prostacyclin

Prostacyclin
Systematic (IUPAC) name
(Z)-5-[(4R,5R)-5-hydroxy-4-((S,E)-3-hydroxyoct-1-enyl)hexahydro-2H-cyclopenta[b]furan-2-ylidene]pentanoic acid
Identifiers
CAS number	35121-78-9
ATC code	B01AC09
PubChem	114805
DrugBank	APRD00949
Chemical data
Formula	C20H32O5
Mol. mass	352.465 g/mol
SMILES	eMolecules & PubChem
Pharmacokinetic data
Bioavailability	?
Metabolism	?
Half life	?
Excretion	?
Therapeutic considerations
Pregnancy cat.	?
Legal status
Routes	?

Prostacyclin (or PGI₂) is a member of the family of lipid molecules known as eicosanoids.

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

Production

Eicosanoid synthesis. (Prostacyclin near bottom center.)

It 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.

Function

Prostacyclin (PGI₂) chiefly prevents formation of the platelet plug involved in primary hemostasis (a part of blood clot formation). 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.

Degradation

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

Mode of action

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 which 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. Notably, PGI₂ and TXA₂ work as physiological antagonists.

Pharmacology

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

• as a vasodilator in severe Raynaud's phenomenon or ischemia of a limb;
• in pulmonary hypertension.

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 would think that the effect of NSAIDs would balance out. 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 primarily released 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.

See also

• Essential fatty acid

References