dosimetry and kinetics
DESCRIPTION
Dosimetry and Kinetics. Oct 17 2007. Casarett and Doull, Chapter 7, pp. 225-237 Timbrell, Chapter 3, pp 48-61. Exposure. External exposure – ambient air, water Dose received by body Dose at target organ Dose at target tissue Dose at target molecule. - PowerPoint PPT PresentationTRANSCRIPT
Dosimetry and Kinetics
Oct 17 2007
Casarett and Doull, Chapter 7, pp. 225-237Timbrell, Chapter 3, pp 48-61
Exposure
• External exposure – ambient air, water
• Dose received by body
• Dose at target organ
• Dose at target tissue
• Dose at target molecule
Exposure – Dose
How are they related ?Can we measure them ?
How can we describe the crucial steps so that we can
estimate what we can’t measure?
The single compartment(one compartment) model
kin kout
Kinetics of absorption
• Absorption is generally a first-order process
• Absorption constant = ka
• Concentration inside the compartment = C
C/t = ka * D where D = external dose
First-Order Processes
• Follow exponential time course
• Rate is concentration-dependent
v = [A]/t = k[A]
• Units of k are 1/time, e.g. h-1
• Unsaturated carrier-mediated processes
• Unsaturated enzyme-mediated processes
Kinetics of elimination
• Elimination is also generally a first-order process
• Removal rate constant k, the sum of all removal processes
C/t = -kC where C = concentration inside compartment
• C = C0e-kt
• Log10C = Log10C0 - kt/2.303
Kinetics of Enzyme-catalyzed Reactions
Michaelis-Menten Equation:
v = Vmax * [S]
Km + [S]
First-order where Km >> [S]
Zero-order where [S] >> Km
Second-Order Processes
• Follow exponential time course
• Rate is dependent on concentration of two reactants
v = [A]/t = k[A]*[B]
First-order elimination
Half-life, t1/2
Units: time
t1/2 = 0.693/k
One compartment system
First-order decay of plasma concentration
Total body burden
• Integration of internal concentration over time
• Area under the curve
Area under the curve (AUC)
A more complex
time-course
The two-compartment model
Centralcompartment
Peripheralcompartment
kin kout
Tissues
Plasma
The three-compartment model
Deepdepot
Peripheralcompartment
kin
kout
Centralcompartment
Slow equilibrium
Rapid equilibrium
The four-compartment model
Mamillary model
Deepdepot
Peripheralcompartment
kin
kout
Centralcompartment
Kidney
The four-compartment model
Catenary model
A B C D
kinkout
Physiologically-Based Pharmacokinetic Modeling
• Each relevant organ or tissue is a compartment• Material flows into compartment, partitionnns
into and distributes around compartment, flows out of compartment – usually in blood
• If blood flow rates, volume of compartment and partition coefficient are known, can write an equation for each compartment
• Assuming conservation of mass, solve equations simultaneously – can calculate concentration (mass) in each compartment at any time
Example of equation
δkidney/δt = (Cak * Qa) – (Ck * Qvk)
IN OUT
Rate of change of the amount in the kidney =
Concentration in (incoming) arterial blood X arterial blood flow
Minus
Concentration in (outgoing) venous blood X venous blood flow
Example of a modelAir inhaled
Lungs
Arterial blood
Venous blood
Urine
Metabolism
Liver
Kidneys
Rest of body