Phosphodiesterase type 5 and high altitude pulmonary hypertension.

Abstract

This study explored phosphodiesterase type 5 (PDE5) inhibition as a strategy for treating high altitude pulmonary arterial hypertension (HAPH).
689 subjects (313 men) of mean (SD) age 44 (0.6) years living above 2500 m were screened for HAPH by medical examination and electrocardiography, and 188 (27%) met the criteria for right ventricular hypertrophy. 44 underwent cardiac catheterisation and 29 (66%) had a resting mean pulmonary artery pressure (PAP) above 25 mmHg. 22 patients with a raised mean PAP were randomised to receive sildenafil (25 or 100 mg) or matching placebo taken 8 hourly for 12 weeks.
At 3 months, patients on sildenafil 25 mg 8 hourly (n = 9) had a significantly (p = 0.018) lower mean PAP (-6.9 mmHg) at the end of the dosing interval than those on placebo (n = 8) (95% CI -12.4 to -1.3). The treatment effect for sildenafil 100 mg 8 hourly (n = 5) compared with placebo was -6.4 mm Hg (95% CI -12.9 to 0.1). Both doses improved 6 minute walk distance, the lower dose by 45.4 m (95% CI 11.5 to 79.4; p = 0.011) and the higher dose by 40.0 m (95% CI 0.2 to 79.8; p = 0.049). Sildenafil was well tolerated. Necroscopic lung specimens from three subjects with HAPH showed abundant PDE5 in the muscular coat of remodelled pulmonary arterioles.
PDE5 is an attractive drug target for the treatment of HAPH and a larger study of the long term effects of PDE5 inhibition in HAPH is warranted.

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PULMONARY HYPERTENSION
Phosphodiesterase type 5 and high altitude pulmonary
hypertension
A A Aldashev, B K Kojonazarov, T A Amatov, T M Sooronbaev, M M Mirrakhimov, N W Morrell,
J Wharton, M R Wilkins
...............................................................................................................................
See end of article for
authors’ affiliations
.......................
Correspondence to:
Professor M R Wilkins,
Experimental Medicine
and Toxicology, Imperial
College London,
Hammersmith Hospital, Du
Cane Road, London W12
0NN, UK; m.wilkins@
imperial.ac.uk
Received 4 February 2005
Accepted 27 April 2005
.......................
Thorax 2005;60:683–687. doi: 10.1136/thx.2005.041954
Background: This study explored phosphodiesterase type 5 (PDE5) inhibition as a strategy for treating
high altitude pulmonary arterial hypertension (HAPH).
Methods: 689 subjects (313 men) of mean (SD) age 44 (0.6) years living above 2500 m were screened
for HAPH by medical examination and electrocardiography, and 188 (27%) met the criteria for right
ventricular hypertrophy. 44 underwent cardiac catheterisation and 29 (66%) had a resting mean
pulmonary artery pressure (PAP) above 25 mm Hg. 22 patients with a raised mean PAP were randomised
to receive sildenafil (25 or 100 mg) or matching placebo taken 8 hourly for 12 weeks.
Results: At 3 months, patients on sildenafil 25 mg 8 hourly (n=9) had a significantly (p=0.018) lower
mean PAP (26.9 mm Hg) at the end of the dosing interval than those on placebo (n=8) (95% CI 212.4 to
21.3). The treatment effect for sildenafil 100 mg 8 hourly (n=5) compared with placebo was
26.4 mm Hg (95% CI 212.9 to 0.1). Both doses improved 6 minute walk distance, the lower dose by
45.4 m (95% CI 11.5 to 79.4; p=0.011) and the higher dose by 40.0 m (95% CI 0.2 to 79.8; p=0.049).
Sildenafil was well tolerated. Necroscopic lung specimens from three subjects with HAPH showed
abundant PDE5 in the muscular coat of remodelled pulmonary arterioles.
