Population pharmacokinetic/pharmacogenetic model for optimization of efavirenz therapy in Caucasian HIV-infected patients.

Despite extensive clinical experience with efavirenz (EFV), unpredictable interindividual variabilities in efficacy and toxicity remain important limitations associated with the use of this antiretroviral. The purpose of this study was to determine the factors affecting EFV pharmacokinetics and to develop a pharmacokinetic/pharmacogenetic (PK/PG) model in a Caucasian population of HIV-infected patients. In total, 869 EFV plasma concentrations from 128 HIV-infected patients treated with EFV were quantitatively assessed using a validated high-performance liquid chromatography technique. All patients were genotyped for 90 single nucleotide polymorphisms (SNPs) in genes coding for proteins involved in the metabolism and transport of EFV, using a MassArray platform provided by Sequenom. The influence of these polymorphisms on EFV pharmacokinetics and the effects of demographic, clinical, biochemical, lifestyle, and concurrent drug covariates were evaluated. Plasma concentrations were fitted by a one-compartment model, with first-order absorption and elimination using nonlinear mixed-effect modeling (NONMEM program). The CYP2B6*6 allele, multidrug resistance-associated protein 4 (MRP4) 1497C → T, and gamma-glutamyltranspeptidase (GGT) were identified as major factors influencing the apparent EFV oral clearance (CL/F), reducing the initial interindividual variability by 54.8%, according to the model CL/F = (12.2 - 0.00279 · GGT) · 0.602(CYP2B6*6 [G/T]) · 0.354(CYP2B6*6 [T/T]) · 0.793(MRP4 1497C → T), where CYP2B6*6 [G/T], CYP2B6*6 [T/T], and MRP4 1497C → T take values of 0 or 1 to indicate the absence or presence of polymorphisms. The detailed genetic analysis conducted in this study identified two of 90 SNPs that significantly impacted CL/F, which might indicate that the remaining SNPs analyzed do not influence this PK parameter, at least in Caucasian populations with characteristics similar to those of our study population.

Clinical repercussions of analytical interferences due to aldosterone antagonists in digoxin immunoassays: an assessment.

Analytical interferences in digoxin immunoassays constitute a well-known problem, with repercussions for therapeutic drug monitoring. Clinically effective doses of spironolactone and potassium canrenoate cross-react in several digoxin immunoassays, producing falsely elevated or lowered concentrations. This study evaluates the interferences caused by these drugs in the microparticle enzyme immunoassay (MEIA III) in comparison with another 3 immunoassays used for digoxin therapeutic drug monitoring: MEIA II, fluorescence polarization immunoassay, and enzyme multiplied immunoassay. The potential clinical implications of assay discrepancies in patient care are also assessed. To evaluate assay performance, in vitro specimens and real patient samples were measured using the 4 assays. Five serum pools were spiked with digoxin to achieve concentrations of 1 and 2.25 ng/mL digoxin and measured with the immunoassays before and after supplementation. Real samples from patients receiving digoxin (n = 39), digoxin and spironolactone (n = 35), or digoxin and potassium canrenoate (n = 4) were also quantified. The influence of ultrafiltration was evaluated in 3 pools from 29 additional patients. The implications of assay discrepancies for dose recommendations were also evaluated. In general, the results obtained for the in vitro and in vivo approaches coincided, confirming statistically significant differences in the assays regardless of the type of sample. MEIA III showed positive interference against the well-known negative interference attributed to MEIA II. According to Bland-Altman analysis, it is not possible to assume the interchangeability of the immunoassays evaluated. Thus, individual patients must be monitored with the same technique even in the absence of potential interferences. Discordant digoxin dose recommendations were estimated in 31% of patients not treated with interfering drugs and in 43% of cotreated patients. From a clinical perspective, analytical interferences in digoxin immunoassays are a real and frequent problem, which seems even more important in view of the lower therapeutic range now recommended.

Influence of the cytochrome P450 2B6 genotype on population pharmacokinetics of efavirenz in human immunodeficiency virus patients.

