Chemistry • Year 12 • Module 8 • Lesson 14
Solubility, Polarity & Drug Delivery
Build Band 5–6 extended-response technique: synthesise polarity principles, log P data and delivery-system knowledge to evaluate real drug-delivery scenarios and interrogate media claims.
1. Data-driven evaluation — choosing a delivery strategy (Band 5–6)
8 marks Band 5–6
Scenario. Drug X is being developed by a CSIRO-partnered team in Australia for treatment of a chronic inflammatory condition. The data below summarises its key physicochemical properties alongside two candidate delivery routes under consideration.
| Property | Drug X | Lipinski guideline |
|---|---|---|
| Molecular weight | 448 Da | < 500 Da |
| log P | 4.8 | < 5 |
| H-bond donors | 4 | < 5 |
| H-bond acceptors | 9 | < 10 |
| Aqueous solubility | Very low (0.003 mg/mL) | — |
| First-pass metabolism | Significant (~60% extracted by liver on first pass) | — |
Hypothetical drug data for educational purposes. Lipinski guidelines adapted from Lipinski et al. (2001).
Candidate routes under consideration: (A) Standard oral tablet (B) Transdermal patch
Q1. Evaluate which delivery route — oral tablet (A) or transdermal patch (B) — is more suitable for Drug X. In your response you must:
- Apply Lipinski’s Rule of Five to the data and identify whether Drug X nominally “passes” the rules.
- Explain how Drug X’s very low aqueous solubility creates a problem specifically for the oral route.
- Explain how the significant first-pass metabolism (60%) affects oral bioavailability.
- Assess whether Drug X’s log P supports or limits its suitability for transdermal delivery.
- Reach an evidence-based judgement identifying the more suitable route and at least one remaining limitation of that choice.
2. Source critique — evaluate a media claim (Band 5–6)
7 marks Band 5–6
“The CSIRO has been researching nanoparticle drug delivery systems, and the science is now clear: any drug can be made water-soluble enough to be swallowed as a pill simply by encapsulating it in a liposome or nanoparticle. The old ideas about polarity and partition coefficients being barriers to oral delivery are basically obsolete now that nanoparticles exist.”
— Composite statement representing claims sometimes found in popular science media, 2024.
Q2. Critically evaluate the claim above. In your response:
- Identify the scientific flaw (or flaws) in the claim.
- Explain what a liposome or nanoparticle can do, and what it cannot fully overcome, with reference to specific chemical or physiological barriers.
- Explain why polarity and partition coefficient (log P) remain relevant considerations even when nanoparticle encapsulation is used.
- Describe how you would test, experimentally, whether a nanoparticle formulation genuinely overcomes a specific polarity-related barrier to oral delivery.
Q1 — Marking criteria (8 marks)
Mark 1: Correctly applies Rule of Five: Drug X meets all four numerical criteria (MW 448 < 500; log P 4.8 < 5; donors 4 < 5; acceptors 9 < 10) so nominally “passes” the rules.
Mark 2: Identifies that very low aqueous solubility (0.003 mg/mL) is a major problem for oral delivery because dissolution in gut fluid must precede absorption — if the drug cannot dissolve, even good membrane permeability (high log P) cannot compensate.
Mark 3: Correctly quantifies or describes the first-pass problem: 60% liver extraction means only approximately 40% of the absorbed dose survives to reach systemic circulation, substantially reducing effective oral dose.
Mark 4: Assesses log P 4.8 as supportive of transdermal delivery because it is within or near the optimal range for skin penetration (stratum corneum is lipid-rich; moderately high log P enables partitioning into and across this layer).
Mark 5: Reaches a justified judgement: transdermal patch is the more suitable route because it bypasses both the aqueous dissolution step in the gut and the first-pass liver extraction, while the high log P supports skin penetration.
Mark 6: Identifies at least one remaining limitation of the transdermal route for Drug X: e.g. very high log P can cause drug to be “trapped” in the stratum corneum (depot effect) or the drug’s lipophilicity may slow diffusion out of the skin reservoir; skin irritation; variable absorption depending on skin thickness/condition; limited dose rate achievable via skin.
Marks 7–8: Quality of synthesis and argument: logical progression from data to recommendation; all criteria considered rather than only one; evidence used, not just asserted.
Q2 — Marking criteria (7 marks)
Mark 1: Identifies the primary flaw: “any drug can be made water-soluble enough” is an overstatement — liposomes and nanoparticles do not chemically change the drug’s own polarity or molecular properties; they encase it in a vehicle but the drug retains its inherent physicochemical character.
Mark 2: Correctly explains what liposomes/nanoparticles can do: improve apparent solubility/dispersion, protect the drug from stomach acid and digestive enzymes, or alter biodistribution and targeting.
Mark 3: Identifies what nanoparticle delivery cannot fully overcome: first-pass metabolism in the liver (the encapsulated drug is still metabolised once the nanoparticle releases it or is taken up by liver cells); intestinal epithelial cell uptake of large nanoparticles may be size-limited; the drug must still escape the endosome/lysosome after cellular uptake.
Mark 4: Explains why log P / polarity remain relevant even with nanoparticle use: the log P of the drug determines how it releases from the nanoparticle in different tissue environments, how well it crosses the endosomal membrane after delivery, and whether it distributes to the target tissue correctly once released.
Mark 5: Proposes a valid experimental test, e.g. measure oral bioavailability (plasma concentration-time area under curve) of the nanoparticle-encapsulated drug versus free drug at equivalent doses, with and without hepatic blood flow inhibitor; or measure drug concentration in target tissue at defined time points in an animal model with appropriate controls.
Marks 6–7: Quality of critique: identifies both strengths (liposomes do have real value) and genuine limitations (not a magic solution); demonstrates understanding that polarity is a fundamental property of the molecule, not just the formulation.