Chemistry and Mechanism of Action

LA Chemistry - Molecular Blueprints

• Core Structure (All LAs):
  • Lipophilic Head: Aromatic ring (e.g., benzene); for lipid solubility, nerve penetration.
  • Intermediate Chain: Ester (-COO-) or Amide (-NHCO-) link; dictates class, metabolism.
  • Amine Tail (tertiary/secondary): Ionized ($BH^+$, active) & non-ionized (B, lipid-soluble) forms.
• Classification:
  • Esters: (e.g., Procaine, Tetracaine, Benzocaine)
    • Metabolism: Plasma pseudocholinesterases.
    • Allergy: ↑ risk (PABA metabolite).
    • 📌 Mnemonic: Esters have one 'i' (e.g., Procaine).
  • Amides: (e.g., Lidocaine, Bupivacaine, Ropivacaine)
    • Metabolism: Hepatic (CYP450 enzymes).
    • Allergy: ↓ risk.
    • 📌 Mnemonic: Amides have two 'i's (e.g., Lidocaine).
• Ionization: LAs are weak bases (pKa 7.5-9.0).
  • Uncharged base (B) crosses membranes; Cationic form ($BH^+$) is active.
  • Lower pKa → faster onset at physiological pH (7.4).

⭐ Amide LAs are metabolized in the liver, while ester LAs are metabolized by plasma pseudocholinesterases. This difference is crucial for understanding potential toxicity in patients with liver disease or pseudocholinesterase deficiency.

Mechanism of Action - Gate Crashers

LAs reversibly block nerve conduction by targeting voltage-gated sodium (Na+) channels.

• Target: Voltage-gated Na+ channels (inner surface).
  • Binding: α-subunit (S6 segment, domain IV).
• Action: Block Na+ influx → prevent depolarization & action potential propagation.
• State-Dependent Blockade (Use/Phasic):
  • Higher affinity for open/inactivated vs. resting channels.
  • ↑ Nerve firing frequency → ↑ block. Effective on active pain fibers.
• pH & LA Form: LAs = weak bases (pKa 7.5-9.0).
  • RN (non-ionized): penetrates nerve.
  • RNH+ (ionized): active form, binds channel intracellularly.
  • ↓ pH (inflammation): ↑ RNH+ extracellularly → ↓ penetration → ↓ efficacy. 📌 "Acidosis Antagonizes Anesthesia"
• Differential Blockade: (Sensitivity Order)
  • C (pain), Aδ (pain, temp) > Aγ > Aβ > Aα (motor).

⭐ LAs exhibit use-dependent blockade: greater affinity for Na+ channels in open/inactivated states, enhancing effect on frequently firing neurons (e.g., pain).

Action Modifiers - The Puppet Masters

• pKa (Dissociation Constant):
  • Governs onset of action. LAs are weak bases (pKa 7.5-9.0).
  • Non-ionized ($RN$) form penetrates nerve membrane; ionized ($RNH^+$) form is active intracellularly.
  • ↓pKa → ↑$RN$ at physiologic pH (7.4) → faster onset.
  • 📌 Lower pKa = Less time to onset.
• Lipid Solubility:
  • Determines LA potency.
  • ↑Lipid solubility → ↑potency (enhanced nerve membrane penetration).
• Protein Binding:
  • Primarily to $\alpha_1$-acid glycoprotein.
  • Determines duration of action.
  • ↑Protein binding → ↑duration (LA reservoir at site).
• pH:
  • Tissue pH:
    • Inflamed/infected tissue: acidic (↓pH) → ↑$RNH^+$ → ↓membrane penetration → delayed/failed block ("ion trapping").

    ⭐ LAs are less effective in infected tissues because the acidic environment (↓pH) increases ionization, trapping the LA extracellularly and reducing its ability to penetrate the nerve membrane.

  • LA Solution pH:
    • Commercial preps acidic (pH 4-6) for stability.
    • Alkalinization (adding $NaHCO_3$) → ↑$RN$ → faster onset.
• Vasoconstrictors (e.g., Adrenaline):
  • Commonly Adrenaline 1:200,000 (5 $\mu g/mL$).
  • Effects:
    • ↓Local blood flow → ↓systemic absorption.
    • ↑Duration of action & ↑block intensity.
    • ↓Systemic toxicity.
  • ⚠️ Caution: Avoid in end-arterial regions (e.g., digits, penis, nose).

High‑Yield Points - ⚡ Biggest Takeaways

• LAs are weak bases: lipophilic aromatic ring, intermediate ester/amide link, hydrophilic amine.
• Amides (e.g., Lidocaine) are liver metabolized; Esters (e.g., Procaine) by plasma pseudocholinesterase (more allergic).
• Mechanism: Block voltage-gated Na+ channels from inside, halting nerve impulse.
• Uncharged base form crosses membrane; charged cation is active intracellularly.
• Lower pKa = faster onset; ↑Lipid solubility = ↑potency; ↑Protein binding = ↑duration.
• Acidic tissue ↓ efficacy (↑external ionization); small pain fibers blocked first.