04a - Drug Receptor

receptor

class of cellular macromolecule in chemical signaling between and within cells, where a drug binds and produces a measurable response'

• determine quantitative relationship between drug concentration and pharmacological effect

• important properties include affinity, receptor number, ligand specificity, signal transduction capacity

  • ligands should have selectivity to a receptor, and a receptor should have ligand specificity to elicit action

chemistry of drug-receptor interactions

drug-receptor binding occurs through chemical forces

• irreversible interactions → covalent bonds; less common

  • undesired unless intentionally used

  • ex: aspirin, anti-tumor drug

• reversible interactions → weak chemical bonds; most common

two-state receptor theory

receptors can exist in two states:

• inactive → R; active → R*

• D + R = D-R* → drug response

• drug shifts receptor towards active or inactive states

affinity vs efficacy

• affinity → ability of a drug to bind a receptor and form D-R complex

• efficacy / intrinsic activity → ability of the drug to trigger a response after binding

agonist vs antagonist

• agonist → binds a receptor and produces a biological response

  • has affinity and efficacy

• antagonist → binds receptor but does not activate it

  • has affinity but no intrinsic activity

  • no effect in absence of an agonist, but can reduce effect of agonist when agonist is present

dose-response relationship

dose-response curve shows how drug effect changes as drug concentration increases

• linear scale → curve is hyperbolic

• semilog scale → curve is sigmoidal

  • spreads out lower concentrations where response changes rapidly and compresses higher concentrations where response changes slowly

  • easier to identify EC₅₀ (concentration that produces 50% of maximum response)

    • lower EC₅₀ = higher potency

ligand-gated ion channel

opens when ligand binds to allow ions to flow across the membrane

• synaptic transmitters → Ach (nicotinic), GABA, excitatory amino acids (glutamate, aspartate)

• ligand binding to receptor causes conformational change that opens the channel to allow ion flow, leading to change in postsynaptic potential and subsequent cellular response

• different ion concentration change leads to different effects

  • increase in Na and Ca influx → excitatory; decrease in Na and Ca → inhibitory

  • increase in K → inhibitory; decrease in K → excitatory

  • increase in Cl → inhibitory; decrease in Cl → excitatory

• important for neural transmitter signaling due to rapid signaling for moment-to-moment transfer of information across synapses

  • generation and propagation of nerve impulses

  • synaptic transmission of neurons

  • muscle contraction

  • hormone release

nicotinic acetylcholine receptor

ligand-gated ion channel that is pentameric and contains 2 alpha subunits, 1 beta subunit, 1 gamma subunit, and 1 delta subunit

1. ligand binds receptor at alpha subunits

2. receptor undergoes conformational change to open channel

3. ions flow through into the cells

4. localized change in synaptic potential

5. subsequent cellular response occurs

clinical drugs targeting ligand-gated ion channels

• nicotine → agonist at nicotinic Ach receptor

• d-Tubocurarine → antagonist at muscle nicotinic receptor; muscle relaxant

• benzodiazepines → potentiate GABA signaling

• muscle relaxants → act on ion channels

voltage-gated ion channel

ion channel that does not need neurotransmitter binding but are controlled by membrane potential

• ex: lidocaine blocks voltage-gated sodium channels

  • results in blockade of neuronal depolarization and local anesthesia

G-protein couple receptors (GPCR)

consists of cell surface receptor, G protein, and effector element

• surface receptor → 7 transmembrane domains, COOH inside, NH₂ outside; conserved

• G protein → sits on cytoplasmic side of membrane

  • Gs (histamine; stimulatory) and Gi (opioids; inhibitory) on adenyl cyclase for cAMP

  • Gq (muscarinic Ach) on phospholipase C to increase IP₃ and DAG for increasing cytoplasmic Ca²⁺

  • cycle between GDP-bound (inactive) and GTP-bound (active)

• effector element → adenyl cyclase, phospholipase C, guanylyl cyclase, ion channels