Conclusions: PDE5 is an attractive drug target for the treatment of HAPH and a larger study of the long
term effects of PDE5 inhibition in HAPH is warranted.
H
among the highland population.1It is estimated that more
than 140 million people reside above 2500 m,2and the
number of temporary visitors to mountains is close to
40 million.3
HAPH is characterised by increased pulmonary vascular
resistance secondary to hypoxia induced pulmonary vaso-
constriction and vascular remodelling of pulmonary arter-
ioles.1 4 5The vascular remodelling involves all cellular
elements of the vessel wall with endothelial dysfunction,
extension of smooth muscle into previously non-muscu-
larised vessels, and adventitial thickening.4 5The result is an
increased pressure load on the right ventricle, reduced
exercise capacity, and premature death from right ventricular
failure.6The structural changes in the pulmonary vasculature
are due, at least in part, to hypoxia associated smooth
muscle cell proliferation and—together with increased
pulmonary vascular tone—represent targets for therapeutic
intervention.6
The biochemical pathways underlying this pathophysiology
are poorly understood, but a reduction in nitric oxide (NO)
production is thought to have a role. In animal studies the
absence of endothelial nitric oxide synthase increases
susceptibility to this condition.7Interestingly, indigenous
Tibetans who are acclimatised to life at 3600 m have twofold
higher NO concentrations in exhaled breath than low-
landers,8and inhaled NO has been shown to have beneficial
effects on pulmonary haemodynamics in HAPH.9
NO has vasorelaxant and antiproliferative effects which are
mediated by cyclic GMP.10Cyclic GMP is hydrolysed by
phosphodiesterases (PDE). PDE5 is the major PDE subtype
present in pulmonary vasculature and is more abundant in
the lung than in other tissues.11This offers the possibility of
igh altitude pulmonary hypertension (HAPH) is a
public health problem in the mountainous areas of
the world, including Kyrgyzstan where it is common
relatively selective pulmonary vasodilatation with little
systemic hypotension. Agents with PDE5 inhibitory activity
reduce pulmonary artery pressure (PAP) in animal models.12–15
In an earlier study we showed that the selective PDE5
inhibitor, sildenafil, has a significant inhibitory effect on the
acute pressor response to hypoxia in healthy subjects.12The
aim of this study was to investigate the role of PDE5 in
regulating PAP and pulmonary vascular resistance (PVR) in
patients with HAPH by exploring the haemodynamic response
to sildenafil and the distribution of PDE5 enzyme in lung
specimens.
METHODS
Subjects
All volunteers (aged 16–86 years) were recruited from the
Naryn region of Kyrgystan, altitude 2500–4000 m above sea
level. All subjects were native to and residents of Naryn, and
spend less than 2 weeks per year below 2500 m. All under-
went health screening by history, physical examination,
spirometry, blood pressure measurement, and electrocardio-
graphy (ECG). Subjects who met ECG criteria for right
ventricle hypertrophy (RVH) were invited for right heart
catheterisation in Bishkek (760 m above sea level). The ECG
criteria for RVH were one of the following patterns: right axis
deviation, defined as a frontal plane QRS axis of >90 degrees;
R wave in lead V1 of >5 mm, an R/S ratio .1, and S.R in V5
or V6; a leftward shift in the transition zone (the precordial
lead with negative deflections of equal amplitude). Right
heart catheterisation was performed with a Swan-Ganz
thermodilution catheter (Baxter Healthcare, Compton, UK)
Abbreviations: CO, cardiac output; HAPH, high altitude pulmonary
hypertension; 6MW, 6 minute walk test; NO, nitric oxide; PAP,
pulmonary artery pressure; PDE5, phosphodiesterase type 5; PVR,
pulmonary vascular resistance; RVH, right ventricle hypertrophy
683
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attached
Compton, USA). Patients with a resting mean PAP of more
than 25 mm Hg and PVR more than 200 dynes.s/cm5were
offered treatment by randomisation to sildenafil or placebo
for 12 weeks. Patients with co-existing medical conditions
(including difficulty walking due to infirmity or arthritis) or
who felt unable to travel to the National Center for
Cardiology in Bishkek were excluded from the randomisation
protocol. Lung specimens were obtained for histological
examination from patients from the same population who
had died from HAPH.1
The study was approved by the local ethics committee of
the National Center of Cardiology and Internal Medicine in
Bishkek and the Hammersmith Hospitals, Chelsea and
Westminster
&
Queen Charlotte’s
Informed consent was obtained from all subjects and
families.
toa VigilanceMonitor (Baxter Healthcare,
HospitalsTrusts.
