Predictability of individualized dosage regimens of carbamazepine and valproate mono- and combination therapy

WHAT IS KNOWN AND OBJECTIVE

Many investigators agree that appropriate rational utilization of therapeutic drug monitoring (TDM) with Bayesian feedback dosage adjustment facilitates epilepsy treatment with carbamazepine (CBZ) and/or valproate (VPA) by increasing the seizure control and safety, as well as by reducing treatment costs. In previous works we have developed and used in clinical practice population pharmacokinetic (PK) models of different dosage forms for VPA and post-induction CBZ behaviour, as well as for combined therapy with CBZ plus another 'old' antiepileptic drug (AED). An important step of external validation is to evaluate how well a procedure of Bayesian individualizing AED dosage regimens based on a proposed population PK model and sparse TDM data 'works', and how helpful it is in real practical clinical settings. The aim of this study was to evaluate the predictability of individualized dosage regimens for monotherapy with CBZ in the post-induction period or with VPA, as well as for CBZ and VPA given as combination therapy based on TDM data of epileptic patients and the earlier developed population models.

METHODS

Four groups of TDM data were analysed using the USC*PACK software for PK/PD analysis: 556 predictions for adult epileptic patients on CBZ monotherapy, 662 predictions for VPA monotherapy, 402 predictions of CBZ serum levels and 430 predictions of VPA serum levels for adult epileptic patients on CBZ+VPA combination therapy. Statistical characteristics of the prediction errors (PE) and weighted PE were used to estimate bias and precision of predictions. Intraindividual and interoccasional variability of predictions were also estimated.

RESULTS AND DISCUSSION

This study demonstrated that in most cases of CBZ and VPA monotherapy and combination therapy, predictions of future AED concentrations based on the earlier developed population PK models, TDM data and patient-specific maximum a posteriori probability Bayesian posterior parameter values provided clinically acceptable estimates. Statistical analysis of the residuals demonstrated that the distributions of residual and weighted residual were close to the normal distribution (Kolmogorov-Smirnov test, P > 0·05) and their mean values did not differ statistically significant from zero (no statistically significant bias, P > 0·05) for all groups of predictions. The observed decreased quality of predictions of VPA concentrations during VPA+CBZ combination therapy, especially when CBZ dosages were changed, might well be explained by their PK interactions. For all groups, in linear regression analysis, the observed trend of decreasing of the prediction quality over various future prediction time horizons was considered statistically significant (P < 0·05). Prediction of serum levels further in future was less precise than those closer to the present for a 1·5- to 3·5-year observation period. No bias in predictions was associated with the time horizons.

WHAT IS NEW AND CONCLUSION

Our validation results suggest good predictive performance of the population models developed earlier, and quite acceptable predictions of future AED serum levels for individualized dosage regimens of CBZ and VPA therapy in real clinical settings.

Population pharmacokinetic modelling of carbamazepine in epileptic elderly patients: implications for dosage

BACKGROUND

Proper use of antiepileptic drugs in the elderly involves knowledge of their pharmacokinetics to ensure a patient-specific balance between efficacy and toxicity. However, populations of epileptic patients on chronic carbamazepine (CBZ) therapy which have been studied have included data of relatively few elderly patients.

AIMS

The aim of the present study was to evaluate the population pharmacokinetics of CBZ in elderly patients on chronic monotherapy.

METHODS

We have used the non-parametric expectation maximization (NPEM) program in the USC*PACK collection of PC programs to estimate individual and population post-induction pharmacokinetics of CBZ in epileptic elderly patients who received chronic CBZ monotherapy. Age-related changes of CBZ population pharmacokinetics were evaluated from routine therapeutic drug monitoring (TDM) data of 37 elderly and 35 younger patients with epilepsy. As a 'historical control' we used previously published population modelling results from 99 young epileptic patients on chronic CBZ monotherapy. In that control group, TDM was performed in the same pharmacokinetic (PK) laboratory, using the same sampling strategy as in the present study, and the same PK population modelling software was used for data analysis.

RESULTS AND CONCLUSIONS

A poor correlation was found between daily CBZ dose and serum concentrations in the elderly patients (r=0.2, P=0.25). Probably statistically significant difference in the median values of the CBZ metabolic rate constant (P<0.001) between elderly and relatively young epileptic patients was found. Our results showed that age-related influences in CBZ pharmacokinetics in elderly patients should be considered in the optimal planning of CBZ dosage regimens. Most elderly patients with epilepsy will usually need CBZ dosages lower than those based on the median population PK parameter values obtained from younger patients. The present population model is also uniquely well suited for the new 'multiple model' design of dosage regimens to hit target therapeutic goals with maximum precision.

Pharmacokinetics of ethambutol in children and adults with tuberculosis

SETTING

Five hospitals in the United States.

OBJECTIVE

To describe ethambutol pharmacokinetics in children and adults with active tuberculosis (TB).

DESIGN

Prospective, open-labeled study in 56 adults and 14 children with active tuberculosis who received ethambutol as part of their multidrug TB regimens.

RESULTS

Most serum samples were collected up to 10 h post dose and assayed using a validated gas chromatography assay with mass selective detection (GC/MS). Concentration data were analyzed using non-compartmental and population pharmacokinetic methods. Drug exposure increased with dose, but less than proportionally at doses >3000 mg. Lower than expected maximum serum concentrations (Cmax <2 microg/ml) were common in adults. Very low Cmax (<1 microg/ml) were common in children, as was delayed absorption (time to Cmax >3 h). Many Cmax were at or below typical TB minimal inhibitory concentrations. Cmax values for HIV-positive patients were 20% lower than HIV-negative patients with daily doses, but were similar with larger twice-weekly doses.

CONCLUSIONS

Adult TB patients often had lower than expected ethambutol serum concentrations, and most pediatric TB patients had very low ethambutol serum concentrations. Higher doses and therapeutic drug monitoring may be indicated for many of these patients.

Population pharmacokinetic modelling of tramadol with application of the NPEM algorithms

BACKGROUND AND OBJECTIVE

Although the kinetic behaviour of tramadol has been described, the present study is the first to our knowledge, to report specifically on the population pharmacokinetic modelling of tramadol hydrochloride.

METHODS

The parametric Iterative Two-stage Bayesian Population Model (IT2B) program followed by the Non-parametric Expectation Maximization Population Model (NPEM2) program was used to determine population pharmacokinetic parameter values of tramadol in 138 postoperative orthopaedic Malaysian patients. All patients had received a 100 mg intravenous dose of tramadol, infused over 2-3 min, as their first postoperative analgesic. Blood was sampled at 0 min and subsequently at 15, 30 min, 1, 2, 4, 8, 16, 20 and 24 h for serum tramadol high-performance liquid chromatography analysis.

RESULTS AND DISCUSSION

The one-compartmental model pharmacokinetic parameters--volume of distribution (Vd), elimination rate constant (kel) and the total clearance rates (ClT)--found were: mean Vd = 167.6 +/- 63.84 L; median Vd = 161.48 L; mean kel = 0.1241 +/- 0.056 h(-1); median kel = 0.1138 h(-1); ClT = 19.57 +/- 9.51 L/h; median ClT =18.12 L/h. The interindividual coefficient of variation of ClT (48.56%) was higher than that of Vd (38.09%), indicating the presence of other possible influencing factors on tramadol's ClT such as CYP2D6 polymorphism, gender and age. Overall, NPEM2 suggested more diversity in the population than did IT2B.

