how drugs act Flashcards
Superfamilies of Receptors
Ligand-Gated Ion Channels
— “ionotropic” receptors
* G-Protein Coupled Receptors
— “metabotropic” receptors, “7 trans-membrane spanning domain” receptors, “heptahelical” receptors, “serpentine” receptors
* Kinase-Linked & Related Receptors
— large and heterogeneous group
— single trans-membrane spanning domain
* Nuclear Receptors
— “steroid superfamily”
- Ligand-Gated Ion Channels structure
G-Protein Coupled Receptors structure
Kinase-Linked & Related Receptors structure
Nuclear Receptors structure
Receptor Subtypes
- receptors in given family generally occur in several molecular varieties or “subtypes”
— similar architecture
— significant differences in amino acid sequence
— often different pharmacologic properties
— nicotinic acetylcholine receptor subtypes occur in different brain regions and these differ from subtype in muscle
- different genes, different phenotypes of receptor subtypes
— different genes may encode for different subtypes
- same gene, different phenotypes of receptors subtypes
how is this possible?
big role in which class of receptors?
— variation may arise from alternative mRNA splicing
* single gene can give rise to more than one receptor isoform
* splicing can result in inclusion or deletion of one or more mRNA coding regions giving rise to short or long forms of protein
* big role in G-protein coupled receptors
— single nucleotide polymorphisms
* often results in different drug-receptor efficacy
Ligand-Gated Ion Channels
share features with what other structures?
additional name?
- share structural features with voltage-gated ion channels
- “ionotropic” receptors
Ligand-Gated Ion Channels examples
— nicotinic acetylcholine receptor (nAChR)
— GABA type A receptor (GABAA): inhibitory neurotransmitter
— glutamate receptors: excitatory neurotransmitter
nAChR; structure/mechanism
— best characterized of all cell-surface receptors
— pentamer: four different polypeptide subunits (5 total)
* 2 a, 1 b, 1 g, and 1 d, MW from 43K-50K each
* each subunit crosses plasma membrane 4 times
— acetylcholine binds sites on a subunits
— conformational change occurs
— transient opening of central aqueous channel
— Na+ flow from outside to inside cell: down electro-chemical gradient
— cell depolarizes
— all occurs in milliseconds
G-Protein Coupled Receptors
- largest superfamily of receptors
- “metabotropic” receptors
- 7 transmembrane spanning domains
G-Protein Coupled Receptors mechanism
- agonist binds to region inside receptor surrounded by 7 TM domains
- conformational change in cytoplasmic side
— spreads cytoplasmic side of 7 TM domains
— opens cavity in receptors cytoplasmic side
— cavity binds critical regulator surface of the G-Protein - G-Protein affinity for nucleotide GDP is reduced
— GDP dissociates - GTP binds
— GTP normally higher in concentration than GDP intracellularly - GTP-bound G-Protein dissociates from the receptor
- GTP-bound G-Protein engages downstream mediators (a.k.a. “effectors”)
G-Protein Coupled Receptors agonsit dissociation and g pro actvity?
dif effects in various tissues?
- agonist binds and dissociates rapidly
— a few milliseconds - activated GTP-bound G-proteins remain active much longer
— up to tens of seconds
— this produces significant signal amplification from one ligand-receptor interaction - heterogeneity of G-proteins allow for substantial diversity in GPCR signaling in various tissues
Opioid Pharmacology
* Agonists of Opioid Receptors
— heroin, morphine, oxycodone,
hydrocodone
Competitive Antagonists of Opioid
Receptors
— naloxone, naltrexone
Opioid m type receptors (mOR) strucutre
Opioid m type receptors (mOR) morphine effects
common side effect of this in GI
K+ out (beta/gamma) and Ca2+ channel(alpha) antagonized=hyperpolarize
can cause constipation in GI
naloxone effect
comp antagonist of morphine= binds same site, can be overwhelmed with increased morphine
naloxone and morphine together and effect on respiratory rate, why useful?
can be used with suspected opiod OD
- Protease Activated Receptors (PAR)
subclass of? how do they differ?
