J Appl Physiol 99: 2363–2368, 2005.
`First published September 1, 2005; doi:10.1152/japplphysiol.00083.2005.
`
`Potent effects of aerosol compared with intravenous treprostinil
`on the pulmonary circulation
`
`Brett L. Sandifer,1,3 Kenneth L. Brigham,1,2,3 E. Clinton Lawrence,1,3
`David Mottola,4 Chris Cuppels,1,2 and Richard E. Parker1,2
`1Division of Pulmonary, Allergy, and Critical Care, 2Center for Translational Research in the Lung,
`and 3McKelvey Center for Lung Transplantation, Emory University School of Medicine, Atlanta,
`Georgia; and 4United Therapeutics Corporation, Research Triangle Park, North Carolina
`
`Submitted 24 January 2005; accepted in final form 20 July 2005
`
`Sandifer, Brett L., Kenneth L. Brigham, E. Clinton Lawrence,
`David Mottola, Chris Cuppels, and Richard E. Parker. Potent
`effects of aerosol compared with intravenous treprostinil on the
`pulmonary circulation. J Appl Physiol 99: 2363–2368, 2005. First pub-
`lished September 1, 2005; doi:10.1152/japplphysiol.00083.2005.—
`Inhaled vasodilator therapy for pulmonary hypertension may decrease
`the systemic side effects commonly observed with systemic adminis-
`tration. Inhaled medications only reach ventilated areas of the lung, so
`local vasodilation may improve ventilation-perfusion matching and
`oxygenation. We compared the effects of intravenous vs. aerosolized
`treprostinil on pulmonary and systemic hemodynamics in an unanes-
`thetized sheep model of sustained acute pulmonary hypertension.
`Acute, stable pulmonary hypertension was induced in instrumented
`unanesthetized sheep by infusing a PGH2 analog, U-44069. The sheep
`were then administered identical doses of treprostinil either intrave-
`nously or by aerosol. Systemic and pulmonary hemodynamics were
`recorded during each administration. Both intravenous and aerosol
`delivery of treprostinil reduced pulmonary vascular resistance and
`pulmonary arterial pressure, but the effect was significantly greater
`with aerosol delivery (P ⬍ 0.05). Aerosol delivery of treprostinil had
`minimal effects on systemic hemodynamics, whereas intravenous
`delivery increased heart rate and cardiac output and decreased left
`atrial pressure and systemic blood pressure. Aerosol delivery of the
`prostacyclin analog treprostinil has a greater vasodilatory effect in the
`lung with minimal alterations in systemic hemodynamics compared
`with intravenous delivery of the drug. We speculate that this may
`result from treprostinil stimulated production of vasodilatory media-
`tors from pulmonary epithelium.
`
`pulmonary hypertension;
`epoprostenol
`
`transcription factor activator protein-1;
`
`PULMONARY ARTERIAL HYPERTENSION is commonly thought to be
`a consequence of long-standing vascular remodeling charac-
`terized by proliferation of vascular smooth muscle cells, endo-
`thelial cells and extracellular matrix (2, 11, 13). However, it
`can also occur abruptly as is seen in acute lung injury. In such
`setting, it is believed to be because of active vasoconstriction
`rather than remodeling. Local vasoconstriction resulting from
`alveolar hypoxia acts to improve ventilation-perfusion match-
`ing matching. Global pulmonary vasoconstriction may result
`from an imbalance between endogenous vasoconstricting and
`vasodilating mediators (17).
`Intravenous prostacyclin has improved survival and exercise
`tolerance in the chronic forms of pulmonary hypertension and
`
`has been the cornerstone of treatment for the last several years
`(1). However, administration requires a central venous cathe-
`ter, and the short half-life of the drug requires continuous
`infusion. Systemic administration in acute lung injury has been
`shown to cause increasing shunt fraction with worsening oxy-
`genation (24). Systemic effects of prostacyclin include hypo-
`tension, alterations in cardiac output, nausea and vomiting,
`headache, and rash.
`Inhaled delivery of pulmonary vasodilators has potential
`advantages over systemic delivery. Aerosols only reach venti-
`lated areas of the lung, and local vasodilation in those areas
`should improve ventilation-perfusion matching and oxygen-
`ation, thereby complementing the effects of hypoxic vasocon-
`striction. Limited experience in patients with acute lung injury
`has shown that aerosolized prostacyclin can improve shunt
`fraction and oxygenation and reduce pulmonary vascular re-
`sistance (30, 31). If the biological effects are limited to the
`lung, then systemic side effects should be avoided.
