Pharmacodynamics: Signal Transduction

• Pharmacodynamics describes a drug's effects on the body and how drug concentrations affect the magnitude of the response.
• Most drugs interact with specific receptors (target macromolecules) on cell surfaces or within cells to exert their effects.
• The drug-receptor complex then initiates changes in a cell's biochemical activity through signal transduction.

Signal Transduction

• Drugs act as signals, and receptors act as signal detectors.
• An agonist binds to a receptor and activates it, triggering a cascade of reactions that lead to a specific intracellular response.
• Second messengers are effector molecules that translate agonist binding into cellular response.
• Cells have various receptors specific to particular agonists, producing unique responses.
• Cardiac cell membranes have β-adrenergic receptors for epinephrine/norepinephrine and muscarinic receptors for acetylcholine, dynamically controlling heart functions.
• The magnitude of the cellular response is proportional to the number of drug-receptor complexes.
• Not all drugs require receptors; antacids neutralize gastric acid chemically.

Receptor States

• Receptors exist in inactive (R) and active (R*) states in reversible equilibrium, usually favoring the inactive state.
• Agonists shift the equilibrium from R to R*, eliciting a biological effect.
• Antagonists bind to receptors but do not increase the R* fraction, stabilizing the R fraction.
• Partial agonists shift the equilibrium from R to R*, but the fraction of R* is less than that caused by an agonist.
• The magnitude of the biological effect is related to the fraction of R*.

Major Receptor Families

• A receptor is any biologic molecule to which a drug binds and produces a measurable response, including enzymes, nucleic acids, and structural proteins.
• Membrane-bound proteins that transduce extracellular signals are rich sources of receptors.
• Four main families of receptors exist:
  • Ligand-gated ion channels
  • G protein-coupled receptors
  • Enzyme-linked receptors
  • Intracellular receptors
• Hydrophilic ligands interact with cell surface receptors, while hydrophobic ligands interact with intracellular receptors.

Transmembrane Ligand-Gated Ion Channels

• The extracellular part contains a ligand binding site that regulates the pore's shape for ion flow across cell membranes.
• The channel opens briefly (milliseconds) upon agonist activation.
• These receptors mediate neurotransmission and cardiac/muscle contraction, depending on the ion conducted.
• Acetylcholine stimulates nicotinic receptors, which results in sodium influx and potassium outflux, generating an action potential.
• GABA receptor stimulation increases chloride influx and hyperpolarization of neurons.
• Local anesthetics bind to voltage-gated sodium channels, inhibiting sodium influx and neuronal conduction.

Transmembrane G Protein–Coupled Receptors

• The extracellular domain contains the ligand-binding area, while the intracellular domain interacts with a G protein.
• Binding of an agonist increases GTP binding to the α subunit and dissociation of the α-GTP complex from the βγ complex.
• These complexes interact with cellular effectors (enzymes, proteins, or ion channels), leading to further actions within the cell.
• Activated effectors may produce second messengers like cAMP, IP3, and DAG to further activate other effectors in the cell, causing a signal cascade effect.
• Adenylyl cyclase, activated by Gs and inhibited by Gi, produces cAMP.
• Gq activates phospholipase C, which generates IP3 and DAG.
• DAG and cAMP activate protein kinases, leading to physiological effects and IP3 regulates intracellular calcium.

Enzyme-Linked Receptors

• These receptors form dimers or multisubunit complexes.
• Activation causes conformational changes, increasing cytosolic enzyme activity (minutes to hours).
• Common examples include receptors for epidermal growth factor, platelet-derived growth factor, atrial natriuretic peptide, and insulin.
• They often possess tyrosine kinase activity, phosphorylating tyrosine residues on themselves and other proteins, acting as a molecular switch.
• Insulin binding leads to autophosphorylation of the receptor, activating other cellular signals.

Intracellular Receptors

• Receptors are located entirely inside the cell, requiring ligands to diffuse into the cell.
• Ligands must have sufficient lipid solubility to cross the cell membrane.
• Ligand-receptor complexes primarily target transcription factors in the cell nucleus.
• Ligand binding activates the receptor, which translocates to the nucleus, dimerizes, and binds to transcription factors.
• This affects DNA transcription into RNA and RNA translation into proteins (hours to days).
• Steroid hormones act via intracellular receptors.
• Other targets include structural proteins, enzymes, RNA, and ribosomes (tubulin, dihydrofolate reductase, and the 50S subunit of bacterial ribosome).

Characteristics of Signal Transduction

• Signal transduction has two key features:
  • Amplifying small signals
  • Protecting cells from excessive stimulation

Signal Amplification

• G protein-linked and enzyme-linked receptors amplify signal intensity and duration via signal cascades.
• Activated G proteins persist longer than the original agonist-receptor complex.
• Albuterol binding may last milliseconds, but activated G proteins may last hundreds of milliseconds.
• Only a fraction of total receptors need to be occupied to elicit a maximal response due to amplification creating spare receptors.
• About 99% of insulin receptors are spare, while only 5-10% of β3-adrenoceptors in the heart are spare.

Desensitization and Down-Regulation of Receptors

• Repeated agonist exposure can lead to receptor desensitization, resulting in a diminished response = Tachyphylaxis.
• Desensitization can occur through phosphorylation, making receptors unresponsive to the agonist.
• Receptors may be internalized within the cell (down-regulation).
• Ion channels require a recovery period post-stimulation = "refractory",
• Repeated antagonist exposure can lead to up-regulation, increasing the number of available receptors.

