Multiple-dose pharmacokinetics of amikacin and ceftazidime in critically ill patients with septic multiple-organ failure during intermittent hemofiltration

Antibiotic dosing recommendations in critically ill patients receiving new innovative kidney replacement therapy

BACKGROUND

The Tablo Hemodialysis System is a new innovative kidney replacement therapy (KRT) providing a range of options for critically ill patients with acute kidney injury. The use of various effluent rate and treatment durations/frequencies may clear antibiotics differently than traditional KRT. This Monte Carlo Simulation (MCS) study was to develop antibiotic doses likely to attain therapeutic targets for various KRT combinations.

METHODS

Published body weights and pharmacokinetic parameter estimates were used to predict drug exposure for cefepime, ceftazidime, imipenem, meropenem and piperacillin/tazobactam in virtual critically ill patients receiving five KRT regimens. Standard free β-lactam plasma concentration time above minimum inhibitory concentration targets (40-60%fT> MIC and 40-60%fT> MICx4) were used as efficacy targets. MCS assessed the probability of target attainment (PTA) and likelihood of toxicity for various antibiotic dosing strategies. The smallest doses attaining PTA ≥ 90% during 1-week of therapy were considered optimal.

RESULTS

MCS determined β-lactam doses achieving ∼90% PTA in all KRT options. KRT characteristics influenced antibiotic dosing. Cefepime and piperacillin/tazobactam regimens designed for rigorous efficacy targets were likely to exceed toxicity thresholds.

CONCLUSION

The flexibility offered by new KRT systems can influence β-lactam antibiotic dosing, but doses can be devised to meet therapeutic targets. Further clinical validations are warranted.

Aminoglycoside Pharmacokinetics in Critically Ill Patients Undergoing Continuous Renal Replacement Therapy

BACKGROUND

There are few studies describing aminoglycoside pharmacokinetics during continuous renal replacement therapy (CRRT).

OBJECTIVE

To characterize the effect of CRRT on aminoglycoside clearance and volume of distribution (V_d).

METHODS

Retrospective observational pharmacokinetic study of adult critically ill oncologic patients who received a first dose of amikacin or tobramycin during CRRT between February 2012 and May 2017. Study outcomes included aminoglycoside clearance, V_d, and attainment of the target peak: MIC (minimum inhibitory concentration) ratio as a surrogate for dosing appropriateness.

RESULTS

In total, 80 patients were included, sustained low-efficiency dialysis (SLED), n = 49; continuous venovenous hemodialysis (CVVHD), n = 19; continuous venovenous hemofiltration (CVVH), n = 12. Fifty-one patients received amikacin at a median dose of 14.5 mg/kg per actual body weight and achieved a median peak level of 26.7 mg/L. Twenty-nine patients received tobramycin at a median dose of 6.5 mg/kg actual body weight and achieved a median peak level of 10.3 mg/L. The median aminoglycoside clearance was 63.1 mL/min and was similar between CRRT modality groups (P = 0.97). The median V_d was 0.47 L/kg and was different between the SLED and CVVH groups (P = 0.007). Attainment of target peak: MIC occurred in 29% in the total study population and 44% in the subgroup of 23 patients with isolates tested for aminoglycoside susceptibility.

CONCLUSION AND RELEVANCE

Critically ill oncology patients undergoing CRRT exhibited reduced clearance and expanded V_d that was not significantly different between CRRT modalities. Current dosing regimens led to low peak concentrations and poor attainment of pharmacokinetic targets.

An optimal extended-infusion dosing of cefepime and ceftazidime in critically ill patients with continuous renal replacement therapy

PURPOSE

This study aimed to determine optimal extended-infusion dosing regimens for cefepime and ceftazidime in critically ill patients receiving continuous renal replacement therapy using Monte Carlo Simulations (MCS).

MATERIALS AND METHODS

Pharmacokinetic models were built using published pharmacokinetic/demographic data to predict drug disposition in 5000 virtual critically ill patients receiving continuous venovenous hemofiltration (CVVH) with the standard (20-30 mL/kg/h) and a higher (40 mL/kg/h) effluent rates. MCS was performed to assess the probability of target attainment (PTA) of four cefepime and ceftazidime doses administered over 4-h with the target of ≥60% fT > 4×MIC. The lowest dose attaining PTA ≥90% during the first 48-h was considered optimal. Additionally, risk of drug toxicity was assessed at 48-h using suggested neurotoxicity thresholds.