`Iloprost is a carbacyclin analog of prostacyclin that is cur-
`rently used in Europe. It has been administered by aerosol and
`intravenously to children with pulmonary hypertension due to
`congenital heart disease. Delivery by either route decreased
`mean pulmonary arterial pressure and pulmonary vascular
`resistance. Given intravenously, the drug caused a significant
`decrease in systemic blood pressure that was not observed with
`aerosol (9). In a study of 35 patients with primary pulmonary
`hypertension, inhaled iloprost reduced mean pulmonary arte-
`rial pressure and pulmonary vascular resistance significantly
`more than inhaled nitric oxide (10). Prostacyclin delivered by
`aerosol to patients with primary pulmonary hypertension or
`scleroderma-associated pulmonary hypertension reduced both
`pulmonary vascular resistance and pulmonary arterial pressure
`significantly, but it also decreased systemic vascular resistance
`and increased cardiac output (20).
`Another prostacyclin analog, treprostinil (Remodulin, United
`Therapeutics), has been recently introduced for treatment of pul-
`monary hypertension. The drug is delivered by subcutaneous
`infusion, eliminating the need for an indwelling catheter; how-
`ever, pain at the injection site is a significant problem. Treprostinil
`also has a longer half-life than prostacyclin and iloprost, leading to
`less rebound if the medication is abruptly discontinued.
`Because of the potential advantages of aerosol delivery and
`the encouraging clinical studies with prostacyclin and other
`analogs, we conducted studies to compare the vasodilatory
`
`Address for reprint requests and other correspondence: R. E. Parker, 615
`Michael St., Suite 250, Whitehead Research Bldg., Atlanta, GA 30322 (e-mail:
`reparke@emory.edu).
`
`The costs of publication of this article were defrayed in part by the payment
`of page charges. The article must therefore be hereby marked “advertisement”
`in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
`
`http://www. jap.org
`
`8750-7587/05 $8.00 Copyright © 2005 the American Physiological Society
`
`2363
`
`IPR2021-00406
`United Therapeutics EX2087
`
`

`

`2364
`
`PULMONARY VASCULAR EFECTS OF VASODILATORS
`
`activity of aerosol vs. intravenous treprostinil. We developed a
`model of stable acute pulmonary hypertension in chronically
`instrumented unanesthetized sheep and determined effects of
`identical doses of treprostinil delivered either by aerosol or
`intravenously.
`
`METHODS
`
`Surgical preparation. Six yearling sheep (3 males, 3 females;
`21–37 kg) were fasted for 18 –24 h then sedated with thiopental to
`allow for intubation. Surgical procedures were performed with the
`sheep receiving 1.5–2.5% halothane. A left thoracotomy was per-
`formed, and a Transonic blood flow probe (Transonic Bloodflow
`Meter, Ithaca, NY) was placed around the main pulmonary artery, and
`Silastic catheters were placed in the main pulmonary artery and left
`atrium. Sheep were allowed to recover for 7 days. Subsequently, the
`sheep were reanesthetized and a catheter was inserted into the left
`carotid artery, a Cordis Introducer Sheath was inserted in the left
`jugular vein, and a tracheostomy was performed. The sheep were
`allowed to recover for an additional 3–5 days before experimentation.
`This instrumentation was used to measure pulmonary arterial pres-
`sure, left atrial pressure, central venous pressure, systemic arterial
`pressure, heart rate, and cardiac output. Cardiac output was measured
`on a Transonic Systems T101 Ultrasonic Bloodflow Meter. Pressures
`were monitored with Hewlett-Packard transducers (model 1290A),
`recorded on Astromed MT-9500 Stripchart Recorder, and digitally
`recorded with Easy Data Acquisition Software. Drug aerosolization
`was performed with a Healthline Medical AM-601 Medicator Aerosol
`Delivery System. Intravenous infusions were via a Manostat Cas-
`sette Pump. Sheep procurement, housing, surgical procedures and
`experimental protocols were approved by the Vanderbilt University
`Animal Care Committee and overseen by the Vanderbilt Division of
`Animal Care.
