## Lipophilicity and Pharmacokinetics of Beta Blockers **Key Point:** Propranolol is the most lipophilic beta blocker, which results in extensive hepatic metabolism (first-pass effect), high bioavailability variability, and significant CNS penetration. ### Lipophilicity Classification | Characteristic | Lipophilic (e.g., Propranolol) | Hydrophilic (e.g., Atenolol, Nadolol) | | --- | --- | --- | | **Absorption** | Variable (food-dependent) | Predictable, incomplete | | **Metabolism** | Hepatic (extensive) | Renal elimination (unchanged) | | **CNS Effects** | High (crosses BBB) | Minimal (poor BBB penetration) | | **Bioavailability** | Low & variable (5–30%) | Higher (40–50%) | | **Half-life** | Shorter (3–6 hrs) | Longer (6–24 hrs) | **High-Yield:** Lipophilic beta blockers (propranolol, metoprolol, labetalol) cause more CNS side effects (fatigue, depression, nightmares, sexual dysfunction) and have greater drug–drug interactions due to hepatic metabolism. Hydrophilic agents (atenolol, nadolol, sotalol) are safer in renal failure and cause fewer CNS effects. **Clinical Pearl:** Propranolol's high first-pass hepatic metabolism explains why oral doses are much higher than IV doses (oral 40–80 mg vs. IV 1–3 mg). This also makes propranolol prone to significant inter-individual variability. **Mnemonic:** **"Lipophilic = Liver; Hydrophilic = Kidney"** — propranolol (lipophilic) is metabolized by the liver; atenolol and nadolol (hydrophilic) are excreted unchanged by the kidneys.
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