Prostacyclin Analogs

Publication Details

OVERVIEW

Introduction

Prostacyclin is a small lipid eicosanoid molecule also known as prostaglandin I-2 (PGI-2) that is produced by endothelial cells and acts locally, inhibiting platelet activation and causing vasodilation. Analogs of prostacyclin have been developed as therapies of pulmonary arterial hypertension (PAH).

Background

Prostacyclin is synthesized in endothelial cells and released in response to local factors such as inflammation and injury. Prostacyclin acts locally on receptors present on endothelial cells and platelets activating adenyl cyclase and thereby increasing intracellular cyclic adenine monophosphate (cAMP), a signaling molecule that activates multiple downstream pathways. In platelets, cAMP inhibits their activation, thus limiting the formation of platelet clots and decreasing hemostasis. In endothelial cells, intracellular cAMP activate pathways to induce smooth muscle relaxation and vasodilation. Prostacyclin is a paracrine signaling molecule, acting locally and with a very short half-life. When used as a drug, it is referred to as epoprostenol which is a potent vasodilator, particularly of the pulmonary vascular bed. Because of its short half-life and relative instability, epoprostenol requires continuous intravenous infusion and special handling. Several analogs of prostacyclin have been produced that have a longer half-life and greater stability, resulting in more favorable pharmacokinetics and bioavailability allowing administration by subcutaneous and oral routes or by inhalation. Prostacyclin analogs in clinical use in the United States include epoprostenol, iloprost and treprostinil. All three are used to treat pulmonary arterial hypertension (PAH), a severe and progressive disease marked by vasoconstriction and smooth muscle cell proliferation of the pulmonary arterioles. Importantly, PAH is associated with a relative increase in mediators of vasoconstriction (endothelin-1, thromboxane A) and a relative decrease in mediators of vasodilation (prostacyclin, nitric oxide). Modern therapy of PAH often combines antagonists of vasoconstrictors with agonists of the vasodilatory mediators. All three of the vasodilatory prostacyclin analogs require careful and monitored dose adjustments and should be administered only by physicians and health care providers trained and experienced in the management of PAH. None of the prostacyclin analogs have been convincingly linked to instances of clinically significant acute liver injury.

Epoprostenol (e" poe pros' ten nol) is the pharmaceutical name for prostacyclin, a naturally occurring metabolite of arachidonic acid that causes pulmonary arterial vasodilation and inhibits platelet activation. Epoprostenol must be given parenterally and has a short half-life (~6 minutes), for which reasons it is administered as a continuous intravenous infusion. In multiple clinical trials in patients with PAH, prolonged epoprostenol infusions led to an increase in exercise capacity (6 minute walk distance), improvement in symptoms and delay in clinical progression in patients with idiopathic and inheritable forms of PAH (WHO Group 1) and connective tissue disease-associated PAH. Epoprostenol was approved for use in the United States in 1995 and remains available generically and under the brand name Flolan. Epoprostenol is provided as a sterile powder in single use vials of 0.5 or 1.5 mg. The typical dose is 2 ng per kg body weight per minute administered via an implanted central venous catheter. The dose can be increased in increments of 1 to 2 ng/kg/min based upon clinical response and tolerance. Common side effects include dizziness, jaw pain, headache, muscle pain, gastrointestinal upset, nausea and vomiting, effects commonly associated with vasodilatory therapies. Uncommon, but potentially severe adverse events include pulmonary edema, hypotension, syncope, and bleeding. Sudden discontinuation can lead to rebound pulmonary hypertension. Side effects of the intravenous catheter required for administration include local pain, swelling and infection at the site of injection and catheter-associated bacterial infections that can be severe and result in fatality. Epoprostenol has not been associated with serum enzyme elevations above the rate in control subjects and has not been linked to instances of clinically apparent acute liver injury.

Likelihood score: E (unlikely to cause clinically apparent liver injury).

