Drug Receptor Interactions Notes

Drug Receptor Interactions

Introduction

• Drugs act as signals, and receptors act as signal detectors.
• Receptors signal ligand binding by initiating reactions leading to specific intracellular responses.
• Ligand: A small molecule that binds to a site on a receptor protein (naturally occurring or a drug).
• Secondary messenger molecules (effector molecules): Part of the cascade that translates ligand binding into a cellular response.
• Cells have different types of receptors, each specific for a particular ligand and producing a unique response.
  • Example: The heart contains membrane receptors for epinephrine/norepinephrine and muscarinic receptors for acetylcholine, interacting to control heart functions.
• The magnitude of the response is proportional to the number of drug-receptor complexes.

Drug-Receptor Complex

• Drug (D) + Receptor (R) = DR complex = biological effect
• Similar to enzyme-substrate or antigen-antibody interactions.
• Features:
  • Specificity of the receptor for a given ligand.
  • Ability to transduce binding into a response (conformational change or biochemical effect).
• Receptors are named after the drug/chemical they interact with best (e.g., histamine receptor).
• Receptor States:
  • Classical view: Ligand binding causes a shift from inactive to an activated state.
  • Modern view: Receptors exist in at least two states (active and inactive) in reversible equilibrium.
  • In the absence of an agonist, active receptors are a small fraction of the total.
  • Drugs can stabilize the receptor in a given conformational state, shifting the equilibrium.
  • Some drugs induce a different change than the endogenous ligand.

Major Receptor Families

• Pharmacology defines a receptor as any biologic molecule to which a drug binds and produces a measurable response (enzymes, nucleic acids, structural proteins).
• Proteins that transduce extracellular signals into intracellular responses are the richest source of therapeutically useful receptors.
• Four families:
  1. Ligand-gated ion channels
  2. G protein-coupled receptors
  3. Enzyme-linked receptors
  4. Intracellular receptors
• The type of receptor a ligand interacts with depends on the ligand's chemical nature.
  • Hydrophilic ligands interact with cell surface receptors.
  • Hydrophobic ligands enter cells to interact with intracellular receptors.

Ligand-Gated Ion Channels

• Regulate ion flow across cell membranes.
• Activity is regulated by ligand binding.
• Mediate neurotransmission, cardiac conduction, and muscle contraction.
• Example: Acetylcholine stimulates nicotinic receptors, resulting in sodium flux, action potential generation, and skeletal muscle contraction.
• Voltage-gated ion channels (e.g., sodium channels) are important drug receptors for some drug classes, including local anesthetics.

G Protein-Coupled Receptors

• Comprise a single α-helical peptide with 7 transmembrane regions.
• The extracellular domain contains the ligand-binding area.
• Intracellularly linked to a G protein (Gs, Gi, etc.) with three subunits: α (binds GTP) and βγ.
• Ligand binding activates the G protein, replacing GDP with GTP on the α subunit.
• The α-GTP subunit and βγ subunit interact with cellular effectors (enzymes, proteins, or ion channels).
• These effectors activate second messengers, causing further actions within the cell.
• Stimulation results in responses lasting seconds to minutes.
• Most abundant type of receptors, accounting for the actions of most therapeutic agents.
• Important processes: neurotransmission, olfaction, and vision.

Second Messengers

• Essential for conducting and amplifying signals from G protein-coupled receptors.
• A common pathway involves activation of adenylyl cyclase by α-GTP subunits, producing cyclic adenosine monophosphate (cAMP), a second messenger regulating protein phosphorylation.
• G proteins also activate phospholipase C, generating other second messengers.

Enzyme-Linked Receptors

• Proteins that span the membrane once and may form dimers or multisubunit complexes.

• Have cytosolic enzyme activity as an integral part of their structure and function.

• Ligand binding to an extracellular domain activates or inhibits cytosolic enzyme activity.

• Responses last minutes to hours.

• Control metabolism, growth, and differentiation.

• The most common type has tyrosine kinase activity.

• Ligand binding causes receptor subunits to undergo conformational changes, activating kinases.

• The activated receptor auto-phosphorylates and then phosphorylates tyrosine residues on specific proteins.

• Phosphate group addition modifies the target protein's 3D structure, acting as a molecular switch.

• Example: Insulin binding to receptor subunits causes auto-phosphorylation, which leads to phosphorylation of target molecules (insulin receptor substrate peptide), activating other cellular signals like inositol triphosphate and mitogen-activated protein (MAP) kinase system.

