The rate of renal drug elimination depends on whether the drug is ionized or nonionized in the urine:

• Ionized drugs are water-soluble → trapped in urine → excreted faster.
• Nonionized (neutral) drugs are lipid-soluble → can be reabsorbed in renal tubules → slower elimination.

Weak Acids

Examples: phenobarbital, methotrexate, aspirin (salicylates)

• Mechanism: Weak acids become ionized in alkaline urine → trapped → excreted faster.
• Overdose management: Alkalinize urine with sodium bicarbonate to accelerate elimination.

Chemical Reaction:

Weak Bases

Examples: tricyclic antidepressants (TCAs), amphetamines

• Mechanism: Weak bases are ionized in acidic urine → trapped → excreted faster.
• Urine acidification therapy: Ammonium chloride can be used in severe alkalosis to increase elimination.
• Note on TCA toxicity: Sodium bicarbonate is used not to increase elimination, but to stabilize cardiac sodium channels during toxicity.

Chemical Reaction:

pKa and Drug Ionization

• pKa: pH at which 50% of the drug is ionized and 50% is nonionized.
• Indicates the strength of a weak acid or base and helps predict renal elimination under different urine pH conditions.

Summary Table: Urine pH Manipulation for Overdose

Learning Objective (Step 1):

Explain how urine pH affects renal drug elimination, predict which drugs can be trapped by ionization, and describe clinical interventions to enhance elimination in overdose scenarios.