RESULTS

Cefepime 2 g loading dose (LD), then extended-infusion of 2 g q8hr was optimal in CVVH at 20 mL/kg/h and the same ceftazidime dose was optimal in CVVH at 20-30 mL/kg/h. Higher cefepime and ceftazidime doses were required to be optimal at higher effluent rates. This optimal dose particularly for cefepime likely increases neurotoxicity risk in most virtual patients with all CVVH settings.

CONCLUSIONS

Cefepime and ceftazidime 2 g LD, followed by extended-infusion 2 g q8hr may be optimal in CVVH with standard effluent rates.

Size Matters: The Influence of Patient Size on Antibiotics Exposure Profiles in Critically Ill Patients on Continuous Renal Replacement Therapy

(1) Purpose of this study: To determine whether patient weight influences the probability of target attainment (PTA) over 72 h of initial therapy with beta-lactam (cefepime, ceftazidime, piperacillin/tazobactam) and carbapenem (imipenem, ertapenem, meropenem) antibiotics in the critical care setting. This is the first paper to address the question of whether patient size affects antibiotic PTA in the ICU. (2) Methods: We performed a post hoc analysis of Monte Carlo simulations conducted in virtual critically ill patients receiving antibiotics and continuous renal replacement therapy. The PTA was calculated for each antibiotic on the following pharmacodynamic (PD) targets: (a) were above the target organism’s minimum inhibitory concentration (≥%fT≥1×MIC), (b) were above four times the MIC (≥%fT≥4×MIC), and (c) were always above the MIC (≥100%fT≥MIC) for the first 72 h of antibiotic therapy. The PTA was analyzed in patient weight quartiles [Q1 (lightest)-Q4 (heaviest)]. Optimal doses were defined as the lowest dose achieving ≥90% PTA. (3) Results: The PTA for fT≥1×MIC led to similarly high rates regardless of weight quartiles. Yet, patient weight influenced the PTA for higher PD targets (100%fT≥MIC and fT≥4×MIC) with commonly used beta-lactams and carbapenems. Reaching the optimal PTA was more difficult with a PD target of 100%fT≥MIC compared to fT≥4×MIC. (4) Conclusions: The Monte Carlo simulations showed patients in lower weight quartiles tended to achieve higher antibiotic pharmacodynamic target attainment compared to heavier patients.

Drug dosing considerations in continuous renal replacement therapy

Acute kidney injury (AKI) is a common complication in critically ill patients, which is associated with increased in-hospital mortality. Delivering effective antibiotics to treat patients with sepsis receiving continuous renal replacement therapy (RRT) is complicated by variability in pharmacokinetics, dialysis delivery, lack of primary literature, and therapeutic drug monitoring. Pharmacokinetic alterations include changes in absorption, distribution, protein binding (PB), metabolism, and renal elimination. Drug absorption may be significantly changed due to alterations in gastric pH, perfusion, gastrointestinal motility, and intestinal atrophy. Volume of distribution for hydrophilic drugs may be increased due to volume overload. Estimation of renal clearance is challenged by the effective delivery of RRT. Drug characteristics such as PB, volume of distribution, and molecular weight impact removal of the drug by RRT. The totality of these alterations leads to reduced exposure. Despite our best knowledge, therapeutic drug monitoring of patients receiving continuous RRT demonstrates wide variability in antimicrobial concentrations, highlighting the need for expanded monitoring of all drugs. This review article will focus on changes in drug pharmacokinetics in AKI and dosing considerations to attain antibiotic pharmacodynamic targets in critically ill patients receiving continuous RRT.

Antibiotic Exposure Profiles in Trials Comparing Intensity of Continuous Renal Replacement Therapy

OBJECTIVES

To determine whether the probability of target attainment over 72 hours of initial therapy with beta-lactam (cefepime, ceftazidime, piperacillin/tazobactam) and carbapenem (imipenem, meropenem) antibiotics were substantially influenced between intensive and less-intensive continuous renal replacement therapy groups in the Acute Renal Failure Trial Network trial and The RENAL Replacement Therapy Study trial.

DESIGN

The probability of target attainment was calculated using pharmacodynamic targets of percentage of time that free serum concentrations (fT): 1) were above the target organism's minimum inhibitory concentration (≥ fT > 1 × minimum inhibitory concentration); 2) were above four times the minimum inhibitory concentration (≥ % fT > 4 × minimum inhibitory concentration); and 3) were always above the minimum inhibitory concentration (≥ 100% fT > minimum inhibitory concentration) for the first 72 hours of antibiotic therapy. Demographic data and effluent rates from the Acute Renal Failure Trial Network and RENAL Replacement Therapy Study trials were used. Optimal doses were defined as the dose achieving greater than or equal to 90% probability of target attainment.

