Solubility, Polarity & Drug Delivery
In 1998, AstraZeneca reformulated omeprazole (Prilosec) as a magnesium salt prodrug (esomeprazole/Nexium) — increasing water solubility from 0.36 mg/mL to 5 mg/mL and lifting oral bioavailability from 35% to 64%. The reformulation generated $6 billion in annual revenue and demonstrated that solubility chemistry, not just biological activity, drives commercial pharmaceutical development.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
A patient needs a medicine that must act reliably, but the drug is partly broken down during its first trip through the liver after being swallowed.
- Why might an oral tablet be a poor choice for this medicine?
- What alternative delivery routes or formulations might solve that problem?
Know
- How polarity and hydrogen bonding affect drug solubility
- The meaning of first-pass metabolism, prodrug and controlled-release formulation
- The main drug delivery systems named in the course
Understand
- Why aqueous and lipid environments favour different molecular features
- How Lipinski's Rule of Five guides oral bioavailability thinking
- Why delivery route is tied to chemistry, not just convenience
Can Do
- Apply like-dissolves-like to predict drug solubility
- Match a delivery system to a drug-solubility or metabolism problem
- Explain why first-pass metabolism reduces oral bioavailability
Core Content
A drug must fit both water and membrane chemistry
Drug solubility is a chemical balancing act. Blood plasma is aqueous, but cell membranes are largely lipid-like, so the same molecule may behave very differently in different parts of the body.
Polar functional groups and the ability to form hydrogen bonds usually improve solubility in water. Less polar hydrocarbon-rich regions generally favour interaction with lipid environments. This is why medicinal chemistry often involves compromise rather than an "ideal" single property.
Like dissolves like — polar drugs dissolve more easily in aqueous environments such as blood plasma; non-polar drugs dissolve more easily in lipid environments such as cell membranes. Drug design often balances these competing solubility requirements.
Pause — copy the highlighted rule into your book.
A guide for oral bioavailability, not a guarantee
We just saw that polarity governs how a drug dissolves in aqueous vs lipid environments. That raises a question: how do medicinal chemists quickly judge whether a new molecule might work orally? This card answers it → Lipinski's Rule of Five provides practical heuristic limits on four molecular properties.
Medicinal chemists use heuristic rules to estimate whether a drug is likely to be suitable for oral delivery. One of the best-known is Lipinski's Rule of Five.
Lipinski's Rule of Five: MW < 500 Da, logP < 5, H-bond donors < 5, H-bond acceptors < 10. These are guides for likely oral absorption — not guaranteed rules. Other factors (metabolism, formulation) also matter.
Pause — copy the highlighted Rule of Five criteria into your book.
Inactive first, active later
We just saw that the Rule of Five guides whether a molecule might be orally absorbed. That raises a question: even if a drug is absorbed, what can still reduce how much active drug reaches the body? This card answers it → first-pass metabolism in the liver can convert or destroy a significant fraction before it reaches general circulation.
Sometimes a drug is deliberately designed not to be the final active form when swallowed. Instead, the body converts it into the active compound after administration.
A prodrug is an inactive or less active compound that is metabolised in vivo to the active drug. Examples named in the course include codeine → morphine and aspirin → salicylic acid.
First-pass metabolism refers to the metabolism of a drug in the gut wall and especially the liver after absorption from the digestive tract but before the drug reaches general circulation in full concentration. This can lower oral bioavailability because less active drug reaches the rest of the body unchanged.
First-pass metabolism is the breakdown of a drug in the gut wall and liver before it reaches systemic circulation, reducing oral bioavailability. A prodrug is an inactive precursor metabolised in vivo to the active drug (e.g., codeine → morphine).
Pause — copy the highlighted definitions into your book.
An orally absorbed drug reaches the liver before the general circulation. If substantial metabolism happens there, less unchanged active drug reaches the rest of the body, lowering oral bioavailability.
Matching route to chemistry and clinical need
We just saw that first-pass metabolism can limit oral bioavailability. That raises a question: what alternatives exist, and how is the choice of route tied to the drug's chemistry? This card answers it → different delivery routes each have specific solubility, stability and bioavailability requirements linked to molecular properties.
Drug delivery systems are not interchangeable packaging choices. Each route is chosen because it suits particular solubility, stability or bioavailability constraints.
Oral: needs dissolution and survival through digestion + first-pass metabolism. Transdermal: drug must cross lipid-rich skin barriers; bypasses first-pass. Intravenous: immediate systemic delivery, bypasses first-pass entirely. Liposome: specialised encapsulation for targeted transport.
Pause — copy the highlighted delivery routes summary into your book.
For example, a transdermal patch works best for drugs that can cross skin barriers effectively, while an intravenous formulation requires strong compatibility with aqueous delivery.
Chemistry that slows and spreads out delivery
We just saw that different delivery routes suit different chemistry and clinical needs. That raises a question: can an oral dose itself be designed to release over time rather than all at once? This card answers it → controlled-release formulations use coatings, diffusion barriers or matrix materials to slow drug release.
A patient takes a standard morphine tablet every four hours for post-surgical pain — four doses, four peak-and-trough cycles in the bloodstream, four periods of side-effect risk. A controlled-release morphine tablet (e.g., MS Contin) releases the drug over 12 hours, flattening the concentration curve to a stable plateau — two doses, dramatically fewer peaks, the same total exposure. That outcome is not pharmacology; it is polymer coating chemistry controlling diffusion rates. Some medicines are designed not to release all at once.
Their chemical basis often involves coatings, diffusion barriers, or matrix materials that slow how quickly the drug dissolves or escapes into body fluids. The result can be more stable drug concentration over time and fewer doses needed.
