MedChem Exam 3

a. The potency is 10 nM

b. The potency is 10 micromolar

c. The potency is 10^-8 nanomolar

d. The potency is 10 mg/kg

a

Evolutionary context:

• _____________ → specialization

• Specialization requires _________

• ________ → coordination → ________

• Multicellularity → specialization

• specialization requires coordination

• Cell-cell communication → coordination → complexity

Mechanisms of cellular communication:

1. mechanical

2. local (diffusion) or global (circulation)

3. electrical signals

1. direct contact

2. chemical signals

a) endocrine signaling → long distances on the inside (exocrine → sweat and gut glands)

b) autocrine signaling → secretion of 1 cel acts on itself

c) paracrine signaling → affects nearby cell (synapse)

Derivatives of amino-acids – Epinephrine, DOPA

Derivatives of cholesterol – Steroid hormones

Derivatives of Lipids – Prostaglandins, retinoids

Nucleic acid derivatives – Nucleotides, nucleosides, ATP, ADP

Peptides – Insulin

Proteins – Growth hormone

Gases – NO

Ions – Ca2+

electrical signal

Na/K ATPase

action potential propagation

• The process whereby information presented extracellularly is translated into a meaningful intracellular form

• Can vary depending on nature and mechanism of receptor

signal transduction

• hydrophobic (steroids, retinoids, thyroxine)

• hydrophilic (small molecules, peptides, proteins)

transducer → G protein

amplifier → effector

Signal transduction can be linked in chains to further _______

amplification (2nd, 3rd, 4th messenger, etc.)

Epinephrine effect on gluconeogenesis is an example of what?

• Contributes to tissue responsiveness

  – and hence to coupling and receptor reserve

Signal amplification

The followings are examples of?

• Light detection in retina

• Receptor mediated smooth muscle contraction

signal amplification

• Light detection in retina → GPCR, large number of receptors are sensitive to 1 single photon

Signal Termination

Agonist metabolic degradation

– Example: Aetylcholine
• Degraded by __________

Acetylcholinesterase

Signal termination

Agonist reuptake
– Example: Norepinephrine

• Removed by _________

norepinephrine transporter (NET) - has both autocrine and paracrine signaling

Muscarinic or nicotinic cholinergic, α or β-adrenergic are what type of receptor classification?

Pharmacological

Receptor classification?

• Rhodopsin, muscarinic cholinergic, α- and β-adrenergic (7TM)

• Nicotinic cholinergic, glycine or GABA A receptors (Cys-Loop)

Structural (amino acid) homologies

Receptor classification?

• Proximal signal transduction mechanism

• Ionotropic, metabotropic

Functional responses

Code	Receptor Structural classes
1.0
2.0
3.0	_______ associated receptors – subunits with 1 TM span
4.0

1. Ion channels and ligand-gated channels

2. 7 transmembrane; GPCR

3. Enzyme

4. Transcription factor receptors; Nuclear receptors

1. nicotinic acetylcholine receptors

2. GABA A receptors

3. ionotropic glutamate receptors

1. Adrenergic receptors

2. muscarinic cholinergic

1. insulin receptor

2. EGF receptor

1. Steroid hormone receptors

2. Thyroid hormone receptor

3. Cholesterol and bile acid receptors

1. ligand-gated ion channels (ionotropic receptors) → fastest

2. GPCR (metabotropics)

3. kinase-linked receptors

4. Nuclear receptors

ALL can change gene expression

which receptors change existing components of cell?

1. ligand-gated ion channels (ionotropic receptors)

   1. nicotinic ACh receptor

2. GPCR (metabotropics)

   1. muscarinic ACh receptor

Which receptors change the composition of the cell (cell needs to grow)?

1. Kinase-linked receptors (cytokine)

2. nuclear receptors (oestrogen receptor)

Example circuitry for which ligand?

ACh

Example circuitry for?

NE (multiple GPCR but depends on receptor → diff response)

• Activation of GPCRs can result in activation of the MAP kinase pathway

• Activation of LGICs can result in activation of the MAP kinase pathway

crosstalk (Signaling pathways interconnect)

Phosphorylation circuits

• 2nd messengers

• kinases

• phosphatases

Intracellular _____ is a 2nd messenger signaling by amplitude and frequency (levels oscillate)

calcium

hormones signaling → frequency (can occur at cellular or organ level)

1. Many cell-surface initiated signal transduction pathways culminate in changes in ________

2. slower _____

3. ______ term changes

1. gene expression

2. slower kinetics

3. long

Signal transduction is _________

Prolonged receptor activation leads to reduction in responsiveness

self-limiting

• Multiple terms:

  • Adaptation

  • Desensitization

  • Tachyphylaxis

  • Fade

  • Tolerance

Rank the fastest to slowest desensitization

Different time frames

• Fast ( ms to seconds) → Uncoupling, conf. change

• Medium (minutes/hours) → Internalization, sequestration

• Slow (hours/days) → Down-regulation

Reduce receptors = DEC response

LGIC mechanisms for desensitization(2)?

