-based cefuroxime (20 mg/kg) dosing approach provides comparable perioperative plasma and target tissue T > MIC (4 μg/mL) in animals between 50-100 kg body weight, and thus a comparable prophylaxis of surgical site infections.

Clinical Application and Evaluation of Vancomycin Dosing in Adults

Objective:

To evaluate the accuracy of a vancomycin dosing method (study method) in estimating volume of distribution (Vd) and clearance of vancomycin (Clvanco) and the frequency with which it would provide serum vancomycin concentrations (SVCs) within a specified range compared with other published methods.

Methods:

One hundred and seven patients with 108 pairs of SVCs were used to calculate patient-specific Vd and Clvanco. Based on these patient-specific Vd and Clvanco values, the predictive ability of the estimated parameters from the study dosing method (Vd [L] = 0.17 [age] + 0.22 [actual body weight] + 15; Clvanco = creatinine clearance) was evaluated against three previously published pharmacokinetic dosing methods, using predictive performance analysis (precision and bias). Furthermore, the patient-specific pharmacokinetic parameters were used to simulate steady-state peak and trough SVCs, using first-order pharmacokinetic equations from doses derived from the study method, the three different pharmacokinetic methods, and two other published dosing schemes. The frequency with which each method would have achieved target peak and trough SVCs was determined.

Results:

The study method was found to be more precise and less biased (p < 0.05) than comparative methods in predicting vancomycin Vd and Clvanco. The study method also resulted in a higher frequency of steady-state peak and trough SVCs within the target range specified.

Conclusions:

The study method presented here provided a reliable estimation of vancomycin pharmacokinetic parameters that was easily applied to various patient populations to individualize vancomycin dosing, and frequently yielded SVCs within a predictable and desired range.

An Evaluation of Systemic Vancomycin Dosing in Obese Patients

We retrospectively identified 67 patients with severe or morbid obesity (body mass index ≥35 kg/m²) who had received intravenous vancomycin at our institution. We observed that an initial dose of 45 to 65 mg/kg vancomycin per day based upon ideal body weight rather than actual body weight was more predictive of initial trough concentrations between 15 and 20 mcg/mL.

A Review of the Toxicologic Implications of Obesity

The increasing prevalence of obesity in developed nations has far-reaching implications for medical toxicology. The management of obese patients is complicated by comorbid illnesses, changes in cardiovascular and respiratory physiology, alterations in pharmacokinetics, and a lack of studies to identify appropriate dosing for current therapeutics and antidotes. In this review article, we examine obesity-associated physiologic and pharmacokinetic changes that may increase the vulnerability of obese patients to overdose. Further research is needed to characterize the relationship between drug toxicity and obesity.

Vancomycin-Associated Nephrotoxicity: The Obesity Factor

BACKGROUND

Current recommendations suggest that vancomycin dosing utilize actual rather than ideal body weight in obese patients. Thus, obese patients may be at greater risk for nephrotoxicity. The purpose of this study was to compare the incidence of nephrotoxicity in vancomycin-treated obese and lean patients at our institution, where unadjusted, actual body weight-based dosing (capped at 2 g per dose twice daily) is used. We expected obese patients to experience a greater incidence of nephrotoxicity than lean patients.

METHODS

This study examined a retrospective cohort of patients treated with vancomycin for gram-positive or mixed infections in our facility from 2005-2009 who were not receiving hemodialysis at the time of admission. Patients were stratified by body mass index (BMI; obese ≥30 kg/m(2) vs. lean <30 kg/m(2)). Relative risk (RR), 95% confidence intervals (CIs), and p values were computed using a generalized estimating equation to accommodate a correlated data structure corresponding to multiple episodes of infection per individual. Multivariable analysis was performed.

RESULTS

A total of 530 patients (207 obese; 323 lean) with 1,007 episodes of infection were treated with vancomycin. Patient demographics, co-morbidities, sites of infection, and infecting organisms were similar in the two groups. Female gender (p=0.042), diabetes mellitus (DM) (p=0.018), and hypertension (HTN) (p=0.0009) were more often associated with obesity, whereas allografts (p=0.022) and peripheral vascular disease (p=0.036) were more often present in lean patients. The Acute Physiology and Chronic Health Evaluation II score >21 was the only variable associated with nephrotoxicity (p=0.039). After adjusting for statistically significant variables, obesity was found not to be associated with a greater risk of nephrotoxicity (RR=0.98; 95% CI=0.93-1.04; p=0.59).

