Population pharmacokinetics and concentration-effect relationships of capecitabine metabolites in colorectal cancer patients

AIMS: To assess the relationship between systemic exposure to capecitabine metabolites and parameters of efficacy and safety in patients with advanced or metastatic colorectal cancer from two phase III studies. METHODS: Concentration-effect analyses were based on data from 481 patients (248 males, 193 females; age range 27-86 years) in two phase III studies. Plasma concentration-time data for 5'-deoxy-5-fluorouridine (5'-DFUR), 5-fluorouracil (5-FU) and alpha-fluoro-beta-alanine (FBAL) were obtained from sparse blood samples collected within the time windows 0.5-1.5 h, 1.5-3.0 h, and 3.0-5.0 h after capecitabine administration (1250 mg m(-2)) on the first day of cycles 2 (day 22) and 4 (day 64), respectively. Systemic exposure based on plasma concentrations of capecitabine and its metabolites was determined using individual parameter estimates derived from a population pharmacokinetic model constructed for this purpose in NONMEM. Logistic regression analysis was conducted for selected safety parameters (all treatment-related grade 3-4 adverse events, treatment-related grade 3-4 diarrhoea, grade 3 hand-foot syndrome (HFS) and grade 3-4 hyperbilirubinaemia) and for tumour response. Cox regression analysis was used for the analysis of time-to-event data (time to disease progression and duration of survival). RESULTS: Statistically significant relationships between covariates and PK parameters were found as follows. A doubling of alkaline phosphatase activity was associated with a 11% decrease in 5-FU clearance and a 12% increase in its AUC. A 50% decrease in creatinine clearance was associated with a 35% decrease in FBAL clearance, a 53% increase in its AUC, a 24% decrease in its volume of distribution, and a 41% increase in its Cmax. A 30% increase in body surface was associated with a 24% increase in the volume of distribution of FBAL and a 19% decrease in its Cmax. There was a broad overlap in systemic drug exposure between patients regardless of the occurrence of treatment-related grade 3-4 adverse events or response to treatment, leading to weak relationships between systemic exposure to capecitabine metabolites and the safety and efficacy parameters. Of 42 concentration-effect relationships investigated, only five achieved statistical significance. Thus, we obtained a positive association between the AUC of FBAL and grade 3-4 diarrhoea (P = 0.035), a positive association between the AUC of 5-FU and grade 3-4 hyperbilirubinaemia (P = 0.025), a negative association between the Cmax of FBAL and grade 3-4 hyperbilirubinaemia (P = 0.014), a negative association between the AUC of 5-FU (in plasma) and time to disease progression (hazard ratio (HR) = 1.626, P = 0.0056), and a positive association between the Cmax of 5'-DFUR and survival (HR = 0.938, P = 0.0048). Additionally, there were inconsistencies when concentration-effect relationships were compared across the two studies. CONCLUSIONS: Systemic exposure to capecitabine and its metabolites in plasma is poorly predictive of safety and efficacy. The present results have no clinical implications for the use of capecitabine and argue against the value of therapeutic drug monitoring for dosage adjustment.     

Population pharmacokinetic parameters in patients treated with oral mexiletine

Summary A new data analysis approach, NONMEM, proposed by Sheiner and Beal, has been employed to estimate the population pharmacokinetic parameters of oral mexiletine in patients treated for arrhythmias. 452 serum concentration measurements in 58 patients were available for analysis. 27 patients had congestive heart failure and 8 had abnormal liver function tests at the time of the study. The population averages of the pharmacokinetic parameters and their interindividual variability were: oral total body clearance (Cl) 0.38 l/h/kg±43% (C. V.), apparent volume of distribution (Vd) 5.3 l/kg±40%, absorption rate constant 3.1 h^–1±205%, absorption time-lag 0.3 h. Congestive heart failure and sex did not show a significant effect on Cl and Vd; the number of patients with severe liver function impairment was too small for a definite conclusion. Normalizing Cl and Vd for body weight significantly decreased their interindividual variability. Based on these results, a dosage regimen is recommended which is expected to produce a therapeutic serum concentration (0.8–2 mg/l) in over 60% of patients. Because of its unique features, which allow estimation of pharmacokinetic parameters and their variability from fragmentary patient data, the NONMEM system has great potential applicability to clinical pharmacokinetic studies.Presented in part at the First European Congress on Biopharmaceutics and Pharmacokinetics, Clermont-Ferrand, April 1981     

Population pharmacokinetic and pharmacogenetic analysis of 6-mercaptopurine in paediatric patients with acute lymphoblastic leukaemia

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• The cytotoxic effects of 6-mercaptopurine (6-MP) were found to be due to drug-derived intracellular metabolites (mainly 6-thioguanine nucleotides and to some extent 6-methylmercaptopurine nucleotides) rather than the drug itself.
• Current empirical dosing methods for oral 6-MP result in highly variable drug and metabolite concentrations and hence variability in treatment outcome.