Clinical and pharmacological risk factors for acute graft-versus-host disease after paediatric bone marrow transplantation from matched-sibling or unrelated donors

The aim of this study was to identify the risk factors for acute graft-versus-host disease (aGVHD) in children transplanted from a matched-sibling donor (MSD) or an unrelated donor (UD). In all, 87 children consecutively underwent allogeneic bone marrow transplantation (BMT) from MSD (n=36), and UD (n=51). GVHD prophylaxis included CsA alone (n=33) or with MTX (n=51). ATG was added in UD-BMT and thalassemic recipients. CsA whole-blood concentrations were measured by EMIT and the dosing regimen was monitored by Bayesian pharmacokinetic modelling. Trough blood concentration (TBC) during the first 2 weeks post transplantation was lower in children who developed grade II-IV aGVHD than those developing no GVHD or only grade I (57+/-9 vs 94+/-8 ng/ml, P=0.007), whereas peak blood concentration and area under concentration curve vs time were similar in both groups. TBC <85 ng/ml and 'use of MTX' were associated with aGVHD in MSD-SCT (P=0.003 and 0.007, respectively) as well as in UD-SCT (P=0.006 and 0.003). Donor age >or=8 years was significant only in MSD-BMT. Our results have shown the significant decisive role of pharmacological factors such as CSA TBC or use of MTX in the occurrence of GVHD in MSD as well as in UD paediatric BMT.

Nonparametric population modeling of valproate pharmacokinetics in epileptic patients using routine serum monitoring data: implications for dosage

Therapeutic drug monitoring (TDM) of valproate (VAL) is important in the optimization of its therapy. The aim of the present work was to evaluate the ability of TDM using model-based, goal-oriented Bayesian adaptive control for help in planning, monitoring, and adjusting individualized VAL dosing regimens. USC*PACK software and routine TDM data were used to estimate population and individual pharmacokinetics of two commercially available VAL formulations in epileptic adult and pediatric patients on chronic VAL monotherapy. The population parameter values found were in agreement with values reported earlier. A statistically significant (P < 0.001) difference in median values of the absorption rate constant was found between enteric-coated and sustained-release VAL formulations. In our patients (aged 0.25-53 years), VAL clearance declined with age until adult values were reached at about age 10. Because of the large interindividual variability in PK behavior, the median population parameter values gave poor predictions of the observed VAL serum concentrations. In contrast, the Bayesian individualized models gave good predictions for all subjects in all populations. The Bayesian posterior individualized PK models were based on the population models described here and where most patients had two (a peak and a trough) measured serum concentrations. Repeated consultations and adjusted dosage regimens with some patients allowed us to evaluate any possible influence of dose-dependent VAL clearance on the precision of total VAL concentration predictions based on TDM data and the proposed population models. These nonparametric expectation maximization (NPEM) population models thus provide a useful tool for planning an initial dosage regimen of VAL to achieve desired target peak and trough serum concentration goals, coupled with TDM soon thereafter, as a peak-trough pair of serum concentrations, and Bayesian fitting to individualize the PK model for each patient. The nonparametric PK parameter distributions in these NPEM population models also permit their use by the new method of 'multiple model' dosage design, which allows the target goals to be achieved specifically with maximum precision. Software for both types of Bayesian adaptive control is now available to employ these population models in clinical practice.

Relationship between CsA trough blood concentration and severity of acute graft-versus-host disease after paediatric stem cell transplantation from matched-sibling or unrelated donors

In order to determine optimal CsA trough blood concentrations (TBC) in the early post transplantation period, we analysed relationships between TBC and acute graft-versus-host disease (aGVHD) in paediatric SCT. A total of 94 children consecutively underwent allogeneic stem cell transplantation (SCT) from: matched-sibling (MSD) (n=36), mismatched-related (MMRD) (n=3) and unrelated donors (UD) (n=55). GVHD prophylaxis usually included CsA alone or with methotrexate. Antithymocyte globulin was added in UD-SCT. TBC during the first weeks of post transplantation were estimated retrospectively by a Bayesian pharmacokinetic method and statistically associated with aGVHD. In MSD-SCT, the mean TBC during the first 2 weeks post transplantation were 42+/-10 and 90+/-7 ng/ml, respectively, in patients with grade II-IV and 0-I aGVHD (P=0.001). In SCT from UD and MMRD, TBC were 73+/-4 vs 95+/-8 ng/ml (P=0.284). For TBC >85 ng/ml, no patient developed grade II-IV aGVHD, 10 developed mild aGVHD and 30 had no aGVHD. For TBC <65 ng/ml, 7/11 patients receiving an MSD-SCT and 4/18 receiving an UD- or MMRD-SCT developed grade II-IV aGVHD. The mean TBC corresponding to each grade were: no GVHD: 101+/-10 ng/ml, mild: 77+/-11 ng/ml, moderate: 61+/-13 ng/ml, severe: 56+/-15 ng/ml (P <0.001). These results reveal a strong relationship between TBC during the early post transplantation period and the severity of aGVHD in paediatric SCT.

[PK/PD modeling of aminoglycoside nephrotoxicity]

Aminoglycosides are bactericidial antibiotics with a serum concentration-dependent activity. They are mainly eliminated by the kidneys and the main difficulty arising in clinical use is their uptake by the renal cortex which leads to nephrotoxicity. An ototoxicity is also reported. We propose a PK/PD modelling of aminoglycoside nephrotoxicity which unifies more fourty years of physiological knowledge. This deterministic model successively describes the pharmacokinetics of aminoglycosides, their storage into renal cortex, their effect on renal cells, their consequences on the renal function through tubuloglomerular feedback and the changes in the serum concentrations of creatinine that is considered as a toxicity marker. The simulation of the model displays the leading effect of the shape and daily-time of administration schedule on the search for minimizing toxicity.

Distribution and elimination of tobramycin administered in single or multiple daily doses in adult patients with cystic fibrosis

Aminoglycosides are often prescribed as part of the treatment regimen for acute pulmonary exacerbations due to their potent activity and low potential for development of resistance. Preliminary evidence from randomized controlled trials in patients with cystic fibrosis (CF) suggests that once-daily administration of aminoglycosides results in similar efficacy and a low risk for toxicity compared with traditional dosing. The pharmacokinetics of aminoglycosides administered once daily in CF patients are currently not well described. In this study we compare the distribution and elimination patterns of traditional dosing (3.3 mg/kg q8h) versus once-daily dosing (10 mg/kg q24h) of tobramycin in six adult patients with CF. The pharmacokinetics of tobramycin administered either once daily or every 8 h were best described by a two-compartment model. No statistically significant differences in any of the pharmacokinetic parameter values between regimens were noted. The distribution phase half-lives of 32 and 24 min following the q8h and q24h regimens were longer than expected. The use of a one-compartment model requires clinical peak levels to be drawn 2 h after initiation of either a 30 min infusion for multiple daily dosing or a 60 min infusion with once-daily dosing, to ensure completion of the distribution phase. Our data indicate that a dose of 10 mg/kg/day provides post-distributional phase peak concentrations that achieve the desired goal for susceptible organisms (>20 mg/L) and AUC(24) values at the upper end of the desired range (70-100 mg.h/L).