CONCLUSION

No difference in nephrotoxicity was observed between lean and obese patients treated with vancomycin at our institution.

Impact of hospital guideline for weight-based antimicrobial dosing in morbidly obese adults and comprehensive literature review

WHAT IS KNOWN AND OBJECTIVE

Obesity is a significant burden on the healthcare system in the United States, and determining the appropriate antimicrobial dosing regimen in morbidly obese patients is challenging. Morbidly obese patients have documented differences in pharmacokinetic and pharmacodynamic properties compared to normal-weight patients, which impact antibiotic efficacy and toxicity. The Food and Drug Administration does not recognize obesity as a special population and does not require pharmaceutical companies to perform studies specific to obese patients. However, there are an increasing number of post-approval studies in obese patients, and this manuscript reviews available clinical and pharmacokinetic literature regarding weight-based antimicrobial agents. Additionally, we describe a single-centre approach to optimize dosing in morbidly obese patients.

METHODS

A comprehensive literature search was performed on 15 weight-based antimicrobials in the setting of obesity: acyclovir, aminoglycosides, amphotericin B, cidofovir, colistimethate, daptomycin, flucytosine, foscarnet, ganciclovir, quinupristin/dalfopristin, trimethoprim/sulfamethoxazole, vancomycin and voriconazole. A weight-based antimicrobial dosing guideline for morbidly obese patients was developed. An analysis of guideline compliance and cost analysis were performed following guideline implementation.

RESULTS AND DISCUSSION

This review describes the pharmacokinetic changes that occur in obese patients, including increased volume of distribution, altered hepatic metabolism, renal excretion and changes in protein binding. The majority of weight-based antimicrobials result in increased serum concentrations in morbidly obese patients compared to normal-weight patients when the calculated dose is based on actual body weight.

WHAT IS NEW AND CONCLUSION

This review demonstrates different antibiotic pharmacokinetic properties are altered in obese patients that could impact efficacy and toxicity. A single-centre guideline for weight-based antimicrobial dosing in obesity was developed and provides recommendations for using ideal body weight, adjusted body weight or actual body weight when calculating antimicrobial doses. However, more research is needed to better elucidate optimal dosing of weight-based antimicrobials in obesity, with particular focus on efficacy and toxicity.

Preventing surgical site infections after bariatric surgery: value of perioperative antibiotic regimens

Bariatric surgery for obesity has emerged as an effective and commonly used treatment modality. This paper reviews the surgical site infections (SSIs) that occur post bariatric surgery and SSI prevention. The benefit of bariatric surgery resulting in profound weight loss brings with it consequences in the form of postoperative complications that can have profound effects on morbidity and mortality in these patients. This paper sets out to define different types of SSIs that occur following bariatric surgery and to discuss existing literature on the critical aspects of SSI prevention and the appropriate use of surgical antimicrobial prophylaxis for bariatric surgery.

Drug disposition in obesity and protein–energy malnutrition

Clinical response to medication can differ between patients. Among the known sources of variability is an individual's nutrition status. This review defines some pharmacokinetic terms, provides relevant body size metrics and describes the physiologic influences of protein–energy malnutrition and obesity on drug disposition. Weight-based drug dosing, which presumes a healthy BMI, can be problematic in the protein–energy malnourished or obese patient. The use of total body weight, lean body weight, or an adjusted body weight depends on the drug and how it is differently handled in malnutrition or obesity. Most of the recognized influences are seen in drug distribution and drug elimination as a result of altered body composition and function. Distribution characteristics of each drug are determined by several drug-related factors (e.g. tissue affinity) in combination with body-related factors (e.g. composition). Drug elimination occurs through metabolic and excretory pathways that can also vary with body composition. The current data are limited to select drugs that have been reported in small studies or case reports. In the meantime, a rational approach to evaluate the potential influences of malnutrition and obesity can be used clinically based on available information. Antimicrobials are discussed as a useful example of this approach. Further advancement in this field would require collaboration between experts in body composition and those in drug disposition. Until more data are available, routine monitoring by the clinician of the protein–energy malnourished or obese patient receiving weight-based drug regimens is necessary.

