RE: questions about IOV
Dear Jia,
Firstly I would say make sure you do want to model IOV. As your patients
have higher doses on the second occasion, then you need to be sure the PK is
linear, otherwise changes in CL would better be modelled with nonlinearity
in CL. As for what parameters to try, my approach in the past has been to
sequentially add IOV to each parameter, and see which gave the biggest OFV
drop/reduction in residual and interindividual variability. Then have IOV
on just that parameter, although you may then want to test for additional
IOV on other parameters.
As for your coding, for each occasion you need an eta (modelled with the
same omega using SAME option - see below), and one for IIV - so in your case
3 etas for every parameter with IIV and IOV. You could code it like this:
OCC=1
IF(TIME.GE.100)OCC=2
IOV = 0
IF(OCC.EQ.1) IOV=ETA(2)
IF(OCC.EQ.2) IOV=ETA(3)
IIVCL = ETA(1)
ETCL = IIVCL + IOV
..
$OMEGA 0.1 ; IIV ON CL
$OMEGA BLOCK(1) 0.05
$OMEGA BLOCK(1) SAME ; IOV ON CL
If you are using NONMEM 7, there are some suggestions in the manual as to
how to code it with MU parameterisation.
Best wishes,
Joe
_____
Quoted reply history
From: [email protected] [mailto:[email protected]] On
Behalf Of Jia Ji
Sent: den 11 november 2009 03:03
To: [email protected]
Subject: [NMusers] questions about IOV
Dear All,
I am a new NONMEM user and now trying to model clnical PK data with a
two-compartment model. More than half of patients in our trial had escalated
dose in the second cycle. So I should have inter-occasion variability. But I
got a couple of questions here.
First, what parameters should be estimated with IOV? I have seen some models
with IOV on CL, but not on others (maybe because I have seen too few
models). Now I have IOV on CL, V1 and Q and it run successfully. But when I
run with either two of them, I got minimization problem. But, am I having
too many IOVs in the model?
Second, I was wondering how to model IOV. Now I have the code as
; Define IOV, DESC = DOSE ESCALATION
DESC=0
IF (TIME.GE.100) DESC=1
ETCL = ETA(1) + DESC*ETA(5)
ETV1 = ETA(2) + DESC*ETA(6)
ETQ = ETA(3) + DESC*ETA(7)
ETV2 = ETA(4)
So IOV is modeled as additive relationship to IIV. But what about
multiplying relationship? Like
; Define IOV, DESC = DOSE ESCALATION
DESC=0
IF (TIME.GE.100) DESC=1
ETCL = ETA(1)*(1+DESC*ETA(5))
ETV1 = ETA(2)*(1+DESC*ETA(6))
ETQ = ETA(3)*(1+DESC*ETA(7))
ETV2 = ETA(4)
When I run with this multiplying relationship, I got increased OFV and
minimization terminated due to rounding error. But I didn't understand why
it is not working.
Thank you so much for your patience and time!
Jia