Improved clinical outcome of paediatric bone marrow recipients using a test dose and Bayesian pharmacokinetic individualization of busulfan dosage regimens

In order to control busulfan pharmacokinetic variability and toxicity, a specific monitoring protocol was instituted in our bone marrow transplant BMT paediatric patients including a test dose, daily Bayesian forecasting of busulfan plasma levels, and Bayesian individualization of busulfan dosage regimens. Twenty-nine children received BMT after a busulfan-based conditioning regimen. Individual pharmacokinetic parameters were obtained following a 0.5 mg*kg test dose and were used for daily individualization of dosage regimens during the subsequent 4-day course of treatment. Doses were adjusted to reach a target mean AUC per 6 h between 4 and 6 microg.h.ml(+1). Plasma busulfan assays were performed by liquid chromatography. Pharmacokinetic analysis used the USC*PACK software. The performance of the test dose to predict AUC during the busulfan regimen was evaluated. Incidence of toxicity, chimerism and relapse, overall Kaplan-Meier survival, and VOD-free survival were compared after matching our patients (group A) with patients conditioned by using standard doses of busulfan (group B). Busulfan doses were decreased in 69% of patients compared to conventional doses. Expected AUC was significantly correlated with observed AUC and predictability of the test dose was 101.9 +/- 17.9%. Incidence of VOD in group A was 3.4% vs 24.1% in group B, while the incidence of stomatitis was similar. Engraftment was successful in all patients in group A. The rate of full engraftment at 3 months post-BMT was higher in group A (P = 0.012). Long-term overall survival did not differ between the two groups, in contrast to the 90-day survival. VOD-free survival was higher in group A (P = 0.026). Pharmacokinetic monitoring and individualization of busulfan dosage regimen are useful in improving clinical outcome and reducing early mortality in paediatric bone marrow transplant recipients.

Population pharmacokinetics of intravenous and intramuscular streptomycin in patients with tuberculosis

STUDY OBJECTIVES

To determine population pharmacokinetic parameters of streptomycin after administration of multiple intramuscular and intravenous doses.

DESIGN

Prospective, unblinded clinical study.

SETTING

Two medical centers in Denver, Colorado.

PATIENTS

Thirty patients with tuberculosis.

INTERVENTION

Patients received multiple doses of streptomycin as part of their tuberculosis treatment. They received concurrent drugs based on in vitro susceptibility data.

MEASUREMENTS AND MAIN RESULTS

Serum samples were collected over a 10-hour period and assayed by validated high-performance liquid chromatography Concentration-time data were analyzed using population methods. Streptomycin concentrations increased linearly with increasing intravenous doses. The intramuscular doses did not produce as linear a relationship, presumably because of variability in rates of and, potentially, completeness of absorption. Streptomycin elimination decreased with declining renal function. Higher, intermittent doses were well tolerated and appeared to maximize the peak concentration:minimal inhibitory concentration ratio.

CONCLUSION

Overall, pharmacokinetic parameters of streptomycin were comparable with those previously published for streptomycin and other aminoglycosides. Higher, intermittent doses maximize pharmacodynamic parameter estimates and might have advantages for treatment of tuberculosis.

Population pharmacokinetics of amikacin at birth and interindividual variability in renal maturation

OBJECTIVE

Pharmacokinetic (PK) interindividual variability in amikacin has been shown to be wide in neonates. This study evaluated the evolution of this variability with gestational age (GA) at birth in relation to renal maturation.

METHODS

Population PK values of amikacin were studied in 131 newborns (postnatal age 1 day, GA 24-41 weeks) divided into 16 groups, defined by GA, from 24 to 41 weeks (with a mean of 8.2 infants per group). PK variables were Kel/Vol, Ks/Vs, Cl/Vol. Cls/ where: Kel = Kslope x GA + Kintercept, Cl = Clslope x GA + Clintercept, and Vol = Vs x body weight. Ki and Cli were held as constants. The nonparametric distribution of the probability density function (PDF) was obtained, as were mean, median, and SD values of each PK variable for each GA group.

RESULTS

Amikacin elimination increased linearly with GA, showing that GA is a good covariate of renal elimination. Amikacin volume of distribution increased with body weight up to a GA of about 38 weeks and then decreased for highest GA values. However, the PDF for the individual GA groups showed a multimodal PK distribution. Kel, Vol, Vs, Cl, and Cl, standard deviations increased linearly with GA, showing differential renal maturation. The higher the GA, the more interindividual PK variability increased.

CONCLUSIONS

These results show that amikacin elimination and the volume of distribution are dependent upon GA, and that differential renal maturation in neonates is responsible for the wider PK interindividual variability with high GA. Dosage regimens of amikacin and other aminoglycosides should be revised in newborns with high GA. Bayesian adaptive control of therapeutics might be particularly indicated to obtain efficacy for each neonate as early as the first dose.

Once-a-day individualized amikacin dosing for suspected infection at birth based on population pharmacokinetic models

Amikacin is widely used in the treatment of suspected or confirmed neonatal infections. However, dosage regimens are not well defined in this group of patients because of a wide inter-individual pharmacokinetic variability. An individualized goal-oriented amikacin dosage design was applied using population pharmacokinetic data. A dosing chart was developed for neonates during the first 2 days of life, by using population pharmacokinetic parameter values and USCPACK software. This dosing chart based on gestational age (GA) and body weight gives a once-a-day amikacin dosage regimen involving an injection every 24 h. Validation was performed in 57 neonates less than 2 days old, divided into three GA groups and prospectively treated using the dosing chart. Target peak serum levels of amikacin were obtained in 62-80% of patients after the first dose and in 80-100% after the second dose, and trough concentrations were obtained in 100%. This study has confirmed the need for individualization of amikacin dosage regimens in neonates.

Population pharmacokinetic modelling of carbamazepine by using the iterative Bayesian (IT2B) and the nonparametric EM (NPEM) algorithms: implications for dosage

OBJECTIVE

To estimate individual and population postinduction pharmacokinetics of carbamazepine (CBZ) in epileptic adult and paediatric patients who received chronic CBZ monotherapy.

METHODS

We have used the USC*PACK collection of PC programs for the estimations. The preinduction CBZ metabolism was also estimated in 16 volunteers after a single dose of CBZ (200 mg). We used a linear one-compartmental model with oral absorption and found the pharmacokinetic parameter values of CBZ behaviour to be in good agreement with those reported earlier.

RESULTS

Serum CBZ concentrations correlated poorly with daily doses in both the adult and child populations. Because of the diversity within the population, use of the mean population model without knowledge of an individual patient's pharmacokinetic characteristics gives poor prediction. In contrast, the individual Bayesian posterior models gave good prediction for all subjects in the population, due to the removal of the interindividual variability.

