MED CHEM QUIZ 8 💊

Resting Platelets

• Act as vascular sentries monitoring endothelial integrity

• In absence of injury → circulate freely without interaction

• Endothelium prevents contact with vessel wall substances that cause activation

• Injury exposes subendothelial collagen → platelets adhere & activate

Step 1: Platelet Contact with Damaged Endothelium

• Unactivated platelet contacts exposed collagen

• Mediated by:

  • Gp Ib/IX/V complex binding to von Willebrand factor (vWF)

  • vWF tethers platelet to collagen

  • Additional receptors: Gp VI and Integrin α2β1 (Gp Ia/IIa)

Step 2: Platelet Activation

• Binding to collagen → activation → morphological change (spiky pseudopods)

• Activated platelets secrete granular contents via exocytosis:

  • ADP

  • Thromboxane A₂ (TxA₂)

  • Serotonin

  • Platelet-activating factor

  • Thrombin

Step 2: Propagation of Platelet Activation

• Released mediators (ADP, TxA₂, etc.) bind to receptors on nearby platelets
  → ↑ intracellular Ca²⁺, ↓ cAMP
  → activates more platelets

• Final Common Pathway: Activation of Gp IIb/IIIa receptors, allowing fibrinogen binding

Step 3: Platelet Aggregation

• Fibrinogen binds to Gp IIb/IIIa receptors on multiple platelets → cross-linking → aggregation

• Creates initial hemostatic plug → trapped by fibrin

• Amplification loop: each activated platelet activates more

Major Classes of Antiplatelet Agents

COX-1 Inhibitors

Inhibit TxA₂ Biosynthesis

• Platelet activation → phospholipase activation → releases arachidonic acid (AA)

• COX-1 converts AA → PGH₂, → TxA₂

• TxA₂ promotes more platelet activation

• COX-1 inhibitors (e.g. Aspirin) → inhibit TxA₂ → stop activation

• COX-1 = constitutive; COX-2 = inducible

Aspirin MOA

• MoA: Irreversible COX-1 inhibitor → ↓ TxA₂ synthesis and acetylates Ser529 on COX-1.

• Platelets can’t resynthesize COX-1 → effect lasts 10 days

• Unique: Only NSAID with antithrombotic efficacy (others reversible)are

____ is the only NSAID that has antithrombotic efficacy

Aspirin

Aspirin 

• MOA: Irreversible COX-Enzyme inhibitor by acetylating the serine on the COX-enzyme. 

• Indications:

  • Acute coronary syndrome (ACS) → ↓ CV mortality by 23%

  • Secondary prevention post-MI/stroke

  • Primary prevention: Risk vs benefit (bleeding risk)

• AEs:

  • GI bleeding

  • ↑ bleeding with other antithrombotics

• Aspirin resistance: failure to inhibit TxA₂ → ↑ CV risk 

ADP (P2Y₁₂) Receptors

• P2X₁: ATP-activated → Ca²⁺ mobilization

• P2Y₁: ADP-activated → shape change (low [ADP])

• P2Y₁₂: ADP-activated → sustained aggregation (high [ADP])

How does low concentrations of ADP bring about platelet shape change, whereas higher concentration aggregation?

At low concentration P2Y1 is activated (EC50 = 0.3 uM) →shape change.

At higher concentration P2Y12 is activated (EC50 = 2 uM) → aggregation.

P2Y₁₂ Antagonists (ADP Antagonists)

• Irreversible: Ticlopidine, Clopidogrel, Prasugrel

• Reversible: Cangrelor, Ticagrelor (-grelor)

Ticlopidine

• MoA: Irreversible P2Y₁₂ antagonist (prodrug → CYP activation → covalent disulfide bond)

• Use:

  • Prevent stroke/TIA

  • Adjunct with aspirin after stent placement

• AEs:

  • TTP, aplastic anemia, neutropenia, thrombocytopenia, bleeding

• Notes: Replaced by clopidogrel

Ticlopidine

Clopidogrel

• MoA: Irreversible P2Y₁₂ antagonist (prodrug → CYP activation → disulfide bond)

• Use:

  • Secondary prevention after MI, stroke, PAD

  • ACS prophylaxis with aspirin

  • PCI (with or without stent)

• AEs: Bleeding, rare TTP

• Resistance: CYP2C19/CYP3A4 polymorphism → ↓ activation

• Preferred over ticlopidine (safer, better data)

Clopidogrel

Activation of Clopidogrel

CC: Thienopyridines

Common Structural Motif for the thienopyridines (Clopidogrel, Ticaglerol).

