Evidence for a second receptor for prostacyclin on human airway epithelial cells that mediates inhibition of CXCL9 and CXCL10 release

Randomized, Placebo-controlled Trial of Inhaled Treprostinil for Patients at Risk for Acute Respiratory Distress Syndrome

Rationale: Inhaled treprostinil may improve oxygenation and have additional antiinflammatory effects in early acute hypoxemic respiratory failure, potentially preventing or reducing the severity of acute respiratory distress syndrome (ARDS).Objectives: To determine whether administration of inhaled treprostinil to patients at risk for ARDS is feasible, safe, and efficacious.Methods: We performed a double-blind, placebo-controlled, single-center randomized pilot trial at a quaternary care academic medical center. Patients with acute hypoxemia due to pneumonia or signs of low-pressure pulmonary edema with a unilateral or bilateral infiltrate on chest imaging and a 4 L/min supplemental oxygen requirement not requiring positive pressure ventilation were evaluated. Randomized patients received study drug or placebo (2:1 ratio). Treatment was initiated at 6 breaths every 4 hours and titrated up to 12 breaths. Subjects were maintained on treatment for 7 days and then tapered off over a period of 4 days. Study drug was stopped if positive pressure ventilation was required (invasive or noninvasive).Results: Fourteen patients were enrolled over a period of 31 months. Baseline characteristics were not significantly different between treatment groups with respect to age, sex, race, Acute Physiologic Assessment and Chronic Health Evaluation score, lung injury prediction score, or baseline mean oxygen saturation as measured by pulse oximetry (Sp_O2):fraction of inspired oxygen (Fi_O2) ratio. Trends in daily baseline and 30-minute postdose Sp_O2:Fi_O2 ratio for all treatment points were not significantly different between placebo and treprostinil. Four patients required positive pressure ventilation in the treprostinil group versus one in the placebo group.Conclusions: Inhaled treprostinil administration is feasible in patients at risk for ARDS but was not associated with improvement in the Sp_O2:Fi_O2 ratio relative to placebo. Drug-associated adverse events were not severe nor unexpected based on the known adverse effect profile of inhaled treprostinil. The clinical benefit of this intervention is unclear at this time in the absence of larger studies.Clinical trial registered with Clinicaltrials.gov (NCT02370095).

International Union of Basic and Clinical Pharmacology. CIX. Differences and Similarities between Human and Rodent Prostaglandin E2 Receptors (EP1-4) and Prostacyclin Receptor (IP): Specific Roles in Pathophysiologic Conditions

Prostaglandins are derived from arachidonic acid metabolism through cyclooxygenase activities. Among prostaglandins (PGs), prostacyclin (PGI2) and PGE2 are strongly involved in the regulation of homeostasis and main physiologic functions. In addition, the synthesis of these two prostaglandins is significantly increased during inflammation. PGI2 and PGE2 exert their biologic actions by binding to their respective receptors, namely prostacyclin receptor (IP) and prostaglandin E2 receptor (EP) 1-4, which belong to the family of G-protein-coupled receptors. IP and EP1-4 receptors are widely distributed in the body and thus play various physiologic and pathophysiologic roles. In this review, we discuss the recent advances in studies using pharmacological approaches, genetically modified animals, and genome-wide association studies regarding the roles of IP and EP1-4 receptors in the immune, cardiovascular, nervous, gastrointestinal, respiratory, genitourinary, and musculoskeletal systems. In particular, we highlight similarities and differences between human and rodents in terms of the specific roles of IP and EP1-4 receptors and their downstream signaling pathways, functions, and activities for each biologic system. We also highlight the potential novel therapeutic benefit of targeting IP and EP1-4 receptors in several diseases based on the scientific advances, animal models, and human studies. SIGNIFICANCE STATEMENT: In this review, we present an update of the pathophysiologic role of the prostacyclin receptor, prostaglandin E2 receptor (EP) 1, EP2, EP3, and EP4 receptors when activated by the two main prostaglandins, namely prostacyclin and prostaglandin E2, produced during inflammatory conditions in human and rodents. In addition, this comparison of the published results in each tissue and/or pathology should facilitate the choice of the most appropriate model for the future studies.