actions that occur for activation
G-protein coupled receptor subclass
— activation of GPCR is normally a result of diffusible ligand in solution acting on a receptor
— but GPCR receptor activation can occur as a result of protease activation
* protease cleaves off part of N-terminal domain of receptor
* “tethered agonist”: remaining attached domain is free to interact with ligand-binding domain
Protease Activated Receptors (PAR) mechanism
pacman cleaves and new domain formed activates receptor
G-Protein Coupled Receptors
* Desensitization
— applicable to all GPCRs
— occurs via 2 main mechanisms:
* receptor phosphorylation
* receptor internalization
less receptors will req more drug for effect
G-Protein Coupled Receptors
* Further Intricacies
* receptor subtypes
* cross-talk
* more to be learned
* new drug targets
- receptor subtypes
» one gene can give rise to multiple subtypes of receptors through alternative mRNA splicing - single nucleotide polymorphisms
» one amino acid change can result in different phenotypes of receptor - cross-talk and collaboration between two different GPCRs or GPCR with receptor tyrosine kinase (RTK)
- much, much more to be learned
- many new drug targets
Kinase-Linked & Related Receptors
— involved mainly in what cell actions?
— act indirect/direct?
— signal transduction generally involves?
— all have what domains?
— involved mainly in events controlling cell growth and differentiation
— act indirectly by regulating gene transcription
— signal transduction generally involves dimerization of two receptor molecules followed by autophosphorylation of tyrosine residues
— all have large extracellular ligand-binding domain connected via single membrane spanning domain to an intracellular domain which has enzymatic activity
Kinase-Linked & Related Receptors
* three major families
— Receptor Tyrosine Kinases (RTKs)
— Serine/Threonine Kinases
— Cytokine Receptors
- Receptor Tyrosine Kinases (RTKs)
examples?
— intracellular domain has tyrosine kinase activity to phosphorylate proteins at Tyr
— e.g. EGF receptor, NGF, TLRs, insulin receptors
— insulin receptor
RTK example
* activates PI3 kinase pathway
» turns on or off gene expression
» activates glycogen synthesis
* activates MAP Kinase pathway
» turns on or off gene expression
- Serine/Threonine Kinases
group size?
structure/function?
example ligand?
— smaller group than RTKs
— structurally and functionally very similar to RTKs
— phosphorylate serine and threonine
— e.g. transforming growth factor (TGF)
- Cytokine Receptors
ligands?
lack what?
mechanism?
— interleukins, interferons, chemokines, etc
— lack intrinsic enzymatic activity in intracellular domains!! (NO INTRINSIC KINASE)
— associate and activate other kinases
* binds and activates Janus Kinase (Jak)
* Jak binds and activates Signal Transducers and Activators of Transcription
» a.k.a. the “Jak-STAT” pathway
* downstream turns on or off gene expression
- Nuclear Receptors (“Steroid Superfamily”)
— ligand-activated transcription factors
— ligand examples:
* estrogens, progestins, androgens, glucocorticoids, mineralocorticoids, vitamin D, vitamin A (retinoid receptors), fatty acids, etc.
— two main locations in the cell
* cytoplasmic
* nuclear
— ligand-binding and DNA-binding domains
Nuclear Receptor mechanism
cyto and nuclear receptors
— cytoplasmic mechanism
* most are bound to Heat Shock Proteins when no ligand is present
* most form homodimers upon ligand binding (e.g. steroid receptors)
* some form heterodimers with Retinoid X Receptor (e.g. thyroid hormone)
* translocate to nucleus to regulate gene expression
— nuclear mechanism (e.g. fatty acid receptors)
* constitutively present in nucleus
* form heterodimers with Retinoid X Receptor (RXR)
— both interact with hormone response elements on genes to regulate gene expression
androgen receptor mechanism
- androgen will cross cell membrane (lipophilic), bound to SHBG in blood
- bind to receptor, displace HSP
- 5a-R will convert androgen to DHT, DHT binds receptors with greater affinity
- Dna binding domain translocates to nuc and bind to response elements, turning on gene transcription (recruit RNA polymerases)
Enzymes as Drug Targets
what are the best ones to target?
most common strategy?
- many enzymes serve as drug targets
— enzymes that are key rate-limiting steps in biochemical reactions are the best drug targets
— strategy is most often to reduce enzyme activity through drug inhibition
non-competitive enzyme inhibitors
examples?
— drug may covalently modify enzyme
* aspirin acetylates Cyclooxygenase 1 and 2 to non-competitively and irreversibly inhibit
competitive enzyme inhibitors
— drug is often a structural analog of the naturally occurring substrate
— example: HMG-CoA Reductase Inhibitors
- HMG-CoA Reductase Inhibitors
how they work?
examples?
