Eicosanoids

Question 1

Nomenclature:

Answer 1

• ‘eicosanoids’ describes the families of prostaglandins, leukotrienes and other related compounds
• 8,11,14-eicosatrienoic acid (di-homo-γ-linolenic acid)
• 5,8,11,14-eicosatetraenoic acid (arachidonic acid)
  
  • most abundant precursor of eicosanoids
• 5,8,11,14,17-eicosapentaenoic acid (found primarily in fish oils)

Question 2

How is arachidonic acid released into the blood?

Answer 2

• concentration of free AA in cells is very low
• arachidonic acid is found esterified to membrane phospholipids
• Phospholipase A₂ is a calcium-dependent enzyme that hydrolyzes the sn-2 ester bond of phospholipid and releases arachidonic acid

Question 3

What are the two cyclooxygenases isoforms? How do they differ?

Answer 3

• COX-1 and COX-2:
  
  • heme proteins
  • membrane-bound
• How they differ:
  
  1. COX-1: constitutive
     
     • Expressed in all tissues
  2. COX-2: inducible
     
     • Has a promoter region that
       binds transcription factors
     • bigger, more
       accessible active site
     • commonly seen in
       inflammation
     • Nf-kB, C/EBP, ERK1/2,
       MAPK

Question 4

What are the two activites of cyclooxygenases?

Answer 4

1. oxygenates and cyclizes the precursor fatty acid to form cyclic endoperoxide, PGG₂
2. peroxidase activity that converts PGG₂ ⇒ PGH₂

Question 5

What is the fate of PGH₂?

Answer 5

• transformed enzymatically into a variety of products including PGI₂, T_XA₂, PGE₂, PGF_2α or PGD₂
• PGE₂ vs PGE₁
  
  • subscript refers to number of double bonds in molecule

Question 6

Lipooxygenases:

Answer 6

• catalyze the oxygenation of fatty acids to corresponding lipid hydroperoxides
• 5-lipoxygenase leads to synthesis of leukotrienes:
  
  • ​most important
  • activation of 5-lipoxygenase requires calcium and 5-lipoxygenase activating protein (FLAP)

Question 7

What type drugs inhibit Phospholipase A₂?

Answer 7

• drugs that reduce the availability of
  calcium
• glucocorticoids induce synthesis of a group of proteins called annexins (lipocortin) that inhibit phospholipase A₂ activity

Question 8

What type drugs inhibit cyclooxygenase?

Answer 8

• Aspirin/ other related NSAIDs inhibit COX- 1 and COX-2
• COX-2 selective drugs
• Glucocorticoids decrease expression of COX-2, but not COX-1

Question 9

Which drugs inhibit lipooxygenases (LOXs)?

Answer 9

1. zileuton (Zyflo®)
2. zafirlukast (Accolate®): cysteinyl leukotriene receptor antagonists

Question 10

Zileuton (Zyflo®)

• Pharmacokinetics:
• Mechanism:
• Adverse Effects:
• Therapeutic Use:

Answer 10

5-lipoxygenase inhibitor:

• Pharmacokinetics: oral administration; half-life of 2.5 hrs metabolized by CYP enzymes
• Mechanism: inhibits cys-LTs (bronchoconstriction and increase vascular permeability and LTB4 (chemotaxis)
• Adverse Effects: few; increase liver enzymes
• Therapeutic use: prophylactic treatment of mild asthma

Question 11

Zafirlukast (Accolate®):

• Pharmacokinetics:
• Mechanism:
• Adverse Effects:
• Therapeutic Use:

Answer 11

• Pharmacokinetics:
  
  • Oral administration
  • Metabolized CYP2C9
• Mechanism:
  
  • Cysteinyl leukotriene receptor
    antagonist
• Adverse effects:
  
  • Minimal
• Therapeutic use:
  
