Evaluation of safety, pharmacokinetics, and pharmacodynamics of apixaban in pediatric subjects at risk of venous or arterial thrombotic disorder

Abstract

Apixaban is an oral small-molecule, direct factor Xa (FXa) inhibitor approved in adults for treatment of deep vein thrombosis and pulmonary embolism, and for reducing risk of venous thromboembolism recurrence after initial anticoagulant therapy. This phase I study (NCT01707394) evaluated the pharmacokinetics (PKs), pharmacodynamics (PDs), and safety of apixaban in pediatric subjects (<18 years), enrolled by age group, at risk of venous or arterial thrombotic disorder. A single apixaban dose, targeting adult steady-state exposure with apixaban 2.5 mg, was administered using two pediatric formulations: 0.1 mg sprinkle capsule (age <28 days); 0.4 mg/ml solution (age 28 days to <18 years; dose range, 1.08–2.19 mg/m²). End points included safety, PKs, and anti-FXa activity. For PKs/PDs, four to six blood samples were collected ≤26 h postdosing. A population PK model was developed with data from adults and pediatric subjects. Apparent oral clearance (CL/F) included fixed maturation function based on published data. From January 2013 to June 2019, 49 pediatric subjects received apixaban. Most adverse events were mild/moderate, and the most common was pyrexia (n = 4/15). Apixaban CL/F and apparent central volume of distribution increased less than proportionally with body weight. Apixaban CL/F increased with age, reaching adult values in subjects aged 12 to <18 years. Maturation affected CL/F most notably in subjects aged <9 months. Plasma anti-FXa activity values were linearly related to apixaban concentrations, with no apparent age-related differences. Pediatric subjects tolerated single apixaban doses well. Study data and population PK model supported phase II/III pediatric trial dose selection.

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Study Highlights

WHAT IS THE CURRENT KNOWLEDGE ON THE TOPIC?

Apixaban is approved for the initial treatment of deep vein thrombosis and pulmonary embolism, and for prevention of recurrent events in adults.

WHAT QUESTION DID THIS STUDY ADDRESS?

How to evaluate the safety, pharmacokinetics (PKs), and pharmacodynamics (PDs) of apixaban in pediatric subjects at risk of venous or arterial thrombotic disorder events.

WHAT DOES THIS STUDY ADD TO OUR KNOWLEDGE?

Single oral doses of apixaban were well-tolerated by the 49 pediatric subjects (neonate/infant to age <18 years). Apixaban apparent oral clearance was influenced by maturation and body weight; apparent volume of distribution of the central compartment was influenced by body weight; distributional clearance, and absorption were influenced by age. The push-pull method is a viable method for obtaining blood samples in pediatric subjects.

HOW MIGHT THIS CHANGE DRUG DISCOVERY, DEVELOPMENT, AND/OR THERAPEUTICS?

This study characterizes the PK/PD of apixaban in pediatric subjects and will support dosing recommendations for future studies of apixaban in pediatric patients.

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INTRODUCTION

For the treatment of venous thromboembolism (VTE) and prevention of recurrent VTE, the standard-of-care antithrombotics in pediatric subjects include unfractionated heparin/low-molecular-weight heparin (LMWH), aspirin, and vitamin K antagonists (VKAs).¹ In addition, the direct oral anticoagulants (DOACs) rivaroxaban² and dabigatran³ have been recently approved as antithrombotic agents for pediatric patients. However, based on the results of two phase II studies and the phase III EINSTEIN-Jr trial, body-weight–adjusted rivaroxaban (taken with food) has been approved for the treatment of VTE and reduction in the risk of recurrent VTE in pediatric patients from birth to <18 years old and for thromboprophylaxis in pediatric patients aged ≥2 years old with congenital heart disease after the Fontan procedure.² In addition, based on the results of the phase III DIVERSITY trial, the US Food and Drug Administration has approved weight-based dabigatran for use in the treatment of VTE events in pediatric patients aged 8 to <18 years who have been treated with a parenteral anticoagulant for ≥5 days, and to reduce the risk of recurrence of VTE in pediatric patients aged 8 to <18 years who have been previously treated for VTE.³

Apixaban is an orally active, small-molecule, direct factor Xa (FXa) inhibitor approved in adults (a) to reduce the risk of stroke and systemic embolism in patients with nonvalvular atrial fibrillation; (b) for the prophylaxis of deep vein thrombosis (DVT), which may lead to pulmonary embolism (PE) in patients who have undergone hip or knee replacement surgery; and (c) for the treatment of DVT and PE and the reduction in risk of recurrent DVT and PE following initial anticoagulant therapy.^4,5 Apixaban is eliminated via multiple routes, including metabolism, biliary excretion, and direct intestinal excretion, with ~27% of total apixaban clearance occurring via renal excretion,⁶ and may be an effective and convenient oral treatment option with an acceptable safety profile in pediatric subjects. Given the large age and weight ranges associated with pediatric patients that could benefit from this agent, it is important to characterize the pharmacokinetics (PK) of apixaban in this population to support future dosing recommendations.

