A Model for Predicting the Interindividual Variability of Drug-Drug Interactions

Pharmacokinetic drug-drug interactions are frequently characterized and quantified by an AUC ratio (Rauc). The typical value of the AUC ratio in case of cytochrome-mediated interactions may be predicted by several approaches, based on in vitro or in vivo data. Prediction of the interindividual variability of Rauc would help to anticipate more completely the consequences of a drug-drug interaction. We propose and evaluate a simple approach for predicting the standard deviation (sd) of Ln(Rauc), a metric close to the interindividual coefficient of variation of Rauc. First, a model was derived to link sd(Ln Rauc) with the substrate fraction metabolized by each cytochrome and the potency of the interactors, in case of induction or inhibition. Second, the parameters involved in these equations were estimated by a Bayesian hierarchical model, using the data from 56 interaction studies retrieved from the literature. Third, the model was evaluated by several metrics based on the fold prediction error (PE) of sd(Ln Rauc). The median PE was 0.998 (the ideal value is 1) and the interquartile range was 0.96-1.03. The PE was in the acceptable interval (0.5 to 2) in 52 cases out of 56. Fourth, a surface plot of sd(Ln Rauc) as a function of the characteristics of the substrate and the interactor has been built. The minimal value of sd(Ln Rauc) was about 0.08 (obtained for Rauc = 1) while the maximal value, 0.7, was obtained for interactions involving highly metabolized substrates with strong interactors.

[Identification of knowledge deficits of pharmacy students at the beginning of the fifth year of pharmacy practice experience: Proposals to change the content of academic programs]

INTRODUCTION

In France, community pharmacy students performed a hospital pharmacy practice experience during the 5th year of the university curriculum. The purpose of a part of the content of the academic teaching program delivered before this practice experience is to prepare the students for their future hospital activities. It should enable them for the practical use of knowledge in order to improve pharmacotherapy, laboratory diagnosis and monitoring of patients' care. The aim of this study was to show if there are gaps in this program.

METHODS

Fourteen students performing their clerkship in a teaching hospital were invited to highlight these gaps when they were gradually immersed in the pharmaceutical care. They did so under the careful observation of hospital pharmacist preceptors. These practitioners referred to professional guidelines, documentary tools used in daily clinical practice and publications supporting their pharmaceutical care practices.

RESULTS

Shortcomings and gaps identified were: how to communicate with other healthcare professionals and the content of verbal exchanges, how to conduct a patient-centered consultation, documentation tools required for relevant pharmacist' interventions, codification of pharmacist's interventions, risks related to drug packaging and benefit risk assessment of health information technologies.

DISCUSSION

These gaps represent a handicap by delaying the process that led to move from student to healthcare professional. Hospital pharmacist preceptors have to fill in these gaps before engaging students in pharmaceutical care.

CONCLUSION

These results invite to revise partly the content of the academic teaching program delivered before the 5th year hospital pharmacy practice experience.

Modeling and predicting optimal treatment scheduling between the antiangiogenic drug sunitinib and irinotecan in preclinical settings

We present a system of nonlinear ordinary differential equations used to quantify the complex dynamics of the interactions between tumor growth, vasculature generation, and antiangiogenic treatment. The primary dataset consists of longitudinal tumor size measurements (1,371 total observations) in 105 colorectal tumor‐bearing mice. Mice received single or combination administration of sunitinib, an antiangiogenic agent, and/or irinotecan, a cytotoxic agent. Depending on the dataset, parameter estimation was performed either using a mixed‐effect approach or by nonlinear least squares. Through a log‐likelihood ratio test, we conclude that there is a potential synergistic interaction between sunitinib when administered in combination with irinotecan in preclinical settings. Model simulations were then compared to data from a follow‐up preclinical experiment. We conclude that the model has predictive value in identifying the therapeutic window in which the timing between the administrations of these two drugs is most effective.

A Joint Model for the Kinetics of CTC Count and PSA Concentration During Treatment in Metastatic Castration-Resistant Prostate Cancer

Assessment of treatment efficacy in metastatic castration-resistant prostate cancer (mCRPC) is limited by frequent nonmeasurable bone metastases. The count of circulating tumor cells (CTCs) is a promising surrogate marker that may replace the widely used prostate-specific antigen (PSA). The purpose of this study was to quantify the dynamic relationships between the longitudinal kinetics of these markers during treatment in patients with mCRPC. Data from 223 patients with mCRPC treated by chemotherapy and/or hormonotherapy were analyzed for up to 6 months of treatment. A semimechanistic model was built, combining the following several pharmacometric advanced features: (1) Kinetic-Pharmacodynamic (K-PD) compartments for treatments (chemotherapy and hormonotherapy); (2) a latent variable linking both marker kinetics; (3) modeling of CTC kinetics with a cell lifespan model; and (4) a negative binomial distribution for the CTC random sampling. Linked with survival, this model would potentially be useful for predicting treatment efficacy during drug development or for therapeutic adjustment in treated patients.