WHAT THIS STUDY ADDS

• The first population pharmacokinetic model has been developed for 6-MP active metabolites in paediatric patients with acute lymphoblastic leukaemia and the potential demographic and genetically controlled factors that could lead to interpatient pharmacokinetic variability among this population have been assessed.
• The model shows a large reduction in interindividual variability of pharmacokinetic parameters when body surface area and thiopurine methyltransferase polymorphism are incorporated into the model as covariates.
• The developed model offers a more rational dosing approach for 6-MP than the traditional empirical method (based on body surface area) through combining it with pharmacogenetically guided dosing based on thiopurine methyltransferase genotype.

AIMS

To investigate the population pharmacokinetics of 6-mercaptopurine (6-MP) active metabolites in paediatric patients with acute lymphoblastic leukaemia (ALL) and examine the effects of various genetic polymorphisms on the disposition of these metabolites.

METHODS

Data were collected prospectively from 19 paediatric patients with ALL (n = 75 samples, 150 concentrations) who received 6-MP maintenance chemotherapy (titrated to a target dose of 75 mg m⁻² day⁻¹). All patients were genotyped for polymorphisms in three enzymes involved in 6-MP metabolism. Population pharmacokinetic analysis was performed with the nonlinear mixed effects modelling program (nonmem) to determine the population mean parameter estimate of clearance for the active metabolites.

RESULTS

The developed model revealed considerable interindividual variability (IIV) in the clearance of 6-MP active metabolites [6-thioguanine nucleotides (6-TGNs) and 6-methylmercaptopurine nucleotides (6-mMPNs)]. Body surface area explained a significant part of 6-TGNs clearance IIV when incorporated in the model (IIV reduced from 69.9 to 29.3%). The most influential covariate examined, however, was thiopurine methyltransferase (TPMT) genotype, which resulted in the greatest reduction in the model''s objective function (P < 0.005) when incorporated as a covariate affecting the fractional metabolic transformation of 6-MP into 6-TGNs. The other genetic covariates tested were not statistically significant and therefore were not included in the final model.

CONCLUSIONS

The developed pharmacokinetic model (if successful at external validation) would offer a more rational dosing approach for 6-MP than the traditional empirical method since it combines the current practice of using body surface area in 6-MP dosing with a pharmacogenetically guided dosing based on TPMT genotype.     

A human capecitabine excretion balance and pharmacokinetic study after administration of a single oral dose of 14C-labelled drug

An excretion balance and pharmacokinetic study was conducted in cancer patients with solid tumors who received a single oral dose of capecitabine of 2000 mg including 50 Ci of ¹⁴C-radiolabelled capecitabine. Blood, urine and fecal samples were collected until radioactive counts had fallen to below 50 dpm/mL in urine, and levels of intact drug and its metabolites were measured in plasma and urine by LC/MS-MS (mass spectrometry) and ¹⁹F-NMR (nuclear magnetic resonance) respectively. Based on the results of the 6 eligible patients enrolled, the dose was almost completely recovered in the urine (mean 95.5%, range 86–104% based on radioactivity measurements) over a period of 7 days after drug administration. Of this, 84% (range 71–95) was recovered in the first 12 hours. Over this time period, 2.64% (0.69–7.0) was collected in the feces. Over a collection period of 24–48h, a total of 84.2% (range 80–95) was recovered in the urine as the sum of the parent drug and measured metabolites (5-DFCR, 5-DFUR, 5-FU, FUH₂, FUPA, FBAL). Based on the radioactivity measurements of drug-related material, absorption is rapid (t_max 0.25–1.5 hours) followed by a rapid biphasic decline. The parent drug is rapidly converted to 5-FU, which is present in low levels due to the rapid metabolism to FBAL, which has the longest half-life. There is a good correlation between the levels of radioactivity in the plasma and the levels of intact drug and the metabolites, suggesting that these represent the most abundant metabolites of capecitabine. The absorption of capecitabine is rapid and almost complete. The excretion of the intact drug and its metabolites is rapid and almost exclusively in the urine.     

