Inflammatory MedChem Flashcards
What do antimetabolites do and how?
Molecules that disrupt the metabolic function in the body:
- affect biochemical synthesis of functional molecules
- affect biochemical metabolism and recycling
Describe the functional groups in methotrexate which is essential for function:
NH2 at top of double aromatic group- stronger binding as increases H and ionic bonding so increase in potency
Middle tertiary amide- increase potency and decrease in side effects
Describe the role of folic acid:
Involved in the denovo synthesis of purines and pyrimidines therefore in DNA and RNA synthesis
Series of enzymatic steps lead to N5N10-methylenetetrahydrofolic acid, a CH2 donor
Main enzyme used is dyhydrofolate reductase
Describe the steps into how folic acid changes to its wanted product:
Folic acid-> dihydrofolate-> tetrahydrofolate-> N5N10-methylenetetrahydrofolic acid
Last step via a pyroxidal enzyme (serine into glycine)
How is N5N10 involved in the synthesis of purines?
It needs to be metabolised to N10 formyltetrahydrofolic acid (aldehyde) as it is an important precursor in the formation of the purine ring
This also affects guanine and adenosine synthesis
How is N5N10 involved in the synthesis of pyrimidines?
The conversion of uridylic acid to thymidine via thymidylate synthase
dUMP (Deoxyuridine monophosphate) to dTMP (deoxythymidine monophosphate )
How does methotrexate work?
It is a competitive reversible inhibitor of dihdrofolate reductase, so decreases formation of N5N10
The effect is larger on pyrimidine synthesis than purine
It has a stronger binding affinity (1nm) than folic acid but still reversible
Describe the metabolism of methotrexate:
Metabolised in vivo (like folic acid) to the polyglutamate which increases its size and shape and traps it inside the cells and therefore prolonges action
What is the rescue therapy for methotrexate and how?
Leucovorin- as methotrexate is toxic it acts an alternative CH2 donor molecule
Name other dihydrofolate reductase inhibitors and how do they work?
Trimethoprim-binds to bacterial dihydrofolate reductase
Co-trimoxazole (trimethoprim and sulfamethoxazole)- attacks synthesis pathway in two places
Pyrimethamine (antimalerial)-exploits difference between A/S of dihydrofolate reductase in mammals and plasmodium
What is mercaptopurine?
A prodrug of azathioprine
Mercaptopurine was readily metabolised by xanthine oxidase
What are two inactive metabolites of azathioprine and how are these produced?
Azathioprine is activated by glutathione conjugation to give mercaptopurine
Thiouric acid via xanthine oxidase metabolism
Methyl mercaptopurine via Thiopurine S-methyl transferase introducing a methyl group
Describe a pharmacogenomic effect which can cause azathioprine to become toxic:
In the Thiopurine S-methyl transferase pathway (TPMT), some people can have less of this enzyme so there is more levels of azathioprine
Describe the MoA of how azathioprine is converted to mercaptopurine:
Glutathione can attack the double bond on the imidazole ring
This is facilitated by having an electron withdrawing group present on the ring, so is electron deficient, so the carbon is more ready to react with the nucleophile
The electron is passed to the N group and back again
Describe the purine salvage pathway:
Mercaptopurine undergoes further metabolism and is the major pathway as an immune suppressant
Mercaptopurine acts as a substrate for Hypoxanthine-guanine phosphoribosyltransferase (HPRT) which converts mercaptopurine to Thionosine monophosphate (TIMP) to TGMP (guanine) and the DNA unit TdGTP
So acts as a substrate for DNA polymerase and incorporated into DNA/RNA
Why does azathioprine take a while to work?
The cell must replicate in order to see an effect
Describe de-novo purine synthesis:
Mercaptopurine undergoes further metabolism by inhibiting new purine synthesis (minor)
TPMT can metabolise TIMP to MeTIMP, which inhibits an early step in purine de novo synthesis, the synthesis of the ribose unit
What is the basic explanation of azathioprine MoA?
Affects DNA activity due to an incorporation of a false nucleotide
How does allopurinol work?
Inhibitor of xanthine oxidase as affects the biosynthesis of uric acid
It has 15-30x affinity of xanthine oxidase than xanthine
Means there is a build up of hypoxanthine and xanthine but these are harmless and readily excreted
Describe the structure of allopurinol:
Isomer of a purine (hypoxanthine)
Five membered ring
N positioned differently
Describe how uric acid is formed:
From the breakdown of adenine and guanine
Adenine (adenine deaminase) into hypoxanthine (xanthine oxidase) into xanthine
Guanine (guanine deaminase) into xanthine
Xanthine (xanthine oxidase) uric acid
How does uric acid cause gout?