Pharmacology of morphine in obese patients: clinical implications

Morphine is an analgesic drug used to treat acute and chronic pain. Obesity is frequently associated with pain of various origins (e.g. arthritis, fibromyalgia, cancer), which increases the need for analgesic drugs. Obesity changes drug pharmacokinetics, and for certain drugs, specific modalities of prescription have been proposed for obese patients. However, scant data are available regarding the pharmacokinetics and pharmacodynamics of morphine in obesity. Prescription of morphine depends on pain relief but the occurrence of respiratory adverse effects correlates with obesity, and is not currently taken into account. Variations in the volume of distribution, elimination half-life and oral clearance of morphine, as well as recent advances in the respective roles of drug-metabolizing enzymes, catechol-O-methyltransferase and the mu opioid receptor in morphine pharmacokinetics and pharmacodynamics, may contribute to differences between obese and non-obese patients. In addition, drug-drug interactions may alter the disposition of morphine and its glucuronide metabolites, which may either increase the risk of adverse effects or reduce drug efficacy.

Implementation of an antibiotic nomogram improves postoperative antibiotic utilization and safety in patients undergoing coronary artery bypass grafting

Background

Routine, initial, empiric vancomycin dosing by clinicians in postoperative coronary artery bypass grafting (CABG) patients was identified as a potential patient safety issue in the Cardiovascular Intensive Care Unit (CVICU) because the rate of postoperative acute renal insufficiency (ARI) and average patient Body Mass Index (BMI) > 35 kg/m² were significantly higher in our institution than those of the Society of Thoracic Surgeons (STS) database. A vancomycin dosing nomogram was derived from the local patient population in the attempt to improve patient safety by convincing clinicians to use an evidence-based approach to vancomycin prescription.

Methods

We analyzed two different treatment strategies that were applied consecutively to an intensive care unit population. CABG patients dosed empirically with vancomycin (group 1, pre-nomogram) were compared with CABG patients dosed using a vancomycin dosing nomogram (group 2, post-nomogram) derived from the hospital population using an Internet program that facilitated creation of a local nomogram. The two groups were analyzed as to age, sex, body mass index, creatinine clearance, and vancomycin dosage using logistic regression and testing for continuous and categorical variables.

Results

Nomogram use decreased the number of patients receiving the customary dose of one gram every 12 hours in those group 2 patients with diminished CrCl as compared with those in group 1 with diminished CrCl (group 2, 2/21 vs. group 1, 14/21, p < .0001), as well as in those with a normal creatinine clearance, (group 2, 2/15 vs. group 1, 26/34, p < .0001). Therefore, nomogram use affected the customary dose of one g vancomycin every 12 hours between the two groups overall (group 1, 40/55 vs. group 2, 4/36, p < .001), whereby 32/36 (88.9%) of group 2 patients had their dosing altered when compared to what would have been formerly prescribed, p < .0001. Furthermore, nomogram use resulted in fewer doses of antibiotics per year resulting in a cost savings to the hospital with no increase in the rates of infection.

Conclusion

Implementation of the nomogram resulted in a more appropriate antibiotic utilization, regardless of creatinine clearance, that decreased costs without increasing infection rates.

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.

Pharmacokinetic and Pharmacodynamic Aspects of Antibiotic Use in High-Risk Populations

The study of pharmacokinetics includes the absorption, distribution, metabolism, and elimination of drugs. The pharmacologic effect that a medication has on the body is known as pharmacodynamics. With antimicrobials, pharmacokinetic and pharmacodynamic parameters become especially important because of the association between host drug concentrations, microorganism eradication, and resistance. This article focuses on the pharmacokinetic changes that can occur with antimicrobials when they are used in patients at high risk of infections and how they influence pharmacodynamic effects. The populations described here include patients with obesity and diabetes mellitus, renal or hepatic failure, chronic lung disease, severe burns, and long-term prosthetic devices and the elderly.

Resuscitation of the morbidly obese patient

Obesity is a major health care problem in the United States. The body mass index (BMI) is the standard measure of obesity. A BMI >25 kg/m2 is defined as overweight and obesity as a BMI > 30 kg/m2. Recent surveys indicate that 54% of adults, or roughly 97 million people, are overweight. Given the incidence of obesity in the general population, it is likely that EM physicians will be involved in the emergency care of critically ill or injured obese patients. The objective of this article is to present the clinical problems associated with the resuscitation of the critically ill or injured obese patient and their potential solutions.

Soft tissue concentrations of ciprofloxacin in obese and lean subjects following weight-adjusted dosing

OBJECTIVE

To investigate whether weight adjusted ciprofloxacin dosing results in comparable target site concentrations in obese and lean subjects.

DESIGN

Comparative study in two populations.

