Challenging Dogma - The Diverse Nature of Drugs and Their Receptors Flashcards
what are dirty drugs?
one drug that can bind to many targets or receptors in the body - tend to have a wide range of effects and drug reactions
what are examples of dirty drugs? (don’t need to know all details of examples)
Aspirin - would never be approved today.. (too dirty).
Inhibiting COX2 (wanted effect - pain relief) as well as other stuff e.g. inhibiting COX1
Highly selective COX-2- inhibitors were produced but there were problems because they were two selective
Cyclooxygenase‐2‐inhibitors -
problems due to their high selectivity ‐tipping the balance of pro‐and antithrombotic mediators in an unfavourable way
Steroid hormones - act by binding to intracellular “nuclear receptors”
but also activate membrane‐bound GPCRs!!! (very recent findings)
The effects of steroids often result from the interplay of the two mechanisms
Quetiapine – an antipsychotic - reacts with all below receptors.
Dosage can cause several different effects as though it is a different drug.
Dopamine D1-D5 receptor antagonist
Serotonin 5‐HT1A receptor partial agonist,
5‐HT2A, 5‐HT2B… receptor antagonist,
5‐HT1B, 5‐HT1D… receptor ligand
Most drugs affect cell function via receptors.
E.g. artificially made hormone molecule may directly be used as a drug e.g. thyroxine
what does two different doses of aspirin do?
75mg once a day low‐dose aspirinas = blood thinner
300mg or 500mg, only taken once = a pain killer
that means two different dosings of the same drug are actually equivalent to TWO different drugs
what are the 7 different drug receptors?
- Enzymes – e.g. aspirin inhibits COX 2 enzymes which catalyse synthesis prostaglandins (inflammatory regulator)
- Ion channels – i.e. Ca2+ channels blocked by nifedipine (used to treat high blood pressure)
- Transporters (pumps, transport proteins) – i.e. Noradrenaline transporter blocked by cocaine
- ‘Physiological receptors’ – receptors for hormones and neurotransmitters e.g. Acetyl choline, histamine, insulin
- Substrates: If asparagine is taken up by cancer cell it causes them to grow. However an enzyme can be given (asparaginase) which degrades asparagine which slows down the growth of the tumour
- DNA/RNA and ribosomes (human and bacterial ribosomes are different so easy to target them) – i.e. cisplatin, many antibiotics and DNA-ase (in Cystic Fibrosis)
- Targets of monoclonal antibodies – i.e. epidermal growth factor targeted by cetuximab slows down growth of vessels in a cancer cell
NB: Many drugs which we do not know the mechanisms of action
what is the nomenclature around the effect of drugs?
In pharmacology, distinct words are used for
drugs, depending on the effect they have on a certain receptor.
what are the ways of regulating cell function?
Below there are 3 different pathways that can be altered by drugs to cause a significant effect:
Altered membrane potential - change in activity (tetrodotoxin)
Enzyme activity - Aspirin
Gene expression - control proteins thus controlling nature of the cell (acridine dyes)
NB: these drugs affect these pathways directly rather than via receptors
define agonism
various way in which drugs produce their effects by binding to receptors.
define antagonism and desensitisation
various ways in which effects are blocked or reduced by drugs binding to receptors.
explain the patch clamp technique procedure
Measuring responses generated at receptors requires patch-clamp technique
1. Glass pipette filled with liquid and an electrode
2. Slightly suck up part of a cell ideally so you have one receptor
3. Apply another electrode to the rest of the cell so you have an electric circuit in which you can record electric current.
4. Response is now a quantifiable current
why are these results achieved from a patch clamp procedure?
The binding of an agonist opens the channel
Peaks show channels are open
Sometime doubling (at end) shows you may have two receptors in the clamp
Partial agonists are less good because they decrease frequency of opening.
what are the different types of agonists when bound to a ligand gated ion channel?
Full, high efficacy agonist - channel opens when agonist binds => receptors capable of max response
Partial, low efficacy agonist - channel doesn’t open as readily when bound by agonist => receptors not capable of max response
Neutral antagonist / no agonist - should be no response (although some spontaneous channel opening occurs)
Inverse agonist - binds as an agonist and not only antagonises the effects of an agonist but exerts the opposite effect - channels much less likely to open => spontaneous openings reduced
what is the high throughput model?
a method to screen thousands of drugs/ compounds in one go
explain the process of high throughput model?
- Unstable protein (PAPSS2) is cloned and fused to a green florescence protein
- Integrate PAPSS2-GFP into genome of human cell line
- Initially there is a low expression of green florescence due to unstable protein
- Add the GFP cell line and different drugs into each well.
- The drugs that stabilise the protein will show a green well as there is an increased expression of the protein as so the GFP.
what are allosteric effects of a drug on receptors?
substances that bind to a receptor to change that receptor’s response to stimulus
explain the allosteric effects of drugs on the GABA receptor
- Ligand-gated Cl- ion channels in the brain have different types of GABA receptors
- GABA binds to one of the GABA receptors which causes the opening of a Cl- ions channel.