The objective of our study was to evaluate the PKs, pharmacodynamics (PDs), safety, and tolerability of apixaban in pediatric subjects at risk of venous or arterial thrombotic disorder and to support dose selection for apixaban pediatric program trials. An additional exploratory objective was to assess a “push-pull” method for blood draws from the central venous catheter as a means of conserving blood volume needed for PK and PD sample collection.

METHODS Study design

This phase I, open-label study (NCT01707394) enrolled pediatric subjects from birth to <18 years old and at risk of venous/arterial thrombotic disorder into groups by postnatal age (Figure 1). The postnatal age groups were as follows: birth to 27 days, 28 days to <9 months, 9 months to <2 years, 2 to <6 years, 6 to <12 years, and 12 to <18 years. Enrollment of older age groups (2 to <6 years, 6 to <12 years, and 12 to <18 years) started in parallel, with interim analyses performed prior to enrollment of younger age groups. A single apixaban dose was administered using a dose range of 1.08–2.19 mg/m². Specifically, apixaban was administered as a 0.1 mg sprinkle capsule (neonates only) or as a ready-to-use oral solution containing 0.4 mg/ml apixaban. The oral solution of apixaban was administered via dosing syringe and was administered prior to other concomitant medications either orally or via nasogastric/gastrostomy tube. The 0.1 mg dose of apixaban was administered as sprinkle capsules dispersed in breast milk or formula for neonates. Enrollment was phased, to allow for iterative enrollment based on updates and refinement of the doses for younger age groups, based on data from older age groups (Figure 1).

FIGURE 1. Study design. *Based on 16 subjects aged 2 to [less than]6 years, 6 to [less than]12 years, and 12 to [less than]18 years to confirm or adjust the dose for subjects aged 9 months to [less than]2 years. †Based on four subjects aged 9 months to [less than]2 years and subjects from older age groups to confirm or adjust the dose for subjects aged 28 days to [less than]9 months. ‡Based on all subjects aged 28 days to [less than]18 years to confirm or adjust the dose from birth to age 27 days. §Based on all subjects in all groups who were enrolled prior to study termination.

Apixaban dosing was based on body surface area (BSA) targeted to achieve adult exposures with apixaban 2.5 mg. The BSA was calculated using the Mosteller method: BSA (m²) = square root of (height [cm] × weight [kg]/3600). The mean absolute doses administered per cohort (dose per BSA per subjects in age group) were: birth to 27 days: 0.1 mg; 28 days to <9 months (1.08 mg/m²) = 0.29 mg; 9 months to <2 years (2.43 mg/m²) = 0.59 mg; 2 to <6 years (1.17 mg/m²) = 0.71 mg; 6 to <12 years (1.80 mg/m²) = 1.82 mg; and 12 to <18 years (2.19 mg/m²) = 3.22 mg.

Safety end points included incidence of adverse events (AEs), serious AEs (SAEs), AEs leading to discontinuation, and death, as well as results of clinical laboratory tests, vital sign measurements, and physical examinations. PK end points included area under the concentration–time curve from zero to infinity (AUC_inf), apparent plasma clearance (CL/F), maximum plasma concentration (C_max), absorption rate constant (K_a), time to maximum plasma concentration (T_max), and apparent volume of distribution of the central compartment (V_c/F). PD activity was measured using anti-FXa activity versus apixaban concentration.⁷ The exploratory outcome compared sample collection methods using the push-pull method versus duplicate samples.

Subjects were monitored closely throughout the study for nonserious AEs and SAEs. The collection of nonserious AE information began at apixaban dosing, whereas SAEs were recorded from when the subject's written consent was obtained during the screening period until 30 days after dosing. Subjects were not discharged from the study until the investigator had determined that all AEs had completely resolved or were not of clinical significance. Treatment-emergent AEs were defined as events that began or worsened any time after initiation of apixaban dosing.

