slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Physiologically Based Pharmacokinetics – Lecture II PowerPoint Presentation
Download Presentation
Physiologically Based Pharmacokinetics – Lecture II

Loading in 2 Seconds...

play fullscreen
1 / 36

Physiologically Based Pharmacokinetics – Lecture II - PowerPoint PPT Presentation


  • 141 Views
  • Uploaded on

Physiologically Based Pharmacokinetics – Lecture II. Melvin Andersen CIIT Centers for Health Research October 27, 2006 University of North Carolina. TODAY’S TOPICS. PBPK Models for the Metabolism of Methylene Chloride and Application in Risk Assessment - Easy

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Physiologically Based Pharmacokinetics – Lecture II' - aaron


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

Physiologically Based Pharmacokinetics – Lecture II

Melvin Andersen

CIIT Centers for Health Research

October 27, 2006

University of North Carolina

slide2

TODAY’S TOPICS

  • PBPK Models for the Metabolism of Methylene Chloride and Application in Risk Assessment - Easy
  • Thinking about Pharmacokinetics while thinking more deeply about terms such as exposure and mixture? - Harder
slide3

Metabolism of Inhaled Dihalomethanes In Vivo:

  • Two Pathways – How can we measure them?
slide4

Gas Chromatograph

5 mL Gas

Sampling Loop

Stainless

Steel

Bellows

Pump

Particulate

Filter

O2 Monitor

CO2

Scrubber

INTEGRATOR

Pressure

Gauge

Injection

Port

~ 2.0

L/min

~ 100 mL/min

Ice Filled Pan for

H2O Condensation

Desiccator Jar

Chamber

Experiments to Get Some of the Needed Parameters for a PBPK Model

Gas Uptake and Metabolic Parameters

Vial equilibration and Partitioning

slide6

Chamber Volume

Qalv

Qalv

Alveolar Space

Calv (Cart/Pb)

Cinh

Qc

Qc

Lung Blood

Cven

Cart

Qt

Fat Tissue Group

Cvt

Cart

Qm

Muscle Tissue

Group

Cart

Cvm

Qr

Richly Perfused

Tissue Group

Cart

Cvr

Liver

Metabolizing

Tissue Group

Ql

(

)

Cvl

Cart

Vmax

Metabolites

kf

Km

Adding a Different Exposure Scenario

slide7

Gas Uptake Methods Estimate Metabolic Parameters:

– Vmax and Km

- kf

Linear process

Chamber loss with CH2BrCl

Saturable process

slide8

Motivation for Studying Bromide

  • We studied bromide concentrations in plasma after 4-hr exposures of rats to dibromomethane at various concentrations.
  • From data, determined production rates of bromide ion during exposure and thereby estimated biochemical constants (Vmax and Km and kfc) for metabolism of CH2Br2 or CH2BCl in rats.
  • Examine the effect of inducers and inhibitors on bromide production curves to see if we can discover the biochemical pathways of dihalomethane metabolism.
slide9

kf

Formation and Distribution of Bromide from Oxidation of Dibromomethane

A complete distributional model for CH2Br2 with hepatic metabolism via Cytochrome P450 2E1 oxidation and GST.

slide10

2,3-EP reduces liver GSH

  • Pyrazole blocks microsomal oxidation

Plasma inorganic bromide concentrations on ambient concentrations of CH2BrCl following 4-hr exposures using naive, 2,3-EP, and pyrazole-pretreated rats. The smooth curves were generated by the PBPK.

slide11

What about carbon monoxide?

HbCO concentrations in naïve, 2,3-EP-, and pyrazole-pretreated animals following 4-hr exposures to CH2BrCl.

slide13

METHYLENE CHLORIDE - 1987

  • Causes cancer in mouse lung and mouse liver by inhalation, but not in mice exposed via drinking water.
  • Metabolized by two pathways, each producing a reactive metabolite. Oxidation to formyl chloride and GST-conjugation to chloromethylglutathione
  • Either pathway could produce a mutagenic metabolite. Which is it?
slide14

METHYLENE CHLORIDE - 1987

The two pathways contribute differentially at high and low exposure concentrations in rodents, as noted in studies with bromide release.

slide15

METHYLENE CHLORIDE - 1987

  • Responses related to intensity of tissue exposure to short-lived, spontaneously reactive intermediates.
  • Dose metric for PBPK modeling was estimated by
  • (amount metabolite formed )/tissue volume/time
  • Evaluate relationship between daily tissue exposure and cancer in the two-year mouse bioassay.
  • Provide an approach to extrapolate to lower doses, dose routes, and between mouse and humans.
slide16

QP·CI

QP·CX

QC·CV

QC·CA

Lung Gas

Exchange

Lung Tissue

MFO

GST

QR·CVR

QR·CA

Moderately

Perfused Tissues

Richly Perfused

Tissues

QM·CVM

QM·CA

Slowly Perfused

Tissues

QS·CVS

QS·CA

QF·CVF

QF·CA

GI Tract

Fat

QG·CA

Drink

QG·CVG

(QL+QG)·CVL

QL·CA

Liver

MFO

GST

PBPK Model Structure for Methylene Chloride

Attributes:

- Tissue Volumes

- Blood Flows

- Partition Coefficients

- Metabolic Constants

- Breathing Rate

- Water Intake

- Tissue metabolism

slide17

Tissue Doses for CYP2E21 and GST

Pathways of Metabolism

slide18

Interspecies Dose Comparison for Metabolites from the Glutathione Transferase Pathway

LIVER DOSE LUNG DOSE

  • The solid curve is calculated from the PBPK model for the mouse; the dashed curve is calculated for the human. The straight line is extrapolated based on a linear relationship, as was previously assumed. The difference between the upper and lower lines is about 70 to 80 fold.
slide19

Using PBPK Models - 1987

Identify toxic effects in animals and/or people

Evaluate available data on mode(s) of action, metabolism, chemistry of compound, metabolites and related chemicals

Describe potential mode(s) of action

Propose relation between response and tissue dose

Develop a PBPK model to calculate tissue dose(s)

Estimate tissue dose during toxic exposures with model

Estimate risk in humans assuming similar tissue response for equivalent target tissue dose

slide20

Develop PBPK parameters for CO portion of model in absence of DHM exposures.