A population pharmacokinetic model for efavirenz has been developed from therapeutic drug monitoring data in human immunodeficiency virus (HIV)-positive patients by using a nonlinear mixed-effect model. The efavirenz plasma concentrations (n = 375) of 131 patients were analyzed using high-performance liquid chromatography with UV detection. Pharmacokinetic parameters were estimated according to a one-compartment model. The effects of sex, age, total body weight, height, body mass index, and HIV treatment were analyzed. In a subgroup of 32 patients, genetic polymorphisms of the cytochrome P450 2B6 gene (CYP2B6), CYP3A4, and MDR1 were also investigated. Efavirenz oral clearance and the apparent volume of distribution were 9.50 liters/h and 311 liters, respectively. The model included only the effect of CYP2B6 polymorphisms on efavirenz clearance; this covariate reduced the intersubject variability of clearance by about 27%. Patients showing G/T and T/T CYP2B6 polymorphisms exhibited efavirenz clearances that were about 50% and 75% lower than those observed in the patients without these polymorphisms (G/G). Accordingly, to obtain EFV steady-state concentrations within the therapeutic range (1 to 4 mg/liter), it would be advisable to implement a gradual reduction in dose to 400 or 200 mg/day for patients that are intermediate or poor metabolizers, respectively. However, the remaining interindividual variability observed in the pharmacokinetic parameters of the model highlights the need for dose individualization to avoid inadequate exposure to efavirenz and suggests that these recommended doses be used with caution and confirmed by therapeutic drug monitoring and clinical efficacy. The population model can be implemented in pharmacokinetic clinical software for dosage optimization by using the Bayesian approach.

Vancomycin dosage optimization in patients with malignant haematological disease by pharmacokinetic/pharmacodynamic analysis.

BACKGROUND: The use of vancomycin against Staphylococcus aureus is currently debated because of the increasing resistance developed by this pathogen. Nevertheless, antibacterial effectiveness is a limited resource that must be protected and restored. Novel dosage strategies based on pharmacokinetic/pharmacodynamic analyses are needed to retain effectiveness that could improve drug exposure in patients infected with such pathogens. OBJECTIVE: The aim of this study was to assess whether standard or higher vancomycin dosages are required to increase the probability of attaining a target pharmacokinetic/pharmacodynamic index for several staphylococcal strains and thus to estimate the minimum vancomycin daily dose related to a high probability of effective treatment in patients with malignant haematological disease. METHODS: Monte Carlo simulation was performed to calculate the cumulative fraction of response (CFR) for different vancomycin daily dosages, using a population pharmacokinetic model previously defined in patients with malignant haematological disease and the minimum inhibitory concentration (MIC) distribution for vancomycin against several staphylococcal species (vancomycin-susceptible S. aureus and vancomycin-intermediate S. aureus [VISA], S. epidermidis, S. haemolyticus and coagulase-negative Staphylococcus [CNS] species) obtained from the European Committee on Antimicrobial Susceptibility Testing (EUCAST) in order to predict the dose that would achieve the pharmacokinetic/pharmacodynamic index value associated with efficacy (the area under the concentration-time curve from 0 to 24 hours divided by the MIC [AUC(24)/MIC >/=400]). RESULTS: CFR values showed dependence on the renal function of the patient and the causative pathogen. Only in patients with a creatinine clearance (CL(CR)) <60 mL/min did the standard vancomycin dosage (2000 mg/day) induce CFRs >60% for all staphylococci, except the VISA strains. CFRs for S. aureus of 90.6%, 47.3% and 31.2% for CL(CR) values of <60, 60-120 and >120 mL/min, respectively, were obtained, whereas for the VISA strains, the corresponding values were only 14.0%, 0.3% and 0%. The impact of potential pathogens on CFRs is also significant. According to our pharmacokinetic/pharmacodynamic analysis, in patients with normal renal function (CL(CR) between 60 and 120 mL/min) vancomycin 2000 mg/day leads to a risk of not achieving the recommended AUC(24)/MIC breakpoint of 52.7%, 70.4%, 74.9% and 80.3% for S. aureus, S. haemolyticus, CNS and S. epidermidis, respectively. Application of our results to clinical practice graphically allows us to obtain the recommended dose for any a priori-selected probability of attaining the AUC(24)/MIC ratio of >/=400 and to evaluate the CFRs for any dosing regimen used in this population group, depending on the patients' renal function. CONCLUSIONS: Application of pharmacokinetic/pharmacodynamic analysis based on Monte Carlo simulation offers an excellent tool for selecting the therapeutic option with the highest probability of clinical success in patients with malignant haematological disease. Thus, for vancomycin-susceptible S. aureus, if a CFR >/=80 is assumed as clinically acceptable, vancomycin doses of 1500, 3000 and 4000 mg/day for a CL(CR) of <60, 60-120 and >120 mL/min, respectively, will be required.