  • Prophylaxis and chronic
    treatment of asthma
  • Not appropriate or indicated for the reversal of bronchospasm in acute asthma attacks
• DOES NOT INHIBIT BIOSYNTHESIS
• Inhibits the effect of the CYS-containing leukotrienes

Question 12

Eicosanoid Catabolism:

Answer 12

• Rapidly inactivated
  
  • infuse PGE1: approximately 95% inactivated during one passage through the pulmonary circulation
• Two Steps:

1. initial step; rapid:
   
   • oxidation of the 15-OH group ⇒ ketone by 15-OH PG dehydrogenase
   • followed by reduction catalyzed by Δ¹³ PG -reductase
   • most biological activity lost
2. second step; relatively slow:
   
   • β and ω oxidation of side chains giving a polar compound that is excreted

Question 13

Cellular Mechanism of Action:

Prostaglandins

Answer 13

• wide diversity of effects
• explained by a number of distinct receptors
• all receptors are coupled to effector mechanisms through G proteins

Question 14

Cellular Mechanism of Action:

Leukotrienes

Answer 14

• LTB4 receptors:
  
  • BLT1 and BLT2
  • chemotaxis
• LTC4, LTD4 & LTE4 recpetors:
  
  • Cysteinyl leukotriene recpetors
  • cysLT1 and cys LT2
  • Bronchoconstriction
  • Increase vascular permeability
• activation increases intracellular calcium

Question 15

EICOSANOIDS CAN BE FORMED BY _________________.

Answer 15

VIRTUALLY EVERY CELL

Question 16

How are PGs involved in pain?

Answer 16

1. Periphery:
   
   • PGE₂, PGI₂ sensitize afferent nerve endings to effects of chemical and mechanical stimuli by lowering the threshold of nociceptors
     
     • “hyperalgesia”
   • PGs potentiate pain-producing activity of bradykinin and other autocoids
2. CNS:
   
   • Neuronal sensitization
   • COX-2 is expressed in the dorsal horn of the spinal cord
   • Increases during inflammation
   • Centrally generated PGE₂ activates spinal neurons and also microglia that contribute to neuropathic pain

Question 17

Do PGs alone cause pain?

Answer 17

• ONLY in high enough concentrations
• PGs decrease the threshold for pain so that lower concentrations of other mediators (like histamine, bradykinin, substance P) activate the pain fibers

Question 18

How do eicosanoids contribute to fever?

Answer 18

1. Increased formation of cytokines
2. This increases synthesis of PGE₂ in areas of brain associated with temperature control
3. PGE₂ increases cAMP
4. Triggers hypothalamus to elevate body temperature
5. Hypothalamus regulates the set point at which body temperature maintained

Question 19

Which eicosanoids induce fever?

Answer 19

• PGE₂ synthesis is stimulated by endogenous (e.g. interleukin-1) or exogenous (e.g. lipopolysaccharide) pyrogens
• Exogenous PGF_2α and PGI₂: induce fever but do not contribute to the pyretic response
• PGD₂ and TxA₂: do not induce fever

Question 20

How do eicosanoids interact with platelets and endothelium?

Answer 20

• Activation of platelet membrane PLA₂ ⇒ release of arachidonic acid ⇒ transformation into TXA₂ by COX-1
• TXA₂: promotes platelet aggregation (physiological effect) by stimulating the TP receptor
  
  • TP receptor is coupled to increase in intracellular calcium (cellular mechanism of action)
• PGI₂: inhibits platelet aggregation & promotes vasodilation (physiological effect) via stimulation of IP receptor that couples to cAMP (cellular mechanism of action)
  
  • Cellular source of PGI₂ is endothelial cell, not platelet
  • Activation of endothelial membrane PLA₂ ⇒ release of arachidonic acid ⇒ metabolism by COX-1 and/or COX-2 (inflammation)

Question 21

Reproduction and Parturition:
Uterus

1. PGI₂:
2. PGE₂:
3. PGF_2α:
4. PGF_2α:

Answer 21

1. PGI₂:
   
   • keeps uterus in quiescent state during early pregnancy
   • increase of cAMP
2. PGE₂:
   