Prostaglandin I2 Suppresses Proinflammatory Chemokine Expression, CD4 T Cell Activation, and STAT6-Independent Allergic Lung Inflammation

Allergic airway diseases are immune disorders associated with heightened type 2 immune responses and IL-5 and IL-13 production at the site of inflammation. We have previously reported that cyclooxygenase (COX) inhibition by indomethacin augmented allergic airway inflammation in a STAT6-independent manner. However, the key COX product(s) responsible for restraining indomethacin-mediated STAT6-independent allergic inflammation is unknown. In this study, using the mouse model of OVA-induced allergic airway inflammation, we identified that PGI2 receptor (IP) signaling was critical for indomethacin-induced, STAT6-independent proallergic effects. We demonstrated that IP deficiency increased inflammatory cell infiltration, eosinophilia, and IL-5 and IL-13 expression in the lung in a STAT6-independent manner. The augmented STAT6-independent allergic inflammation correlated with enhanced primary immune responses to allergic sensitization and elevated production of multiple inflammatory chemokines (CCL11, CCL17, CCL22, and CXCL12) in the lung after allergen challenge. We also showed that the PGI2 analogue cicaprost inhibited CD4 T cell proliferation and IL-5 and IL-13 expression in vitro, and IP deficiency diminished the stimulatory effect of indomethacin on STAT6-independent IL-5 and IL-13 responses in vivo. The inhibitory effects of PGI2 and the IP signaling pathway on CD4 T cell activation, inflammatory chemokine production, and allergic sensitization and airway inflammation suggest that PGI2 and its analogue iloprost, both Food and Drug Administration-approved drugs, may be useful in treating allergic diseases and asthma. In addition, inhibiting PGI2 signaling by drugs that either block PGI2 production or restrain IP signaling may augment STAT6-independent pathways of allergic inflammation.

Recent advances in targeting the prostacyclin pathway in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a severe disease characterised by increased pulmonary vascular resistance, which leads to restricted pulmonary arterial blood flow and elevated pulmonary arterial pressure. In patients with PAH, pulmonary concentrations of prostacyclin, a prostanoid that targets several receptors including the IP prostacyclin receptor, are reduced. To redress this balance, epoprostenol, a synthetic prostacyclin, or analogues of prostacyclin have been given therapeutically. These therapies improve exercise capacity, functional class and haemodynamic parameters. In addition, epoprostenol improves survival among patients with PAH. Despite their therapeutic benefits, treatments that target the prostacyclin pathway are underused. One key factor is their requirement for parenteral administration: continuous intravenous administration can lead to embolism and thrombosis; subcutaneous administration is associated with infusion-site pain; and inhalation is time consuming, requiring multiple daily administrations. Nevertheless, targeting the prostacyclin pathway is an important strategy for the management of PAH. The development of oral therapies for this pathway, as well as more user-friendly delivery devices, may alleviate some of the inconveniences. Continued improvements in therapeutic options will enable more patients with PAH to receive medication targeting the prostacyclin pathway.

Inhibition of cyclooxygenase-2 causes a decrease in coronary flow in diabetic mice. The possible role of PGE2 and dysfunctional vasodilation mediated by prostacyclin receptor

