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2014 20: 355 originally published online 11 December 2013CLIN APPL THROMB HEMOSTJoost DeJongh, Johan Frieling, Simon Lowry and Henk-Jan Drenth

Hereditary Antithrombin DeficiencyPharmacokinetics of Recombinant Human Antithrombin in Delivery and Surgery Patients With

  

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Original Article

Pharmacokinetics of RecombinantHuman Antithrombin in Deliveryand Surgery Patients With HereditaryAntithrombin Deficiency

Joost DeJongh, PhD1, Johan Frieling, MD, PhD2,Simon Lowry, MD2, and Henk-Jan Drenth, PhD1

AbstractPopulation pharmacokinetic (PK) analyses were conducted to refine dosing recommendations for recombinant humananti-thrombin therapy in surgery and delivery patients with hereditary antithrombin deficiency (HD). Single-dose PK data frompatients with HD and nonlinear mixed-effects modeling were used to devise a dosing regimen to target antithrombin (AT) activitylevels between 80% and 120% of normal. External validation with data from a phase 3 trial confirmed the correctness of acovariate-free model for surgery patients, but dosing adjustment was necessary for delivery patients. After different covariateswere tested, the model was updated to incorporate the influential covariate, delivery. Simulations were used to develop a ther-apeutic drug-monitoring scenario that results in steady state AT activity levels within the target range as quickly as practicallyfeasible. Data from a second clinical trial provided additional external validation and confirmed the accuracy of the dosing modelfor both groups of patients.

Keywordsrecombinant antithrombin, dosing, model validation, population pharmacokinetics

Introduction

Venous thromboembolism (VTE), which consists of deep vein

thrombosis and pulmonary embolism, is a major health problem

in the United States and worldwide, especially among subgroups

such as pregnant women and patients who are undergoing

surgical procedures.1,2 Risk factors for VTE are generally cumu-

lative2 and include deficiencies of natural coagulation inhibitors

(eg, antithrombin [AT]) and thrombophilias.3

Hereditary AT deficiency (HD), a well-characterized,

almost always heterozygous, autosomal dominant disorder that

is caused by either a reduction in AT production (type I HD) or

the presence of a dysfunctional form of AT (type II HD), is

associated with a high risk of VTE.4 Although HD is rare, with

an overall prevalence between 1:2000 and 1:5000 in the gen-

eral population,5 affected individuals have a �50% lifetime

risk of developing VTE, which is the highest risk among all

inherited thrombophilias.6 As in healthy individuals, the

probability of VTE may be further elevated in patients with

HD during high-risk situations such as surgery and delivery.

Approximately half of the VTEs in patients with hypercoagula-

tion disorders such as HD are thought to be provoked in these

high-risk situations.7,8 Primary, long-term anticoagulant pro-

phylaxis is recommended only for symptomatic individuals

with AT deficiency.9 However, short-term AT replacement

therapy is recommended to prevent VTE in patients with HD

during high-risk situations. The goal of such therapy is to

reduce the risk of VTE by maintaining AT activity levels

between 80% and 120% of normal.6

Several pooled human plasma-derived AT concentrates and

1 recombinant human AT concentrate (ATryn;10 rEVO Biolo-

gics, Framingham, Massachusetts) are available for the preven-

tion of VTE in connection with surgical or obstetric procedures

in patients with HD. Dosing of pooled human plasma-derived

AT consists of individualized bolus loading and bolus mainte-

nance doses at least once a day. The loading dose is based on

the patient’s pretreatment AT activity level, with the goal of

maintaining trough plasma AT activity levels above 80% of

normal. Subsequent doses are based on the individual trough

levels measured during treatment. Results from a pharmacoki-

netic (PK) study in healthy volunteers11 clearly suggested that

1 LAP&P Consultants BV, Leiden, the Netherlands2 rEVO Biologics, Framingham, MA, USA

Corresponding Author:

Johan Frieling, Clinical Development, rEVO Biologics, Inc. 175 Crossing Blvd,

Framingham, MA 01702, USA.

Email: johan.frieling@revobiologics.com

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the PK characteristics of recombinant human AT are different

from those reported for pooled human plasma-derived concen-

trates,12 necessitating a different approach to dosing.

