S TAT E OF THE ART R EV I EW

CARD IOVASCULAR D I S EASE

Pulmonary Arterial Hypertension: Challenges inTranslational Research and a Vision for ChangeGopinath Sutendra and Evangelos D. Michelakis*

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Pulmonary arterial hypertension (PAH) is a vascular remodeling disease with a relentless course toward heart failureand early death. Existing PAH therapies, all of which were developed originally to treat systemic vascular diseases,cannot reverse the disease or markedly improve survival and are expensive. Although there has been a recentincrease in the number of potential new therapies emerging from animal studies, less than 3% of the activePAH clinical trials are examining such therapies. There are many potential explanations for the translational gapin this complex multifactorial disease. We discuss these challenges and propose solutions that range from includingclinical endpoints in animal studies and improving the rigor of human trials to conducting mechanistic early-phasetrials and randomized trials with innovative designs based on personalized medicine principles. Global,independent patient and tissue registries and enhanced communication among academics, industry, and regulatoryauthorities are needed. The diversity of the mechanisms and pathology of PAH calls for broad comprehensivetheories that encompass emerging evidence for contributions of metabolism and inflammation to PAH tosupport more effective therapeutic target identification.

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INTRODUCTION

Pulmonary arterial hypertension (PAH) is a complex vascular diseasethat results in heart failure and premature death. Although relativelyrare, its impact on society is significant: PAH is a deadly disease thatkills patients at a productive age and at a tremendous financial cost.The prevalence of PAH is estimated to be 15 to 50 per million, but thisis likely an underestimation (1, 2). Within certain high-risk groups,the prevalence is higher: Among patients with HIV infection, the prev-alence is 0.5% (3), and among patients with scleroderma, the prev-alence is as high as 12% (4).

Although none of the currently approved therapies can reverse orcure PAH, the care and the quality of life of PAH patients have im-proved over the past 20 years, as described in recent reviews that sum-marize the progress in therapy and management of PAH (2), subjectsonly briefly discussed here. Nevertheless, an explosion of knowledgefrom preclinical models of PAH is being translated into humans ata frustratingly slow rate. We will discuss the many challenges for clini-cians, pathologists, and scientists in the PAH field and will proposepotential solutions to improve translation of basic discoveries to PAHtreatment.

THE CLINICAL PROBLEM AND ITS IMPACT

PAH is a syndrome defined by an increase in the mean pulmonaryartery pressure to more than 25 mmHg at rest with normal left ven-tricular filling pressures, in the absence of any secondary causes suchas parenchymal or thromboembolic lung disease (2) (Table 1). PAH ischaracterized by decreased vascular perfusion (Fig. 1A), a result ofproliferative remodeling in the medium- and small-sized pulmonaryarteries that progressively decreases the cross-sectional area of thevascular lumen (Fig. 1B). The resulting rise in pulmonary vascular re-sistance stresses the right ventricle (RV), which, after short-lived com-

Department of Medicine, University of Alberta, Edmonton, Alberta T6G 2B7, Canada.*Corresponding author. E-mail: em2@ualberta.ca

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pensatory hypertrophy (5), progresses to a decompensation phase anda decrease in cardiac output, progressive heart failure, and death. Be-cause the pulmonary circulation has a higher capacitance and lower

A

B

Normal PAH

PAH PA histology

Media thickening Plexiform lesionIntima thickening

Normal PA histology

MRI

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PA

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Fig. 1. PAH is a complex vascular remodeling disease. (A) Magneticresonance imaging (MRI) angiogram of the lungs in a healthy control in-

dividual (left panel) and a patient with PAH (right panel) showing enlargedpulmonary arteries and severely attenuated vascular perfusion of the distalpulmonary arteries in PAH. PA, pulmonary artery, RA, right atrium, modifiedwith permission from (19). (B) Immunohistochemistry of distal pulmonaryarteries shows a cross section from a healthy control (left panels), with anormal thin-walled vessel (arrow) around a wide lumen, surrounded byalveoli [hematoxylin and eosin (H&E) staining]. Sections from a patient withiPAH (right panels) show, from left to right, intima thickening (H&E staining;nuclei, dark blue; cytoplasm, pink), media thickening (immunohisto-chemistry of smooth muscle cells; smooth muscle actin, brown), and aplexiform lesion (H&E staining) showing “anarchous” growth of cells thatcompletely obliterates the vascular lumen. Scale bars, 100 mm.

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pressure than systemic vascular beds, vascular remodeling can increasemarkedly before clinical symptoms are apparent. PAH, therefore, be-comes apparent late in the course of the disease. The symptoms areprimarily a result of decreased cardiac output and so tend to be non-specific: shortness of breath, weakness, syncope with exertion, andfluid retention.

PAH can be idiopathic (iPAH) or heritable (hPAH) (6), or can beassociated with other conditions (aPAH) such as scleroderma or other

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autoimmune collagen diseases, congenital heart disease, HIV infection,or anorexigen drug use (Table 1). The morbidity of PAH results fromboth the progressive vascular remodeling and the inability of the thin-walled RV to compensate adequately for higher pulmonary resistance(in comparison to the compensation of the left ventricle in responseto systemic hypertension) (2). The exact survival rates in PAH are notknown. Results from an unbiased, prospective 1980’s National Insti-tutes of Health registry indicated 3- and 5-year survival rates of 48%and 34% (7), respectively, but these data predate current therapies andDoppler echocardiography, a major screening tool. More recent projec-tions from smaller, country-specific registries suggest that the present5-year survival is about 50%, and the largest U.S. database REVEALindicates a 5-year survival of 57% (2, 8). This apparent improvementmaybe because patients are now diagnosed at an earlier stage. Subpopulationsof aPAH patients (PAH associated with scleroderma or HIV infection)have 5-year survival rates of less than 40% or 20%, respectively (2).

Existing therapies cannot reverse the disease or significantly reducemortality, although they improve patients’ quality of life and symp-toms (9, 10). Three classes of medications are available: prostacyclinsynthetics/analogs, endothelin receptor antagonists (ERAs), and phos-phodiesterase type 5 inhibitors (PDE5i). Current practice specifies theuse of an ERA or PDE5i as monotherapy in patients at earlier stagesof the disease [World Health Organization (WHO) functional class IIor III], with the use or addition of a parenteral prostacyclin synthetic/analog in advanced disease (WHO functional class IV) (2). Eventually,lung or lung/heart transplantation may be required for the many pa-tients who continue to progress. The treatment of PAH is extremelyexpensive. For example, in the UK, the annual cost to treat a patientwith advanced PAH with bosentan (an ERA) and epoprostenol (a syn-thetic prostacyclin) is $216,953 (current conversion rates) (11). In theUnited States, the annual cost of a patient treated with bosentan (anERA) can reach $79,404 (12, 13). A lung/heart transplantation addsmarkedly to the cost of the disease, and the fact that women of repro-ductive age are the primary victims (2) magnifies its societal impact.

