Pharmacodynamics Lecture Notes PDF

Pharmacodynamics MOHAMED BAHR; MD, PHD Mohamed Bahr; MD, PhD  Types of Drug Receptors Mohamed Bahr; MD, PhD  Intracellular Receptors for Lipid-Soluble Agents Several biologic ligands are sufficiently lipid-soluble to cross the plasma membrane a...

Pharmacodynamics MOHAMED BAHR; MD, PHD Mohamed Bahr; MD, PhD  Types of Drug Receptors Mohamed Bahr; MD, PhD  Intracellular Receptors for Lipid-Soluble Agents Several biologic ligands are sufficiently lipid-soluble to cross the plasma membrane and act on intracellular receptors. One class of such ligands includes steroids (corticosteroids, mineralocorticoids, sex steroids, vitamin D) and thyroid hormone, whose receptors stimulate the transcription of genes by binding to specific DNA sequences (often called response elements) near the gene whose expression is to be regulated. All of these hormones produce their effects after a characteristic lag period of 30 minutes to several hours The effects of these agents can persist for hours or days after the agonist concentration has been reduced to zero. The persistence of effect is primarily due to the relatively slow turnover of most enzymes and proteins, which can remain active in cells for hours or days after they have been synthesized. Mohamed Bahr; MD, PhD  Ligand-Regulated Transmembrane Enzymes Including Receptor Tyrosine Kinases The receptor polypeptide has extracellular and cytoplasmic domains, depicted above and below the plasma membrane. Upon binding of EGF, the receptor converts from its inactive monomeric state (left) to an active dimeric state (right), in which two receptor polypeptides bind noncovalently. The cytoplasmic domains become phosphorylated (P) on specific tyrosine residues (Y), and their enzymatic activities are activated, catalyzing phosphorylation of substrate proteins (S). Mohamed Bahr; MD, PhD  Cytokine Receptors Cytokine receptors, like receptor tyrosine kinases, have extracellular and intracellular domains and form dimers. However, after activation by an appropriate ligand, separate mobile protein tyrosine kinase molecules (JAK) are activated, resulting in phosphorylation of signal transducers and activation of transcription (STAT) molecules. STAT dimers then travel to the nucleus, where they regulate transcription. Mohamed Bahr; MD, PhD  Ion Channels The nicotinic acetylcholine (ACh) receptor, a ligand-gated ion channel. The receptor molecule is depicted as embedded in a rectangular piece of plasma membrane, with extracellular fluid above and cytoplasm below. Composed of five subunits (two α, one β, one γ, and one δ), the receptor opens a central transmembrane ion channel when ACh binds to sites on the extracellular domain of its α subunits. Receptors that mediate excitatory neurotransmission at CNS synapses bind glutamate through a large appendage domain that protrudes from the receptor and has been called a “flytrap” because it physically closes around the glutamate molecule; the glutamate-loaded flytrap domain then moves as a unit to control pore opening. Drugs can regulate the activity of such glutamate receptors by binding to the flytrap domain, to surfaces on the membrane-embedded portion around the pore, or within the pore itself. Mohamed Bahr; MD, PhD  G Proteins & Second Messengers The guanine nucleotide-dependent activation- inactivation cycle of G proteins. he agonist activates the receptor (R→R*), which promotes release of GDP from the G protein (G), allowing entry of GTP into the nucleotide binding site. In its GTP-bound state (G-GTP), the G protein regulates activity of an effector enzyme or ion channel (E→E*). The signal is terminated by hydrolysis of GTP, followed by return of the system to the basal unstimulated state. Open arrows denote regulatory effects. (Pi, inorganic phosphate.) Mohamed Bahr; MD, PhD  Transmembrane topology of a typical “serpentine” GPCR. The receptor’s amino (N) terminal is extracellular, and its carboxyl (C) terminal intracellular, with the polypeptide chain “snaking” across the membrane 7 times. Agonist (Ag) approaches the receptor from the extracellular fluid and binds to a site surrounded by the transmembrane regions of the receptor protein. G protein interacts with cytoplasmic regions of the receptor. Lateral movement of these helices during activation exposes an otherwise buried cytoplasmic surface of the receptor that promotes guanine nucleotide exchange on the G protein and thereby activates the G protein. The receptor’s cytoplasmic terminal tail contains numerous serine and threonine residues whose hydroxyl (-OH) groups can be phosphorylated. This phosphorylation is associated with diminished receptor-G protein coupling and can promote receptor endocytosis. Mohamed Bahr; MD, PhD  Many extracellular ligands act by increasing the intracellular concentrations of second messengers such as cAMP, calcium ion, or the phosphoinositides. In most cases, they use a transmembrane signaling system with three separate components. First, the extracellular ligand is selectively detected by a cell-surface receptor. The receptor in turn triggers the activation of a GTP-binding protein (G protein) located on the cytoplasmic face of the plasma membrane. The activated G protein then changes the activity of an effector element, usually an enzyme or ion channel. This element then changes the concentration of the intracellular second messenger. For cAMP, the effector enzyme is adenylyl cyclase, a membrane protein that converts intracellular adenosine triphosphate (ATP) to cAMP. The corresponding G protein, Gs, stimulates adenylyl cyclase after being activated by hormones and neurotransmitters that act via specific Gs-coupled receptors. Mohamed Bahr; MD, PhD  Well-Established Second Messengers A. Cyclic Adenosine Monophosphate (cAMP) B. Phosphoinositides and Calcium C. Cyclic Guanosine Monophosphate (cGMP) Mohamed Bahr; MD, PhD  G proteins and their receptors and effectors. Mohamed Bahr; MD, PhD  Relation Between Drug Concentration & Response Mohamed Bahr; MD, PhD  Chemistry of Drug