Oral anticoagulation with warfarin is significantly influenced by steroids and CYP2C9 polymorphisms in children with cancer

Personalised Warfarin Dosing in Children Post-cardiac Surgery

Warfarin dosing is challenging due to a multitude of factors affecting its pharmacokinetics (PK) and pharmacodynamics (PD). A novel personalised dosing algorithm predicated on a warfarin PK/PD model and incorporating CYP2C9 and VKORC1 genotype information has been developed for children. The present prospective, observational study aimed to compare the model with conventional weight-based dosing. The study involved two groups of children post-cardiac surgery: Group 1 were warfarin naïve, in whom loading and maintenance doses were estimated using the model over a 6-month duration and compared to historical case-matched controls. Group 2 were already established on maintenance therapy and randomised into a crossover study comparing the model with conventional maintenance dosing, over a 12-month period. Five patients enrolled in Group 1. Compared to the control group, the median time to achieve the first therapeutic INR was longer (5 vs. 2 days), to stable anticoagulation was shorter (29.0 vs. 96.5 days), to over-anticoagulation was longer (15.0 vs. 4.0 days). In addition, median percentage of INRs within the target range (%ITR) and percentage of time in therapeutic range (%TTR) was higher; 70% versus 47.4% and 83.4% versus 62.3%, respectively. Group 2 included 26 patients. No significant differences in INR control were found between model and conventional dosing phases; mean %ITR was 68.82% versus 67.9% (p = 0.84) and mean %TTR was 85.47% versus 80.2% (p = 0.09), respectively. The results suggest model-based dosing can improve anticoagulation control, particularly when initiating and stabilising warfarin dosing. Larger studies are needed to confirm these findings.

UNUSUAL WARFARIN DOSE TO ACHIEVE THERAPEUTIC INR IN A 4-MONTH OLD CHILD: NON-GENETICS RISK FACTORS ARE STILL A CHALLENGE

Objective:

To report a case of a 4-month old girl that required 0.7 mg/kg/day (5 mg) of warfarin and discuss relevant risk factors for requiring higher doses.

Case Description:

In November 2015, a 5 kg female infant (36-week preterm) was admitted to the hospital due to status epilepticus and fever. Diazepam, phenytoin and ceftriaxone were prescribed. Cerebrospinal fluid contained 7 leukocytes, 150 mg/dL proteins, 1 mg/dL glucose and gram positive cocci were observed. Cranial tomography suggested hypodense signs in the cerebellum, right temporal lobe and left basal nuclei, which was consistent with pneumococcal meningitis-induced infectious vasculitis. She required low molecular weight heparin and warfarin for post-encephalitis thrombosis. About 10 days were required to achieve therapeutic INR, and warfarin was adjusted five times since the initial prescription.

Comments:

The risk factors for higher warfarin doses were age and enteral tube feeding. Phenobarbital and prednisone might also have contributed with one of the highest warfarin dose ever reported. Despite current importance given to genetics testing, clinicians should attempt to identify common contributing factors for prolonged non-therapeutic INR, to minimize the risk of coagulation, and to reduce costs of hospital stay and laboratory exams.

Frequency of CYP2C9 and VKORC1 gene polymorphisms and their influence on warfarin dose in Egyptian pediatric patients

INTRODUCTION

Warfarin is a widely used anticoagulant that shows a high inter-individual variability in the dose needed to achieve target anticoagulation. In adults, common genetic variants in the cytochrome P450-2C9 (CYP2C9) and vitamin K epoxide reductase complex (VKORC1) enzymes, in addition to non-genetic factors, explain this dose variability. In children, data about warfarin pharmacogenetics are limited and inconsistent.

METHODS

CYP2C9 (*2 and *3) alleles and the VKORC1 (C1173T and G-1639A) polymorphisms were studied by multiplex real time polymerase chain reaction in 41 pediatric patients who received stable warfarin maintenance dose.

RESULTS

The allele frequency of the studied genes was CYP2C9*2 (0.085), CYP2C9*3 (0.12), VKORC1 1173T (0.52), and VKORC1 -1639A (0.54). In univariate analysis, patients' age, weight, and height were significantly (p < 0.0001) associated with warfarin maintenance dose. However, CYP2C9 and VKORC1 gene polymorphisms did not affect warfarin dose. In multivariate analysis, age was found to be the only significant determinant of daily warfarin maintenance dose (p = 0.045).