SETTING

Monte Carlo simulations using demographic data from Acute Renal Failure Trial Network and RENAL Replacement Therapy Study trials.

PATIENTS

Virtual critically ill patients requiring continuous renal replacement therapy.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The pharmacodynamic target of fT greater than 1 × minimum inhibitory concentration led to similarly high rates of predicted response with antibiotic doses often used in continuous renal replacement therapy. Achieving 100% fT greater than minimum inhibitory concentration is a more stringent benchmark compared with T greater than 4 × minimum inhibitory concentration with standard antibiotic dosing. The intensity of effluent flow rates (less intensive vs intensive) did not substantially influence the probability of target attainment of antibiotic dosing regimens regardless of pharmacodynamic target.

CONCLUSIONS

Antibiotic pharmacodynamic target attainment rates likely were not meaningfully different in the low- and high-intensity treatment arms of the Acute Renal Failure Trial Network and RENAL Replacement Therapy Study Investigators trials.

A Monte Carlo Simulation Approach for Beta-Lactam Dosing in Critically Ill Patients Receiving Prolonged Intermittent Renal Replacement Therapy

Cefepime, ceftazidime, and piperacillin/tazobactam are commonly used beta-lactam antibiotics in the critical care setting. For critically ill patients receiving prolonged intermittent renal replacement therapy (PIRRT), limited pharmacokinetic data are available to inform clinicians on the dosing of these agents. Monte Carlo simulations (MCS) can be used to guide drug dosing when pharmacokinetic trials are not feasible. For each antibiotic, MCS using previously published pharmacokinetic data derived from critically ill patients was used to evaluate multiple dosing regimens in 4 different prolonged intermittent renal replacement therapy effluent rates and prolonged intermittent renal replacement therapy duration combinations (4 L/h × 10 hours or 5 L/h × 8 hours in hemodialysis and hemofiltration modes). Antibiotic regimens were also modeled depending on whether drugs were administered during or well before prolonged intermittent renal replacement therapy therapy commenced. The probability of target attainment (PTA) was calculated using each antibiotic's pharmacodynamic target during the first 48 hours of therapy. Optimal doses were defined as the smallest daily dose achieving ≥90% probability of target attainment in all prolonged intermittent renal replacement therapy effluent and duration combinations. Cefepime 1 g every 6 hours following a 2 g loading dose, ceftazidime 2 g every 12 hours, and piperacillin/tazobactam 4.5 g every 6 hours attained the desired pharmacodynamic target in ≥90% of modeled prolonged intermittent renal replacement therapy patients. Alternatively, if an every 6-hours cefepime regimen is not desired, the cefepime 2 g pre-prolonged intermittent renal replacement therapy and 3 g post-prolonged intermittent renal replacement therapy regimen also met targets. For ceftazidime, 1 g every 6 hours or 3 g continuous infusion following a 2 g loading dose also met targets. These recommended doses provide simple regimens that are likely to achieve the pharmacodynamics target while yielding the least overall drug exposure, which should result in lower toxicity rates. These findings should be validated in the clinical setting.

Antibiotic Dosing in Continuous Renal Replacement Therapy

Appropriate antibiotic dosing is critical to improve outcomes in critically ill patients with sepsis. The addition of continuous renal replacement therapy makes achieving appropriate antibiotic dosing more difficult. The lack of continuous renal replacement therapy standardization results in treatment variability between patients and may influence whether appropriate antibiotic exposure is achieved. The aim of this study was to determine if continuous renal replacement therapy effluent flow rate impacts attaining appropriate antibiotic concentrations when conventional continuous renal replacement therapy antibiotic doses were used. This study used Monte Carlo simulations to evaluate the effect of effluent flow rate variance on pharmacodynamic target attainment for cefepime, ceftazidime, levofloxacin, meropenem, piperacillin, and tazobactam. Published demographic and pharmacokinetic parameters for each antibiotic were used to develop a pharmacokinetic model. Monte Carlo simulations of 5000 patients were evaluated for each antibiotic dosing regimen at the extremes of Kidney Disease: Improving Global Outcomes guidelines recommended effluent flow rates (20 and 35 mL/kg/h). The probability of target attainment was calculated using antibiotic-specific pharmacodynamic targets assessed over the first 72 hours of therapy. Most conventional published antibiotic dosing recommendations, except for levofloxacin, reach acceptable probability of target attainment rates when effluent rates of 20 or 35 mL/kg/h are used.