Physical examinations were performed at screening, on day 1, and on day 2 prior to study discharge. Physical measurements (including height, weight, and calculated body surface area) were obtained at screening and day 1. If the screening physical examination or measurements were performed within 72 h prior to dosing on day 1, then a single examination counted as both the screening and predose evaluation. The day 1 physical examination must have been performed within 72 h prior to study drug administration. Four to six blood samples were collected ≤26 h after apixaban dosing (Table S1). A sample size of 40 total subjects with the sparse optimized blood sampling scheme (age 9 months to <2 years, n = 12; all other age groups, n = 8) was decided based on simulation and expected to result in estimation of the population mean AUC_inf for each age group, with a 95% probability of being within 60–140% of the true mean.⁸ The evaluation of this sample size was conducted using a clinical trial simulation prior to study from a compartmental population PK model, derived from adult data, accounting for changes in PKs as a function of age and body size using allometric scaling. Treated subjects were defined as all subjects who received at least one dose of apixaban.

For PK/PD analyses, four to six blood samples were collected ≤26 h after dosing from a peripheral vein or through a central venous catheter or arterial line. Following collection, samples were immediately transferred to a 1.0 ml tube containing 3.2% sodium citrate as the anticoagulant. The tube was then gently inverted eight to 10 times for complete mixing with the anticoagulant. Within 1 h following collection, the sample was centrifuged for 15 min at ~1600 × g at room temperature. Immediately following centrifugation, two aliquots of plasma were transferred to two 1.8 ml screw-capped polypropylene transport tubes (one for measurement of apixaban concentration and one for measurement of anti-FXa activity level) and stored at −20°C until being shipped on dry ice to the central laboratory.

Apixaban in human plasma was assayed using two validated (and cross-validated) liquid chromatography tandem mass spectrometry methods (Intertek for samples analyzed through March 2015 and PPD Inc for samples analyzed after November 2016) during the period of known analyte stability, as previously described.⁹

Anti-FXa activity level was measured using a validated method at Covance Esoterix Coagulation Laboratory using the Diagnostic Stago Rotachrom or STA-Liquid Anti-Xa Heparin assay (for samples collected after November 2015) on a STA-Compact analyzer.⁷

A push-pull method (Methods S1) for blood draws from the central venous catheter was investigated (up to a total of 8 subjects from the 6 to <12 years and 12 to <18 years groups) as a means of conserving blood volume needed for PK and PD sample collection. An additional PK and PD sample was obtained on day 2 utilizing venipuncture, or blood-drawing catheter as a reference.

Subject population

Male and female pediatric subjects (birth to age <18 years), with or without a central venous catheter or arterial line, and at risk of a venous or arterial thrombotic disorder, were included. Neonates were included if they had a gestational age ≥34 weeks or post-conceptual age ≥37 weeks. Subjects were excluded if they had a history or evidence of abnormal bleeding, including in a first-degree relative; an active bleed or high risk of bleeding; central nervous system lesions; or risk factors for intracranial bleeding. Subjects were also excluded if they received an anticoagulant within 24 h (except warfarin, which required a 5-day washout period), antiplatelet therapy within 5 days of starting apixaban (except for subjects with Kawasaki's disease on low-dose aspirin), treatment with a potent inhibitor and/or inducer of CYP3A4 or P-gp within 2 weeks of enrollment, alanine aminotransferase >3 times the upper limit of normal (ULN) and/or aspartate aminotransferase >3 times the ULN and/or direct (conjugated) bilirubin >2 two times the ULN, renal function <50% of normal for age and size as determined by Schwartz formula, or a platelet count <50,000/mm³.

Population PK analysis

A population PK (PopPK) model, developed with data from adults and pediatric subjects (Table S2), was used to evaluate PKs. The PopPK model was developed in three stages. First, a base model was developed to describe the PKs of apixaban in adult and pediatric subjects without consideration of covariate effects. Second, covariate models were tested by incorporating the effect of relevant covariate-parameter relationships using a stepwise covariate modeling approach including forward selection (p < 0.05) and backward deletion (p < 0.01; Table S3). Third, the final model was developed by retaining the statistically significant covariate–parameter relationships.

The final model was selected to provide the best description of the data after incorporating the effect of subject covariates and maturation function to explain interindividual variability in structural model parameters. The interindividual variability model was also re-assessed as part of final model development, as correlations between the parameter values of individuals could be affected by the inclusion of covariates in the model. The final model was further refined for the evaluation of maturation function and covariate effects and was re-estimated using the final analysis dataset.

In order to improve the estimation of PK parameters of apixaban in pediatrics, data from adult phase I studies were pooled with pediatric data from this study and another phase I pediatric study. The PKs of apixaban for this analysis was developed using plasma concentration versus time data. Population mean and individual PK parameters of the PopPK model were estimated using a nonlinear mixed-effects population estimation algorithm (first-order conditional estimation with interaction). Guided by the PopPK models developed in adults,⁶ a two-compartment model with first-order absorption and elimination was used to characterize the PKs of apixaban, parameterized in terms of absorption rate constant, CL/F, V_c/F, apparent intercompartmental clearance (Q/F), and apparent peripheral volume of distribution. The mean and individual PK parameters of the PopPK model were estimated using a nonlinear mixed-effects population estimation algorithm. CL/F was estimated and included a fixed maturation function,¹⁰ in addition to body weight.

