Pharmacology: Receptors & Receptor Theory

Basic Principles of Pharmacology: Receptors

• Regulatory Proteins: Mediate neurotransmitter/hormone action like steroid and acetylcholine receptors
• Enzymes: Dihydrofolate reductase (for methotrexate, an anticancer drug), and trimethoprim (antibiotic) can act as receptors for drugs
• Membrane Transport Proteins: Control ion movement like Na+/K+ ATPase
• Structural Proteins: Tubulin (for colchicine), vinca alkaloids, and paclitaxel (anticancer drug) provide cellular framework
• Membrane Lipids: Amphotericin B targets these
• Nucleic Acids: DNA/RNA can act as receptors (e.g., doxorubicin, cyclophosphamide)
• Orphan Receptors: Identified from the human genome but lack a known endogenous ligand

Receptor Theory

• Neurotransmitter/Hormone: Binds to the receptor to produce a response
• Agonist: Facilitates the pharmacological response
• Antagonist: Blocks receptor and inhibits the response
• Receptor Reserve: Maximal response occurs even with low agonist binding (<1%) due to spare receptors, enhancing reserve capacity
• Spare in number: Full response occurs even when not all receptors are occupied
• Temporal Spareness: Response lasts longer than receptor binding due to downstream signaling

Graded Dose-Response Relationship

• Drug response is plotted as percentage of maximal response versus log dose
• ED50 (Effective Dose 50%): The dose required to achieve 50% of the maximal response
• Shifts in curves indicate potency and efficacy differences among drugs

Quantal Dose-Response Relationship

• Describes the cumulative percentage of subjects showing an all-or-none effect, plotted against the log dose
• Therapeutic index (TI): The ratio between the median lethal dose (LD50) and the median effective dose (ED50)
• Certain Safety Factor (CSF) / Margin of Safety (MOS): The ratio between the dose that is lethal in 1% of subjects (LD1) and the dose that produces a therapeutic effect in 99% of subjects (ED99)

Dose-Response Curve/Concentration-Effect Curve

• Shows the relationship between drug concentration (C) and drug effect (E) or receptor-bound drug (B)
• EC50: Drug concentration that produces 50% of the maximum effect (Emax)
• Kd (dissociation constant): Drug concentration where 50% of receptors are occupied (Bmax)
• Lower EC50 or Kd = Higher potency

Drug-Receptor Interaction

• Drug ([D]) binds to receptor ([R]) to form [D-R] complex, leading to an effect
• Kd (Dissociation Constant): Ratio of k2 (dissociation rate) to k1 (association rate); measures drug affinity
• Lower Kd = Higher affinity; the drug binds more tightly to receptor
• Highly selective drugs have low Kd values

Drug-Receptor Interaction: Agonists

• Activates receptor, mimicking the natural ligand
• Full Agonist: Produces maximal response like the natural ligand
• Partial Agonist: Has reduced efficacy at the receptor relative to the natural ligand; binding (affinity) is not necessarily reduced
• Inverse Agonist (negative antagonist): Decreases the rate of signal transduction upon binding; the signal transduction proceeds at a basal rate in the absence of any ligand binding to the receptor

Partial Agonists

• Full receptor occupancy by a partial agonist results in lower maximal response than a full agonist
• Increasing partial agonist concentration reduces the response of a full agonist, competing for receptor binding
• As more receptors bind to the partial agonist, the overall response decreases to its maximum potential

Drug-Receptor Interaction: Antagonists

• Prevents agonist action
  • Competitive antagonist (Reversible): Competes with agonist; higher agonist concentration needed to overcome inhibition, causing a rightward shift in the dose-response curve
  • Noncompetitive antagonist (Irreversible): Binds permanently, reducing maximal agonist effect without necessarily competing for binding site
• Curve A represents agonist response without antagonist
• Curve B has a low antagonist concentration, shifts the curve right, and maintains maximal response because available receptors are still more than the number required
• Curve C has higher antagonist concentration, no spare receptors, but mediates an undiminished maximal response
• Curves D & E have higher antagonist concentration, reducing the number of available receptors