RECENT PROGRESS

Before the 1990s, we already had a basic understanding of pulmonaryvascular biology [including the discovery of nitric oxide (NO), pros-tacyclin, and endothelin as major regulators]. Nevertheless, it was notuntil after the publication of seminal papers on human tissues and onpatients with PAH that progress in treating PAH was made (Fig. 2). In1993, the endothelin-1 axis was found to be up-regulated in lungs ofpatients with PAH (14), and in 1995, NO synthase was shown to bedown-regulated in patients’ lungs (15). Along with previous animalstudies, these results formed the basis of the “vasoconstrictor hypoth-esis” of PAH, which proposed that reversal of the apparent imbalancebetween vasoconstrictors (endothelin) and vasodilators (NO) in thepulmonary vasculature could be therapeutic. This was a welcome ideabecause there were no available therapies. A randomized trial showingthat a vasodilator (intravenous epoprostenol) improved PAH (16)sparked an increase in publications, and about 10 years after the 1993endothelin-1 paper, oral vasodilator therapy with the ERA bosentanwas shown to improve PAH in a randomized trial (17). Both epoprostenoland bosentan were subsequently approved for use in the clinic, al-though the trials were short (3 months) and the efficacy was modest(an increase in the distance covered in a 6-min walk, the primary

Table 1. Clinical classification of pulmonary hypertension (DanaPoint, 2008) (6).

1. Pulmonary arterial hypertension

1.1. iPAH

1.2. hPAH

BMPRII

ALK1, endoglin (with or without hereditary hemorrhagic telangiectasia)

Unknown

1.3. Drug- and toxin-induced

1.4. aPAH

Connective tissue disease

HIV infection

Portal hypertension

Congenital heart diseases

Schistosomiasis

Chronic hemolytic anemia

1.5. Persistent pulmonary hypertension of the newborn

1* Pulmonary veno-occlusive disease and/or pulmonarycapillary hemangiomatosis

2. Pulmonary hypertension due to left ventricular dysfunction

2.1. Systolic dysfunction

2.2. Diastolic dysfunction

2.3. Valvular disease

3. Pulmonary hypertension due to lung diseases and/or hypoxia

3.1. Chronic obstructive pulmonary disease

3.2. Interstitial lung disease

3.3. Other pulmonary diseases with mixed restrictive andobstructive pattern

3.4. Sleep-disordered breathing

3.5. Alveolar hypoventilation disorders

3.6. Chronic exposure to high altitude

3.7. Developmental abnormalities

4. Chronic thromboembolic pulmonary hypertension

5. Miscellaneous

5.1. Hematologic disorders: myeloproliferative disorders, splenectomy

5.2. Systemic disorders: sarcoidosis, pulmonary Langerhanscell histiocytosis

5.3. Metabolic disorders: glycogen storage disease, Gaucher disease,thyroid disorders

5.4. Others: tumoral obstruction, fibrosing mediastinitis

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endpoint, of 47 and 36 m, respectively). These successes have certainlyenhanced the lives of PAH patients. Nevertheless, vasodilators are notas useful as initially hoped, because they do not treat the cause of thedisease: PAH is not a result of vasoconstriction but rather a result ofproliferative vascular remodeling (18, 19).

The vasodilator therapies, including the later-approved PDE5i, whichactivate the NO–soluble guanylyl cyclase (sGC)–cyclic guanosinemonophosphate (cGMP) axis (20), were not developed primarily asPAH therapies. Epoprostenol and ERAs had failed in congestive heartfailure and hypertension trials (because of worse patient outcomes)(21–23). Sildenafil (a PDE5i), initially formulated as a systemic vaso-dilator, was tested in PAH when patents for its popular use in erectiledysfunction were due to expire.

In 2001, patients with hPAH were found to carry loss-of-functionmutations in the bone morphogenetic protein receptor II (BMPRII)(24, 25). Signaling downstream of this receptor proved to be complex,and this work has not yet led to any definitive conclusions or promis-ing therapeutic targets. Such mutations are found in most patientswith hPAH (although these are rare) but in only a few patients withthe more common sporadic (nonheritable) PAH (2).

Over the past 15 years, a myriad of molecular factors have beenshown to contribute to PAH in animals, and many have been effec-tively targeted in preclinical studies, as recently reviewed (18, 19, 26).These factors are diverse, ranging from potassium channels (27) toserotonin receptors (28, 29), vasoactive intestinal peptide (VIP) (30),survivin (31), Notch (32), tyrosine kinases (33), vascular elastases (34),mitochondrial metabolism (35–38), apelin (39), or transcription factorslike nuclear factor of activated T cells (NFAT) (40), hypoxia-induciblefactor 1a (HIF1a) (41), peroxisome proliferator–activated receptor g(PPARg) (42), or signal transducer and activator of transcription 3

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(STAT3) (43, 44). This diverse array of targets makes it difficult toidentify the most promising to translate to the bedside.

In the face of this plethora of possible drug targets, how are weperforming in this critical stage of translational research? Of the 598clinical trials for PAH registered at clinicaltrials.gov (Fig. 3), 393 explorePAH therapies. However, 328 of these test drugs are from one of the threealready approved classes. Only 65 test new therapies, and of these, mostare for drugs that are either generic or already used for other indications;none has been developed specifically for PAH. Overall, only 2.5% ofall current trials test drugs that reverse the specific vascular pathologyof PAH in preclinical studies. These include pyruvate dehydrogenase(PDH) kinase inhibitors (dichloroacetate), 5-hydroxytryptamine (5-HT)reuptake or receptor antagonists (PRX-08066, fluoxetine, and escitalopram),tyrosine kinase inhibitors (imatinib, sorafenib, and nicotinib), VIP, en-dothelial progenitor cell therapy, dehydroepiandrosterone (DHEA),PPARg agonists (pioglitazone), CD20 antibodies (rituximab), andinterleukin-1 receptor antagonists (anakinra) (Fig. 3). Most of these weredeveloped primarily for other indications including depression, cancer,and rheumatologic or metabolic diseases. Why are there no drugs de-veloped specifically for PAH, and why do so few trials investigate drugsthat specifically address the current understanding of the pathology ofPAH? The answers may lie in the challenges faced by clinicians, pa-thologists, scientists, and industry as they translate preclinical resultsinto clinical applications.

WANTED: A UNIFYING THEORY OF PAH

One outstanding hurdle to progress is that we lack a comprehensivetheory(s) that explains the diverse features of PAH. The modest effi-cacy of the currently approved therapies for PAH may be a result oftheir targeting only one of several involved signaling pathways and failto address the fundamentals of PAH pathology. However, to developPAH-specific therapeutics, a unified understanding of the etiologyand pathology of the disease is required.

Any comprehensive theory for PAH must explain the underlyingfeatures of this disease. These include an antiapoptotic and pro-proliferative diathesis, as well as a strong inflammatory diathesis thatcharacterizes the remodeled vascular wall in medium and small pul-monary arteries. This diathesis is the response of the pulmonary vesselsto diverse stimuli or disease triggers (hypoxia, germline loss-of-functionmutations, infection with viruses like herpes simplex or HIV, etc.). Anadditional key feature of PAH is that, although many of the triggersare systemic, only the pulmonary vasculature is affected; systemic ves-sels are typically spared.