`Induction of pulmonary hypertension. Acute pulmonary hyperten-
`sion was induced with an infusion of the PGH2 analog, U-44069
`(9,11-dideoxy-9␣,11␣-epoxymethanoprostaglandin F2␣). This sub-
`stance is similar to endogenously formed thromboxane A2 and can be
`titrated to induce the desired degree of pulmonary vasoconstriction.
`U-44069 was mixed with sterile normal saline and was protected from
`light by wrapping the saline bag with aluminum foil. Previous exper-
`iments had determined that U-44069 infused at 1,000 ng 䡠 kg⫺1 䡠 min⫺1
`elevated the pulmonary vascular resistance to approximately four
`times baseline. Pulmonary vascular resistance was calculated as
`(mean pulmonary arterial pressure ⫺ left atrial pressure)/cardiac
`output. After a 30-min period of baseline hemodynamic measure-
`ments, four sheep received U-44069 at 1,000 ng 䡠 kg⫺1 䡠 min⫺1 for 180
`min to demonstrate its ability to maintain a steady-state increase in
`pulmonary vascular resistance.
`Experimental protocol. Each sheep underwent 30 min of baseline
`measurements followed by a U-44069 infusion at 1,000 ng 䡠 kg⫺1 䡠
`min⫺1. After each sheep was allowed to reach steady state for 30 – 60
`min, treprostinil was infused at 250, 500, and 1,000 ng 䡠 kg⫺1 䡠 min⫺1.
`Each infusion lasted 30 – 60 min. The experiment was repeated with
`the same dose of U-44069 but with the treprostinil delivered via
`aerosol at 0.28 ml/min in escalating doses of 250, 500, and 1,000
`ng 䡠 kg⫺1 䡠 min⫺1. Pulmonary and systemic hemodynamic measure-
`ments were recorded at each dose for each route of administration.
`To evaluate the duration of action of vasodilator aerosols, we
`delivered treprostinil for 30 min at 1,000 ng 䡠 kg⫺1 䡠 min⫺1 after reach-
`ing a steady-state elevation in pulmonary vascular resistance with
`U-44069. At the end of 30 min, the treprostinil was stopped, and the
`U-44069 infusion was continued for an additional 30 min to estimate
`the duration of action of the medication by following the return
`toward the steady-state pulmonary vascular resistance. As a compar-
`ison, this experiment was repeated using aerosol epoprostenol at 1,000
`
`ng 䡠 kg⫺1 䡠 min⫺1. In addition, arterial blood gases were drawn to
`follow changes in oxygenation.
`Statistical analysis. One-way repeated-measures ANOVA and
`Dunnett’s method were used to test for statistical significance during
`the U-44069 steady-state experiment. Two-way repeated-measures
`ANOVA and the Student-Newman-Keuls test were used to compare
`data for the remaining experiments. Significance was assumed for
`values of P ⬍ 0.05 for all experiments.
`
`RESULTS
`
`As shown in Fig. 1, infusion of the thromboxane analog,
`U-44069, at 1,000 ng 䡠kg⫺1 䡠min⫺1 produced a stable increase
`in pulmonary vascular resistance to almost four times the
`baseline value; the steady-state pulmonary hypertension re-
`mained constant throughout the 180-min infusion. Similarly,
`U-44069 caused a significant increase in pulmonary arterial
`pressure as illustrated in Fig. 2B. The U-44069 also had
`significant effects on systemic hemodynamics. As shown in
`Fig. 3A there was a significant drop in cardiac output during the
`infusion of U-44069 before administration of the medication.
`Similarly, there was, on average, a decrease in heart rate during
`U-44069 that did not reach statistical significance (Fig. 3B).
`With the decreased cardiac output and heart rate there was, on
`average, an increase in the directly measured left atrial pressure
`that did not reach significance (Fig. 3C). The vasoconstricting
`properties of intravenously administered U-44069 also in-
`creased the average systemic arterial blood pressure, but this
`did not reach statistical significance (Fig. 3D).
`During a stable period of pulmonary hypertension produced
`by infusing the thromboxane analog, sheep received treprosti-
`nil either intravenously or by aerosol. The same doses of the
`drug were delivered by either route, although the actual amount
`of drug delivered to the lungs with aerosol was considerably
`less than that delivered intravenously because of the ineffi-
`ciency of aerosol delivery systems. Figures 2 and 3 summarize
`the hemodynamic effects of treprostinil.