The objective of the present report is to describe the devel-

opment of the current dosing recommendations for recombi-

nant human AT in surgery and delivery patients. Single-dose

PK data from patients with HD were used for nonlinear

mixed-effects modeling to establish an initial dosing regimen.

After the predictive strength of the model was tested in a

second study of surgery and delivery patients, delivery was

observed to be a significant covariate for PK, and the model

was updated. A final external validation was performed on both

surgery and delivery patients from a third study.

Methods

Studies and Patients

A total of 47 patients with HD who received recombinant

human AT treatment during 3 trials formed the study popula-

tion for this PK analysis. An open-label, single-dose PK study

(AT III 009-00) included nonpregnant HD patients not in high-

risk situations who were sufficiently healthy to undergo the

necessary tests and procedures. The female–male ratio was

13:2, and the mean baseline AT activity was approximately

51%. The 2 clinical trials, AT III 01002 (clinicaltrials.gov

NCT00056550)13 and AT HD 012-04 (clinicaltrials.gov

NCT00110513; M. J. Paidas, MD, accepted for publication)

were similar in design and included a total of 32 patients with

HD in high-risk situations (21 delivery and 11 surgery).

Patients were treated with recombinant human AT during the

perioperative or peripartum period to prevent occurrence of

VTE. Treatment was started the day before the procedure (or

as soon as possible after confirmation of active labor in

delivery patients) and continued for at least 3 days. Approvals

for all studies were obtained from the applicable authorities and

institutional review boards/ethics committees. All patients pro-

vided informed consent before any study-related procedure was

performed.

Study Drug Administration

Patients in the initial PK study received a single-recombinant

human AT intravenous bolus dose of either 50 IU/kg (n ¼ 9)

or 100 IU/kg (n¼ 6). In study AT III 01002, loading and main-

tenance doses were determined by formulas that accounted for

both individual baseline AT activity and body weight. In AT

study HD 012-04, dosing calculation formulas also accounted

for the type of high-risk situation (i.e., surgery or delivery).

Loading doses were administered as 15-minute bolus infusions

and were followed immediately by maintenance dosing via

continuous infusion (CI).

Blood Sampling

In the PK study, blood samples were collected at the screening

visit and at 1 hour before and 15 minutes before recombinant

human AT dosing. Postdose samples were collected at the fol-

lowing times after infusion cessation: 0, 5, 10, 15, 30, 45, and

60 minutes and 2, 4, 6, 8, 24 (+2), 48 (+2), and 72 (+2)

hours. In the clinical trials, predose blood samples were

collected at screening and baseline. In study AT III 01002,

samples were collected for AT activity measurements 0.5 hour

after the start of the maintenance infusion. If AT activity was

outside the target range, the dose was adjusted, and another

sample was drawn for AT activity measurement 0.5 hour later.

If no dose adjustment was needed, AT activity was measured 4

hours later and at least every 24 hours for the duration of the

infusion. In the AT HD 012-04 study, samples were collected,

and AT activity was measured 2 hours after the start of the infu-

sion and again 2 hours later (if the dose was adjusted) or 4 hours

later (if the dose was not adjusted). Subsequently, samples were

collected 1 or 2 times daily during the infusion, with dosing

adjustment as needed to maintain AT activity in the target

range.

Population PK Analysis

A population PK model was developed on the basis of PK study

data from 15 patients with HD. This model was used to create a

dosing algorithm to achieve plasma AT activity levels between

80% and 120% of normal. On the basis of results from a

previous study of recombinant human AT PK in healthy volun-

teers,11 a 2-compartment model with linear elimination from

the central compartment was chosen as a starting point.

Pretreatment AT activity was considered as a separate, inde-

pendent model parameter at baseline, and any increased AT

activity due to recombinant human AT infusion was modeled

additive to this baseline. The population PK analysis and exter-

nal validation were performed using nonlinear mixed-effects

modeling in NONMEM (versions V.2 and 7.2; Icon PLC,

Dublin, Ireland), which describes the population PK, simulta-

neously accounting for interindividual and intraindividual

variabilities in 1 joint model optimization run. First-order condi-

tional estimation with interaction (METHOD ¼ 1 INTER) was

used as a minimization method for all model runs. Subsequent

postprocessing of NONMEM results was performed using the

statistical software package S-Plus for Windows (versions 6.1

and 8; TIBCO, Somerville, Massachusetts). The model was

sequentially optimized for interindividual variability (IIV) of

each PK parameter until no further improvement in goodness

of fit of the model to the data was observed, and all relevant

random effects for IIV had been assigned. Description of the

residual variability between model predictions and observations

was optimized on the basis of a proportional error model.