CONCLUSION

This approach permits one to individualize drug therapy for patients even when only sparse therapeutic drug monitoring (TDM) data are available. Future individual CBZ serum level predictions were acceptable from a clinical point of view (mean absolute error = 13.2 +/- 9.7%). The optimal sampling strategy approach helped to design an optimal cost-effective TDM protocol for CBZ therapy management.

Mixed pharmacokinetic population study and diffusion model to describe ciprofloxacin lung concentrations

The pharmacokinetics of ciprofloxacin in plasma and lung tissue at steady-state (500 mg b.i.d.) were studied in 38 patients subjected to lung surgery for bronchial epithelioma. The mean characteristics of the patient population were: age = 60 years (range: 48-70), weight=70kg (47-95), height=165cm (range 160-170) and serum creatinine=85microM (range 62-168). Plasma samples, two for each patient and lung samples, one for each patient, were obtained and analyzed. Seven groups were made according to the time of sampling after ingestion of the 5th dose. A three-compartment model was used to describe ciprofloxacin kinetics in plasma and lung. The non-linear mixed effect model approach was used to estimate the mean and variance of the pharmacokinetic parameters. The mean +/- SD of the estimates (coefficient of variation of interindividual variability as a percentage) were central volume of distribution, 39.45+/-52.47l(133%); steady-state volume of distribution, 145.86+/-97.51l(60%), clearance of influx into lung tissue, 35.83+/-22.57l/h(63%), extrapolated elimination rate constant, 0.173+/-0.25/h and extrapolated elimination half-life, 4.02+/-0.89h. The mean +/- SD ciprofloxacin concentration versus time curve in plasma and lung at steady state was simulated using pharmacokinetic parameters and lung physiological parameters, another approach was studied to model the transport of ciprofloxacin into the lung tissue by diffusion. Ciprofloxacin concentration-time history was obtained both by experiments or simulations. The ciprofloxacin level in the lung tissue followed the ciprofloxacin plasma level with a lag time resulting from the time necessary for ciprofloxacin to diffuse through the lung.

Failure of traditional trough levels to predict tacrolimus concentrations

The objective of this study was to estimate tacrolimus population parameter values and to evaluate the ability of the maximum a posteriori probability (MAP) Bayesian fitting procedure to predict tacrolimus blood levels, using the traditional strategy of monitoring only trough levels, for dosage individualization in liver transplant patients. Forty patients treated with tacrolimus after liver transplantation were studied during the early posttransplant phase (first 2 weeks). This phase was divided into four time periods (1-4 days, 5-7 days, 8-11 days, 12-14 days). Tacrolimus was administered twice daily. Approximately one determination of a tacrolimus trough level on whole blood was performed each day. The NPEM2 program was used to obtain population pharmacokinetic parameter values. With each individual pharmacokinetic parameter estimated by the MAP Bayesian method for a given period, the authors evaluated the prediction of future levels of tacrolimus for that patient for the next period. This evaluation of Bayesian fitting predictive performance was performed using the USC*PACK clinical software. Mean pharmacokinetic parameter values were in the same general range as previously published values obtained with richer data sets. However, during each period, the percentage of blood levels predicted within 20% did not exceed 40%. The traditional strategy of obtaining only trough whole blood levels does not provide enough dynamic information for the MAP Bayesian fitting procedure (the best method currently available) to be used for adaptive control of drug dosage regimens for oral tacrolimus. The authors suggest modifying the blood concentration monitoring scheme to add at least one other concentration measured during the absorptive or distributive phase to obtain more information about the behavior of the drug. D-Optimal design and similar strategies should be considered.

Pharmacokinetics of tobramycin in adults with cystic fibrosis: implications for once-daily administration

Once-daily administration of aminoglycosides is routinely used in many institutions. However, comparative efficacy data for patients with cystic fibrosis (CF) are lacking. The purpose of the present study was to compare the predicted pharmacodynamic activity of tobramycin at 10 mg/kg of body weight/day administered every 24 h (q24h), q12h, and q8h. Pharmacokinetic (PK) data were derived from analysis of data on the drug concentration in sera from 60 adult CF patients. Individual maximum a posteriori probability Bayesian PK parameter values were used to construct serum concentration-versus-time curves and to determine various indices (peak concentration/MIC ratio [peak/MIC], area under the concentration-time curve/MIC ratio [AUC/MIC], and time that the concentration was less than the MIC [T<MIC]) for the three regimens described above. MICs of 1, 2, and 4 microg/ml for Pseudomonas aeruginosa were assumed in the simulations. Irrespective of the MIC, significantly lower peak/MIC but shorter T<MIC were noted when regimens of q8h versus q12h (P < 0.001), q8h versus q24h (P < 0.001), and q12h versus q24h (P < 0.001) were compared. This analysis suggests that the potential benefit of achieving a greater peak/MIC with once-daily aminoglycoside administration may be offset by the significantly greater T<MIC in CF patients compared with that achieved with multiple-daily-dosing regimens. Clinical trials are necessary to determine if once daily aminoglycoside administration is efficacious in the CF population before its routine use can be recommended.

Pharmacokinetics of ethambutol under fasting conditions, with food, and with antacids

Ethambutol (EMB) is the most frequent "fourth drug" used for the empiric treatment of Mycobacterium tuberculosis and a frequently used drug for infections caused by Mycobacterium avium complex. The pharmacokinetics of EMB in serum were studied with 14 healthy males and females in a randomized, four-period crossover study. Subjects ingested single doses of EMB of 25 mg/kg of body weight under fasting conditions twice, with a high-fat meal, and with aluminum-magnesium antacid. Serum was collected for 48 h and assayed by gas chromatography-mass spectrometry. Data were analyzed by noncompartmental methods and by a two-compartment pharmacokinetic model with zero-order absorption and first-order elimination. Both fasting conditions produced similar results: a mean (+/- standard deviation) EMB maximum concentration of drug in serum (Cmax) of 4.5 +/- 1.0 micrograms/ml, time to maximum concentration of drug in serum (Tmax) of 2.5 +/- 0.9 h, and area under the concentration-time curve from 0 h to infinity (AUC0-infinity) of 28.9 +/- 4.7 micrograms.h/ml. In the presence of antacids, subjects had a mean Cmax of 3.3 +/- 0.5 micrograms/ml, Tmax of 2.9 +/- 1.2 h, and AUC0-infinity of 27.5 +/- 5.9 micrograms.h/ml. In the presence of the Food and Drug Administration high-fat meal, subjects had a mean Cmax of 3.8 +/- 0.8 micrograms/ml, Tmax of 3.2 +/- 1.3 h, and AUC0-infinity of 29.6 +/- 4.7 micrograms.h/ml. These reductions in Cmax, delays in Tmax, and modest reductions in AUC0-infinity can be avoided by giving EMB on an empty stomach whenever possible.

Pharmacokinetics of pyrazinamide under fasting conditions, with food, and with antacids

STUDY OBJECTIVES

To determine intrasubject and intersubject variability in, and the effects of food and antacids on, the pharmacokinetics of pyrazinamide (PZA).

DESIGN

Randomized, four-period, crossover phase I study.

SUBJECTS

Fourteen healthy men and women volunteers.

INTERVENTIONS

Subjects ingested single doses of PZA 30 mg/kg under fasting conditions twice, without a high-fat meal and with an aluminum-magnesium antacid. They also received standard dosages of isoniazid, rifampin, and ethambutol.