Influence of Renal Replacement Modalities on Amikacin Population Pharmacokinetics in Critically Ill Patients on Continuous Renal Replacement Therapy

The objective of this study was to describe amikacin pharmacokinetics (PK) in critically ill patients receiving equal doses (30 ml/kg of body weight/h) of continuous venovenous hemofiltration (CVVH) and continuous venovenous hemodiafiltration (CVVHDF). Patients receiving amikacin and undergoing CVVH or CVVHDF were eligible. Population pharmacokinetic analysis and Monte Carlo simulation were undertaken using the Pmetrics software package for R. Sixteen patients (9 undergoing CVVH, 11 undergoing CVVHDF) and 20 sampling intervals were analyzed. A two-compartment linear model best described the data. Patient weight was the only covariate that was associated with drug clearance. The mean ± standard deviation parameter estimates were 25.2 ± 17.3 liters for the central volume, 0.89 ± 1.17 h(-1) for the rate constant for the drug distribution from the central to the peripheral compartment, 2.38 ± 6.60 h(-1) for the rate constant for the drug distribution from the peripheral to the central compartment, 4.45 ± 2.35 liters/h for hemodiafiltration clearance, and 4.69 ± 2.42 liters/h for hemofiltration clearance. Dosing simulations for amikacin supported the use of high dosing regimens (≥25 mg/kg) and extended intervals (36 to 48 h) for most patients when considering PK/pharmacodynamic (PD) targets of a maximum concentration in plasma (Cmax)/MIC ratio of ≥8 and a minimal concentration of ≤2.5 mg/liter at the end of the dosing interval. The mean clearance of amikacin was 1.8 ± 1.3 liters/h by CVVHDF and 1.3 ± 1 liters/h by CVVH. On the basis of simulations, a strategy of an extended-interval high loading dose of amikacin (25 mg/kg every 48 h) associated with therapeutic drug monitoring (TDM) should be the preferred approach for aminoglycoside treatment in critically ill patients receiving continuous renal replacement therapy (CRRT). (This study is a substudy of a trial registered at ClinicalTrials.gov under number NCT01403220.).

An open prospective study of amikacin pharmacokinetics in critically ill patients during treatment with continuous venovenous haemodiafiltration

Background

The objectives of the current study were to determine amikacin pharmacokinetics in patients undergoing treatment with continuous venovenous haemodiafiltration (CVVHDF) in an Intensive Care Unit (ICU), and to determine whether peak and trough concentration data could be used to predict pharmacokinetic parameters. An open prospective study was undertaken, comprising five critically ill patients with sepsis requiring CVVHDF.

Methods

Peak and trough plasma concentrations and multiple serum levels in a dosage interval were measured and the latter fitted to both a one- and two-compartment model. Blood and ultrafiltrate samples were collected and assayed for amikacin to calculate the pharmacokinetic parameters; total body clearance (TBC), elimination rate constant (k) and volume of distribution (V_d). The concentration of amikacin in ultrafiltrate was used to determine the clearance via CVVHDF. CVVHDF was performed at prescribed dialysate rates of 1-2l h^-1 and ultrafiltration rate of 2l h^-1. Blood was pumped at 200ml/min using a Gambro blood pump and Hospal AN69HF haemofilter. Amikacin dosing was according to routine clinical practice in the Intensive Care Unit.

Results

The multi serum level study indicated that the one compartment model was adequate to characterize the pharmacokinetics in these patients suggesting that peak and trough plasma level data may be used to estimate individual patient pharmacokinetic parameters and to optimise individual patient dosing during treatment with CVVHDF. CVVHDF resulted in an amikacin k of 0.109+/−0.025 h, t_1/2 of 6.74 +/− 1.69h, TBC of 3.39+/−0.817 h^-1, and V_d of 31.4 +/− 3.27. The mean clearance due to CVVHDF of 2.86 l h^-1 is similar to the creatinine clearance of 2.74 +/−0.4 lh^-1. Amikacin was significantly cleared by CVVHDF, and its half life in patients on CVVHDF was approximately 2–3 times that reported in subjects without renal impairment and not undergoing haemodiafiltration for any reason.