CONCLUSION

Age was the most significant determinant of warfarin dosage in this preliminary study including Egyptian pediatric patients. Further studies involving larger numbers of children are warranted to determine the true impact of genetic factors on warfarin doses in pediatric patients.

Pharmacokinetics and pharmacodynamics of anticoagulants in paediatric patients

Given the rising incidence of thrombotic complications in paediatric patients, understanding of the pharmacologic behaviour of anticoagulant drugs in children has gained importance. Significant developmental differences between children and adults in the haemostatic system and pharmacologic parameters for individual drugs highlight potentially unique aspects of anticoagulant pharmacology in this special and vulnerable population. This review focuses on pharmacologic information relevant to the dosing of unfractionated heparin, low molecular weight heparin, warfarin, bivalirudin, argatroban and fondaparinux in paediatric patients. The bulk of clinical experience with paediatric anticoagulation rests with the first three of these agents, each of which requires higher bodyweight-based dosing for the youngest patients, compared with adults, in order to achieve comparable pharmacodynamic effects, likely related to an inverse correlation between age and bodyweight-normalized clearance of these drugs. Whether extrapolation of therapeutic ranges targeted for adult patients prescribed these agents is valid for children, however, is unknown and a high priority for future research. Novel oral anticoagulants, such as dabigatran, rivaroxaban and apixaban, hold promise for future use in paediatrics but require further pharmacologic study in infants, children and adolescents.

VKORC1 and CYP2C9 genotypes are predictors of warfarin-related outcomes in children

BACKGROUND

Despite substantial evidence supporting a pharmacogenetic approach to warfarin therapy in adults, evidence on the importance of genetics in warfarin therapy in children is limited, particularly for clinical outcomes. We assessed the contribution of CYP2C9/VKORC1/CYP4F2 genotypes and variation in other genes involved in vitamin K and coagulation pathways to warfarin dose and related clinical outcomes in children.

PROCEDURE

Clinical and genetic data for 93 children (age ≤ 18 years) who received warfarin therapy were obtained. DNA was genotyped for 93 selected single nucleotide polymorphisms using a custom assay.

RESULTS

With a median age of 4.8 years, our cohort included more young children than most previous studies. Overall, 76.3% of dose variability was explained by weight, indication, VKORC1-1639G/A and CYP2C9 *2/*3, with genotypes accounting for 21.1% of variability. There was a strong correlation (R(2) = 0.68; P < 0.001) between actual and predicted warfarin dose using a pediatric genotype-based dosing model. VKORC1 genotype had a significant impact on time to therapeutic international normalized ratio (INR) (P = 0.047) and time to over-anticoagulation (INR > 4; P = 0.024) during the initiation of therapy. CYP2C9*3 carriers were also at increased risk of major bleeding while receiving warfarin (adjusted OR = 11.28). An additional variant in CYP2C9 (rs7089580) was significantly associated with warfarin dose (P = 0.020) in a multivariate clinical and genetic model.

CONCLUSIONS

This study confirms the importance of VKORC1/CYP2C9 genotypes for warfarin dosing in a young pediatric cohort and demonstrates an impact of genetic factors on clinical outcomes in children. Furthermore, we identified an additional variant in CYP2C9 of potential relevance for warfarin dosing in children.

Vitamin K antagonists in children with heart disease: height and VKORC1 genotype are the main determinants of the warfarin dose requirement

Managing vitamin K antagonist (VKA) therapy is challenging in children because of a narrow therapeutic range and wide inter- and intra-individual variability in dose response. Only a few small studies have investigated the effect of nongenetic and genetic factors on the dose response to VKAs in children. In a cohort study including 118 children (median age 9 years; range, 3 months-18 years) mostly with cardiac disease, we evaluated by multivariate analysis the relative contribution of nongenetic factors and VKORC1/CYP2C9/CYP4F2 genotypes on warfarin (n = 83) or fluindione (n = 35) maintenance dose and the influence of these factors on the time spent within/above/below the range. The results showed that height, target international normalized ratio and VKORC1 and CYP2C9 genotypes were the main determinants of warfarin dose requirement, accounting for 48.1%, 4.4%, 18.2%, and 2.0% of variability, respectively, and explaining 69.7% of the variability. Our model predicted the warfarin dose within 7 mg/wk in 86.7% of patients. None of the covariates was associated with the time spent above or below the international normalized ratio range. Whether this model predicts accurately the effective maintenance dose is currently being investigated.

