USMLE Step 1 Review/Pharmacology

Pharmacodynamic and pharmacokinetic processes, general principles.

Pharmacokinetics

The four stages of chemicals passing through the body

Absorption - the process of the intake of the drug into the body
Distribution - the process of the dispersion of the drug into the blood stream and tissues
Metabolism - the process of the parent compounding into daughter metabolites
Excretion - the process of eliminating the drug from the body

Dosage intervals

Zero order pharmacokinetics, the rate of reaction is independent of the concentration of the reactants:

r = k

Drugs following zero order pharmacokinetics; Mnemonic: 'Peas & WHEATS' (A Pea would look like a zero and denotes the order). Pea also stands for 'Pee'-ing out a constant amount as in zero order kinetics constant amount of the drug is eliminated per unit time. Rate of elimination is constant and does not depend on or vary with the drug intake or plasma concentration of the drug.

Phenytoin, Phenylbutazone
Warfarin
Heparin
Ethanol
Aspirin
Theophylline, Tolbutamide
Salicylates

Absorption

Many abbreviations are used in writing administration routes and frequency:

p.o.: by mouth
IM: intramuscular injection
SC: subcutaneous injection
IV: intravenous
PR: per rectum
h.s.: at hour of sleep (bedtime)
ac: before meals
pc: after meals
q: every, ie, q 8 h means every 8 hours
q.d.: every day
b.i.d.: twice/day
t.i.d.: three times/day
q.i.d.: four times/day
q.o.d.: every other day

Distribution

The volume of distribution is given by the following equation:

{V_{D}}={\frac {\mathrm {total\ amount\ of\ drug\ in\ the\ body} }{\mathrm {drug\ blood\ plasma\ concentration} }}

Metabolism

Drugs undergoing complete first pass metabolism when taken orally: 'FILTHy'

First pass metabolism
Isoprenaline
Lignocaine (Lidocaine)
Testosterone
Hydrocortisone

Drugs metabolized by acetylation, acetylator polymorphism: Some individuals are slow acetylators and some are fast acetylators.The drugs metabolized by acetylation may become ineffective in fast acetylators and cause toxicity in slow acetylators. Mnemonic: 'SHIP'

Sulfonamides including dapsone
Hydralazine
Isoniazid (INH)
Procainamide

Enzyme inducers: 'SCRAP Bar'

Sulphonylureas
Carbemazepine
Rifampicin
Alcohol
Pheytoin
Barbituates

Enzyme inhibitors 'COMIC CASE'

Cimetidine
Omeprazole
Metronidazole
Isoniazid
Chloramphenicol
Ciprofloxacin
Allopurinol
Sulphonamides
Erythromycin

Elimination

Three sites where drug excretion occurs: The kidneys, the liver and the lungs.

Drug actions

mechanisms of drug action, structure-activity relationships (including anticancer drugs)
concentration- and dose-effect relationships, types of agonists and antagonists and their actions
individual factors altering pharmacokinetics and pharmacodynamics
mechanisms of drug adverse effects, overdosage, toxicology
mechanisms of drug interactions
regulatory issues
signal transduction, including structure/function of all components of signal transduction pathway such as receptors, ligands
cell cycle/cell cycle regulation