Conclusions

CVVHDF contributes significantly to total clearance of amikacin. The use of pharmacokinetic parameter estimates obtained from two steady state serum-drug concentrations (peak and trough) can be used to guide individualised dosing of critically ill patients treated with CVVHDF. This is considered a useful strategy in this patient cohort, particularly in avoiding the risk of underdosing.

A Kinetic Model for Simultaneous Fit of Clozapine and Norclozapine Concentrations in Chronic Schizophrenic Patients during Long-Term Treatment

OBJECTIVE

The pharmacokinetic profiles of clozapine and its main metabolite, norclozapine, were investigated in 18 chronic schizophrenic inpatients during long-term treatment.

PATIENTS

Patients received stable daily doses (between 300 and 900mg) for at least 1 month. Plasma drug concentrations were determined by high performance liquid chromatography. The pharmacokinetic parameters were calculated from both noncompartmental and compartmental approaches with zero-order input rate using a kinetic model for simultaneous fit of clozapine and norclozapine (active metabolite) concentrations.

RESULTS

Large interpatient variations in pharmacokinetic parameters of the two drugs were observed. Plasma clozapine concentration peaked on average at 2 hours. The mean elimination rate constants from compartments 1 (k(10)) and 2 (k(20 ), elimination rate constant of norclozapine) were 0.087 and 0.156h(-1), respectively. The rate of formation of norclozapine, k(12), averaged 1.25h(-1). The mean fraction of the administered dose converted to norclozapine was estimated to be 66%. The apparent clearance of clozapine (CL/F) averaged 44.7 L/h and the volume of distribution (V(c)/F) was 7.00 L/kg. The pharmacokinetics of clozapine after multiple doses were linear over the range of clozapine plasma concentrations of 145 to 1411 microg/L.

CONCLUSION

This is the first study assessing the pharmacokinetic profile of clozapine plus norclozapine in plasma during long-term treatment. This pharmacokinetic model can be used to determine the population pharmacokinetic parameters of clozapine and norclozapine in order to optimise individual dosage regimens using a Bayesian methodology.

Antibiotic dosing in critically ill patients with acute kidney injury

A common cause of acute kidney injury (AKI) is sepsis, which makes appropriate dosing of antibiotics in these patients essential. Drug dosing in critically ill patients with AKI, however, can be complicated. Critical illness and AKI can both substantially alter pharmacokinetic parameters as compared with healthy individuals or patients with end-stage renal disease. Furthermore, drug pharmacokinetic parameters are highly variable within the critically ill population. The volume of distribution of hydrophilic agents can increase as a result of fluid overload and decreased binding of the drug to serum proteins, and antibiotic loading doses must be adjusted upwards to account for these changes. Although renal elimination of drugs is decreased in patients with AKI, residual renal function in conjunction with renal replacement therapies (RRTs) result in enhanced drug clearance, and maintenance doses must reflect this situation. Antibiotic dosing decisions should be individualized to take into account patient-related, RRT-related, and drug-related factors. Efforts must also be made to optimize the attainment of antibiotic pharmacodynamic goals in this population.

Drug dosing during intermittent hemodialysis and continuous renal replacement therapy : special considerations in pediatric patients

Chronic renal failure is, fortunately, an unusual occurrence in children; however, many children with various underlying illnesses develop acute renal failure, and transiently require renal replacement therapy - peritoneal dialysis, intermittent hemodialysis (IHD), or continuous renal replacement therapy (CRRT). As children with acute and chronic renal failure often have multiple comorbid conditions requiring drug therapy, generalists, intensivists, nephrologists, and pharmacists need to be aware of the issues surrounding the management of drug therapy in pediatric patients undergoing renal replacement therapy. This article summarizes the pharmacokinetics and dosing of many drugs commonly prescribed for pediatric patients, and focuses on the management of drug therapy in pediatric patients undergoing IHD and CRRT in the intensive care unit setting. Peritoneal dialysis is not considered in this review. Finally, a summary table with recommended initial dosages for drugs commonly encountered in pediatric patients requiring IHD or CRRT is presented.