`Intravenous delivery of treprostinil caused a dose-related
`decrease in both pulmonary vascular resistance and pulmonary
`arterial pressure in a dose-dependant manner (Fig. 2, open
`symbols). Effects were seen even at the lowest dose (250
`ng 䡠kg⫺1 䡠min⫺1) infused, and further vasodilation occurred
`with increasing doses. However, even at the highest dose
`
`Fig. 1. U-44069 infusion (1,000 ng 䡠 kg⫺1 䡠 min⫺1) induces an increase in
`pulmonary vascular resistance (PVR) to ⬃4 times baseline. The increase is
`maintained at this level throughout a 180-min infusion (#P ⬍ 0.05). Values are
`means ⫾ SE; n ⫽ 4 animals.
`
`J Appl Physiol • VOL 99 • DECEMBER 2005 • www.jap.org
`
`

`

`PULMONARY VASCULAR EFECTS OF VASODILATORS
`
`2365
`
`to baseline levels even though infusion of the vasoconstrictor
`continued. This marked pulmonary vasodilation was achieved
`with minimal effects on systemic hemodynamics. As shown in
`Fig. 3 (solid symbols), aerosol delivery of the drug caused no
`significant changes in cardiac output or heart rate even at the
`highest dose. Systemic arterial pressure also did not change
`significantly even at the highest dose of the drug. The only
`significant hemodynamic effect that we observed was a small
`increase in left atrial pressure that occurred at the lowest dose
`and did not change further with higher doses of the drug.
`The duration of action of aerosol treprostinil was much
`greater than that observed with aerosol epoprostenol as seen in
`Fig. 4. Within 10 min of stopping the epoprostenol, the pul-
`monary vascular resistance was almost back to steady state.
`However, 30 min after stopping treprostinil, the pulmonary
`vascular resistance remained less than steady state. The slope
`of the off-transient indicates that the duration of effect of
`treprostinil was about three times that of epoprostenol. Arterial
`blood-gas data showed that the oxygen saturation remained
`above 90% throughout the experiments.
`
`DISCUSSION
`
`Although, at least in its later stages when usually diagnosed,
`pulmonary hypertension is characterized by extensive remod-
`eling of the pulmonary vascular bed. The hypertension is
`sometimes partially reversible by administration of vasodila-
`tors like nitric oxide and iloprost by inhalation or prostacyclin
`by either intravenous infusion or inhalation (1, 10, 20). In the
`primary form of the disease, urinary concentrations of prosta-
`noids show increased production of the pulmonary vasocon-
`strictor thromboxane relative to the vasodilator prostacyclin,
`implicating this prostanoid imbalance as a possible cause of
`some degree of persistent pulmonary vasoconstriction (3).
`Improved survival from chronic administration of prostacyclin
`in patients with primary pulmonary hypertension suggests
`effects on the progressive remodeling process. Although not
`demonstrated in humans with pulmonary hypertension after
`prolonged treatment, in vitro studies suggest that prostacyclin
`can alter smooth muscle proliferation (4). Whether the mech-
`anism of the effects of chronic prostacyclin administration is
`similar to that for acute vasodilation is unknown.
`In acute lung injury,
`in which pulmonary hypertension
`contributes to hypoxemia, inhaled vasodilators have shown
`significant improvements in pulmonary vascular resistance,
`shunt fraction, and oxygenation (30, 31). In this setting, pul-
`monary hypertension is initially caused by hypoxic vasocon-
`striction and an imbalance of endogenous vasoactive sub-
`stances. However, in the later stages of acute lung injury
`vascular, remodeling occurs and is characterized by concentric
`deposition of fibrin, hyperplasia of endothelial cells, and me-
`dial hypertrophy. This has been shown to occur in the small
`muscular arteries, veins, and lymphatics (29).