A critical review of the analytical approaches for circulating tumor biomarker kinetics during treatment

Changes in serum tumor biomarkers may indicate treatment efficacy. Traditional tumor markers may soon be replaced by novel serum biomarkers, such as circulating tumor cells (CTCs) or circulating tumor nucleic acids. Given their promising predictive values, studies of their kinetics are warranted. Many methodologies meant to assess kinetics of traditional marker kinetics during anticancer treatment have been reported. Here, we review the methodologies, the advantages and the limitations of the analytical approaches reported in the literature. Strategies based on a single time point were first used (baseline value, normalization, nadir, threshold at a time t), followed by approaches based on two or more time points [half-life (HL), percentage decrease, time-to-events…]. Heterogeneities in methodologies and lack of consideration of inter- and intra-individual variability may account for the inconsistencies and the poor utility in routine. More recently, strategies based on a population kinetics approach and mathematical modeling have been reported. The identification of equations describing individual kinetic profiles of biomarkers may be an alternative strategy despite its complexity and higher number of necessary measurements. Validation studies are required. Efforts should be made to standardize biomarker kinetic analysis methodologies to ensure the optimized development of novel serum biomarkers and avoid the pitfalls of traditional markers.

Prediction of tumour response induced by chemotherapy using modelling of CA-125 kinetics in recurrent ovarian cancer patients

Background:

The main objective of the present study was to establish the relationships between CA-125 kinetics and tumour size changes during treatment.

Methods:

The data from the CALYPSO-randomised phase III trial, comparing two platinum-based regimens in recurrent ovarian cancer (ROC) patients, was randomly split into a ‘learning data set' to estimate model parameters and a ‘validation data set' to validate model performances. A kinetic–pharmacodynamic semi-mechanistic model was built to describe tumour size and CA-125 kinetics during chemotherapy. The ability of the model to predict tumour response induced by chemotherapy, based on CA-125 values, was assessed.

Results:

Data from 535 ROC patients were used to model CA-125 kinetics and tumour size changes during the first 513 days after treatment initiation. Using the validated model, we could predict with accuracy the tumour size changes induced by chemotherapy based on the baseline imaging assessment and longitudinal CA-125 values (mean prediction error: 0.3%, mean absolute prediction error: 10.6%).

Conclusions:

Using a semi-mechanistic model, the dynamic relationships between tumour size changes and CA-125 kinetics induced by chemotherapy were established in ROC patients. A modelling approach allowed CA-125 to be assessed as a biomarker for tumour size dynamics, to predict treatment efficacy for research and clinical purposes.

Sorafenib in advanced melanoma: a critical role for pharmacokinetics?

BACKGROUND

Inter-patient pharmacokinetic variability can lead to suboptimal drug exposure, and therefore might impact the efficacy of sorafenib. This study reports long-term pharmacokinetic monitoring of patients treated with sorafenib and a retrospective pharmacodynamic/pharmacokinetic analysis in melanoma patients.

PATIENTS AND METHODS

Heavily pretreated patients with stage IV melanoma were started on sorafenib 400 mg twice daily (bid). In the absence of limiting toxicity, dose escalation of 200 mg bid levels was done every 2 weeks. Plasma sorafenib measurement was performed at each visit, allowing a retrospective pharmacodynamic/pharmacokinetic analysis for safety and efficacy.

RESULTS

In all, 19 of 30 patients underwent dose escalation over 400 mg bid, and 28 were evaluable for response. The overall disease control rate was 61% (95% confidence interval (CI): 42.6-78.8), including three confirmed responses (12%). Disease control rate and progression-free survival (PFS) were improved in patients with high vs low exposure (80% vs 32%, P=0.02, and 5.25 vs 2.5 months, P=0.005, hazard ratio (HR)=0.28 (95% CI: 0.11-0.73)). In contrast, drug dosing had no effect on PFS. In multivariate analysis, drug exposure was the only factor associated with PFS (HR=0.36 (95% CI: 0.13-0.99)). Diarrhoea and anorexia were correlated with drug dosing, while hypertension and hand-foot skin reaction were correlated with drug exposure.

CONCLUSIONS

Although sorafenib had modest efficacy in melanoma, these results suggest a correlation between exposure and efficacy of sorafenib. Therefore, dose optimisation in patients with low exposure at standard doses should be evaluated in validated indications.

Overdosed prescription of paracetamol (acetaminophen) in a teaching hospital

INTRODUCTION

Paracetamol is the most commonly used analgesic and antipyretic. Reviews of hospital use of paracetamol are scarce. Little is known about the appropriateness of the dose of paracetamol prescribed for hospitalized adults. The aim of this study was to report on the nature and the frequency of the overdosed prescription of paracetamol observed in adult patients over a 4.5-year period in a teaching hospital.

METHODS

Prescription analysis by pharmacists was performed once a week in six medical and three surgical departments and daily in a post-emergency unit. In cases of prescription error, the pharmacist notified the physician through an electronic alert when a computerized prescription order entry system was available or otherwise by face-to-face discussion. For each drug-related problem detected, the pharmacists recorded relevant details in a database.

RESULTS

From October 2006 to April 2011, 44,404 prescriptions were reviewed and 480 alerts related to the overdosed prescription of paracetamol were made (1% of analyzed prescriptions). The extent of errors of dosage was within the intervals [90-120 mg/kg/d] and greater than 120 mg/kg/d for 87 and 11 patients respectively, who were prescribed a single non-combination paracetamol containing product. Sixty alerts concerned co-prescription of at least two paracetamol containing products with similar frequency for computerized (1.4/1000) or handwritten (1.2/1000) prescriptions.