The final PopPK model included the covariate effects of the following: baseline body weight on CL/F and V_c/F, age on Q/F, age groups (9 months to <18 years) on K_a, dose, and neonatal formulation on relative bioavailability (F1), and CYP3A4 maturation as a function of age on CL/F. The exact parameter-covariate relationships in the final model are shown in the following equations:[Image Omitted. See PDF][Image Omitted. See PDF][Image Omitted. See PDF][Image Omitted. See PDF][Image Omitted. See PDF][Image Omitted. See PDF][Image Omitted. See PDF][Image Omitted. See PDF]where θ represents fixed effect parameters and η represents subject-specific random effects; BBWT_i is the individual subject's baseline body weight; AGE_i is the individual subject's age; PED9M is an indicator variable, which is equal to 0 for subjects aged <9 months and ≥18 years and 1 for subjects aged 9 months to <18 years; TVF1_i is the relative bioavailability; $\left(\frac{\exp \left({\theta }_7\right)}{1+\exp \left({\theta }_7\right)}\right)$ is the fraction constraining the reduction in the F1 between 0 and 1; θ₆ is the shape parameter that controls the relationship between dose and TVF1_i; a value of 47.5 in the denominator constrains the dose-dependency in the reduction between 0 and 1 (i.e., 1 for a maximum administered dose of 50 mg); and θ₁₄ is F1 for neonatal formulation, which is equal to 1.1 for subjects receiving neonatal formulation and 1 for others.

Parameter values were estimated using pharmacometric methods by a PopPK analysis program (NONMEM 7.3.0) using actual times of apixaban dosing and PK sample collection. The following derived individual PK parameters were estimated: C_max (model-estimated maximum plasma concentration) in each subject, which was estimated from the predicted concentration–time profile for each subject based on the final PK model and individual-specific estimates of parameters; and AUC_inf (model-based area under the concentration–time curve), which was calculated using the following equation:[Image Omitted. See PDF]where CL/F is apparent clearance of an orally administered dose and the apparent clearance value used for the calculation was the individual Bayesian post hoc estimate obtained from the PopPK model.

PK/PD analysis

The PK/PD analysis population was defined as all subjects who received one dose of apixaban and had evaluable apixaban plasma concentration and PK/PD data available. Apixaban concentration values less than the lower limit of quantification (LLOQ; 1 ng/ml) were listed as less than the LLOQ in the concentration listing, treated as 1/2 the LLOQ or missing in related figures, and treated as missing in the summary statistics. Post-treatment anti-FXa activity values less than LLOQ (0.1 U/ml) were analyzed as 1/2 the LLOQ (0.05 U/ml) in the summary statistics.

For the PK/PD analysis, a linear mixed effect/regression analysis was performed to estimate the slope on the observed apixaban plasma concentration versus observed anti-FXa activity level. The linear mixed effect model was developed to estimate the interindividual variability in regression parameters. The linear regression for the PD analysis was performed by the pediatric age group and for the overall pediatric dataset. The maximum anti-FXa activity for each pediatric subject in the analysis dataset was calculated using PD regression parameters and individual C_max.

R version 3.0.2 or higher was used for regression analysis for PD modeling, statistical summaries, tabulations, and graphical presentations.

Study conduct

This study was conducted in accordance with the International Conference on Harmonisation Good Clinical Practice guidelines and the Declaration of Helsinki. The protocol, amendments, and subject informed consent received appropriate approval by an institutional review board/independent ethics committee prior to initiation of study at the site. All subjects/caregivers gave written informed consent to participate in the study. The last visit occurred on June 18, 2019, prior to the coronavirus disease 2019 (COVID-19) pandemic; therefore, no protocol deviations occurred due to COVID-19 and the pandemic was judged to have had no impact on the integrity of the results.

RESULTS Subject population

From January 2013 to June 2019, 63 subjects were enrolled, and 49 pediatric subjects aged 9 days to <18 years received a dose of apixaban (birth to 27 days, n = 1; 28 days to <9 months, n = 11; 9 months to <2 years, n = 9; 2 to <6 years, n = 8; 6 to <12 years, n = 10; and 12 to <18 years, n = 10). Fourteen subjects were not administered apixaban because they did not meet all of the inclusion criteria. One subject (aged 6 to <12 years) was excluded from the PopPK analysis due to vomiting within 45 min of drug administration.