Evaluation of population pharmacokinetic models for amikacin dosage individualization in critically ill patients.

OBJECTIVES: The aim of this study was to evaluate the reliability for dosage individualization and Bayesian adaptive control of several literature-retrieved amikacin population pharmacokinetic models in patients who were critically ill. METHODS: Four population pharmacokinetic models, three of them customized for critically-ill patients, were applied using pharmacokinetic software to fifty-one adult patients on conventional amikacin therapy admitted to the intensive care unit. An estimation of patient-specific pharmacokinetic parameters for each model was obtained by retrospective analysis of the amikacin serum concentrations measured (n = 162) and different clinical covariates. The model performance for a priori estimation of the area under the serum concentration-time curve (AUC) and maximum serum drug concentration (C(max)) targets was obtained. KEY FINDINGS: Our results provided valuable confirmation of the clinical importance of the choice of population pharmacokinetic models when selecting amikacin dosages for patients who are critically ill. Significant differences in model performance were especially evident when only information concerning clinical covariates was used for dosage individualization and over the two most critical determinants of clinical efficacy of amikacin i.e. the AUC and C(max) values. CONCLUSIONS: Only a single amikacin serum level seemed necessary to diminish the influence of population model on dosage individualization.

Pharmacokinetic/pharmacodynamic analysis of vancomycin in ICU patients.

AIMS: To identify the variables affecting vancomycin pharmacokinetics in medical ICU patients and to evaluate the potential efficacy of dosage schedules by PK/PD analysis. DESIGN: A retrospective pharmacokinetic analysis of serum levels obtained in routine vancomycin monitoring was performed. SETTING: A 12-bed general ICU of a university teaching hospital. PATIENTS: Forty-six vancomycin-treated ICU patients fitting the following criteria: over 18 years old; more than three concentration data per patient; absence of renal replacement support, cardiac surgery and neoplastic disorders. INTERVENTIONS: Clinical information was collected from the patients' medical records. Details of vancomycin therapy, dosage and blood sampling times were obtained from pharmacokinetic reports. Population analysis were made by the standard two-stage approach. MEASUREMENTS AND MAIN RESULTS: Vancomycin clearance and distribution volume were estimated individually assuming a one-compartment pharmacokinetic model. PK/PD analysis was performed by Monte Carlo simulation. In the ICU patients, higher Vd (nearly twice the quoted value of 0.72 l/kg) and different vancomycin clearance-creatinine clearance relationship were found. Renal function, the APACHE score, age and serum albumin accounted for more than 65% of drug clearance variability. Vancomycin standard dosages led to a 33% risk of not achieving the recommended AUC(24h)/MIC breakpoint for Staphylococcus aureus. CONCLUSIONS: The population kinetics and PK/PD analyses based on Monte Carlo simulation procedures offer an excellent tool for selecting the therapeutic option with the highest probability of clinical success in ICU patients.

Economic evaluation of voriconazole versus caspofungin for the treatment of invasive aspergillosis in Spain.

BACKGROUND AND OBJECTIVE: Invasive fungal infections are becoming increasingly prevalent and are more frequently the aetiological agents responsible for nosocomial infections. Since mid-2002, two new antifungal drugs - voriconazole, a third-generation azole, and caspofungin, a member of a new class of drugs called echinocandins - have been marketed in Spain. Both drugs have proven [corrected] efficacy in the treatment of aspergillosis, are better tolerated than amphotericin B and are cheaper [corrected] than liposomal amphotericin B. The objective of this study was to conduct an economic evaluation of voriconazole versus caspofungin for the treatment of invasive aspergillosis in Spain. METHODS: This was a cost-minimisation analysis (2006 costs) from the hospital perspective. Duration of treatment and bodyweight of patients were obtained from the Fungcost study and the incidence of adverse events was obtained from different published sources. Only direct costs were considered. Mean expected cost and incremental cost were calculated, and univariate and bivariate (bodyweight/treatment duration) sensitivity analyses were conducted. RESULTS: The mean expected cost per episode was 6041.93 euro (intravenous treatment acquisition cost 5524.75 euro) for voriconazole and 7174.05 euro (intravenous treatment acquisition cost 6672.80 euro) for caspofungin in invasive aspergillosis; the incremental cost was 1132.18 euro. Results were robust for any treatment duration and sensitive to bodyweights <103.42 kg. CONCLUSION: Voriconazole is a more cost-effective option than caspofungin in invasive aspergillosis in patients with a bodyweight <103.42 kg.