MEASUREMENTS AND MAIN RESULTS

Serum was collected for 48 hours and assayed by gas chromatography with mass selective detector. Data were analyzed by noncompartmental methods and a compartmental analysis using nonparametric expectation maximization. Both fasting conditions produced similar results: mean PZA Cmax 53.4+/-10.4 microg/ml, Tmax 1.43+/-1.06 hours, and AUC(0-infinity) 673+/-79.7 microg x hr/ml. Fasting results are similar to those in previous reports. In the presence of antacids, subjects had a mean Cmax of 55.6+/-9.0 microg/ml, Tmax of 1.43+/-1.23 hours, and AUC(0-infinity) of 628+/-88.4 microg x hr/ml. In the presence of the high-fat meal, mean Cmax was 45.6+/-9.44 pg/ml, Tmax 3.09+/-1.74 hours, and AUC(0-infinity) 687+/-116 microg x hr/ml.

CONCLUSIONS

These small changes in Cmax, Tmax, and AUC(0-infinity) can be avoided by giving PZA on an empty stomach whenever possible.

Comparisons between antimicrobial pharmacodynamic indices and bacterial killing as described by using the Zhi model

Various suggestions have been made for empirical pharmacodynamic indices of antibiotic effectiveness, such as areas under the drug concentration-time curve in serum (AUC), AUC > MIC, AUC/MIC, area under the inhibitory curve (AUIC), AUC above MIC, and time above MIC (T > MIC). In addition, bacterial growth and killing models, such as the Zhi model, have been developed. The goal of the present study was to compare the empirical behavior of the Zhi model of bacterial growth and killing with the other empirical pharmacodynamic indices described above by using simulated clinical data analyzed with the USC*PACK PC clinical programs for adaptive control of drug therapy, with one model describing a concentration-dependent antibiotic (tobramycin) and another describing a concentration-independent antibiotic (ticarcillin). The computed relative number of CFU was plotted against each pharmacodynamic index, with each axis parameterized over time. We assumed that a good pharmacodynamic index should present a clear and continuous relationship between the time course of its values and the time course of the bacterial killing as seen with the Zhi model. Preliminary work showed that some pharmacodynamic indices were very similar. A good sensitivity to the change in the values of the MIC was shown for AUC/MIC and also for T > MIC. In addition, the time courses of some other pharmacodynamic indices were very similar. Since AUC/MIC is easily calculated and shows more sensitivity, it appeared to be the best of the indices mentioned above for the concentration-dependent drug, because it incorporated and used the MIC the best. T > MIC appeared to be the best index for a concentration-independent drug. We also propose a new composite index, weighted AUC (WAUC), which appears to be useful for both concentration-dependent and concentration-independent drugs.

A population pharmacokinetic model of cyclosporine in the early postoperative phase in patients with liver transplants, and its predictive performance with Bayesian fitting

The availability of personal computer programs to individualize drug regimens has stimulated interest in modeling population pharmacokinetics. This study used the NPEM2 software to determine cyclosporine population pharmacokinetic parameter values and distributions in a first group of 25 recipients of liver transplants during their first postoperative week. On a second group of 25 patients, the authors used these values to evaluate Bayesian predictive performance of cyclosporine blood concentrations with the USC*PACK PC program. During the study period, all the patients have been treated by continuous intravenous infusion. The one-compartment model pharmacokinetic parameter-the slope of volume to body weight (Vs) and the elimination rate constant (Kel) values found (mean values: Vs = 2.177 l/kg, Kel = 0.235 h(-1); median values: Vs = 1.559 l/kg, Kel = 0.163 h(-1); the percent coefficient of variation (Vs = 92%, Kel = 79%) appear reasonable and show the ability of NPEM2 to deal with sparse data. When the predictions were studied with day 1, day 2, or day 3 concentrations, predictive bias was respectively -0.030, -0.013, and 0.013 microg/ml, suggesting a greater clearance of cyclosporine immediately after surgery, the clearance decreasing in the days after. With the first three blood levels and the Bayesian fitting procedure, it was possible to predict at least half the subsequent measured blood levels of each patient accurately (within 20%) in more than three-quarters (76%) of the second group of recipients of transplants, and for 40% of patients the authors obtained accurate predictions in 100% of the subsequent blood levels. For a few patients (12%) they found quite poor predictions. The reason for this is unclear. The results suggest that this population model and the Bayesian fitting procedure using two or three blood levels can be reasonably and carefully used to control, in real time, cyclosporine blood levels in a majority of new patients with liver transplants.

Model-based, goal-oriented, individualised drug therapy. Linkage of population modelling, new 'multiple model' dosage design, bayesian feedback and individualised target goals

This article examines the use of population pharmacokinetic models to store experiences about drugs in patients and to apply that experience to the care of new patients. Population models are the Bayesian prior. For truly individualised therapy, it is necessary first to select a specific target goal, such as a desired serum or peripheral compartment concentration, and then to develop the dosage regimen individualised to best hit that target in that patient. One must monitor the behaviour of the drug by measuring serum concentrations or other responses, hopefully obtained at optimally chosen times, not only to see the raw results, but to also make an individualised (Bayesian posterior) model of how the drug is behaving in that patient. Only then can one see the relationship between the dose and the absorption, distribution, effect and elimination of the drug, and the patient's clinical sensitivity to it; one must always look at the patient. Only by looking at both the patient and the model can it be judged whether the target goal was correct or needs to be changed. The adjusted dosage regimen is again developed to hit that target most precisely starting with the very next dose, not just for some future steady state. Nonparametric population models have discrete, not continuous, parameter distributions. These lead naturally into the multiple model method of dosage design, specifically to hit a desired target with the greatest possible precision for whatever past experience and present data are available on that drug--a new feature for this goal-oriented, model-based, individualised drug therapy. As clinical versions of this new approach become available from several centers, it should lead to further improvements in patient care, especially for bacterial and viral infections, cardiovascular therapy, and cancer and transplant situations.

Population pharmacokinetic modeling of isoniazid, rifampin, and pyrazinamide

Isoniazid (INH), rifampin (RIF), and pyrazinamide (PZA) are the most important drugs for the treatment of tuberculosis (TB). The pharmacokinetics of all three drugs in the plasma of 24 healthy males were studied as part of a randomized cross-over phase I study of two dosage forms. Subjects ingested single doses of INH at 250 mg, RIF at 600 mg, and PZA at 1,500 mg. Plasma was collected for 36 h and was assayed by high-performance liquid chromatography. The data were analyzed by noncompartmental, iterative two-stage maximum a posteriori probability Bayesian (IT2B) and nonparametric expectation maximization (NPEM) population modeling methods. Fast and slow acetylators of INH had median peak concentrations in plasma (C[max]) of 2.44 and 3.64 microg/ml, respectively, both of which occurred at 1.0 h postdose (time of maximum concentrations of drugs in plasma [T(max)]), with median elimination half-lives (t1/2) of 1.2 and 3.3 h, respectively (by the NPEM method). RIF produced a median C(max) of 11.80 microg/ml, a T(max) of 1.0 h, and a t1/2 of 3.4 h. PZA produced a median C(max) of 28.80 microg/ml, a T(max) of 1.0 h, and a t1/2 of 10.0 h. The pharmacokinetic behaviors of INH, RIF, and PZA were well described by the three methods used. These models can serve as benchmarks for comparison with models for other populations, such as patients with TB or TB with AIDS.