VKORC1 and CYP2C9 genotype and patient characteristics explain a large proportion of the variability in warfarin dose requirement among children

Although genetic and environmental factors explain approximately half of the interindividual variability in warfarin dose requirement in adults, there is limited information available in children. In a cross-sectional study of anticoagulated children from 5 tertiary care centers, 120 children with a stable warfarin dose were genotyped for VKORC1 (-1639G > A; rs9923231), CYP2C9 (*2 and *3 alleles; rs1799853 and rs1057910), and CYP4F2 (V433M; rs2108622) polymorphisms. Clinical and demographic features were recorded. Multiple regression analysis of the data showed that, although CYP4F2 made no contribution to the dose model, 72.4% of the variability in warfarin dose requirement is attributed to by patient height, genetic polymorphisms in VKORC1 and CYP2C9, and indication for warfarin. The recently published International Warfarin Pharmacogenetics Consortium pharmacogenetic-based warfarin dosing algorithm (based on data derived from anticoagulated adults) consistently overestimated warfarin dose for our cohort of children. A similar proportion of the interindividual variability in warfarin dose is explained by genetic factors in children compared with adult patients, although height is a greater predictor in children. A pharmacogenomic approach to warfarin dosing has the potential to improve the efficacy and safety of warfarin therapy in children. However, algorithms should be derived from data in children if their potential benefit is to be realized.

Profound changes in drug metabolism enzymes and possible effects on drug therapy in neonates and children

INTRODUCTION

There are profound changes that take place in drug metabolism enzymes during fetal and postnatal development. These changes may significantly impact drug therapy in children.

AREAS COVERED

A combination of focused and comprehensive literature searches using PubMed and reference lists (from inception to 7 November 2009) is undertaken to identify reports on in vitro and in vivo development of drug metabolism enzymes as well disposition of selected drugs and their effect in children. The article provides an update on development of drug metabolism enzymes and their impact on drug substrate disposition and disease, which may aid to improve clinical practice and optimally design clinical trials in children.

EXPERT OPINION

Drug metabolism enzyme activity changes profoundly throughout the continuum of postnatal development and often results in different disposition pathways than in adults. Genetics and co-morbidity interact significantly with these developmental changes. Translation of existing knowledge into age-adjusted dosing guidelines and clinical trial design is highly needed for there to be an improvement in drug therapy in children.

In pediatric patients, age has more impact on dosing of vitamin K antagonists than VKORC1 or CYP2C9 genotypes

Anticoagulation with vitamin K antagonists (VKAs) is problematic because of difficulties in safely managing dosing. Polymorphisms in cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase genes (VKORC1) have been shown to affect VKA dosing in adults. The association of these polymorphisms on VKA dosing in children has not been investigated. The objective of the study was to assess associations of CYP2C9 and VKORC1 polymorphisms and clinical variables on VKA dosing in children. A nonselected cohort of pediatric patients receiving VKA were tested for CYP2C9 and VKORC1 polymorphisms, and clinical data were collected. Multiple linear regression modeling was used to assess relationships of VKA dose with genetic and clinical variables. Fifty-nine patients were recruited; 55.9% were receiving warfarin, and 44.1% were on phenprocoumon. There was a negative association of age with VKA dose (P < .001). Comparing VKORC1 genotypes, the AA group required significantly lower daily doses than GG group (P = .011). In the full model including age, VKORC1 and CYP2C9 genotypes accounted for 38% of dose variation. Age explained 28.3% of VKA dose variations; VKORC1 and CYP2C9 explained only 3.7% and 0.4%, respectively. In children, the most critical factor in determining VKA dose is age. VKORC1/CYP2C9 genotypes only marginally explain dose variations.