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232 SEcTIoN II Pharmacology ` PHARMACOLOGY—PHARMACOKINETICS ANd PHARMACOdYNAMICS Pharmacology ` PHARMACOLOGY—PHARMACOKINETICS ANd PHARMACOdYNAMICS

Drug metabolism Geriatric patients lose phase I first. Patients who are slow acetylators have  side effects from
certain drugs because of  rate of metabolism (eg, isoniazid).

Lipophilic drug

Phase I
(cytochrome P-450 dependent) Phase II

Reduction Oxidation Hydrolysis Methylation Glucuronidation Acetylation Sulfation

Slightly polar (water-soluble) metabolite Very polar metabolite
(active or inactive) (inactive and renally excreted)

Elimination of drugs

Zero-order Rate of elimination is constant regardless of C p Capacity-limited elimination.
elimination (ie, constant amount of drug eliminated per PEA (a pea is round, shaped like the “0” in
unit time). C p  linearly with time. Examples zero-order).
of drugs—Phenytoin, Ethanol, and Aspirin (at
high or toxic concentrations).
First-order elimination Rate of First-order elimination is directly Flow-dependent elimination.
proportional to the drug concentration (ie,
constant Fraction of drug eliminated per unit
time). C p  exponentially with time. Applies to
most drugs.

Zero-order elimination First-order elimination
Elimination rate (=slope) Elimination rate (=slope)
4 U/h Time of t 1/2 is constant
Drug plasma concentration First t 1/2 > 2 U/h Time of t 1/2 as ↑ Drug plasma concentration First t 1/2 == as concentration ↑
2 U/h
concentration
↑
2 U/h

Second t 1/2 > 2 U/h Second t 1/2 = 1 U/h 0.5 U/h
Third t 1/2 Third t 1/2
Time (h) Time (h)

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