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First order kinetics
The process that is directly proportional to the drug
concentration available at that time.
• Linear kinetic process
• Concentration dependent process
• Rate of process increase linearly with increase in
drug concentration
• Never comes to an end
Drug metabolism
First order kinetics
Constant fraction per unit
time is metabolized
Rate increases if drug conc.
increases
General expression
• dC/dt = - kC
Here ,
dC = The change in concentration
dt = Time interval
dC/dt = The velocity of the reaction
k = Rate constant
C = Concentration
General equation: Log C = Log C0 - kt / 2.303
Half-life : t1/2 = 0.693 / K
- Concentration independent
- Constant value
Rate constant units : Time‾ 1
www.chem.purdue.edu
This is the same curve as the one above
but the y-axis is the natural logarithm of conc.
This allows the calculation of t½.
http://schoolbag.info/chemistry/mcat_2/
mcat_2.files/image252.jpg
Pseudo-order reaction
• Following administration of a drug, it may beeliminated from the body only after reacting withtissue components.
-Tissue components present in highconcentration.
-Are not used up to any degree.
-Show pseudo-monomolecular reactions
-Drug eliminates from the body according to firstorder kinetics.
Example:
100 mg drug orally
Absorption rate: 10% per hour
First hour : 10 % of 100 mg(10 mg)
Second hour:10% of remaining 90mg (9 mg )
Third hour : 10 % 0f remaining 81 mg (8.1 mg) and will continue in this process.
http://www.aboutkidshealth.ca//En/Asse
ts/PST_child_medicine_syringe_EN.jpg
References
• IUPAC, Compendium of Chemical Terminology, 2nd
ed. (the "Gold Book") (1997). Online corrected
version: (2006)
• http://www.pharmainfo.net
• http://www.chemteam.info
• http://chem.libretexts.org
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