Although eight subjects were planned for each age group, enrollment into the birth to 27 day group was suspended in April 2017 because the amount of propylene glycol in the apixaban oral solution formulation exceeded the safety limits in neonates described in the European Medicines Agency guidelines.¹¹ Levels of propylene glycol did not exceed the limits in any other age group. Based on the results of an interim analysis, an additional four subjects were included in the 9 months to <2 years group at a higher dose than that administered to the first four subjects. In anticipation of having to replace subjects enrolled at one site for which institutional review board approval was suspended and subsequently terminated, an additional four subjects were enrolled (12 to <18 years, n = 2; 6 to <12 years, n = 1; and 28 days to <9 months, n = 1). Because the US Food and Drug Administration did not deem the subjects from this site ineligible, data from all subjects were included in the study. Baseline characteristics, including the mean age of each group and baseline physical measurements, are shown in Table 1.

TABLE 1 Baseline characteristics.

Characteristic	Birth to 27 days (n = 1)	28 days to <9 months (n = 11)	9 months to <2 years (n = 9)	2 to <6 years (n = 8)	6 to <12 years (n = 10)	12 to <18 years (n = 10)	All (N = 49)
Age	9.0 (−) daysa	126.9 (67.2) days	13.3 (4.2) months	3.6 (1.2) years	7.4 (1.2) years	13.7 (1.4) years	5.1 (5.2) years
Female sex, n (%)	1 (100)	6 (55)	5 (56)	4 (50)	6 (60)	6 (60)	28 (57)
Race, n (%)
White	1 (100)	9 (82)	9 (100)	8 (100)	8 (80)	7 (70)	42 (86)
Black	0	0	0	0	2 (20)	3 (30)	5 (10)
Other	0	2 (18)	0	0	0	0	2 (4)
Ethnicity, n (%)
Hispanic/Latino	1 (100)	5 (46)	0	1 (12)	3 (30)	0	10 (20)
Not Hispanic/Latino	0	3 (27)	4 (44)	2 (25)	5 (50)	6 (60)	20 (41)
Not reported	0	3 (27)	5 (56)	5 (63)	2 (20)	4 (40)	19 (39)
Weight, kg	3.1 (−)	4.9 (1.7)	8.3 (2.4)	15.9 (4.7)	29.8 (11.1)	49.9 (17.1)	21.5 (19.3)
Body surface area, m2	0.2 (−)	0.3 (0.1)	0.4 (0.1)	0.7 (0.1)	1.0 (0.2)	1.5 (0.3)	0.8 (0.5)
Body mass index, kg/m2	11.9 (−)	14.4 (2.4)	15.6 (2.6)	14.9 (1.8)	17.7 (4.3)	19.4 (4.5)	16.3 (3.8)

Note: Data are mean (standard deviation) unless otherwise stated.

^aEnrollment/screening procedures began when the subject was 9 days old; at the time of study drug administration, the subject was 15 days old.

Safety

Treatment-emergent AEs occurred in 15 subjects (30.6%), the majority of which were mild to moderate in intensity and resolved with minimal or no intervention. No deaths or discontinuations due to AEs occurred. The most common AEs were pyrexia (n = 4) and vomiting (n = 2). The apixaban-related AEs that occurred in four subjects were prolonged activated partial thromboplastin time (n = 1; not associated with bleeding), diaphoresis (n = 1), bleeding gums (n = 1), and headache and vomiting (n = 1). Two subjects (both aged 6 to <12 years) experienced SAEs, both of which were unrelated to apixaban (one subject with a history of seizures had a seizure on day 2, and one subject had a venous thrombosis of the right brachial vein on day 8). There were no changes in vital signs or physical examination findings with apixaban administration.

PK results

The final PopPK model parameter estimates are shown in Table 2, and individual estimated exposures by age group are summarized in Table 3. The final PopPK model demonstrated that the two-compartment model with first-order absorption, first-order elimination, and dose-dependent relative bioavailability (F1) described the data adequately for the pediatric subjects (aged 28 days to <18 years) included in the analysis. The final PopPK model included the covariate effects of baseline body weight on CL/F and V_c/F, age on Q/F, age groups (9 months to <18 years) on K_a, dose and neonatal formulation on F1, and CYP3A4 maturation as a function of age on CL/F. The results of the covariate evaluation demonstrated that apixaban CL/F and V_c/F increased primarily with body weight, although less than proportionally, and that apixaban CL/F increased with age, reaching adult values in subjects aged 12 to <18 years. In addition, fixed maturation was shown to adequately describe CL/F, most notably in subjects aged <9 months. Age was a statistically significant predictor of apixaban K_a and Q/F. Compared with adults, apixaban K_a was 142% higher in pediatric subjects aged 9 months to <18 years. Apixaban Q/F increased with increasing age. The observed apixaban concentration–time profiles were generally consistent across age groups (Figure 2). Apixaban CL/F increased with increasing age and reached adult clearance values in adolescents (Figure 3). The visual predictive check (VPC) by age group was performed for the model evaluation. The 5th and 95th percentiles for the observed data were based on a limited sample size for each age group. However, the VPC by age group still suggested that the model structure and covariate effects adequately described the observed data in adult and pediatric populations (Figure S1).