Prasugrel

• MoA: Irreversible P2Y₁₂ antagonist (prodrug; activated by esterase → stable metabolite)

• Use: ACS (with aspirin)

• Better PK and stronger effect than clopidogrel

• AEs: Major bleeding (⚠ BBW)

  • Contraindicated: history of TIA/stroke or age >75

-Grelor

Reversible P2Y₁₂ antagonist

Prasugrel

Metabolism is by esterases (**differs from the other thienopyridines)

All irreversible thienopyridines are ___

prodrugs and is activated by CYPs (exception: Prasugrel which is activated by esterases).

Cangrelor

• Brand Name: Kengreal®

• MoA: Reversible P2Y₁₂ antagonist (IV only)

• Use: During PCI to reduce periprocedural MI

• AEs: Bleeding

Cangrelor

Ticagrelor

• Brand Name: Brilinta

• MoA: Reversible allosteric P2Y₁₂ antagonist

• Does NOT require metabolic activation

• Use: Secondary prevention in ACS

• AEs: Dyspnea, bleeding

• Aspirin restriction: High-dose aspirin (>100 mg) ↓ effectiveness

• Active metabolite: 30–40% plasma concentration of parent drug 

Ticagrelor

Gp IIb/IIIa Antagonists

• MoA: Block final common pathway of platelet aggregation (fibrinogen binding)

• Very potent but ↑ bleeding risk

• Drugs:

  • Abciximab – monoclonal antibody

  • Eptifibatide – cyclic peptide

  • Tirofiban – small-molecule non-peptide

• All IV formulations

Abciximab

• Type: Chimeric monoclonal antibody (human + murine Fab)

• MoA: Binds Gp IIb/IIIa → blocks fibrinogen & vWF binding

• Use:

  • Adjunct in PCI (with heparin/aspirin)

  • Unstable angina, MI prophylaxis

• t½: 72 hours (effects 24–48 h post-infusion)

• AEs: Major bleeding

• Expensive due to antibody production

Eptifibatide 

Eptifibatide

• Type: Cyclic heptapeptide (snake venom analog)

• MoA: Reversible inhibition of fibrinogen/vWF binding to Gp IIb/IIIa

• Use: ACS, PCI

• Formulation: IV

• t½: ~2.5 hours

• AEs: Bleeding

Tirofiban

• Type: Low MW non-peptide reversible Gp IIb/IIIa antagonist

• Formulation: IV

• t½: 2 hours

• Use: ACS – reduces death, MI, ischemia

• AEs: Bleeding

Outcome results of GPIIb/IIIa antagonists

Thrombotic Thrombocytopenic Purpura (TPP) is a major ADR of which thienopyridine?

Ticlopidine

Vorapaxar 

MOA: PAR-1 Antagonist that blocks platelet activation by inhibiting thrombin’s PAR-1 receptor.

Brand Name: Zontivity®

• Does NOT inhibit thrombin’s ability to convert fibrinogen → fibrin (JUST INHIBITING PLATELET AGGERATION)

• Works by preventing the “tethered ligand” from activating PAR-1

• Derived from structural studies of natural product himbacine 

MOA of Vorapaxar

Vorapaxar

Vorapaxar was discovered from what?

Natural product Himbacine which is a muscarinic antagonist.

Himbacine has no PAR-1 activity on it’s one

Himbacine → Muscarinic Antagonist that lead to Vorapaxar → Stereochemistry is opposite to that Vorapaxar

Vorapaxar Indications

• dded to aspirin + ADP antagonist for secondary prevention in:

  • Prior MI

  • Peripheral arterial disease (PAD)

• Contraindicated in history of stroke or intracranial bleeding

Given orally

Vorapaxar Pharmacokinetics

• Oral bioavailability ≈ 100%

• Long effective half-life: 3–4 days

• Metabolism: CYP3A4, CYP2J2

• Active metabolite: M20 (~20% exposure)

• Inactive metabolite: free amine

Of Vorapaxar

The major metabolite is inactive!

