Back to basics: understanding drugs in children: pharmacokinetic maturation

The Blind Spot of Pharmacology: A Scoping Review of Drug Metabolism in Prematurely Born Children

The limit for possible survival after extremely preterm birth has steadily improved and consequently, more premature neonates with increasingly lower gestational age at birth now require care. This specialized care often include intensive pharmacological treatment, yet there is currently insufficient knowledge of gestational age dependent differences in drug metabolism. This potentially puts the preterm neonates at risk of receiving sub-optimal drug doses with a subsequent increased risk of adverse or insufficient drug effects, and often pediatricians are forced to prescribe medication as off-label or even off-science. In this review, we present some of the particularities of drug disposition and metabolism in preterm neonates. We highlight the challenges in pharmacometrics studies on hepatic drug metabolism in preterm and particularly extremely (less than 28 weeks of gestation) preterm neonates by conducting a scoping review of published literature. We find that >40% of included studies failed to report a clear distinction between term and preterm children in the presentation of results making direct interpretation for preterm neonates difficult. We present summarized findings of pharmacokinetic studies done on the major CYP sub-systems, but formal meta analyses were not possible due the overall heterogeneous approaches to measuring the phase I and II pathways metabolism in preterm neonates, often with use of opportunistic sampling. We find this to be a testament to the practical and ethical challenges in measuring pharmacokinetic activity in preterm neonates. The future calls for optimized designs in pharmacometrics studies, including PK/PD modeling-methods and other sample reducing techniques. Future studies should also preferably be a collaboration between neonatologists and clinical pharmacologists.

Dose Optimization of Vancomycin Using a Mechanism-based Exposure-Response Model in Pediatric Infectious Disease Patients

PURPOSE

This study aimed to determine the appropriate vancomycin dosage, considering patient size and organ maturation, by simulating the bacterial count and biomarker level for drug administration in pediatric patients with gram-positive bacterial (GPB) infections.

METHODS

Natural language processing for n-gram analysis was used to detect appropriate pharmacodynamic (PD) markers in infectious disease patients. In addition, a mechanism-based model was established to describe the systemic exposure and evaluate the PD marker simultaneously in pediatric patients. A simulation study was then conducted by using a mechanism-based model to evaluate the optimal dose of vancomycin in pediatric patients.

FINDINGS

C-reactive protein (CRP) was selected as a PD marker from an analysis of ~270,000 abstracts in PubMed. In addition, clinical results, including the vancomycin plasma concentrations and CRP levels of pediatric patients (n = 93), were collected from electronic medical records. The vancomycin pharmacokinetic model with allometric scaling and a maturation function was built as a one-compartment model, with an additional compartment for bacteria. Both the effects of vancomycin plasma concentrations on the destruction of bacteria and those of bacteria on CRP production rates were represented by using a maximum achievable effect model (E_max model). Simulation for dose optimization was conducted not only by using the final model but also by exploring the possibility of therapeutic failure based on the MICs of vancomycin for GPB. Clinical cure was defined as when the CRP level fell below the upper limit of the normal range. Our dose optimization simulations suggested a vancomycin dosage of 10 mg/kg every 8 h as the optimal maintenance dose for pediatric patients with a postconceptual age <30 weeks and 10 mg/kg every 6 h for older children, aged up to 12 years. In addition, the MIC of 3 μg/mL was assessed as the upper concentration limit associated with successful vancomycin treatment of GPB infections.

IMPLICATIONS

This study confirmed that the changes in bacterial counts and CRP levels were well described with mechanistic exposure-response modeling of vancomycin. This model can be used to determine optimal empiric doses of vancomycin and to improve therapeutic outcomes in pediatric patients with GPB.

Mass Drug Administration With High-Dose Ivermectin and Dihydroartemisinin-Piperaquine for Malaria Elimination in an Area of Low Transmission With High Coverage of Malaria Control Interventions: Protocol for the MASSIV Cluster Randomized Clinical Trial

BACKGROUND

With a decline in malaria burden, innovative interventions and tools are required to reduce malaria transmission further. Mass drug administration (MDA) of artemisinin-based combination therapy (ACT) has been identified as a potential tool to further reduce malaria transmission, where coverage of vector control interventions is already high. However, the impact is limited in time. Combining an ACT with an endectocide treatment that is able to reduce vector survival, such as ivermectin (IVM), could increase the impact of MDA and offer a new tool to reduce malaria transmission.

OBJECTIVE

The study objective is to evaluate the impact of MDA with IVM plus dihydroartemisinin-piperaquine (DP) on malaria transmission in an area with high coverage of malaria control interventions.