TABLE 2 Parameter estimates from the final model.

Parameter name (unit)	Symbol	Estimatea	Standard error (%RSE)	95% CIb
Ka: Absorption rate constant (1/h)	θ 1	0.502	0.0347 (6.91)	0.434–0.57
CL/F: Apparent total clearance (L/h)	θ 2	4.7	0.115 (2.45)	4.48–4.93
Vc/F: Apparent central volume of distribution (L)	θ 3	31.3	1.1 (3.53)	29.1–33.4
Q/F: Apparent intercompartmental clearance (L/h)	θ 4	1.4	0.11 (7.91)	1.18–1.61
Vp/F: Apparent peripheral volume of distribution (L)	θ 5	18	1.21 (6.69)	15.7–20.4
F1: Shape factor for reduction in F1 at dose >2.5 mg (−)	θ 6	0.884	0.0758 (8.58)	0.735–1.03
F1: LOGIT maximum reduction in F1 at dose >2.5 mg (−)	θ 7	−0.465	0.0279 (6)	−0.52 to −0.41
Vc/F: Allometric exponent for body weight (−)	θ 8	0.879	0.023 (2.61)	0.834–0.924
CL/F: Allometric exponent for body weight (−)	θ 9	0.71	0.0273 (3.84)	0.656–0.763
Q/F: Allometric exponent for age (−)	θ 10	0.663	0.065 (9.79)	0.536–0.791
Ka: Proportional shift in 9 months to 18 years (−)	θ 11	1.42	0.267 (18.8)	0.897–1.94
CL/F: Ontogeny function shape parameter (−)	θ 12	0.83 FIXED	NA	NA
CL/F: Ontogeny function age 50 (years)	θ 13	0.244 FIXED	NA	NA
F1: Relative F1 for a new neonatal formulation (−)	θ 14	1.1 FIXED	NA	NA
CL/F: Ontogeny function FBIRTH (−)	θ 15	0.05 FIXED	NA	NA
Random effects
ZKA (−)	ω 1,1	0.296 (0.544)	0.0338 (11.4)	0.229–0.362
ZCL (−)	ω 2,2	0.0864 (0.294)	0.0097 (11.2)	0.0674–0.105
ZVC (−)	ω 3,3	0.034 (0.184)	0.0105 (31)	0.0133–0.0546
ZVP (−)	ω 4,4	0.525 (0.725)	0.0769 (14.6)	0.374–0.676
ZQ (−)	ω 5,5	0.433 (0.658)	0.098 (22.6)	0.241–0.625
ZVC:ZCLc	ω 3,2	0.0468 (0.216)	0.00665 (14.2)	0.0337–0.0598
ZVP:ZCLc	ω 4,2	0.0789 (0.281)	0.0224 (28.4)	0.0349–0.123
ZVP:ZVCc	ω 4,3	0.05 (0.224)	0.0242 (48.5)	0.00245–0.0975
ZQ:ZCLc	ω 5,2	0.105 (0.324)	0.0246 (23.4)	0.0569–0.153
ZQ:ZVCc	ω 5,3	0.0924 (0.304)	0.0251 (27.2)	0.0432–0.142
ZQ:ZVPc	ω 5,4	0.393 (0.627)	0.0797 (20.3)	0.236–0.549
Residual effects
Exponential	σ 1,1	0.0844 (0.29)	0.000839 (0.994)	0.0827–0.086

Note: The condition number was 146. ETA shrinkage: ETA KA: 12.8%, ETA CL/F: 4.91%, ETA V_c/F: 12.0%, ETA V_p/F: 16.5%, and ETA Q/F: 15.6%. Minimum value of the objective function = −5805.653.

Abbreviations: %RSE, relative standard error (standard error as a percentage of estimate); CI, confidence interval; NA, not applicable.

^aRandom effect and residual error parameter estimates are shown as variance (standard deviation) for diagonal and off-diagonal elements.

^bCIs of random effects and residual error parameters are for variance or covariance.