Cilostazol MoA

• PDE III inhibition → ↑ cAMP in platelets & vessels

  • Platelets: inhibits aggregation

  • Vascular tissue: vasodilation

• Blocks adenosine uptake → decreased aggegration

• Improves lipid profile (↓ triglycerides, ↑ HDL)

Cilostazol

Cilostazol Uses

• FDA: Intermittent claudication in PAD

• Non-FDA: Buerger’s disease, diabetic vascular sclerosis, chronic cerebral ischemia

Cilostazol PK / AEs / Warnings

• Oral; food ↑ absorption

• Two active metabolites

• Contraindicated in heart failure

• Use caution with other PDE-3 inhibitors or cardiac disease

Dipyridamole

Dipyridamole MOA

Dual mechanism (same as Cilostazol):

• PDE-III inhibition → ↑ cAMP → ↓ platelet aggregation

• Blocks adenosine reuptake

• Also PDE-V inhibition → peripheral vasodilation

Dipyridamole Use

• Commonly combined with aspirin

• Combined with warfarin to prevent emboli from prosthetic heart valves (ONLY FDA indication)

Dipyridamole + Warfarin

Prevent emboli from prosthetic heart valves (ONLY FDA indication)

Aggrenox 

Dipyridamole + aspirin, used to prevent stroke after TIA/stroke

Dipyridamole AE

Avoid as monotherapy in elderly (GI issues, fainting/orthostasis)

Fibrinolytic Drug Purpose

• Dissolve fibrin clots (polymerized fibrin)

• Used in MI, PE, DVT, arterial thrombosis

• Used when PCI unavailable

• High bleeding risk (lyse normal + pathological clots)

Endogenous Plasmin System

• Plasminogen converted → plasmin (fibrin-digesting enzyme)

• Major activator: tPA from endothelium

• tPA regulated by PAI-1 and PAI-2

Thrombolytic Drugs MOA

Act like t-PAs and convert plasminogen to plasmin to dissolve clots.

Suffix: “teplase”

Types of Thrombolytic Drugs 

First Generation: Streptokinase 

Second Generation: Alteplase

Third Generation: Tenecteplase; Reteplase  

Streptokinase

• Bacterial protein from β-hemolytic streptococci

• Forms 1:1 complex with plasminogen → converts free plasminogen → plasmin (act like tPA)

• NOT fibrin-specific → systemic fibrinolysis

• Uses: PE, DVT, MI, arterial thrombosis, shunt occlusion

• Cheap (cost-effective)

Streptokinase AEs

• Bleeding due to non-specific lysis activity and degrrades fibrinogen (precursor to fibrin) and Factors V and VII.

• Also more plasmin means that more clots are dissolved and increased bleeding risk.

• Hypersensitivity (rash/fever/anaphylaxis)

• Ineffective if antibodies from prior strep infection neutralize it (since it’s a foregin protein).

• Antibody presence → need high dose to overcome resistance

Aminocaproic Acid

Antidote if too much bleeding occurs from streptokinase administration → promotes a thromobitic state. 

Streptokinase PK

• IV formulation

• Very short t½ < 30 min

Anistreplase

• Prodrug (acylated with anisoyl group at the active serine site plasminogen–streptokinase complex)

• Longer half-life: ~90 min

• More clot-specific than streptokinase

• Given as rapid IV bolus (3–5 min)

• Better reperfusion due to longer action

Urokinase

• Human enzyme (not antigenic) → like Streptokinase 

• Directly converts plasminogen → plasmin

• Also directly degrades fibrin & fibrinogen (*ADDITIONAL EFFECT THEN OTHER KINASES*).

• t½ ~15 min; cleared by liver

• Use: PE, coronary artery thrombosis (off-label MI, DVT)

• AE: Bleeding (most common) 

“from UR body" → not antigenic” 

Alteplase

MOA: Second-generation recombinant t-PA 

• Fibrin-selective at low doses (this can lead to hemorrhage) 

• Activates plasminogen bound to fibrin

• Short t½ < 5 min

• Use: MI, massive PE, acute ischemic stroke (within 3 hours)

• AE: GI & intracranial bleeding 

Reteplase

• Brand Name: Retavase

• Deletion mutant of tPA (lacks first 172 aa)

• ↓ fibrin specificity but longer half-life: 14–18 min

• Use: Acute MI

• Off-label: catheter occlusions, DVT

CC: First vs Second Generation Thrombolytic Agents

Tenecteplase

Brand Name: TNKase®

• Modified tPA with 3 point mutations and inhibits PAI-1 (plasminogen inhibitor)

• ↑ t½ (17 min)

• 15× higher fibrin specificity vs alteplase

• Use: Acute MI (give ASAP)

• Off-label: ischemic stroke

Tranexamic Acid

Used as an antidote for too much bleeding/hemorrhage caused by fibrinolytic therapy