METHODS

The study is a cluster randomized trial in the Upper River Region of The Gambia and included 32 villages (16 control and 16 intervention). A buffer zone of ~2 km was created around all intervention clusters. MDA with IVM plus DP was implemented in all intervention villages and the buffer zones; control villages received standard malaria interventions according to the Gambian National Malaria Control Program plans.

RESULTS

The MDA campaigns were carried out from August to October 2018 for the first year and from July to September 2019 for the second year. Statistical analysis will commence once the database is completed, cleaned, and locked.

CONCLUSIONS

This is the first cluster randomized clinical trial of MDA with IVM plus DP. The results will provide evidence on the impact of MDA with IVM plus DP on malaria transmission.

TRIAL REGISTRATION

ClinicalTrials.gov NCT03576313; https://clinicaltrials.gov/ct2/show/NCT03576313.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

DERR1-10.2196/20904.

Vancomycin Dosage and Its Association with Clinical Outcomes in Pediatric Patients with Gram-Positive Bacterial Infections

Aim

The aim of this study was to evaluate whether vancomycin trough concentrations at initial steady state are associated with clinical and microbiological outcomes along with vancomycin-related nephrotoxicity in pediatric patients with Gram-positive bacterial (GPB) infections.

Methods

A retrospective cohort study of pediatric patients who received vancomycin for ≥72 hours during 2008–2016 was conducted. Study patients were divided into three cohorts in accordance with their first trough levels at steady state: <5 mg/L (lower-trough), 5–10 mg/L (low-trough), and >10 mg/L (high-trough; reference) cohorts.

Results

Of the 201 patients eligible for study inclusion, 60 patients in the lower- and low-trough cohorts, respectively, were idect 3ntified via propensity score matching and analyzed against 30 high-trough patients in each comparison pair (neonates were excluded due to small sample size). Lower-trough patients were at a greater risk for prolonged therapy, retreatment, and dose adjustment than high-trough patients. Final steady-state troughs remained substantially lower in both the lower- and low-trough cohorts (p<0.001 and p=0.005, respectively), despite greater dose up-titration in the lower-trough cohort and percent change in daily dose in both the lower- and low-trough cohorts than in the high-trough cohort (p<0.001 for all). Clinical cure and death risk, along with the risks of isolation of resistant strains and renal events, were not significantly different between cohorts in both comparison pairs.

Conclusion

Vancomycin troughs of <5 mg/L at initial steady state were associated with significantly compromised clinical outcomes in terms of risk of therapy prolongation, retreatment, and aggressive dose up-titration, compared to >10 mg/L troughs in pediatric patients with GPB infections.

Age, Weight, and CYP2D6 Genotype Are Major Determinants of Primaquine Pharmacokinetics in African Children

Low-dose primaquine is recommended to prevent Plasmodium falciparum malaria transmission in areas threatened by artemisinin resistance and areas aiming for malaria elimination. Community treatment campaigns with artemisinin-based combination therapy in combination with the gametocytocidal primaquine dose target all age groups, but no studies thus far have assessed the pharmacokinetics of this gametocytocidal drug in African children. We recruited 40 children participating in a primaquine efficacy trial in Burkina Faso to study primaquine pharmacokinetics. These children received artemether-lumefantrine and either a 0.25- or a 0.40-mg/kg primaquine dose. Seven blood samples were collected from each participant for primaquine and carboxy-primaquine plasma levels determinations: one sample was collected before primaquine administration and six after primaquine administration according to partially overlapping sampling schedules. Physiological population pharmacokinetic modeling was used to assess the impact of weight, age, and CYP2D6 genotype on primaquine and carboxy-primaquine pharmacokinetics. Despite linear weight normalized dosing, the areas under the plasma concentration-time curves and the peak concentrations for both primaquine and carboxy-primaquine increased with age and body weight. Children who were CYP2D6 poor metabolizers had higher levels of the parent compound, indicating a lower primaquine CYP2D6-mediated metabolism. Our data indicate that primaquine and carboxy-primaquine pharmacokinetics are influenced by age, weight, and CYP2D6 genotype and suggest that dosing strategies may have to be reconsidered to maximize the transmission-blocking properties of primaquine. (This study has been registered at ClinicalTrials.gov under registration no. NCT01935882.)