An ideal PAH therapy should inhibit proliferation and induce ap-optosis (to reverse established lesions without the inflammatory con-sequence of nonapoptotic cell death) in the affected pulmonary vesselsbut not in systemic vessels. For example, a therapy that induces pulmo-nary vascular apoptosis nonspecifically may also cause aneurysms insystemic arteries. Also, an effective and pulmonary-selective therapyshouldnot adversely affect theRVor, ideally, should improve its function.

Unique features of the pulmonary vascular systemThe selective susceptibility of the pulmonary vasculature is a keyfeature that must be explained by any theory of PAH etiology. Onedistinction is its response to hypoxia. Because lungs are “distributors”of oxygen and the peripheral organs are “consumers” of oxygen, low

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1. First observation of increased endothelin-1 expression in lungs of PAH patients2. First observation of reduced endothelial NOS expression in lungs of PAH patients 3. First successful randomized clinical trial in PAH (intravenous epoprostenol)4. First demonstration of BMPRII mutations in familial PAH 5. First successful randomized clinical trial in PAH with an oral agent (bosentan)

Fig. 2. Number of publications on experimental PAH therapies afterpivotal human studies. The number of publications addressing exper-

imental therapies in PAH began increasing in 1995 after critical publica-tions in human PAH tissues and PAH randomized clinical trials (see textand Supplementary Materials).

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218 Industry-funded trials

110 Non-industry-funded trials

ERAs(15%)

Prostacyclinanalogs(21%)

Combination(1%)

ERAs (33%)

Prostacyclinanalogs(31%)

Combination(11%)

Outcomes, biomarkers and supportive therapies

Classes of drugs approved by FDA

New therapies

328

65

205

NO-cGMP axis (25%)(sGC activators, PDE5 inhibitors)

NO-cGMP axis (63%)(sGC activators, PDE5 inhibitors)

21 Industry-funded trials 44 Non-industry-funded trials

Drug intervention Drug target Clinical trial sponsor and collaborator

PRX-08066 Serotonin receptor antagonist

Epix Pharmaceuticals

EPC transplantation

Stem cell therapy

Northern Therapeutics, St. Michael’s Hospital, Sir Mortimer B. Davis (Jewish General Hospital)

Imatinib, nilotinib, sorafenib

Tyrosine kinase inhibitor

Novartis, Bayer, University of Chicago

Ferricarboxymaltose Iron-regulated proteins

VU University Medical Center, Vifor Pharma

Ranolazine Fatty acid oxidation inhibitor

Northwestern University, Gilead

Drug intervention

Drug target Clinical trial sponsor and collaborator

DCA PDK inhibitor University of Alberta, Imperial College

Fluoxetine, Escitalopram

Selective serotonin re-uptake inhibitor

University of Texas SW Medical Center

Tacrolimus NFAT inhibitor Stanford University

N-Acetylcysteine Antioxidant University of Washington

Apelin Ligand to APJ receptor Golden Jubilee National Hospital, NHS Lothian, Imperial College Healthcare NHS Trust

Pioglitizone PPAR activator Stanford University

Simvastatin, Atorvastatin, Rosuvastatin

HMG-CoA reductase inhibitor

Imperial College, MRC,Cardiovascular Institute & Fuwai Hospital

Anakinra Interleukin-1 receptor antagonist

Virginia Commonwealth University

DHEA AKT pathway inhibitor University Hospital, Bordeaux

VIP SMC proliferation inhibitor

Medical University of Vienna

Carbon Monoxide

eNOS activator NIH, NHLBI

Metformin AMPK activator Nantes University Hospital

Rituximab Antibody to CD20 National Institute of Allergy and Infectious Disease

Anastrozole Aromatase inhibitor University of Pennsylvania

Hydroxyurea Ribonucleotide reductase inhibitor

Children’s Memorial Hospital

Fig. 3. Current clinical trials in PAH. There are 598 ongoing clinicaltrials on pulmonary hypertension (clinicaltrials.gov). Currently approvedclasses of drugs for PAH—prostacyclin synthetics/analogs, ERAs, and ac-tivators of the NO-cGMP axis [including phosphodiesterase 5 inhibitors(PDE5i) or soluble guanylyl cyclase (sGC) activators]—are being tested in328 of these clinical trials. Two hundred five PAH clinical trials investi-gate clinical outcomes in cohorts or registries of PAH patients (survival,quality of life, and number of hospitalizations), biomarkers, and support-ive therapies, whereas only 65 clinical trials test new PAH therapies. Ofthese 65 trials, which are based on strong preclinical evidence of rever-sal of the proliferative vascular remodeling, 21 are funded by industryand 44 by other funding sources. Most of these therapies have beendeveloped to treat other common diseases, not PAH, such as depres-sion, rheumatologic diseases, or cancer. ERAs, endothelin receptor an-tagonists; NO, nitric oxide; GC, guanylyl cyclase; DCA, dichloroacetate; EPC,

endothelial precursor cells; SMC, smooth muscle cells; APJ, apelin receptor.

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oxygen levels cause the pulmonary arteries to constrict and the sys-temic arteries to dilate. This hypoxic pulmonary vasoconstriction(HPV), intrinsic only to the lungs, is present in all mammals and main-tains ventilation-perfusion match, because constriction of a pulmonaryartery perfusing a hypoxic area redirects the blood to better-oxygenatedareas of the lung (45). The mitochondria of pulmonary arterial smoothmuscle cells (PASMCs), which serve as oxygen sensors in many tis-sues, are the basis of this pulmonary response (45). In hypoxia, themitochondria alter the production of mitochondria-derived reactiveO2 species (mROS), which diffuse to regulate redox-sensitive targets.These targets, such as the plasma membrane voltage-gated potassiumchannels (Kv) or redox-sensitive components of the HIF1a signalingpathway, induce acute PASMC contraction (via the intracellular calciumincrease that follows Kv channel inhibition) or initiate the response to amore sustained hypoxia by the activation of the many HIF1a-regulatedgenes. PASMC mitochondria are quite distinct from mitochondria insystemic arteries (for example, they have altered expression of com-plex I proteins and manganese superoxide dismutase), potentiallyexplaining the restriction of HPV to the pulmonary circulation (46).Mitochondria are also critical regulators of apoptosis, can induceproliferative and inflammatory signals, and at the same time can in-tegrate other molecular signals such as oncogenes, growth factors,and endoplasmic reticulum (ER) stress (47). These unique featuresofmitochondriamay contribute to PAHand could therefore be targetsfor selective and effective proapoptotic and antiproliferative therapiesfor PAH (48).

The contributions of metabolic dysfunction andinflammation to PAHMitochondrial function, specifically glucose oxidation, is suppressed inboth PAHPASMCs and endothelial cells (37, 38, 49). The consequencesof this suppression are multiple.

(i) The suppression of glucose oxidation and a metabolic switchtoward glycolysis is associated with a state of resistance to apoptosisvia several mechanisms, reviewed elsewhere (50). Mitochondria be-come hyperpolarized and mROS decreases (37, 38, 51–54). This resultsin an increase in the opening threshold of the mitochondria transitionpore, a mega channel through which proapoptotic factors (like cyto-chrome c or apoptosis-inducing factor) move to the cytoplasm (55).The inefficiency of adenosine triphosphate (ATP) production that fol-lows the suppression of mitochondrial function results in a secondaryup-regulation of glucose uptake and glycolysis to generate ATP inthe cytoplasm. This is in keeping with higher uptake of a radiolabeledglucose analog in the lungs of PAH patients than in those of healthycontrols (Fig. 4A) (49). Many up-regulated glycolytic enzymes alsohave secondary antiapoptotic properties (50).