Receptor Binding The ionic bonds: is strong but reversible and responsible for most drug receptor interactions. The hydrogen bonds: is weak and reversible bond. The covalent bonds: is very strong and irreversible at body temperature. Mohamed Bahr; MD, PhD  Concentration-Effect Curves & Receptor Binding of Agonists Mohamed Bahr; MD, PhD  Relations between drug concentration and drug effect (A) or receptor-bound drug (B). The drug concentrations at which effect or receptor occupancy is half-maximal are denoted by EC50 and Kd, respectively. Mohamed Bahr; MD, PhD  Competitive & Irreversible Antagonists Receptor antagonists bind to receptors but do not activate them; the primary action of antagonists is to reduce the effects of agonists (other drugs or endogenous regulatory molecules) that normally activate receptors. While antagonists are traditionally thought to have no functional effect in the absence of an agonist, some antagonists exhibit “inverse agonist” activity because they also reduce receptor activity below basal levels observed in the absence of any agonist at all. Antagonist drugs are further divided into two classes depending on whether or not they act competitively or noncompetitively relative to an agonist present at the same time. Mohamed Bahr; MD, PhD  Drugs may interact with receptors in several ways. The effects resulting from these interactions are diagrammed in the dose-response curves. Drugs that alter the agonist (A) response may activate the agonist binding site, compete with the agonist (competitive inhibitors, B), or act at separate (allosteric) sites, increasing (C) or decreasing (D) the response to the agonist. Allosteric activators (C) may increase the efficacy of the agonist or its binding affinity. The curve shown reflects an increase in efficacy; an increase in affinity would result in a leftward shift of the curve. Mohamed Bahr; MD, PhD  Competitive Antagonists In the presence of a fixed concentration of agonist, increasing concentrations of a competitive antagonist progressively inhibit the agonist response; high antagonist concentrations prevent the response almost completely. Conversely, sufficiently high concentrations of agonist can surmount the effect of a given concentration of the antagonist; that is, the Emax for the agonist remains the same for any fixed concentration of antagonist. Because the antagonism is competitive, the presence of antagonist increases the agonist concentration required for a given degree of response, and so the agonist concentration-effect curve is shifted to the right. Mohamed Bahr; MD, PhD  Noncompetitive Antagonists Once a receptor is bound by such a noncompetitive antagonist, agonists cannot surmount the inhibitory effect irrespective of their concentration. In many cases, noncompetitive antagonists bind to the receptor in an irreversible or nearly irreversible fashion, sometimes by forming a covalent bond with the receptor. After occupancy of some proportion of receptors by such an antagonist, the number of remaining unoccupied receptors may be too low for the agonist (even at high concentrations) to elicit a response comparable to the previous maximal response. Therapeutically, such irreversible antagonists present distinct advantages and disadvantages. Once the irreversible antagonist has occupied the receptor, it need not be present in unbound form to inhibit agonist responses. Consequently, the duration of action of such an irreversible antagonist is relatively independent of its own rate of elimination and more dependent on the rate of turnover of receptor molecules. (Example: Phenoxybenzamine in pheochromocytoma) Mohamed Bahr; MD, PhD Mohamed Bahr; MD, PhD  Antagonists can function noncompetitively in a different way; that is, by binding to a site on the receptor protein separate from the agonist binding site; in this way, the drug can modify receptor activity without blocking agonist binding. Although these drugs act noncompetitively, their actions are often reversible. Such drugs are called negative allosteric modulators because they act through binding to a different (ie, “allosteric”) site on the receptor relative to the classical (ie, “orthosteric”) site bound by the agonist and reduce activity of the receptor. Not all allosteric modulators act as antagonists; some potentiate rather than reduce receptor activity. For example, benzodiazepines are considered positive allosteric modulators because they bind to an allosteric site on the ion channels activated by the neurotransmitter GABA and potentiate the net activating effect of GABA on channel conductance. Benzodiazepines have little activating effect on their own, and this property is one reason that benzodiazepines are relatively safe in overdose; even at high doses, their ability to increase ion conductance is limited by the release of endogenous neurotransmitter. Allosteric modulation can also occur at targets lacking a known orthosteric binding site. Mohamed Bahr; MD, PhD  A model of drug-receptor interaction. The hypothetical receptor is able to assume two conformations. In the Ri conformation, it is inactive and produces no effect, even when combined with a drug molecule. In the Ra conformation, the receptor can activate downstream mechanisms that produce a small observable effect, even in the absence of drug (constitutive activity). In the absence of drugs, the two isoforms are in equilibrium, and the Ri form is favored. Conventional full agonist drugs have a much higher affinity for the Ra conformation, and mass action thus favors the formation of the Ra–D complex with a much larger observed effect. Partial agonists have an intermediate affinity for both Ri and Ra forms. Conventional antagonists, according to this hypothesis, have equal affinity for both receptor forms and maintain the same level of constitutive activity. Inverse agonists, on the other hand, have a much higher affinity for the Ri form, reduce constitutive activity, and may produce a contrasting physiologic result. Mohamed Bahr; MD, PhD Mohamed Bahr; MD, PhD