Because of the small population, a formal, model-based analysis

of demographic covariates was not performed.

For model development, the simplest possible model that

adequately described the data was accepted as final, using the

forward addition/backward deletion principle. The effects of

adding structural or stochastic parameters to the model on

model fit were assessed by inspection of both the diagnostic

plots of observations versus individual/population predictions

356 Clinical and Applied Thrombosis/Hemostasis 20(4)

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and the weighted residuals versus time/population predictions,

as well as statistical considerations. Internal model validation

using visual predictive check (VPC) and bootstrapping con-

firmed that model accuracy and precision were adequate for

simulation of expected AT activity during prolonged clinical

treatment with recombinant human AT.

Data from both clinical studies were used for external vali-

dation of the initial model. For each study, baseline AT activ-

ities were described by optimization of 1 fixed and 1 random

effect (i.e., mean baseline and IIV) to the validation data. All

other population PK model parameters were fixed to their pre-

viously derived values. Residual variability (ie, the difference

between individually observed and predicted AT activity val-

ues) was also optimized to the data. Predicted versus actual

observed AT activity values were visually inspected for ran-

dom distribution around the identity line. Patient plasma AT

activity levels were modeled on the basis of patient infusion

rate records. The final population PK model was used to infer

the most likely value of baseline AT activity while allowing for

specification of IIV in the baseline values. The results were

used to describe the observed AT activity in the population for

study AT III 01002. Predicted AT activity values were com-

pared with observed values, and the corresponding residuals for

predicted/observed activities were determined. The influence

of relevant covariates on the residuals was analyzed in an

exploratory way by plotting the residuals against each of the

covariates (ie, age, gender, body weight, study center, high-

risk situation, and concomitant medications). Potentially

distinct covariates were incorporated into the model to test

diminished bias in the predictions. Categorical covariates were

included in the model by assuming different typical values

(population means) of the structural model parameters (ys) for

different categories.

Simulations to Optimize Therapeutic Drug Monitoring

Two dosing models were used for simulations (in 10 000

patients) to determine the optimal number of dose adjustments

needed for loading and infusion of recombinant human AT to

attain AT activity in the target range within the shortest time.

One set of model parameters was derived for surgical patients

and validated by a dataset from AT III 01002, and the other was

derived for delivery patients. A typical body weight of 76 kg,

and normal distribution were assumed, with a variance of

204 kg.2 The influence of the following factors was investi-

gated: initial and second sampling times, cutoff values, and dif-

ferent dosing algorithms or dose adjustments factors.

Results

Study Descriptions and PK of Recombinant Human AT

Of the 47 patients included in the development and external

validation of the population PK model in patients with HD,

15 were not in high-risk situations. The remaining 32 patients

treated with recombinant human AT in the 2 clinical trials were

exposed to high-risk situations (either surgery or delivery).

Table 1 shows the total and maintenance recombinant human

AT doses and the treatment durations for patients in the 2 clin-

ical studies.

Determination of the Population PK Model (AT III 009-00 Study)

Descriptive PK parameters from the AT III 009-00 study are

shown in Table 2. Initially, a 2-compartment model with 5

structural PK model parameters (ie, plasma clearance [CL],

steady state volume of distribution [Vss], central volume of dis-

tribution [Vcentral], intercompartmental clearance [Q], and AT

activity level at baseline [ATBL]) was optimized to the data.

Next, random effects of IIV were sequentially added to each

of the structural PK model parameters. Overall, the best model

fit was obtained when IIV coefficients were assigned to Vss,

CL, ATBL, and Q. To account for possible correlation between

IIV coefficients, a full 4 � 4 covariance matrix was implemen-

ted for the IIV coefficients for these parameters, which resulted

in a small, but significant, improvement in the goodness-of-fit

model. To optimize the description of the IIV coefficients and

their interdependence, Q was expressed as a multiple of CL.