DISCUSSION

Prescriptions of paracetamol for hospitalized adults frequently exceed the recommended dosage.

CONCLUSION

These results highlight the need for increased awareness of unintentional paracetamol overdose and support the initiation of an educational program aimed at physicians and nurses.

Predictive values of hCG clearance for risk of methotrexate resistance in low-risk gestational trophoblastic neoplasias

BACKGROUND

Early identification of patients at high risk for chemoresistance among those treated with methotrexate (MTX) for low-risk gestational trophoblastic neoplasia (GTN) is needed. We modeled human chorionic gonadotropin (hCG) decline during MTX therapy using a kinetic population approach to calculate individual hCG clearance (CL(hCG)) and assessed the predictive value of CL(hCG) for MTX resistance.

PATIENTS AND METHODS

A total of 154 patients with low-risk GTN treated with 8-day MTX regimen were retrospectively studied. NONMEM was used to model hCG decrease equations between day 0 and day 40 of chemotherapy. Receiver operating characteristic curve analysis defined the best CL(hCG) threshold. Univariate/multivariate survival analyses determined the predictive value of CL(hCG) and compared it with published predictive factors.

RESULTS

A monoexponential equation best modeled hCG decrease: hCG(t) = 3900 x e(-0.149 x t). Median CL(hCG) was 0.57 l/day (quartiles: 0.37-0.74). Only choriocarcinoma pathology [yes versus no: hazard ratio (HR) = 6.01; 95% confidence interval (CI) 2.2-16.6; P < 0.001] and unfavorable CL(hCG) quartile (< or =0.37 versus >0.37 l/day: HR = 6.75; 95% CI 2.7-16.8; P < 0.001) were significant independent predictive factors of MTX resistance risk.

CONCLUSION

In the second largest cohort of low-risk GTN patients reported to date, choriocarcinoma pathology and CL(hCG) < or =0.37 l/day were major independent predictive factors for MTX resistance risk.

Investigation of PK-PD drug-drug interaction between acenocoumarol and amoxicillin plus clavulanic acid

A pharmacokinetic-pharmacodynamic (PK-PD) drug-drug interaction between acenocoumarol and amoxicillin + clavulanic acid antibiotic was assessed in eight healthy volunteers, using a population PK-PD) model. Each subject received at day 1 a single dose of 8 mg of acenocoumarol. Then 1 g of amoxicillin + 250 mg of clavulanic acid was given from days 3 to 9. On day 8, each subject received a single dose of 8 mg of acenocoumarol concomitantly with the antibiotic combination. Eleven blood samples were taken during 48 h following each acenocoumarol administration. Acenocoumarol plasma concentrations and prothrombin time were measured at each sampling time. We first identified the structural PK model by pooling data from this trial with individual data from other acenocoumarol PK trials. An indirect response model was used to fit PD data. Models were built using a non-linear mixed effect modelling approach with nonmem software. Covariates were tested on PK and PD parameters, including antibiotic treatment. Acenocoumarol PK data were fitted by a two-compartment, first-order input model with log normal inter-individual variability. Weight and antibiotic treatment were found to improve significantly the fit of PK data with a 15% decrease in acenocoumarol clearance with concomitant antibiotics (P < 0.05). An indirect response model was successfully applied to the PK-PD data of acenocoumarol. No covariate, including antibiotic treatment effect, significantly affected PT. Drug-drug interaction was demonstrated at the PK level, without any PD corollary.

Major prognostic value of modeled AUChCG-AFP, a dynamic kinetic marker characterizing tumor marker decline of nonseminomatous germ cell tumors (NSGCT) intermediate-poor-risk patients according to the IGCCCG

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Background: The level of human chorionic gonadotrophin (hCG) and alpha-foetoprotein (AFP) serum tumor marker is well established in NSGCT as prognostic factor, the relevance of marker kinetic analysis under treatment is still unclear. This may be due to the inaccuracy of methods employed so far, simplifying complex exponential decrease curves by a median half-life (HL). We propose to model patient's AFP and hCG decline profiles in order to calculate area under the curve of marker concentrations versus time (AUChCG-AFP) and to test its prognostic value. Methods: Our retrospective study involved 65 pts treated by 4 cycles of bleomycin-etoposide-cisplatin (BEP) regimen for an intermediate-poor-risk group NSGCT in the same center between 1997 and 2008. A kinetic population approach with NONMEM software was used to model equations of hCG and AFP individual decrease profiles between day 7 (D7) and D42 after the first BEP cycle. AUChCG and AUCAFP were calculated between day D0 and D42 as: AUC0–42=AUC0–7+AUC7–42 where AUC0–7 = trapezium area between D0 and D7 while AUC7–42=integral of modeled equation. Survival univariate and multivariate analyses tested the prognostic value of AUChCG-AFP regarding PFS. Results: Mono-exponential models best fitted AFP and hCG decreases: CAFP (t) = 381*e - 0.14 *t +3.27 and ChCG (t) = 1230*e - 0.25 *t +1.22. Three prognostic groups (AUChCG-AFP) were determined according to AUCAFP median and AUChCG terciles: good if AUCAFP<=11729.4 and AUChCG0–42<=6670; intermediate if AUCAFP>11729 and/or if 6670<=AUChCG<18178 and poor risk if AUChCG>18178 whatever AUCAFP. AUChCG-AFP was a significant prognostic factor in the univariate analysis on the 2 year PFS (100% vs 73.8% vs 67.7%, p = 0.035) as well as IGCCCG score (poor/intermediate risk groups), primary site (mediastinal/other) and HLhCG-AFP. Yet AUChCG-AFP was the only significant independent factor in the multivariate Cox model (HR = 3.3, 95%CI = [1.2–9.2], p = 0.032). Conclusions: Modeled AUChCG-AFP is a dynamic kinetic marker characterizing NSGCT patient marker decline during BEP treatment. These results must be validated in a prospective cohort. It may be a major prognostic factor.