Iloprost (eye' loe prost) is a synthetic prostacyclin analog that is more stable and has a longer half-life than epoprostenol and can be given by inhalation. Chronic inhalation therapy using iloprost has been shown to improve symptoms, exercise capacity and pulmonary function in patients with PAH, WHO type 1. Iloprost was approved for use in the United States in 2004 and is usually given in combination with other agents for PAH. Iloprost is available in ampules of 10 or 20 µg/mL under the brand name Ventavis and is administered via a nebulizer in doses of 2.5 to 5.0 µg, 6 to 9 times daily. Because of its short half-life, iloprost must be administered frequently (minimum of 2 hours between doses), a requirement that limits its acceptability. Common side effects include flushing, cough, headache, dizziness, insomnia, palpitations, jaw pain, muscle pain, palpitations, nausea and vomiting. Uncommon, but potentially severe adverse events include hypotension, hemoptysis, pulmonary edema, bronchospasm and pneumonia. Iloprost has not been associated with serum enzyme elevations above the rate in control subjects and has not been linked to instances of clinically apparent acute liver injury.

Likelihood score: E (unlikely to cause clinically apparent liver injury).

Treprostinil (tre pros’ ti nil) is a tricyclic benzidine analog of prostacyclin that can be administered orally, by inhalation, subcutaneously or by intravenous infusion. It has a longer half-life (3 to 4 hours) than epoprostenol and iloprost and is stable at room temperature and neutral pH, making it more convenient to administer when given parenterally. Intravenous and subcutaneous forms of treprostinil have been found to be better accepted than other intravenously administered prostacyclin analogs and to have similar long term efficacy. Treprostinil was initially approved for use in PAH (WHO Group 1) in the United States in 2002 and is available in solution for injection in vials of 1.0, 2.5, 5.0 and 10.0 mg/mL under the brand name Remodulin, the typical dose being 1.25 to 2.5 ng/kg per minute. Treprostinil is also available as a solution for inhalation under the brand name Tyvaso in 2.9 mL ampules with 1.74 mg/ampule, the usual dose being 3 to 9 breaths four times daily. Finally, oral treprostinil tablets of 0.125, 0.25, 1 and 2.5 mg are available under the brand name Orenitram, the initial dose being 0.125 three times daily or 0.25 twice daily, with careful adjustment of dose thereafter based upon tolerance. Side effects of treprostinil are similar to those of iloprost and epoprostenol and include headache, diarrhea, nausea and vomiting, flushing, dizziness, paresthesia, muscle and jaw pain. Sudden withdrawal of therapy can cause rebound worsening of pulmonary hypertension. Inhalation therapy can cause cough. Chronic intravenous therapy with treprostinil is associated with a high rate of infusion site pain and erythema and with catheter-associated bacterial infections that can be severe and have resulted in death in rare instances. The oral form of treprostinil has limited efficacy and is considered a second line therapy of PAH generally given with other oral agents for PAH or to replace intravenous or subcutaneous treprostinil in patients with poor tolerance of the parenteral therapies. Treprostinil therapy has not been associated with serum enzyme elevations above the rate in control subjects and has not been linked to instances of clinically apparent acute liver injury.

Likelihood score: E (unlikely to cause clinically apparent liver injury).

Mechanism of Injury

The mechanism by which prostacyclin analogs might cause liver injury is not known. All three are eicosanoid molecules that have a relatively rapid half-life and are metabolized locally. Furthermore, the prostacyclin analogs are typically administered in low daily doses (<1 mg), doses at which direct and even idiosyncratic liver injury are rare. The prostacyclin analogs have little or no effect on cytochrome P450 enzymes and are not prone to causing drug-drug interactions. On the other hand, they are often administered with other drugs used to treat PAH such as the endothelin-1 receptor antagonists (ambrisentan, bosentan, macitentan), guanylate cyclase simulators (riociguat) or prostacyclin receptor agonists (selexipag), some of which have been linked to idiosyncratic, symptomatic acute liver injury.

Drug Class: Pulmonary Arterial Hypertension Agents

Other Drugs in the Class: Ambrisentan, Bosentan, Macitentan, Riociguat, Selexipag

PRODUCT INFORMATION

REPRESENTATIVE TRADE NAMES

Epoprostenol – Generic, Flolan®, Veletri®

Iloprost – Ventavis®

Treprostinil – Orenitram®, Remodulin®, Tyvaso®

DRUG CLASS

Pulmonary Arterial Hypertension Agents

COMPLETE LABELING

Product labeling at DailyMed, National Library of Medicine, NIH

CHEMICAL FORMULAS AND STRUCTURES

ANNOTATED BIBLIOGRAPHY

References updated: 25 November 2016

Abbreviations used: PAH, pulmonary arterial hypertension; sc, subcutaneous; iv, intravenous.