GPCR mechanism of activation

1. ligand binds GPCR and causes conformational change

2. GPCR binds inactive G protein

3. GDP released from alpha subunit, and GTP binds

4. alpha-GTP separates from beta-gamma subunits

   1. alpha-GTP is the main signal transducer that interacts with downstream effectors

5. alpha-GTP activates effector protein

6. effector protein changes second messenger production

7. GTP hydrolyzed back to GDP once ligand is removed

8. signal turns off

clinical drugs targeting GPCR

• antagonists:

  • prazosin → selective alpha-1 adrenoblocker for hypertension, anxiety, and PTSD

  • atropine → muscarinic receptor blocker for anti-cholinesterase poisoning

  • phentolamine / OraVerse → non-selective alpha blocker for soft tissue anesthesia reversal

• agonists:

  • opiods (oxycodone, methadone, codeine, heroin, morphine) → on mu-opioid receptor to increase endorphins and provide analgesic effects

spare receptors

spare receptors exist when a maximal response can occur without occupying all receptors, occurring because signaling pathways amplify the signal

• small fraction of receptors can produce a large response

• separation between recognition (ligand binding receptor) and signal transduction/amplification (intracellular response)

GPCR regulation

repeated receptor stimulation can reduce response

• desensitization/resensitization → short exposure of seconds to minutes; reversible

• downregulation → longer exposure of hours/days; less reversible due to receptor degradation

  • tachyphylaxis → rapidly diminishing response to repeated drug dose

mechanism of GPCR desensitization

1. agonist binds GPCR

2. receptor COOH-terminal tail changes conformation

3. receptor binds to Gs and activates GRK

4. GRK phosphorylates serine or threonine residues in COOH

5. phosphorylation increases β-arrestin binding

6. β-arrestin-receptor complex promotes internalization into coated pits

7. receptor can be dephosphorylated and recycled to membrane

   1. desensitization → response can return

8. OR receptor can be sent to lysozome for degradation

   1. downregulation → receptor number decreases

clinical relevance of GPCR regulation

• β-adrenergic receptors → downregulated in chronic heart failure due to excessive stimulation

  • β-blcokers reverse maladaptive signaling and reduce cardiac remodeling

• leuprolide → superactive agonist at GnRH receptors

  • desensitizes/downregulates GnRH receptors with continuous stimulation

  • suppresses testosterone production; used in testosterone-dependent prostate cancer

ligand-regulated transmembrane receptors

tyrosine kinases (insulin, EGF, VEGF, PDGF receptors), serine/threonine kinases (TGF-β), guanylate cyclases (ANP receptors)

1. ligand binds receptor

2. receptor undergoes conformational change

3. receptors dimerize and become active

4. kinase domains autophosphorylate or cross-phosphorylate

5. phosphorylated receptor activates downstream proteins

6. signal amplification occurs, even in absence of ligand

7. receptor endocytosis regulates intensity and duration

drugs targeting tyrosine kinase receptors

• agonists → activates receptors

• monoclonal antibodies (-mab) → blocks receptor or traps ligand

• decoy receptors → soluble receptor domain that traps ligand

• small molecule tyrosine kinase inhibitor (-inib) → blocks tyrosine kinase active site

cytokine receptors: JAK-STAT

cytokine receptors do not have intrinsic kinase activity, but rather associate with separate kinases called JAKs

1. ligand binds cytokine receptor

2. receptors dimerize

3. covalently-bound JAKs activate

4. JAKs phosphorylate tyrosine residues on receptor

5. STAT proteins bing phosphorylated tyrosines

6. JAKs phosphorylate STATs

7. phosphorylated STATs dimerize by attaching at tyrosine phosphates

8. STAT dimer leaves receptor and translocates to nucleus to act as transcriptional factor

9. STAT regulates gene transcription

clinically relevant drugs targeting cytokine receptors

• GH → dwarfism

• GM-CSF → neutropenia, bone marrow transplantation

• erythropoietin → anemia

• interferon-⍺ → cancer and hepatitis

• IL-2 → thrombocytopenia

intracellular receptors

bind lipid-soluble ligands that can cross the cell membrane including corticosteriods, mineralocorticoids, sex steroids, vitamin D, thyroid hormone, and retinoic acid