Controlled-release formulations slow drug delivery using coatings, diffusion barriers or matrix materials — producing more stable drug concentration over time and requiring fewer doses. Contrast with immediate-release formulations which prioritise fast availability.
Pause — copy the highlighted controlled-release point into your book.
The principle 'like dissolves like' means that polar drugs tend to dissolve in ____ solvents such as water, while non-polar drugs dissolve in ____ solvents or lipid membranes.
Lipinski's Rule of Five predicts good oral bioavailability when molecular weight is below ____ Da and the drug has fewer than ____ hydrogen-bond donors.
A prodrug is an inactive form converted to the ____ form by metabolic processes in the body.
Complete the Learn phase to unlock Practice.
Activities
Use polarity and hydrogen bonding to predict where the drug is more comfortable: aqueous plasma or lipid membranes.
1. A drug has several -OH groups and multiple hydrogen-bonding sites.
2. A drug has a large hydrocarbon region and very few polar groups.
3. Explain how "like dissolves like" helps predict whether a drug formulation will need help dissolving in blood plasma.
Choose the most suitable delivery or formulation strategy and justify it.
1. A drug is effective, but only a small fraction reaches circulation after being swallowed because of liver metabolism.
2. A medicine needs to be delivered steadily for many hours rather than in one sharp burst.
3. A compound is designed to be converted in the body into the active drug.
Check Your Understanding
1. Which statement best applies the principle "like dissolves like"?
2. Which set of conditions matches Lipinski's Rule of Five most closely?
3. What is first-pass metabolism?
4. Why might a transdermal patch be chosen instead of an oral tablet?
5. Which statement best describes a prodrug?
1. Explain how polarity and hydrogen bonding influence the solubility of a drug in aqueous plasma compared with lipid membranes. (4 marks)
2. Explain first-pass metabolism and analyse how it affects oral drug bioavailability. (5 marks)
3. Evaluate the most suitable delivery strategy for a heart medicine that is strongly affected by first-pass metabolism and needs a steady therapeutic concentration. In your answer, compare an oral tablet with at least one alternative route or formulation. (5 marks)
Show All Answers
Activity 1
1. This suggests stronger aqueous solubility because multiple -OH groups and hydrogen-bonding sites increase polarity and interaction with water.
2. This suggests stronger lipid affinity and weaker aqueous solubility because the large hydrocarbon region is relatively non-polar.
3. Like dissolves like means polar drugs dissolve more easily in polar plasma, while less polar drugs may need formulation support if aqueous dissolution is poor.
Activity 2
1. A non-oral route such as transdermal or intravenous may be better because it can reduce or avoid first-pass metabolism.
2. A controlled-release formulation is suitable because it releases drug more gradually and helps maintain a steadier concentration.
3. This is a prodrug. Codeine converting to morphine is a named example, and the design can help optimise delivery or activation.
Multiple Choice
1. B — polar drugs generally dissolve more easily in aqueous environments.
2. D — these are the Rule of Five guide values given in the course.
3. A — first-pass metabolism occurs before a swallowed drug fully reaches general circulation.
4. C — a patch can bypass first-pass metabolism and provide steadier delivery.
5. B — a prodrug is metabolised in vivo to the active drug.
Short Answer Model Answers
Q1 (4 marks): Polar groups and hydrogen-bonding ability increase a drug's interaction with water, so they usually improve solubility in aqueous plasma. Less polar or more hydrocarbon-rich regions are more compatible with lipid environments such as membranes. This means a drug that is very polar may dissolve well in plasma but cross membranes less easily, while a less polar drug may interact better with membranes but dissolve less well in water.
Q2 (5 marks): First-pass metabolism is the metabolism of a drug in the gut wall and especially the liver after it is absorbed from the digestive tract but before it reaches full systemic circulation. This reduces oral bioavailability because some of the drug is converted to other forms before the rest of the body receives it. As a result, a swallowed dose may deliver less active drug than expected. The stronger the first-pass effect, the poorer the efficiency of oral delivery may become.
Q3 (5 marks): An ordinary oral tablet may be a poor choice because first-pass metabolism can reduce the amount of active drug reaching general circulation. If the medicine also needs a steady therapeutic concentration, a transdermal patch or controlled-release non-oral formulation may be more suitable. A transdermal route can reduce first-pass metabolism and provide slower, sustained delivery over time. An IV route can bypass first-pass metabolism completely, but it may be less practical for routine long-term dosing. Overall, for a medicine needing both reduced first-pass loss and sustained delivery, a transdermal or controlled-release alternative is often more suitable than a standard oral tablet.
Return to the 1981 glyceryl trinitrate (GTN) transdermal patch story. Now that you understand polarity, first-pass metabolism, and delivery routes, explain why the patch solved the bioavailability problem that the oral tablet could not.
- What polarity property of GTN makes it suitable for transdermal delivery — and why would a highly polar, ionised drug molecule fail to cross skin using the same approach?
- How does the transdermal route bypass the liver's first-pass metabolism — and why does this allow a much lower total dose to achieve the same therapeutic blood concentration as a swallowed tablet?
- How does a controlled-release coating (like that used in MS Contin) solve a different problem from a transdermal patch — even though both produce a steadier drug concentration over time?
Review
What does "like dissolves like" predict about a polar drug in blood plasma?
What are the four Lipinski Rule of Five criteria?
Define first-pass metabolism and explain its effect on oral bioavailability.
Name a prodrug example from the course and identify the active form produced.
Why is a transdermal patch a better choice than an oral tablet for a drug strongly affected by first-pass metabolism?