% HbCO

Rats exposed to 500 ppm CO for 2 hr.

What about carbon monoxide? Can we develop a PBPK model as well? Sure…

slide21

Develop parameters for CO portion of PBPK model for humans.

Human volunteers were exposed to 50, 100, 200, and 500 ppm CO (Stewart et al., 1975).

slide22

Link DHM metabolism to CO production

Human exposure to 50 and 350 ppm dichloromethane.

slide23

Link DHM metabolism to CO production

Human exposure to 50 and 350 ppm dichloromethane: time course of blood carboxyhemoglobin.

slide24

What can we evaluate with a model of HbCO production from a Dihalomethane?

  • Metabolism to CO for methylene chloride first noted in a human volunteer study on carbon monoxide.
  • Volunteer doing paint stripping at home with solvents had high blood HBCO in the morning.
  • How could this happen?
slide25

Experimental design in rat study !!!

What’s going on here? Why do the compounds have different time courses?

Blood carboxyhemoglobin levels after half-hour exposures to 5159 ppm dichloromethane or 5000 ppm bromochloromethane (BCM). Triangles are for BCM; circles are for DCM.

slide27

II. Atrazine Metabolism & Inhibition in vitro

Incubations conducted at 4 different atrazine concentrations for 90 min.

All 4 chlorinated triazines were followed in the incubation media.

What do you expect from a study of this kind?

Ethyl

Iso

DACT

McMullin, T.S. (2005). Integrating tissue dosimetry and mode of action to evaluate atrazine dose response. PhD Thesis, Colorado State University. In press at Toxicology in vitro

slide28

Results with atrazine metabolism in rat hepatocytes look quite odd...

What’s going on here? Any Ideas?

44 & 98

266

1.7

slide29

Accounting for inhibition of metabolic pathways by multiple substrates

RAMatra = (Vmaxatra*Catra) / (Catra + Katra*(1+Ciso/Kiso + Cethyl/Kethyl + Cdact/Kdact))

RAMatra = (Vmaxatra*Catra) / (Catra + Katra*(1+Ciso/Kiso + Cethyl/Kethyl + Cdact/Kdact))

Atrazine

Isopropyl

Ethyl

DACT

RAMiso = (Vmaxiso*Ciso) / (Ciso + Kiso*(1+ Cethyl/Kethyl + Catra/Katra + Cdact/Kdact))

RAMethyl=(Vmaxethyl*Cethyl)/ (Cethyl + KEthyl*(1+Catra/Katra+ Ciso/Kiso + Cdact/Kdact))

slide30

Could it be competitive inhibition???

DACT dose-response at 90 minutes comparing model simulations (A) without and (B) with competitive metabolic inhibition terms. Lines represent model simulations.

The non-linear behavior of DACT formation required a model that included competitive inhibition, where high [ATRA] inhibit further metabolism of Iso or Ethyl.

slide31

Hexane (Hx) Exposures & Mixtures

Hx induces changes in mean nerve conduction velocity.

It is more potent at 1000 ppm than at 3000 ppm!

What gives rise to this behavior?

slide32

Hexane exposures produce 2,5-HD – the actual neurotoxicant.

Blood concentrations of 2,5-HD are complexly related to inhaled Hx and have very unintuitive relationships over time.

After cessation of Hx exposure in the 2 higher concentration groups, 2,5-HD actually increases over time.

Where have you seen this behavior?

Baker and Rikert (1979)

slide33

Hexane PBPK Modeling

CYP 2E1

Interactions arise from two primary sources:

– competition for a common enzyme required for sequential steps in Hx oxidation

- differential properties of Hx (low blood:air partitioning) versus m-n-BK (much higher blood:air partitioning)

CYP 2E1

Clewell & Andersen (1984)

slide34

Hexane

Exposure

Complex dose and time dependencies

At higher Hx exposures, 2,5-HD increases after exposure cessation.

Inhibitory interactions present during exposure are released as Hx is rapidly exhaled.

Looks a lot like the atrazine story from the in vitro studies…

Do you believe me? Do you want to buy a bridge?

How could we test if this idea worked with other situations?

- Clewell & Andersen (1984)

slide35

Designer Mixture – A lipophilic compound (DBM) metabolized to CO and a poorly soluble anesthetic, isoflurane (ISO), in the air.

What happens to CO?

Develop a PBPK model with inhibition between ISO and DBM. Can you explain why you see the bump?

What do we mean by exposure?

slide36

You can be wrong!

Air

Metabolic Constants

Tissue Solubility

Tissue Volumes

Blood and Air Flows

Experimental System

Lung

Body

Tissue Concentration

X

Fat

X

X

X

X

X

X

Liver

X

Model Equations

Time

Define Realistic Model

Make

Predictions

Collect Needed

Data

Refine Model Structure

Physiologically Based Pharmacokinetic (PBPK) Modeling