`To test the acute vasodilatory effects of treprostinil, a pros-
`tacyclin analog that is in clinical use, we produced stable
`pulmonary vasoconstriction in chronically instrumented un-
`anesthetized sheep by infusing an analog of thromboxane,
`U-44069 (23). Constant infusion of this drug produces a stable
`increase in pulmonary vascular resistance and pulmonary ar-
`terial pressure that is directly related to the infusion rate of the
`drug, permitting testing of vasodilator responses in the precon-
`
`Fig. 2. A: U-44069 infusion causes a significant increase in PVR. Intravenous
`(IV) treprostinil caused a dose-dependent decrease in PVR that remained
`significantly elevated above baseline (#P ⬍ 0.05). Aerosol treprostinil caused
`a dose-dependent decrease in PVR that was significantly lower compared with
`intravenous delivery (*P ⬍ 0.05). B: U-44069 caused a significant increase in
`mean pulmonary arterial pressure. Intravenous treprostinil caused a dose-
`dependent decrease in pulmonary arterial pressure that remained significantly
`elevated above baseline (#P ⬍ 0.05). Aerosol delivery caused a dose-depen-
`dent decrease in pulmonary arterial pressure that was significantly lower
`compared with intravenous delivery (*P ⬍ 0.05). Values are means ⫾ SE; n ⫽
`6 animals.
`
`(1,000 ng 䡠kg⫺1 䡠min⫺1), neither pulmonary vascular resistance
`nor pulmonary arterial pressure returned to baseline levels.
`Effects of intravenous treprostinil on systemic hemodynamics
`are summarized in Fig. 3 (open symbols). Infusion of the drug
`caused a dose-related increase in cardiac output and heart rate
`and a dose-related decrease in left atrial pressure and systemic
`arterial pressure. At doses that were necessary to cause sub-
`stantial pulmonary vasodilation, there were significant alter-
`ations in systemic hemodynamics.
`The effectiveness of treprostinil as a pulmonary vasodilator
`was much greater when the drug was delivered by aerosol than
`when it was delivered intravenously. As shown in Fig. 2 (solid
`symbols), aerosol treprostinil reduced both pulmonary vascular
`resistance and pulmonary arterial pressure significantly even at
`a dose of 250 ng 䡠kg⫺1 䡠min⫺1. At the highest dose (1,000
`ng 䡠kg⫺1 䡠min⫺1), aerosol delivery of the drug returned both
`pulmonary vascular resistance and pulmonary artery pressure
`
`J Appl Physiol • VOL 99 • DECEMBER 2005 • www.jap.org
`
`

`

`2366
`
`PULMONARY VASCULAR EFECTS OF VASODILATORS
`
`Fig. 3. U-44069 caused a decline in cardiac output (#P ⬍ 0.05) and heart rate (not significant), whereas it caused nonsignificant increases in left atrial pressure
`and systemic arterial pressure. A: intravenous treprostinil caused a dose dependent increase in cardiac output, whereas there was no change from aerosol
`treprostinil. B: intravenous treprostinil caused a dose-dependent increase in heart rate that was significantly elevated above baseline at the highest dose (#P ⬍
`0.05), whereas aerosol delivery caused no change. C: intravenous treprostinil caused a significant decrease in left atrial pressure relative to U-44069 (&P ⬍ 0.05)
`and baseline at the higher doses (#P ⬍ 0.05). Aerosol delivery caused an increase in left atrial pressure that was significantly above U-44069 (&P ⬍ 0.05) and
`baseline (#P ⬍ 0.05) at the lower doses. D: intravenous and aerosol treprostinil caused a nonsignificant dose-dependent decrease in systemic arterial pressure.
`Values are means ⫾ SE; n ⫽ 6 animals. bpm, Beats/min.
`
`stricted pulmonary vascular bed. Such data may be relevant to
`acute lung injury-associated pulmonary hypertension in hu-
`mans because an imbalance between endogenous vasoconstric-
`tors such as thromboxane and vasodilators such as nitric oxide
`
`Fig. 4. U-44069 was infused at 1,000 ng 䡠kg⫺1 䡠min⫺1 to achieve a steady-state
`elevation of PVR for up to 70 min. The sheep received either epoprostenol or
`treprostinil aerosol at 1,000 ng 䡠kg⫺1 䡠min⫺1 for a period of 30 min. Within 10
`min, the PVR had almost returned to baseline with epoprostenol. After 30 min, the
`PVR had increased but not returned to steady state after treprostinil administration.
`
`may facilitate the elevation in pulmonary arterial pressures.
`However, this acute model does not reproduce the structural
`alterations in the pulmonary vascular bed typical of pulmonary
`arterial hypertension. This approach is similar to that others
`have used to evaluate vasodilator potency (12, 15, 26).
