Population pharmacokinetics and maximum a posteriori probability Bayesian estimator of abacavir: application of individualized therapy in HIV-infected infants and toddlers

Abacavir Drug Exposures in African Children Under 14 kg Using Pediatric Solid Fixed Dose Combinations According to World Health Organization Weight Bands

BACKGROUND

The pharmacokinetics of abacavir (ABC) in African children living with HIV (CLHIV) weighing <14 kg and receiving pediatric fixed dose combinations (FDC) according to WHO weight bands dosing are limited. An ABC population pharmacokinetic model was developed to evaluate ABC exposure across different World Health Organization (WHO) weight bands.

METHODS

Children enrolled in the LIVING study in Kenya and Uganda receiving ABC/lamivudine (3TC) dispersible tablets (60/30 mg) according to WHO weight bands. A population approach was used to determine the pharmacokinetic parameters. Monte Carlo simulations were conducted using an in silico population with demographic characteristics associated with African CLHIV. ABC exposures (AUC0-24) of 6.4-50.4 mg h/L were used as targets.

RESULTS

Plasma samples were obtained from 387 children. A 1-compartment model with allometric scaling of clearance (CL/F) and volume of distribution (V/F) according to body weight best characterized the pharmacokinetic data of ABC. The maturation of ABC CL/F was characterized using a sigmoidal Emax model dependent on postnatal age (50% of adult CL/F reached by 0.48 years of age). Exposures to ABC were within the target range for children weighing 6.0-24.9 kg, but children weighing 3-5.9 kg were predicted to be overexposed.

CONCLUSIONS

Lowering the ABC dosage to 30 mg twice daily or 60 mg once daily for children weighing 3-5.9 kg increased the proportion of children within the target and provided comparable exposures. Further clinical study is required to investigate clinical implications and safety of the proposed alternative ABC doses.

Open-source maximum a posteriori-bayesian dosing AdDS to current therapeutic drug monitoring: Adapting to the era of individualized therapy

Recent updates in the therapeutic drug monitoring (TDM) guidelines for vancomycin have rekindled interest in maximum a posteriori-Bayesian (MAP-Bayesian) estimation of patient-specific pharmacokinetic parameters. To create a versatile infrastructure for MAP-Bayesian dosing of vancomycin or other drugs, a freely available, R-based software package, Advanced Dosing Solutions (AdDS), was created to facilitate clinical implementation of these improved TDM methods. The objective of this study was to utilize AdDS for pre- and post-processing of data in order to streamline the therapeutic management of vancomycin in healthy and obese veterans. Patients from a local Veteran Affairs hospital were utilized to compare the process of full re-estimation versus Bayesian updating of priors on healthy adult and obese patient populations for use with AdDS. Twenty-four healthy veterans were utilized to train (14/24) and test (10/24) the base pharmacokinetic model of vancomycin while comparing the effects of updated and fully re-estimated priors. This process was repeated with a total of 18 obese veterans for both training (11/18) and testing (7/18). Comparison of MAP objective function between the original and re-estimated models for healthy adults indicated that 78.6% of the subjects in the training and 70.0% of the subjects in the testing datasets had similar or improved predictions by the re-estimated model. For obese veterans, 81.8% of subjects in the training dataset and 85.7% of subjects in the testing dataset had similar or improved predictions. Re-estimation of model parameters provided more significant improvements in objective function compared with Bayesian updating, which may be a useful strategy in cases where sufficient samples and subjects are available. The generation of bespoke regimens based on patient-specific clearance and minimal sampling may improve patient care by addressing fundamental pharmacokinetic differences in healthy and obese veteran populations.

Population pharmacokinetics of abacavir and lamivudine in severely malnourished human immunodeficiency virus-infected children in relation to treatment outcomes

AIMS

Describe the pharmacokinetics (PK) of the antiretroviral drugs abacavir and lamivudine in malnourished paediatric patients and relate to viral load outcomes after 12 and 48 weeks of treatment.

METHODS

Severely malnourished human immunodeficiency virus-infected children were randomized to early (within 14 days) or delayed (after nutritional recovery) initiation of antiretroviral treatment (ART) using World Health Organization weight-band dosages. Abacavir and lamivudine concentrations were measured as a secondary objective on day 1 and day 14 and patients were followed-up to week 48. Population PK of abacavir and lamivudine were described using NONMEM.

RESULTS

In total, 623 abacavir and 627 lamivudine concentrations were collected from 75 paediatric patients aged 0.1-10.8 (median 1.4) years. Abacavir PK was described by a 2-compartment model, patients randomized to early ART showed increased bioavailability of 31%. Apparent clearance (CL/F, L/h/7 kg) of abacavir increased from day 1 to day 14 from 3.33 (95% confidence interval 2.71-4.12) to 5.86 (95% confidence interval 4.78-7.3). A 1-compartment model described lamivudine PK, variability on CL/F was explained by maturation with age, with age at half-matured CL/F being 4 months. For both drugs allometrically scaled total body weight was related to CL/F and apparent volume of distribution. PK exposure did not correlate with virological outcomes or death at 12 or 48 weeks.

CONCLUSION

Increases in Abacavir's CL/F between day 1 to day 14, bioavailability and PK variability with early start of ART was found in this cohort of severely malnourished children; however, these changes did not influence virological outcomes. The study supports the use of weight-band dosage tables.