Population pharmacokinetics of ondansetron: a covariate analysis

Aims To construct a population model to account for the variability in ondansetron pharmacokinetics and to evaluate methods for the efficient development of population models.
Methods Population models were developed using 99 subjects consisting of paediatric patients, young, elderly and aged volunteers. A two compartment pharmacokinetic model with a zero order input was used to describe the pharmacokinetics of ondansetron. Three stepwise methods were proposed and used alongside a three step approach to develop population models with both rich and sparse data sets. The stepwise methods were based on obtaining empirical Bayes posterior estimates of pharmacokinetic parameters within a nonlinear mixed effect modelling (NONMEM) program. The parameters were then regressed against covariates in a stepwise procedure. Variance parameters were obtained by fitting the proposed population model to the data in one further NONMEM run. The population model was validated against a test data set of 54 subjects, including children, young and elderly patients and volunteers.
Results The population model adequately described the differences in ondansetron pharmacokinetics between paediatric patients, young, elderly and aged volunteers. Different covariates were identified by the various methods. Weight was found to have a strong positive linear relationship with all four pharmacokinetic parameters. Clearance showed a weak negative relationship with age. Males were found to have a greater clearance than females after weight adjustment.
Conclusions The stepwise search procedures potentially are capable of considerably reducing the time required to develop population pharmacokinetic models. The model developed for ondansetron gave accurate predictions of both the concentration-time profile and variability in an independent data set.     

Population pharmacokinetic model of digoxin in older Chinese patients and its application in clinical practice

Aim:

To establish a population pharmacokinetic (PPK) model of digoxin in older Chinese patients to provide a reference for individual medication in clinical practice.

Methods:

Serum concentrations of digoxin and clinically related data including gender, age, weight (WT), serum creatinine (Cr), alanine aminotransferase (ALT), aspartate aminotransferase (AST), blood urea nitrogen (BUN), albumin (ALB), and co-administration were retrospectively collected from 119 older patients taking digoxin orally for more than 7 d. NONMEM software was used to get PPK parameter values, to set up a final model, and to assess the models in clinical practice.

Results:

Spironolactone (SPI), WT, and Cr markedly affected the clearance rate of digoxin. The final model formula is Cl/F=5.9×[1–0.412×SPI]×[1–0.0101×(WT–62.9)]×[1–0.0012×(Cr–126.8)] (L/h); Ka=1.63 (h⁻¹); V_d/F=550 (L). The population estimates for Cl/F and V_d/F were 5.9 L/h and 550 L, respectively. The interindividual variabilities (CV) were 49.0% for Cl/F and 94.3% for V_d/F. The residual variability (SD) between observed and predicted concentrations was 0.365 μg/L. The difference between the objective function value and the primitive function value was less than 3.84 (P>0.05) by intra-validation. Clinical applications indicated that the percent of difference between the predicted concentrations estimated by the PPK final model and the observed concentrations were −4.3%–+25%. Correlation analysis displayed that there was a linear correlation between observated and predicted values (y=1.35x+0.39, r=0.9639, P<0.0001).

Conclusion:

The PPK final model of digoxin in older Chinese patients can be established using the NONMEM software, which can be applied in clinical practice.     

用NONMEM法建立西酞普兰群体药代动力学模型

目的建立中国人西酞普兰(抗抑郁药)的群体药代动力学(PPK)模型,为临床个体化给药提供参考。方法用群体药代动力学方法,对西酞普兰生物等效性研究中23例受试者的血药浓度和临床资料进行分析,用NON-MEM软件求算西酞普兰的PPK参数值,建立西酞普兰的PPK模型,并进行模型验证。结果经NONMEM法处理,所有因素中,年龄、体重以及CYP2C19基因型对中央隔室清除率有显著性的影响;体重对分布容积有显著性的影响。年龄和体重的增加对清除率影响分别为-0.39L·h-1.a-1和0.18L.h-1·kg-1。结论用NONMEM软件拟合获得的西酞普兰群体药代动力学最终模型,经验证稳定可靠。     

Pharmacokinetic Modelling of 5-FU Production from Capecitabine—A Population Study in 40 Adult Patients with Metastatic Cancer