Pharmacokinetic and pharmacodynamic considerations when treating patients with sepsis and septic shock

Sepsis and septic shock are accompanied by profound changes in the organism that may alter both the pharmacokinetics and the pharmacodynamics of drugs. This review elaborates on the mechanisms by which sepsis-induced pathophysiological changes may influence pharmacological processes. Drug absorption following intramuscular, subcutaneous, transdermal and oral administration may be reduced due to a decreased perfusion of muscles, skin and splanchnic organs. Compromised tissue perfusion may also affect drug distribution, resulting in a decrease of distribution volume. On the other hand, the increase in capillary permeability and interstitial oedema during sepsis and septic shock may enhance drug distribution. Changes in plasma protein binding, body water, tissue mass and pH may also affect drug distribution. For basic drugs that are bound to the acute phase reactant alpha(1)-acid glycoprotein, the increase in plasma concentration of this protein will result in a decreased distribution volume. The opposite may be observed for drugs that are extensively bound to albumin, as the latter protein decreases during septic conditions. For many drugs, the liver is the main organ for metabolism. The determinants of hepatic clearance of drugs are liver blood flow, drug binding in plasma and the activity of the metabolic enzymes; each of these may be influenced by sepsis and septic shock. For high extraction drugs, clearance is mainly flow-dependent, and sepsis-induced liver hypoperfusion may result in a decreased clearance. For low extraction drugs, clearance is determined by the degree of plasma binding and the activity of the metabolic enzymes. Oxidative metabolism via the cytochrome P450 enzyme system is an important clearance mechanism for many drugs, and has been shown to be markedly affected in septic conditions, resulting in decreased drug clearance. The kidneys are an important excretion pathway for many drugs. Renal failure, which often accompanies sepsis and septic shock, will result in accumulation of both parent drug and its metabolites. Changes in drug effect during septic conditions may theoretically result from changes in pharmacodynamics due to changes in the affinity of the receptor for the drug or alterations in the intrinsic activity at the receptor. The lack of valid pharmacological studies in patients with sepsis and septic shock makes drug administration in these patients a difficult challenge. The patient's underlying pathophysiological condition may guide individual dosage selection, which may be guided by measuring plasma concentration or drug effect.

Pharmacokinetic-pharmacodynamic evaluation of ceftazidime continuous infusion vs intermittent bolus injection in septicaemic melioidosis

AIMS

Experimental studies have suggested that constant intravenous infusion would be preferable to conventional intermittent bolus administration of beta-lactam antibiotics for serious Gram-negative infections. Severe melioidosis (Burkholderia pseudomallei infection) carries a mortality over 40% despite treatment with high dose ceftazidime. The aim of this study was to measure the pharmacokinetic and pharmacodynamic effects of continuous infusion of ceftazidime vs intermittent bolus dosing in septicaemic melioidosis.

METHODS

Patients with suspected septicaemic melioidosis were randomised to receive ceftazidime 40 mg kg(-1) 8 hourly by bolus injection or 4 mg kg(-1) h(-1) by constant infusion following a 12 mg kg(-1) priming dose and pharmacokinetic and pharmacodynamic parameters were compared.

RESULTS

Of the 34 patients studied 16 (59%) died. Twenty patients had cultures positive for B. pseudomallei of whom 12 (60%) died. The median MIC90 of B. pseudomallei was 2 mg l(-1), giving a minimum target concentration (4*MIC) of 8 mg l(-1). The median (range) estimated total apparent volume of distribution, systemic clearance and terminal elimination half-lives of ceftazidime were 0.468 (0.241-0. 573) l kg(-1), 0.058 (0.005-0.159) l kg(-1) h(-1) and 7.74 (1.95-44.71) h, respectively. Clearance of ceftazidime and creatinine clearance were correlated closely (r = 0.71; P < 0.001) and there was no evidence of significant nonrenal clearance.

CONCLUSIONS

Simulations based on these data and the ceftazidime sensitivity of the B. pseudomallei isolates indicated that administration by constant infusion would allow significant dose reduction and cost saving. With conventional 8 h intermittent dosing to patients with normal renal function, plasma ceftazidime concentrations could fall below the target concentration but this would be unlikely with a constant infusion. Correction for renal failure, which is common in patients with meliodosis is Clearance = k(*) creatinine clearance where k = 0.72. Calculation of a loading dose gives median (range) values of loading dose, DL of 18.7 mg kg(-1) (9.5-23) and infusion rate I = 3.5 mg k(-1) h(-1) (0.4-13) (which equals 84 mg kg(-1) day(-1)). A nomogram for adjustment in renal failure is given.

