Pharmacology Drug Absorption Quiz

Drug Absorption

• Absorption is the process by which drugs enter the systemic circulation from the administration site.
• Drugs must cross biological membranes to be absorbed.
• This process applies to all routes of administration except topical and intravenous.
• Orally administered drugs are absorbed from the small intestine, travel through the portal vein, liver, and then enter the systemic circulation.

Processes of Absorption

• Passive diffusion: Drugs move down their concentration gradient across the membrane. The membrane itself doesn't actively participate. This is the most common method.
• Lipid diffusion: Lipophilic drugs dissolve in the lipid components of cell membranes.
• Aqueous diffusion/filtration: Drugs pass through aqueous pores in the cell membranes. Drugs with small molecular weights can cross these pores more easily.

Factors Affecting Drug Absorption

• Particle size: Smaller particles absorb faster.
• Drug concentration: Higher concentrations absorb faster.
• Lipid solubility: Lipid-soluble drugs absorb easily through passive diffusion.
• Water solubility: Water-soluble drugs absorb poorly.
• Drug solubility: Aqueous solutions dissolve more quickly than oily solutions.
• Dosage form: Tablets and capsules need to disintegrate and dissolve before absorption can occur—small particle size post-dissolution improves absorption.
• Area of absorbing surface: Larger surface area (e.g., small intestine) facilitates absorption.
• Vascularity: Increased blood flow increases absorption.
• Route of administration: Oral absorption is slow and erratic, while intravenous is immediate and complete.
• Presence of food: Food can delay absorption.
• Ionization: Non-ionized drugs are absorbed better than ionized drugs. pH plays a key role in ionization. Acidic drugs primarily absorb in acidic environments, and basic drugs in basic environments.
• First-pass hepatic metabolism: The metabolism of a drug in the liver before reaching systemic circulation. This decreases the bioavailability of a drug.

Bioavailability

• Bioavailability is the fraction of a drug that reaches the systemic circulation in unchanged form.
• Oral bioavailability is often less than 100% due to incomplete absorption and first-pass metabolism.
• Intravenous and sublingual routes have 100% bioavailability.
• Factors that increase absorption increase bioavailability and vice versa.
• The liver is the primary site of drug metabolism.
• Some drugs are metabolized in the gut wall, skin, or lungs.

Factors Affecting Bioavailability

• First-pass hepatic metabolism: when a drug enters the portal circulation before the systemic circulation, the liver metabolizes it, thus reducing the amount of unchanged drug reaching the systemic circulation.
• Solubility: Hydrophobic drugs absorb more readily, leading to increased bioavailability.
• Chemical instability: some drugs are unstable in the stomach, decreasing their bioavailability. Insulin is an example.
• Particle size: Smaller particle sizes lead to better absorption.

Plasma Half-Life (t_1/2)

• t_1/2 is the time it takes for the drug concentration in the plasma to decrease by half.
• It impacts the dosing frequency needed for therapeutic effectiveness.
• A drug with a short half-life requires more frequent dosing than a drug with a long half-life.

Significance of Plasma t_1/2

• Indicates how quickly the drug is removed from the plasma via biotransformation or excretion.
• Determines the frequency of dosing for therapeutic efficacy
• Determines the duration of drug action

Points to Remember

• Absorption occurs across cell membranes through passive diffusion, lipid diffusion, and aqueous diffusion.
• Factors like lipid solubility, ionization, pH, and surface area all affect absorption.
• Bioavailability relates to the portion of a drug reaching the circulation in its unchanged form.
• Plasma half-life affects drug dosing frequency.

Revision:

• Alkalinization hastens the excretion of weakly basic drugs
• Drug diffusion across membranes depends on the cell's metabolic activity, but is also affected by ionization and concentration of chemically related drugs
• Lipid-insoluble drugs cannot be absorbed through passive diffusion.