Aims: To model the biotransformation steps of 5-FU production from capecitabine and identify patient characteristics that may influence the drug disposition. Methods: Blood samples and demographic data were collected from two phase I studies in which adult patients received oral capecitabine for various malignancies. Capecitabine, 5′-deoxy-5-fluorocytidine (5′-DFCR), 5′-deoxy-5-fluorouridine (5′-DFUR) and 5-fluorouracile (5-FU) concentration-time data were analysed via a population approach using NONMEM. Results: Forty patients and 75 pharmacokinetic time-courses were available for analysis. Capecitabine pharmacokinetics was ascribed to a one compartment model from which 5′-DFCR, 5′-DFUR and 5-FU were sequentially produced. Capecitabine oral absorption was characterized by a rapid first order input (K_a=2.1 ± 0.3 hr⁻¹) with a lag time (0.28 ± 0.11 hr), but related inter-occasion (IOV) and inter-subject (ISV) variabilities for these parameters, 167% and 110%, indicated that this oral absorption was highly variable. The capecitabine CL (CL₁₀ = 218± 18 L/hr, ISV = 18%) and 5′-DFUR elimination rate constant (K₃₄ = 5.3 ± 2.0 hr⁻¹, ISV = 25%) were influenced by total bilirubin (BILT). The elimination rate constant of plasma 5-FU (K₄₀) was 66 ± 24 hr⁻¹ (ISV = 34%).The final pharmacokinetic model was validated using 2000 bootstrap runs and provided non-parametric statistics of the parameters (median, 2.5th and 97.5th percentiles). Conclusions: This study supported the possibility of modelling a complex sequential metabolic pathway which produces pharmacologicaly active compounds from a prodrug. Only BILT significantly influenced the pharmacokinetics but this effect was not considered as relevant for dosing adjustment.     

Evaluation of the autoinduction of ifosfamide metabolism by a population pharmacokinetic approach using NONMEM

AIMS: This study investigated the population pharmacokinetics of ifosfamide in 15 patients treated for soft tissue sarcoma with 9 or 12 g m-2 ifosfamide by means of a 72 h continuous i.v. infusion. METHODS: A model was developed using nonlinear mixed effects modelling (NONMEM) to describe the nonlinear pharmacokinetics of ifosfamide by linking the ifosfamide plasma concentrations to the extent of the autoinduction. RESULTS: The proposed model revealed the effect of autoinduction on the disposition of ifosfamide. The initial clearance, volume of distribution, rate constant for enzyme degradation, induction half-life of the enzyme and the ifosfamide concentration at 50% of the maximum inhibition of enzyme degradation were estimated at 2.94 +/- 0.27 l h-1, 43.5 +/- 2.9 l, 0.0546 +/- 0. 0078 h-1, 12.7 h and 30.7 +/- 4.8 microM, respectively. Interindividual variabilities of initial clearance, volume of distribution, rate constant for enzyme degradation were 24.5, 23.4 and 22.7%, respectively. Proportional and additive variability not explained by the model were 13.6% and 0.0763 microM, respectively. CONCLUSIONS: The absence of a lag time for the autoinduction of ifosfamide metabolism could be the result of an immediate inhibition of the enzymatic degradation of CYP3A4 by ifosfamide. By application of the autoinduction model individual pharmacokinetic profiles of patients were described with adequate precision. This model may therefore be used in the future development of a model to individualize dose selection in patients.     

Population pharmacokinetic and dynamic analysis of the topoisomerase I inhibitor lurtotecan in phase II studies

Population pharmacokinetic-dynamic analysiswas prospectively integrated in a broadphase II program of lurtotecan (GI147211),a novel camptothecin derived topoisomeraseI inhibitor, to determine the populationpharmacokinetic profile in a largerpopulation, to estimate individualpharmacokinetic parameters and toinvestigate relationships with clinicaloutcome. A sparse sampling method wasapplied during course one, which involvedtwo sampling time-points. A Bayesianalgorithm was used to estimate individualpharmacokinetic parameters, in particulartotal plasma clearance (CL) and volume ofdistribution. In total, samples werecollected of 109 (63%) of 173 patients.Pharmacokinetic-dynamic evaluation could becarried out successfully in 85 (78%) ofthe sampled patients. CL of lurtotecanshowed substantial variability (mean 87± 28 L/h) and was of the same magnitudeas in the phase I studies where fullpharmacokinetic curves were used. Residualvariability in the population estimate ofCL was 9.9%. No significant relationshipswere observed between exposure parametersand toxicity nor likelihood of tumorresponse, however the latter relationshipmay well have been obscured by theheterogeneity of the studied population.Prospective implementation of large scalepopulation pharmacokinetic-dynamic analysisis feasible and important to establishwhether interpatient variability in drugexposure is a major determinant of toxicityor activity.     

