Last updated 14 August 2020
NextDose estimates
pharmacokinetic (PK) and pharmacodynamic (PD) parameters using a Bayesian
method. These individual parameters are known as empirical Bayes estimates
(EBEs). The EBEs are used to individualize dose predictions and to predict the
time course of concentrations and biomarkers such as INR.
The EBEs are determined in
part by population parameters and in part by any observations that are
available. The population parameters describe a standard individual based on
patient factors such as weight and renal function (e.g. weight=70 kg, renal
function=1). These patient factors are known as covariates. Covariates are used
to predict a group parameter value using covariate effects. For example, group
clearance can be predicted from weight (WTKG) and renal
function (RF0 and population clearance by assuming clearance is linearly proportional
to RF and a theory based allometric function of WTKG (Equation 1):

Equation 1 
RF is calculated from the
ratio of the creatinine clearance (CLcr) calculated from serum creatinine to
the glomerular filtration rate (GFR) predicted for an individual of the same
size and maturation. Note that CLcr is the value for that individual without
any standardization based on for size, maturation or sex. NextDose calculates
CLcr based on the ratio of creatinine production rate (CPR) to steady state Scr.
CPR is predicted using a model developed from data in (Rhodin,
Anderson et al. 2009). This model uses fat free mass, postmenstrual
age and sex to calculate CPR relative to a 70 kg, 176 cm, 20 y old adult male. When
a series of Scr measurements are available the Scr time course is used to
predict CLcr without assuming steady state.
Note that Equation 1 is usually an oversimplification
by assuming that clearance is entirely proportional to RF. Even for drugs that
are thought to be almost completely renally eliminated (such as gentamicin) a
substantial part of the observable clearance is not predictable from RF (Matthews,
Kirkpatrick et al. 2004). In that case the clearance can be
split into a component not predictable from renal function () and a component predictable from
renal function (). The component
may actually be due to renal elimination but it cannot linked to RF based on CLcr.
An example is shown in Equation 2.

Equation 2 
Individual estimates (EBEs)
of the parameter are then predicted from the group parameter value and an
individual specific random effect. This random effect is made up of a between
subject variability component (BSV) and a within subject variability component
(WSV). The within subject variability is usually estimated with reference to an
interval known as an occasion and WSV is therefore commonly described as
between occasion variability (BOV). The definition of an occasion in NextDose
is a dosing interval which includes one or more observations.
Random effects are commonly
assumed to be lognormally distributed so that the individual clearance is
predicted using Equation 3:

Equation 3 
Figure 1 shows the
concentration predictions in a patient treated with vancomycin who had three
predose vancomycin concentration observations.
Figure 1
The Bayesian
dose predictions for these 3 observation occasions are shown in Figure 2
Figure 2
NextDose TCI vancomycin NNNN:20200617131151_conc_holfordGAV2020_AVG
Target: AUC 300 mg/L*h per 12 hours at steady state
Bayesian 
Route 
Predicted Dose 
Actual Dose 