This led to a model structure with IIV coefficients for CL and

Vss, linked by a 2 � 2 covariance matrix, and a single indepen-

dent IIV coefficient for ATBL. Goodness of fit was only

marginally decreased when compared with the previous model

the previous model, but numerical stability was acceptable. The

original description of residual variability was also challenged,

but the optimal description was always proportional to the mod-

eled AT value.

The accuracy of the model was internally evaluated by com-

paring the actual AT activity values (AT III 009-00 study data-

set) with the 10th and 90th percentiles of the simulated data and

using a VPC (excluding the baseline values; Figure 1). In the

50- and 100-IU/kg recombinant human AT dose groups, 84%

Table 1. AT III 01002 and AT HD 012-04 Study Dosing.

AT III 01002 AT HD 012-04

Number of patients, no. 14 18Surgery group 5 6Delivery group 9 12

Screening AT activity, mean (range), %Surgery group 50 (37-62) 54 (45-65)Delivery group 46 (33-58) 49 (29-65.7)

Total dose, median (range), IU/kgSurgery group 1194 (1041-3346) 523 (377-1913)Delivery group 973 (518-3018) 770 (208-3372)

Total maintenance dose, median (range), IU/kgSurgery group 1147 (1003-3311) 509 (359-1885)Delivery group 954 (493-2970) 726 (174-3359)

Treatment duration; median (range), dSurgery group 11.5 (8-19) 3.0 (3-14)Delivery group 3.1 (3-10) 3.6 (1-14)

Abbreviations: AT, antithrombin; HD, hereditary AT deficiency.

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and 94% of the observations were within the 10th and 90th per-

centiles. Hence, the model appeared to be free from bias. The

precision of the model parameters for prospective simulation

of AT activity in patient populations during prolonged clinical

treatment with recombinant human AT was also confirmed by

comparing bootstrap results with the standard errors initially

derived from the covariance matrix of the final model.

Optimal Maintenance and Loading Dosage Calculations

Several dosing regimens were simulated, and the results were

compared to identify the optimal dose regimen that provided

an AT activity level at steady state in the 80% to 120% target

range within the shortest possible time for a typical patient with

HD. This was achieved by administering a loading dose

followed immediately by maintenance CI.

The following loading and maintenance dosages were

selected for use during the initial clinical trial:

Loading dose IUð Þ :

100� pretreatment AT activity level

2:3� body weight kgð Þ

Maintenance dose IU=hð Þ :

100� pretreatment AT activity level

10:2� body weight kgð Þ

Figure 1. Internal validation (visual predictive check) of the population PK model for recombinant human AT-treated patients. Scatter,observed plasma AT activity; thick line, geometric mean of model simulations; dashed lines, 90th and 10th percentiles of model simulations. Note:Double log scale was used for improved visibility. AT indicates antithrombin; PK, pharmacokinetics.

Table 2. Descriptive PK Parameters in Patients With HD.

Study (Location) Treatment Dose

Baseline Correcteda Mean PK Parameters

Cmax, % (% CV)Incremental Recovery,

%/IU/kg (% CV) tmax, h t1/2, h (% CV) AUC0-inf, % h (% CV)

AT III 009-00 (EU) IV bolus rhAT50 IU/kg

112.0 (20.2) 2.24 (20.2) 0.22 11.6 (84.7) 595.5 (44.5)

IV bolus rhAT100 IU/kg

193.8 (14.8) 1.94 (14.8) 0.25 17.7 (60.9) 1413.8 (15.6)

Abbreviations: AUC0-inf, area under the curve to infinity; Cmax, maximum concentration; CV, coefficient of variation; EU, European Union; HD, hereditary antith-rombin deficiency; IV, intravenous; PK, pharmacokinetics; rhAT, recombinant human AT; tmax, time of maximum concentration; t1/2, elimination half-life.aPK parameters were calculated by subtracting each observed postdose value from the individual’s average baseline AT activity value (except for incrementalrecovery).