Population pharmacokinetic analysis of carboxyhaemoglobin concentrations in adult cigarette smokers

AIMS

To develop a population-based model to describe and predict the pharmacokinetics of carboxyhaemoglobin (COHb) in adult smokers.

METHODS

Data from smokers of different conventional cigarettes (CC) in three open-label, randomized studies were analysed using NONMEM (version V, Level 1.1). COHb concentrations were determined at baseline for two cigarettes [Federal Trade Commission (FTC) tar 11 mg; CC1, or FTC tar 6 mg; CC2]. On day 1, subjects were randomized to continue smoking their original cigarettes, switch to a different cigarette (FTC tar 1 mg; CC3), or stop smoking. COHb concentrations were measured at baseline and on days 3 and 8 after randomization. Each cigarette was treated as a unit dose assuming a linear relationship between the number of cigarettes smoked and measured COHb percent saturation. Model building used standard methods. Model performance was evaluated using nonparametric bootstrapping and predictive checks.

RESULTS

The data were described by a two-compartment model with zero-order input and first-order elimination with endogenous COHb. Model parameters included elimination rate constant (k₁₀), central volume of distribution (Vc/F), rate constants between central and peripheral compartments (k₁₂ and k₂₁), baseline COHb concentrations (c0), and relative fraction of carbon monoxide absorbed (F1). The median (range) COHb half-lives were 1.6 h (0.680–2.76) and 30.9 h (7.13–367) (α and β phases, respectively). F1 increased with increasing cigarette tar content and age, whereas k₁₂ increased with ideal body weight.

CONCLUSION

A robust model was developed to predict COHb concentrations in adult smokers and to determine optimum COHb sampling times in future studies.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• The pharmacokinetics of carboxyhaemoglobin have been reported previously, primarily with regard to poisoning and toxicity.
• Most of these reports have involved noncompartmental analysis of data obtained where the actual dose of carbon monoxide was not known.

WHAT THIS STUDY ADDS

• This study presents a comprehensive population pharmacokinetic model for carboxyhaemoglobin in adult cigarette smokers.
• Since carboxyhaemoglobin is a marker of cigarette smoke exposure, model-based evaluations can be used for simulation and other evaluations of the kinetics of this agent.

     

基于NONMEM法建立紫杉醇群体药动学模型

目的：建立国人紫杉醇（paclitaxel,PTX）群体药动学（population pharmacokinetic,PPK）模型,为制定个体化给药方案提供理论支持。方法：收集138例接受紫杉醇治疗的肿瘤患者（建模组105例,验证组33例）210个血样,HPLC法测定紫杉醇血药浓度,PCR-RFLP法检测MDR1 C3435T。应用非线性混合效应模型（NONMEM）法,考察MDR1 C3435T基因多态性、合并用药及病理生理因素对紫杉醇药动学参数的影响,建立紫杉醇PPK模型。对模型进行拟合优度诊断、自举法（Bootstrap）内部验证,正态预测分布误差法（NPDE）及外部验证考察模型预测能力。结果：紫杉醇清除率（CL）和表观分布容积（V_d）的群体典型值分别为64.7 L·h^-1和1 240 L,患者内生肌酐清除率（CLcr）和给药速率（RATE）显著影响紫杉醇清除率。最终模型Bootstrap法验证结果与模型计算值相符,拟合优度、准确度及精密度均优于最简模型。结论：紫杉醇PPK最终模型稳定、有效,可结合Bayesian反馈法为临床优化给药方案提供科学依据。     

老年心衰患者口服地高辛群体药动学模型的建立

目的:应用非线性混合效应模型计算国人老年心衰患者口服地高辛(digxion)群体药动学参数,以促进个体化给药。方法:采用荧光偏振免疫法(FPIA)测定84例老年患者120例次地高辛的血清浓度并收集相关临床指标,运用NONMEM软件建立群体药动学模型。结果:地高辛的药动学符合一室线性开放模型,固定效应参数中,体质量、剂量、血肌酐及尿素氮对参数有影响。最终回归模型中地高辛血药浓度估算值与实测浓度间线性关系良好。结论:用群体药动学模型分析常规监测数据可为老年患者个体化给药提供依据。     

