Introduction The discovery of pharmacologic agents by modern pharmaceutical companies and universities often involves the use of receptor-ligand binding techniques. Following the synthesis of a series of new chemically related compounds, which can constitute hundreds to thousands of compounds, the determination of the desired biologic activity was once a rather daunting task. Today, receptor-ligand binding techniques, such as high-throughput screening (HTS), are used to narrow large numbers of compounds down to those that display the greatest affinity for a receptor, thereby significantly decreasing the time and cost associated with identifying “lead” compounds.

Interaction When a drug interacts with a receptor, multiple chemical interactive forces, both weak and strong, between two molecules (a drug and its receptor) are believed to be responsible for the initial interaction. Such forces having a role in ligand–receptor binding include covalent, ionic, and hydrogen bonds and hydrophobic interactions.

An Example Molecular recognition of the bacterial leucine transporter (LeuT, pdb:3GWW) with cocrystallized leucine substrate (shown as ball-and-stick). Side-chain residues within 4.5 Å (line rendering) of the leucine molecule are shown, along with two Na+ ions (black spheres), which are required for transport. Hydrogen bonding and hydrophobic interactions between LeuT and substrate are shown as dotted lines, respectively.

Interaction types Covalent Bond The strongest of bonds involved in drug–receptor interactions is the covalent bond, in which two atoms, one from the ligand and one from the receptor, share a pair of electrons to form a covalent bond. Because of the significant strength of the covalent bond (50 to 150 kcal/mol), covalent bonding often produces a situation in which the ligand is irreversibly bound by the receptor and, thus, leads to the receptor’s eventual destruction via endocytosis and chemical destruction. Full recovery of cellular function therefore requires the synthesis of new receptors.

Interaction types Ionic Bond When two ions of opposite charge are attracted to each other through electrostatic forces, an ionic bond is formed. The strength of this type of bond varies between 5 and 10 kcal/mol, and it decreases proportionally to the square of the distance between the two atoms. The ability of a drug to bind to a receptor via ionic interactions therefore increases significantly as the drug molecule diffuses closer to the receptor. Additionally, the strength associated with the ionic bond is strong enough to support an initial transient interaction between the receptor and the drug, but unlike the covalent bond, the ionic bond is not so strong as to prevent dissociation of the drug–receptor complex.

Interaction types Ionic Bond The tendency of an atom to participate in ionic bonding is determined by its degree of electronegativity. Hydrogen, as a standard, has an electronegativity value of 2.2; fluo- rine is 4.2, chlorine 2.9 and nitrogen 3.1 (Linus Pauling units). Fluorine and chlorine atoms, as well as hydroxyl, sulfhydryl, and carboxyl groups, form strong ionic bonds because of a stronger attraction for electrons compared with that of hydrogen. On the other hand, alkyl groups do not participate in ionic bonds because of a weaker ten- dency to attract electrons compared with that of hydrogen.

Interaction types Hydrogen Bond A hydrogen bond (or hydrogen bonding) is a strong electrostatic dipole– dipole interaction between a hydrogen atom and an electronegative atom, such as oxygen, nitrogen, or fluorine. The hydrogen bond is extremely strong because oxygen, nitrogen, and fluorine are extremely good at attracting the relative positive charge of hydrogen, resulting in an extreme dipole situation. This type of bond can occur between molecules (intermolecular hydrogen bonds) or within the same molecule (intramolecular hydrogen bonds). At 2 to 5 kcal/mol, a single hydrogen bond is stronger than a van der Waals interaction, but weaker than covalent or ionic bonds, and thus would not be expected to support a drug–receptor interaction alone. However, when multiple intermolecular hydrogen bonds are formed between drugs and receptors, as typically is the case, a substantial amount of stability is conferred on the drug–receptor interaction, an essential requirement for drug–receptor interactions. For example, a water molecule behaves as an electronic dipole and can easily form intermolecular hydrogen bonds with other water molecules, which gives water its high boiling point of 100°C. Intramolecular hydrogen bonding is partly responsible for the secondary, tertiary, and quaternary structures of proteins and nucleic acids.

Interaction types Hydrophobic Interactions Hydrophobic interactions (hydrophobic effect; fear of water) are intermolecular interactions or dispersion forces that occur between nonpolar organic molecules and contribute to the binding forces that attract a ligand to its receptor, other than ionic, covalent, or hydrogen bonds. These interactions are often referred to as van der Waals forces or London forces, which require two nonpolar molecules to come in close range to one another, or between groups within the same molecule. Dispersion forces (or London dispersion forces) are induced dipole–dipole electrostatic interactions between atoms/molecules at close distances and occur over a large surface area (i.e., at the interface of the ligand and binding site) and thus contribute to receptor binding. London forces are weaker than van der Waals forces. These forces tend to align the atoms/molecules in order to increase their interaction, thereby reducing their potential energy. Theorists have suggested that for these forces to operate, a momentary dipolar structure needs to exist to allow such association. This induced dipolar interaction results from a temporary imbalance of charge distribution between or within molecules. These forces are very weak (0.5 to 1 kcal/mol) and decrease proportionally to the seventh power of the interatomic distance. The hydrophobic effect is the ability of polar water molecules to exclude (repel) nonpolar hydrocarbon-like molecules.

Interactions: Comparison Various drug–receptor bonds. (A) Covalent. (B) Ionic. (C) Hydrogen. (D) Hydrophobic.

DOSE–RESPONSE RELATIONSHIPS Equation below illustrates the interaction of a drug ([D]) with a receptor ([R]), which results in a drug–receptor complex ([DR]) and a biologic response. The interaction between most therapeutically useful drugs and their receptors is generally reversible:

DOSE–RESPONSE RELATIONSHIPS After administration of a drug, one can monitor the biologic responses produced. Plotting the dose or concentration of the drug versus the effect produced (% response) yields a rectangular hyperbolic function.

DOSE–RESPONSE RELATIONSHIPS Dose–response curves are typically plotted to determine both quantitative and qualitative parameters of potency and efficacy. Potency is inversely related to the dose required to produce a given response (typically half-maximum), and efficacy is the ability of a drug to produce a full response (100% maximum).

DOSE–RESPONSE RELATIONSHIPS drug X is equally efficacious to drug Y. drug X is more potent than drug Y. both drug X and drug Y produce a 100% response, but drug X reaches that response at a lower dose. Those curves positioned to the left are more potent than those positioned to the right. drug Z is more potent than drug Y, and drug Z is equipotent to drug X. the greater the maximum response (i.e., efficacy), the higher the maximum point on the dose–response curve. drug X and drug Y are of equal efficacy, and drug X and drug Y are of greater efficacy than drug Z.

Major classes of drug receptors. (A) Transmembrane ligand-gated ion channel receptor. (B) Transmembrane G protein–coupled receptor (GPCR). (C) Transmembrane catalytic receptor or enzyme-coupled receptors. (D) Intracellular cytoplasmic/nuclear receptor.

Agonist spectrum

A prerequisite for an inverse agonist response is that the receptor must have a constitutive (also known as intrinsic or basal) level activity in the absence of any ligand. An agonist increases the activity of a receptor above its basal level, whereas an inverse agonist decreases the activity below the basal level. A neutral antagonist has no activity in the absence of an agonist or inverse agonist but can block the activity of either constitutiveintrinsic ligandneutral antagonist

An agonist is a chemical that binds to a receptor and activates the receptor to produce a biological response. Whereas an agonist causes an action, an antagonist blocks the action of the agonist and an inverse agonist causes an action opposite to that of the agonist. receptorantagonistinverse agonist