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    (Textbook of hepatotoxicity published in 1999; does not discuss epoprostenol, iloprost or treprostinil).
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    (Textbook on drug induced liver injury; does not discuss the prostacyclin analogs).
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    (Among 24 patients with PAH treated with aerosolized iloprost for 1 year, exercise capacity and hemodynamics improved, while side effects tended to be mild and liver tests “remained stable in all patients”).
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    (Concise review of the mechanism of action, efficacy, safety and cost of subcutaneous (sc) treprostinil shortly after its approval for use in PAH in the US; mentions side effects of infusion site reactions, pump complications as well as diarrhea, jaw pain, flushing, edema and gastrointestinal hemorrhage, but does not mention hepatotoxicity).
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    (Among 470 patients with PAH treated with continuous sc treprostinil or placebo for 12 weeks, exercise capacity, symptoms and hemodynamics improved with treprostinil while adverse events included infusion site pain [85% vs 27%], headache, diarrhea, nausea, jaw pain and edema and 3 patients had gastrointestinal bleeding; no mention of ALT elevations or hepatotoxicity).
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    (Among 203 patients with PAH treated with inhaled iloprost [6-9 times daily] or placebo for at least 12 weeks, side effects included cough, headache, flushing and jaw pain, but serum enzyme elevations and hepatotoxicity were not mentioned).
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    (In 3 trials of iv or sc treprostinil in comparison to iv epoprostenol or placebo in a total of 65 patients with PAH, both iv and sc treprostinil had similar efficacy to epoprostenol and was superior to placebo in improving exercise capacity and hemodynamics, while side effects included infusion site pain, erythema or induration in most patients receiving treprostinil [88% to 94%]; no mention of ALT elevations or hepatotoxicity).
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    (Review of prostacyclin analog therapy of PAH including iv epoprostenol, iv and sc treprostinil and inhaled iloprost; no mention of serum enzyme elevations during therapy or hepatotoxicity).
  • Barst RJ, Galie N, Naeije R, Simonneau G, Jeffs R, Arneson C, Rubin LJ. Long-term outcome in pulmonary arterial hypertension patients treated with subcutaneous treprostinil. Eur Respir J 2006; 28: 1195-203. [PubMed: 16899485]
    (Among 860 patients with PAH treated with sc treprostinil for up to 4 years, survival was 68-87%, much higher than the predicted rates of 38-69% and side effects were similar to those reported in short term trials; “all mean chemistry…values were within normal ranges throughout the 4 years of treatment”).
  • McLaughlin VV, Oudiz RJ, Frost A, Tapson VF, Murali S, Channick RN, Badesch DB, et al. Randomized study of adding inhaled iloprost to existing bosentan in pulmonary arterial hypertension. Am J Respir Crit Care Med 2006; 174: 1257-63. [PubMed: 16946127]
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    (In a retrospective review of bloodstream infections at 7 centers in the US between 2003 and 2006, 144 cases [including 2 deaths] were identified in patients with PAH receiving infusions of prostacyclin analogs, rates being higher for treprostinil than epoprostenol).
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    (Review of the clinical efficacy and safety of inhaled iloprost in PAH mentions that side effects are generally mild and similar to those with other prostacyclin analogs).
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    (Surveys of physicians and nurses specializing in PAH care found that prostacyclin pump or dosing errors in epoprostenol and treprostinil administration were common and potentially serious, and were thought to have contributed to 9 patient deaths).
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    (Among 350 patients with PAH on stable therapy with an endothelin receptor antagonist or phosphodiesterase 5 inhibitor who were treated with oral treprostinil or placebo for 16 weeks, symptoms and exercise capacity improved with treprostinil and side effects included headache, nausea, diarrhea, flushing, jaw pain and myalgia, but there were no changes in “laboratory parameters” and no discontinuations for liver related events).
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    (Among 310 patients with PAH on stable therapy with oral agents who were treated with oral treprostinil or placebo, exercise tolerance and symptoms did not improve with treprostinil, while side effects included headache [71%], diarrhea [55%], nausea [46%], flushing [35%], and jaw pain [25%]; there were no liver related serious adverse events).