No significant financial relationships to disclose.

Correlation of sorafenib plasma concentrations and clinical toxicity: A prospective population pharmacodynamic and pharmacokinetic study

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Background: Sorafenib is an angiogenesis inhibitor recently approved for the treatment of metastatic renal cell carcinoma and hepatocarcinoma. This oral multi-targeted kinase inhibitor blocks the VEGF, PDGF receptors, BRAF and c-kit. In clinical trials, severe toxicities included rash, hand-foot syndrome and diarrhea. We investigated the relationship between severe toxicity and sorafenib plasmatic exposure. Patients and methods: From February 2008 to December 2008, 32 patients (pts) were treated with sorafenib (800 mg given daily). Clinical and blood exams were performed at baseline and every 2 weeks. Toxicity events were graded according to National Cancer Institute Common Toxicity Criteria 3.0. The sorafenib area under the plasma concentration-time curve over 12 hours (AUC0–12) was determined every 2 weeks by gradient elution liquid chromatography, data analysis with one-compartment disposition and a population approach using NONMEM software. Grade 3 toxicity free survival was estimated by Kaplan-Meier method. Results: Pts (23 males), ECOG 0–1 (27 pts), median age 62.8 years (range 37–78), with metastatic hepato-carcinoma (11), melanoma (6), thyroid cancer (8), renal cell carcinoma (7), received a median treatment duration of 94 days (range : 7–330). 20 pts experienced grade 3 toxicities with 33 events: hand-foot hyperkeratosis (14), diarrhea (5), cutanous rash (4), asthenia (2), anorexia (2), dyspnea (1), scrotal toxicity (1), post surgical complication (1), hypertension (1), increase creatinine level (1), digestive hemorrhage (1). The median AUC0–12 was 62.4 mg/L.h, ranging from 28.7 to 202.2 mg/L.h. Amongst pts experiencing grade 3 toxicity, two groups of 16 pts were defined with the median AUC0- 12 as cut off: low exposure (Lw) and high exposure (Hi). Six Lw pts experienced grade 3 toxicity versus 14 Hi pts (exact Fisher Test p=0.009). The probability of free grade 3 toxicity survival was higher for Lw (Logrank test p=0.004). Conclusions: These results show that grade 3 toxicity occurrence may be related to high plasma sorafenib exposure. This supports the need for therapeutic drug monitoring to prevent toxicity grade 3 occurrence and therefore optimize the clinical management of patients under sorafenib.

No significant financial relationships to disclose.

Evaluation of drugs in pediatrics using K-PD models: perspectives

Some pharmacodynamic (PD) models, called K-PD models, have been developed for the description of drug action kinetics in the absence of drug concentration measurements. Because blood samples for drug measurements are not needed, these models may be very useful in pediatric studies, by reducing their invasiveness. In addition, a number of PD measurements are also non-invasive and specific devices exist for measures in children. Therefore, the kinetics of drug action may be characterized with minimal invasiveness. A brief description of the key features of these models is given, and a number of examples of application are presented. K-PD models are expected to be most useful when the drug kinetics is simple (i.e. when the one-compartment model is a reasonable description), or when the response kinetics is slow compared with drug kinetics. K-PD models have already demonstrated their usefulness in animal and adult studies. They are very attractive for pediatric studies and they should facilitate the assessment of drug efficacy and safety.

Quinine pharmacokinetics and pharmacodynamics in children with malaria caused by Plasmodium falciparum

The aim of the present study was to assess the pharmacokinetics and the efficacy of a shorter than usual 5-day quinine treatment given orally to children in Cameroon with malaria caused by Plasmodium falciparum. Quinine (8.3 mg of base per kg of body weight every 8 h) was administered as a 2% formiate salt syrup for 5 days to 30 children (age range, 0.55 to 6.7 years) with uncomplicated falciparum malaria (initial parasitemia, 1.4 x 10(3) to 1.8 x 10(5)/microl). Quinine concentrations in plasma samples (five to nine per patient) were measured by liquid chromatography on days 1 to 3. Parasitemia was counted on days 0, 1, 2, 3, 4, 7, and 14. Pharmacokinetic and pharmacodynamic data were analyzed by population approaches by using NONMEM and WinBugs, respectively. The kinetics of quinine were best described by a one-compartment model with time-varying protein binding. Clearance and the volume of distribution were positively correlated with body weight and increased over time. Parasitemia was undetectable from day 3 to 14 in all children. The time to a 4-log reduction of the initial level of parasitemia (Ter) was related to the average quinine concentration from 0 to 72 h (Cav) as Ter = Tmin [1 + (C50/Cav)s], where sigmoidicity (s) is equal to 2, Tmin is the time to eradication at infinite Cav, and C50 is the value of Cav for which Ter is twice Tmin. The C50 distribution was unimodal, and all C50 values were less than 8 mg/liter, while Cav ranged from 5.9 to 18.3 mg/liter. The median (10th to 90th percentile) Ter was 47 h (range, 39 to 76 h). The efficacy of a 5-day treatment course should be evaluated in a larger clinical trial.