Design of study
Suitable patients were randomised to receive sildenafil,
25 mg or 100 mg, or matching placebo every 8 hours for
12 weeks. The study was double blind. The primary end point
was the change in mean PAP from baseline (week 0) after
12 weeks of treatment. Other measures of efficacy were the
change in PVR (mean PAP (mm Hg) – pulmonary capillary
wedge pressure (mm Hg)/cardiac output (CO, l/min) 6 80
dynes.s/cm5), cardiac output (l/min), and 6 minute walk
(6MW) distance from week 0 after 12 weeks of treatment.
Thephysical limitation domain
Cardiomyopathy Questionaire16was used to assess activity.
Both the baseline (week 0) and 12 week measurements
were made at 760 m in the period 7–10 days after arrival at
the hospital. Patients returned to their villages between
assessments. Compliance with the protocol was assessed by
tablet counts. The 6 minute walk and blood tests were made
24 hours before cardiac catheterisation. Baseline cardiopul-
monary haemodynamic measurements were made following
standard operating procedures after 30 minutes rest. At
12 weeks these measurements were made 8–10 hours after
administration of study medication (trough measurements)
and repeated 60 minutes after a dose of study medication
was given by mouth (1 hour post-dose measurements).
of theKansasCity
Histological study
Lung tissue sections were examined from three ethnic Kyrgyz
highlanders who had died from HAPH and right ventricular
hypertrophy and who had histological evidence of pulmonary
vascular remodelling, and from two healthy individuals of
similar age from the same region who had died in road traffic
accidents.1Dewaxed and rehydrated sections were subjected
to proteolytic antigen retrieval with 0.1% trypsin in 0.1%
calcium chloride (pH 7.6) at 37˚C for 8 minutes17and
immunostained using the avidin-biotin-peroxidase complex
(ABC Elite, Vector) method with 3,39-diaminobenzidine as
substrate. Antisera were raised in rabbits against the N-
terminal 1–12 amino acid residues (MERAGPSFGQQR) of
human PDE5A1 (code LIP-1) and a C-terminal 341 amino
acid sequence (code aPRO) common to all three PDE5 splice
variants.18 19Smooth muscle and endothelial cells were
demonstrated with mouse monoclonal antibodies to a-
smooth muscle actin (a-SMA; clone IA4, Sigma) and CD31,
respectively (clone JC/70A, Dako, Ely, UK). The specificity of
PDE5 immunostaining was confirmed by the absence of
staining after pre-absorption of diluted antiserum (1:1000)
with peptide antigen (0.001–100 mg/ml) and after omission of
the primary antisera. Sections were counterstained with
haematoxylin, dehydrated, cleared in xylene, mounted in
Pertex (CellPath, Hemel Hempstead, UK) and examined by
light microscopy.
Statistical analysis
The data are presented as mean (SD). Changes from baseline
to week 12 in the primary end point (mean PAP) and the
secondary end points (PVR, CO, physical symptom score, and
6MW) were compared between the sildenafil and placebo
groups using analysis of variance (ANOVA). The differences
between all three treatment groups were compared using
ANOVA. If a significant difference was observed between the
three treatment groups, comparisons of each treatment group
with placebo were conducted. In addition, the combined
sildenafil dose groups (sildenafil 25 mg 8 hourly and 100 mg
8 hourly) were compared with placebo by ANOVA.