1. conformational change

2. localization (move out of nucleus)

GPCR mechanisms for desensitization(3)?

1. post-translational modifications

2. changes in protein complexes

3. localization

Nuclear receptors mechanism for desensitization?

localization

homologous desensitization

ligand gated ion channels

4 TM domains

2 orthosteric (ACh) binding sites

Receptors subunit composition

• Dictates?

• Is different?

Dictates: Permeability and sensitivity to toxins

Different: in specific sites (Brain vs muscle)

shuffle subunit to change composition

• Binding of Ach causes rotation in the extracellular region

• Transmitted to TM domains

• Splaying or bloom of TMs causes opening

Twist and Bloom of the receptor’s subunit

heterologous desensitization (negative feedback)

RTK mechanisms for desensitization (2)?

1. Post-translational modifications

2. changes in protein complexes

Example circuitry?

Same receptor can modulate multiple effectors

Example circuitry?

Effectors can be different in different cells

What receptor does both paracrine and autocrine signaling for acetylcholine?

nicotinic ACh receptor

equipotent

Drug X is more efficacious than drug Y

Reasoning: The maximal response to X is higher than that of Y

True

Under conditions of high receptor reserve, the effects of a negative allosteric modulator that reduces orthosteric agonist intrinsic efficacy are surmountable

true

Under conditions of low receptor reserve, the effects of a negative allosteric modulator that reduces orthosteric agonist intrinsic efficacy are surmountable

– True/False/Cannot be determined

False

A neutral antagonist can antagonize the effects of an inverse agonist.

– True/false/Cannot be determined

True

Can a neutral antagonist have clinical efficacy? In two sentences or less explain why.

Yes. A neutral antagonist can have clinical efficacy by competing for an endogenous ligand at a specific receptor if such competitive antagonism has therapeutic value. For example, beta adrenergic receptor neutral antagonists are useful to reduce heart rate.

Two drugs that bind to the same receptor reduce blood pressure In hypertensive patients. Drug A has a Kd of 10 nM and drug B has a Kd of 300 nM. Based on this information, select the best description:

a) A has a lower affinity than B

b) B has lower potency than A

c) A has a higher affinity than B

d) B has lower efficacy than A

e) None of the above can be concluded with certainty based on the available data.

a) F

b) Cannot be determined

c) T

d) Cannot be determined

• Since the drugs are tested in the same system, the number of receptors and tissue responsiveness is the SAME for both ligands.

• We only know the affinity of the new ligand. No information about its intrinsic efficacy is provided or can be deduced from the data given.

• If the new compound is an agonist with intrinsic efficacy equal to that of Y, the maximal response will be the same as Y and the statement would be false.

• If the new compound is an antagonist or inverse agonist the maximal response would be lower than that of Y. Under these conditions the statement would be true.

• Therefore, the correct answer is Cannot be determined.

clinical efficacy

1 mg/kg

Ability to generate a desired response in a patient

– Lower blood pressure

– Promote growth

clinical efficacy

– Same target mediates the desired and undesired effects

– Receptors in different organs

– Excessive effects

ON target side effects

safe and effective dose

Margin of safety is lower than therapeutic index

Yes (even when the MS is low), if that’s the only drug you have

Precautions with low therapeutic index

Toxic effects are OFF target

– Improve selectivity of the drug for the desired target

• Higher potency for target

• Lower affinity for target of toxicity (β2 vs β1 selective agonist for asthma)

Toxic effects are ON target

• If responses are due to different effectors, could consider__________

• If therapeutic vs toxic responses are in different tissues, can consider specific ___________ or _________ to access the therapeutic but not undesirable tissue

• biased agonist (SERMs)

• routes of administration or differential distribution

Lower variability = INC MS

• Drugs can have effects at multiple targets

• Drugs are selective not absolutely specific

• Side effects may be due to unintentionally engaging a different target

Off target side effects

• Allosteric sites are less conserved, so can provide receptor subtype specificity

• Modulators enhance/inhibit only the actions of __________ signaling molecules

  • Spatial specificity

  • Temporal specificity

endogenous

Effects of negative modulators are_________

– Increased safety

saturable

Effects of negative modulators that affect intrinsic efficacy can be __________ by an orthosteric agonist

– Can be clinically advantageous

insurmountable

Stimulus is proportional to _________

fractional occupancy

What parameter describes the amount of stimulus generated per occupied receptor?