Suggestions for Model-Informed Precision Dosing to Optimize Neonatal Drug Therapy

Evidence for dosing, efficacy, and safety of most medications used to treat neonates is sparse. Thus, dosing is usually derived by extrapolation from adult and pediatric pharmacologic data with scaling by body weight or body surface area. This may lead to drug dosing that is unsafe or ineffective. However, new strategies are being developed and studied to dose medications in critically ill neonates. Mass spectroscopy technology capable of quickly analyzing drug levels is readily available. Software that integrates population pharmacokinetics and pharmacodynamics with data from sparse samples from neonates allows for timely adjustments of dosing to achieve the desired effect while minimizing adverse outcomes. Some genetic polymorphisms that affect drug response in neonates have also been reported. This review highlights aspects of drug response and how it is impacted by prematurity, assesses pharmacogenomic studies in neonates, and offers suggestions for innovative pharmacokinetic/pharmacodynamic model-based approaches that combine population- or physiology-based pharmacology data, Bayesian analysis, and electronic decision support tools for precision dosing in neonates while illustrating examples where this approach can be used to optimize medical therapy in neonates. Barriers to implementing precision dosing in neonates and how to overcome them are also discussed.

Integrated semi-physiological pharmacokinetic model for both sunitinib and its active metabolite SU12662

AIMS

Previously published pharmacokinetic (PK) models for sunitinib and its active metabolite SU12662 were based on a limited dataset or lacked important elements such as correlations between sunitinib and its metabolite. The current study aimed to develop an improved PK model that circumvented these limitations and to prove the utility of the PK model in treatment optimization in clinical practice.

METHODS

One thousand two hundred and five plasma samples from 70 cancer patients were collected from three PK studies with sunitinib and SU12662. A semi-physiological PK model for sunitinib and SU12662 was developed incorporating pre-systemic metabolism using non-linear mixed effects modelling (nonmem). Allometric scaling based on body weight was applied. The final model was used for simulation of the PK of different treatment regimens.

RESULTS

Sunitinib and SU12662 PK were best described by a one and two compartment model, respectively. Introduction of pre-systemic formation of SU12662 strongly improved model fit, compared with solely systemic metabolism. The clearance of sunitinib and SU12662 was estimated at 35.7 (relative standard error (RSE) 5.7%) l h(-1) and 17.1 (RSE 7.4%) l h(-1), respectively for 70 kg patients. Correlation coefficients were estimated between inter-individual variability of both clearances, both volumes of distribution and between clearance and volume of distribution of SU12662 as 0.53, 0.48 and 0.45, respectively. Simulation of the PK model predicted correctly the ratio of patients who did not reach proposed PK targets for efficacy.

CONCLUSIONS

A semi-physiological PK model for sunitinib and SU12662 in cancer patients was presented including pre-systemic metabolism. The model was superior to previous PK models in many aspects.

Advances in pediatric pharmacology, therapeutics, and toxicology

In the United States, passage of the FDASIA legislation made BPCA and PREA permanent, no longer requiring reauthorization every 5 years. This landmark legislation also stressed the importance of performing clinical trials in neonates when appropriate. In Europe the Pediatric Regulation, which went into effect in early 2007, also provides a framework for expanding pediatric clinical research. Although much work remains, as a result of greater regulatory guidance more pediatric data are reaching product labels.

Population pharmacokinetics of abacavir in pregnant women

For the first time, a population approach was used to describe abacavir (ABC) pharmacokinetics in HIV-infected pregnant and nonpregnant women. A total of 266 samples from 150 women were obtained. No covariate effect (from age, body weight, pregnancy, or gestational age) on ABC pharmacokinetics was found. Thus, it seems unnecessary to adapt the ABC dosing regimen during pregnancy.

Population pharmacokinetics of abacavir in infants, toddlers and children

AIMS

To characterize the pharmacokinetics of abacavir in infants, toddlers and children and to assess the influence of covariates on drug disposition across these populations.

METHODS

Abacavir concentration data from three clinical studies in human immunodeficiency virus-infected children (n = 69) were used for model building. The children received either a weight-normalized dose of 16 mg kg(-1) day(-1) or the World Health Organization recommended dose based on weight bands. A population pharmacokinetic analysis was performed using nonlinear mixed effects modelling VI. The influence of age, gender, bodyweight and formulation was evaluated. The final model was selected according to graphical and statistical criteria.

RESULTS

A two-compartmental model with first-order absorption and first-order elimination best described the pharmacokinetics of abacavir. Bodyweight was identified as significant covariate influencing the apparent oral clearance and volume of distribution. Predicted steady-state maximal plasma concentration and area under the concentration-time curve from 0 to 12 h of the standard twice daily regimen were 2.5 mg l(-1) and 6.1 mg h l(-1) for toddlers and infants, and 3.6 mg l(-1) and 8.7 mg h l(-1) for children, respectively. Model-based predictions showed that equivalent systemic exposure was achieved after once and twice daily dosing regimens. There were no pharmacokinetic differences between the two formulations (tablet and solution). The model demonstrated good predictive performance for dosing prediction in individual patients and, as such, can be used to support therapeutic drug monitoring in conjunction with sparse sampling.

CONCLUSIONS

The disposition of abacavir in children appears to be affected only by differences in size, irrespective of the patient's age. Maturation processes of abacavir metabolism in younger infants should be evaluated in further studies to demonstrate the potential impact of ontogeny.

Practical Considerations For Developing Nucleoside Reverse Transcriptase Inhibitors

Nucleoside reverse transcriptase inhibitors (NRTI) remain a cornerstone of current antiretroviral regimens in combinations usually with a non-nucleoside reverse transcriptase inhibitor (NNRTI), a protease inhibitor (PI), or an integrase inhibitor (INI). The antiretroviral efficacy and relative safety of current NRTI results from a tight and relatively specific binding of their phosphorylated nucleoside triphosphates (NRTI-TP) with the HIV-1 reverse transcriptase which is essential for replication. The intracellular stability of NRTI-TP produces a sustained antiviral response, which makes convenient dosing feasible. Lessons learned regarding NRTI pharmacology screening, development, and use are discussed. NRTI and prodrugs currently under clinical development are outlined.