(ii) The levels of mROS and diffusible mitochondria-derived me-diators like a-ketoglutarate (aKG) decrease. This has multiple down-stream effects. For example, plasma membrane redox-sensitive Kvchannels are inhibited, leading to a secondary influx of calcium tothe cell (45, 56). Many pro-proliferative, calcium-sensitive transcriptionfactors are activated, including NFAT. Also suppressed is NFAT’s in-hibitor, the metabolic enzyme glycogen synthase kinase 3b (GSK3b),a direct result of the metabolic switch toward glycolysis (37, 57). NFATnot only activates proliferative signals but also down-regulates Kv chan-nels and promotes global mitochondrial suppression, closing a positivefeedback loop (58). NFAT is critical for PAH, and its inhibition canreverse PAH in animal models (40).

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The decrease in mROS and aKG also promotes activation of HIF1a,another master transcription factor that is activated in animal and hu-man PAH, even in the absence of hypoxia (41). HIF1a can be activateddirectly by suppressed mitochondrial signaling via both redox-sensitiveand redox-insensitive mechanisms (54). Like NFAT, HIF1a also sup-presses Kv channels and directly promotes proliferation (41).

(iii) Although yet unexplored in PAH, mitochondrial suppressionmay also activate the inflammasome (59), which could cause a cascadeof events that increases autocrine or paracrine cytokines (60) and re-cruits inflammatory cells to the diseased pulmonary circulation (61).

Thus, mitochondrial suppression could explain diverse, apparentlyunrelated, features of the PAH phenotype: hyperpolarized mitochon-dria, activated NFAT and HIF1a, inhibition of Kv channels, increasedcytosolic calcium levels, and activation of inflammation. If this sup-pression is global and includes, for example, peripheral muscles, it couldalso explain the insulin resistance (not related to obesity or diabetes)recently described in PAH patients (62). Indeed, suppressed oxidativephosphorylation in skeletal muscle mitochondria contributes to themetabolic syndrome (63, 64).

What is the proximal cause (or upstreammediators) of this mitochon-dria suppression in PAH? One candidate is the gate-keeping enzymefor glucose oxidation, mitochondrial PDH, which is inhibited in severalPAH models (35–38, 53). This suppression may be caused by upstreamregulation by its inhibitor PDH kinase (PDK), a HIF1a-inducible anda tyrosine kinase receptor–activated enzyme (65, 66), and an importanttherapeutic target (35–37). Another regulator of PDH is mitochondrialcalcium, which, when suppressed, decreases both PDH and general mito-chondrial function. The ER stress that accompanies many of the trig-gers of PAH (infections, BmprII mutations, and toxins) (3, 67–71)would decrease calcium entry from the ER as part of stress-induced ana-tomic remodeling of the ER-mitochondria unit (38). There are also fewermitochondria in PAH tissues, a result of abnormalities in the masterregulator PGC1a or in proteins like mitofusin-2 that regulate mitochon-dria networking during biogenesis (72). Genetic polymorphisms inmitochondrial regulators such as Sirt3 and Ucp2 have also been linkedto metabolic and vascular diseases in humans (73, 74). Intriguingly, theabsence of Ucp2 (uncoupling protein 2; a mitochondrial protein thatregulates calcium entry into the mitochondria) causes mitochondrialsuppression in pulmonary artery smooth muscle cells. Ucp2-deficientmice, which have features of insulin resistance, develop spontaneouspulmonaryhypertension, associatedwith activationofNFATandHIF1a,even under normoxia (75).

Normalization of these causes of mitochondrial suppression canreverse PAH in animal models. Activation of PDH, directly (with thesmall-molecule PDK inhibitor dichloroacetate) (35, 37, 76) or indirectly(with activators of fatty acid oxidation) (37), mitofusin-2, or PGC1a (72)or inhibition of ER stress with phenylbutyrate or salubrinal (61, 77) nor-malizes both the cellular and the in vivo phenotype of the disease, includ-ing seeminglyunrelated abnormalities such asNFATorHIF1a activationor inhibition of Kv channels, promoting apoptosis and reversal of thePAH vascular remodeling selectively in the pulmonary circulation.

Another overarching hypothesis for PAH that can potentially ex-plain several PAH-related abnormalities is inflammation, as discussedin a recent review (60). Sterile inflammation characterizes animal pul-monary hypertension models (like the hypoxia and monocrotalinemodels) and is an increasingly recognized feature of clinical PAH(78–80). In addition, PAH patients with generalized inflammation(for example, connective tissue diseases and HIV) have worse prognosis

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and clinical outcomes than those without inflammation (81). Immuno-suppressive therapy causes improvement in a subset of PAH patients,supporting the relevance of inflammation to PAH (82). Remodeled vas-cular walls and the perivascular region of PAH lungs contain inflam-

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matory infiltrates (83): T cells (84), Bcells (85), macrophages (85), neutro-phils (86), andmast cells (87) (Fig. 4B).Inflammatory cytokines such as in-terleukins, stromal-derived factor–1,monocyte chemoattractant protein–1,and tumor necrosis factor–a (TNFa)are found in the serum of PAH pa-tients (88, 89), often correlatingwithdisease severity and prognosis (88).Targeting these immune cells or cy-tokines [like T helper 2 (TH2) cells,interleukin-1, and TNFa] can reversePAH in animals (53, 90, 91).

Because suppressed mitochondriacan induce the inflammasome (92),both metabolic defects and inflam-mation may contribute to the patho-genesis of PAH (Fig. 4C). Many ofthe inflammatory mediators elevatedin PAH can also suppress mitochon-drial function. For example, TNFacan inhibit PDH in PASMCs and in-duce the PAH cellular phenotype innormal PASMCs (53). Inhibition ofTNFa with etanercept normalizesthe metabolic phenotype in animalPAH and reverses the disease in vivo(53). On the other hand, activated im-munecells showsuppressedmitochon-drial function and develop a glycolyticphenotype, similar to PAH vascularcells (93, 94). As in PAH PASMCsin animal models, the circulating im-mune cells of PAH patients show ERstress (95), a state that is associatedwith increased synthesis of cytokines(95). These findings suggest that thereis a mutually reinforcing interplay be-tween the vascular cells and activatedimmune cells that amplify and sustainthe PAH phenotype in vivo.

Patients with the familial diseaseDursun syndrome exhibit iPAH, ab-normal white blood cells, and atrialseptal defects in children; these pa-tients carry a causal homozygousmu-tation inG6PC3, the gene encoding theglucosemetabolism enzyme glucose-6-phosphatase (96, 97). The abnormal-ities in inflammatory cells and glucosehomeostasis in these patients may berelated to their PAH, mimicking thesuppressed glucose oxidation and

activated inflammation in both humans and animals with PAH. Thecharacteristics of PAH seen in patients withDursun syndrome supportan underlying combined metabolic and inflammatory deficit in thisdisease.