Other Mechanisms of Drug Antagonism

• Chemical Antagonism: One drug binds to another, preventing it from binding to its target (receptor)
• Example: Protamine (+ve charge) binds to heparin (-ve charge), neutralizing its anticoagulant effect
• Physiological Antagonism: Two drugs act on different receptors but have opposing physiological effects (one increases while the other reduces)
• Example 1: Glucocorticoids increase blood sugar, while insulin lowers it.
• Example 2: Histamine stimulates acid secretion, while omeprazole inhibits the proton pump, reducing acid production

Signaling Mechanisms & Drug Action

• Receptor binding followed by activation of intracellular molecular mechanisms (transmembrane signaling/signal transduction)
• Most mechanisms generate and amplify signals by using chemical second messengers

Transmembrane Signaling Mechanisms

• Lipid-soluble drugs cross membranes and bind to intracellular receptors (nuclear receptor type 1/type 2)
• Signal binds to the extracellular domain of a transmembrane enzyme, activating an enzymatic activity of its cytoplasmic domain
• Signal binds to the extracellular domain of a transmembrane receptor, activating a separate protein tyrosine kinase
• Signal binds to regulate ion channel opening
• Signal binds to a cell-surface receptor linked Transmembrane signaling via a G protein

Intracellular Receptor Signaling (Nuclear Receptors)

• hsp90 stabilizes the glucocorticoid receptor
• Steroid binding releases hsp90, exposing the DNA-binding and transcription-activating domains
• Activated receptor alters gene transcription, leading to a biological response
• Lag period before response.
• Long-lasting effect even after agonist removal due to slow enzyme/protein turnover

Type I Nuclear Receptors

• Ligands: Sex hormones (androgen, estrogen, progesterone), glucocorticoids, mineralocorticoids receptors Mechanism: Bound to heat-shock proteins (hsp) in the cytosol
• On activation, the hsp dissociates and two steroid-receptors proteins dimerize and translocate to the nucleus

Type II Nuclear Receptors

• Ligands: Nonsteroids (thyroid hormone, vitamins)

Transmembrane Enzymes (receptor)

• Examples: Insulin, EGF, PDGF, TGF-β, ANP. Structure:
  • Extracellular ligand-binding domain
  • Cytoplasmic enzyme domain linked by a hydrophobic region
• Upon binding of a ligand, 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 amino acid residues
• EGF (Epidermal Growth Factor)
• PDGF (Platelet-Derived Growth Factor)
• TGF-β (Transforming Growth Factor Beta)
• ANP (Atrial Natriuretic Peptide)
• Five types
  • Receptor guanylate cyclase, e.g., ANP binds with the receptor and activates the guanylate cyclase (no dimerization), cGMP activates cGMP-dependent kinases
  • Receptor tyrosine kinase, e.g., EGF, insulin
  • Tyrosine kinase-associated receptor
  • Receptor tyrosine phosphatase
  • Receptor serine/threonine kinase

Cytokine Receptors

• Similar to transmembrane enzymes, they have extracellular and intracellular domains and form dimers
• After activation by an appropriate ligand, separate mobile protein tyrosine kinase molecules 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

Ligand-Gated Ion Channels

• Examples: GABA, ACh, serotonin, glutamate
• Structure: Tetrameric/Pentameric proteins regulating ion flow across the membrane
• Function: Fast synaptic transmission via depolarization/hyperpolarization
• Drug binding at an orthosteric or allosteric site causes a conformational change
• Nicotinic ACh receptor: 5 subunits opens central transmembrane ion char Na+

Glutamate Receptors

• NMDA Receptors
  • Bind glutamate, glycine, Mg2+, Zn2+,polyamines
  • Structure: 7 subunits
  • Function: Forms ion channels highly permeable
• AMPA & Kainate Receptors
  • Bind glutamate & specific agonists
  • Function: Channels permeable to Na+ & K+
• Metabotropic Glutamate Receptors (mGluRs): G-protein coupled receptors triggering second messenger cascades
  • Found in pre- & post-synaptic neurons and microglia