Tailored botulinum toxin type A injections in aesthetic medicine: consensus panel recommendations for treating the forehead based on individual facial anatomy and muscle tone.

BACKGROUND: Facial lines and wrinkles are strongly influenced by individual differences in anatomy and muscle activity and no single injection protocol will suit all patients. However, there is only limited information in the published literature on how to develop a tailored approach to botulinum toxin treatment. METHODS: An expert panel of physicians was convened to establish a consensus on developing an individualized approach to treatment of the forehead with incobotulinumtoxinA. Separate treatment protocols were developed for men and women and subdivided by background level of muscle activity: kinetic, hyperkinetic, and hypertonic. Each muscle tone category was then further subdivided to take account of individual characteristics that can influence treatment. RESULTS: Consensus members describe how to perform a dynamic assessment to optimize the dose and injection technique for each patient. A tailored treatment protocol is described for men and women with a wide range of forehead presentations. For each presentation, units of toxin as well as the precise location of injection points were defined by creating a 12-zone map of the forehead. CONCLUSION: These recommendations depart from traditional consensus documents by providing detailed incobotulinumtoxinA injection protocols for the forehead based on the major parameters that differ between patients, including muscular anatomy, size, and tone. It is expected that the use of this document will lead to more satisfactory, natural, and individualized aesthetic outcomes for patients.

[Preventable adverse drug events in hospitalized patients]. / Acontecimientos adversos prevenibles causados por medicamentos en pacientes hospitalizados.

BACKGROUND AND OBJECTIVE: To determine the incidence of adverse drug events (ADE) in hospitalized patients, identify those that were potentially preventable, and asses the drug classes involved, the clinical symptoms and the type of medication errors that led to the preventable ADE. PATIENTS AND METHOD: An observational study of ADE prevalence in hospitalized patients in internal medicine, pneumology, gastroenterology, nephrology and neurology wards, over a six-month period, at a tertiary university hospital. ADE were prospectively detected through physician and nurses reporting fostered by daily visits of a clinical research and retrospectively through review of medical records using event codes as defined by the IDC-9-CM system. RESULTS: In a total of 2,643 hospitalized patients, 191 (7.2%) ADE were detected. Of these, 38 cases (19.9%) were classified as preventable, of which 21.1% were mild; 60.5% moderate and 18.4% serious or life-threatening. Preventable ADE were frequently associated with anti-infective drugs (22.9%), diuretics (18.8%) and digoxin (16.7%). Inadequate therapy monitoring (28.3%), excessive dosage (21.7%), selection of an inappropriate drug according to patient characteristics and/or to diagnosis (15.0%), lack of prescription of a necessary drug (15.0%) and drug-drug interactions (11.7%) were the most common identified type of errors leading to preventable ADE. CONCLUSIONS: 1.4% of hospitalized patients in medical wards experienced potentially preventable ADE. Healthcare professionals and administrators must be made aware of the scope of this problem so that they will implement effective safety practices directed to reduce the incidence of medication errors, particularly prescription and monitoring errors.

Pharmacokinetics and pharmacodynamics of levofloxacin in intensive care patients.

OBJECTIVE: A prospective pharmacokinetic study was performed in Caucasian patients from an intensive care unit with respiratory support to evaluate the influence of this circumstance on the pharmacokinetic behaviour of levofloxacin. PATIENTS AND METHODS: A standard dosage regimen of 500 mg/day was administered to nine Caucasian patients included in the study, irrespective of their demographic characteristics. The experimental data on plasma concentrations were analysed by independent-modelling techniques to estimate the following pharmacokinetic parameters: area under the plasma concentration-time curve (AUC), volume of distribution at steady state (V(ss)), plasma clearance (CL), maximum plasma concentration at steady state (C(max)(,)(ss)) and elimination half-life (t((1/2))(beta)). Multiple regression analysis was applied to establish the type of correlation between the pharmacokinetic parameters and patient characteristics; the Monte Carlo simulation technique was implemented for the pharmacokinetic/pharmacodynamic analysis based on the probability distribution of the values of AUC/minimum inhibitory concentration (MIC) and C(max)(,)(ss)/MIC observed in this group of patients. RESULTS AND CONCLUSION: The results show that for AUC the simplest linear model with creatinine clearance as the only independent variable fits the data at a 99% confidence level, explaining more than 85% of the observed variability in this parameter. The volume of distribution, however, showed a statistical correlation with the severity of the illness (Simplified Acute Physiology Score II), although total bodyweight also explains a high percentage of variability of these parameters. Since the group of patients included in the study was small and also included obese individuals, it is difficult to estimate with precision the contribution of each circumstance (overweight or illness severity) to the pharmacokinetic behaviour of levofloxacin.