— competitively inhibit rate-limiting step in cholesterol biosynthesis in liver
* liver upregulates LDL receptors thereby reducing plasma LDL concentrations
— structurally similar to HMG-coenzyme A
— Statins:
* lovastatin, atorvastatin, fluvastatin, pitavastatin, pravastatin, rosuvastatin, simvastatin
lovastatin mech
prevents melvalonate formation to reduce plasma cholesterol by inhibiting HMG-CoA reductase
Infliximab and adalimumab
— monoclonal antibodies that recognizes TNF-a
— bind TNF-a removing it from circulation
anti-inflammatory
TNF-a can be useful for inflammation and immune response so adverse effects include sus to M. tuberculosis
Disease and/or Symptoms Produced by Elevated TNF-A
lowering this?
drugs to do so?
- Tumor Necrosis Factor-a
— elevated in rheumatoid arthritis, Crohn’s disease, psoriasis, ankylosing spondylitis
— elevated in severe cases of aphthous ulcers - Lowering TNF-a in RA or Crohn’s patients decreases symptoms and may delay progression
- Infliximab and adalimumab
Ion and Small Molecule Transporters
- important in moving substances across lipid bilayer membranes
- often good drug targets as they regulate key cellular events
Carrier Molecules (Transporters) drug targets
Ion and Small Molecule Transporters
Small Molecule Transporters examples
- neurotransmitter uptake (norepinephrine, 5-HT, glutamate, etc.)
- organic ion transporters (organic acids and bases)
- p-glycoprotein (Multi-Drug Resistance)
- p-glycoprotein (Multi-Drug Resistance)
role?
pharm implication?
— protective role in moving potential toxicants out of gastrointestinal epithelial cells back into lumen to prevent absorption
— overexpressed in certain tumor cells leading to drug resistance
— can be blocked by drugs
* could increase absorption of some drugs
* could potentially increase activity of anti-cancer drugs
Ion Transporters, important where? why?
- many transporters important in renal tubules
— drives water reabsorption and concentration of urine
Na+/K+ ATPase
important where?
purpose?
requires?
drug to inhibit for HF tx?
- Na+/K+ ATPase important everywhere
— establishes electrochemical gradient by moving Na+ out and K+ in against concentration gradient
— requires energy (ATP) to function
— key in all muscle contraction, nervous conduction, ion gradient establishment, etc
— often provides the driving force for other ion transporters
— can be inhibited by drugs (e.g. digoxin, a cardiac glycoside used for heart failure)
Voltage-Gated Ion Channels
structure?
types?
- structure— very similar in structure and function
to ligand-gated ion channel receptors - Ca++ channels (L, T, N types)
- Na+ channels (fast and slow types)
- K+ channels (voltage- and ligand-gated types)
— produce at least 9 different K+ currents in heart, vascular smooth muscle, and other tissues such as pancreas
Voltage-Gated Ion Channels
* voltage-dependent
can these be bound?
voltage-dependent— channels open or close depending upon the electrical gradient (voltage) across the plasma membrane
* resting membrane potential ~ -90 mV
* depolarized membrane potential ~ 0 mV
— channels change opened/closed or activated/resting states as electrical potential changes from -90 mV to +10 mV (inside relative to outside)
— channels often susceptible to binding by various compounds, including xenobiotics
- verapamil
effects?
— Ca++ Channel Blocker
* binds to L-type Ca++ channels in heart and vascular smooth muscle
* blocks movement of Ca++ from outside to inside
* reduces cardiac contraction (negative inotropic effect)
* slows cardiac conduction (negative chronotropic effect)
* reduces vascular smooth muscle contraction
* reduces blood pressure
Excitation-Contraction Coupling
Mechanism of Action
— contraction of cardiac muscle
* depolarization of the membrane via Na leads to calcium influx through L-type calcium channels
* results in an increase in intracellular calcium
* calcium stimulates further calcium release from the sarcoplasmic reticulum (CICR) to further increase intracellular calcium via binding ca gated ca channel (RyR = ryanodine receptor)
* contractility increases with the increased availability of calcium for contraction
* repolarization via K channels opening and Na/K pump and SERCA moving intracell Ca back into SR
- Cardiac Muscle
— calcium channel blockers, name?
VERMAPRIL
* Ltype calcium channel blockers reduce calcium influx into
the cardiac muscle
* thereby reduce intracellular calcium and force of
contraction
» negative inotropic effect
* through similar blockade of calcium channels in pace-
maker (SA node), AV node, and Purkinje fibers in the
heart, calcium channel blockers reduce depolarization
and slow conduction of depolarizing waves through
the heart
» negative chronotropic effect