BMPR2 mutationsInflammation

HypoxiaViruses

PAH pulmonary arterysmooth muscle cell

Activated inflammatory cell

ER stress

ER stress

Cytokines

Cytokines

Cytokines

ER/Mitodisruption

ApoptosisImmune cell recruitment

Inflammasome

Proliferation UPR

UPR

NFAT

NFAT

Ca2+

Ca2+

HIF1 mROS

C

A B Infiltration of inflammatory cells in PAH

0.0151 2 3 4 5 6 7 Ctrl

Ctrl IPAHIPAH

0.025

0.035

0.045

averagevalues

Increased 18-FDG uptake in PAH

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pta

ke (S

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)

Fig. 4. An emerging comprehensive theory for PAH: metabolism and inflammation. (A) Metabolic

alterations in PAH. Four iPAH patients show higher glucose uptake in the lungs than do three healthycontrols, as assessed by 18-fluorodeoxyglucose uptake (18-FDG), suggesting a cancer-like switch in metab-olism toward glycolysis. The lung standardized uptake values (SUV) were normalized to lung tissue fraction.Mean data are shown on the right (*P < 0.01, multiple comparison test). Modified with permission from(49). (B) Inflammation in PAH. Cross sections of pulmonary arteries from PAH lung are remodeled, withdense inflammatory cell infiltrates (left panel, black arrow) or with concentric intima thickening (right panel,red arrow) surrounded by a dense infiltration of inflammatory cells (right panel, black arrow) (Russel-Movatpentachrome stain; nuclei, black; smooth muscle, red; mucin, blue). Modified with permission from (80). Scalebar, 100 mm. (C) Diagram showing ER stress and mitochondrial dysfunction in PAH. Suppressed mitochondrialfunction in smooth muscle cells of PAH pulmonary arteries causes increased intracellular calcium (Ca2+) anddecreased generation of mitochondrial-derived reactive oxygen species (mROS), resulting in activation ofpro-proliferative transcription factors like NFAT and HIF1a. Suppressed mitochondrial function can resultthrough one of two ways: (i) ER stress (which causes a disruption of the ER-mitochondrial unit and decreasedmitochondrial calcium, in turn inhibiting many calcium-sensitive mitochondrial enzymes such as pyruvatedehydrogenase), or (ii) suppression of pyruvate dehydrogenase by inflammatory cytokines such as TNFa,resulting in apoptosis resistance. ER stress can also increase the production of cytokines by inducing the un-folded protein response (UPR), resulting in the recruitment and activation of inflammatory cells. Inflammatory cells(characterized by a similar suppression of mitochondrial function) are activated and recruited to the lungs of PAHpatients and can increase the cytokines that induce ER stress or directly suppress mitochondrial function in PAHsmooth muscle cells of PAH pulmonary arteries. Cytokine production by PAH inflammatory cells can also haveautocrine effects on mitochondrial function. PAH immune cells also show ER stress, which activates NFAT and up-regulatesmany cytokines. This interplay of vascular and immune/inflammatory cells in PAH further reinforces andsustains the molecular phenotype of the disease.

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TRANSLATIONAL CHALLENGES FOR THE PATHOLOGIST

The pathology of PAH has not beensystematically describedObliterative intima and media thickening, consisting mainly of theproliferation of smooth muscle cells and myofibroblasts, as well asof plexiform lesions (disorganized growth of proliferative endothelialcells), form the foundation of PAH pathology (Fig. 1B). PAH pathol-ogy was originally described in one cohort by Heath and Edwards in1958 (98), and since then in 10 other cohorts, a total of 609 cases(80). Although the heterogeneity of the vascular lesions within a lungof individual patients has been recognized, the sampling strategy hasnot been standardized and consistent. Additionally, there has beenno systematic attempt to make clinical-pathological correlations, aparticular problem because the definition of PAH has been expandingover the past 20 years. Many pathologists still use the Heath-Edwardsgrading of vascular lesions from 1958, which was based on a cohort of67 lungs with PAH secondary to congenital heart disease and only 2lungs from patients with iPAH. This classification described worsen-ing vascular pathology, from grade 1 (medial hypertrophy) and grade2 (intimal hypertrophy) to grade 5 (plexogenic lesions), giving the(untested) impression that this describes the natural history of diseaseprogression. This oversimplification, along with scarce tissue for study,forced the field away from the clinicopathological approach to the dis-ease, one of the cornerstones of medicine.

To address several of these limitations, in 2006, the PulmonaryHypertension Breakthrough Initiative established a network of 13 multi-disciplinary research and transplant centers. Whole lungs from trans-plant surgery (rather than lung samples or biopsies), cells, and bloodwere retrieved along with relevant validated clinical data. This systematicanalysis of 62 PAH and 28 control lungs (80) showed that plexogeniclesions were found in most but not all PAH samples, whereas mediaand intima thickening were found in all. On several occasions, someslides from the same patient showed no plexogenic lesions, whereasothers showed as many as 27 lesions. This confirmed the notion thatthe pathology of PAH is heterogeneous. Media thickening, but notplexogenic arteriopathy, correlated with mean pulmonary artery pres-sure and pulmonary vascular resistance. No differences in any of thelesions were found with current PAH therapies, with the exceptionof prostacyclin and its analogs, which correlated with the number ofplexiform lesions. In some patients, plexiform lesions were present inthe absence of media or intima thickening, challenging the notionthat the plexiform lesions are the most advanced pathologic findingin PAH in a disease progression continuum. Many of the control lungsalso showed media and intima thickening, supporting the need forlarger sample sizes and biobanks of PAH tissues from patients withreliable and detailed clinical data.

Another important finding from this study (80) was the unexpect-edly strong perivascular and interstitial inflammation (infiltration withlymphocytes, macrophages, and neutrophils) in all PAH lungs). Al-though the sample sizewas relatively small, specific patterns did emerge:Muscular remodeling (media and intima thickening) plus plexiformlesions showed 94% concordance with the clinical diagnosis of iPAH.In contrast, interstitial matrix remodeling, muscularization, inflam-mation, and fibrosis showed 42% concordance with the clinical diagno-sis of scleroderma-associated PAH. Different molecular phenotypesmay underlie these pathology patterns, undermining the idea that atherapy showing efficacy in one type of PAH will be beneficial in

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another. The use of more than one animal model in preclinical researchmay be necessary to cover the spectrum of disease pathology.

Response: Systematically collect and analyze human PAH tissues.Expansion of the methodology of the Pulmonary Hypertension Break-through network, including methods to preserve macromolecules formechanistic correlation studies, should be a priority for the field. Broadernational and international networks for tissue harvesting and process-ing need to be developed and synchronized to establish large tissuebanks accessible to all investigators. At this point, the harvesting ofPAH tissues is limited to transplant surgery centers, although cathetersfor percutaneous endovascular biopsies to enable longitudinal studiesin living PAH patients are under development (99).