G Protein Coupled Receptors (GPCR)

• Structure: 4 extracellular, 7 transmembrane, and 4 intracellular domains
• N-terminal (extracellular) Ligand binding & selectivity
• C-terminal (intracellular) Interacts with G proteins
• G proteins have 3 subunits: Ga, Gß, and Gy
• Ga subunit: site of guanosine-5'-triphosphate (GTP) hydrolysis
• Gß and Gy subunits are tightly bound together. Separation from Ga on ligand-receptor activation alters K+ or Ca2+ channel conductance

G Proteins and Their Receptors and Effectors

• Gs: Receptors for β-Adrenergic amines, glucagon, histamine, serotonin, hormones. ↑Adenylyl cyclase → ↑ cAMP
• Gi1, Gi2, Gi3: Receptors for α2-Adrenergic amines, acetylcholine (muscarinic), opioids, serotonin. ↓Adenylyl cyclase → ↓ cAMP, Open cardiac K+ channels → ↓ heart rate
• Gq: Receptors for Acetylcholine (muscarinic), α₁-Adrenergic amines, bombesin, serotonin. ↑ Phospholipase C → ↑ IP3, diacylglycerol, cytoplasmic Ca2+

Processes of Absorption (Permeation)

• Lipid diffusion is the most important limiting factor for drug permeation.

Some Transport Molecules Important in Pharmacology

• Norepinephrine transporter (NET): Norepinephrine reuptake from synapse; Target of cocaine and some tricyclic antidepressants
• Serotonin reuptake transporter (SERT): Serotonin reuptake from synapse; Target of selective serotonin reuptake inhibitors and some tricyclic antidepressants
• Multidrug resistance protein-1 (MDR1): Transport of many xenobiotics out of cells; Increased expression confers resistance to certain anticancer drugs; inhibition increases blood levels of digoxin

Pharmacokinetic Principles

• the area of pharmacology that focuses on how drugs move within the body
• Absorption, distribution, metabolism and excretion are the four processes

Absorption

• The process of a drug moving from the site of administration into the bloodstream Process:
  • Disintegrate drug
  • Drug dissolves into fluid
  • Drug passes into the blood stream
• Flux (molecules/unit time) = (C1-C2) x (area x permeability coefficient / thickness)

Distribution

• After absorption into the blood, drugs are distributed via the circulatory system
• Some of the drug binds to circulating plasma proteins
• Large molecules have indentations in their molecular surface which permit drug molecules to bind to them
• This makes them pharmacologically inactive as they are carried through the blood circulation
• Other portions that do not bind to plasma proteins, moves through circulatory system, passes through walls of capillaries into body tissues
• As this portion leaves the blood, some of the bound drug is released by the plasma proteins so as to maintain the equilibrium of unbound drug in the blood Drugs that move into body tissues contact with a cell membrane to exerts an effect by interacting with a receptor

Factors affecting distribution

• Organ Blood Flow: Higher part rapidly in brain, heart, liver and kidney with the lower part slowly in skeletal muscle, skin, bone and adipose tissue
• Plasma Protein Binding: Depends on the drug's affinity for protein-binding sites
• Molecular Size: Larger molecules, like heparin, stay mostly in the plasma
• Lipid Solubility: Affects how easily drugs pass through cell membranes
• Blood brain barrier (BBB): Limits drug distribution to the brain. Exists between the capillary walls of blood vessels in the brain and the surrounding brain tissues
• Blood placental barrier - Protects the fetus, allowing only certain substances to pass.