Tranexamic Acid

• Blocks lysine sites on plasminogen → prevents conversion to plasmin

• Reverses fibrinolytic bleeding

• Oral/IV; renal elimination; t½ ≈ 2 h

• Uses: heavy menses, dental bleeding on anticoagulants, fibrinolytic reversal

• AE: thrombosis, hypotension, muscle necrosis

ε-Aminocaproic Acid

• Same MoA as tranexamic acid

• Oral/IV, renal elimination, t½ ≈ 2 h

• Use: active bleeding due to fibrinolysis, postop/control bleeding

• AE: thrombosis, hypotension, muscle necrosis 

ε-Aminocaproic Acid

What are the 3 major steps involved in platelet aggregation?

Platelet adhesion, platelet activation, and platelet aggregation via GpIIb/IIIa cross-linking.

What is the mechanism of propagation of platelet activation?

Activated platelets release mediators (ADP, TxA₂, serotonin, thrombin, PAF) that bind receptors on nearby resting platelets, increasing Ca²⁺ and decreasing cAMP → activating more platelets.

What are the major biochemical mediators involved in the propagation of platelet activation?

ADP, thromboxane A₂ (TxA₂), serotonin, thrombin, and platelet-activating factor (PAF).

What is the final convergent pathway in platelet activation (that leads to platelet aggregation)?

Prothrombin → Thrombin

Activation of GpIIb/IIIa receptors, allowing fibrinogen to cross-link platelets.

What is the antiplatelet mechanism of action of aspirin?

Irreversible inhibition of COX-1 → decreased thromboxane A₂ formation → reduced platelet activation.

What is unique about the mode of binding (and inactivation) of aspirin on COX enzymes?

Aspirin covalently acetylates Ser529 on COX-1, making the inhibition irreversible for the platelet’s lifespan.

What are the 2 major antiplatelet indications for aspirin?

Acute coronary syndrome (ACS) and secondary prevention after MI or stroke.

What is known about the relative risk of patients with aspirin resistance?

Aspirin-resistant patients have higher cardiovascular risk.

What are the 3 major purinergic receptors and what are their main activators?

• P2X1: Activated by ATP

• P2Y1: Activated by ADP → platelet shape change

• P2Y12: Activated by ADP → sustained aggregation

Which ADP receptors cause platelet shape change? Which cause aggregation?

• Shape change: P2Y1

• Aggregation: P2Y12

Which are the irreversible P2Y12 antagonists? Which are the reversible ones?

• Irreversible: Ticlopidine, clopidogrel, prasugrel

• Reversible: Cangrelor, ticagrelor

What is the molecular mechanism of irreversible P2Y12 antagonism?

Formation of a covalent disulfide bond with receptor cysteine residues.

What are the two major AEs of ticlopidine?

TTP and hematologic toxicity (neutropenia, aplastic anemia).

What are the two major indications of ticlopidine?

Prevention of TIA/stroke and adjunct therapy with aspirin after stent placement.

Know the prodrug nature of ADP antagonists such as ticlopidine.

Ticlopidine, clopidogrel, and prasugrel must be metabolically activated into their active thiol metabolites.

Is the major circulating metabolite of clopidogrel active?

No — the major carboxylic acid metabolite is inactive.

What chemical bond causes irreversible inactivation of ADP receptors?

A disulfide covalent bond to receptor cysteines.

What are the 3 major indications for clopidogrel?

Secondary prevention after MI/stroke/PAD, ACS with aspirin, and PCI (with or without stent).

Are any ADP antagonists indicated for primary prevention?

No

What is the reason for clopidogrel resistance?

CYP2C19 and CYP3A4 genetic polymorphisms or drug interactions reducing activation.

How is prasugrel activation different from ticlopidine/clopidogrel? Is its active metabolite stable?

Prasugrel uses esterase activation (not CYP), and its active metabolite is stable.

What is the use restriction for prasugrel?

Must be used with aspirin.

What is the major indication for prasugrel?

Treatment of ACS.

What are the 2 boxed warnings for prasugrel?

Major bleeding risk and contraindicated in prior TIA/stroke or age >75.

What is the half-life of cangrelor? What is it used for?

t½ ≈ 2.6 minutes; used during PCI to prevent periprocedural MI.

Is ticagrelor reversible or irreversible? Is it a prodrug?

Reversible; not a prodrug.

Is ticagrelor a competitive or allosteric P2Y12 binder?

Allosteric binder — binds a different site from ADP.