1 
IV 
1008 mg every 12 hours 
1250 mg 
10/06/2020 03:55 

2 
IV 
863 mg every 12 hours 
1250 mg 
12/06/2020 03:50 

3 
IV 
863 mg every 12 hours 
900 mg 
14/06/2020 04:05 

Proposed IV maintenance dose 911 mg every 12 hours (Bayesian Average)
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
3.36 
20 
24.9 
0.2 
1 
0 
38.9 
100 
2.87 
20.1 
24.9 
0.2 
1 
0 
38.9 
85.4 
2.88 
20 
24.9 
0.2 
1 
0 
38.9 
85.4 
Following the dose
predictions is a table of individual parameters (EBEs) and covariate effects.
NextDose shows 8 values which are displayed for all medicines.
The first 6 describe the parameters
clearance (CL), central volume of distribution (V) and bioavailability (F).
Following the EBE for each parameter is the fractional change (f) from the
group parameter value expressed as a percentage. When a medicine is given
parenterally then F is 1 and there is no random effect.
The EBE for
CL appears to decrease from 3.36 l/h to 2.87 L/h. This is explained by the
change in RF% from 100% to 85.4%. The first prediction of renal function was
made before a serum creatinine observation was provided so renal function is
assumed to be normal (100%). The second two occasion were associated with a
serum creatinine measurement and a 15% change in RF. The random effect
difference for this patient is 20% lower than the group prediction. For
vancomycin this is primarily due to between subject variability (BSV) because
the between occasion variability (BOV) in this model is small.
The last 2
standard values are the covariate effects, predicted fat free mass (FFM) and
renal function (RF). RF is expressed as a percentage where 100% means normal
renal function relative to predicted normal glomerular filtration rate.
FFM is used
to predict the effect of body size and composition on clearance, volume of
distribution and glomerular filtration rate. RF is used to predict a component
of clearance that is related to RF. The remaining component is not linked to RF
(nonrenal function clearance).
Medicine
specific values are shown with gentamicin, amikacin and vancomycin predictions.
This example is for vancomycin.
CLcr L/h 
Normal GFR L/h 
CPR uM/h 
RFss% 
CLcrss L/h 
CPRss uM/h 
. 
5.33 
. 
. 
. 
. 
4.55 
5.33 
150 
128 
6.55 
216 
4.55 
5.33 
150 
128 
6.55 
216 
The first (CLcr) is the
predicted creatinine clearance which takes into account the time course of
change in serum creatinine (Scr) in order to predict the steady state Scr. The
next value is the predicted normal glomerular filtration rate (GFR). GFR is
predicted from size and maturation (Rhodin,
Anderson et al. 2009). Renal function is calculated from
the ratio of CLcr to normal GFR. CLcr is calculated from the ratio of creatinine
production rate (CPR) to Scr. The NextDose method for calculating CPR is based
on a model derived from combined GFR and SCr data (Rhodin,
Anderson et al. 2009)
The last 3 values are
predictions of RF, CLcr and CPR using methods for CLcr that assume the measured
SCr is at steady state (Schwartz 1992,
Matthews, Kirkpatrick et al. 2004)
There are
no medicine specific values shown for busulfan.
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
10.8 
36.7 
7.94 
8.7 
1 
0 
36.5 
. 
Methotrexate
has additional specific values for observed urine pH and predicted CLcr
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
Urine pH 
CLcr L/h 
7.77 
68.1 
41.2 
73.6 
1 
0 
55.9 
73.2 
7.00 
5.60 
9.25 
62.1 
48.4 
69 
1 
0 
55.9 
73.2 
7.00 
5.60 
There are 6
specific values for tacrolimus. Oral bioavailability is high during the first 2
days after transplant and fall to normal after day 2 (day of transplant is day