Comparison of 99mTc-DTPA and urea for measuring cefepime concentrations in epithelial lining fluid

The efficacy of antimicrobial agents against pulmonary infections depends on their local concentrations in the lung. The aims of the present study were to: 1) compare technetium-99m diethylenetriaminepenta-acetic acid (99mTc-DTPA) and urea as markers of epithelial lining fluid (ELF) dilution for measuring ELF concentrations of pharmaceuticals; 2) quantify ELF cefepime concentrations in normal and injured lung; and 3) measure the increase in permeability to cefepime following oleic acid-induced acute lung injury. A modified bronchoalveolar lavage technique, based on equilibration of infused 99mTc-DTPA, was used to measure ELF volume. Cefepime was administered intravenously at steady plasma levels. Six serial bronchoalveolar lavages were performed 5 h after the beginning of infusion. ELF to plasma cefepime concentration ratios were 95 +/- 17 and 100 +/- 14.5% in normal and injured lung respectively. When urea was used as marker, cefepime concentration ratios were underestimated at 16.4 +/- 2.7 and 73.9 +/- 8.4% respectively. Cefepime blood/ airspace clearance increased from 3.8 +/- 0.7 micro x min(-1) in controls to 39.8 +/- 4.9 microL x min(-1) in acute lung injury. It was concluded that: 1) cefepime concentrations in epithelial lining fluid were in equilibrium with those in plasma in both normal and injured lung after 5 h at steady plasma concentrations; 2) epithelial lining fluid cefepime concentration by the urea method was much less underestimated in injured versus normal lung; and 3) acute lung injury induces a 10-fold elevation of cefepime blood/airspace clearance.

[Clinical applications of intravenous anaesthetics pharmacology: the example of hypnotics and opioids]

OBJECTIVE

To review the general principles of pharmacokinetics and pharmacodynamics models by focusing on intravenous anaesthetics (hypnotics and opioids).

DATA SOURCES

Medline references, lectures from the French congress of anaesthesiology and intensive care medicine, abstracts.

DATA SYNTHESIS

Pharmacokinetic and pharmacodynamic modelling allows simple estimation of becoming of anaesthetic drugs in the body, instead of classical pharmacologic approach. However, pharmacokinetic as well as pharmacodynamic parameters are often considered as resulting from complex mathematic approaches and remain then poorly used in practice by physicians. The aim of this article is to simply expose concepts underlying PK-PD models building and to explain significance of the main PK-PD parameters (first-order rate constants, k(e0), T(1/2)k(e0), T(peak), context-sensitive half-time, context-sensitive decrement times). Clinical consequences for using intravenous anaesthetic drugs (hypnotics and opioids) are exposed either during bolus injection or continuous infusion, when injected alone or co-administered.

Ceftriaxone (1 g intravenously) penetration into abdominal tissues when administered as antibiotic prophylaxis during nephrectomy

A pharmacokinetics study was conducted to determine the effects of ceftriaxone administered before nephrectomy on the occurrence of postoperative wound infection. Ceftriaxone was administered as antibiotic prophylaxis in 11 consecutive patients undergoing nephrectomy who received 1,000 mg intravenously 30 min before surgery. Simultaneous blood and tissue samples were collected at three stages of the surgical procedure: opening of the abdominal cavity, nephrectomy, and closure of the abdominal cavity. Samples of following tissues were assayed: abdominal-wall fat, perirenal fat, kidney cortex and medulla, and urine. During the different stages of surgical procedures, ceftriaxone concentrations remained higher than the MIC90 of the potential pathogens (Staphylococcus aureus, Escherichia coli, and Enterobacteriaceae). In selected patients undergoing nephrectomy and requiring antimicrobial prophylaxis such as malnourished, debilitated, diabetic or immunosuppressed patients, the use of a single dose of ceftriaxone (1,000 mg) makes it possible to offer an optimal bacterial coverage for the prevention of postoperative infection. According to our kinetic evaluation, ceftriaxone is potentially a good antibiotic for prophylaxis in urological surgery.