Response: Understand the natural history of disease progression.The earliest lesion(s) in PAH and its triggers are unknown. The hy-pothesis that the earliest lesion is endothelial cell injury, which leadsto pulmonary arterial endothelial cell apoptosis, followed by patho-logical repair and regeneration of the vasculature and recruitment ofproliferative precursor cells, is intriguing but unproven (85). PAH tissuescollected at the time of transplant will not elucidate this question becausethe disease is then at a highly advanced stage. Molecular imagingapproaches that could detect apoptotic endothelial cells (similar to whatis being developed in oncology) may prove to be useful (100).

TRANSLATIONAL CHALLENGES FOR THE CLINICIAN

All types of PAH are not likely to be equivalentSeveral international meetings have proposed a classification schemefor PAH that groups conditions of diverse origin under the umbrellaof PAH (6) on the basis of the fact that the lung vascular pathology ispresumed to be similar (Table 1). After the pivotal clinical trial ofbosentan (17), which primarily included patients with iPAH, the Foodand Drug Administration approved the drug for several aPAH condi-tions not included in the trial. This presumed similarity between iPAHand aPAH has not been systematically verified, however, and it is notclear that the various types of PAH share the same pathology and thattreatment should be the same among them. For example, hPAH causedby BMPRII loss-of-function mutations (which dysregulate SMADsignaling downstream of the BMPRII receptor) may be distinct fromanorexigen-induced PAH (in which serotonin homeostasis andvoltage-gated potassium channels are dysregulated in pulmonary vas-cular cells) or forms of PAH with activated inflammatory signaling(such as scleroderma-associated PAH). Indeed, as discussed above,systematic analyses of human PAH lung samples are revealing distinctphenotypes. Our present classification of PAH may not be adequate topredict whether a patient with a specific form of PAHwill benefit froma treatment that has shown benefit (or not) in another form of PAH.

Response: Reexamine the clinical classification of PAH. Therecent progress in the molecular understanding of the disease and re-cent reassessments of PAH pathology should be taken into account. A“metabolic” or an “inflammatory” phenotype, for example, may bemore relevant for management and choice of therapy than the currentclassification of PAH.

RV function as a key determinant of PAH outcome is notreflected in clinical practice or in researchThe most important determinant of morbidity and mortality in PAHis not the severity of the vascular pathology, as previously assumed,

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but rather the function of the RV (2, 5), a poorly understood aspect ofPAH. The characteristics of the left ventricle cannot be extrapolated tothe RV because the two chambers have distinct embryology and dif-ferences in metabolism and physiology (101). The response of the RVto pressure overload is very different, and the compensation phase(the phase in which RV remodeling prevents a decrease in cardiac out-put) is much shorter than that of the left ventricle. In some PAH pa-tients, pulmonary vascular resistance decreases in response to therapies(suggesting a positive response to treatment), but the RVs continue todeteriorate and the disease progresses clinically (102). Indeed, MRI in-dices of RV dysfunction predict clinical worsening in patients withPAH, independent of pulmonary artery pressure (103). These datachallenge the generally accepted idea that RV failure is a linear andpredictable consequence of progressive vascular remodeling. RV-intrinsic molecular mechanisms may drive primary RV deterioration,independent of the pulmonary vascular changes (104).

Studies of PAH may not directly assess RV function. Many ap-proved PAH therapies have failed in previous trials of congestive heartfailure, but their effects on the RV were never studied before clinicaldevelopment for PAH. Although it is now becoming clear that it is theRV function and its response to therapy that determine morbidity andmortality in PAH, RV function is not always included in the interpre-tation of clinical research data. Useful echocardiographic indices forRV function are being validated, but for now, costly MRI remainsthe gold standard for assessment of RV function (105).

Response: Study the RV and pulmonary circulation as a unit.It is not enough to show that an experimental therapy improves thepulmonary vascular remodeling or decreases the mean pulmonary ar-tery pressure. Without improvement in RV function and the cardiacoutput, there will be no net benefit for the animal or the patient. Atherapy that suppresses RV function can appear to cause a decrease inpulmonary artery pressure, but in actuality, the response is a result ofRV pump failure not of improvement in pulmonary vascular remodel-ing. Thus, the direct effects of any promising therapy for PAH shouldbe tested on RV myocardium in vivo and ex vivo to make sure thatthere is a net improvement in cardiac output in vivo as well as in func-tional capacity and survival (106). For example, PDE5i and ERAs bothdecrease pulmonary artery pressure, but they have opposite effects onthe RV when the RV is studied ex vivo: PDE5i increase (107) whereasERAs decrease (108) RV myocardial inotropy (Fig. 5, A and B), per-haps explaining why edema (a potential result of suppressed RV con-tractility) is much more common in patients treated with ERAs.

Current drugs do not improve patient survival orhemodynamic function and clinical trials do notuse strong and mechanistic endpointsCurrent clinical guidelines (2) for PAH are based on trials in which theprimary endpoint is the distance that a patient can walk in 6 min. Thistest was chosen for the first randomized trial in PAH (epoprostenol)to reflect the patient’s symptoms and functional performance (16). Alllater trials used the same endpoint, and all current PAH therapies weresubsequently approved on the basis of prolongation of the 6-min walkby about 40 m. Although statistically significant, this increase is notclinically meaningful or predictive of outcomes, including survival,hospitalizations, or need for heart-lung transplantation (109, 110). Asupervised 10-week exercise protocol without any therapy improvesquality of life indices and increased the 6-min walk more than 40 min PAH patients (111).

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Reliance on drugs that only improve the 6-min walk time butnot more meaningful endpoints can give the false impression thatthere are many good therapy options available to PAH patients. Thissituation hinders translational efforts of new and more effectivetherapies.

Response: Develop drugs with new modes of action, relevantto PAH-specific pathology. Most drugs being tested clinically in PAH(outside of the three approved classes of drugs) are either generic orapproved for other, non-PAH–related diagnoses. This may be in-dicative of the barriers to investment in the clinical development ofdrugs for rare diseases such as PAH, although the Orphan Drug Actcan offer assistance (112). Creative approaches to intellectual propertylaw will be important.

Response: Identify and validate biomarkers for PAH. Effortsneed to be made to develop therapy-specific biomarkers, particularlyusing molecular imaging approaches. For example, a metabolism-based imaging biomarker (like glucose uptake studies with positronemission tomography) will be important for the assessment of meta-bolic modulators. An imaging biomarker that can detect apoptosisin vivo could be critical for the assessment of a proapoptotic therapyin vivo (100).

Response: Confirm preclinical animal results in mechanisticearly-phase human trials. Currently, early-phase trials focus onsafety/toxicity and potential efficacy assessed on the basis of non-mechanistic endpoints such as tolerance, toxicity, 6-min walk, andpulmonary artery pressure. One aim of such trials should be to iden-tify the molecular phenotypes of patients most likely to respond to aparticular therapy, with the goal of validation of therapy- and disease-specific biomarkers. Early-phase trialists (clinician scientists that oftenalso perform preclinical work) should be embedded in networks ofpreclinical research to facilitate efficient translation. In addition, suchnetworks can develop and support much-needed industry-independentblood and tissue registries. Trial designs with adaptive protocols andstatistical approaches for rare diseases are being developed and shouldbe considered for PAH trials (113–115).