358 Clinical and Applied Thrombosis/Hemostasis 20(4)

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Results of Monte Carlo simulations of AT activity versus

time suggest that in 79.5% of patients, the infusion regimen

results in AT activity levels 80% to 120% of normal without the

need for further adjustment of the infusion rate. For patients

who are estimated to have AT activity levels that are either too

high (14.5% of patients) or too low (6.4% of patients), thera-

peutic drug monitoring (TDM; adjustment of infusion rate

according to actual plasma AT activity levels) will be needed

to drive these values into the target range.

Pharmacokinetic Model Validation

Dosing information from patients with HD in high-risk situa-

tions (surgery and delivery, from study AT III 01002) was

applied to the model described previously to predict AT activ-

ity. These AT activity values were compared with the actual

observed AT activity data from the study, which revealed that

the initial population PK model tended to overpredict the AT

activity in this cohort.

The influence of relevant covariates on the accuracy of the

model predictions was investigated by plotting the weighted

residuals of the fit of the model against several covariates that

are thought to be relevant (age, gender, body weight, study cen-

ter, high-risk situation [surgery or delivery], and concomitant

medications). Delivery was determined to be the only covariate

that significantly influenced the predictive strength of the model.

As shown in Figure 2, most of the overpredicted AT activity val-

ues below the identity line belong to delivery patients (upper

panel). In contrast, surgery patients in the lower panel are clus-

tered within the 80% to 120% of normal AT activity window and

are very well predicted by the original PK model.

Incremental updates were made to the model structure to

include the influential covariate, delivery. Post hoc estimates

from the original population PK model suggested that CL and

Vss were higher in delivery patients than in surgery patients

Figure 2. Observed versus predicted AT activity in study AT III 01002. Upper panel, delivery patients; lower panel, surgery patients. Plots showdata scatter around a dashed identity line. Solid lines indicate the boundaries of 80% and 120% of normal AT activity. AT indicates antithrombin.

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(Table 3). The NONMEM-estimated lower 95% confidence

boundaries for CL and Vss in delivery patients (1.38 L/h and

14.3 L, respectively) differed substantially from the values for

surgery patients (0.665 L/h and 7.72 L, respectively). On the basis

of a significance test assuming chi-square distribution, the CL and

Vss in delivery patients were considered significantly larger (P <

.05) than the corresponding parameter estimates obtained for the

original patients of the population PK model. Bootstrap analysis

confirmed differences in these PK parameters between surgery

and delivery patients. The updated model provided a superior

description of the new data for delivery patients (Figure 3).

On the basis of the model update, dosing calculations to

achieve 100% AT activity at steady state in delivery patients

were revised as follows:

Loading dose IUð Þ :

100� pretreatment AT activity level

1:3� body weight kgð Þ

Maintenancedose IU=hð Þ :

100� pretreatment AT activity level

5:4� body weight kgð Þ

Therapeutic Drug Monitoring

The aim of TDM during recombinant human AT infusion is to

maintain a steady state AT activity level within the therapeutic

window of 80% to 120% of normal. The time it takes to reach

the 80% of normal AT activity level should be as short as prac-

tically feasible. With use of the algorithm for recombinant

human AT dosing and the population PK models, various TDM

scenarios were tested. The effects of TDM on plasma AT activ-

ity were simulated for 10 000 surgery and 10 000 delivery

patients. Effects of changes in the TDM variables (eg, sampling

times, cutoff values, and dose-adjustment algorithms) were

investigated. Simulations suggested that approximately 18%of all patients would have steady state AT activity levels of

<80% or >120% of normal if the infusion rate remained

unchanged.

Several TDM parameters were varied and evaluated, lead-

ing to a TDM scenario that was deemed effective and practical

(allowing some time between sample draw and availability of

results). In cases where the AT activity level was >120% of

normal, changing the dose by a correction factor of 0.7 times

the current dose rate (ie, a decrease of 30%) was effective for

dose reduction. For patients with AT levels <80% of normal,

a dose increase correction factor of 1.3 (ie, an increase of

30%) appeared to be optimal for achieving steady state AT

activity levels >80% of normal for nearly all patients. Timing

of the first TDM sample should be 2 hours after the start of

treatment; if AT activity is within the target range, a second

sample should be taken after 6 hours. If the AT activity in a

TDM sample is outside the target range, adjustment of the dose

up or down by 30% is suggested; AT activity should be

checked 2 hours after a dose adjustment has been made.

