Drug Distribution, Barriers & Protein Binding

Definition & Fundamentals of Drug Distribution

  • Distribution = reversible transfer of a drug between compartments, driven by blood circulation.
  • Practical meaning: reversible transfer between blood (plasma) and extravascular fluids/tissues until equilibrium is reached.
  • Passive process governed by concentration gradient.

Global Scheme of Disposition

  • After administration:
    • Drug distributes in the body ➜ part remains in plasma (as parent drug + metabolites) ➜ undergoes metabolism or excretion.

Apparent Volume of Distribution (Vd)

  • Hypothetical volume into which drug appears to be distributed.
  • Mathematical relation: Vd=Amount of drug in body (X)Plasma concentration (C)V_d = \frac{\text{Amount of drug in body (X)}}{\text{Plasma concentration (C)}}
  • Units: L or L/kg; indicator of extent of extravascular distribution.

Major Determinants of Drug Distribution

  • Perfusion rate to organ/tissue size.
  • Tissue permeability (drug factors + physiological barriers).
  • Binding to blood or tissue components.
  • Miscellaneous: age, obesity, pregnancy, diet, disease, drug-drug interactions.

I. Physicochemical Properties of Drugs

• Lipid Solubility

  • Cell membrane = phospholipid bilayer ⇒ lipophilic, non-polar, unionized drugs diffuse faster via passive diffusion.
  • Degree of ionization at biological pH (≈7.4) critical; unionized forms penetrate membranes rapidly.
  • Example: non-polar, unionized drug crosses BBB with ease.

• Molecular Size

  • < 500–600 Da500\text{–}600\,\text{Da}: cross capillary membranes easily → ECF.
  • < 50 Da50\,\text{Da} & water-soluble: may enter cells via aqueous channels.
  • Larger molecules need specialized transport systems or remain intravascular.

II. Drug Transporters

  • Two functional classes:
    • Influx transporters: accelerate entry into target cells.
    • Efflux transporters: slow entry / pump drug out.
  • Energy-dependent; operate against concentration gradient.
  • Examples:
    • Sodium–iodide symporter in thyroid for iodide uptake.
    • Transferrin transports iron in blood.

III. Physiological Barriers to Distribution

1. General Capillary Endothelium / Cell Membranes

  • Permit passage primarily by simple diffusion; lipophilicity & size rule.

2. Blood–Brain Barrier (BBB)

  • Specialized tight junctions between brain capillary endothelial cells + astrocytic end-feet & pericytes.
  • Much less permeable to water-soluble drugs; highly lipophilic drugs with high o/w partition coefficient cross readily; moderately lipophilic or partially ionized drugs cross slowly.
  • BBB acts as lipoidal barrier → protein-bound drug does not cross.
  • Illustrative drugs: Levodopa (uses active transport), Carbidopa (does not cross).

3. Blood–Cerebrospinal-Fluid (CSF) Barrier

  • Formed by tight junctions between choroid plexus epithelial (choroidal) cells in ventricular system.
  • Permeability mirrors BBB; highly lipid-soluble drugs pass easily; others slow.

4. Placental Barrier

  • Layers: syncytiotrophoblast, cytotrophoblast, connective tissue stroma, fetal capillary endothelium + basement membranes.
  • Not as restrictive as BBB.
  • Drugs < 1000 Da1000\,\text{Da} & moderate–high lipophilicity (ethanol, sulfonamides, barbiturates, gaseous anesthetics, steroids, narcotics, anticonvulsants, many antibiotics) cross rapidly by diffusion.
  • Nutrients via carrier-mediated transport; immunoglobulins via endocytosis.

5. Blood–Testis Barrier (mentioned list)

  • Tight junctions between Sertoli cells limit passage; important in drug design & reproductive toxicology.

Protein Binding of Drugs

  • Definition: reversible complex formation of drug with proteins (plasma or tissue).
  • Consequence: bound fraction pharmacokinetically & pharmacodynamically inert—neither metabolized, excreted, nor active.
  • Chemical nature: weak bonds (hydrogen, hydrophobic, ionic, van der Waals) ⇒ reversible.
  • Relative affinity sequence in plasma: Albumin > α\alpha1-acid glycoprotein (AAG) > Lipoproteins > Globulins.
  • Acidic drugs ↔ Albumin; Basic drugs ↔ AAG & lipoproteins.

Factors Affecting Protein–Drug Binding

A. Drug-Related

  • Physicochemical nature (↑lipophilicity ⇒ ↑binding; e.g., thiopental localizes in adipose).
  • Drug concentration:
    • Low concentrations → binding sites not saturated → high % bound.
    • High concentrations → saturation → rapid rise in free fraction.
  • Specific affinity for particular protein/tissue (e.g., lidocaine binds AAG > HSA; digoxin prefers cardiac muscle proteins).