`In our studies, treprostinil delivered either by aerosol or
`intravenously caused a dose-dependent decrease in both pul-
`monary arterial pressure and pulmonary vascular resistance in
`the preconstricted pulmonary vascular bed. Surprisingly, aero-
`sol delivery of the drug had a much greater vasodilatory effect
`than intravenous delivery. This difference is especially striking
`in light of the fact that we delivered the same doses of drug by
`either route. Continuous aerosol delivery is notoriously ineffi-
`cient, delivering 0 – 42% of the nebulized dose to the lower
`respiratory tract (5).
`Several studies document that aerosol delivery of prostacy-
`clin can effectively vasodilate the pulmonary vasculature (20,
`27), but few studies have compared aerosol and intravenous
`delivery of such drugs directly. In one such study, Hallioglu et
`al. (8) compared inhaled and intravenous iloprost in children
`with pulmonary hypertension secondary to congenital heart
`disease. They delivered the same dose of drug by either route
`
`J Appl Physiol • VOL 99 • DECEMBER 2005 • www.jap.org
`
`

`

`PULMONARY VASCULAR EFECTS OF VASODILATORS
`
`2367
`
`and found similar decreases in pulmonary arterial pressure and
`pulmonary vascular resistance. However, because aerosol de-
`livery is inefficient, it is likely that the actual amount of drug
`delivered that way was less than when given intravenously, so
`the potency of the drug delivered by aerosol may have been
`greater. The differences between our findings and theirs could
`also be a result of the fact that we used different prostacyclin
`analogs, although most data would indicate that the analogs
`have similar actions to the parent compound. It is also possible
`that sheep react differently than humans or that the existence of
`pulmonary hypertension due to congenital heart disease alters
`how the drug acts. To achieve an effect in sheep, it was
`necessary to administer doses of treprostinil that were much
`higher than those used in treating patients, regardless of the
`route of delivery. Whether this is due to differences in species
`or a requirement for higher doses of vasodilator to overcome
`thromboxane-induced vasoconstriction of the degree we pro-
`duced experimentally is not clear.
`We found that with aerosol delivery, even large doses of
`treprostinil had minimal effects on systemic hemodynamics.
`Intravenous drug caused a dose-related increase in heart rate
`and cardiac output and decrease in left atrial pressure, whereas
`aerosol delivery resulted only in a modest elevation in left
`atrial pressure that was unrelated to dose. With aerosol deliv-
`ery, this was true even though the dose of the drug was
`sufficient to return pulmonary hemodynamics completely to
`normal. This is in contrast to data from studies in humans with
`chronic pulmonary hypertension. For example, alterations in
`systemic hemodynamics were seen in patients receiving intra-
`venous epoprostenol for pulmonary hypertension in a random-
`ized trial (1). Studies with aerosol delivery of prostacyclin in
`patients with pulmonary hypertension also reported altered
`systemic hemodynamics (21, 27). In a recent study by Ol-
`schewski et al (19), the inhaled prostacyclin analog iloprost,
`now available in the United States, was given to patients with
`pulmonary arterial hypertension. They not only demonstrated
`significant improvements in pulmonary vascular resistance but
`also noted significant changes in cardiac output, systemic
`arterial pressure, pulmonary capillary wedge pressure, and
`arterial oxygen saturation. These patients were evaluated over
`12 wk of therapy, so it is difficult to compare these results with
`our acute model of pulmonary hypertension with one-time
`dosing of therapy in otherwise normal sheep with acute pul-
`monary vasoconstriction.
`It is clear from this study and others that aerosolized delivery
`of prostacyclin analogs can reverse acute pulmonary vasocon-
`striction with minimal systemic side effects (9, 10, 20). Fur-
`thermore, when similar doses of intravenous and aerosolized
`medication have been used, the effects of aerosol are similar to
`or greater than systemically administered drug (9). Assuming
`that only a fraction of the aerosol reaches the distal airways and
`alveoli, it appears the aerosol delivery is much more potent. If
`prostacyclin acts directly on lung resistance vessels, this find-
`ing is especially surprising because with intravenous delivery
`the drug directly accesses those vessels and with aerosol
`delivery, the drug would need to traverse epithelial and inter-
`stitial barriers to reach the vessels. It is possible that actual
`concentrations of drug reaching resistance vessels is greater
`with targeted delivery of the drug by aerosol, but given the
`inefficiency of aerosol delivery, that seems unlikely.