Pharmacokinetics of mycophenolic acid in children with clinically stable idiopathic nephrotic syndrome receiving cyclosporine

BACKGROUND

The suitable dosage regime of mycophenolate mofetil (MMF) based on the pharmacokinetics of mycophenoric acid (MPA) for pediatric patients with idiopathic nephrotic syndrome (INS) is controversial. The pharmacokinetics of MPA is influenced by renal function, serum albumin concentration, and concomitant medications, especially calcineurin inhibitors. This study analyzed the pharmacokinetics of MPA in clinically stable children with INS receiving cyclosporine (CyA).

METHODS

This retrospective study enrolled children with INS receiving MMF (Cellcept^®) (30-40 mg/kg/day in two divided doses) combined with CyA (Neoral^®) without relapse and renal dysfunction. Pharmacokinetic parameters, including the area under the concentration-time curve (AUC) calculated by the trapezoid method, were calculated from seven serial blood samples.

RESULTS

Thirty-two patients (22 males) of median age 11.0 years were included; 32 pharmacokinetic studies were performed. The median MMF dose was 16.2 mg/kg/time or 470.4 mg/m²/time. The median AUC0-12 was 44.3 ng h/mL. AUC0-12 of all patients showed excellent correlations with C2 (r ²  = 0.6405, P < 0.0001), resulting in a regression formula of AUC0-12 = 21.971 + 2.6059 C2. Comparisons of dose/body weight-normalized AUC0-12 values among age groups showed a lower value in the youngest group (≤5 years).

CONCLUSION

In children with clinically stable INS receiving CyA, C2 monitoring was the most useful single parameter for estimating MPA pharmacokinetics. Younger children required higher MMF doses.

Developmental pharmacokinetics in pediatric populations

Information on drug absorption and disposition in infants and children has increased considerably over the past 2 decades. However, the impact of specific age-related effects on pharmacokinetics, pharmacodynamics, and dose requirements remains poorly understood. Absorption can be affected by the differences in gastric pH and stomach emptying time that have been observed in the pediatric population. Low plasma protein concentrations and a higher body water composition can change drug distribution. Metabolic processes are often immature at birth, which can lead to a reduced clearance and a prolonged half-life for those drugs for which metabolism is a significant mechanism for elimination. Renal excretion is also reduced in neonates due to immature glomerular filtration, tubular secretion, and reabsorption. Limited data are available on the pharmacodynamic behavior of drugs in the pediatric population. Understanding these age effects provide a mechanistic way to identify initial doses for the pediatric population. The various factors that impact pharmacokinetics and pharmacodynamics mature towards adult values at different rates, thus requiring continual modification of drug dose regimens in neonates, infants, and children. In this paper, the age-related changes in drug absorption, distribution, metabolism, and elimination in infants and children are reviewed, and the age-related dosing regimens for this population are discussed.

Prevalence of renal dysfunction in tacrolimus-treated pediatric transplant recipients: a systematic review

Renal dysfunction after non-renal transplantation in adult tacrolimus-treated transplant patients is well documented. Little is known about its prevalence in children. Age-related changes in both disposition and effect of tacrolimus as well as renal function may preclude extrapolation of adult data to children. To systematically review the literature on renal dysfunction in non-renal pediatric transplant recipients treated with tacrolimus. PubMed/Medline, Embase, and Google were searched from their inception until April 19, 2012, with the search terms "tacrolimus," "renal function," "transplantation," and "children." Eighteen of 385 retrieved papers were considered relevant. Twelve dealt with liver, four with heart transplant, one with heart and lung transplant, and one with intestinal recipients. Reported prevalences of mild and severe chronic kidney disease ranged from 0% to 39% and 0% to 71.4%, respectively, for liver, and from 22.7% to 40% and 6.8% to 46%, respectively, for heart and/or lung transplant recipients. Ranges remained wide after adjusting for follow-up time and disease severity. Possible explanations are inclusion bias and definitions used for renal dysfunction. A considerable proportion of pediatric non-renal transplant patients who receive tacrolimus-based immunosuppression, appear to suffer from chronic kidney disease. This conclusion warrants further research into the real risk, its risk factors, and individualization of immunosuppressant therapy.