External Validation of the Updated PK Model and TDM

External validation using the dataset from the second clinical

trial (AT HD 012-04) confirmed the predictive strength of the

model and the dosing algorithm in both surgery and delivery

patients (Table 4; Figure 4).

The original dosing algorithm was designed to bring the AT

activity level in each patient within the 80% to 120% of normal

target range as quickly as possible while not exceeding 200%of normal. The initial dose for each patient is based on the mea-

surement of his or her individual predose (ie, baseline) AT

activity. After evaluation of the results from the first study

(AT III 01002) in surgery and delivery patients, the dosing

algorithm in delivery patients was adjusted on the basis of the

higher CL and Vss observed for recombinant human AT in this

subpopulation compared with surgery patients (Table 3).

Results of this dose adjustment in delivery patients are most

apparent from the relative numbers of (nonbaseline) plasma

samples with AT activity above the lower limit (>80%) of the

therapeutic range in the 2 studies (Table 4). In the first study

(AT III 01002), which used the original unadjusted dosing

algorithm, 60% of the samples from delivery patients had an

AT activity >80% of normal. In the second study (AT HD

012-04), in which the dosing algorithm had been adjusted to

delivery status, 92% of the samples from delivery patients had

an AT activity >80% of normal. Similar results were observed

for the relative numbers of plasma samples within 80% to

Table 3. Estimated Population PK Parameters for Final Model: CL andVss for Delivery Patients After Adjustment to Study AT III 01002Results.

ParameterSurgeryGroupa

DeliveryGroupa

Fixed effectsCL, value (SE), L/h 0.665 (0.0493) 1.38 (0.161)Vss, value (SE), L 7.72 (1.26) 14.3 (2.6)Vr ¼ (Vss/Vcentral � 1), value (SE) 1.51 (0.331)Q, value (SE), L/h 0.613 (0.646)ATBL, value (SE), % 44.7 (3.03)

Random effects (IIV)oCL(IIV)

2, value (SE) 0.0676 (0.0205)oVss(IIV)

2, value (SE) 0.0521 (0.026)oCL(IIV)

2 � oVss(IIV)2

(o covariance),value (SE)

0.0395 (0.0175)

oATBL(IIV)2, value (SE) 0.0519 (0.0221)

Random effects (residual error)s2, value (SE) 0.0289 (0.00543)

Abbreviations: ATBL, antithrombin activity level at baseline; CL, clearance; IIV,interindividual variability; PK, pharmacokinetics; Q, intercompartmental clear-ance; SE, standard error of parameter estimates; Vcentral, central volume ofdistribution; Vss, steady state volume of distribution; Vr, central distribution vol-ume divisor; o2, random effect for IIV of each parameter; s2, random effect forresidual error.aColumns with only 1 value represent combined results for the surgery anddelivery groups.

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120% of the normal therapeutic AT activity range in the first

(40%) and second (61%) studies.

Discussion

Hereditary AT deficiency is a rare but serious and potentially

life-threatening condition due to the risks of VTEs during the

perioperative and peripartum periods. Thus, prevention strate-

gies should be used. Pooled human plasma AT has been used

for the treatment and prevention of thromboembolic events in

patients with HD.14,15 Pooled human plasma-derived AT has

a relatively long half-life and is administered daily via bolus

injection, with AT activity monitoring recommended at least

every 12 hours. Initial doses are calculated on the basis of pre-

treatment AT activity level and body weight.

Although techniques for screening and removal of infec-

tious agents from plasma-derived products have improved dur-

ing the past few decades, some risk of infection still exists with

the use of these products. For example, results from a recent

study showed that patients who were exposed to plasma-

derived products (alone or in combination with other products)

had higher age-specific prevalence rates of parvovirus B19

Figure 3. Updated model of observed versus predicted AT activity levels in study AT III 01002. Upper panel, delivery patients; lower panel,surgery patients; triangles, delivery patients; circles, surgery patients. Plots show data scatter around a dashed identity line. Solid lines indicatethe boundaries of 80% and 120% of normal AT activity. AT indicates antithrombin.