RESULTS
Subject characteristics
Electrocardiograms were performed on 689 residents at high
altitude (313 men, mean (SD) age 41 (0.6) years). One
hundred and eighty eight subjects (27%, 113 men) had one or
more ECG criteria of RVH. Pulmonary artery haemodynamics
were measured in 44 highlanders with ECG evidence of RVH
and 29 (25 men) had a mean resting PAP of .25 mm Hg.
Assuming that the same proportion (66%) of the 188 with
ECG evidence of RVH had a raised PAP, the estimated
prevalence of HAPH in this study population (689 indivi-
duals) is 18%. Twenty five patients from the catheterised
group with a raised PAP (.25 mm Hg) volunteered for
randomisation to treatment. Three declined further follow up
after randomisation (family commitments prevented them
from travelling to Bishkek). Twenty two patients completed
the study; their ages and lung function data are shown in
table 1 and baseline haemodynamic data in table 2. All
subjects reported dyspnoea on moderate exertion at entry
and most were judged to be in NYHA/WHO functional class
II/III (table 1). There were no clinically significant baseline
differences between the treatment groups.
Response to treatment
Mean PAP
There was a statistically significant difference between the
three groups in changes from baseline to week 12 in mean
PAP measured 8–10 hours post-dose (trough) (p=0.039).
Sildenafil25 mg8 hourly
26.9 mm Hg (95% CI 212.4 to 21.3; p=0.018) compared
with placebo. The treatment effect for the higher dose of
sildenafil compared with placebo was 26.4 mm Hg (95% CI
212.9 to 0.1). When patients receiving sildenafil were
combined into a single treatment group, mean PAP at trough
was significantly reduced by 26.7 mm Hg (95% CI 211.6 to
21.8) compared with placebo (p=0.010).
A statistically significant difference was also observed
between the three groups in changes from baseline to week
12 in mean PAP measured 1 hour post-dose (p,0.001).
Sildenafil 25 mg and 100 mg reduced mean PAP by
211.4 mm Hg (95% CI 216.7 to 26.1) and 211.8 mm Hg
(95% CI 218.0 to 25.6), respectively, compared with placebo.
When patients receiving sildenafil were combined into a
single treatment group, mean PAP at 1 hour post-dose was
significantly reduced by 211.6 mm Hg (95% CI 216.3 to
26.9) compared with placebo (p,0.001).
reducedmean PAPby
PVR
There was no statistically significant difference between the
three groups in changes from baseline to week 12 in PVR
measured at trough levels and at 1 hour post-dose. The
treatment effects of sildenafil 25 mg and 100 mg 8 hourly
compared with placebo at trough levels were 16.2 dyne.s/cm5
(95% CI 2137.3 to 169.7) and 276.9 dyne.s/cm5(95% CI
2257.0 to 103.2). The treatment effect for the lower dose of
sildenafil compared with placebo at 1 hour post-dose was
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289.1 dyne.s/cm5(95% CI 2214.7 to 36.5) and for the higher
dose 2129.9 dyne.s/cm5(95% CI 2277.2 to 17.4). For the
combined sildenafil treatments
217.1 dyne.s/cm5
(95%CI
2103.7 dyne.s/cm5(95% CI 2215.9 to 8.6) for trough and
1 hour post-dose measurements, respectively.
theeffect
to
size was
and
2157.4123.2)
Cardiac output
There was no statistically significant difference between the
three groups in changes from baseline to week 12 in cardiac
output measured at trough levels (p=0.051), so no further
statistical comparisons were carried out. The treatment
effects at trough levels were 21.0 l/min (95% CI 22.1 to
0.1) and 0.5 l/min (95% CI 20.8 to 1.9) for the lower and
higher doses of sildenafil compared with placebo. When
patients receiving sildenafil were combined into a single
treatment group, cardiac output at trough levels was reduced
by was 20.4 l/min (95% CI 21.6 to 0.7) but was not
significantly different from placebo.
