Mechanism of Diffusion for Weak Electrolytes
Most pharmacological agents are chemical weak electrolytes—either weak acids (e.g., Acidum acetylsalicylicum) or weak bases (e.g., Atropinum).
To enter the systemic circulation or target tissues, a molecule must cross cellular membranes. The primary requirement for successful diffusion across the lipid bilayer is lipophilicity. Only the non-ionized form of a substance satisfies this requirement. Ionized molecules become hydrophilic and are repelled by the lipids of the cell membranes.
Effect of Environment and pKa on Ionization
The degree of ionization of a drug is not constant. It depends on two parameters:
- Environmental pH of the compartment where the drug is located (stomach, intestine, blood, urine).
- pKa (the negative logarithm of the dissociation constant $K_a$) — an intrinsic physicochemical property of the substance. The physical meaning of pKa is that when the environmental pH equals the drug's pKa, exactly half (50%) of the molecules are in the ionized state, and the other half are in the non-ionized state.
General equilibrium rules apply:
- Weak acids readily ionize in an alkaline environment (and are poorly absorbed there). The lower the pKa of an acid, the easier it releases a proton and ionizes, even at relatively low pH values.
- Weak bases ionize more strongly in an acidic environment (by accepting a proton), which also hinders their absorption.
Clinical Significance: Gastrointestinal Absorption
Knowing a drug's pKa allows clinicians to predict its absorption in different parts of the gastrointestinal tract.
- Stomach (acidic environment, pH 1–2). Here, weak acids (e.g., aspirin, pKa = 3.5) are predominantly in their non-ionized form, so their absorption begins at this stage. Weak bases are heavily ionized in the stomach and are practically unabsorbed (unless they possess an extremely low pKa, such as diazepam with pKa = 3.0).
- Intestine (alkaline environment, pH ranges from 5–6 to 9–11). This is the ideal environment for the absorption of weak bases because they convert into the lipophilic, non-ionized form. Interestingly, despite the unfavorable pH shift, the small intestine remains the primary site of absorption for weak acids as well (with pKa > 3.0). This is due to the massive surface area of the small intestine, which compensates for the suboptimal pH.
Managing Renal Drug Elimination
The principles of ionization are applied in toxicology to accelerate the elimination of toxins or in drug overdoses. The clinical goal is to prevent reabsorption (back-diffusion) of the substance from the renal filtrate into the blood.
- In weak acid poisoning: The urine is artificially alkalinized (e.g., by administering Natrii hydrocarbonas). In an alkaline environment, weak acids convert into their ionized form, lose the ability to cross the lipid membranes of the renal tubules, and are excreted in the urine.
- In weak base poisoning: The urine is acidified (e.g., using Ammonii chloridum). In an acidic environment, bases become heavily ionized and are "trapped" within the tubular lumen, after which they are excreted.