A population pharmacokinetic study of alminoprofen penetration into synovial fluid

The pharmacokinetics of alminoprofen in plasma and synovial fluid (SF) at steady state (300 mg t.i.d.) was studied in 45 patients with knee effusion. Plasma and SF samples, one each per patient, were obtained. Six groups were made according to the time of sampling after ingestion of the 13th dose: 1h (n = 7), 2h (n = 7), 4h (n = 7), 6h (n = 10), 8h (n = 6), 12h (n = 8). A three-compartment model was used to describe alminoprofen kinetics in plasma and SF, with two parameterizations, a ‘classical’ and a ‘physiological’ one. The non-linear mixed effect model approach was used to estimate the mean and variance of the pharmacokinetic parameters. The mean ±SE of the estimates (coefficient of variation of interindividual variability as a percentage) were volume of distribution, 11.0 ± 1.711 (12%); elimination rate constant, 0.236 ± 0.025 h^?1 (18%); absorption rate constant 2.80 ± 0.31 h^?1 (464%), clearance of influx into SF, 0.29 ± 0.14 mL min^?1; clearance of efflux into plasma, 0.56 ± 0.25 mL min^?1. These two clearances were not significantly different, which indicates that passive diffusion occurs in both directions. The mean ±SD alminoprofen concentration versus time curve in plasma and SF at steady state was simulated and showed that the mean ±SD maximal concentration in SF was 8.1 ± 6.3 mg L^?1 and was obtained 4h after dose administration.     

Population pharmacokinetics of the new antimalarial agent tafenoquine in Thai soldiers

AIMS: To describe the population pharmacokinetics of tafenoquine in healthy volunteers after receiving tafenoquine for malaria prophylaxis. METHODS: The population consisted of 135 male Thai soldiers (mean age 28.9 years; weight 60.3 kg). All soldiers were presumptively treated with artesunate for 3 days plus doxycycline for 7 days to remove any pre-existing malaria infections. After the treatment regime, 104 soldiers (drug group) received a loading dose of 400 mg tafenoquine base daily for 3 days followed by 400 mg tafenoquine monthly for 5 consecutive months. In the placebo group, 31 soldiers were infected with malaria during the study period. They were re-treated with artesunate for 3 days plus doxycycline for 7 days followed by a loading dose of 400 mg tafenoquine daily for 3 days and then 400 mg tafenoquine weekly for prophylaxis. Blood samples were randomly collected from each soldier on monthly and weekly prophylaxis. Plasma tafenoquine concentrations were measured by h.p.l.c. Population pharmacokinetic modelling was performed using NONMEM. RESULTS: A one-compartment model was found best to describe the pharmacokinetics of tafenoquine after oral administration. Age and weight influenced volume of distribution (V/F), and subjects who contracted malaria had higher clearance (CL/F), but none of these factors was considered to have sufficient impact to warrant change in dosing. The population estimates of the first-order absorption rate constant (Ka), CL/F and V/F were 0.694 h(-1), 3.20 l h(-1) and 1820 l, respectively. The intersubject variability in these parameters (coefficient of variation, CV%) was 61.2%, 25.3% and 14.8%, respectively. The absorption and elimination half-lives were 1.0 h and 16.4 days, respectively. The residual (unexplained) variability was 17.9%. CONCLUSIONS: The population pharmacokinetics of orally administered tafenoquine have been determined in Thai soldiers under field conditions. This information, together with its known potent antimalarial activity, portends well for the application of tafenoquine as a useful prophylactic drug or for short-term radical treatment of vivax malaria.     

Population pharmacokinetic analysis of varenicline in adult smokers

AIMS

To characterize the population pharmacokinetics of varenicline and identify factors leading to its exposure variability in adult smokers.

METHODS

Data were pooled from nine clinical studies consisting of 1878 subjects. Models were developed to describe concentration–time profiles across individuals. Covariates were assessed using a full model approach; parameters and bootstrap 95% confidence intervals (CI) were estimated using nonlinear mixed effects modelling.