Neonatal pharmacology: extensive interindividual variability despite limited size

Providing safe and effective drug therapy to neonates requires knowledge of the impact of development on the pharmacokinetics and pharmacodynamics of drugs. Although maturational changes are observed throughout childhood, they are most prominent during the first year of life. Several of these processes overlap, making development an extremely dynamic system in the newborn compared with that in infants, children, or adults. Changes in body composition and porportions, liver mass, metabolic activity, and renal function collectively affect the pharmacokinetic behavior of medications. Instead of simply adapting doses by scaling adult or pediatric doses on the basis of a patient's weight and/or body surface area, integrated knowledge of clinical maturation and developmental pharmacology is critical to the safe and effective use of medications in neonates. Unfortunately, the effects of human ontogeny on both pharmacokinetics and pharmacodynamics have not been well established in these early stages of life, and information regarding the influence of developmental changes on the pharmacodynamics of medications is even more limited. Theoretically, age-dependent variations in receptor number and affinity for drugs have significant potential to influence an individual's response to drug therapy. In this review, some of the relevant covariates of pharmacokinetics and pharmacodynamics in neonates are reviewed and illustrated based on the published literature.

Pharmacokinetic and pharmacogenetic determinants and considerations in chemotherapy selection and dosing in infants

INTRODUCTION

There is a lack of high-quality data regarding optimal chemotherapy dosage regimens among infants. Dosing regimens for chemotherapy during the first year of life are commonly based on empiric recommendations extrapolated from older children; however, balancing efficacy and toxicity is critical as severe adverse drug reactions may lead to treatment failure or reduced adherence to needed medications.

AREAS COVERED

This review describes pharmacokinetic and pharmacogenetic considerations when administering chemotherapeutic agents to infants. Examples of commonly used agents are provided with practical recommendations for dosing adjustments.

EXPERT OPINION

Optimal chemotherapy for children and infants in particular has lagged behind the remarkable progress in cancer treatment and it is clear that far more basic and clinical research are needed with respect to the mechanistic basis of age-dependent differences in pharmacokinetic parameters. More recent studies which have combined pharmacokinetic data with clinical toxicity and outcome data have resulted in a number of more evidence-based guidelines at least for the initial chemotherapy dosing; however, at present, the dosing of chemotherapy drugs in neonates and infants remains largely empiric.

Adverse drug reactions in newborns, infants and toddlers: pediatric pharmacovigilance between present and future

INTRODUCTION

The detection, assessment, understanding and prevention of adverse drug reactions (ADRs) are the primary aims of pharmacovigilance activities. Pediatric patients, especially all newborns and infants, are particularly at risk for experiencing drug-related adverse events.

AREAS COVERED

This review briefly analyzes the physiological peculiarities of pharmacodynamic and pharmacokinetic aspects of drugs in newborns, infants and toddlers and children. It also deals with specific pediatric pharmacovigilance aspects, such as the frequent use of unlicensed and/or off-label drugs in neonatal intensive care units in European countries and in Australia. This review reports on European, American and Canadian data about the incidence and type of pediatric ADRs, particularly focusing on neonates, infants and toddlers.

EXPERT OPINION

The awareness of pediatricians about the importance of reporting ADRs should be stimulated, new reporting systems should be encouraged and pediatric pharmacovigilance activities should be improved, first, by intensifying active post-marketing surveillance methods.

The pharmacokinetic evaluation of artemisinin drugs for the treatment of malaria in paediatric populations

INTRODUCTION

The use of artemisinin combination therapies to treat uncomplicated malaria is growing and, therefore, so is the number of children exposed to these agents. As a result, there is a huge drive to develop paediatric formulations. However, relatively limited data exist regarding the pharmacokinetic properties of these drugs in this vulnerable population.

AREAS COVERED

The article reviews the pharmacokinetic data for artemisinin drugs used for the treatment of malaria in paediatric populations. The authors discuss how developmental and environmental factors can produce significant variation in the pharmacokinetic properties of artemisinin drugs. The authors also discuss how this variation may lead to suboptimal therapeutic drug concentrations with implications on efficacy, safety and the development of parasite resistance to these drugs.

EXPERT OPINION

There is currently a lack of published studies on the pharmacokinetics of artemisinin drugs in children and this subject is complicated by several interdependent variables. Therefore, the construction of a systems-based model of this subject should be a priority area in order to identify gaps in current knowledge to ensure their continued effective and safe use.

Neonatal pharmacology--pharmacokinetics

The goal of drug therapy is a defined clinical response. Drug therapy in the vulnerable preterm and term newborn population is complicated by developmental limitations that affect the pharmacokinetics of most drugs. It is important that, in any therapy, care providers maintain a keen awareness of the expected response while monitoring the infant's clinical response throughout drug administration. This column has addressed pharmacologic principles, especially the pharmacokinetics of drug therapy, to guide clinical assessments of patients' responses to drug therapy.