0). There is a BSV random effect on bioavailability upto day 2 which is 63%
lower than expected. The daily dose of prednisolone (Pred) affects oral
bioavailability and in this case F is 75.6% of F without a steroid effect. The
CYP3A5 genotype can affect both F and CL. In this case there is no genotype
effect. The haematocrit influences the measured whole blood value of tacrolimus
so its value (HCT) is shown here. Finally the predicted plasma to blood
fraction (fu) expressed as a percentage 2.09% at a standardized blood
concentration (HCT=45%) is shown.
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
9.09 
52.3 
43.3 
71.9 
0.720 
4.8 
70.4 
. 
9.09 
52.3 
43.3 
71.9 
0.841 
11.2 
70.4 
. 
Day Tx on F 
diffDTx F% 
Pred on F % 
GT CYP3A5 on F % 
GT CYP3A5 on CL % 
HCT % 
fu% at HCT 45% 
1 
62.687 
75.6 
100 
100 
29.0 
2.09 
1 
62.687 
75.6 
100 
100 
29.0 
2.09 
Linezolid
has 2 specific values. The first is the minimum inhibitory concentration (MIC,
2 mg/L). The second is the baseline platelet count (PLT0, cells/microL).
There is a
marked decrease in RF with a decrease in CL. Nephrotoxicity is a well known
adverse effect of linezolid.
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
MIC 
PLT0 
1.31 
47.6 
10.2 
45.4 
1 
0 
46.7 
23.4 
2.00 
212000 
1.31 
47.6 
10.2 
45.4 
1 
0 
46.7 
23.4 
2.00 
212000 
1.29 
47.6 
10.2 
45.4 
1 
0 
46.7 
20.7 
2.00 
212000 
1.29 
47.6 
10.2 
45.4 
1 
0 
46.7 
20.7 
2.00 
212000 
1.26 
47.6 
10.2 
45.4 
1 
0 
46.7 
15.3 
2.00 
212000 
1.25 
47.6 
10.2 
45.4 
1 
0 
46.7 
13.3 
2.00 
212000 
1.24 
47.6 
10.2 
45.4 
1 
0 
46.7 
13 
2.00 
212000 
1.24 
47.6 
10.2 
45.4 
1 
0 
46.7 
12.3 
2.00 
212000 
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
2.19 
33.3 
66.9 
5 
1 
0 
42.5 
. 
There are
many covariates that affect voriconazole PK. Ritonavir (RTV), St John’s Wort
(SJW), prednisolone (or prednisone) (Pred), methyl prednisolone (MePRed),
dexamethasone (DEX). Phenytoin (PHE) and rifampicin (RIF) are both inducing
agents. The CYP2C19 genotype may be associated with a reduced F (87.7% of
normal) and CL (56.3% of normal). The minimum inhibitory concentration (MIC,
mg/L) is also shown.
RTV 
SJW 
Pred 
MePred 
DEX 
PHE or RIF 
GT CYP2C19 on F % 
GT CYP2C19 on CL % 
MIC mg/L 
0 
0 
0 
0 
0 
0 
87.8 
56.3 
1 
The
warfarin model estimates both PK and PD parameters.
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
0.141 
4.8 
29.8 
0.9 
1 
0 
66.0 
. 
The predicted baseline prothrombin complex
activity (PCA0) is shown here as 97.9% of normal. The PCA half life (T2PCA) EBE
is 11.5 h which is just under 0.5% higher than the group value (fT2PC%). The
potency of the Senantiomer of warfarin for inhibition of PCA synthesis is
0.211 mg/L (C50 S) and is 6.39% of the group value (fC50%). The exponent for
the warfarin inhibition model is 2.75 (Hill) and this is 1.52% higher than the
group value. The potency of the Renantiomer of warfarin as an inhibitor of Swarfarin
is 2.4 mg/L. The VKORC1 genotype affects the Swarfarin C50 so that it is 0.766
of normal. The CYP2C9 genotype effect on Swarfarin clearance is negligible
with a fractional increase of 0.01.
PCA0 % 
T2PCA h 
fT2PC% 
C50S mg/L 
fC50% 
Hill 
fHill% 
C50R mg/L 
FGT VKOR on C50 
FGT CYP2C9 on CLs 
97.9 
11.5 
0.469 
0.211 
6.393 
2.75 
1.52 
2.40 
0.766 
1.01 
Mycophenlate unbound clearance increases substantially
after a renal transplant (from 757 L/h to 1229 L/h over 1 month).
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
ALB g/L 
fu MPA % 
757 
27.4 
3263 
7.7 
0.950 
0 
66.1 
14.7 
32.0 
1.66 
750 
28.1 
3078 
1.6 
0.950 
0 
66.1 
14.7 
32.0 
1.66 