Pharmacokinetic-pharmacodynamic evaluation of ceftazidime continuous infusion vs intermittent bolus injection in septicaemic melioidosis

AIMS

Experimental studies have suggested that constant intravenous infusion would be preferable to conventional intermittent bolus administration of beta-lactam antibiotics for serious Gram-negative infections. Severe melioidosis (Burkholderia pseudomallei infection) carries a mortality of 40% despite treatment with high dose ceftazidime. The aim of this study was to measure the pharmacokinetic and pharmacodynamic effects of continuous infusion of ceftazidime vs intermittent bolus dosing in septicaemic melioidosis.

METHODS

Patients with suspected septicaemic melioidosis were randomised to receive ceftazidime 40 mg kg-1 8 hourly by bolus injection or 4 mg kg-1 h-1 by constant infusion following a 12 mg kg-1 priming dose to perform estimation of pharmacokinetic and pharmacodynamic parameters.

RESULTS

Of the 34 patients studied 16 (59%) died. Twenty patients had cultures positive for B. pseudomallei of whom 12 (60%) died. The median MIC90 of B. pseudomallei was 2 mg l-1, giving a target concentration CT, of 8 mg l-1. The median (range) estimated total apparent volume of distribution, systemic clearance and terminal elimination half-lives of ceftazidime were 0.468 (0.241-0.573) l kg-1, 0.058 (0.005-0.159) l kg-1 h-1 and 7.74 (1.95-44.71) h, respectively. Clearance of ceftazidime and creatinine clearance were correlated closely (r = 0. 71; P < 0.001) and there was no evidence of significant nonrenal clearance.

CONCLUSIONS

Simulations based on these data and the ceftazidime sensitivity of the B. pseudomallei isolates indicated that administration by constant infusion would allow significant dose reduction and cost saving. With conventional 8 h intermittent dosing to patients with normal renal function, plasma ceftazidime concentrations could fall below the target concentration but this would be unlikely with a constant infusion. Correction for renal failure which is common in these patients is Clearance = k * creatinine clearance where k = 0.072. Calculation of a loading dose gives median (range) values of loading dose, DL of 3.7 mg kg-1 (1. 9-4.6) and infusion rate I = 0.46 mg kg h-1 (0.04-1.3) (which equals 14.8 mg kg-1 day-1). A nomogram for adjustment in renal failure is given.

Clozapine and metabolite concentrations during treatment of patients with chronic schizophrenia

Results presented in this article are focused on the variability in pharmacokinetics. The purpose of this study was (1) to investigate intra- and interindividual variabilities of pharmacokinetic parameters of clozapine and its two main metabolites in plasma after multiple oral administration in 8 chronic schizophrenic patients (Study 1) and (2) to gain more information regarding plasma concentrations of these drugs after multiple doses in a group of 25 treatment-responsive patients (Study 2). Patients were treated with clozapine in fixed daily doses (given every 8-12 hours) between 200 and 900 mg. Plasma drug concentrations were determined by high-performance liquid chromatography. The mean volume of distribution and the total plasma clearance of clozapine, uncorrected for bioavailability, were 7 L/kg and 40.5 L/h, respectively. The terminal elimination half-lives averaged 10.5 hours for clozapine, 19.2 hours for norclozapine, and 8.6 hours for the N-oxide metabolite. Significant relationships were observed between clozapine and norclozapine (or clozapine N-oxide) plasma concentrations. Large inter- and intrapatient variations in pharmacokinetics were observed. Clozapine was generally well tolerated by the patients, with sedation, hypersialorrhea, and tiredness as the most common side effects encountered.

Multiple-dose pharmacokinetics of clozapine in patients with chronic schizophrenia

The pharmacokinetic parameters of clozapine and its two main metabolites, N-desmethylclozapine (norclozapine, active metabolite) and clozapine N-oxide, were evaluated, after oral administration, in 19 patients with chronic schizophrenia. Plasma and red blood cell (RBC) drug concentrations were determined by high-performance liquid chromatography. Large interpatient variations in pharmacokinetic parameters of clozapine and its two metabolites were observed. Plasma clozapine concentration peaked, on average, at 2.3 hours. The mean volume of distribution and the total plasma clearance, uncorrected for bioavailability, were 6 L/kg and 38 L/hr, respectively. The terminal elimination half-lives averaged 7.6 hours for clozapine, 13 hours for norclozapine, and 7 hours for the N-oxide metabolite. The mean RBC/plasma concentration ratios were 23, 61, and 81% for clozapine, N-desmethylclozapine, and clozapine N-oxide, respectively. From RBC concentration data, the mean elimination half-lives were 7.6 hours for clozapine, 16 hours for N-desmethylclozapine, and 8 hours for the N-oxide metabolite. The average value for blood clearance of clozapine was 54.7 L/hr. Significant correlations were observed between dose and maximum plasma concentrations and between dose and area under the curve concentrations; these results suggested linear steady-state pharmacokinetics over the range of concentrations studied.

