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Questions and Answers
What is the term used to describe the solvate formed when water is the solvent?
What is the term used to describe the solvate formed when water is the solvent?
What is the relationship between the solvation of a crystal and its solubility and dissolution rate in a solvent identical to the solvation molecules?
What is the relationship between the solvation of a crystal and its solubility and dissolution rate in a solvent identical to the solvation molecules?
What is the term used to describe the process by which a drug forms a complex with a component of gastrointestinal fluids?
What is the term used to describe the process by which a drug forms a complex with a component of gastrointestinal fluids?
What is the term used to describe the amount of drug absorbed into the bloodstream?
What is the term used to describe the amount of drug absorbed into the bloodstream?
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What is the term used to describe the hypothesis that the pH of the gastrointestinal fluids affects drug absorption?
What is the term used to describe the hypothesis that the pH of the gastrointestinal fluids affects drug absorption?
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What is the term used to describe the process by which a drug dissolves in the gastrointestinal fluids?
What is the term used to describe the process by which a drug dissolves in the gastrointestinal fluids?
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What is the factor that affects the concentration of a drug in solution in the gastrointestinal fluids?
What is the factor that affects the concentration of a drug in solution in the gastrointestinal fluids?
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What is the term used to describe the process by which a drug is absorbed into the bloodstream through the lipid bilayer of the gastrointestinal membranes?
What is the term used to describe the process by which a drug is absorbed into the bloodstream through the lipid bilayer of the gastrointestinal membranes?
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What is the term used to describe the process by which a drug forms a complex with an excipient within a dosage form?
What is the term used to describe the process by which a drug forms a complex with an excipient within a dosage form?
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What is the term used to describe the factors that affect the dissolution and solubility of a drug?
What is the term used to describe the factors that affect the dissolution and solubility of a drug?
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Study Notes
Lipid Solubility
- Thiopentone is more lipid soluble than barbitone, resulting in better absorption due to its greater affinity for the gastrointestinal membrane.
- Lipid solubility is a measure of a drug's lipophilicity, which determines its ability to be transported across membranes.
- The partition coefficient (P) is a measure of a drug's lipophilicity, usually expressed as a logarithm due to the large range of values.
- Octanol is commonly used as an organic solvent to mimic the biological membrane.
- Polar molecules with low lipid solubility (log P < 0) are poorly absorbed after oral administration and require injection.
Lipophilicity and Drug Absorption
- Lipophilic drugs with favorable partition coefficients (log P > 0) are usually absorbed after oral administration.
- Drugs with high lipophilicity (log P > 3) tend to be well absorbed but are also more susceptible to metabolism and biliary clearance.
- Within a homologous series, drug absorption usually increases as lipophilicity rises.
Approaches to Enhance Lipid Solubility
- Modify the structure of a compound to achieve lipid solubility while maintaining pharmacological activity.
- Create lipid prodrugs to improve absorption.
- Enalapril is a prodrug that is metabolized in vivo to the active form enalaprilat by various esterases.
Molecular Size and Hydrogen Bonding
- For paracellular absorption, the molecular weight should ideally be less than 200 Da.
- Shape is also an important factor for paracellular absorption.
- For transcellular passive diffusion, a molecular weight of less than 500 Da is preferable.
- Too many hydrogen bonds within a molecule can hinder absorption.
- A molecule should have no more than five hydrogen bond donors and no more than 10 hydrogen bond acceptors for good absorption.
Solvates and Bioavailability
- Solvates can exhibit differences in dissolution rates and bioavailability.
- The faster-dissolving anhydrous form of ampicillin was absorbed to a greater extent than the slower-dissolving trihydrate form.
Physicochemical Factors Influencing Bioavailability
- Factors affecting dissolution and solubility include:
- Physiological factors affecting dissolution rate
- Drug factors affecting dissolution rate
- Factors affecting the concentration of drug in solution in the GI fluids
- Factors affecting drug absorption include:
- pH-partition hypothesis of drug absorption
- Lipid solubility
- Molecular size and hydrogen bonding
Complexation
- Complexation of a drug may occur within the dosage form and/or in the gastrointestinal fluids, and can be beneficial or detrimental to absorption.
- Complexation may occur between the drug and:
- Components of gastrointestinal fluids
- Dietary components
- Excipients within the dosage forms
- Mucin, a normal component of gastrointestinal fluids, can complex with some drugs.
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Description
This quiz covers the importance of lipid solubility in drug absorption, specifically how it affects the transportation of drugs across membranes. Understand the difference in absorption between thiopentone and barbitone.