Several lines of evidence suggest that cyclooxygenase-2 (COX-2) activity can have a beneficial role in the maintenance of vascular tone of the blood vessels in diabetes. Specifically, the increased production of prostacyclin (PGI2) and prostaglandin E2 (PGE2), mediated by COX-2, has been suggested to compensate for decreased synthesis of nitric oxide (NO). The study investigates whether inhibition of COX-2 may reduce the coronary flow in diabetic animals and may also lead to decreased synthesis of prostaglandins. Mice aged 18-20 weeks were used for the study: those with leptin receptor deficiency (db/db) served as a model of diabetes while heterozygous (db/+) mice served as controls. Coronary flow was measured by the Langendorff method, and prostaglandin synthesis by myocardia was assayed in heart perfusates. COX-2 inhibition was found to reduce basal coronary flow in db/db mice but had no effect in db/+ mice. Secretion of PGE2 was found to be higher in db/db mice, while prostacyclin synthesis did not differ. COX-2 inhibition decreased production of both prostaglandins to similar levels in both groups. The use of ONO-1301, a specific agonist for the prostacyclin receptor revealed that vasodilating responses mediated by the receptor were impaired in db/db mice. The expression levels of the receptor in cardiac tissue did not differ between the groups. It is concluded that the increased COX-2 contribution to vasodilation in diabetic animals appears to be partially a result of increased COX-2-dependent synthesis of PGE2 and also may be caused by impaired vasodilation mediated by the prostacyclin receptor.

Assessing the agonist profiles of the prostacyclin analogues treprostinil and naxaprostene, particularly their DP₁ activity

In this study, the inhibitory profiles of the prostacyclin analogues treprostinil and naxaprostene on several isolated smooth muscle preparations have been investigated. Treprostinil was an agonist for prostanoid DP1, EP2 and IP receptors, but not EP4 receptors; its DP1 potency was only 3-4 times less than PGD2 itself. Naxaprostene was much more selective for IP receptors and tended towards partial agonism. Treprostinil is a 13,14-dihydro analogue and the role of conformation around C12-15 in controlling agonist specificity is debated; the synthesis of new analogues is proposed and possible clinical usage discussed. In terms of selective prostanoid antagonists employed, BW-A868C/MK-0524 (DP1), ACA-23 (EP2) and GW-627368 (EP4) were found fit for purpose. However, the IP antagonist RO-1138452 was compromised by α1 and α2-adrenoceptor-mediated contractile activity on rat tail artery and anti-muscarinic activity on mouse trachea. There is a need for IP receptor antagonists with better selectivity and higher affinity.

Role of prostacyclin in pulmonary hypertension

Prostacyclin is a powerful cardioprotective hormone released by the endothelium of all blood vessels. Prostacyclin exists in equilibrium with other vasoactive hormones and a disturbance in the balance of these factors leads to cardiovascular disease including pulmonary arterial hypertension. Since it's discovery in the 1970s concerted efforts have been made to make the best therapeutic utility of prostacyclin, particularly in the treatment of pulmonary arterial hypertension. This has centred on working out the detailed pharmacology of prostacyclin and then synthesising new molecules based on its structure that are more stable or more easily tolerated. In addition, newer molecules have been developed that are not analogues of prostacyclin but that target the receptors that prostacyclin activates. Prostacyclin and related drugs have without doubt revolutionised the treatment and management of pulmonary arterial hypertension but are seriously limited by side effects within the systemic circulation. With the dawn of nanomedicine and targeted drug or stem cell delivery systems it will, in the very near future, be possible to make new formulations of prostacyclin that can evade the systemic circulation allowing for safe delivery to the pulmonary vessels. In this way, the full therapeutic potential of prostacyclin can be realised opening the possibility that pulmonary arterial hypertension will become, if not curable, a chronic manageable disease that is no longer fatal. This review discusses these and other issues relating to prostacyclin and its use in pulmonary arterial hypertension.

