RE: [NMusers] Dilemma with PPK parameters

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_at_farmbio.uu.se]
Sent: Friday, February 05, 2010 10:07 AM
To: Serge Guzy; varsham_at_med.umich.edu; nmusers_at_globomaxnm.com
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_at_globomaxnm.com [mailto:owner-nmusers_at_globomaxnm.com] On Behalf Of Serge Guzy
Sent: den 5 februari 2010 18:17
To: varsham_at_med.umich.edu; nmusers_at_globomaxnm.com
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_at_globomaxnm.com <owner-nmusers_at_globomaxnm.com>
To: nmusers_at_globomaxnm.com <nmusers_at_globomaxnm.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_at_umich.edu

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