- Cl- influx causes hyperpolarisation which inhibits cell function (prevents depolarisation).
- Benzodiazepine agonists bind to GABAA receptors (allosteric site) which increase affinity of GABA to its binding site via conformational change. This increases channel opening and so increases Cl– influx.
what is the glutamate gated chloride receptor and what is its agonist?
Common channels in the nervous system
Important target for anti-parasitic drugs
Pentamer - 5 subunits
works like an aperture - can only be opened or closed
Agonist: Glutamate
what are super agonists?
highly efficacious agonists
can produce a maximal response without being bound to all available receptors, so you get spare receptors
When agonist binds to spare receptors you get Emax of greater than 100%
Super agonists causes shift in response curve to the left - Emax for super-agonists > 100% of natural ligand
what is a partial agonist
Low efficacy, partial agonists cannot produce the cell’s maximal response, even when they have bound to all the available receptors.
what are competitive antagonists?
- bind reversibly (can be substituted) at same site as natural agonist
- produce parallel shift to right of the agonist dose-response curve - higher agonist conc. is needed in presence of competitive antagonist to produce the same response.
i.e. atropine at muscarinic receptors, propranolol at beta-adrenoceptors, & sildenafil at phosphodiesterase-5 (PDE5) competing w/ cGMP
what are irreversible antagonists?
give an example
- bind irreversibly at same site as agonist (covalent bond- cannot be removed)
- decrease max response to agonists => decreases no. of available receptors
- may produce an initial shift to the right of the dose/response curve with no decrease in max - evidence for spare receptors (still enough receptors to produce maximum response).
- receptors that are inactivated by irreversible antagonist get ingested & degraded by the cell
Example: Fulvestrant
used to treat breast cancer in postmenopausal women
selective oestrogen receptor degrader - binds to oestrogen receptor causing it to be ingested by cell and degraded
give an example of experimental proof of the two different classes of ACh receptors
Muscarinic acetylcholine receptor (mAChR) has higher affinity for ACh than Nicotinic acetylcholine receptor (nAChR)
Add ACh - binds to mAChR = BP decreases
Adding large amount of atropine inhibits mAChR - low conc. of ACh causes no BP decrease
A Higher conc. of ACh given => BP rises as ACh binds to nAChR
This is experimental proof of two different classes of acetylcholine(ACh) receptors: “muscarinic” and “nicotinic” ones. Only after the high‐affinity muscarinic receptors ones have been poisoned, the low‐affinity nicotinic ACh receptors become visible as they are no longer competing with the higher affinity mAChR.
what is the difference in structure between muscarinic and nicotinic ACh receptors?
Nicotinic ACh receptor structure – pentamers and have a massively large extracellular extension + membrane bed
Muscarinic ACh receptor structure = GPCRs - Has 7 TM helices with crystallised lipids within.
Completely different receptors, same molecular function
what are integral ion channels?
Integral ion channels e.g. nicotinic receptor glycine receptor
Mechanism: 2 agonists bind to receptors causing the channel pore to open allowing a specific ion (e.g. Cl-) to move through which will cause a biological change
what are integral tyrosine kinases?
Integral tyrosine kinases e.g. insulin receptor
Mechanism: Extracellular ligand binding will typically cause receptor dimerization. This allows a tyrosine in the cytoplasmic portion of each receptor monomer to be trans-phosphorylated by its partner receptor. This allows the propagation of a signal through the plasma membrane e.g. via activated proteins
what are cytokine receptors?
Cytokine Receptors: E.g. prolactin receptor, EPO receptor, growth hormone, TNF Receptor.
Mechanism: (Similar to Tyrosine Kinases) Cytokine receptors lacks catalytic activity (no physically bound enzyme) and hence depend on cytoplasmic tyrosine kinases known as Janus kinases (JAKs). Upon ligand binding, the cytokine receptors dimerises and allow the associated JAKs to cross-phosphorylate each other and activate. The activated JAKs then phosphorylate the cytokine receptors activating the receptor itself.
what are GPCRs?
GPCR e.g. muscarinic rec. adrenoreceptors
Mechanism: Previous Notes
several G subunits
- Gs →stimulating
- Gi →inhibiting (opposite of Gs)
what are nuclear receptors (steroid receptors)?
Nuclear Receptor (Steroid receptors) e.g. oestrogen receptor, androgen receptor:
1. Nuclear receptors are found within the cytoplasm. They have a ligand binding domain.
2. As long as a ligand binding domain does not have a ligand bound the receptor is unfolded.
3. This conformation makes it a substate to an enzyme (HSP) which holds it tightly trapping it in the cytoplasm.
4. Once the receptor is activated, by a ligand binding to it folds and takes it shape and the HSP protein gets off.
5. The Receptor then dimerises and enters the nucleus
6. It then binds to a HRE (hormone response element)
7. And then the DNA binding domain mediates the binding of the receptor to thousands of sites within the genome.
(Steroid Hormones also activate membrane-bound GPCRs)
