Pharmaceutical Chemistry and Drug Structure–Activity Relationships
What the domain tests
This domain examines how functional groups, ionization, stereochemistry, and molecular shape determine drug solubility, permeability, receptor fit, metabolism, and stability. Exam items frequently ask you to compare two analogues, identify a prodrug, or predict whether a molecule is ionized at a given pH.
Core rules and concepts
- The Henderson–Hasselbalch relationship predicts ionization: weak acids are more ionized at higher pH; weak bases are more ionized at lower pH. Ionized species are usually more water-soluble and less membrane-permeable.
- logP describes lipophilicity; logD accounts for ionization at a specified pH. Excess lipophilicity can increase volume of distribution but reduce aqueous solubility.
- Bioisosteres replace a group with similar size, electronics, or hydrogen-bonding (for example, –OH/-NH2 or –CO2H/tetrazole) to retain activity while changing metabolism or solubility.
- Stereochemistry matters: enantiomers can differ in potency, toxicity, and clearance. Thalidomide, warfarin, and omeprazole illustrate chiral or metabolic stereoselectivity.
- Prodrugs improve absorption or target delivery; they require metabolic or chemical conversion to the active species.
Worked example
Enalapril is an ethyl ester prodrug. The ester masks the polar dicarboxylate of the active enalaprilat, improving oral absorption. After absorption, hepatic esterases hydrolyze enalapril to enalaprilat, which inhibits angiotensin-converting enzyme. The reasoning is: the prodrug is not the pharmacologically active form; the ester is a bioavailability strategy; and the active metabolite carries the charge and receptor binding. A common error is to call enalaprilat a prodrug or to assume the ester itself inhibits ACE.
Traps that cost marks
Do not assume all esters are prodrugs; some are stable or designed for sustained release. Do not ignore pH when predicting absorption of weak acids in the stomach versus intestine. Do not treat enantiomers as automatically interchangeable. Do not confuse metabolic stability with chemical stability.
Key points
- Ionization depends on pKa and environmental pH.
- logP and logD describe different lipophilicity measures.
- Bioisosteres can preserve activity while altering PK.
- Prodrugs require conversion to active drug.
- Stereochemistry can change efficacy and safety.