Multiple interactions of cimetidine and probenecid with valaciclovir and its metabolite acyclovir

The effects of probenecid and cimetidine on the pharmacokinetics of valaciclovir and its metabolite acyclovir have been investigated. Twelve healthy male volunteers participated in this open single-dose study with a four-way-crossover randomized and balanced design. At the first of four administrations, volunteers in four groups received 1 g of valaciclovir alone, valaciclovir with 1 g of probenecid, valaciclovir with 800 mg of cimetidine, or valaciclovir with a combination of probenecid and cimetidine. At three subsequent administrations, drug regimens were alternated among groups so that each group received each regimen. Probenecid and cimetidine increased the mean maximum concentrations in serum (C(max)) of valaciclovir by 23 and 53% and the areas under the concentration-time curves (AUC) for valaciclovir by 22 and 73%, respectively; probenecid and cimetidine also increased the mean acyclovir C(max) by 22 and 8% and its AUC by 48 and 27%, respectively. The combination had a greater effect than either drug alone. Their effects may be due to competitive inhibition of membrane transport of valaciclovir and acyclovir in the liver and kidney. Neither cimetidine nor probenecid affected the absorption of valaciclovir. Both probe drugs reduced the rate of valaciclovir metabolism but not its extent. These pharmacokinetic modifications did not affect the tolerability of valaciclovir.

Impact of pharmacokinetic-pharmacodynamic model linearization on the accuracy of population information matrix and optimal design

Influence of experimental design on hyperparameter estimates precision when performing a population pharmacokinetic-pharmacodynamic (PK-PD) analysis has been shown by several studies and various approaches have been proposed for optimizing or evaluating such designs. Some of these methods rely on the optimization of a suitable scalar function of the population information matrix. Unfortunately for the nonlinear models encountered in pharmacokinetics or pharmacodynamics the latter is particularly difficult to evaluate. Under some assumptions and after a linearization of the PK-PD model a closed form of this matrix can be obtained which considerably simplifies its calculation but leads to an approximation. The aim of this paper is to evaluate the quality of the latter and its potential impact, when comparing or optimizing population designs and to relate it to Bates and Watts curvature measures. Two models commonly used in PK-PD were considered and nominal hyperparameter values when chosen for each one. Several population designs were studied and the associated population information matrix was computed for each using the approximate procedure and also using a reference method. Design optimizations were calculated under constraints for each model from the reference and approximate population information matrix. Nonlinearity curvatures were also computed for every model and design. The impact of model linearization when calculating the population information matrix was then examined in terms of lower bound accuracies on the hyperparameter estimates, design criterion variation, as well as D-optimal population designs, these results being related to nonlinearity curvature measures. Our results emphasize the influence of the parameter effects curvature when deriving the lower bounds of the hyperparameter estimates precision for a given design from the approximate population information matrix especially for hyperparameters quantifying the PK-PD interindividual variability. No discrepancies were detected between the population D-optimal designs obtained from the approximate and reference matrix despite some minor differences in criterion variation with respect to the design. More pronounced differences were, however, observed when comparing the amplitudes of criterion variation which can lead to errors when calculating design efficiencies. From a practical point of view, a strategy easily applicable by the pharmacokineticist for avoiding such problems in the context of population design optimization or comparison is then proposed.

Co-amoxiclav pharmacokinetics during posttraumatic hemorrhagic shock

OBJECTIVE

To determine the effects of severe trauma with hemorrhagic shock on amoxicillin and clavulanate concentrations in plasma and their pharmacokinetics.

DESIGN

A prospective, open, descriptive study.

SETTING

A 12-bed, adult surgical intensive care unit in a university-affiliated hospital in France.

SUBJECTS

Subjects were 12 patients (10 men, 2 women) with severe trauma: median (range) Injury Severity Score, 38 (17-48); Acute Physiology and Chronic Health Evaluation II, 16 (7-38); Simplified Acute Physiology Score II, 41 (23-77). Also enrolled were 12 healthy volunteers who were matched on age (+/-5 yrs), gender, and body-surface area (+/-20 cm2). All the trauma patients suffered hemorrhagic shock defined as the association of at least one episode of systolic blood pressure <90 mm Hg and an intravascular volume expansion >2000 mL between trauma and surgery.

INTERVENTION

Prophylactic perioperative administration of 2 g of amoxicillin and 0.2 g of clavulanate in combination during the first 12 hrs posttrauma in patients, and at the start of the pharmacokinetic study in volunteers.

MEASUREMENTS AND MAIN RESULTS

Serial plasma samples (n = 13) were obtained after the first antibiotic administration to measure antibiotic levels by using high-performance liquid chromatography assays. Compared with volunteers, trauma patients had higher plasma amoxicillin and clavulanate concentrations, attributed to a reduction of the volume of distribution (p =.001 and p =.06, respectively) and, to a lesser extent, of the total body clearance (p =.09 and p =.20, respectively). Consequently, amoxicillin and clavulanate elimination half-lives were similar for the two groups of subjects. The interindividual variabilities for all the amoxicillin pharmacokinetic parameters were higher in patients.

CONCLUSIONS

In trauma patients with hemorrhagic shock requiring surgery, the administration of 2 g of amoxicillin and 0.2 g of clavulanate seems adequate, according to the antibiotic concentrations observed in plasma for both drugs. However, further studies exploring antibiotic concentrations in tissues are warranted.