• some receptors are cytoplasmic then move to the nucleus (corticosteroid and mineralocorticoid receptors)

• some receptors are already nuclear (estrogen and other nuclear hormone receptors)

• responsive elements consist of ligand-binding domain, DNA binding domain, and 2 transcription activating domains

intracellular receptor mechanism

inactive receptor is bound to inhibitory proteins such as HSP90 or other inhibitory proteins

1. lipid-soluble ligand diffuses into cell and binds intracellular receptor

2. HSP90 / inhibitory proteins dissociate

3. receptor changes conformation to expose DNA binding domains

4. two receptor-ligand complexes dimerize

5. complex binds response elements on DNA

6. gene transcription changes and new proteins are made

7. cellular effect occurs

therapeutic implications of hormone regulated gene expression

• effects have a lag period of 30 minutes to several hours

  • due to protein synthesis taking time

• effects may persist for hours, days, weeks, or longer if ligand is gone

  • due to low turnover of proteins

  • proteins produced by gene activation may last a long time

• plasma drug concentration does not have a simple relationship with effect

intracellular receptor drug targets

• agonist:

  • triamcinolone → glucocorticoid receptor agonist for anti-inflammatory therapy to relieve discomfort and redness of the mouth

• antagonist:

  • mifepristone → blocks progesterone and glucocorticoid receptors

    • used with misoprostol to induce abortion

cAMP signaling

cAMP formed from ATP by adenyl cyclase, which is activated by Gs

1. ligand activates GPCR

2. Gs activates adenyl cyclase

3. adenyl cyclase converts ATP to cAMP

4. cAMP activates protein kinase A (PKA)

   1. tetrameric kinase with two regulatory R and two catalytic C subunits

   2. cAMP binds R dimer and releases active C chains

5. PKA phosphorylates target proteins

   1. diffuse through cytoplasm and nucleus to transfer phosphate to substrate proteins

6. cellular response occurs

7. action of cAMP stopped via degradation by phosphodiesterase (PDE)

pharmacological intervention of cAMP pathway

milrinone → type 3 PDE inhibitor for heart failure

cGMP signaling

cGMP is produced by guanylyl cyclase to activate cGMP-dependent protein kinases; important in intestinal mucosa and vascular smooth muscle

1. acetylcholine stimulates endothelial cells

2. endothelial nitric oxide synthase makes NO from arginine

3. NO diffuses to neighboring vascular smooth muscle cell

4. NO activates guanylyl cyclase to increase cGMP

5. cGMP causes myosin light chain dephosphorylation

6. vascular smooth muscle relaxes

7. blood vessel dilates

vasodilatory drugs

• nitroglycerin, sodium nitroprusside → generates/mimics NO; cardiac ischemia and acute hypertension

• sildenafil → blocks PDE to inhibit cGMP breakdown; erectile dysfunction and pulmonary hypertension

Ca²⁺ and phosphoinositide signaling

PLC breaks down PIP₂ into IP₃ and DAG

• IP₃ → water-soluble and diffuses through cytoplasm

  • binds receptors on endoplasmic reticulum and causes Ca²⁺ release from storage

• DAG → stay in membrane and activates protein kinase C (PKC)

• Ca²⁺ → binds calmodulin to form complex that activates calcium/calmodulin-dependent kinases

• pathway can be turned off / deactivated:

  • dephosphorylation of IP₃

  • phosphorylation of DAG

  • deacetylation to produce arachidonic acid

  • removal of Ca²⁺ by Ca²⁺ pumps

• EXAMPLE: lithium ion treats bipolar disorder

second messenger interplay

• opposing:

  • vasopressor drugs increase Ca²⁺ → smooth muscle contraction

  • drugs increase cAMP → smooth muscle relaxation

• complementing:

  • both pathways can contribute to glucose release

• reversible phosphorylation allows amplification and flexible regulation