# RE: Dilemma with PPK parameters

From: Serge Guzy <GUZY>
Date: Fri, 5 Feb 2010 10:21:52 -0800

Dear Ulrika.

I of course do agree with you about the relationships. My issue was: What is the relationship if any when the true model is let say a two compartmental model but you are using the one compartmental model with only one half life?

While the relationship Cl=V.K still handle (assuming linear kinetics), I do not think a similar simple relationship applies always between Clearance and this single half-life since your model is misspecified and the half-life you get is some sort of hybrid estimate. You then maximize the likelihood using any optimization procedure and will then get an estimate of this hybrid parameter by suing the formula T=0.693/K.

My point was that may be model misspecification could explain the Clearance issues addressed by our colleague.

Best

Serge Guzy, PhD

President, CEO; POP_PHARM; Inc.

From: Ulrika Simonsson [mailto:ulrika.simonsson
Sent: Friday, February 05, 2010 10:07 AM
To: Serge Guzy; varsham
Subject: RE: [NMusers] Dilemma with PPK parameters

Dear all,

Just a short note:

There is still a relationship between CL and half-lives for a two-compartmental model although not as simple as for a one compartmental model.

For a two-compartmental model you can derive the two half-lives (t1/2alpha and t1/2 beta) through either estimates of macro (CL, V2, V3, Q) or micro constants (K10,K21,K12) through (2 comp iv model):

K10=CL/V1

K12=Q/V1

K21=Q/V2

SUM=K10+K12+K21

ROOT=SQRT(SUM*SUM-4*K21*K10)

Alpha=0.5*(SUM+ROOT)

Beta=0.5*(SUM-ROOT)

t1/2alpha=LN(2)/Alpha

t1/2beta=LN(2)/Beta

Best regards,

Ulrika

Ulrika Simonsson, PhD

Assoc Prof of Pharmacometrics

Uppsala Pharmacometrics

Department of Pharmaceutical Biosciences

Uppsala University

BMC, Box 591, 751 24 Uppsala

Sweden

From: owner-nmusers lobomaxnm.com [mailto:owner-nmusers
Sent: den 5 februari 2010 18:17
To: varsham ; nmusers
Subject: Re: [NMusers] Dilemma with PPK parameters

I did not see the data but could you envisage that you have may be model misspecification. Half life relationship to clearance applies for example to the one compartmental model assumption. What if you have multiple half lives? I would not expect a simple relationship between a model using one half life with clearance when the true model has multiple one.
As far as I know the non compartmental relationship applies to clerance with volume and elimination rate and assumes linear kinetics.
Best
Serge guzy,PhD
President,CEO,POPPHARM

----- Original Message -----
From: owner-nmusers
To: nmusers maxnm.com>
Sent: Fri Feb 05 05:58:01 2010
Subject: [NMusers] Dilemma with PPK parameters

Dear Group:
I am seeking some help to logically explain the difference in CL of a drug between 2 groups who received the same drug for the same indication but one group had induced hypothermia (treatment-N=24) while the other did not (control-N=115).

The half life for control- 111hrs and treatment 129 hrs (roughly)- not statistically different.
However, the parameters:

Control Group Treatment group

CL- 0.000105 L/kg/hr (suspect) CL- 0.00467 L/kg/hr

Vd -0.733 L/kg Vd 0.876 L/Kg

I used the same model for both groups and the half life calc was part of the model. I have run the model for both multiple times using various theta values. each time the minimization is complete. The bootstrap for treatment group gives reasonably good agreement. The bootstrap for control parameters match well for Cl (0.000273 L/kg/hr) but not for Vd (0.416 L/kg).

Given the relationship between Vd and CL how is this possible? How can the half life be so close when there is such a huge difference in CL for both groups? Where am I going wrong?

Would appreciate any help anyone can provide.

Thanks!!

Varsha Mehta, MS(CRDSA), Pharm.D., FCCP
Clinical Associate Professor
Pharmacy, Pediatrics and Communicable Diseases
Clinical Pharmacist Neonatal Critical Care
University of Michigan
(O) 734-936-8985
(F) 734-936-6946
varsham

**********************************************************
Electronic Mail is not secure, may not be read every day, and should not be used for urgent or sensitive issues

________________________________

The information contained in this email message may contain confidential or legally privileged information and is intended solely for the use of the named recipient(s). No confidentiality or privilege is waived or lost by any transmission error. If the reader of this message is not the intended recipient, please immediately delete the e-mail and all copies of it from your system, destroy any hard copies of it and notify the sender either by telephone or return e-mail. Any direct or indirect use, disclosure, distribution, printing, or copying of any part of this message is prohibited. Any views expressed in this message are those of the individual sender, except where the message states otherwise and the sender is authorized to state them to be the views of XOMA.

```--
The information contained in this email message may
contain confidential or legally privileged information and is intended solely
for the use of the named recipient(s).  No confidentiality or privilege is
waived or lost by any transmission error. If the reader of this message is
not the intended recipient, please immediately delete the e-mail and all
copies of it from your system, destroy any hard copies of it and notify the
sender either by telephone or return e-mail.  Any direct or indirect use,
disclosure, distribution, printing, or copying of any part of this message is
prohibited.  Any views expressed in this message are those of the individual
sender, except where the message states otherwise and the sender is
authorized to state them to be the views of XOMA.
```
Received on Fri Feb 05 2010 - 13:21:52 EST

The NONMEM Users Network is maintained by ICON plc. Requests to subscribe to the network should be sent to: nmusers-request@iconplc.com.

Once subscribed, you may contribute to the discussion by emailing: nmusers@globomaxnm.com.