Uric acid is only just soluble in the body, so when there is an excess, it precipitates out and causes pain
Could be caused by excess alcohol intake e.g beer or purine rich foods
Describe the metabolism of allopurinol:
Metabolised to oxypurinol and has a longer half life and is still active- most effect comes from oxypurinol
When can allopurinol be given?
Can’t be started until the patient is asymptomatic
How does Febuxostat work?
Another xanthine oxidase inhibitor
Binds in channel leading to active site and blocks entry of substrate
Binds to both oxidised and reduced form of enzyme so increased affinity and selectivity
More effective than allopurinol
What is Leflunamide used for?
RA
What does Leflunamide target?
Effects de novo synthesis of pyrimidines so lymphocytes can’t be replicated quickly as needed for inflammation
Also have activity against TKs and COX when used at high levels
What is the MoA of Leflunamide?
Blocks the 4th step in de novo prymidine synthesis
Reversible inhibitor of dihydroorotate dehydrogenase (rate limiting step)
Describe prostaglandins:
20 fatty acid (eicosanoids)
General name PGX(y)
X= depends on substituent at 9 and 11
Y= number of double bonds, 1,2 or 3
What is another name for COX?
Prostaglandin synthase
Where are prostaglandins located?
A membrane embedded protein, has a hydrophobic channel (dimer) into A/S so arachidonic acid can directly pass in without touching the membrane and can bind close to A/S, allows release of PGH2
What is archiadonic acid metabolised to?
Cox enzymes into PGG2 which is rapidly degraded to PGH2
Describe the steps for the mechanism for the conversion of arachidonic acid to PGG2:
COX primarily acts as endoperoxide synthase:
Facilitates removal of H from arachidonic acid, stereoselective, forms a radical through resonance
Then reacts with a molecule of oxygen
Radical reacts again through addition of a double bond on alpha chain
Cyclisiation reaction occurring on a and B chain to form a 5 membered ring
Introduction of a second molecule of oxygen
COX secondarily acts as a peroxidase enzyme
Where does aspirin work?
Serine reside 529, which isn’t the A/S or the binding site of archiadonic acid
Describe why COX 1 and 2 has high homology:
Both have:
-hydrophobic binding channel
-catalytic site
-acylation site
-arginine for binding of COOH
-same biological activity but COX 2 less selective
What are the differences between COX 1 and 2?
Isoleucine residues in COX 1 at 434 and 523 altered to smaller valine
Allows access to an additional binding pocket in COX 2 so selectivity
What are the 2 roles of PGG2 in the synthetic pathway?
Acts as an endoperoxide synthase (intro of oxygen and cyclisation)
Peroxidase activity (conversion of peroxide to alcohol = reduction)
Describe the MoA of aspirin:
Carboxylate anion on aspirin increases nucleopholicity, then transesterification of covalent bond with ser529
Try 385 also forms a H bond with acetyl portion of aspirin, so position is in correct place for reaction with serine, Try 348 helps with this
What class of NSAID is aspirin?
Salicylates
How does aspirin work against COX?
It leaves the serine acylated so blocks access of arachiodonic acid to A/S
Can only be overcome by synthesis of new COX enzymes as irreversible
Name the different classes of competitive non-selective NSAIDs:
*Arylalkanoic acids
- N-arylanthranilic acids
- Enolic acids
Describe the importance of the acidic group in competitive non-selective NSAIDs:
Carboxylic acid, enrol, hydroxamic acid or masked acid that can be metabolised
Mimics acid of arachidonic acid and binds to arginine at 120
Also allows accumulation in sites of inflammation
Describe the importance of the one carbon spacer in competitive non-selective NSAIDs:
One carbon spacer between acid group and flat surface
2 or 3 reduces activity
Can be substituted but only by a methyl (branched)
Describe the importance of a flat surface in competitive non-selective NSAIDs:
Can be aromatic or heteroaromatic
Believed to correspond with double bonds at position 5 and 8 of arachidonic acid
Describe the importance of a lipophilic area in competitive non-selective NSAIDs:
Non co planar with aromatic ring
Can be aromatic or aliphatic and can be fused
Believed to correspond with double bond at position 11 of arachidonic acid
How does Flurbiprofen work?
At Arginine 120 there is an ionic interaction which competes for the carboxylic binding site with the arachidonic acid and blocks arachidonic acid from entering/ binding
Name 5 examples of arylalkanoic acids:
Ibuprofen
Diclofenac
Naproxen
Flurbiprofen
Indometacin
What is the subclass of indometacin and solindac?
