Pharmacokinetics questions are short calculations plus a handful of concepts. Learn the formulas below, practise them until they are quick, and these become easy marks.
How this book helps in the exam
- One-step calculations: half-life, clearance, volume of distribution, loading dose.
- Concept questions on bioavailability, bioequivalence and protein binding.
- Renal dose adjustment and therapeutic drug monitoring cases in Gulf exams.
What the book covers
10 high-yield topics. Each has the points most often examined and links to practise them.
1Absorption and bioavailability
- Absolute bioavailability compares oral AUC with IV AUC after dose correction.
- First-pass metabolism lowers oral bioavailability of drugs such as GTN, propranolol and lidocaine.
- Weak acids are less ionised in acid, weak bases in alkaline conditions; the un-ionised form crosses membranes.
- P-glycoprotein pumps drugs back into the gut lumen and limits absorption.
2Distribution and protein binding
- Volume of distribution equals amount in the body divided by plasma concentration.
- A large Vd (digoxin, amiodarone) means the drug sits in tissues; dialysis removes little of it.
- Only the unbound drug is active and can be cleared.
- Acidic drugs bind albumin; basic drugs bind alpha-1 acid glycoprotein.
3Clearance and half-life
- Half-life equals 0.693 × Vd divided by clearance.
- Steady state is reached after about 4–5 half-lives, whatever the dose.
- Elimination rate constant k equals 0.693 divided by half-life.
- Total clearance is the sum of renal, hepatic and other clearances.
4First-order and zero-order kinetics
- In first-order kinetics, a constant fraction is eliminated per unit time.
- In zero-order kinetics, a constant amount is eliminated, and half-life rises with dose.
- Ethanol, phenytoin and high-dose aspirin show saturable, zero-order behaviour.
- A small dose increase in a saturable drug causes a large rise in level.
5Loading and maintenance doses
- Loading dose equals target concentration × Vd, divided by bioavailability.
- Maintenance dose rate equals target concentration × clearance, divided by bioavailability.
- A loading dose is useful for drugs with long half-lives such as amiodarone and digoxin.
- Changing the dose changes the steady-state level, not the time to reach it.
6Multiple dosing and infusions
- The accumulation factor depends on how the dosing interval compares with half-life.
- Peak-trough fluctuation shrinks when the dosing interval is short compared with half-life.
- With a constant infusion, steady state equals infusion rate divided by clearance.
- Extended-release products reduce fluctuation and dosing frequency.
7Renal elimination
- Renal clearance combines filtration, active secretion and reabsorption.
- Clearance above GFR means the drug is secreted; below GFR means it is reabsorbed.
- Alkalinising urine speeds elimination of weak acids such as aspirin and phenobarbital.
- Cockcroft-Gault uses age, weight, serum creatinine and sex.
8Hepatic metabolism
- Phase I reactions (oxidation, reduction, hydrolysis) are mainly by CYP enzymes.
- Phase II reactions (glucuronidation, sulfation, acetylation) usually make drugs more water-soluble.
- High-extraction drugs depend on liver blood flow; low-extraction drugs depend on enzyme activity and binding.
- Slow acetylators are more prone to isoniazid and hydralazine toxicity.
9Bioequivalence
- Two products are bioequivalent when the 90% confidence interval of the AUC and Cmax ratios falls within 80–125%.
- Studies are usually crossover designs in healthy volunteers with a washout period.
- Tmax shows the rate of absorption; AUC shows the extent.
- The BCS classifies drugs by solubility and permeability; Class I drugs may get biowaivers.
10Therapeutic drug monitoring
- Monitor drugs with narrow ranges, variable kinetics and a measurable concentration-effect link.
- Aminoglycosides are dosed once daily for concentration-dependent killing and less toxicity.
- Vancomycin is dosed by AUC or trough targets with renal function in mind.
- Take levels at steady state unless toxicity is suspected.
These are PharmaPrep's own notes on the knowledge this book teaches. No text, tables or figures are reproduced from it. Read the book for full explanations, and check doses and laws against current official sources.
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