   • initiation and progression of labor
   • induces uterine contractility
     
     • EP1/EP3 mediated increase in
       calcium
   • mediates ripening of cervix;
     
     • EP2/EP4 mediated increase in
       cAMP
3. PGF_2α:
   
   • Contracts uterus during labor
     
     • FP mediated increase in calcium
   • primary dysmenorrhea: menstrual pain severe enough to limit normal activities
     
     • non-pregnant uterus
     • concentrations increase in menstrual fluid
     • cause vasoconstriction, uterine contraction, pain
4. COX-1 and COX-2 involvement

Question 22

How do eicosanoids interact with cardiovascular/vascular smooth muscle tissue?

Answer 22

• Prostaglandins:
  
  • Systemic blood pressure generally falls in response to PGE₂
  • Profound hypotension after iv administration of PGI₂
• PGE₂ (EP₂ and EP₄) and PGI₂: predominantly vasodilators
  
  • In some cases, PGE₂ is vasoconstrictor (EP1/EP3 recpetors)
• TXA₂: potent vasoconstrictor
• PGF_2α: vasoconstriction
• PGD₂: predominantly vasodilator except in pulmonary where it causes vasoconstriction

Question 23

How do eicosanoids interact with Bronchial/Tracheal Smooth Muscle?

Answer 23

• when sensitized lung tissue is challenged by antigen; a complex mixture of autocoids is released includes both prostaglandins and leukotrienes
• leukotrienes are the major bronchoconstrictors
• PGEs, PGI₂: relax
• PGF_2α, PGD₂, TXA₂: all constrict
• LTC₄, LTD₄: constrict

Question 24

How do eicosanoids interact with the kidney?

Answer 24

• prostaglandins modulate RBF
• regulate urine formation (direct effects on renal tubules)
• PGE₂, PGI₂:
  
  • increase RBF because of vasodilation
  • promote diuresis, natriuresis

Question 25

How eicosanoids interact with the GI tract?

Answer 25

* COX-1 production of cytoprotective prostaglandins 1. **PGE₂** (EP₃) **and PGI₂** (IP)**:** * inhibit gastric acid secretion 2. **PGE₂** (EP₂/EP₄) and **PGI₂** (IP)**:** * increase gastric mucosal blood flow 3. **PGE₂:** stimulates release of viscous mucus 4. **PGE₂:** stimulates bicarbonate secretion 5. **PGE₂** (EP₁): contracts GI smooth muscle * PGs have a **cytoprotective effect: ** * **​**Suppress gastric ulceration

Question 26

How are eicosanoids involved in inflammation?

Answer 26

* **COX-2: **PG production * PGs: Minor role * Signs and symptoms of inflammation * **PGE₂/PGI₂:** directly increase blood flow and indirectly enhance edema formation and leukocyte infiltration * Increases other mediators that reach the site of injury * **Leukotrienes**: Major role * **LTC₄, LTD₄:** increase vascular permeability * **LTB₄:** chemoattractant for neutrophils * Increased in allergies/asthma

Question 27

What are eicosanoids role in cancer?

Answer 27

* increased concentrations of PGs in certain malignancies * PGs: induce cellular proliferation * COX-2: induced in certain cancers

Question 28

Why is use of PGs therpeutically limited?

Answer 28

* Significant adverse effects * Short half-lives in circulation

Question 29

**Dinoprostone** (Prepidil® or Prostin E₂®)**:** * **Therapeutic Use:** * **Preparation:** * **Mechanism of Action:** * **Adverse Effects:**

Answer 29

**_Synthetic analog of PGE₂_** 1. **Therapeutic Use:** _cervical ripening in pregnancy_ **Preparation:** cervical gel **Mechanism of Action:** _promotes cervical ripening_ (activation of collagenase [breakdown of collagen]) also _relaxes cervical smooth muscle_ **EP₄ receptor subtype**, increases cAMP 2. **Therapeutic Use:** _to terminate an early pregnancy/abortion_ **Preparation:** vaginal suppository **Mechanism of action:** _uterine contractions_ via **EP_1/3 receptors** * **Adverse effects:** * GI-Related (nausea, vomiting, diarrhea) * fever * uterine rupture