Intestinal absorption and metabolism of chlorpheniramine enantiomers in rat

Chlorpheniramine (CPAM) is a chiral antihistaminic drug commercialized as a racemic mixture. The intestinal absorption and metabolism of CPAM have been investigated in rat using in vivo (oral and IV administration), in situ (intestinal loop model), and in vitro (everted sac model) experiments. Oral and IV administrations of 20 mg/kg of the racemic mixture show that the pharmacokinetics of CPAM are stereoselective, with higher AUCs for the (+)-S-enantiomer compared to its antipode. The monodesmethyl metabolite (DCPM) was quantifiable in blood and its pharmacokinetics are stereoselective after oral but not after IV administration. Experiments using intestinal loops and everted sacs showed that the absorption is not stereoselective and that in vivo stereoselective formation of DCPM is presumably due to stereoselective hepatic metabolism. Moreover, the in vitro and in situ absorption of CPAM are not modified by modulators of P-glycoprotein and cytochromes P450 (cyclosporin A, ketoconazole).

Sensitive and rapid method for the simultaneous quantification of the HIV-protease inhibitors indinavir, nelfinavir, ritonavir, and saquinavir in human plasma by reversed-phase liquid chromatography

A rapid, sensitive, and specific liquid chromatography method for the simultaneous determination of four protease inhibitors (indinavir, nelfinavir, ritonavir, and saquinavir) in human plasma is described. After a liquid-liquid extraction with terbutyl methyl ether and a sequential washing of the reconstituted sample with hexane, protease inhibitors are separated on a phenyl column using a simple binary mobile phase of ammonium acetate buffer:acetonitrile (48:52) (pH = 7.5) with an ultraviolet detection at 260 nm. The standard curves are linear in the range 0.025-1 microg/mL for saquinavir, 0.1-4 microg/mL for indinavir and nelfinavir, and 0.25-10 microg/mL for ritonavir, with an average recovery ranging from 79% to 99%, and with both low interday and intraday coefficients of variation (<15%). This assay is simple, rapid (15-minute interval between runs) , and useful for therapeutic monitoring of the protease inhibitors on a routine basis.

Pharmacokinetics of oral acyclovir in neonates and in infants: a population analysis

Acyclovir is approved for the treatment of herpes simplex virus (HSV) and varicella-zoster virus (VZV) infections in children by the intravenous and oral routes. However, its use by the oral route in children younger than 2 years of age is limited due to a lack of pharmacokinetic data. The objectives of the present study were to determine the typical pharmacokinetics of an oral suspension of acyclovir given to children younger than 2 years of age and the interindividual variabilities in the values of the pharmacokinetic parameters in order to support the proposed dosing regimen (24 mg/kg of body weight three times a day for patients younger than 1 month of age or four times a day otherwise). Children younger than age 2 years with HSV or VZV infections were enrolled in a multicenter study. Children were treated for at least 5 days with an acyclovir oral suspension. Plasma samples were obtained at steady state, before acyclovir administration, and at 2, 3, 5, and 8 h after acyclovir administration. Acyclovir concentrations were measured by radioimmunoassay. The data were analyzed by a population approach. Data for 79 children were considered in the pharmacokinetic study (212 samples, 1 to 5 samples per patient). Acyclovir clearance was related to the estimated glomerular filtration rate, body surface area, and serum creatinine level. The volume of distribution was related to body weight. The elimination half-life decreased sharply during the first month after birth, from 10 to 15 h to 2.5 h. Bioavailability was 0.12. The interindividual variability was less pronounced when the parameters were normalized with respect to body weight. Hence, dosage adjustment by body weight is recommended for this population. Simulations showed that the length of time that acyclovir remains above the 50% inhibitory concentration during a 24-h period was more than 12 h for HSV but not for VZV. The proposed dosing regimen seems adequate for the treatment of HSV infections, while for the treatment of VZV infections, a twofold increase in the dose seems necessary for children older than age 3 months.

Antienterococcal antibiotics

The treatment of severe enterococcal infections based on the currently available antibacterial agents is difficult. The help of the microbiology laboratory for determining MICs, MBCs, and most effective synergistic combinations is crucial. There is a need for good prospective multicenter clinical trials to improve the prognosis of such infections by defining therapeutic strategies better. Such a requirement is highly suitable for the treatment of infections caused by enterococci exhibiting acquired resistance mechanisms to the available agents. The current clinical development of new compounds looks promising in these persistently life-threatening infections mostly occurring in deficient hosts.

Ceftriaxone pharmacokinetics during iatrogenic hydroxyethyl starch-induced hypoalbuminemia: a model to explore the effects of decreased protein binding capacity on highly bound drugs

BACKGROUND

Although various drugs used by anesthesiologists highly bind to plasma proteins, the impact of iatrogenically induced hypoproteinemia on their pharmacologic effects has never been investigated. The authors determined the pharmacokinetics of ceftriaxone, a cephalosporin that binds strongly to albumin in postsurgical patients with hydroxyethyl starch-induced hypoalbuminemia.

METHODS

Eleven hypoalbuminemic (serum albumin < 25 g/l) patients and age (+/- 5 yr)-, sex-, and body surface area (+/- 10%)-matched healthy volunteers received a 2-g ceftriaxone dose infused over a 15-min period. Fourteen venous blood samples were collected during the 24-h study period. Free ceftriaxone concentrations were determined by ultrafiltration. Antibiotic concentrations in plasma and ultrafiltrate were measured by ion-paired reversed-phase chromatography. The pharmacokinetic parameters derived from total and free antibiotic concentrations were determined using a noncompartmental method. Data are expressed as median and range.

