Ion trapping of a basic drug moving from blood to the stomach occurs because the stomach has a lower pH than blood, causing the drug to become ionized and trapped in the stomach. What is the consequence for distribution?

Ion trapping of a basic drug moving from blood to the stomach occurs because the stomach has a lower pH than blood, causing the drug to become ionized and trapped in the stomach. What is the consequence for distribution?

• A. Ion trapping increases distribution to brain
• B. Ion trapping reduces plasma levels, decreasing distribution
• C. Ion trapping enhances renal excretion only
• D. Ion trapping has no effect on distribution

Answer: (B)

Ion trapping happens when a drug’s ionization state changes with the pH of the compartment it enters, and the ionized form cannot cross membranes easily. A basic drug moving from blood (neutral form favored at near-physiologic pH) into the stomach (very acidic) becomes protonated and ionized, so it cannot easily diffuse back into the blood. This sequesters the drug in the stomach, reducing how much remains in the circulating central compartment and is available to distribute to other tissues. With less drug in plasma to reach target sites, overall distribution is diminished. The effect is due to a shift in where the drug resides rather than enhanced excretion, and it doesn’t promote increased distribution to the brain or other organs.

Ion trapping happens when a drug’s ionization state changes with the pH of the compartment it enters, and the ionized form cannot cross membranes easily. A basic drug moving from blood (neutral form favored at near-physiologic pH) into the stomach (very acidic) becomes protonated and ionized, so it cannot easily diffuse back into the blood. This sequesters the drug in the stomach, reducing how much remains in the circulating central compartment and is available to distribute to other tissues. With less drug in plasma to reach target sites, overall distribution is diminished. The effect is due to a shift in where the drug resides rather than enhanced excretion, and it doesn’t promote increased distribution to the brain or other organs.