Determination of ceftazidime in plasma using high-performance liquid chromatography and electrochemical detection. Application for individualizing dosage regimens in elderly patients

This study describes a sensitive HPLC-electrochemical detection method for the analysis of ceftazidime, a third-generation cephalosporin, in human plasma. The extraction procedure involved protein precipitation with 30% trichloroacetic acid. The separation was achieved on a reversed-phase column (250X4.6 mm I.D., 5 microm) packed with C18 Kromasil with isocratic elution and a mobile phase consisting of acetonitrile-25 mM KH2PO4-Na2HPO4 buffer, pH 7.4 (10:90, v/v). The proposed analytical method is selective, reproducible and reliable. The assay has a precision of 0.2-15.1% (C.V.) in the range of 5-200 microg mil(-1). (corresponding to 0.5 to 20 ng of ceftazidime injected onto the column), and is optimised for assaying 50 microl of plasma. The extraction recovery from plasma was approximately 100%. The method was highly specific for ceftazidime and there was no interference from either commonly administered drugs or endogenous compounds. This assay was used to measure ceftazidime in elderly patients for therapeutic drug monitoring.

Development and evaluation of a Bayesian pharmacokinetic estimator and optimal, sparse sampling strategies for ceftazidime

Data were gathered during an activity-controlled trial in which seriously ill, elderly patients were randomized to receive intravenous ceftazidime or ciprofloxacin and for which adaptive feedback control of drug concentrations in plasma and activity profiles was prospectively performed. The adaptive feedback control algorithm for ceftazidime used an initial population model, a maximum a posteriori (MAP)-Bayesian pharmacokinetic parameter value estimator, and an optimal, sparse sampling strategy for ceftazidime that had been derived from data in the literature obtained from volunteers. Iterative two-stage population pharmacokinetic analysis was performed to develop an unbiased MAP-Bayesian estimator and updated optimal, sparse sampling strategies. The final median values of the population parameters were follows: the volume of distribution of the central compartment was equal to 0.249 liter/kg, the volume of distribution of the peripheral compartment was equal to 0.173 liter/kg, the distributional clearance between the central and peripheral compartments was equal to 0.2251 liter/h/kg, the slope of the total clearance (CL) versus the creatinine clearance (CLCR) was equal to 0.000736 liter/h/kg of CL/1 ml/min/1.73 m2 of CLCR, and nonrenal clearance was equal to + 0.00527 liter/h/kg. Optimal sampling times were dependent on CLCR; for CLCR of > or = 30 ml/min/1.73 m2, the optimal sampling times were 0.583, 3.0, 7.0, and 16.0 h and, for CLCR of < 30 ml/min/1.73 m2, optimal sampling times were 0.583, 4.15, 11.5, and 24.0 h. The study demonstrates that because pharmacokinetic information from volunteers may often not be reflective of specialty populations such as critically ill elderly individuals, iterative two-stage population pharmacokinetic analysis, MAP-Bayesian parameter estimation, and optimal, sparse sampling strategy can be important tools in characterizing their pharmacokinetics.

Ceftazidime. An update of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy

Ceftazidime is a third generation cephalosporin antibacterial agent which, since its introduction in the early 1980s, has retained a broad spectrum of in vitro antimicrobial activity and clinical utility in serious infections. However, increasing resistance to ceftazidime and other third generation cephalosporins, particularly among Enterobacteriaceae, due to the emergence of plasmid-mediated extended spectrum beta-lactamases and the class I chromosomally mediated beta-lactamases, is of concern. There is now a wealth of information on the pharmacokinetics of the drug. enabling ceftazidime to be used predictably, and with a low potential for adverse effects, in a diversity of patient populations. Overall, ceftazidime remains an effective agent for the treatment of serious infection, particularly those due to major nosocomial pathogens, and respiratory infections in patients with cystic fibrosis. Ceftazidime-containing regimens also remain an important option for the empirical therapy of febrile episodes in neutropenic patients. The tolerability profile of ceftazidime makes the drug a useful option in seriously ill patients who are at risk of developing adverse events with other antibacterial agents. Although patterns of bacterial resistance have changed in the ensuing years since its introduction, judicious use of this important agent will help maintain its present clinical utility.