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Prostacyclins chemical structure
Prostacyclins

(Z)-5-[(4R,5R)-5-hydroxy-4-((S,E)-3-hydroxyoct-1-enyl)hexahydro-2H-cyclopenta[b]furan-2-ylidene]pentanoic acid
IUPAC name
CAS number
35121-78-9
ATC code

B01AC09

PubChem
5282411
DrugBank
DB01240
Chemical formula {{{chemical_formula}}}
Molecular weight 352.465 g/mol
Bioavailability
Metabolism
Elimination half-life
Excretion {{{excretion}}}
Pregnancy category
Legal status
Routes of administration

Prostacyclin (also called prostaglandin I2 or PGI2) is a prostaglandin member of the family of lipid molecules known as eicosanoids. It inhibits platelet activation and is also an effective vasodilator.

As a drug, it is also known as "epoprostenol".[1] The terms are sometimes used interchangeably.[2]

HistoryEdit

During the 1960s, a U.K. research team, headed by Professor John Vane, began to explore the role of prostaglandins in anaphylaxis and respiratory diseases. Working with a team from the Royal College of Surgeons, Sir John discovered that aspirin and other oral anti-inflammatory drugs work by inhibiting the synthesis of prostaglandins. This critical finding opened the door to a broader understanding of the role of prostaglandins in the body.

Sir John and a team from the Wellcome Foundation, had identified a lipid mediator they called “PG-X,” which inhibits platelet aggregation. PG-X, which later would become known as prostacyclin, is 30 times more potent than any other then-known anti-aggregatory agent.

By 1976, Sir John and fellow researchers Salvador Moncada, Ryszard Gryglewski and Stuart Bunting published the first paper on prostacyclin, in the scientific journal Nature. The collaboration produced a synthesized molecule, which was given the name epoprostenol. But, as with native prostacyclin, the structure of the epoprostenol molecule proved to be unstable in solution, prone to rapid degradation. This presented a challenge for both in vitro experiments and clinical applications.

To overcome this challenge, the research team that discovered prostacyclin was determined to continue the research in an attempt to build upon the success they had seen with the prototype molecule. The research team synthesized nearly 1,000 analogues.

Through innovative work done by researcher Lucy Clapp, treprostinil has demonstrated a unique effect on PPAR gamma, a transcription factor important in vascular pathogenesis as a mediator of proliferation, inflammation, and apoptosis. Through a complementary, yet cyclic AMP-independent pathway, treprostinil activates PPARs, another mechanism that contributes to the anti-growth benefits of the prostacyclin class.

ProductionEdit

Eicosanoid synthesis

Eicosanoid synthesis. (Prostacyclin near bottom center.)

Prostacyclin is produced in endothelial cells from prostaglandin H2 (PGH2) by the action of the enzyme prostacyclin synthase. Although prostacyclin is considered an independent mediator, it is called PGI2 (prostaglandin I2) in eicosanoid nomenclature, and is a member of the prostanoids (together with the prostaglandins and thromboxane).

The series-3 prostaglandin PGH3 also follows the prostacyclin synthase pathway, yielding another prostacyclin, PGI3.[3] The unqualified term 'prostacyclin' usually refers to PGI2. PGI2 is derived from the ω-6 arachidonic acid. PGI3 is derived from the ω-3 EPA.

FunctionEdit

Prostacyclin (PGI2) chiefly prevents formation of the platelet plug involved in primary hemostasis (a part of blood clot formation). It does this by inhibiting platelet activation.[4] It is also an effective vasodilator. Prostacyclin's interactions in contrast to thromboxane (TXA2), another eicosanoid, strongly suggest a mechanism of cardiovascular homeostasis between the two hormones in relation to vascular damage.

DegradationEdit

Prostacyclin, which has a half-life of 42 seconds,[5] is broken down into 6-keto-PGF1, which is a much weaker vasodilator.

Mode of actionEdit

Prostacyclin effect Mechanism Cellular response
Classical
functions
Vessel tone ↑cAMP, ↓ET-1
↓Ca2+, ↑K+
↓SMC proliferation
↑Vasodilation
Antiproliferative ↑cAMP
&earr;PPAR
↓Fibroblast growth
↑Apoptosis
Antithrombotic ↓Thromboxane-A2
↓PDGF
↓Platelet aggregation
↓Platelet adherence to vessel wall
Novel
functions
Antiinflammatory ↓IL-1, IL-6
↑IL-10
↓Proinflammatory cytokines
↑Antiinflammatory cytokines
Antimitogenic ↓VEGF
↓TGF-β
↓Angiogenesis
↑ECM remodeling