^cThe calculated correlation coefficients (r²) of the off-diagonal omegas were as follows: 0.745 for ZVC:ZCL, 0.137 for ZVP:ZCL, 0.140 for ZVP:ZVC, 0.295 for ZQ:ZCL, 0.581 for ZQ:ZVC, and 0.678 for ZQ:ZVP.

TABLE 3 Summary of individual estimated exposures and pharmacokinetic parameters, by age group.

		Birth to 27 days (n = 1)	28 days to <9 months (n = 11)	9 months to <2 years (n = 9)a		2 to <6 years (n = 8)	6 to < 12 years (n = 9)	12 to < 18 years (n = 10)	All (N = 48)
Dose		0.1 mg	1.08 mg/m2	1.08 mg/m2	2.43 mg/m2	1.17 mg/m2	1.80 mg/m2	2.19 mg/m2	–
Ka, 1/h	Geometric mean	0.55	0.72	1.34b		1.03	1.15	1.14	1.03
	%CV geometric mean	–	135.0	49.4		50.0	54.9	64.3	76.4
CL/F, L/h	Geometric mean	0.10	0.35	0.76a		1.62	2.69	3.85	1.23
	%CV geometric mean	–	41.3	63.4		60.6	38.4	66.0	151.0
Vc/F, L	Geometric mean	1.82	2.69	4.61b		8.97	15.4	23.6	7.86
	%CV geometric mean	–	27.9	40.4		36.5	39.5	46.5	115.0
Cmax, ng/ml	Geometric mean	45.9	64	59.1	148	49.9	80	96.5	72.3
	%CV geometric mean	–	59.5	37.8	35.1	54.0	28.0	29.6	52.0
AUCinf, ng•h/ml	Geometric mean	1120	806	501	1260	423	662	815	682
	%CV geometric mean	–	31.1	43.8	68.7	67.7	21.7	48.4	53.7
Tmax, h	Median	4.54	2.32	1.60	1.67	1.91	1.71	1.95	1.92
	Range	–	1.46–9.42	1.16–2.66	1.15–2.61	1.24–3.89	1.30–3.46	1.32–6.39	1.15–9.42

Abbreviations: AUC_inf, area under the concentration–time curve from zero to infinity; CL/F, apparent total clearance; C_max, maximum plasma concentration; CV, coefficient of variation; K_a, absorption rate constant; T_max, time to maximum plasma concentration; V_c/F, apparent central volume of distribution.

^an = 6 for 1.08 mg/m² and n = 3 for 2.43 mg/m².

^bPresented as a combined value for both doses in this age group.

FIGURE 2. Observed apixaban concentration versus time on a semilog scale stratified by age group in pediatric subjects.

FIGURE 3. Model-estimated individual apparent clearance versus age.

PK/PD results

Plasma anti-FXa activity was linearly related to apixaban concentration (r² = 0.9504) with no apparent age-related differences in slope in the pediatric age groups (Figure 4). The slope of the exposure–response relationship in pediatric subjects was comparable to that in adults,¹² demonstrating that anti-FXa has similar potential utility and interpretability across the pediatric range.

FIGURE 4. Observed plasma anti-FXa level versus apixaban concentration, stratified by age. FXa, factor Xa.

Push-pull sample collection method comparison

A similar plasma concentration of apixaban was observed between the samples collected by the push-pull collection method and the duplicate samples (% coefficient of variation, 0.475–7.71; r² = 0.997; n = 8). Anti-FXa activity results appeared to be similar between the two sample collection methods (% coefficient of variation, 0.00–6.73; n = 6). However, for two subjects, the duplicate sample was less than the LLOQ; therefore, limited comparisons could be made using data from these subjects.

DISCUSSION

Apixaban was well-tolerated across all pediatric age groups, with no safety signals observed. The PopPK modeling approach used healthy adult subject data to describe pediatric PKs in the context of adult PKs and identified predictors of pediatric exposure. This included a comprehensive and iterative covariate evaluation to quantify the intrinsic/extrinsic factors (Table S3) that influence the exposure of apixaban across the pediatric age range from neonates/infants to young adults.

The PopPK base model for the combined dataset demonstrated that the two-compartment model with first-order absorption, first-order elimination, and dose-dependent relative bioavailability (F1) described the data adequately for the pediatric subjects (aged 28 days to <18 years) included in the analysis. The results of the covariate evaluation demonstrated that apixaban CL/F was influenced primarily by body weight and fixed maturation based on elimination pathways of apixaban, V_c/F was influenced by body weight, and distributional clearance and absorption were influenced by age. CL/F values are generally comparable to those of adults in adolescents aged ≥12 years. Once normalized by body-weight effect, clearances in these pediatric subjects were generally consistent across the pediatric age range, except in subjects aged <9 months who had lower clearances. Similar plasma concentrations and anti-FXa activity were seen using both the push-pull method and the duplicate sample method of blood draws, suggesting this may be a viable method for obtaining blood samples in younger patients.