Drug delivery in biotechnology: present and future.

Drug delivery is becoming a whole interdisciplinary and independent field of research and is gaining the attention of pharmaceutical makers, medical doctors and industry. A targeted and safe drug delivery could improve the performance of some classical medicines already on the market and, moreover, will have implications for the development and success of new therapeutic strategies, such as peptide and protein delivery, glycoprotein administration, gene therapy and RNA interference. Many innovative technologies for effective drug delivery have been developed, including implants, nanotechnology, cell and peptide encapsulation, microfabrication, chemical modification and others. On the long way from the clinic to market, however, several issues will have to be addressed, including suitable scientific development, specific financial support as a result of altered scientific policy, government regulations and market forces.

Immunosuppressive therapy for paediatric transplant patients: pharmacokinetic considerations.

Immunosuppressive therapy in paediatric transplant recipients is changing as a consequence of the increasing number of available immunosuppressive agents. Generic and other new formulations are now emerging onto the market, clinical experience is growing, and it is expected that clinicians should tailor immunosuppressive protocols to individual patients by optimising dosages and drugs according to the maturation and clinical status of the child. Most information about the clinical pharmacokinetics of immunosuppressive drugs in paediatrics is centred on cyclosporin, tacrolimus and mycophenolate mofetil in renal and liver transplant recipients; data regarding other immunosuppressants and transplant types are limited. Although the clinical pharmacokinetics of these drugs in paediatric transplant recipients are still under investigation, it is evident that the pharmacokinetic parameters observed in adults may not be applicable to children, especially in younger age groups. In general, patients younger than 5 years old show higher clearance rates irrespective of the organ transplanted or drug used. Another important factor that frequently affects clearance in this patient population is the post-transplant time. In accordance with these findings, and in contrast with the usual under-dosage in children, the need for higher dosages in younger recipients and during the early post-transplant period seems evident. To achieve the best compromise between prevention of rejection and toxicity, dosage individualisation is required and this can be achieved through therapeutic drug monitoring (TDM). This approach is particularly useful to ensure the cost-effective management of paediatric transplant recipients in whom the pharmacokinetic behaviour, target concentrations for clinical use and optimal dosage strategies of a particular drug may not yet be well defined. Although TDM may be a tool for improving immunosuppressive therapy, there is little information concerning its positive contribution to clinical events, including outcomes, for paediatric patients. Substantial information to support the use of TDM exists for cyclosporin and, to a lesser extent, for tacrolimus, but a diversity of options affects their implementation in the clinical setting. The role of TDM in therapy with mycophenolate mofetil and sirolimus has yet to be defined regarding both methods and clinical indications. Pharmacodynamic monitoring appears more suited to other immunosuppressants such as azathioprine, corticosteroids and monoclonal or polyclonal antibodies. If coupled with pharmacokinetic measurements, such monitoring would allow earlier and more precise optimisation of therapy. Very few population pharmacokinetic studies have been carried out in paediatric transplant patients. This type of study is needed so that techniques such as Bayesian forecasting can be applied to optimise immunosuppressive therapy in paediatric transplant patients.

Modification of the EMIT immunoassay for the measurement of unbound mycophenolic acid in plasma.

OBJECTIVES: Evaluation of the performance of a modified Enzyme Multiplied Immunoassay Technique for therapeutic drug monitoring of plasma free mycophenolic acid (fMPA) concentrations. DESIGN AND METHODS: A fMPA assay was developed on a Viva-E analyzer. A study of prior ultrafiltration conditions and analytical validation of the EMIT assay were performed. RESULTS: The method was reliable and reproducible. CONCLUSIONS: fMPA levels can be monitored using this EMIT assay with the advantage of being an automated method.

Costs of eprosartan versus diuretics for treatment of hypertension in a geriatric population: an observational, open-label, multicentre study.