`
`Airway epithelial cells can produce “relaxant factors” that
`have been studied mostly in relation to airway rather than
`vascular function (7). Whether prostacyclin stimulated lung
`epithelial cells to produce vasodilatory mediators that amplify
`the direct effects of the drug has not been investigated. How-
`ever, in response to hydrostatic pressure, prostacyclin produced
`in bone cells activates the transcription factor activator pro-
`tein-1, and the prostacyclin analog iloprost caused a similar
`response in cultured bone cells (8). The activator protein-1
`family of transcription factors is involved in numerous pro-
`cesses in the lung, including inflammation, apoptosis, and cell
`proliferation (21, 28). Activator protein-1 also increases ex-
`pression of inducible nitric oxide synthase that could enhance
`vasodilation (15). Activator protein-1 has been found to be
`involved in the signal transduction of bone morphogenetic
`protein (14), and mutations of a bone morphogenetic protein
`receptor (BMPR2) is causally implicated in a familial form of
`primary pulmonary hypertension (18). We have preliminary
`data indicating that prostacyclin increases expression of acti-
`vator protein-1 and several activator protein-1-regulated genes
`in human airway epithelial cells in culture (25).
`Our studies and what other data are available indicate that
`prostacyclin analogs are more potent pulmonary vasodilators
`when delivered by aerosol than when given intravenously.
`Systemic hemodynamic effects are also minimized by aerosol
`administration of the drug, making this approach especially
`appealing. Duration of action of prostacyclin is short, requiring
`an unrealistic frequency of administration for clinical use (6,
`33), but development of analogs (e.g., treprostinil) or formu-
`lations that are long acting could make this approach feasible.
`Also given the high cost of these medications, delivery via
`aerosol may provide a monetary benefit if less total drug can be
`given on a daily basis by using intermittent inhalation com-
`pared with continuous infusion. The reason for an enhanced
`pulmonary vasodilatory effect with aerosol administration is
`not yet clear, but we speculate that the effect may be mediated
`by effects of prostacyclin on epithelial cells, possibly a conse-
`quence of activation of the transcription factor activator pro-
`tein-1.
`
`DISCLOSURES
`
`This work was supported by a grant from United Therapeutics Corporation
`to Vanderbilt University and was conducted in the Division of Allergy,
`Pulmonary, and Critical Care Medicine, Department of Medicine, Vanderbilt
`University School of Medicine.
`
`REFERENCES
`
`1. Barst RJ, Rubin LJ, Long WA, McGoon MD, Rich S, Badesch DB,
`Groves BM, Tapson VF, Bourge RC, Brundage BH, Koerner SK,
`Langleben D, Keller CA, Murali S, Uretsky BF, Clayton LM, Jobsis
`MM, Blackburn SD, Shortino D, Crow JW, and the Primary Pulmo-
`nary Hypertension Study Group. A comparison of continuous intrave-
`nous epoprostenol (prostacyclin) with conventional therapy for primary
`pulmonary hypertension. N Engl J Med 334: 296 –301, 1996.
`2. Budhiraja R, Tuder RM, and Hassoun PM. Endothelial dysfunction in
`pulmonary hypertension. Circulation 109: 159 –165, 2004.
`3. Christman BW, McPherson CD, Newman JH, King GA, Bernard GR,
`Groves BM, and Loyd JE. An imbalance between the excretion of
`thromboxane and prostacyclin metabolites in pulmonary hypertension.
`N Engl J Med 327: 70 –75, 1992.
`4. Clapp LH, Finney P, Turcato ST, Rubin LJ, and Tinker A. Differential
`effects of stable prostacyclin analogs on smooth muscle proliferation and
`cyclic AMP generation in human pulmonary artery. Am J Respir Cell Mol
`Biol 26: 194 –201, 2002.
`
`J Appl Physiol • VOL 99 • DECEMBER 2005 • www.jap.org
`
`

`

`2368
`
`PULMONARY VASCULAR EFECTS OF VASODILATORS
`
`5. Dhand R and Tobin MJ. Inhaled bronchodilator therapy in mechanically
`ventilated patients. Am J Respir Crit Care Med 156: 3–10, 1997.
`6. Flolan (epoprostenol) Package Insert. GlaxoSmithKline, Research Tri-
`angle Park, NC.
