HSC Exam Practice

Sample response. The partition coefficient (log P) is the logarithm of the ratio of a drug’s equilibrium concentration in octanol (a lipid-like solvent) to its concentration in water. A log P of +4 indicates the drug is approximately 10,000 times more concentrated in the octanol phase than in water, meaning it is strongly lipophilic. In the body, such a drug would preferentially partition into lipid cell membranes and fatty tissues, giving good membrane permeability but potentially very poor aqueous solubility, which could limit oral bioavailability if dissolution in gut fluid is a bottleneck.

Marking notes. 1 mark for correct definition of log P (octanol–water ratio); 1 mark for correctly interpreting log P = +4 as strongly lipophilic/favours lipid phase; 1 mark for a relevant body consequence (e.g. good membrane permeability, partitions into lipid tissues, or potential aqueous solubility problem).

Sample response. Lipinski’s Rule of Five is a set of four heuristic criteria used to predict whether a drug candidate is likely to have good oral bioavailability: (1) molecular weight < 500 Da; (2) log P < 5 (not excessively lipophilic); (3) fewer than 5 hydrogen-bond donors; (4) fewer than 10 hydrogen-bond acceptors. A drug that violates more than one of these criteria is predicted to have poor oral absorption. The rule is a guide for medicinal chemists, not an absolute guarantee.

Marking notes. 1 mark for any two criteria correctly stated; 1 mark for the remaining two criteria correctly stated; 1 mark for stating what the rule predicts (poor oral absorption if criteria violated). Full marks require all four criteria and the prediction.

Sample response. A prodrug is a compound that is itself inactive or less active when administered, but is converted by metabolic processes inside the body into the pharmacologically active drug. Named HSC examples: aspirin (acetylsalicylic acid is hydrolysed in vivo to the active salicylic acid) or codeine (metabolised in the liver to the active morphine).

Marking notes. 1 mark for correct definition; 1 mark for a correctly named example with the conversion stated. Accept either aspirin or codeine.

Sample response. First-pass metabolism occurs when an orally absorbed drug travels from the gut, via the hepatic portal vein, to the liver before reaching systemic circulation. In the liver, enzymes metabolise (chemically modify or break down) a fraction of the drug. As a result, the concentration of unchanged active drug that eventually reaches the rest of the body is lower than the absorbed amount. For drugs that undergo extensive first-pass metabolism, the effective systemic concentration can be much less than the swallowed dose, reducing oral bioavailability significantly.

Marking notes. 1 mark for identifying the liver (or gut wall and liver) as the site; 1 mark for explaining that metabolic conversion reduces the amount of active drug reaching general circulation; 1 mark for linking this to reduced oral bioavailability (less active drug available systemically).

Sample response. An immediate-release formulation is designed to dissolve rapidly once swallowed, delivering the drug quickly into the gut for absorption — resulting in a rapid rise in plasma concentration. No special barrier material is present; the tablet or capsule shell dissolves readily in gastric or intestinal fluid. A controlled-release formulation contains a polymer matrix, coating or diffusion barrier that slows dissolution or diffusion of the drug into surrounding fluid. The drug is released gradually over hours rather than all at once, maintaining a more steady plasma concentration within the therapeutic window and reducing dosing frequency.

Marking notes. 1 mark for immediate-release: rapid dissolution, quick onset, no special chemical barrier; 1 mark for controlled-release: slow/sustained release due to polymer matrix, coating or diffusion barrier; 1 mark for the chemical mechanism specifically (polymer/coating/matrix slows dissolution or diffusion).

Sample response. Onset: IV delivers drug directly into the bloodstream, giving almost immediate onset; transdermal delivery is slow, often taking hours to reach steady state as drug diffuses through skin layers. First-pass: Both routes bypass first-pass metabolism because neither passes through the gut–liver portal system. Polarity suitability: IV is suitable for water-soluble (polar, low log P) drugs that can be formulated as aqueous solutions; transdermal requires the drug to be lipophilic enough (moderately high positive log P) to partition into the lipid-rich stratum corneum barrier of the skin.

Marking notes. 1 mark per criterion correctly compared for both routes (max 3): onset, first-pass avoidance, polarity suitability.

Drug E (log P = 7.2, aqueous solubility <0.001 mg/mL) is least likely to succeed by the oral route. Its aqueous solubility is extremely low, meaning it will not dissolve readily in gut fluid; dissolution is a prerequisite for oral absorption. Its log P of 7.2 also exceeds the Lipinski limit of 5, indicating excessive lipophilicity that further predicts poor oral bioavailability. Despite its low MW (395 < 500 Da), the combination of negligible water solubility and very high log P makes oral delivery very unlikely to succeed at a useful dose. Drug B would also be problematic (log P 3.9, solubility 0.01 mg/mL) but Drug E is the worst-case. Award to Drug E: 2 marks (1 identification, 1 using data); award 1 bonus mark if both poor solubility and log P>5 cited; max 4 for part a.