Prostacyclin (PGI2) is released by healthy endothelial cells and performs its function through a paracrine signaling cascade that involves G protein-coupled receptors on nearby platelets and endothelial cells. The platelet Gs protein-coupled receptor (prostacyclin receptor) is activated when it binds to PGI2. This activation, in turn, signals adenylyl cyclase to produce cAMP. cAMP goes on to inhibit any undue platelet activation (in order to promote circulation) and also counteracts any increase in cytosolic calcium levels that would result from thromboxane A2 (TXA2) binding (leading to platelet activation and subsequent coagulation). PGI2 also binds to endothelial prostacyclin receptors and in the same manner raise cAMP levels in the cytosol. This cAMP then goes on to activate protein kinase A (PKA). PKA then continues the cascade by phosphorylating and inhibiting myosin light-chain kinase, which leads to smooth muscle relaxation and vasodilation. It can be noted that PGI2 and TXA2 work as physiological antagonists.

Members[6]Edit

PROSTACYCLINS
Flolan
(epoprostenol sodium)
for Injection
Continuously infused 2 ng/kg/min to start, increased by 2 ng/kg/min every 15 minutes or longer until suitable efficacy/tolerability balance is achieved Class III
Class IV
Veletri
(epoprostenol)
for Injection
Continuously infused 2 ng/kg/min to start, increased by 2 ng/kg/min every 15 minutes or longer until suitable efficacy/tolerability balance is achieved Class III
Class IV
Remodulin SC§
(treprostinil sodium)
Injection
Continuously infused 1.25 ng/kg/min to start, increased by up to 1.25 ng/kg/min per week for 4 weeks, then up to 2.5 ng/kg/min per week until

suitable efficacy/tolerability balance is achieved

Class II
Class III
Class IV
Ventavis
(iloprost)
Inhalation Solution
Inhaled 6–9 times daily 2.5 µg 6–9 times daily to start, increased to 5.0 µg 6–9 times daily if well tolerated Class III
Class IV

PharmacologyEdit

File:Prostacyclin2.png

Synthetic prostacyclin analogues (iloprost, cisaprost) are used intravenously, subcutaneously or by inhalation:

Its production is inhibited indirectly by NSAIDs, which inhibit the cyclooxygenase enzymes COX1 and COX2. These convert arachidonic acid to prostaglandin H2 (PGH2), the immediate precursor of prostacyclin. Since thromboxane (an eicosanoid stimulator of platelet aggregation) is also downstream of COX enzymes, one might think that the effect of NSAIDs would act to balance. However, prostacyclin concentrations recover much faster than thromboxane levels, so aspirin administration initially has little to no effect but eventually prevents platelet aggregation (the effect of prostaglandins predominates as they are regenerated). This is explained by understanding the cells that produce each molecule, TXA2 and PGI2. Since PGI2 is primarily produced in a nucleated endothelial cell, the COX inhibition by NSAID can be overcome with time by increased COX gene activation and subsequent production of more COX enzymes to catalyze the formation of PGI2. In contrast, TXA2 is released primarily by anucleated platelets, which are unable to respond to NSAID COX inhibition with additional transcription of the COX gene because they lack DNA material necessary to perform such a task. This allows NSAIDs to result in PGI2 dominance that promotes circulation and retards thrombosis.

In patients with pulmonary hypertension, inhaled epoprostenol reduces pulmonary pressure, and improves right ventricular stroke work in patients undergoing cardiac surgery. A dose of 60 µg is hemodynamically safe, and its effect is completely reversed after 25 minutes. No evidence of platelet dysfunction or an increase in surgical bleeding after administration of inhaled epoprostenol has been found.[7]

See alsoEdit

ReferencesEdit

  1. Template:DorlandsDict
  2. Kermode J, Butt W, Shann F (August 1991). Comparison between prostaglandin E1 and epoprostenol (prostacyclin) in infants after heart surgery. British heart journal 66 (2): 175–8.
  3. Fischer S, Weber PC (1985). Thromboxane (TX)A3 and prostaglandin (PG)I3 are formed in man after dietary eicosapentaenoic acid: identification and quantification by capillary gas chromatography-electron impact mass spectrometry. Biomed. Mass Spectrom. 12 (9): 470–6.
  4. Pathologic Basis of Disease, Robbins and Cotran, 8th ed. Saunders Philadelphia 2010
  5. Cawello W, Schweer H, Muller R, et al. Metabolism and pharmacokinetics of prostaglandin E1 administered by intravenous infusion in human subjects. Eur J Clin Pharmacol 1994;46:275-7
  6. ^ REM_RefGuideWC_AUG07v.1
  7. Haché M, Denault A, et al. Inhaled epoprostenol (prostacyclin) and pulmonary hypertension before cardiac surgery. J Thorac Cardiovasc Surg 2003;125:642-649

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