Table 4. Predictive Performance of the Population PK Model for Recombinant Human AT in Surgery and Delivery in the 2 Clinical Trials Basedon AT Activity Observed in Non-baseline Plasma Samples.

Study Patient Type AT Activity Number (%) of Observations

AT III 01002 Surgery Total 69 (100)>80% AT activity 64 (93)>80% and <120% AT activity 63 (91)

AT III 01002 Delivery Total 121 (100)>80% AT activity 72 (60)>80% and <120% AT activity 48 (40)

AT HD 012-04 Surgery Total 42 (100)>80% AT activity 35 (83)>80% and <120% AT activity 34 (81)

AT HD 012-04 Delivery Total 79 (100)>80% AT activity 73 (92)>80% and <120% AT activity 48 (61)

Abbreviations: AT, antithrombin; HD, hereditary AT deficiency; PK, pharmacokinetics.

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(B19V) infection and were more likely to have antibodies to

B19V than were patients with no exposure to blood or blood

products.16 Moreover, there is currently no method for screen-

ing blood products for prions,16 so an efficacious product that

avoids the potential risks of human blood-borne infections is

needed. Recombinant human AT is a novel biologic agent that

was developed to avoid these risks and to be independent of the

human plasma supply for its manufacture.

Results from the 2 trials of recombinant human AT treat-

ment in patients with HD demonstrated the efficacy of the

agent in preventing VTE during high-risk situations13 (M. J.

Paidas, MD, unpublished data, 2012). The noninferiority of

recombinant human AT to pooled human plasma AT for the

prevention of VTE in patients with HD has been established

and was the basis for the US Food and Drug Administration’s

approval of recombinant human AT. This study describes how

the dosing regimens used in these trials were developed from a

single-dose PK study in patients with HD not in high-risk

situations and were able to successfully provide AT supple-

mentation within the normal range.

One initial observation was the difference in half-life

between pooled human plasma-derived AT and recombinant

human AT. The half-life of pooled human plasma-derived

AT (2.5 days) is substantially longer than the half-life of

recombinant human AT observed in the PK study of patients

with HD who were not in high-risk situations (11.6 and 17.7

hours after doses of 50 and 100 IU/kg, respectively). Differ-

ences in PK characteristics between native and recombinant

plasma proteins are not uncommon. The production of

recombinant proteins in a biologic system may result in a

distinctive glycosylation profile, which in turn may lead to

PK differences (despite the fact that the amino acid sequence

is the same, as is the case for recombinant human AT and

pooled human plasma AT).4,9,17,18

From the initial PK study in patients with HD not in high-

risk situations (AT III 009-00), a population PK model for

recombinant human AT was determined. Simulations of poten-

tial dosing regimens were compared, and an initial 15-minute

loading dose followed by a CI maintenance dose regimen was

devised for clinical evaluation. This dosing algorithm was

applied in the AT III 01002 study with HD patients in high-

risk situations (ie, surgery or delivery). The individual AT

activity data and number of dose adjustments suggested that the

algorithm for calculating the dosing regimen worked well for

surgery patients but not for pregnant patients, with the latter

needing more frequent dosage adjustments to maintain normal

AT activity levels. The availability of dosing and AT activity

data in the target population allowed for external validation

of the initial population PK model. Comparison of the pre-

dicted AT activity values with the actual AT activity values

observed in the study suggested that the model overpredicted

AT activity in pregnant women. This phenomenon appeared

to result from significantly larger Vss and CL in pregnant

patients (14.3 L and 1.38 L/h, respectively) than in surgery

patients (7.72 L and 0.665 L/h, respectively). Therefore, the

model was updated to include these new results, and a separate

dosing algorithm was devised for pregnant patients. No differ-

ence in PK characteristics between pregnant and nonpregnant

Figure 4. Updated model of observed versus predicted AT activity levels in study AT HD 012-04. Upper panel, delivery patients; lower panel,surgery patients; triangles, delivery patients; circles, surgery patients. Plots show data scatter around a dashed identity line. Solid lines indicatethe boundaries of 80% and 120% of normal AT activity. AT indicates antithrombin; HD, hereditary AT deficiency.