Single-dose pharmacokinetics of famciclovir in infants and population pharmacokinetic analysis in infants and children

A multicenter, open-label study evaluated the single-dose pharmacokinetics and safety of a pediatric oral famciclovir (prodrug of penciclovir) formulation in infants aged 1 to 12 months with suspicion or evidence of herpes simplex virus infection. Individualized single doses of famciclovir based on the infant's body weight ranged from 25 to 175 mg. Eighteen infants were enrolled (1 to <3 months old [n = 8], 3 to <6 months old [n = 5], and 6 to 12 months old [n = 5]). Seventeen infants were included in the pharmacokinetic analysis; one infant experienced immediate emesis and was excluded. Mean C(max) and AUC(0-6) values of penciclovir in infants <6 months of age were approximately 3- to 4-fold lower than those in the 6- to 12-month age group. Specifically, mean AUC(0-6) was 2.2 microg h/ml in infants aged 1 to <3 months, 3.2 microg h/ml in infants aged 3 to <6 months, and 8.8 microg h/ml in infants aged 6 to 12 months. These data suggested that the dose administered to infants <6 months was less than optimal. Eight (44.4%) infants experienced at least one adverse event with gastrointestinal events reported most commonly. An updated pharmacokinetic analysis was conducted, which incorporated the data in infants from the present study and previously published data on children 1 to 12 years of age. An eight-step dosing regimen was derived that targeted exposure in infants and children 6 months to 12 years of age to match the penciclovir AUC seen in adults after a 500-mg dose of famciclovir.

An open-label study of aripiprazole: pharmacokinetics, tolerability, and effectiveness in children and adolescents with conduct disorder

OBJECTIVES

This study evaluated flexible-dose pharmacokinetics, safety, and effectiveness of aripiprazole in children and adolescents with conduct disorder (CD).

METHODS

This open-label, 15-day, three-center study with an optional 36-month extension enrolled a total of 23 patients: 12 children (6-12 years) and 11 adolescents (13-17 years) with CD and a score of 2-3 on the Rating of Aggression Against People and/or Property (RAAPP). Initially, the protocol used the following dosing: subjects <25 kg, 2 mg/day; subjects 25-50 kg, 5 mg/day; subjects >50-70 kg, 10 mg/day; and subjects >70 kg, 15 mg/day. Due to vomiting and sedation, this schedule was revised to: <25 kg, 1 mg/day; 25-50 kg, 2 mg/day; >50-70 kg, 5 mg/day; and >70 kg, 10 mg/day.

RESULTS

Aripiprazole pharmacokinetics were linear, and steady state appeared to be attained within 14 days. Both groups demonstrated improvements in RAAPP scores and Clinical Global Impressions-Severity (CGI-S) scores. Adverse events were similar to the known profile for aripiprazole in adults.

CONCLUSION

The pharmacokinetics of aripiprazole in children and adolescents are linear and comparable with those in adults. Aripiprazole was generally well-tolerated in patients with CD, particularly after protocol adjustments, with improvements in aggressive behavior.

Optimization of Immunosuppressive Drug Monitoring in Children

BACKGROUND

The metabolism of drugs in children differs from adults, although pediatric pharmacokinetic (PK) studies remain scarce. Many of the drugs are metabolized by polymorphically expressed enzymes (cytochrome P450 [CYP450]; glucuronyl transferase [GT]) and/or transported by drug transporters (ABC and SLC families). In children, there is added complexity because of the age dependency of drug metabolism. This review addresses the age dependency of drug metabolism in childhood on the basis of routine PK monitoring.

METHODS

Standard pharmacokinetic studies in pediatric renal transplant recipients were analyzed to study drug-drug interactions between mycophenolic acid and cyclosporine on the one hand, and tacrolimus and sirolimus on the other hand. The exposure was compared with age. We also studied sirolimus metabolites, both by mass spectrometry as well as using human liver microsomes.

RESULTS

We demonstrated age dependency for MPA exposure. Independent of the concomitant medication, infants required approximately twice as much drug for the same exposure. The drug-drug interaction between sirolimus and tacrolimus demonstrated age dependency. Sirolimus metabolites showed a remarkably different pattern in children. Whereas 39-O-desmethyl sirolimus is the most prevalent metabolite in adults, we found 77.5% hydroxylated metabolites in children. Similarly, pediatric human liver microsomes produced 86.1% hydroxylated metabolites.

CONCLUSIONS

Our long-term objective is to develop evidence-based guidelines for age-appropriate drug dosing of all drugs commonly used during childhood and adolescence, based on pharmacokinetically/pharmacogenetically determined drug exposure to maximize therapeutic yield while minimizing toxicity. The potential need for lifelong medications warrants efforts to minimize toxicity in chronically ill pediatric patients.