ABSTRACT

Complexation is a phenomenon that involves covalent or noncovalent interactions between one or more molecules of two compounds-a ligand and a substrate. The ligand generally has the ability to complex different types of substrates with similar molecular size, geometry, charge distribution, and other physicochemical characteristics. A drug molecule can be either a ligand or a substrate. For example, complex formation of theophylline (substrate) with ethylenediamine (ligand) leads to the bronchodilator drug aminophylline (Figure 5.1) with higher drug solubility. On the other hand, aqueous solubility of oxytetracycline (drug, ligand, Figure 5.2) decreases when it complexes with calcium ions (substrate) leading to contraindication due to low absorption of this antibiotic with dairy products. Thus, upon complexation, properties of the drug such as solubility, stability, partitioning (hydrophilicity/lipophilicity), and absorption are altered.