Nostatistically significant
between the three groups in changes from baseline to week
12 in cardiac output measured at 1 hour post-dose. The
respective treatment effects for the lower and higher doses of
sildenafil compared with placebo at 1 hour post-dose were
20.6 l/min (95% CI 21.8 to 0.6) and 20.2 l/min (95% CI
21.2 to 1.6). For the combined sildenafil treatments the
effect size was 20.3 l/min (95% CI 21.4 to 0.8) for 1 hour
post-dose.
differencewas observed
6MW distance
There was a statistically significant difference between the
three groups in changes from baseline to week 12 in 6MW
distance (p=0.028). The increase on sildenafil 25 mg
8 hourly was 45.4 m (95% CI 11.5 to 79.4; p=0.011) and
on sildenafil 100 mg 8 hourly it was 40.0 m (95% CI 0.2 to
79.8; p=0.049) compared with placebo. For the combined
sildenafil treatment groups there was a statistically signifi-
cant treatment effect size (p=0.007) of 43.5 m compared
with placebo (95% CI 13.4 to 72.6).
Other parameters
There was a statistically significant difference between the
three groups in changes from baseline to week 12 in physical
symptom score (p=0.024). The treatment effect for the
lower and higher doses of sildenafil compared with placebo
were 8.4 (95% CI 20.2 to 17.0) and 14.0 (95% CI 4.0 to 24.1;
p=0.009), respectively. For the combined sildenafil treat-
ments the statistically significant treatment effect size
(p=0.012) was 10.4 compared with placebo (95% CI 2.5 to
18.3). There was no significant treatment effect on systemic
blood pressure. Sildenafil was well tolerated. All subjects
reported some improvement in well being and no adverse
effects were reported.
Histological study
Lungs from three Kyrgyz highlanders who had died from
HAPH showed typical features of hypoxia induced vascular
Table 1
or placebo treatment
Mean (SD) baseline characteristics of subjects randomised to receive sildenafil
Sildenafil 25 mg tds
(n=9)
Sildenafil 100 mg tds
(n=5)
Placebo
(n=8)
Age (years)
Mean
Range
Weight (kg)
BMI (kg/m2)
FEV1(l)
FVC (l)
PO2(kPa)
PCO2(kPa)
Hb (g/d1)
Haematocrit (%)
NYHA/WHO functional class
(I/II/III)
6 minute walk (m)
61 (8)
44–67
75 (10.6)
30 (4.8)
2.4 (0.7)
2.9 (0.9)
8.6 (0.6)
4.9 (0.5)
16.4 (2.4)
51 (6.7)
2/2/5
60 (8)
54–72
82 (19.2)
29.5 (4.1)
2.5 (1)
3.3 (1.1)
9.1 (1.2)
4.8 (0.4)
15.7 (1.5)
48 (5)
1/2/2
59 (8)
49–65
70 (10.6)
26.1 (5.2)
1.9 (0.7)
2.6 (0.8)
8.4 (1.8)
5.5 (1.0)
16.3 (1.7)
52 (7.3)
1/2/5
420 (85) 449 (84) 422 (69)
BMI, body mass index; FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity; PO2, PCO2, oxygen
and carbon dioxide tension; Hb, haemoglobin.