362 Clinical and Applied Thrombosis/Hemostasis 20(4)

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patients with HD has been described for human pooled plasma

AT, although Weiner et al19 described a shorter half-life for AT

III concentrate in pregnant women without HD than in healthy

nonpregnant patients. Recent analyses of dosing and AT

activity from a retrospectively derived dataset of patients

treated with pooled human plasma-derived AT also suggest

substantial differences in PK parameters between pregnant and

nonpregnant patients with HD (J. Frieling, MD, PhD, unpub-

lished data, 2012). The exact reason for this difference between

pregnant and nonpregnant patients is not clear and probably

multifactorial. However, it is well known that significant

changes in intra- and extravascular volume occur during

pregnancy, also potentially leading to the increased Vss for antith-

rombin. In addition, hepatic clearance is known to have been

changed during pregnancy.20

Other potentially relevant covariates (age, gender, body

weight, study center, and concomitant medications) did not

have a significant influence on the accuracy of the model pre-

dictions. Therefore, the loading and maintenance infusion

doses were increased in the algorithm for pregnant patients to

ensure that target AT levels are more easily achieved, thus

managing the risk of VTE around the time of delivery.

In addition to the update of the dosing algorithm, simulations

were performed to design a practical TDM scenario. The model

allows for simulation of data from large numbers of patients (in

this case, AT activity data for 10 000 surgery and 10 000 deliv-

ery patients) and use of this information to choose the TDM

scenario that maximizes the percentage of patients who achieve

the target range of AT activity and minimizes the amount of time

spent outside of that range. However, the TDM scenario should

be practical for implementation in current medical practice,

accounting for factors such as turnaround time for AT activity

analysis. The simulations showed that if a sample is taken too

soon (e.g., after 1 hour), the result is not sufficiently predictive

of the patient’s steady state AT activity level to allow appropri-

ate changes in the infusion rate. Therefore, the first TDM sample

should be taken 2 hours after the start of the infusion. The simu-

lations also indicated that for patients with AT activity levels

outside the target range at the time of the first TDM sample,

increasing or decreasing the dose by 30% will bring the level

within the target range in almost all cases. With the recom-

mended TDM, maintenance of AT activity levels within the lim-

its of the therapeutic window (80%-120%) is clinically feasible.

Subsequent use of the updated dosing algorithm and TDM

scenario in the second clinical trial (AT HD 012-04) confirmed

that tight control of AT activity levels was possible in both

pregnant and nonpregnant patients with HD. Very few dose

adjustments were needed to achieve and maintain target AT

activity levels. External validation showed that the final model

had great predictive strength for both surgery and delivery

patients and confirmed the proposed TDM scenario.

Conclusions

Population PK analysis is a strong tool for developing a recom-

binant human AT dosing regimen for patients in high-risk

situations. Using single-dose PK data, an algorithm for recom-

binant human AT dosing over several days in patients with HD

in high-risk situations was established. External validation con-

firmed the model’s correctness for surgery patients and

revealed important PK differences with use of recombinant

human AT in pregnant patients. Adjustment of the dosing regi-

men for delivery patients led to successful use in a second

study. The devised dosing regimen achieves AT activity in the

therapeutic range in most patients with HD within a short time

frame.

Author’s Note

Data described herein were submitted in abstract form for presentation

at the 2013 Annual Meeting of the Society for Gynecologic Investiga-

tion. Manuscript preparation, including medical writing and editorial

assistance, was provided by MedLogix Communications and was sup-

ported by rEVO Biologics. This article reflects the concepts of the

authors and is their sole responsibility. The authors reviewed and edi-

ted this article to ensure accuracy.

Declaration of Conflicting Interests

The author(s) declared the following potential conflicts of interest

with respect to the research, authorship, and/or publication of this arti-

cle: Henk-Jan Drenth and Joost DeJongh are employees of LAP&P

Consultants and were hired by rEVO Biologics to perform the popu-

lation PK model analysis and simulations described in this work.

Johan Frieling and Simon Lowry are employees of rEVO Biologics.

Funding

The author(s) disclosed receipt of the following financial support for

the research, authorship and/or publication of this article: The research

was funded by rEVO Biologics.

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