1449 
33.3 
3264 
7.8 
0.950 
0 
66.1 
14.7 
31.0 
1.71 
1576 
27.4 
3995 
31.9 
0.950 
0 
66.1 
14.7 
31.0 
1.76 
1229 
43.4 
2947 
2.7 
0.950 
0 
66.1 
14.7 
33.0 
1.38 
Both albumin and renal function influence mycophenolate
plasma protein binding. The serum albumin (ALB, g/L) is shown along with RF.
The unbound fraction of mycophenolate (unbound/total plasma) decreased from
1.66% to 1.38% in the month after transplant.
Caffeine is
used in premature neonates to reduce the risk of apnea of prematurity
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
0.008 
0.9 
1.71 
19.7 
1 
0 
1.22 
. 
The number of days after birth (postnatal age,
PNA) for this neonate is 0. Height (HT) is 40 cm. The allometric fraction of
size affecting CL relative to a 70 kg adult is 0.05. The fractional maturation
of nonrenal function CL is 0.025 while the fractional maturation of renal
clearance is 0.171 (relative to adult). The birth method was vaginal (0) rather
than Caesarian (1).
PNA days 
HT cm 
FSIZCL 
FMAT CLnrf 
FMAT CLrf 
BIRTH V=0 C=1 
0 
40.0 
0.050 
0.025 
0.171 
0 
Hydroxychloroquine is used for the treatment of
lupus erythematosus and rheumatoid arthritis. Whole blood pharmacokinetics are
described based on a standard hematocrit (HCT) of 45%.
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
BMI kg/m^2 
ALB g/L 
HCT % 
VENT 
11.6 
25.2 
777 
5.7 
0.746 
0 
63.2 
52.7 
26.8 
35.0 
45.0 
1 
11.6 
25.2 
777 
5.7 
0.746 
0 
63.2 
52.7 
26.8 
35.0 
45.0 
1 
11.6 
25.2 
777 
5.7 
0.746 
0 
63.2 
52.7 
26.8 
35.0 
45.0 
1 
Body mass index (BMI), serum albumin (ALB),
haematocrit (HCT) and use of artificial ventilation (0=no ventilator,
1=ventilator) are the medicine specific factors.
The RELY
2011 PK model (Liesenfeld, T.
et al. 2011) has random effects on V and F but
not on CL so fCL % is always zero.
CL L/h 
fCL% 
V L 
fV% 
F 
fF% 
FFM kg 
RF% 
66.0 
0 
698 
0.8 
1.08 
8.1 
62.8 
43.8 
CLcr, Normal GFR, CPR, RFss%, CLcrss, CPRss
have the same meaning as described for gentamicin, amikacin and vancomycin.
The RELY 2011 PK model predicts CL with a
sigmoid Emax model using CLcrss. The fractional effect of CLcrss (F CLcrss CL)
is relative to the size scaled asymptotic CL (infinite CLcrss). The combined
effects of three concomitant medications (proton pump inhibitors, amiodarone,
verapamil) on bioavailability (F) is expressed as the % difference from no
concomitant medication effect.
CLcr L/h 
Normal GFR L/h 
CPR uM/h 
RFss% 
CLcrss L/h 
CPRss uM/h 
F CLcrss CL 
F ConMed F1 % 
3.32 
7.57 
308 
50.5 
3.56 
331 
0.514 
0 
Liesenfeld, K.H., L. T., C. Dansirikul, P. A. Reilly, S. J. Connolly, M. D. Ezekowitz, S. Yusuf, L. Wallentin, S. Haertter and A. Staab (2011). "Population pharmacokinetic analysis of the oral thrombin inhibitor dabigatran etexilate in patients with nonvalvular atrial fibrillation from the RELY trial." Journal of Thrombosis and Haemostasis 9(11): 21682175.
Matthews, I., C. Kirkpatrick and N. Holford (2004). "Quantitative justification for target concentration interventionparameter variability and predictive performance using population pharmacokinetic models for aminoglycosides." Br J Clin Pharmacol 58(1): 819.
Rhodin, M. M., B. J. Anderson, A. M. Peters, M. G. Coulthard, B. Wilkins, M. Cole, E. Chatelut, A. Grubb, G. J. Veal, M. J. Keir and N. H. Holford (2009). "Human renal function maturation: a quantitative description using weight and postmenstrual age." Pediatr Nephrol 24(1): 6776.
Schwartz, G. J. (1992). "Does kL/PCr estimate GFR, or does GFR determine k?" Pediatr Nephrol 6(6): 512515.
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Holford 20122020