Response: Identify specific subgroups of patients. Defined mo-lecular phenotypes may allow later-phase trials in specific subgroups ofpatients with higher chance for positive response. For example, patientswho show evidence of decreased PDH activity [measured in vivo by eitherhyperpolarized carbon-13 MRI (116) or by a breath test of 13C-labeledpyruvate (117)] may be the best candidates for a drug that increasesPDH activity (such as the small-molecule dichloroacetate). Patients with ageneralized metabolic abnormality (for example, insulin resistance basedon standard blood tests) may be better candidates for a systemicallydelivered metabolic modulator; patients with BMPRII mutations maybe the best candidates for a therapy that enhances downstreamBMPRIIsignaling.

Response: Late-phaseclinical trials shouldseek reversalofdisease.The objective of clinical trials should be the true reversal of diseasewith minimal toxicity in prespecified patient subgroups with specificmolecular phenotypes (118, 119). Protocols need to be extended to atleast a year so that survival, sustainability of the effect, and long-termtoxicity can be explored in a meaningful manner (118).

Response: Base clinical practice on trials with strong end-points. Future treatment guidelines should be based on clinicaltrials in which efficacy is supported by strong endpoints such as sur-vival or improvement in hemodynamics (pulmonary vascularresistance measured invasively or RV function).

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TRANSLATIONAL CHALLENGES FOR THE SCIENTIST

All animal models for PAH have deficienciesThe current animal models of PAH do not completely recapitulate thehuman disease (120, 121). Most common are the monocrotaline(MCT) model in rats and chronic hypoxia in rats and mice. MCT ratsdo exhibit an inflammatory lung environment, pulmonary vascularremodeling (media hypertrophy but not plexogenic lesions) with lessinvolvement of systemic arteries (the toxic metabolite of MCT is me-tabolized in the liver and preferentially enters the pulmonary circula-tion), and rapid RV deterioration, mimicking clinical PAH. Thechronic hypoxic model does not properly recapitulate the humanPAH disease (mild to moderate increase in pressure, media hypertro-phy but no plexogenic lesions) but is a good model to study the com-mon form of pulmonary hypertension that results from chronichypoxia and chronic obstructive pulmonary disease.

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Another model, which is being increasing-ly used, better mimics human PAH histology(severe intima thickening and severe oblitera-tive vascular lesions) but requires an injectionof a vascular endothelial growth factor (VEGF)inhibitor (the Sugen compound) in tandemwith hypoxia (122). However, in PAH, VEGFis up-regulated in the remodeled pulmonaryvessels (123), and kinase inhibitors that inhibitVEGF are being tested as potential therapies forclinical PAH (124, 125). It is therefore not clearwhether VEGF inhibitors promote or inhibitPAH, creating confusion when this model isused for therapeutic development with suchdrugs.

One of the few animal models that developspontaneous PAH is the fawn-hooded (FH)rat in which spontaneous PAH is acceleratedby chronic hypoxia (41, 126). The precise mo-lecular mechanism of PAH in these animals isunknown. A small subset (<5%) of S100A4/Mts1-overexpressing mice, which develop metastaticmouse mammary adenocarcinoma, also de-velop plexogenic-like lesions similar to humanPAH (127), but the phenotype is very rare,making use of these mice to study PAH im-practical. Although one study suggested thatBmprII+/− mice developed moderately elevatedmean pulmonary artery pressures (128), othersusing the same mice showed no significantchanges (129, 130). In addition, mice with con-stitutively decreased BmprII expression did notexhibit changes in pulmonary vascular resist-ance (131). In contrast, a mouse strain withforced overexpression of a human BmprIImu-tation developed significant PAH (132), al-though the forced overexpression likely causesER stress, confounding the underlying mecha-nism (humans do not overexpress the mutatedprotein).

There is no perfect animal model of hu-man PAH, although several aspects of human

PAH can be recreated (121). The lack of plexogenic arteriopathy in theMCT, chronic hypoxia, or FH rat models may not be a fatal weaknessbecause the plexogenic lesion may not be the sine qua non of humanPAH.

Response: Use multiple animal models for preclinical research.The increasingly apparent heterogeneity of PAH pathology necessitatesthe use of different models because none reproduces all of the aspects ofPAH pathology. At least two animal models and human tissues shouldbe used in each preclinical study. It is also important to prove that atherapyworks through the samemechanism in rodent as in human vas-cular cells (ideally derived from patients with PAH or healthy tissues).

Clinically relevant endpoints are rarely used inpreclinical PAH studiesMost preclinical investigations of PAH examine the response of ro-dents to an experimental therapy after 3 to 4 weeks of treatment. This

B Normal RV Hypertrophied RV

ETR-A MHC Merge + DAPI ETR-A MHC Merge + DAPI

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PDE5 inhibitor

ERA

MHC Merge + DAPIPDE5 PDE5 MHC Merge + DAPI

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Fig. 5. PDE5i and ERAs have opposing effects on the hypertrophied RV. (A) PDE5i have apositive inotropic effect on the hypertrophied RV (left), whereas ERAs have a negative inotropic

effect on the hypertrophied RV (right). RV contractile pressure is measured in mmHg. Each blackline (spike) represents an individual contraction. A modified Langendorf model was used inwhich the pulmonary artery in an isolated heart was ligated, and the heart was perfused withinhibitors. A latex balloon filled with water and attached to a pressure transducer was inserted inthe RV via the right atrium to measure RV pressures. In this model, the afterload of the RV is fixed(the pulmonary artery is ligated) and the preload remains stable (the volume of the balloon), andthus, changes in contractile pressure reflect changes in RV myocardial contractility. Modified withpermission from (107, 108). (B) (Upper row) Images from an explanted heart from a normal humanRV and from a hypertrophied RV of a PAH patient. PDE5 (green) colocalized to RV cardiomyocytes[red, myosin heavy chain (MHC) as a marker] is shown in yellow (merge). Nuclei, blue. Very low levelsof PDE5 are seen in the normal RV. (Lower row) Endothelin receptor type A (ETR-A, green) in normaland hypertrophied RV from a PAH patient. Yellow indicates colocalization of ETR-A with cardiomyo-cyte MHC (red) (merge). The normal human RV shows very little ETR-A. This up-regulation of PDE5and ETR-A in the hypertrophied RV explains why inhibitors of these proteins have direct effects onthe hypertrophied (but not normal) RV myocardium. Modified with permission from (107, 108). DAPI,4′,6-diamidino-2-phenylindole.

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period is not adequate to determine whether the effects are sustainedor temporary, to detect long-term toxicity, or to explore effects onsurvival. Nevertheless, assessment of these endpoints is essential forinformative preclinical studies, as is exploration of bone marrow andliver toxicities, to avoid the emergence of these problems later. Fur-thermore, the functional capacity of the treated animals is not typicallymeasured (for example, with rodent treadmill tests). Cardiac output isoften not reported. An additional weakness of many preclinical studiesis the measurement of RV systolic pressure instead of mean pulmo-nary artery pressure, a result of the fact that it is much easier to inserta high-fidelity catheter in the RV than in the main pulmonary artery.Although the RV systolic pressure is usually the same as the systolicpulmonary artery pressure, this method does not measure true meanpulmonary artery pressure, precluding accurate calculation of pulmo-nary vascular resistance [(mean pulmonary artery pressure − pulmonaryartery wedge pressure)/cardiac output], an essential endpoint in bothanimal and clinical studies (133). Promising PAH therapies shouldcause a measured decrease in pulmonary, but not systemic, vascularresistance. Possible direct effects of potential therapies on the RV arealso rarely assessed, despite the recognized importance of RV functionin the overall prognosis of PAH (134). In summary, preclinical animalstudies do not focus on endpoints that are clinically relevant, includingexercise capacity, survival, pulmonary vascular resistance, RV func-tion, and toxicity.