Molecular mechanisms regulating the vascular prostacyclin pathways and their adaptation during pregnancy and in the newborn

Prostacyclin (PGI(2)) is a member of the prostanoid group of eicosanoids that regulate homeostasis, hemostasis, smooth muscle function and inflammation. Prostanoids are derived from arachidonic acid by the sequential actions of phospholipase A(2), cyclooxygenase (COX), and specific prostaglandin (PG) synthases. There are two major COX enzymes, COX1 and COX2, that differ in structure, tissue distribution, subcellular localization, and function. COX1 is largely constitutively expressed, whereas COX2 is induced at sites of inflammation and vascular injury. PGI(2) is produced by endothelial cells and influences many cardiovascular processes. PGI(2) acts mainly on the prostacyclin (IP) receptor, but because of receptor homology, PGI(2) analogs such as iloprost may act on other prostanoid receptors with variable affinities. PGI(2)/IP interaction stimulates G protein-coupled increase in cAMP and protein kinase A, resulting in decreased [Ca(2+)](i), and could also cause inhibition of Rho kinase, leading to vascular smooth muscle relaxation. In addition, PGI(2) intracrine signaling may target nuclear peroxisome proliferator-activated receptors and regulate gene transcription. PGI(2) counteracts the vasoconstrictor and platelet aggregation effects of thromboxane A(2) (TXA(2)), and both prostanoids create an important balance in cardiovascular homeostasis. The PGI(2)/TXA(2) balance is particularly critical in the regulation of maternal and fetal vascular function during pregnancy and in the newborn. A decrease in PGI(2)/TXA(2) ratio in the maternal, fetal, and neonatal circulation may contribute to preeclampsia, intrauterine growth restriction, and persistent pulmonary hypertension of the newborn (PPHN), respectively. On the other hand, increased PGI(2) activity may contribute to patent ductus arteriosus (PDA) and intraventricular hemorrhage in premature newborns. These observations have raised interest in the use of COX inhibitors and PGI(2) analogs in the management of pregnancy-associated and neonatal vascular disorders. The use of aspirin to decrease TXA(2) synthesis has shown little benefit in preeclampsia, whereas indomethacin and ibuprofen are used effectively to close PDA in the premature newborn. PGI(2) analogs have been used effectively in primary pulmonary hypertension in adults and have shown promise in PPHN. Careful examination of PGI(2) metabolism and the complex interplay with other prostanoids will help design specific modulators of the PGI(2)-dependent pathways for the management of pregnancy-related and neonatal vascular disorders.

Binding and activity of the prostacyclin receptor (IP) agonists, treprostinil and iloprost, at human prostanoid receptors: treprostinil is a potent DP1 and EP2 agonist

The prostacyclin analogues, iloprost and treprostinil are extensively used in treating pulmonary hypertension. Their binding profile and corresponding biochemical cellular responses on human prostanoid receptors expressed in cell lines, have now been compared. Iloprost had high binding affinity for EP1 and IP receptors (Ki 1.1 and 3.9 nM, respectively), low affinity for FP, EP3 or EP4 receptors, and very low affinity for EP2, DP1 or TP receptors. By contrast, treprostinil had high affinity for the DP1, EP2 and IP receptors (Ki 4.4, 3.6 and 32 nM, respectively), low affinity for EP1 and EP4 receptors and even lower affinity for EP3, FP and TP receptors. In functional assays, iloprost had similar high activity in elevating cyclic AMP levels in cells expressing the human IP receptor and stimulating calcium influx in cells expressing EP1 receptors (EC50 0.37 and 0.3 nM, respectively) with the rank order of activity on the other receptors comparable to the binding assays. As with binding studies, treprostinil elevated cyclic AMP with a similar high potency in cells expressing DP1, IP and EP2 receptors (EC50 0.6, 1.9 and 6.2 nM, respectively), but had low activity at the other receptors. Activation of IP, DP1 and EP2 receptors, as with treprostinil, can all result in vasodilatation of human pulmonary arteries. However, activation of EP1 receptors can provoke vasoconstriction, and hence may offset the IP-receptor mediated vasodilator effects of iloprost. Treprostinil may therefore differ from iloprost in its overall beneficial pulmonary vasorelaxant profile and other pharmacological actions, especially in diseases where the IP receptor is down-regulated.