Wattle Pharmacy Prep

Solubility and partitioning
Practice questions with explanations

Solubility and partitioning belongs to Medicinal chemistry and biopharmaceutics, which makes up about 10% of the Overseas Pharmacist Readiness Assessment (OPRA®). Answer each question yourself before opening the explanation.

1. What is the role of a cosolvent such as propylene glycol in an injection?

  1. It adjusts tonicity to physiological levels
  2. It acts as a preservative
  3. It increases the viscosity for slower injection
  4. It reduces the polarity of the vehicle so a poorly water-soluble drug stays in solution
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D. It reduces the polarity of the vehicle so a poorly water-soluble drug stays in solution

Cosolvents lower the dielectric constant of the vehicle, allowing lipophilic drugs to dissolve at useful concentrations. The risk is precipitation on dilution into an aqueous infusion or on injection into blood, which is why some such products must be given slowly or through a filter. Propylene glycol itself causes toxicity in large cumulative doses, particularly with intravenous lorazepam or diazepam infusions.

2. How does food generally affect the absorption of a poorly soluble lipophilic drug?

  1. Food converts the drug to an inactive form
  2. A fatty meal often increases absorption by stimulating bile secretion and providing lipid to dissolve the drug
  3. Food has no effect on lipophilic drugs
  4. Food always reduces absorption of every drug
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B. A fatty meal often increases absorption by stimulating bile secretion and providing lipid to dissolve the drug

Bile salts and dietary lipid solubilise poorly soluble drugs and delay gastric emptying, giving more time for dissolution, which is why some antifungals and antiretrovirals are taken with food and why the instruction matters clinically. The opposite applies to drugs chelated by food components or degraded by acid, where an empty stomach is specified.

3. Which factor determines whether a drug crosses the blood-brain barrier passively?

  1. Small size, lipophilicity, low protein binding and not being a substrate for efflux transporters
  2. A high degree of ionisation at plasma pH
  3. High water solubility and a large molecular weight
  4. Extensive plasma protein binding
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A. Small size, lipophilicity, low protein binding and not being a substrate for efflux transporters

Tight junctions between cerebral endothelial cells force molecules through the cells rather than between them, so lipophilic small molecules pass while polar ones do not, unless they use a carrier as levodopa does. P-glycoprotein actively expels many substrates, which is why loperamide, an opioid, produces no central effect at normal doses. Inflammation, as in meningitis, increases permeability.

These are 3 of the 20 questions on solubility and partitioning in the app.

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More topics in Medicinal chemistry and biopharmaceutics

Dosage form design · Drug stability and degradation · Functional groups and drug classes · Pharmaceutical excipients · Sterile products · Structure-activity relationships