A population pharmacokinetic model of trimethoprim in patients with pneumocystis pneumonia, made with parametric and nonparametric methods

A population pharmacokinetic model of intravenously and orally administered trimethoprim in patients with acquired immunodeficiency syndrome and Pneumocystis carinii pneumonia has been made using a parametric iterative two-stage Bayesian and a nonparametric expectation maximization computer program. When good information was present in the serum level data, both methods obtained similar results. With the nonparametric expectation maximization program, the median apparent rate constant for absorption (Ka) was 1.602 hr-1, median slope (Ks) of the relationship between creatinine clearance and elimination was 0.001168 hr-1, median apparent volume of distribution (Vs) was 1.058 l/kg, and median fraction of oral dose absorbed (Fa) was 0.955. These results permit dosage individualization adjusted to body weight and renal function to achieve chosen serum level peak and trough goals. Peak goals of 9 ug/ml and trough goals of 5 ug/ml appear reasonable for most patients in this population, and should permit most to complete an effective course of therapy with a reduced risk for treatment-terminating hematologic toxicity. However, therapeutic goals should always be selected based on each patient's apparent need for the drug and the risk of toxicity that is justifiably acceptable to obtain the expected benefits of the drug.

[Individual variability of pharmacokinetic parameters of amikacin in the elderly: retrospective studies]

In previous works, we have shown: i) good parameter predictive performances of the USC*PACK Clinical Programs for amikacin therapy in the elderly, ii) no significant difference generally detected between estimated parameter values at days 7 and 14 after the beginning of therapy, iii) assurance of neither accumulation nor toxicity during therapy up to 14 days or more, in our conditions. The objectives of this study were to explore which elements best explained differences found in the pharmacokinetic parameters (PK) of the elderly patients, who had received several courses of amikacin therapy.

METHODS

patients' pharmacokinetic data and their medical records were retrospectively analyzed. Only patients who received amikacin therapy with at least a 2-month washout between their courses were studied. Two parameterizations of the 1-compartment PK model were used: one without covariates: Kel-Vol, where Kel = elimination rate constant and Vol = distribution volume, and another including covariates: Ks-Vs, with: Kel = Ks. CCr + Ki, where: Ks = renal fraction of Kel, Ki - non renal elimination, CCr = estimated creatinine clearance, and Vs = Vol/W, where Vs = distribution volume per kg and W = weight.

RESULTS

14 patients, 3 men and 11 women, fulfilled the criteria (4 of them satisfied the condition with 3 courses). They were 66 to 89 years old, their mean weight was 53.86 +/- 11.03 kg (46-71.5), their CCr averaged: 55.45 +/- 17.16 mL/min (14.84-96.27).

CONCLUSION

Among patients exhibiting changes (67%) in PK parameters between different courses of therapy, 42% could have the variability related to covariate (W, CCr) changes; in the others 58% the residual variability could be explained by different factors: severity of infection, immune system deficiency and/or particularly parenteral nutrition. Based on these result, we suggest including septic choc and parenteral nutrition as covariates during amikacin adaptive control.

How many patients and blood levels are necessary for population pharmacokinetic analysis? A study of a one compartment model applied to cyclosporine

OBJECTIVE

This paper describes a method to determine the number of patients and the number of blood levels which are appropriate for a pharmacokinetic population analysis.

METHODS

We studied this question by performing 203 runs of population analysis, using the NPEM algorithm with a one compartment model, starting with only one patient and only one blood level, then 2 patients with one blood level each, until reaching 38 patients each with 5 blood levels. Data were obtained from liver transplant patients treated with cyclosporine.

RESULTS

For 2, 3, 4 or 5 blood levels, the values of median clearance (CL) converged and became almost equal after about 10 patients were studied. The value then remained stable and the variation was fairly small. With only one blood level per patient, the variation was greater. In contrast, with one blood level, median CL became similar to groups having 2, 3, and 4 blood levels only after about 35 patients had been studied, versus about 10. Similar results were found for the median values of the volume of distribution (V). For a one compartment model with parameters of V and CL, from 15 to 20 patients with 2 blood levels may be enough to perform a reasonable population pharmacokinetic analysis; the values of the pharmacokinetic parameters were very similar to those obtained with 3 to 5 blood levels and with more patients. However, a subpopulation probably requires more patients and at least 4 or 5 blood levels per patient to be recognised.

CONCLUSION

Examination of converging pharmacokinetic parameter values by stepwise increases in the number of patients and blood levels appears to be a pragmatic and empirical approach to determine the possible number of patients and blood levels required for population pharmacokinetic analysis.

Population pharmacokinetic models: effect of explicit versus assumed constant serum concentration assay error patterns upon parameter values of gentamicin in infants on and off extracorporeal membrane oxygenation

Prior authors had hypothesized (but not clearly found) an increased apparent volume of distribution (Vd) for gentamicin in neonates undergoing extracorporeal membrane oxygenation (ECMO). We chose to study the question in our own clinical setting. To develop population pharmacokinetic models of the drug, we used the nonparametric expectation and maximization population modeling method and data from 11 neonates who received gentamicin on ECMO, including 6 infants who received gentamicin both on and off ECMO for severe respiratory failure. We found an increased Vd for gentamicin on ECMO and attributed much of the difference from prior investigations to our use of an explicitly determined laboratory assay error pattern for the measured serum concentrations rather than using constant weighting of the serum level data points. For six infants, while on ECMO their median Vd was 0.748 L/kg compared with a median Vd of 0.471 L/kg after ECMO was discontinued. The median clearance of gentamicin in the six infants while undergoing ECMO was 0.239 L/h compared with 0.350 L/h after ECMO was discontinued. The median half-time (T1/2) was 9.24 h while on ECMO compared with 3.87 h when off ECMO. We conclude that while undergoing ECMO, neonates have a higher volume of distribution for gentamicin, a lower clearance, and a much longer half-life.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptive control of drug dosage regimens: basic foundations, relevant issues, and clinical examples

In this paper we examine several of the fundamental foundations and relevant clinical issues in adaptive control of drug dosage regimens for patients. Truly individualized therapy with drugs having narrow margins of safety first requires a practical pharmacokinetic/dynamic model of the behavior of a drug. Past experience with a drug is stored in the form of a population model. Next, using the information in such a model and its relationship to the incidence of adverse reactions, a specific, explicit therapeutic goal must be selected by the responsible clinician, based on the patient's need for the drug and the risk of adverse reactions felt to be justified by each patient's need, small, moderate, or great. Individualized drug therapy thus begins with the selection of individualized therapeutic goals (low, moderate, or high) for each patient. Using subsequent feedback from the patient's serum drug levels, and using Bayesian fitting, the model is then linked to each patient as a patient-specific model. Control of the model by the dosage regimen increasingly controls the patient, to better obtain the desired explicit therapeutic goals. This process is essentially similar to that of a flight control or missile guidance system.