BACKGROUND: Diuretics are considered to be agents of first choice when treating hypertension in the elderly because of their clinical efficacy and, in particular, their low cost. Indeed, the latter consideration has been used by health resource managers to promote the use of diuretics. However, when considering the costs of treating hypertension in a population it is also necessary to assess the adverse effects that diuretics produce, particularly in elderly people. OBJECTIVE: To compare the overall expenditure associated with the treatment of hypertension (specifically the angiotensin II type 1 receptor antagonist eprosartan vs diuretics) in an elderly population, taking into consideration not only the drug acquisition costs but also the adverse effects of treatment and the costs associated with such adverse effects. METHODS: This was a prospective, observational, nonrandomized, open-label, multicentre study based in eight community health centres and the Hypertension Unit of the University Hospital of Salamanca, Spain. The study included 220 hypertensive geriatric outpatients (males and females aged >or=65 years) referred from general practitioners and the Hypertension Unit, with a mean age of 71.8 years and distributed into two groups: one (n = 90) treated with diuretics and the other (n = 130) treated with eprosartan. Following an initial clinical assessment of patients at the beginning of the study, monitoring of treatment continued for 1 year with follow-up consultations scheduled for 3, 6 and 12 months. Both the costs relating to acquisition of the drugs and the costs derived from secondary adverse effects of drug treatment were included in the analysis. RESULTS: The response to the antihypertensive therapy was similar in both groups. In patients taking diuretics, adverse events resulted in increased use of healthcare resources because of urinary incontinence, purchase of adsorbents, hyponatraemia and the need to admit two patients to hospital. The patient/day cost was euro 1.05 for the group treated with diuretics and euro 0.98 for the group treated with eprosartan (year of costing 2006). CONCLUSION: In the geriatric population, the acquisition cost of the prescribed diuretics is not representative of the actual antihypertensive treatment expenditure. According to the results obtained in our study, the overall costs of eprosartan therapy were no different to those of diuretics, despite the fact that eprosartan had a higher acquisition cost. This is consistent with a more favourable safety profile for eprosartan, which may possibly contribute to improved prescription compliance. This conclusion should be taken into consideration when evaluating economic restrictions on the use of drugs.

Hyperhidrosis in association with efavirenz.

Hyperhidrosis may be an adverse drug event (ADE) induced by the effect on any of the components of human thermoregulation. Some of our efavirenz (EFV)-treated patients have reported excessive nocturnal sweating that resolved after dose reduction. A representative clinical case of a male patient being treated with a night-time 600-mg dose of EFV who reported severe nocturnal sweating is reported here. His EFV plasma concentrations were always above normal and he was homozygous for a deficient function-allele of CYP2D6; for this reason, his EFV dose was reduced to 400mg=d. Simultaneous with this reduction, the patient described a progressive decrease in nocturnal sweating until its complete disappearance 15-20 days after this new drug dosage. The mechanism explaining sweating could be similar to the one suggested for hyperhidrosis related to serotonin uptake inhibitors, because this hyperhidrosis is episodic, nocturnal, and dose dependent. Hyperhidrosis could correspond to a dose-dependent ADE induced by EFV, therefore, a reduction of EFV from 600 to 400mg/d seems to control it. EFV crosses the hematoencephalic barrier and reaches a mean concentration in the cerebroespinal fluid equivalent to 0.69% of the plasma concentration. The ability of EFV to accessing the central nervous system (CNS) could explain an effect on thermoregulation. Hyperhydrosis is not easily discovered through a routine anamnesis because it is not noted on the EFV package insert, so its incidence may be higher than expected. Additionally, hyperhidrosis may be an indicator of elevated EFV plasma concentrations and hence may be controlled through a reduction of dose.

Population pharmacokinetics of lamotrigine with data from therapeutic drug monitoring in German and Spanish patients with epilepsy.

