Lecture 11 Clinical Pharmacology

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22 Apr 2012
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Lecture 11 PCL201 Clinical Pharmacology
Bioavailability (F)
Fraction of uncharged drug reaching systemic circulation
Measure as AUC
Measures extent of absorption and first pass
F = f(1-E) (f = absorption)
IV; 100%, rapid onset
IM; 75-100%, larger volume than SC
SC; 75-100%, smaller volume than IM
PO; 5-100%, convenient, 1st pass
PR; 30-100%, less 1st pass than PO
Inhalation; 5-100%, rapid onset
Transdermal; 80-100%, slow onset, prolonged duration
Oral dosing
Capsule disintegrates, dissolves and absorbed via GIT
1st pass effect
reduced bioavailability
[Dr]plasma slow to increase (while absorption takes place)
[Dr]plasma decreases slightly as it equilibrates with all compartments
[Dr]plasma decreases with elimination
1. [Dr]plasma rising slope not as steep as IV
2. equivalent IV/PO dose PO has lower Cmax
3. PO longer to reach Cther after reaching Cmax
Cmax: peak [drug]
Tmax: time at Cmax
AUC: []/time: extent of absorption, amount of drug in plasma bioavailability
Measure bioavailability compare AUC to IV
Bioavailability = (AUC X)/(AUC IV)
Used to establish bioequivalence of generic drug: (AUC trade)/(AUC generic) must be 80-
120%
Bioavailability
Compare bioequivalence
[drug] that reaches systemic unchanged
measure extent of absorption
compare routes of administration
Half life t ½
time to decrease [drug] to half of previous value
t ½ = 0.693/k
Rule of thumb: 5 half lives for 96.9% drug removal
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