Models of drug distribution and elimination

(a) No elimination: The drug is placed in the body but doesn't leave. The drug level rises quickly to its highest peak, then levels off at a plateau (b) With elimination: The drug enters the body, and there is a way for it to be removed. The drug rises quickly to its peak, then decreases over time (c) Two compartments (blood and extravascular): The drug quickly spreads between the blood and second compartment (d) Realistic model (elimination + distribution): The drug quickly spreads between blood and the body, with elimination happening simultaneously. Consists of a phase of rapid distribution, followed by a phase of slower elimination

Metabolism

• Mainly happens in the liver and is the total of all chemical reactions in the body that change substances, like food and drugs, into a different form
• This is the chemical transformation of drugs into new forms making the drug more or less active
• Prodrugs are metabolized into an active form once in the body
• Example 1: Dipivefrin eye drop used for glaucoma
• Example 2: Enalapril used for high blood pressure

Excretion

• Removes drugs from the body
• Main routes
  • Renal (urine)
  • Biliary (feces)
• Minor routes
  • Pulmonary (lung)
  • Salivary (saliva)
  • Dermal (sweat)
  • Mammary (breast milk)
  • Tears

Renal mechanisms in drug excretion

• Active tubular secretion
  • Transporter:organic anion transporter
  • Organic cation transporter
• Passive tubular reabsorption
  • Modulated by urine pH
  • Modulated by drug pKa

Excretion Summary

• Weak acids are excreted faster in alkaline (basic) urine and weak bases are excreted faster in acidic urine
• Ionized drugs are trapped in the urine, so they can't be reabsorbed and are more easily excreted.
• Biliary Excretion: MW > 300 and both polar & lipophilic groups, especially conjugated metabolites After bile empties into intestine, bacteria can break down conjugated drugs into free drugs, which can be reabsorbed into circulation and return to the liver (enterohepatic cycling)

Biotransformation

• Phase I: Converts the parent drug into a more polar metabolite by adding or unmasking functional groups The metabolites are often inactive
• Phase II: The Phase 1 metabolites often undergo a conjugation reaction with substances like glucuronic acid, forming highly polar conjugate that results in the drug being easier to eliminate

Phase I Reactions

• Drug-metabolizing enzymes are found in the smooth endoplasmic reticulum of the liver and other tissues forming microsomes
• Cytochrome P450 (CYP) is a hemoprotein found in microsomal Rifampin
• Inducers increase concentration of some enzyme, while inhibitors decrease the concentration of some enzyme

Phase 1 metabolism-oxidation reaction

• Drug (RH) enters the cycle – the substrate that needs modification
• Cytochrome P450 (CYP) receives electrons (e¯) from NADPH-P450 reductase
• Oxygen (O2) is added to the drug, making it more polar
• The result is ROH (hydroxylated drug), which is now ready for Phase II metabolism or excretion

Enzyme Induction

• increase the production of P450 enzymes, speeding up their own metabolism. Benzo[a]pyrene is an example
• Induction of drug metabolism by nuclear receptor-mediated signal transduction: Atorvastatin, increases metabolism of atorvastatin meaning it is less effective in the body
• P450 enzymes = reduces the amount of time the drug stays in the body meaning there are less side effects

Enzyme Inhibition

• Imidazole-containing drugs bind to the heme iron of cytochrome P450, slowing down metabolism. Cimetidine and ketoconazole are examples
• Macrolide antibiotics produce metabolites that bind to cytochrome P450, inactivating it and stopping drug metabolism resulting in more side effects for the drugs in the body

Phase II Reactions (transferases)

• Conjugation involves high-energy intermediates and specific transferase enzymes.
• Phase I metabolites or parent drugs undergo conjugation with endogenous substances to form drug conjugates.
• Phase II reactions produce conjugates which are more polar and can be easily excreted
• Phase II reactions include Glucuronidation, Glycine conjugation, Sulfation, Acetylation and Methylation Since endogenous substrates come from the diet, nutrition is important in regulating drug conjugation

Metabolism to Toxic Products

• Too much acetaminophen is used which cause glucuronidation and sulfation pathways to get saturated. This lead to the cytochrome P450 pathway becoming active
• Pathway produces a toxic metabolite liver damage. Administering N-acetylcysteine within 8-10 hours can prevent liver failure and death after an acetaminophen overdose