RESULTS

A two-compartment model with first-order absorption and elimination best described varenicline pharmacokinetics. The final population parameter estimates (95% CI) were: CL/F, 10.4 l h⁻¹ (10.2, 10.6); V₂/F, 337 l (309, 364); V₃/F, 78.1 l (61.9, 98.9); Q/F, 2.08 l h⁻¹ (1.39, 3.79); K_a, 1.69 h⁻¹ (1.27, 2.00); and A_lag, 0.43 h (0.37, 0.46). Random interindividual variances were estimated for K_a[70% coefficient of variation (CV)], CL/F (25% CV), and V₂/F (50% CV) using a block covariance matrix. Fixed effect parameters were precisely estimated [most with % relative standard error < 10 and all with % relative standard error < 25], and a visual predictive check indicated adequate model performance. CL/F decreased from 10.4 l h⁻¹ for a typical subject with normal renal function (CLcr = 100 ml min⁻¹) to 4.4 l h⁻¹ for a typical subject with severe renal impairment (CLcr = 20 ml min⁻¹), which corresponds to a 2.4-fold increase in daily steady-state exposure. Bodyweight was the primary predictor of variability in volume of distribution. After accounting for renal function, there was no apparent effect of age, gender or race on varenicline pharmacokinetics.

CONCLUSIONS

Renal function is the clinically important factor leading to interindividual variability in varenicline exposure. A dose reduction to 1 mg day⁻¹, which is half the recommended dose, is indicated for subjects with severe renal impairment.     

Population pharmacokinetic/pharmacodynamic modeling of valproate in Chinese children with epilepsy

AIM： To establish a population pharmacokinetic/pharmacodynamic （PK/PD） model for valproate （VPA） in children with epilepsy in China, and promote reasonable use of antiepileptic drug in clinical practice. METHODS： Sparse data of VPA serum concentrations from 417 pediatric children were collected. These patients were divided into three groups： Population PK-Index group, n = 317; Population PK-Valid group, n = 100; 115 of the total 417 subjects were also included in the Population PD group. Population PK parameters of VPA were estimated based on the data from population PK-Index group. In the validation procedure, the serum concentrations of VPA from the population PK-Valid group were predicted by base and final models respectively. To assess the accuracy and precision of the predictions, mean prediction error （MPE）, mean squared prediction error （MSPE）, root mean squared prediction error （RMSPE）, weight-residues （WRES） and its 95 % confidence intervals （95 % CI） were all calculated, then compared between the two models. For population PD group, all of the ! 15 patients were VPA monotherapy. Efficacy of epilepsy treatment was divided into 5 grades according to the percentage of seizure frequency decreased （ PSFD% ）. The value of PSFD% 100%, 75% - 100%, 50% - 75%, 25% - 50%, or less than 25 % are corresponding to grade 1 to 5. For population PD group, the quantitative relationship between the VPA serum concentrations and the probability for its efficacy score was characterized by logistic regression.[第一段]     

卡培他滨乳腺组织穿透及向氟脲嘧啶转化的药动学研究

目的描述卡培他滨在血浆、乳腺肿瘤组织和正常组织中向氟脲嘧啶转化的药物动力学过程。方法27名乳腺癌化疗患者口服卡培他滨1 255 mg·m-2,同步采集血浆、肿瘤组织和相邻正常组织样品,并用HPLC法测定卡培他滨和5-FU的浓度。使用自建的药物动力学模型拟合卡培他滨和5-FU的药时曲线,并利用拟合参数计算血浆和乳腺组织中卡培他滨和5-FU的组织分布因子,以及卡培他滨向5-FU的转化率。结果卡培他滨在肿瘤组织中的组织分布因子(AUCTumor/AUCplasma)为0.637 1,正常组织中的组织分布因子(AUCH-tissue/AUCplasma)为0·851 4;5-FU在乳腺肿瘤的组织分布因子AUCTumor/AUCplasma为3.992 6,正常组织的组织分布因子AUCH-tissue/AUCplasma为2.438 0。卡培他滨向5-FU的转化率在肿瘤、邻近正常组织和血浆中分别为1.026、0.489 5和0.163。结论血浆、乳腺肿瘤组织和相邻正常组织的药时曲线拟合对卡培他滨转化为5-FU及5-FU消除过程进行了较好的描述。卡培他滨在血浆、乳腺肿瘤组织和相邻正常组织中的分布较为接近,其活性代谢产物5-FU在肿瘤组织中分布为血浆的10·14倍,为正常组织的3·41倍。卡培他滨在肿瘤组织中向5-FU的转化率较高。