Computation of drug concentrations in endocardial vegetations in patients during antibiotic therapy

The treatment of endocarditis often requires prolonged antibiotic therapy. Individualized drug dosage regimens have made such therapy possible even in patients with impaired renal function. However, the problem of efficacy remains. Especially for aminoglycosides, it would be a useful guide to have at least an approximate idea of the concentration of an antibiotic within an endocardial vegetation. This study was designed to develop software to model the drug concentrations at different layers within spherical vegetations to provide a guide during clinical therapy of patients with endocarditis. A general model describing the diffusion of antibiotics in spheres has now been developed and interfaced with the USC*PACK PC Clinical Programs in order to compute and plot concentrations, within the vegetation, based on the regimen given to the patient and the diffusitivity of the antibiotic into the vegetation. Some preliminary results of this research, which are still in progress, are presented. Diffusion into simulated spherical vegetations has been computed for different treatment regimens for endocarditis: amikacin or netilmicin and vancomycin were given to three elderly patients (3 women, 74, 75 and 92 years old, with initial estimated creatinine clearances of 51, 36, and 31 ml/min/1.73 m2, respectively). Although Amikacin has a low diffusivity, the concentrations, even in the center of the vegetation, appear to be effective. The effects of various regimens, including a 'once-a-day' aminoglycoside regimen, are presented.

Comparison of two software programs to be used for the calculation of population pharmacokinetic parameters

The pharmacokinetic parameters of apomorphine, a potent dopamine agonist, were calculated after subcutaneous and intranasal administration. Two available software packages for TDM were used (USC*PACK and MW/PHARM). The findings for the calculated parameters Kel, Vslope, Ka and bioavailability were compared. Although small, sometimes significant differences were found, the NPEM program from the USC*PACK collection provides most information about the population under investigation. We found a median Vslope of 1.7408 +/- 0.8461 (S.D.) 1/kg, and a mean Kel of 1.5341 +/- 0.3748 h-1. The absorption after intranasal administration was extremely rapid: Ka = 20.5 +/- 1.96 h-1, comparable with that found with subcutaneous absorption: Ka = 23.04 +/- 2.20 h-1.

[Adaptive control of therapeutics using amikacin in very old patients: retrospective analysis of efficacy and toxicity]

The authors previously showed the precision of adaptive control of amikacin therapy in elderly patients. The present retrospective study evaluated the effects of such therapy on outcomes. 48 patients, aged 80 +/- 5 years, with estimated creatinine clearance (eCCR) of 48 +/- 15 ml/mn, received amikacin initial dosage of 13.3 +/- 3.5 mg/kg/d, alone or with other drugs. Efficacy outcomes were: E1 = changes in dosage during therapy; E2 = fever reduction within 3 days after therapy; E3 = eradication of infection by culture data; E4 = reduction of white blood cell count (WCB) to normal; E5 = overall recovery. Toxicity outcomes were: T1 = subjective ototoxicity; T2 = nephrotoxicity, variation of serum creatinine low (between 18 et 44 mumol/l) or high (over 0.5 mg/dl).

RESULTS

E1: final dose = 11.8 +/- 5.1 mg/kg/d (NS), 57% reduced, 33% increased, during 15.1 +/- 9.3 days in therapy, with 88% having effective peaks over 15 micrograms/ml. E2: fever reduced within 3 days 16/37; after 12/37; no change, 9/37. E3: cultures became negative, 13/28. E4: WBC fell early, 10/21; late, 7/21; no change, 2/21. E5: recovery 36; death, 8; change in therapy, 3. T1: no clinical signs of ototoxicity. T2: low(+), 9/51; low(-), 11/51; high(-), 7/51. final eCCR: 48 +/- 14 ml/mn (NS); no nephrotoxicity. These results suggest that adaptive control of amikacin regimens yields good efficacy and avoid toxicity in the Elderly. However, prospective controlled clinical trials should be done for confirmation.

[Aminoglycoside determinations calculated in endocarditis vegetations. Relations with clinical practices during infectious endocarditis treatment with amikacin]

Using a new computer program SPHERE, amikacin concentrations have been computed at various layers of simulated endocardial vegetations. Inputs are the computed serum (central compartment) concentrations of either population pharmacokinetic models or of individualized patient-specific models utilizing Bayesian fitting to data of doses given and measured serum levels, using the USC*PACK PC Clinical Programs. The vegetation is modeled as an isotropic homogeneous sphere. Fick's second law of radial diffusion was applied to compute the in situ antibiotic concentrations. Examination of factors affecting concentrations in vegetations shows that in situ peak concentrations are less when the vegetation is larger, and when the antibiotic dose, serum concentrations and diffusivity are all less. The results show that early and aggressive treatment of infectious endocarditis is required with high doses of concentration-dependent antibiotics, such as aminoglycosides, to achieve the desired high peak serum levels and to reach effective concentrations deep inside the vegetations.

Pharmaco-informatics: more precise drug therapy from "multiple model" (MM) stochastic adaptive control regimens: evaluation with simulated vancomycin therapy

MM stochastic control of dosage regimens permits essentially full use of information, either in a population pharmacokinetic model or a Bayesian updated MM parameter set, to achieve and maintain selected therapeutic goals with optimal precision. The regimens are visibly more precise than those developed using mean parameter values. Bayesian MM feedback has now also been implemented.

A graphical user interface to facilitate patient-specific drug dosing

This paper describes the development of a graphical user-interface (GUI) designed to facilitate the process of generating patient-specific drug doses using bayesian modelling software. The graphical user interface was developed in Visual Basic (Microsoft, Inc.) and runs under the Windows 3.1 (Microsoft, Inc.) operating system. Patient demographic data are stored in a relational database (Access, Microsoft, Inc.). The GUI and the database communicate via dynamic data exchange links. The largely object-oriented nature of the language allowed us to change the entire look and feel of the system with a few simple changes to the underlying code. Use of the relational database to store patient demographic information allows greater flexibility in searching for and displaying patient specific information. The GUI reduces the time required to enter data. The prototype has allowed us to experiment with different presentation methods, greatly improving the clinical acceptance of the dosing programs.

Individualizing drug dosage regimens: roles of population pharmacokinetic and dynamic models, Bayesian fitting, and adaptive control

The role of population pharmacokinetic modeling is to store experience with drug behavior. The behavior of the model is then correlated with the clinical behavior of the patients studied, permitting selection of a specific serum level therapeutic goal that is based on each individual patient's need for the drug and on the risk of adverse reactions, both of which must be considered. A dosage regimen is then computed to achieve that goal with maximum precision. The patient should not run a greater risk of toxicity than is justified, and should obtain the maximum possible benefit within the acceptable risk. The regimen is given and the patient monitored.

Pharmacoinformatics: equations for serum drug assay error patterns; implications for therapeutic drug monitoring and dosage

Pharmacoinformatics is the area of Medical Informatics concerned with modeling and simulation of the behavior of drugs, and control of such behavior by individualized dosage regimens for each patient to achieve explicitly chosen therapeutic goals. The credibility of serum concentration data is a major factor in such modeling. The present report examines a more precise way of describing the credibility of such data with a collection of polynomial equations, developed from routine survey data of the College of American Pathologists, which improve the description of the credibility of serum level results when compared to the usual practice of describing the assay coefficient of variation in the usual manner and then not using such information either in population pharmacokinetic modeling or in actual therapeutic drug monitoring. These equations can be used until each laboratory can develop its own assay error patterns with its own similar polynomial equations.