• This cascade amplifies the initial signal.

Intracellular Receptors

• Receptors are entirely intracellular, and the ligand must diffuse into the cell.
• Ligands must have sufficient lipid solubility to cross the cell membrane.
• Lipid-soluble ligands are transported in the body attached to plasma proteins (e.g., albumin).
• Primary targets are transcription factors.
• Activation/inactivation of transcription factors causes transcription of DNA into RNA and translation of RNA into proteins.
• Example: Steroid hormones act via this mechanism.
• Ligand binding activates the receptor through dissociation from various proteins.
• The activated ligand-receptor complex migrates to the nucleus, binds to specific DNA sequences, and regulates gene expression.
• Cellular responses are observed after a considerable time, and the duration is much greater than that of other receptor families.
• Other targets include structural proteins, enzymes, RNA, and ribosomes.
  • Example: Tubulin (protein) is the target of antineoplastic agents like paclitaxel.

Characteristics of Signal Transduction

• Two important features: signal amplification and protection against excessive stimulation.

Signal Amplification

• Many receptors (hormones, neurotransmitters, peptides) can amplify signal duration and intensity.
• G protein-linked receptors exemplify possible responses initiated by ligand binding.
• Two phenomena account for amplification:
  1. A single ligand-receptor complex can interact with many G proteins.
  2. Activated G proteins persist longer than the original ligand-receptor complex.
• Example: Albuterol binding may last milliseconds, but activated G proteins may last hundreds of milliseconds.
• Further amplification is mediated by the interaction between G proteins and their intracellular targets.
• Due to amplification, only a fraction of the total receptors may need to be occupied to elicit a maximal response.
• Systems exhibiting this behavior are said to have spare receptors.
  • Example: Insulin receptors have been estimated to have 99% spare receptors.
  • This constitutes an immense functional reserve that ensures that adequate amounts of glucose enter the cell.
  • The human heart has about 5 to 10% spare β-adrenoreceptors.
  • Little functional reserve exists in the failing heart because more receptors must be occupied to obtain maximum contractility.

Desensitization and Down-Regulation

• Repeated/continuous agonist administration may lead to changes in receptor responsiveness.
• Mechanisms evolved to protect the cell from excessive stimulation.
• Tachyphylaxis: Diminished effect upon repeated drug administration.
  • Receptors are present on the cell surface but unresponsive to the ligand.
• Down-regulation: Receptors are internalized (endocytosis) and sequestered within the cell upon continual stimulation.
  • Receptors may be recycled to the cell surface (restoring sensitivity) or degraded (decreasing the total number of receptors).
• Some receptors (voltage-gated channels) require a rest period after stimulation before they can be activated again (refractory period).

Dose-Response Relationships

• Agonist: An agent that binds to a receptor and elicits a biologic response (mimics the action of the original endogenous ligand).
• The magnitude of the drug effect depends on the drug concentration at the receptor site, determined by the dose and pharmacokinetic profile (absorption, distribution, metabolism, and elimination).

Graded Dose-Response Relations

• As drug concentration increases, the magnitude of its pharmacologic effect also increases.
• The response is a graded effect (continuous and gradual).
• Plotting response magnitude against increasing doses produces a graded dose-response curve (rectangular hyperbola).
• Two important drug properties determined by these curves: potency and efficacy.
• $EC_{50}$ is the concentration of the drug that produces a response equal to 50% of the maximal response.

Potency

• A measure of the amount of drug necessary to produce an effect of a given magnitude.
• The concentration of drug producing an effect that is 50% of the maximum is the $EC_{50}$.
• Therapeutic preparations of drugs will reflect the potency. E.g. candesartan used to treat hypertension is more potent than irbesartan which is used for the same purpose Dose range for candesartan is 4-32 mg compared to 75-300 mg for irbesartan

Efficacy

• The ability of a drug to elicit a response when it interacts with a receptor.
• Dependent on the number of drug-receptor complexes formed and the efficiency of receptor activation coupling to cellular responses.
• The maximum response ($E_{max}$ ) or efficacy is more important than drug potency.
• A drug with greater efficacy is more therapeutically beneficial than one that is more potent.
• Maximum efficacy assumes all receptors are occupied, and no increase in response will result from adding more drug.