Question 30

**Carboprost Tromethamine** (Hemabate®)**:** * **Therapeutic Use:** * **Preparation:** * **Mechanism of Action:** * **Adverse Effects:**

Answer 30

**_PGF_2α analog_** * **Therapeutic Use:** 1. _Termination of pregnancy during the second trimester between weeks 13 and 20 of gestation_ 2. to _control postpartum hemorrhage_ that is not responding to conventional treatment methods * Preparation: I.M. * Mechanism of action: 1. _stimulates uterine contractility_ by action at FP receptors * provides hemostasis at the site of placenta formation 2. increases Ca²⁺ * **Adverse Effects:** * GI-Related (nausea, vomiting, diarrhea) * fever * uterine rupture

Question 31

**Misoprotol** (Cytotec®)**:** * **Therapeutic Use:** * **Preparation:** * **Mechanism of Action:** * **Adverse Effects:**

Answer 31

**_PGE₁ analog:_** * **Therapeutic Use:** * primarily used as **“replacement therapy”** * prevention of ulcers caused by _long-term administration with NSAIDs_ * **Preparation:** **oral administration** * **Mechanism of action:** * stimulates **EP₃ receptors** ⇒ suppresses gastric acid secretion * decrease in cAMP * increase mucin and bicarbonate secretion * increase mucosal blood flow * EP_2/4 receptors * increases cAMP * **Adverse effect:** * diarrhea-common * 100% contraindicated in pregnancy

Question 32

**Alprostadil** (Caverject® or Muse® Pellet; Prostin VR Pediatric®)**:** * **Therapeutic Use:** * **Preparation:** * **Mechanism of Action:** * **Adverse Effects:**

Answer 32

**_PGE₁:_** 1. **Therapeutic Use:** Impotence/Erectile Dysfunction * *Preparation:** _intracavernous injection_ * *Mechanism of action:** _increase in cAMP_ which relaxes smooth muscle of corpus cavernosum * *Adverse effect:** pain at the site of injection (reason for intra-urethral formulation) * *priapism:** prolonged erection 2. **Therapeutic Use:** maintenance of patent ductus arteriosus * *Preparation:** infused **intravenously** * *Mechanism of action:** _cAMP-mediated relaxation_ of ductus arteriosus smooth muscle * *Adverse effect:** apnea

Question 33

**Epoprostenol** (Flolan®): * **Therapeutic Use:** * **Preparation:** * **Mechanism of Action:** * **Adverse Effects:**

Answer 33

**_PGI₂_****:** * **Therapeutic effect:** primary pulmonary hypertension * **Preparation:** continuous **intravenous** infusion * **Mechanism of action:** _cAMP-mediated dilation of pulmonary artery_ vascular smooth muscle * **Adverse effects:** nausea, vomiting, headache, flushing

Question 34

**Bimatoprost** ( Lumigan™; Latisse™)**:** * **Therapeutic Use:** * **Preparation:** * **Mechanism of Action:** * **Adverse Effects:**

Answer 34

**_PGF_2α_****:** 1. **Therapeutic Use:** glaucoma * *Preparation:** **ophthalmic solution** * *Mechanism of action:** increases outflow of aqueous humor * *Adverse effects:** eye redness, itching, may cause permanent changes in eye color (increased brown pigment), eyelid skin; may increase length and number of eyelashes 2. **Therapeutic Use:** eyelash hypotrichosis * *Preparation:** **ophthalmic solution** * *Mechanism of action:** increases the percent and duration of hairs in the growth phase * *Adverse effects:** excess, unwanted hair growth, brown iris pigmentation, eye redness, itching