This study develops a population pharmacokinetic model for lamotrigine (LTG) in Spanish and German patients diagnosed with epilepsy. LTG steady-state plasma concentration data from therapeutic drug monitoring were collected retrospectively from 600 patients, with a total of 1699 plasma drug concentrations. The data were analyzed according to a one-compartment model using the nonlinear mixed effect modelling program. The influences of origin (Germany or Spain), sex, age, total body weight, and comedication with valproic acid (VPA), levetiracetam, and enzyme-inducing antiepileptic drugs (phenobarbital [PB], phenytoin [PHT], primidone [PRM], and carbamazepine [CBZ]) were investigated using step-wise generalized additive modelling. The final regression model for LTG clearance (CL) was as follows: CL(L/h) = 0.028*total body weight*e(-0.713*VPA)*e0.663*PHT*e0.588*(PB or PRM)*e0.467*CBZ*e0.864*IND, where IND refers to two or more inducers added to LTG treatment; this factor as well as VPA, PHT, PB, PRM, and CBZ take a value of zero or one according to their absence or presence, respectively. The administration of inducers led to a significant increase in mean LTG CL (values of 0.045-0.070 L/h/kg vs. 0.028 L/h/kg being reached in monotherapy), whereas VPA led to a significant decrease in CL (0.014 L/h/kg). Thus, comedication with these analyzed drugs can partly explain the interindividual variability in population LTG CL, which decreased from the basic model by more than 40%. The proposed model may be very useful for clinicians in establishing initial LTG dosage guidelines. However, the interindividual variability remaining in the final model (clearance coefficient of variation close to 30%) make these a priori dosage predictions imprecise and justifies the need for LTG plasma level monitoring to optimize dosage regimens. Thus, this final model allows easy implementation in clinical pharmacokinetic software and its application in dosage individualization using the Bayesian approach.

Population pharmacokinetic analysis of vancomycin in patients with hematological malignancies.

This study determines vancomycin (VAN) population pharmacokinetics (PK) in adult patients with hematological malignancies. VAN serum concentration data (n = 1,004) from therapeutic drug monitoring were collected retrospectively from 215 patients. A one-compartment PK model was selected. VAN pharmacokinetics population parameters were generated using the NONMEM program. A graphic approach and stepwise generalized additive modeling were used to elucidate the preliminary relationships between PK parameters and clinical covariates analyzed. Covariate selection revealed that total body weight (TBW) affected V, whereas renal function, estimated by creatinine clearance, and a diagnosis of acute myeloblastic leukemia (AML) influenced VAN clearance. We propose one general and two AML-specific models. The former was defined by CL (liters/h) = 1.08 x CL(CR(Cockcroft and Gault)) (liters/h); CV(CL) = 28.16% and V (liters) = 0.98 x TBW; CV(V) =37.15%. AML models confirmed this structure but with a higher clearance coefficient (1.17). The a priori performance of the models was evaluated in another 59 patients, and clinical suitability was confirmed. The models were fairly accurate, with more than 33% of the measured concentrations being within +/-20% of the predicted value. This therapeutic precision is twofold higher than that of a non-customized population model (16.1%). The corresponding standardized prediction errors included zero and a standard deviation close to unity. The models could be used to estimate appropriate VAN dosage guidelines, which are not clearly defined for this high-risk population. Their simple structure should allow easy implementation in clinical software and application in dosage individualization using the Bayesian approach.

Pharmacokinetic basis for the use of extended interval dosage regimens of gentamicin in neonates.

OBJECTIVES: To analyse the pharmacokinetic basis for the use of extended-interval dosage regimens of gentamicin in neonates using population pharmacokinetics. PATIENTS AND METHODS: The population pharmacokinetics of gentamicin was studied retrospectively in a population of 113 neonates divided into two groups: one for computing the population model (n=97) and another for validation (n=36). A one-compartment pharmacokinetic model and non-linear mixed-effects modelling were used to assess the population pharmacokinetic model. RESULTS: Weight (W) and postnatal age (PA) were the covariates that influenced the pharmacokinetic parameters of gentamicin. The final population model obtained was: distribution volume, V (L)=0.636 x W (kg)0.852; clearance, Cl (L/h)=0.032 x W (kg)1.482+0.0024 x PA (days). The predictive performance of the model in the population validation was adequate for clinical purposes. The optimized population model allowed us to simulate gentamicin serum levels and their variability, in this kind of patient, when extended-interval dosage administration regimens were implemented. CONCLUSIONS: According to our pharmacokinetic population model, initial doses of gentamicin of 10 mg/kg, and dosage intervals between 36-48 h, appear to be appropriate to achieve target peak and trough serum levels of 15-20 and <0.5 mg/L, respectively, when extended-interval dosage regimens are implemented in newborns. The half-life of gentamicin in premature babies of very low weight and gestational age <31 weeks is long. Thus, to achieve serum concentrations in the 1-10 mg/L range, the use of dosage regimens of 5 mg/kg at 36-48 h dosage intervals seems suitable.