SUBJECTS

Twelve obese subjects (mean weight 122+/-22.6 kg, 28-52 y, male&ratio;female ratio 4&ratio;8) and 12 age- and sex-matched lean controls (mean weight 59+/-8.6 kg).

METHODS

Sampling of interstitial space fluid by means of calibrated in vivo microdialysis after a weight-adjusted intravenous bolus dose of 2.85 mg/kg ciprofloxacin. Analysis of drug concentration by high pressure liquid chromatography.

RESULTS

We found significantly higher peak and trough levels of ciprofloxacin in plasma for obese subjects (9.97+/-5.64 and 0.44+/-0.10 microg/ml vs 2.59+/-1.06 and 0.19+/-0.09 microg/ml in lean subjects, P<0.05), while concentration-time curves of interstitial fluid of muscle and subcutaneous fat did not differ between the groups. Tissue penetration, expressed as AUC(tissue)/AUC(plasma) ratio was significantly lower in obese subjects (0.45+/-0.27 vs 0.82+/-0.36, P<0.01).

CONCLUSION

We conclude that the penetration process into the interstitial space fluid is impaired in obese subjects. Therefore antibiotic doses need not be adjusted for an increase in fat/water ratio. Weight-adjusted dosing based on actual body weight will yield adequate tissue levels for ciprofloxacin.

Effects of obesity on pharmacokinetics implications for drug therapy

Obesity is a worldwide problem, with major health, social and economic implications. The adaptation of drug dosages to obese patients is a subject of concern, particularly for drugs with a narrow therapeutic index. The main factors that affect the tissue distribution of drugs are body composition, regional blood flow and the affinity of the drug for plasma proteins and/or tissue components. Obese people have larger absolute lean body masses as well as fat masses than non-obese individuals of the same age, gender and height. However, the percentage of fat per kg of total bodyweight (TBW) is markedly increased, whereas that chrome P450 isoforms are altered, but no clear overview of drug hepatic metabolism in obesity is currently available. Pharmacokinetic studies provide differing data on renal function in obese patients. This review analyses recent publications on several classes of drugs: antibacterials, anticancer drugs, psychotropic drugs, anticonvulsants, general anaesthetics, opioid analgesics, neuromuscular blockers, beta-blockers and drugs commonly used in the management of obesity. Pharmacokinetic studies in obesity show that the behaviour of molecules with weak or moderate lipophilicity (e.g. lithium and vecuronium) is generally rather predictable, as these drugs are distributed mainly in lean tissues. The dosage of these drugs should be based on the ideal bodyweight (IBW). However, some of these drugs (e.g. antibacterials and some anticancer drugs) are partly distributed in adipose tissues, and their dosage is based on IBW plus a percentage of the patient's excess bodyweight. There is no systematic relationship between the degree of lipophilicity of markedly lipophilic drugs (e.g. remifentanil and some beta-blockers) and their distribution in obese individuals. The distribution of a drug between fat and lean tissues may influence its pharmacokinetics in obese patients. Thus, the loading dose should be adjusted to the TBW or IBW, according to data from studies carried out in obese individuals. Adjustment of the maintenance dosage depends on the observed modifications in clearance. Our present knowledge of the influence of obesity on drug pharmacokinetics is limited. Drugs with a small therapeutic index should be used prudently and the dosage adjusted with the help of drug plasma concentrations.

Dosage adjustments for antibacterials in obese patients: applying clinical pharmacokinetics

Obesity is associated with physiological changes that can alter the pharmacokinetic parameters of many drugs. Vancomycin and the aminoglycosides are the only antibacterials that have been extensively investigated in the obese population. The apparent volume of distribution (Vd) and total body clearance of vancomycin are increased in obese patients and have a better correlation with total bodyweight (TBW) than with ideal bodyweight (IBW). The Vd of aminoglycosides is increased in obesity and can be estimated from an adjusted bodyweight that accounts for a fraction of the excess bodyweight (TBW - IBW). These observed changes in pharmacokinetic parameters of vancomycin and aminoglycosides in obese patients may necessitate a deviation from the commonly recommended dosages administered to non-obese individuals. There are limited data regarding the pharmacokinetics of other antibacterial classes in obese patients. The available information for cephalosporins suggests that dosages may need to be increased in obese patients in order to obtain similar serum and tissue concentrations as in non-obese patients. Additional pharmacokinetic studies of other antibacterial classes are required in this special patient population.