Aryl and heteroaryl acetic acids
Describe the structure and mechanism of diclofenac:
More active than indomethacin
Chloro substituents restrict rotation and ensure the two lipophilic areas are non-coplanar
MoA is also inhibition of lipoxgenase therefore decreases production of leukotrienes
What is the subclass of the ‘profens’?
Aryl and heteroaryl propionic acids
Describe the structure of Ibuprofen:
Methyl substituent on the one carbon spacer, makes the molecule chiral
Marketed as the racemate
S form is the active form but the R form undergoes some in vivo conversion to the S form
Name an example of N-arylanthranillic acids:
Mefanamic acid (‘fenamic acids’)
Name 2 examples of Enolic acids and which one has better COX 2 selectivity?
‘Oxicams’:
Piroxicam
Meloxicam- better COX 2 selectivity
What is the difference between Enolic acids and other NSAIDS?
They are non-COOH derivative NSAIDs
How does Piroxicam work?
Similar activity to indomethacin as an anti-inflammatory but decreased analgesic affect
What is the MoA of Enolic acids?
Adopt a conformation similar to the proxy radical precursor of PGG2
Describe the actions of Coxibs on each of the COX enzymes:
Inhibit COX 2 rather than COX 1
Competitive mechanism and reversible for COX 1
Irreversible for COX 2
50 fold higher selectivity for COX 2
Name 3 examples of Coxibs:
Celecoxib
Parecoxib
Etoricoxib
Describe why you would need to administer a higher dose of Celocoxib?
Less selective for COX 2 but nm activity between anti-inflammatory effects and gastro SEs
How does Celocoxib interact with COX 1?
No ionic interaction
Bulky molecule blocks entry of arachidonic acid
How does Celocoxib interact with COX 2?
No ionic interaction
There is a hydrophilic side pocket which the sulphonamide group has H bonding in
So no entry of arachidonic acid
Can’t be overcome by increasing concentration of arachidonic acid as they do not compete for the same site
Describe nicotinic agonists for MG:
The enchantment of Ach
Not therapeutically used due to low activity
Intro of a methyl group into Ach to the a position results in a selective nicotinic agonist
Describe the steps in the mechanism for the hydrolysis of Ach:
- Nucleophilic Ser can attack delta positive base of carbonyl forming a charged tetrahedral intermediate
2+3. The histidine facilitates the movement of a proton from Ser to Ach to provide a good leaving group within the Ach - Electrons then fed back into oxygen and ester of Ach is broken
- This destroys Ach but also inactivates enzyme as Ser has become acylated, need to regenerate the enzyme
Describe the steps in the mechanism for the regeneration of AchE:
- Achieved by water, nucleophilic attack on the carbonyl
2+3. Histidine facilitates the movement of a proton from the water molecule to the Ser residue to make it the best leaving group - Ester breaks
- Ser nucleophile reformed so can now degrade another Ach
What should the properties of a good anticholinesterse drug have?
Contain a leaving group which is equal in efficiency of dissociation to the acetyl group in Ach
Leaving group should produce a residual group bound to sereine which is less susceptible to hydrolysis (must be slower to degrade)
Contain a +ve charge motif to fix the molecule in the correct orientation in the A/S= carbamate
Why is a carbamate used as a anticholineasterase?
Resonance stabilisation
Carbonyl less electrophilic
Regeneration by nucleophile (water) more difficult but still possible so reversible
What are the good properties of pyridostigmine?
Charged N for enzyme binding
Carbamate for enzyme inactivation
Aromatic group for additional enzyme binding interactions
What affect do irreversible anti cholinesterase inhibitors have?
Death through asphyxia as muscles cant be controlled
Forms a covalent bond, a phosphate ester is formed rather than an ester which is more resistant to hydrolysis
Name other carbamates and their effects and why:
Miotine- crosses BBB as only charged at physiological pH
Neostigmine- protonated originally so difficult to cross BBB
Pyridostigmine- protonated originally so difficult to cross the BBB so reduced CNS effects
What occurs when water enters the irreversible anticholinesterase?
Two options:
Hydrolysis of bond to enzyme
‘aging’ reaction- hydrolysis of other phosphorous oxygen bond, which strengthens the bond to the enzyme making it almost impossible to regenerate
What needs to be administered to a patient and when if they have been poisoned by an irreversible anti cholinesterase?
Pralidoxime (charged oxime)
Reversibility only possible prior to ‘ageing’ which can take places in minutes
Most effective if given within a few hours of exposure
Acts faster than water