Dose-Response Relationship

• Agonist drugs mimic endogenous ligands.
• Drug effect depends on receptor sensitivity and drug concentration, determined by dose and pharmacokinetic profile (absorption, distribution, metabolism, elimination).

Graded Dose-Response Relationship

• As drug concentration increases, the pharmacologic effect increases until all receptors are occupied (maximum effect).
• Potency and efficacy are determined by graded dose-response curves.

Potency

• Potency measures the amount of drug needed to produce an effect.
• EC50 (concentration for 50% of maximum effect) is used to determine potency.
• Drug A is more potent than Drug B if it has a lower EC50 value.
• Therapeutic preparations reflect potency (candesartan is more potent than irbesartan)
• Semilogarithmic plots are used because drug concentrations that cause 1%-99% maximal response can span orders of magnitude.

Efficacy

• Efficacy is the magnitude of response a drug causes when it interacts with a receptor.
• Dependent on the number of drug-receptor complexes and the drug's intrinsic activity.
• Maximal efficacy (Emax) assumes all receptors are occupied.
• The maximal response differs between full and partial agonists.
• Antagonists have an Emax of zero.
• Efficacy is more clinically useful than potency.

Intrinsic Activity

• A drug's intrinsic activity determines its ability to fully or partially activate receptors.

Full Agonists

• Bind to a receptor and produce a maximal biologic response like the endogenous ligand.
• Stabilize the receptor in its active state and have an intrinsic activity of one.
• All full agonists for a receptor population should produce the same Emax.
• Phenylephrine is a full agonist at α1-adrenoceptors.
• The dose–response curves for receptor binding and each of the biological responses are comparable.

Partial Agonists

• Intrinsic activities are between zero and one.
• Cannot produce the same Emax as a full agonist with complete receptor saturation.
• May have greater, lesser, or equivalent affinity to a full agonist.
• In the presence of a full agonist, a partial agonist can act as an antagonist by decreasing the Emax.
• Aripiprazole is a partial agonist at selected dopamine receptors.
• Dopaminergic pathways that are overactive tend to be inhibited by aripiprazole, whereas pathways that are underactive are stimulated.

Inverse Agonists

• Stabilize the inactive R form causing R* to convert to R, decreasing activated receptors below levels observed without the drug.
• Intrinsic activity is less than zero, reversing receptor activity.

Antagonists

• Bind to a receptor with high affinity but possess zero intrinsic activity.
• Decrease the effect of an agonist by blocking its ability to bind or activate the receptor.

Competitive Antagonists

• Bind reversibly to the same site as the agonist.
• Interfere with agonist binding and maintain the receptor in its inactive state.
• Terazosin competes with norepinephrine at α1-adrenoceptors, decreasing vascular smooth muscle tone.
• Increasing agonist concentration can overcome inhibition, shifting the agonist dose-response curve to the right (increased EC50) without affecting Emax.

Irreversible Antagonists

• Bind covalently to the active site, permanently reducing the number of available receptors.
• Causes a downward shift of the Emax, with no shift of EC50 values.
• Addition of more agonist does not overcome the effect.

Allosteric Antagonists

• Binds to a site (allosteric site) other than the agonist-binding site, preventing receptor activation.
• Causes a downward shift of the Emax of an agonist, with no change in the EC50 value.
• Picrotoxin binds to the inside of the GABA-controlled chloride channel.

Functional Antagonism

• Acts at a separate receptor, initiating effects opposite those of the agonist i.e "physiologic antagonism."
• Epinephrine antagonizes histamine-induced bronchoconstriction by binding to B2-adrenoceptors on bronchial smooth muscle, causing relaxation.

Quantal Dose-Response Curve

• Relates drug dose to the proportion of a population that responds.
• Responses are quantal, meaning they either occur or do not.
• Useful for determining doses to which most of the population responds.
• Similar shapes as log dose-response curves.
• ED50 is the drug dose that causes a therapeutic response in half of the population.

Therapeutic Index

• Ratio of the dose that produces toxicity in half the population (TD50) to the dose that produces a clinically desired effect in half the population (ED50):
  • TI = TD50/ED50
• Measures a drug's safety; a larger value indicates a wide margin.
• Determined using drug trials and clinical experience.
• Reveal a range of effective doses and toxic doses.
• Some drugs with low therapeutic indices are used to treat serious diseases.

Clinical Usefulness of the Therapeutic Index

• Warfarin: an anticoagulant, has a small therapeutic index

  • As the dose of warfarin is increased, a greater percentage of the patients respond (desired response is a two- to threefold increase in the international normalized ratio [INR]) until, eventually, all patients respond.
  • At higher doses of warfarin, anticoagulation can happen resulting in hemorrhage
  • Agents with a small therapeutic index are drugs for which dose is critically important and bioavailability greatly alters the therapeutic effects

• Penicillin: an antimicrobial drug, has a large therapeutic index

  • For drugs such as penicillin, it is safe and common to give doses in excess of that which is minimally required to achieve a desired response without the risk of adverse effects.
  • In this case, bioavailability does not critically alter the therapeutic or clinical effects.