PAR Antagonists Flashcards
(12 cards)
PAR antagonists - why?
anticoagulants block thrombin production
- e.g. warfarin
- this prevents thrombosis
- but also blocks anti-inflammatory affects and normal wound healing
blocking PAR only prevents platelet activation
- as is further down in the coagulation cascade
- reduced side effects
PAR signalling
- hirudin-like binding site
- thrombin binds
- results in cleavage
- cleavage produces a tethered ligand
- ligand binds orthosteric site
PAR signalling - cleavage
- thrombin, Xa and plasmin cleave the same site
- MMP1 cleaves further away
- elastase and APC cleave closer
- different tethered ligands result in different intracellular signalling pathways
PAR signalling - tethered ligands and intracellular signalling
thrombin (canonical)
- Gq = calcium = PC = platelet activation
- G12/12 = RhoA = platelet activation
MMP1 = different tethered ligand
- Gq only weakly activated
- G12/13 = RhoA = weak platelet activation
- G12/13 = MAPKs = vascular smooth muscle differentiation
APC = different tethered ligand
- beta-arrestins = Rad and AKt
= anti-inflammatory protective response in endothelial cells
Human PARs
PAR1
- sub nM thrombin
- rapid
- transient
PAR4
- higher nM thrombin
- slower
- sustained
PAR1+4 both activated by thrombin
- at low thrombin, PAR1 is more important
- as stimulation levels increases, PAR4 becomes more important
- so need to block both for effective therapuetic!
Challenges to overcoming when targeting PAR
- tethered ligand has a steric advantage
- difficult for drug to compete
- thrombin is able to cleave many PAR1 (catalytic)
- drug needs to be able to overcome this
PAR1 antagonists
vorapaxar
- small molecule, orally active
- competitive inhibitor of PAR1
- reversible but very slow off rate
- originally developed as a muscarinic antagonist so now need to remove the effects of this
- slow off rate as the routes out of the binding pocket are all very small so drug moves around until in the correct orientation to get out (in silica and molecular dynamics studies)
- pseudo-irreversible inhibition
TRACER trials
- aspirin + clopidogrel vs these + vorapaxar
- no significant reduction in endpoint
- significant increase in major bleeds so trial ended early
- other trials showed decrease MI when used for secondary prevention
- FDA approved use of vorapaxar for prevention of MI but only in patients with no history of stroke or major bleeds
- withdrawn as not commercially viable
Bleeding risk e.g. vorapaxar
- vorapaxar does not increase bleeding time
- aspirin + vorapaxar = increased bleeding time
- vorapaxar + clopidogrel = increased bleeding time
- increased incidence of major bleeds in trials is likely due to the number of drugs being used, rather than vorapaxar itself being unsafe
- but vorapaxar cannot be trialled alone due to ethical considerations
PAR4 antagonists
- phase II clinical trials
- trials on non-human primates as cannot use mice as they don’t have PAR1
- BMS-986120 = dose-dependent decrease in MI
Variation in PAR4 signalling - smoking
- F2RL3 encodes PAR4
- methylation of promoter region dependent on smoking
- smoking decreases methylation
- increased gene expression
- smokers = increased PAR4 expression so PAR4 antagonists more likely to be beneficial
- but smokers have an increased risk of CV disease
Smoking, PAR4 methylation and CVD risk
a) smoking = reduced methylation = increased PAR4 = altered CV biology = increased CVD risk
b) smoking = altered CV biology = increased CVD risk and decreased methylation
c) smoking = decreased methylation
and smoking = altered CV biology = increased CVD risk
Variation in PAR4 signalling - ethnicity
- platelet activation via PAR4 greater in self-reported black volunteers
- different miRNA/mRNA profiles explain some of this variation
SNP in PAR4 broadly segregates with ethnicity
- GG (white) inhibited by antagonist
- AA (black) not inhibited
- SNP not in orthosteric site but still affect binding
- drug discovery needs to be carried out across ethnicities
- vorapaxar very effective in white people as PAR4 has a small effect so PAR1 greater
- but in black people, PAR1 has a smaller effect due to the influence of PAR4, so vorapaxar effect is reduced