Anti-FXa can be easily measured in the laboratory and is used as a surrogate marker for apixaban plasma concentrations.^13,14 Endogenous concentrations of FXa differ with age and are significantly lower in neonates and infants than those observed in adults^15,16; however, the direct relationship between apixaban plasma concentration and anti-FXa level is expected to be preserved. For the PK/PD analysis, linear regression was used to estimate the slope of the relationship between the observed apixaban plasma concentration and the observed anti-FXa level where: anti-FXa activity = intercept + slope × apixaban concentration. The relationship between observed anti-FXa activity and observed apixaban plasma concentrations in pediatric data was linear (overall slope estimate of 0.0155 IU/ng), with no apparent age-related differences in slope across the pediatric age groups, and was comparable to that in adults (overall slope estimate of 0.0159 IU/ng).¹⁷

The benefits of DOACs over VKAs in adult patients are oral dosing, decreased food and drug interactions, and decreased need for therapeutic monitoring. This is especially important in pediatric patients for whom administration of subcutaneous unfractionated heparin or low-molecular-weight heparin can cause stress, anxiety, and pain, making subsequent medication administration difficult.¹⁸ Specially developed and tested pediatric formulations of DOACs have allowed accurate and reliable dosing, oral administration, stable PKs/PDs, and fewer drug or food interactions.¹⁸

A further advantage of apixaban is that food does not have a clinically significant effect on the bioavailability of apixaban (i.e., it can be taken with or without food).⁶ Multiple forms of administration have been investigated for apixaban that maintain exposure similar to that of intact oral tablets, including crushed tablets alone, with apple sauce, and via nasogastric tube. These different ways of taking apixaban are important for patients who may have difficulties swallowing tablets, such as hospitalized or pediatric patients.¹⁹ In addition, the 0.4 mg/ml liquid formulation used in this study, as well as a dissolvable mini-tablet (0.5 mg) formulation, have been developed specifically for use in pediatric patients.¹⁸ Apixaban has been shown to have low rates of bleeding in adult populations compared with LMWH and VKAs.^20,21 Although there is limited experience using apixaban in the pediatric population, no known association with increased bleeding has been shown to date and no new or unexpected safety signals were observed in this study. A phase IV study investigating the safety and efficacy of apixaban in children with acute VTE is currently ongoing.²²

One strength of this study is that it covers a wide range of ages, from neonates/infants to children aged <18 years. A limitation is that the study included a small population and was conducted in a controlled, clinical setting. In the real world, factors such as comorbidities and polypharmacy may need to be considered.

CONCLUSIONS

Single oral doses of apixaban were generally well-tolerated by the 49 pediatric subjects, regardless of age. In addition to body weight, maturation played a role in the clearance of apixaban most notably in the youngest pediatric subjects (age <9 months), who had lower clearance. The push-pull method was a viable approach to address pediatric blood volume restrictions for clinical studies, as PK measurements were consistent with the original method. The results from this model-based evaluation, along with data from other ongoing pediatric studies of apixaban (NCT02464969, NCT02981472, and NCT02369653), will guide dosing recommendations in pediatric patients.

AUTHOR CONTRIBUTIONS

All authors wrote the manuscript. S.J.M., W.B., Y.T.P., B.H., D.M., and B.M. designed the research. S.J.M., W.B., A.E., and B.H. performed the research. Y.T.P., V.P., and B.M. analyzed the data.

ACKNOWLEDGMENTS

Professional medical writing and editorial assistance was provided by Claire Line, PhD, at Caudex, funded by Bristol Myers Squibb and Pfizer.

FUNDING INFORMATION

This study was funded by Bristol Myers Squibb and Pfizer.

CONFLICT OF INTEREST STATEMENT

S.J.M., A.E., D.M., V.P., B.H., and B.M. are employees of and shareholders in Bristol Myers Squibb. W.B. was an employee of and shareholder in Pfizer at the time the study took place (currently an employee at Amador Bioscience, Ann Arbor, MI, USA). Y.T.P. reports Cognigen Corp received support from Bristol Myers Squibb to perform the model-based analysis described in the manuscript. W.C. is an employee of Bristol Myers Squibb.

PREVIOUS PUBLICATION

These results were previously presented at the American Heart Association Scientific Sessions: November 13–15, 2021. P2930.