FIRST ORDER KINETICS

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