`7. Folkerts G and Nijkamp FP. Airway epithelium: more than just a
`barrier! Trends Pharmacol Sci 19: 334 –341, 1998.
`8. Glanstchnig H, Varga F, Rumpler M, and Klaushofer K. Prostacyclin
`(PGI2): a potential mediator of c-fos expression induced by hydrostatic
`pressure in osteoblastic cells. Eur J Clin Invest 26: 544 –548, 1996.
`9. Hallioglu O, Dilber E, and Celiker A. Comparison of acute hemody-
`namic effects of aerosolized and intravenous iloprost in secondary pulmo-
`nary hypertension in children with congenital heart disease. Am J Cardiol
`92: 1007–1009, 2003.
`10. Hoeper M, Olschewski H, Ghofrani HA, Wilkens H, Winkler J, Borst
`MM, Niedermeyer J, Fabel H, Seeger W, and the German PPH Study
`Group A comparison of the acute hemodynamic effects of inhaled nitric
`oxide and aerosolized iloprost in primary pulmonary hypertension. J Am
`Coll Cardiol 35: 176 –182, 2000.
`11. Humbert M, Morrell NW, Archer SL, Stenmark KR, MacLean MR,
`Lang IM, Christman BW, Weir EK, Eickelberg O, Voelkel NF, and
`Rabinovitch M. Cellular and molecular pathobiology of pulmonary arte-
`rial hypertension. J Am Col Cardiol 43: s13–s24, 2004.
`12. Ichinose F, Erana-Garcia J, Hromi J, Raveh Y, Jones R, Krim L,
`Clark MWH, Winkler JD, Bloch KD, and Zapol WM. Nebulized
`sildenafil is a selective pulmonary vasodilator in lambs with acute pulmo-
`nary hypertension. Crit Care Med 29: 1000 –1005, 2001.
`13. Jeffrey TK and Wanstall JC. Pulmonary vascular remodeling: a target
`for therapeutic intervention in pulmonary hypertension. Pharmacol Ther
`92: 1–20, 2001.
`14. Lai CF and Cheng SL. Signal transductions induced by bone morpho-
`genetic protein-2 and transforming growth factor beta in normal human
`osteoblastic cells. J Biol Chem 277: 15514 –15522, 2002.
`15. Lam CF, van Heerden PV, Ilett KF, Caterina P, and Filion P. Two
`aerosolized nitric oxide adducts as selective pulmonary vasodilators for
`acute pulmonary hypertension. Chest 123: 869 – 874, 2003.
`16. Lee JK, Choi SS, Won JS, and Suh HW. The regulation of inducible
`nitric oxide synthase gene expression induced by lipopolysaccharide and
`tumor necrosis factor-alpha in C6 cells: involvement of AP-1 and NK
`kappa B. Life Sci 73: 595– 609, 2003.
`17. Moloney ED and Evans TW. Pathophysiology and pharmacological
`treatment of pulmonary hypertension in acute respiratory distress syn-
`drome. Eur Respir J 21: 720 –727, 2003.
`18. Newman JH, Wheeler L, Lane KB, Loyd E, Gaddipati R, Phillips JA
`III, and Loyd JE. Mutation in the gene for bone morphogenetic protein
`
`receptor II as a cause of primary pulmonary hypertension in a large
`kindred. N Engl J Med 345: 319 –324, 2001.
`19. Olschewski H, Simonneau G, Galie N, Higenbottam T, Naeije R,
`Rubin LJ, Nikkho S, Speich R, Hoeper MM, Behr J, Winkler J, Sitbon
`O, Popov W, Ghofrani HA, Manes A, Kiely DG, Ewert R, Meyer A,
`Corris PA, Delcroix M, Gomez-Sanchez M, Siedentop H, and Seeger
`W for the Aerosolized Iloprost Randomized Study Group. Inhaled
`iloprost for severe pulmonary hypertension. N Engl J Med 347:322–329,
`2002.
`20. Olschewski H, Walmrath D, Schermuly R, Ghofrani HA, Grimminger
`F, and Seeger W. Aerosolized prostacyclin and iloprost in severe pulmo-
`nary hypertension. Ann Intern Med 124: 820 – 824, 1996.
`21. Reddy SPM and Mossman BT. Role and regulation of activator pro-
`tein-1 in toxicant-induced responses in the lung. Am J Physiol Lung Cell
`Mol Physiol 283: L