Introduction to Complexation and Protein Binding
Complexation fundamentally alters drug properties – solubility, stability, activity, and absorption. Cyclodextrin inclusion complexes are now used in marketed products to solubilize poorly water-soluble drugs. Drug-protein binding affects how much ‘free’ drug is available to produce pharmacological effects – a critical concept for rational drug therapy.
Syllabus & Topics
- 1Introduction to Complexation: Definition of a complex (coordination compound).
- 2Classification of Complexes:
- 3a) Coordination complexes (metal complexes): Drug + Metal ion. E.g., Cisplatin (Pt complex – anticancer), EDTA chelate.
- 4b) Organic molecular complexes: Charge-transfer complexes (donor-acceptor).
- 5c) Inclusion complexes (Clathrates): Host-guest complexes without covalent bonds.
- 6– Cyclodextrin inclusion complexes: Cyclodextrins (α, β, γ) are toroidal molecules with hydrophobic cavity. Drug fits inside cavity. β-CD used to enhance solubility of poorly soluble drugs. Examples: Piroxicam-β-CD, Itraconazole-HP-β-CD (Sporanox injection).
- 7– Channel-type inclusion complexes: Urea channel complexes.
- 8d) Polymer complexes: Polyvinylpyrrolidone (PVP) complexes.
- 9Applications of Complexation in Pharmacy: Enhancing drug solubility, stability (protection from oxidation/hydrolysis), masking taste/odour, reducing drug toxicity, modifying drug release.
- 10Methods of Analysis of Complexes: Solubility method (phase solubility analysis – Higuchi and Connors), Spectrophotometry, Job’s method (method of continuous variations), Potentiometry.
- 11Protein Binding: Drug-protein binding equilibrium (Drug + Protein ⇌ Drug-Protein complex).
- 12Types of proteins drug binding: Albumin (acidic drugs), α1-acid glycoprotein (basic drugs), Globulins, Tissue proteins.
- 13Factors affecting protein binding: Drug concentration, protein concentration, temperature, pH, disease states (hypoalbuminemia in liver/kidney disease).
- 14Significance of protein binding: Only free (unbound) drug is pharmacologically active. Drug displacement interactions (one drug displacing another from protein binding sites – e.g., Aspirin displaces Warfarin).
- 15Complexation and drug action: Chelation therapy (EDTA for heavy metal poisoning, Deferoxamine for iron poisoning).
- 16Crystalline structures of complexes.
- 17Thermodynamic treatment of stability constants: Ka (association constant), Kd (dissociation constant), ΔG° = -RT ln Ka.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. What are Cyclodextrin Inclusion Complexes and their Pharmaceutical Use?
Cyclodextrins (CDs) are cyclic oligosaccharides characterized by a hydrophobic inner cavity and a hydrophilic outer surface. A lipophilic drug molecule can partially or completely fit into this cavity without forming a covalent bond, resulting in an inclusion complex. This complexation significantly enhances the aqueous solubility and stability of poorly water-soluble drugs. Among them, β-cyclodextrin is most commonly used in pharmaceutical formulations. A widely used derivative is hydroxypropyl-β-cyclodextrin (HP-β-CD), which is employed in Voriconazole IV injection to improve its solubility for parenteral administration.
Q2. What is the Pharmaceutical Significance of Protein Binding?
Protein binding plays a crucial role in drug distribution and pharmacological response. Only the free (unbound) fraction of a drug is pharmacologically active, metabolized, and excreted. Drugs with high protein binding tend to have a longer duration of action because the bound portion acts as a reservoir. However, drug displacement interactions can occur when one drug displaces another from plasma protein binding sites, leading to a sudden increase in free drug concentration and potential toxicity. For example, Aspirin can displace Warfarin from protein binding sites, increasing the risk of bleeding.
Q3. What is Chelation Therapy?
Chelation therapy involves the use of chelating agents, which are ligands capable of forming multiple coordinate bonds with a metal ion to create a stable chelate ring. This therapy is primarily used in heavy metal poisoning to enhance metal excretion. For example, Ethylenediaminetetraacetic acid (EDTA) is used in lead and mercury poisoning, Deferoxamine is used for iron overload, Dimercaprol (BAL) is used for arsenic and mercury poisoning, and Penicillamine is used in Wilson’s disease (copper accumulation).
Q4. What is Phase Solubility Analysis (Higuchi and Connors)?
Phase solubility analysis, developed by Higuchi and Connors, is a method used to study inclusion complex formation and determine the stability constant of the complex. In this technique, excess drug is added to solutions containing increasing concentrations of a ligand (such as cyclodextrin). As the ligand concentration increases, drug solubility increases due to complex formation. A graph plotted between drug solubility and ligand concentration is called a phase solubility diagram, and the slope of the linear portion is used to calculate the stability constant (Kc) of the complex.
Q5. What Factors Decrease Protein Binding of a Drug?
Several factors can decrease protein binding of a drug. Disease states such as hypoalbuminemia (seen in liver cirrhosis, nephrotic syndrome, and malnutrition) reduce albumin levels, thereby decreasing binding capacity. Elderly patients may also have lower albumin levels, leading to reduced binding. Drug interactions can cause competitive displacement from protein binding sites. In uremia, endogenous substances compete for binding sites, lowering drug binding. Additionally, high drug concentrations may saturate protein binding sites, increasing the proportion of free drug in circulation.