Population pharmacokinetics of gentamicin. Use of the nonparametric expectation maximisation (NPEM) algorithm

A new nonparametric expectation maximisation (NPEM) algorithm for the estimation of population pharmacokinetic parameter values was evaluated. The algorithm, in the form of a personal computer program, was used to compute population pharmacokinetic parameter densities of gentamicin in a group of 9 patients with indicators of malnutrition. The 1-compartment parameter values for clearance (CL), volume of distribution (Vd) and elimination rate constant (k) were compared with values generated using a standard 2-stage (STS) approach. NPEM was used with a full data set (72 gentamicin concentrations) and a sparse data set (only peak and trough concentrations for each patient; 18 in total). There were no differences in parameter value estimations between the STS and NPEM with all the data (p greater than 0.05) or with the sparse data (p greater than 0.05). Mean parameter value estimates from the STS and NPEM (with sparse data) were used as a priori data sets in the USC*PACK gentamicin Bayesian program to predict concentrations in 8 subsequent patients with similar indicators of malnutrition. There were no differences in predicted gentamicin concentrations between STS (3.75 +/- 2.06 mg/L) and NPEM (3.75 +/- 2.17 mg/L). NPEM was able to generate population pharmacokinetic parameter values for gentamicin in a defined population of patients using sparse routine clinical data. It was also shown to function with only a single data point per patient.

Individualising gentamicin dosage regimens. A comparative review of selected models, data fitting methods and monitoring strategies

The various components required for individualising clinical drug dosage regimens are reviewed, including a study of 3 types of fitting procedures, 2 types of gentamicin pharmacokinetic model and the utility of D-optimal times for obtaining serum gentamicin concentrations. The combination of the current Bayesian fitting procedure, the kslope pharmacokinetic model [in which the elimination rate constant (kel) can change from dose to dose with changing creatinine clearance] and the explicit measurement of the assay error pattern yielded predictions of future serum gentamicin concentrations which were (a) slightly better than those found using weighted nonlinear least squares; (b) somewhat better than those found with Bayesian fitting and a fixed-kel model; (c) better than those found using the traditional linear regression fitting procedure and a fixed kel model. D-Optimally timed pairs of concentrations also predicted future concentrations at least as well, and more cost effectively.

Gentamicin population pharmacokinetic models for low birth weight infants using a new nonparametric method

Availability of personal computer programs for control of drug regimens has stimulated interest in modeling population pharmacokinetics. In this study, we found parameter values for gentamicin in two infant populations with low birth weights. The models were developed by use of a parametric (i.e., standard two-stage algorithm) and with a new nonparametric expected maximum algorithm. Data for the two populations (i.e., infants less than or equal to 31 weeks' and greater than 31 but less than or equal to 34 weeks' gestational age) were obtained from infants admitted to the University of Texas Medical Branch intensive care nursery between August 1, 1988, and July 31, 1989. The new nonparametric method was demonstrated to be not only the equal of the standard two-stage method for population modeling but better, especially in use of sparse data sets (e.g., single serum levels). It also obviates the need for selecting proper starting conditions for the least-squares fitting procedure used in the standard two-stage method.

Sodium hyaluronate as a carrier for intravitreal gentamicin. An experimental study

Fifty mcg of gentamicin was combined with saline or with 0.8% sodium hyaluronate and injected into the vitreous cavity of rabbit eyes with moderate to severe Staphylococcus aureus endophthalmitis. Endophthalmitis was controlled in 9 of 10 eyes. There was no evidence of toxicity with either treatment regimen. Although the clearance study demonstrated statistical differences at all time points studied, the half-lives of both treatment regimens were similar (3.3 h for aqueous gentamicin and 3.6 h for sodium hyaluronate/gentamicin). These results suggest that the vitreous played a role in keeping the aqueous gentamicin in the eye for a longer time, as similar half-lives were shown with both types of treatment. Thus, if a vitrectomy has to be done for the treatment of endophthalmitis, as much as possible of the vitreous should be left in situ to maintain the drug for longer periods in the eye. Also, if it is necessary to remove all vitreous during vitrectomy, it may be more effective to administer the drug with sodium hyaluronate so as to prolong its action inside the eye.

In vitro and in vivo studies with sodium hyaluronate as a carrier for intraocular gentamicin

We compared the in vitro growth of common intraocular pathogens Staphylococcus aureus, Staphylococcus epidermidis and Pseudomonas aeruginosa in rabbit vitreous and in sodium hyaluronate (SH) with and without gentamicin. The minimal inhibitory concentration for gentamicin/SH was 0.5, 0.062 and 2.0 mcg/ml for these pathogens, respectively. After posterior capsulotomy, P. aeruginosa was inoculated into the anterior vitreous and all 15 untreated eyes developed endophthalmitis. In a similar group, aqueous gentamicin administered in the anterior chamber reduced the incidence of endophthalmitis to 10 of 15 eyes. Under similar circumstances, the SH/gentamicin combination lowered the incidence of endophthalmitis significantly to 4 of 15 eyes. The half-life of aqueous gentamicin was 0.9 h, which was shorter than the 2.2 h for SH/gentamicin combination. These results suggest that SH may be a useful carrier for intraocular drug therapy.

The USC*PACK PC programs for population pharmacokinetic modeling, modeling of large kinetic/dynamic systems, and adaptive control of drug dosage regimens

Programs for PC's and compatibles provide nonparametric (NPEM) population pharmacokinetic modeling, BOXES for compartments and arrows for pathways, to make large kinetic and dynamic models, and clinical software for Bayesian adaptive control of drug dosage regimens, with D-optimal sampling strategies and explicit determination of assay error patterns.

Application of a Bayesian method to monitor and adjust vancomycin dosage regimens

A Bayesian method for monitoring vancomycin concentrations and adjusting regimens in patients with unstable renal function by using a two-compartment population model was evaluated with a personal computer. The population model was derived from data from 12 cardiac outpatients who received single doses of vancomycin. The performance of the method was then tested in 27 acutely ill patients who received multiple doses of vancomycin. Significant renal impairment was observed in 15 patients. Renal function changed in 15 patients. The vancomycin concentrations in the patients with changing renal function were not at steady state during the observation times. Two concentrations in serum (peak and then trough, or trough and then peak) were fitted along with the population model to individualize the parameter values for each patient. All the subsequent concentrations in serum for each patient were then predicted by using the parameter values for each patient. Future concentrations of 118 serum samples were predicted. The mean absolute prediction error was 3.6 +/- 4.5 micrograms/ml, and the mean prediction error was -0.7 +/- 5.3 micrograms/ml. These results confirm that a two-compartment pharmacokinetic model can be sufficiently individualized with the knowledge of just two concentrations of drug in patient serum; it is possible to predict closely subsequent concentrations in serum, and dosing regimens for individual patients can be well adjusted to achieve the chosen therapeutic goals.