Clinical Relevance of Drug Metabolism

• Individual differences of treatment due to differences in drug distribution, metabolism, and elimination
• Influenced by genetics and non-genetic factors like age, sex, liver function and nutrition
• Exposure to substances that induce or inhibit drug metabolism also plays a role

Pharmacokinetics

• The study of what the body does to a drug Includes:
• Volume of distribution
• Clearance
• Half life
• Bioavailability
• Thearapeutic range

Volume of distribution

The volume of fluid need for a drug to have the same conc as in the plasma It is the space in the body where the drug can be distributed

Clearance

The volume of plasma cleared of the drug per unit of time. It measures the bodily function to eliminate a drug

Half life

The time it takes for the amount of drug in the body to decrease by half during elimination

Bioavailability

The fraction of the unchanged drug that reaches the systemic circulation after administration

Thearapeutic range

The plasma conc of the drug that is high enough for thearapeutic affects but low enough to avoid toxicity

CYP2D6 Genetic Polymorphism

• CYP2D6 is a type of enzyme that helps break down the drug debrisoquine, and there are difference versions of this depending on how fast or slow the processing
• Metabolizers: -Poor metabolizers (PM, red bars) – slow -Extensive metabolizers (EM, blue bars)– normal -Ultrarapid metabolizers (URM, green bars)- very fast
• It is important for doctors so they can adjust dosages so its safe

Volume of Distribution

• Describes how a drug spreads in the body and tells us if a drug stays in the blood or moves into tissues Key Concepts:
• Not a real physical volume
• If high = Drug is mostly in tissues
• If low = Drug stays mainly in blood
• Week bases has a large volume
• Must be easily ionized meaning it is highly unlikely to be stored in the blood

2.Clearance (CL)

• Represents how efficiently organs (like kidneys and liver) remove the drug from plasma = 200 ml/min Higher CL = Faster drug removal. Lower CL = Slower drug removal

Renal Excretion (Penicillin G)

• GFR (Glomerular Filtration Rate) = 100 mL/min, Total drug excretion rate = 1200 mg/min Reabsorption is minimal and excretions rate is higher

Types of Renal Clearance

• Filtrations equals glomerular filtration rate
• Filtration equals renal plasma clearance

Drug Elimination

• Removal of the drug from the body mainly by the kidneys but also can be renal or due to urine

First-order kinetics

• Happens through drug is proportional to the drugs concentration, which is why is declines overtime

Zero order kinetics

• Drug is being eliminated at a fixed rate

Flow-Dependent Elimination

• Some drugs are eliminated so quickly that their removal mainly depends on how much blood flows to the organ responsible for elimination (usually the liver)
• These drugs are called "high-extraction" drugs because a large portion is removed from the blood on the first pass through the liver

Half-life

• The time is takes for the drug concentration to reduce in the body
• High half-life = the drug is not being eliminated properly

Drug Accumulation

• If the drug is given to regular if don’t have time to clear and there will be an accumulation and building up in the body
• Higher the steady state concentration

Drug Accumulation: Maintenance Dose Rate

• Keeping Drug Levels Stable Maintenance Required= Required plasma concentration X Clearances

Drug Accumulation: Rapid Effect

• Large initial dose to desired level instead with filling 4-5 half life
• To figure out dosage you will need required plasma concentration and distributions

Bioavailability

• Is the % of the area that the drug can reach

Bioavailability: First-Pass Effect

• This is when the drug is partially cut down in the body but will increase the % of bioavailability
• It is known when the guts and the liver can reduce the bioavailability and make a lot the

8.0 Pharmacodynamics

• Factors affecting drug effectiveness are: Drug Concentration & Dose, Dosing frequency and Food-Drug interactions
• Genetics, excretion rate, half-life, and medical conditions also affect this
• People who are more dangerous for certain drug interactions are those with a low index.

Tachyphylaxis

-Repeated exposure to agonists in order to trigger desensitization.

• GPCR phosphorylation causes phosphorylation which in turn causes down regulation
• Repeated exposure to antagonies will cause supersentitivy which leads to an increase of receptors