1. intrinsic efficacy (e) - property of complex

2. receptor numbers (R_tot) - property of system

What parameter describes the amount of stimulus required to generate a response to a particular system?

Sensitivity (S₅₀) - property of system

Which parameter describes a particular drug-receptor interaction?

affinity (Kd) - property of complex

______ is determined by ALL 4 parameters (R_tot, e, S₅₀, Kd)

potency (EC₅₀) - property of receptor and system

𝑺_𝟓𝟎 is the stimulus required to generate a

response of 50% of the system maximal

capacity

Sensitivity

Lower S₅₀ = more sensitive system

The higher the _________ or _______, the stronger the stimulus at any given fractional occupancy

1. intrinsic efficacy

2. receptor number

As intrinsic efficacy (e) increases?

maximum response increases

potency increases

As affinity (Kd) increases?

maximum unchanged

potency increases

As sensitivity increases?

maximum response increases

potency increases

When EC₅₀ < Kd, low fractional occupancy can generate a significant responsen

receptor reserve

Normal maximal response can be achieved by higher concentrations of agonist.

surmountable antagonist

B is a partial agonist

As concentration of partial agonist B increases,

– The agonist A is displaced by B

• RA is reduced and RB increased

– Response is inhibited

• But only to the level that the partial agonist generates on its own

• Both liganded forms contribute to the response

Competitive Antagonism

As concentration of antagonist increases

• Agonist apparent affinity ______

• Potency is ______

• Can be surmounted by more _____

1. agonist Kd decreases

2. EC₅₀ reduced

3. agonist

Receptor in the absence of ligand can have spontaneous activity

– Usually very low
– Receptor alone has small but non-zero intrinsic efficacy

Constitutive activity

Consitutive activity can generate detectable response if?

High receptor expression

high tissue responsiveness

A. Full agonist

B. Partial agonist

C. Neutral antagonist

D. Inverse agonist

A functional antagonist can elicit a response through a different receptor/mechanism that counteracts the effect of the agonist.

• An agonist for a Gi coupled receptor can inhibit Adenylyl cyclase and antagonize the effect of an agonist for a Gs coupled receptor

• A vasodilator agent can antagonize the effects of a vasoconstricting agent through very different mechanisms

Physiologic Antagonism

• Agents that bind to endogenous ligands or other drugs and block their effects

• Frequent mechanism for Biologics

  • Specific Monoclonal antibodies

  • Denosumab (Osteoporosis)

    • Binds and neutralizes RANKL

  • Ustekinumab (Psoriasis)

    • Neutralizes IL-12 and IL-23

• Complexing agents – Sugammadex

• Binds and neutralizes neuromuscular blocking agents and promotes excretion

• Used for reversal after surgical procedures

antagonism by neutralization

Inactivate target
– Covalent interaction

• Phenoxybenzamine
– α adrenergic antagonist

• Omeprazole
– Proton pump inhibitor

Irreversible Orthosteric Antagonists

If RB has lower __________than that of the free receptor

inverse agonist; intrinsic efficacy

• As concentration of antagonist B increases,

  – The receptor is occupied by B

• If coupling by RB is exactly the same as that of the free receptor,

  – The basal response is unaffected

neutral antagonist

Each species can:

– Have own intrinsic efficacy (ε) 

– Bind orthosteric ligands with a different affinity (Kd)

Usually have little or no effect on their own

allosteric modulators

Enhance
– The affinity of orthosteric agonists and/or

– The intrinsic efficacy of the agonist bound complex

Positive allosteric modulators

Alter affinity and coupling in opposite directions

– Increase in affinity but decrease intrinsic efficacy (vice versa)

Mixed allosteric modulators

Reduce
– The affinity of orthosteric agonists and/or

– The intrinsic efficacy of the agonist bound complex

Negative allosteric modulators

As concentration of antagonist B increases,

– The receptor is occupied by B

If RB has lower intrinsic efficacy than that of the free receptor,

– The response is reduced lower than the basal

inverse agonist

B is a competitive antagonism (surmountable)

As the concentration of antagonist B increases,

– The agonist A is displaced by B

– Since RB does not generate stimulus

– Response is inhibited

Agonist A and antagonist B compete for the same site

• Fractional occupancy by A is affected by B (and vice versa)

Competitive Reversible Antagonism

Depression of the maximal response of the agonist and a non-parallel displacement of the dose/response

Insurmountable antagonist

Maximal response depends on?

intrinsic efficacy (e)

receptor number (R_tot)

sensitivity (S₅₀) to stimulus

Receptor reserve can happen when?

high intrinsic efficacy (e)

high receptor number (R_tot)

high sensitivity to stimulus