Prostacyclin

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

When used as a drug, it is also known as epoprostenol. The terms are sometimes used interchangeably.

Function

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. It is also an effective vasodilator. Prostacyclin's interactions contrast with those of thromboxane (TXA₂), another eicosanoid. Both molecules are derived from arachidonic acid, and work together with opposite platelet aggregatory effects. These strongly suggest a mechanism of cardiovascular homeostasis between these two hormones in relation to vascular damage.

Medical uses

It is used to treat pulmonary arterial hypertension (PAH), pulmonary fibrosis, as well as atherosclerosis. Specifically, epoprostenol is given to patients with class III or class IV PAH.

Degradation

Prostacyclin, which has a half-life of 42 seconds, is broken down into 6-keto-PGF₁, which is a much weaker vasodilator.
A way to stabilize prostacyclin in its active form, especially during drug delivery, is to prepare prostacyclin in alkaline buffer. Even at physiological pH, prostacyclin can rapidly form the inactive hydration product 6-keto-prostaglandin F1α.

Mechanism

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As mentioned above, 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, raises cAMP levels in the cytosol. This cAMP then goes on to activate protein kinase A (PKA). PKA then continues the cascade by promoting the phosphorylation of the myosin light chain kinase, which inhibits it and leads to smooth muscle relaxation and vasodilation. It can be noted that PGI₂ and TXA₂ work as physiological antagonists.

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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.
* in primary pulmonary hypertension (PPH)

The production of prostacyclin is inhibited by the action of NSAIDs on 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 volume 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. The drug has been known to cause flushing, headaches and hypotension.

Synthesis

Biosynthesis

Prostacyclin is produced in endothelial cells, which line the walls of arteries and veins, 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). PGI2, derived primarily from COX-2 in humans, is the major arachidonate metabolite released from the vascular endothelium. This is a controversial point, some assign COX 1 as the major prostacyclin producing cyclooxygenase in the endothelial cells of the blood vessels.

The series-3 prostaglandin PGH₃ also follows the prostacyclin synthase pathway, yielding another prostacyclin, PGI₃. 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.

Artificial synthesis

Prostacyclin can be synthesized from the methyl ester of prostaglandin F_2α. After its synthesis, the drug is reconstituted in saline and glycerin.

Because prostacyclin is so chemically labile, quantitation of their inactive metabolites, rather than the active compounds, is used to assess their rate of synthesis.

History

During the 1960s, a UK 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, Vane 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.

A team at The Wellcome Foundation led by Salvador Moncada had identified a lipid mediator they called "PG-X," which inhibits platelet aggregation. PG-X, later known as prostacyclin, is 30 times more potent than any other then-known anti-aggregatory agent. They did this while searching for an enzyme that generates a fellow unstable prostanoid, Thromboxane A2

In 1976, Vane and fellow researchers Salvador Moncada, Ryszard Gryglewski

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