Response: Use clinically relevant endpoints in animal research.For optimal translation, preclinical studies should incorporate com-prehensive hemodynamic assessment, functional capacity, long-termstudies for toxicity, and survival assessment. Only then can an exper-imental therapy be properly assessed for potential translation to hu-man studies, increasing the confidence of trialists and sponsors ineventual clinical success.

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THE FUTURE

Preclinical progress in PAH research can be accelerated with a trans-parent and robust multicenter approach. Networks of centers with ex-pertise in PAH preclinical research should be assembled in a mannersimilar to the network established for the collection and study oftransplant material from PAH patients. Such networks could prioritizeresearch questions and aggregate data and allow for network-widemonitoring and adherence to good preclinical research principles(record keeping, quality of data, reproducibility among different cen-ters, etc.). Results on a potential therapy confirmed in several modelsand with much higher sample sizes than are typically seen in singlelaboratory studies will be more robust and reliable, addressing poten-tial investigator-based scientific biases (Table 2).

Implementation of the changes recommended above would alsobe facilitated with the guidance of a clinical trial networks similar tothe Thrombolysis In Myocardial Infarction groups for myocardialinfarction trials (135) or the National Cancer Institute clinical trialsnetworks (136, 137). Trials conducted with the guidance of such net-works have other advantages, including the ownership and transpar-ency of the trial data. Currently, data are available only to the sponsorcompany, particularly if the results are not positive. Such an organi-zation would allow synergistic partnerships with industry but wouldalso facilitate the trials with generic/orphan drugs that are increasinglypromising for diseases such as PAH (Fig. 3). Increased communica-

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tion among the stakeholders, including scientists, trialists, industry,and regulatory authorities will be catalytic in allowing the investmentfor trials testing such compounds.

Efforts to increase the communication among all PAH stake-holders have begun. For example, the Pulmonary Vascular ResearchInstitute (PVRI) (an international nonprofit network of scientists andclinicians with a mission to promote research and awareness of PAHglobally) has started hosting meetings between PAH experts, indus-tries, and regulatory authorities from the United States, Canada, andthe European Union. The development of a common language amongall stakeholders will be important for the future.

Most cases of PAH occur in developing countries, where random-ized trials are more difficult to conduct and access to therapies isimpossible. For example, one of the most common causes of PAHglobally is schistosomiasis, which is almost nonexistent in the de-veloped countries, and thus, it is rarely discussed or considered inclinical trials (138). PAH associated with HIV or with uncorrectedcongenital heart disease is also much more prevalent in developingthan developed countries (138). One of the goals of PVRI is to estab-lish an international, unbiased, and independent clinical database tounderstand the true incidence and prevalence of the disease globally,including in developing countries (139). This will both facilitate thedesign of truly global trials and increase the potential users of the de-veloped therapies, benefiting industry.

SUPPLEMENTARY MATERIALSwww.sciencetranslationalmedicine.org/cgi/content/full/5/208/208sr5/DC1Methods

REFERENCES AND NOTES1. A. J. Peacock, N. F. Murphy, J. J. McMurray, L. Caballero, S. Stewart, An epidemiological

study of pulmonary arterial hypertension. Eur. Respir. J. 30, 104–109 (2007).2. V. V. McLaughlin, S. L. Archer, D. B. Badesch, R. J. Barst, H. W. Farber, J. R. Lindner,

M. A. Mathier, M. D. McGoon, M. H. Park, R. S. Rosenson, L. J. Rubin, V. F. Tapson,J. Varga, R. A. Harrington, J. L. Anderson, E. R. Bates, C. R. Bridges, M. J. Eisenberg,V. A. Ferrari, C. L. Grines, M. A. Hlatky, A. K. Jacobs, S. Kaul, R. C. Lichtenberg, D. J. Moliterno,

Table 2. Translational priorities for the future of PAH research.

Preclinical research

1. Clinically relevant endpoints in animal research

2. Comprehensive approach to the RV-PA unit

3. Systematic collection/analysis of human tissues from national/globalregistries

4. Pursuit of comprehensive theories for PAH

5. Networks of excellence in preclinical PAH research

Clinical research

6. Simple but global independent registries (incidence, prevalence, survival)

7. Reexamination of the clinical classification of PAH

8. Early-phase mechanistic studies following personalized medicineprinciples

9. Global clinical trials networks for testing therapies, including generic drugs

10. New trial designs, longer-term protocols, prespecified molecularphenotypes

11. Common Academia-Industry-Regulatory roadmap for future PAH trials

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8. R. L. Benza, D. P. Miller, R. J. Barst, D. B. Badesch, A. E. Frost, M. D. McGoon, An evaluationof long-term survival from time of diagnosis in pulmonary arterial hypertension from theREVEAL Registry. Chest 142, 448–456 (2012).

9. A. Macchia, R. Marchioli, G. Tognoni, M. Scarano, R. Marfisi, L. Tavazzi, S. Rich, Systematicreview of trials using vasodilators in pulmonary arterial hypertension: Why a newapproach is needed. Am. Heart J. 159, 245–257 (2010).

10. A. Macchia, R. Marchioli, R. Marfisi, M. Scarano, G. Levantesi, L. Tavazzi, G. Tognoni, Ameta-analysis of trials of pulmonary hypertension: A clinical condition looking for drugsand research methodology. Am. Heart J. 153, 1037–1047 (2007).

11. Y. F. Chen, S. Jowett, P. Barton, K. Malottki, C. Hyde, J. S. Gibbs, J. Pepke-Zaba, A. Fry-Smith,J. Roberts, D. Moore, Clinical and cost-effectiveness of epoprostenol, iloprost, bosentan,sitaxentan and sildenafil for pulmonary arterial hypertension within their licensed indica-tions: A systematic review and economic evaluation. Health Technol. Assess. 13, 1–320(2009).

12. M. Angalakuditi, E. Edgell, A. Beardsworth, E. Buysman, T. Bancroft, Treatment patternsand resource utilization and costs among patients with pulmonary arterial hypertensionin the United States. J. Med. Econ. 13, 393–402 (2010).

13. R. Copher, A. Cerulli, A. Watkins, M. Laura Monsalvo, Treatment patterns and healthcaresystem burden of managed care patients with suspected pulmonary arterial hypertensionin the United States. J. Med. Econ. 15, 947–955 (2012).

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Competing interests: The authors declare that they have no competing interests.

Submitted 29 November 2012Accepted 3 October 2013Published 23 October 201310.1126/scitranslmed.3005428

Citation: G. Sutendra, E. D. Michelakis, Pulmonary arterial hypertension: Challenges intranslational research and a vision for change. Sci. Transl. Med. 5, 208sr5 (2013).

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