Sample response. Drug A has good aqueous solubility and low MW, so it would dissolve and be absorbed readily from the gut. However, if it undergoes significant first-pass metabolism in the liver, a large fraction of the absorbed drug would be metabolised before reaching systemic circulation. As a result, the effective systemic dose would be much lower than the swallowed amount, reducing oral bioavailability despite the good dissolution properties. 1 mark for correctly identifying first-pass metabolism as the problem; 1 mark for explaining the reduction in systemic drug concentration.

Sample response. Both Drug B and Drug C are similarly lipophilic (log P 3.9 and 4.3 respectively) with very low aqueous solubility. The “like dissolves like” principle states that lipophilic substances dissolve more readily in lipid environments. The skin’s stratum corneum is composed largely of ceramides and lipids. Drug C (log P 4.3) has sufficient lipophilicity to partition into this lipid-rich layer and diffuse through it, reaching dermal capillaries — making the transdermal route viable. Drug B (log P 3.9) is proposed as oral, but because it is also very lipophilic and essentially insoluble in water, it would struggle to dissolve in the gut’s aqueous fluid: this is a flaw in the proposed route for Drug B (it would be chemically more suitable for transdermal or an alternative non-aqueous oral formulation). Marks: 1 for referencing “like dissolves like”; 1 for identifying stratum corneum as lipid; 1 for explaining Drug C’s partitioning; 1 for identifying the inconsistency for Drug B.

Sample response. Formulation P (immediate-release) produces a sharp, high peak that rises above the MTC (toxic concentration) before falling below the MEC (minimum effective concentration) within approximately 4–5 hours — it is both transiently toxic and eventually sub-therapeutic. Formulation Q (controlled-release) produces a flatter, broader curve that stays largely within the therapeutic window for approximately 6–8 hours. 1 mark for describing P (rapid peak above MTC, falls below MEC); 1 mark for describing Q (flatter, sustained within therapeutic window).

Sample response. Formulation P dissolves rapidly because it has no special release barrier; the drug is released and absorbed quickly, producing a sudden high plasma concentration. Formulation Q contains a polymer matrix, coat or diffusion barrier that slows the rate at which the drug dissolves or diffuses into gut fluid. Drug release is spread over several hours. As a result, drug enters the bloodstream at a lower, steadier rate that maintains concentration within the therapeutic window, avoiding both toxic peaks and sub-therapeutic troughs. 1 mark for P: rapid dissolution, no barrier; 1 mark for Q: polymer/matrix/coating slows dissolution; 1 mark for consequence linking slower release to sustained therapeutic window.

Sample response. Polarity and the partition coefficient are central to every delivery-route decision in medicinal chemistry. The principle of “like dissolves like” explains the chemical foundation: polar drugs interact through hydrogen bonds and dipole forces with the aqueous environments of blood plasma and gut fluid, while non-polar drugs interact more favourably with lipid environments such as cell membranes or the stratum corneum of skin. Lipinski’s Rule of Five encodes this knowledge into four practical criteria — MW < 500, log P < 5, H-bond donors < 5, H-bond acceptors < 10 — that collectively identify whether a drug is likely to dissolve adequately in the gut and cross intestinal membranes. A drug that satisfies all criteria, such as aspirin (log P = 1.2, MW = 180), is a good oral candidate because it dissolves in gut fluid and can cross intestinal epithelial membranes. First-pass metabolism complicates oral delivery further: even a drug with favourable Rule-of-Five properties may lose a substantial fraction of its dose in the liver via the hepatic portal system before reaching general circulation, reducing effective oral bioavailability. For drugs with significant first-pass loss, a non-oral route bypasses this problem. The transdermal route, for instance, delivers drug through the lipid-rich stratum corneum directly into dermal capillaries, completely bypassing the gut and liver. This route requires a moderately lipophilic drug (log P typically 1–4) to partition into the skin barrier; a very polar drug (log P < 0) cannot penetrate the stratum corneum and is blocked, while an excessively lipophilic drug may be trapped in the skin depot rather than released into circulation. Intravenous delivery, by contrast, requires the drug to be formulated as an aqueous solution and avoids both the skin barrier and first-pass metabolism entirely, delivering drug directly at 100% bioavailability — but it requires the drug to be water-compatible. In evaluating the overall role of polarity: it determines whether a drug can dissolve in the medium it must cross (aqueous gut fluid or lipid skin), whether it can cross the relevant membrane, and whether it will survive long enough in first-pass environments to act. No single route is universally superior — the appropriate route depends on the drug’s specific polarity profile, log P, susceptibility to first-pass metabolism and the clinical context.

Marking notes. 1 mark — applies “like dissolves like” correctly to the aqueous and lipid environments relevant to drug delivery. 1 mark — states all four Lipinski criteria correctly. 1 mark — correctly links log P to a specific delivery route choice (e.g. low–moderate log P for oral; moderate–high for transdermal; water-compatible for IV). 1 mark — explains first-pass metabolism as a route-selection factor for oral delivery. 1 mark — analyses at least two named delivery routes (e.g. oral, IV, transdermal, inhalation) with reference to polarity requirements. 1 mark — uses at least one named drug example with relevant property data. 1 mark — reaches an explicit evaluative conclusion (no single route is universally best; polarity profile and clinical context drive the decision).