Effect of Drug Concentration on Receptor Binding

• The quantitative relationship between drug concentration and receptor occupancy applies the law of mass action. Drug (D) + Receptor (R) = DR complex = biological effect
• Assuming that the binding of one drug molecule does not alter the binding of subsequent molecules and applying the law of mass action, we can mathematically express the relationship between the % of bound receptors and the drug concentration
• $\frac{[DR]}{[R<em>t]} = \frac{[D]}{[D] + K</em>d}$
• Where:
  • [D] is the concentration of free drug
  • [DR] is the concentration of bound drug
  • [Rt] is the total concentration of receptors (sum of unbound and bound receptors)
  • $K_d$ is the equilibrium dissociation constant for the drug from the receptor
• Affinity describes the strength of the interaction between a ligand and its receptor.
  • Higher $K_d$ = weaker interaction, lower affinity.
  • Lower $K_d$ = stronger interaction, higher affinity.
• As free drug concentration increases, the ratio of bound receptors to total receptors approaches unity.
• Drug binding initiates events leading to a measurable biologic response.

Relationship of Drug Binding to Pharmacologic Effect

• The mathematical model describing drug concentration and receptor binding can be applied to dose and response if:
  1. The magnitude of the response is proportional to the amount of receptors bound.
  2. The $E_{max}$ occurs when all receptors are bound.
  3. Binding of the drug to the receptor exhibits no cooperativity.
• $E = \frac{E<em>{max} \cdot [D]}{[D] + EC</em>{50}}$
  • Where [E] is the effect of the drug at concentration [D] and [Emax] is the maximal effect of the drug

Agonists

• Agonist: Binds to a receptor and produces a biologic response, mimicking the endogenous ligand or eliciting a different response.

Full Agonists

• Produces a maximal biological response that mimics the endogenous ligand.
• Stabilizes the receptor in its active state.

Partial Agonists

• Have efficacies greater than zero but less than a full agonist (
• Cannot produce $E_{max}$ as great as a full agonist, even with full receptor occupancy.
• Can act as an antagonist of a full agonist under appropriate conditions.

Inverse Agonists

• Some receptors spontaneously convert from inactive to active without an agonist (constitutive activity).
• Inverse agonists stabilize the inactive receptor form.
• They decrease the number of activated receptors below that observed in the absence of the drug.
• Reverse the constitutive activity of receptors and exert the opposite pharmacological effect of receptor agonists.

Antagonists

• Drugs that decrease or oppose the actions of another drug or endogenous ligand.
• Have no effect if an agonist is not present.
• Antagonism may occur in several ways.
• Antagonists bind avidly to target receptors because they possess strong affinity, but have no intrinsic activity.

Competitive Antagonists

• Both agonist and antagonist bind to the same site on a receptor.
• The competitive antagonist prevents an agonist from binding to its receptor and maintains the receptor in its inactive form.
• Characteristically causes a shift of the agonist dose-response curve to the right.

Irreversible Antagonists

• Causes a downward shift of the maximum with no shift of the curve on the dose axis unless spare receptors are present.
• The effects of competitive antagonists can be overcome by adding more agonist.
• Irreversible antagonists cannot be overcome by adding more agonist.
• Competitive antagonists increase the $ED_{50}$ whereas irreversible antagonists do not.
• Two mechanisms by which an agent can act as a non-competitive antagonist.
  • The antagonist can bind covalently or with high affinity to the active site of the receptor.
  • This irreversibility in binding to the active site reduces the amount of receptors available to the agonist.
  • The agonist cannot out compete the antagonist even if the dose increases.
  • The second type of antagonist binds to a site (allosteric site) other than the agonist binding site.
  • Preventing the receptor from being activated even if the agonist binds to the site.
  • Termed allosteric antagonist.

Functional and Chemical Antagonisms

• Functional (physiologic) antagonists act at a completely separate receptor, initiating effects opposite those of the agonist.
• Chemical antagonists prevent agonist actions by modifying the agonist, preventing receptor binding and activation.

Therapeutic Index

• The therapeutic index (TI) of a drug is the ratio of the dose that produces toxicity to the dose that produces a clinically desired or effective response.
• $TI = \frac{TD<em>{50}}{ED</em>{50}}$
  • Where $TD_{50}$ is the drug dose that produces a toxic effect in 50% of the population
  • $ED_{50}$ is the drug dose that produces a desired response in 50% of the population.
• TI measures the drug's safety.
• Determination of TI: Measuring the frequency of desired and toxic responses at various drug doses via drug trials and clinical experience.
  • Warfarin: drug with small TI.
  • Penicillin: large TI (safe to use in excess).