Regarding phenytoin, true is: (DNB June 2009)
|A||It is excreted unchanged|
|B||It is not teratogenic|
|C||Shows zero order kinetics|
|D||Does not induce liner enzyme|
a. Phenytoin is most widely used diphenyl-hydantoin derivative.
b. The drug is well absorbed orally and its metabolism is saturable. That means, the metabolic capacity of drug is fixed.
c. It follows pseudo-zero order kinetics in high dose.
d. Zero order means, a fixed amount of the drug is getting eliminated per unit time. For example, ethanol is eliminated at a fixed rate of 10 grams per hour.
e. Since enzymes metabolizing drugs in this manner are saturable; this is also known as saturable kinetics. Both half-life and clearance change in zero order; while both remain constant.
f. Suppose if someone consumes 20 gram of ethanol with in one hour. Then, 10 gram will be eliminated in one hour.
g. Half-life of rest of the amount will increase. Hence, clearance will reduce.
h. Some drugs follow zero order but at high dose. This is known as pseudo zero order (see below).
Zero order kinetics:
i. Ethanol ii. Phenytoin iii. Tolbutamide
iv. Theophylline v. Heparins vi. Warfarin
vii. Ketoconazole viii. Aspirin ix. Caffeine
Ref: (BG Katzung, pp-404, 11th Edition, 2009)