PHARMACOKINETICS
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Volume of distribution (Vd):
Vd = amount of drug/concentration of drug in plasma
Vd = dose
Cp
AUC: the area under the plasma or blood-concentration:time curve.
Bioavailability: F = AUC (oral)
AUC (IV)
MEC: usually a minimum effective concentration (i.e., MTC minimum therapeutic concentration) of drug in the blood or plasma is needed for a therapeutic effect.
Clearance:
A. The rate of elimination of a drug by all routes relative to the concentration of the drug in a biological fluid (usually blood or plasma).
CL = rate of elimination / concentrationplasma
B. CL = KVd
K = elimination rate constant
Elimination kinetics:
A. 1st order kinetics:
1. Half-life:
t2 = 0.693Vd / CL
CL = KVd so t2 = 0.693
K
B. 0 order kinetics:
A constant amount of the drug is eliminated in a given period of time.
A half-life cannot be described.
Extraction ratio:
1. ER = (Ci - Co / Ci )
ER= extraction ratio
Ci = concentration of drug in the blood entering an organ
Co = concentration of drug in the blood exiting an organ
2. CLorgan = Q x ER
Q = blood flow
DOSAGE REGIMEN DESIGN
Single dose:
. Estimates can be made from the graph
a. Half-life estimated from the slope
b. Volume of distribution from extrapolation of second phase of curve back to y-intercept.
Multiple Doses:
A. Maintenance dose
1. For IV dose: dosing rate = CLp x Cp
2. For multiple intermittent doses:
a. (F x Dose) / dosing interval = CLp x Cp
b. Peak and trough drug concentrations.
Peak: Cp, max = (F x dose / Vd )) / fraction loss in a dosing interval
Trough: Cp, min = Cp, max x fraction remaining after dosing interval
From: Katzung
Concentration kinetics:
log C = log C0 -k (t)
2.3
-k = log C1 - log C
2.3 t1 - t2
Plateau levels of a drug (given such that administration rate equals rate of elimination) are reach after four and one-half-lives (i.e., 4.5 x t2)
B. Loading dose
LD = Vd x Cp
Renal disease: Dr = (D) CLsr
CLs
Tr = (T) CLs
CLsr
creatinine clearance = 160 - age (years) x weight (kg)
(L/h) 22 x serum creatinine (mg/dL) 70
The predicted clearance in women is 90% of the value calculated above.
Category: Pharmacology Notes
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