RESULTS

The pharmacokinetic parameters derived from total ceftriaxone concentrations were similar for the two groups, except for the median corrected volume of distribution at steady state, which was increased (P = 0.05) to 0.18 l/kg (range, 0. 11-0.29 l/kg) in patients, compared with 0.15 l/kg (range, 0.13-0.22 l/kg) in volunteers. The area under the free ceftriaxone concentration-time curve was twice as high in patients as in volunteers (median 192, range 114-301 vs. median 122, range 84-169 h. mg-1. l-1;P = 0.03). Moreover, the free ceftriaxone concentration remained more than 4 mg/l during more time in patients (median, 16. 7; range, 12.6-21.4 vs. median, 11.1; range, 6.0-19.0 h; P = 0.03).

CONCLUSIONS

Compared with healthy volunteers, patients with iatrogenic hypoalbuminemia have higher free ceftriaxone concentrations during the 24 h after antibiotic administration. This modification increases drug distribution into extravascular space and may enhance effectiveness.

Pharmacokinetics of cyclophosphamide (CP) and 4-OH-CP/aldophosphamide in systemic vasculitis

Cyclophosphamide given in association with corticosteroids has markedly improved the prognosis of systemic vasculitis. Little information has been reported on cyclophosphamide pharmacokinetics in these diseases and data evaluating its metabolite, 4-hydroxycyclophosphamide/aldophosphamide, pharmacokinetics and concentrations are lacking. Cyclophosphamide was administered as a 1-h intravenous infusion every 3 weeks for six cycles to ten vasculitis patients. Serum cyclophosphamide and 4-hydroxycyclophosphamide/aldophosphamide concentrations were assayed on the first cycle of the treatment by reversed-phase high-pressure liquid chromatography with ultraviolet detection. The mean (+/- SD) 4-hydroxycyclophosphamide/aldophosphamide and cyclophosphamide areas under the serum concentration-time curves were, respectively, 1.86 +/- 1.12 and 154.1 +/- 62.7 mg/L x h with a ratio of 1.30 +/- 0.76%. The mean maximum serum 4-hydroxycyclophosphamide/aldophosphamide was reached 2.3 h after cyclophosphamide administration. The mean (+/- SD) cyclophosphamide and 4-hydroxycyclophosphamide/aldophosphamide half-lives were, respectively, 5.5 +/- 3.1 and 7.6 +/- 2.3 h. The results are consistent with those obtained for cancer patients, in spite of a wide interpatient variability of concentrations and pharmacokinetic parameters.

Clinical pharmacokinetics and pharmacodynamics of isepamicin

Isepamicin is an aminoglycoside antibacterial with properties similar to those of amikacin, but with better activity against strains producing type I 6'-acetyltransferase. The antibacterial spectrum includes Enterobacteriaceae and staphylococci. Anaerobes, Neisseriaceae and streptococci are resistant. The lower and upper break-points are 8 and 16 mg/L. Like other aminoglycosides, isepamicin exhibits a strong concentration-dependent bactericidal effect, a long post-antibiotic effect (several hours) and induces adaptive resistance. Isepamicin is administered intravenously or intramuscularly at a dosage of 15 mg/kg once daily or 7.5 mg/kg twice daily. Isepamicin is not bound to plasma proteins, and it distributes in extracellular fluids and into some cells (outer hair cells, kidney cortex) by active transport. Isepamicin is not metabolised and is eliminated solely via the renal route with an elimination half-life (t 1/2 beta) of 2 to 3 hours in adults with normal renal function. The clearance of isepamicin is reduced in neonates, and 7.5 mg/kg once daily is recommended in children <16 days old. Clearance is also reduced in the elderly, but no dosage adjustment is required. In patients with chronic renal impairment, isepamicin clearance is proportional to creatinine clearance (CLCR); the recommended regimen is 8 mg/kg with an administration interval of 24 hours in moderate impairment, 48 hours in severe impairment, 72 hours for CL(CR) 0.6 to 1.14 L/h (10 to 19 ml/min) and 96 hours for CL(CR) 0.36 to 0.54 L/h (6 to 9 ml/min). In end-stage renal failure, isepamicin is eliminated by haemodialysis, but the administration interval should be determined by monitoring the plasma concentration. Compared with healthy volunteers, patients in the intensive care unit or with neutropenic cancer have an increased volume of distribution and a lower clearance, but the 15 mg/kg once daily regimen remains adequate. Isepamicin kinetics are linear in the range 7.5 to 25 mg/kg, so that dosage adjustments, if necessary, are straightforward. Isepamicin can induce nephro-, vestibulo- and oto-toxicity. However, animal and clinical studies show that isepamicin is one of the less toxic aminoglycosides. The usefulness of maintaining serum aminoglycoside concentrations within a therapeutic range remains controversial. With isepamicin, it is proposed to achieve a 1-hour concentration (30 minutes after a 30-minute infusion) >40 mg/L to maximise bactericidal efficacy, and a 'trough' concentration (at the end of the administration interval) <5 mg/L to minimise toxicity. These thresholds should be modified on an individual basis, considering covariates such as concomitant treatment, underlying disease, nature of bacterial strain and site of infection.