B. Protein/Tissue-Related

  • Physicochemical nature of protein (albumin acidic & basic pockets; pH determines availability of anionic/cationic groups).
  • Protein concentration: varies with disease; albumin ↓ in renal failure, liver disease, hypothyroidism; AAG limited capacity.
  • Number of binding sites: Albumin = high-capacity, multi-site; AAG = low-capacity.

C. Drug Interactions

  1. Competition between drugs (displacement)
    • Example: Phenylbutazone displaces warfarin from HSA → ↑free warfarin → hemorrhage risk.
  2. Competition with endogenous substances
    • Free fatty acids (↑ in fasting, diabetes, MI, post-heparin, caffeine) displace benzodiazepines, propranolol (↓ binding) & warfarin (↑ binding).
    • Bilirubin displacement in neonates by sodium salicylate, benzoate, sulfonamides ⇒ free bilirubin crosses BBB → kernicterus.
  3. Allosteric changes in protein structure altering affinity.

D. Patient-Related

  • Age:
    • Neonates: low albumin → ↑free drug; immature bilirubin conjugation.
    • Young infants: Digoxin dose (4–6× adult per kg) due to higher binding & renal clearance.
    • Elderly: albumin ↓, AAG ↑ ⇒ higher free acidic drugs, altered basic drug binding.
  • Intersubject genetic/environmental variability.
  • Disease states: hypoalbuminemia in aging, CHF, trauma, burns, inflammation, renal/hepatic disorders, pregnancy, surgery, cancer etc.; hyperlipoproteinemia in hypothyroidism, obstructive liver disease, alcoholism.

Significance / Clinical Implications of Protein & Tissue Binding

  • Dose: higher binding ⇒ larger dose needed (only free drug active).
  • Absorption: binding in blood maintains gradient → enhances passive absorption.
  • Distribution: highly bound drugs have low VdV_d (remain in plasma); protein bound drugs cannot cross BBB.
  • Metabolism: only free drug metabolized; binding acts as reservoir → prolonged action; highly bound drugs metabolized more slowly.
  • Excretion: bound fraction not filtered at glomerulus → slower renal elimination.
  • Drug Targeting: exploiting binding property to albumin or specific tissues for controlled delivery.

Kinetics of Protein Binding

  • Reversible equilibrium: P+D⇌PDP + D \rightleftharpoons PD
  • Law of mass action:
    K<em>a=[PD][P][D]K<em>a = \frac{[PD]}{[P][D]}[PD]=K</em>a[P][D][PD] = K</em>a [P][D]
  • Total protein concentration PT=[PD]+[P]P_T = [PD] + [P]
  • Fractional binding (r = moles drug bound per mole protein):
    r=[PD]P<em>T=K</em>a[D]Ka[D]+1r = \frac{[PD]}{P<em>T} = \frac{K</em>a[D]}{K_a[D] + 1} (for single binding site, 1:1 complex)
  • For N identical independent sites:
    r=NK<em>a[D]K</em>a[D]+1r = \frac{N K<em>a[D]}{K</em>a[D] + 1}
  • Determination of KaK_a & N via plots: Direct, Scatchard, Klotz, Hitchcock.

Exam-Relevant Lists

• Physiological barriers:

  • General endothelium, cell membranes, BBB, Blood-CSF barrier, Placental barrier, Blood-testis barrier.
    • Binding sites on HSA: Sudlow Site I (warfarin zone), Site II (benzodiazepine zone), fatty acid sites, metal-binding sites, etc.
    • Short-note topics frequently asked: significance of binding, BBB, VdV_d, kinetics, clinical applications, pre-systemic metabolism.

Miscellaneous Concepts & Examples

  • Prodrugs: inert precursors converted to active drug (e.g., nicotinic acid hydrazide → nicotinic acid; aspirin → salicylic acid; chloramphenicol succinate → chloramphenicol).
  • Drug interactions definition: alteration of pharmacologic activity of an “object” drug by a “precipitant” (drug, food, chemical). Example: tetracycline or ciprofloxacin + antacids ↓ absorption.
  • Pharmacodynamic interactions: tissue-level effects; agonistic, antagonistic, additive, synergistic.
  • HSA-binding drugs (sample list): warfarin, penicillin, diazepam, clindamycin, furosemide, glipizide, ibuprofen, phenytoin, propranolol, verapamil.

Summary Pointers for Essay/Short Answer Preparation

  • Define protein binding & enumerate factors (drug, protein, interactions, patient).
  • Enumerate physiological distribution barriers & elaborate BBB.
  • Discuss binding to blood components (albumin, AAG, globulins, lipoproteins, RBC binding, tissue proteins).
  • Describe kinetics equations & graphical methods.
  • Articulate clinical significance in absorption, distribution, metabolism, excretion, dosing, toxicity, drug interactions.