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    (Among 59 patients with PAH receiving iv iloprost over a 5 year period, 15 episodes of catheter related blood stream infections occurred in 11 patients for a rate of 0.65 per 1000 treatment days; there were no deaths).
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    (Among 349 patients with PAH on no therapy who were treated with oral treprostinil or placebo for 12 weeks, exercise capacity improved, but there were no changes in symptoms or clinical status and side effects were common; there were no liver related serious adverse events or deaths).
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    (Among 30 patients with PAH treated with 1 of 2 formulations [arginine- vs glycine-mannitol excipients] of iv epoprostenol for 28 days, exercise capacity and hemodynamics improved similarly in both groups and side effect rates were not different).
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    (Review of recently approved drugs for PAH including macitentan, riociguat and oral treprostinil mentions that dosing of oral treprostinil is complex and can result in suboptimal therapeutic levels, making the oral form of treprostinil not recommended as a first line agent for PAH).
  • Chalasani N, Bonkovsky HL, Fontana R, Lee W, Stolz A, Talwalkar J, Reddy KR, et al.; United States Drug Induced Liver Injury Network. Features and outcomes of 899 patients with drug-induced liver injury: The DILIN Prospective Study. Gastroenterology 2015; 148: 1340-52. [PMC free article: PMC4446235] [PubMed: 25754159]
    (Among 899 cases of drug induced liver injury enrolled in a US prospective study between 2004 and 2013, one was attributed to bosentan, but none to other agents used primarily to treat pulmonary artery hypertension).
  • Das A, Shabbir A, Sehgal S, Highland KB. Cutaneous hypersensitivity and eosinophilia associated with treprostinil. Pulm Pharmacol Ther 2015; 35: 17-8. [PubMed: 26407925]
    (59 year old woman with scleroderma and PAH on sildenafil and macitentan was treated with iv followed by oral treprostinil for 3 months when she developed rash and eosinophilia [22%], that did not improve on switching back to the iv formulation, but resolved promptly when treprostinil was stopped; no mention of systemic signs or liver test abnormalities).
  • Bourge RC, Waxman AB, Gomberg-Maitland M, Shapiro SM, Tarver JH 3rd, Zwicke DL, Feldman JP, et al. Treprostinil administered to treat pulmonary arterial hypertension using a fully implantable programmable intravascular delivery system: results of the DelIVery for PAH Trial. Chest 2016; 150: 27-34. [PubMed: 27396777]
    (Among 60 patients with severe PAH treated with iv treprostinil using a fully implantable deliver device for 12 months, 6 catheter complications arose, but no bloodstream infections or thromboses; no mention of serum enzyme elevations or hepatotoxicity).
  • O'Connell C, Amar D, Boucly A, Savale L, Jaïs X, Chaumais MC, Montani D, et al. Comparative safety and tolerability of prostacyclins in pulmonary hypertension. Drug Saf 2016; 39: 287-94. [PubMed: 26748508]
    (Review of the clinical efficacy and safety of the prostacyclin analogs used to treat PAH including iloprost, epoprostenol and treprostinil; no mention of serum enzyme elevations occurring during therapy or hepatotoxicity).
  • Parikh KS, Rajagopal S, Fortin T, Tapson VF, Poms AD. Safety and tolerability of high-dose inhaled treprostinil in pulmonary hypertension. J Cardiovasc Pharmacol 2016; 67: 322-5. PubMed Citation. [PMC free article: PMC4824653] [PubMed: 26828324]
    (Among 80 patients treated with inhaled treprostinil [>9 breaths, 4 times daily] for an average of 20 months, chronic side effects included cough, headache, flushing, nausea, throat irritation, jaw pain, diarrhea, dizziness, syncope and hypotension; no mention of serum enzyme elevations or hepatotoxicity).
  • Grünig E, Benjamin N, Lange TJ, Krueger U, Klose H, Neurohr C, Wilkens H, et al. Safety, tolerability and clinical effects of a rapid dose titration of subcutaneous treprostinil therapy in pulmonary arterial hypertension: a prospective multi-centre trial. Respiration 2016; 92: 362-70. [PubMed: 27784026]
    (Among 39 patients with PAH on stable doses of oral therapies, addition of sc treprostinil in a rapid dose titration was associated with improved exercise capacity and hemodynamics and “a good overall safety profile”, although all patients had adverse events including infusion site pain in 97%, diarrhea in 77% and headache in 69%; no mention of ALT elevations or hepatotoxicity).