Table 2
Response to treatment
Measurement
Sildenafil 25 mg (n=9) Sildenafil 100 mg (n=5)Placebo (n=8)
Pre-
treatment Week 12
Pre-
treatment Week 12
Pre-
treatmentWeek 12
BaselineBaseline1 h post-dose Baseline Baseline1 h post-dose BaselineBaseline 1 h post-dose
mPAP (mm Hg)
Syst PAP (mm Hg)
CO (l/min)
PVR (dyne.s/cm5)
WP (mm Hg)
HR (beats/min)
SBP (mm Hg)
DBP (mm Hg)
SaO2(%)
36 (8)
53 (11)
5.3 (1.1)
441 (242)
9 (1)
66 (8)
127 (22)
77 (10)
94 (3)
30 (8)
47 (13)
5.0 (1.4)
391 (243)
9 (2)
63 (10)
125 (19)
78 (7)
95 (2)
25 (7)
39 (10)
5.4 (1.7)
284 (204)
9 (2)
62 (9)
124 (17)
78 (7)
95 (2)
32 (3)
46 (5)
5.5 (1.2)
328 (58)
9.6 (2.1)
74 (12)
132 (20)
80 (7)
94 (2)
26 (3)
41 (6)
6.6 (0.6)
185 (49)
11 (2.8)
70 (14)
127 (8)
81 (2)
95 (1)
21 (2)
33 (4)
6.4 (0.8)
130 (39)
11 (2.8)
69 (14)
125 (7)
81 (2)
95 (1)
34 (6)
51 (7)
4.6 (1.2)
457 (146)
9 (2)
61 (5)
118 (8)
75 (5)
94 (5)
35 (12)
55 (19)
5.3 (1.7)
390 (205)
10 (3)
63 (8)
123 (13)
75 (6)
94 (5)
35 (11)
55 (18)
5.3 (1.6)
388 (208)
10 (3)
62 (8)
123 (13)
76 (6)
94 (5)
mPAP, mean pulmonary artery pressure; SystPAP, systolic pulmonary artery pressure; CO, cardiac output; PVR, pulmonary vascular resistance; WP, wedge pressure; HR, heart
rate; SBP, systolic blood pressure; DBP, diastolic blood pressure; SaO2, systemic arterial oxygen saturation.
PDE5 inhibition and high altitude pulmonary hypertension 685
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remodelling, including the occurrence of longitudinally
orientated intimal cells and distal muscularisation of
arterioles accompanying alveolar ducts and alveolar walls.1 5
All these cells displayed PDE5 immunostaining (fig 1), as did
smooth muscle cells distributed throughout the pulmonary
vasculature. PDE5 immunostaining was abolished following
pre-absorption of primary antisera with peptide antigen and
was not observed in the endothelial or adventitial layers.
Lungs from healthy individuals showed a normal distribution
of vascular smooth muscle, which also expressed immuno-
reactive PDE5.
DISCUSSION
This study shows that the lungs of patients with HAPH have
an abundance of PDE5 in the muscularised coat of
remodelled pulmonary arteries. Chronic treatment with the
PDE5 inhibitor sildenafil was associated with a reduction in
mean PAP and an improvement in exercise capacity and
physical symptom score. The small number of patients per
treatment group resulted in a low power to detect statistically
significant differences in other parameters.
Previous studies have provided evidence of therapeutic
benefit from sildenafil in patients with pulmonary arterial
hypertension, idiopathic or associated with collagen lung
disease,20–22and in chronic thromboembolic disease.23Most of
these studies have been open label. One study of five patients
(four with idiopathic pulmonary arterial hypertension) in
which haemodynamic measurements were made by cardiac
catheterisation reported a fall in mean PAP and PVR of
18 mm Hg and 706 dyne.s/cm5, respectively, with sildenafil
50 mg three times daily for 3 months.21Another study of nine
patients (seven with pulmonary arterial hypertension)
reported a 5 mm Hg fall in mean PAP with sildenafil 50 mg
taken every 8 hours for 3 months.20In 12 patients with
chronic thromboembolic disease, sildenafil 50 mg given
8 hourly reduced mean PAP by 7.7 mm Hg and PVR by
574 dyne.s/cm5.23A recent double blind, placebo controlled,
crossover study of sildenafil 25–100 mg 8 hourly (dose
according to body weight) for 6 weeks in 22 patients with
idiopathicpulmonary arterial
7 mm Hg decrease in mean PAP calculated from Doppler
echocardiography derived measurements.22In this context, a
fall in mean PAP from untreated levels in our study of around
6 mm Hg at the end of a dosing interval and 11 mm Hg
1 hour after dosing is consistent with these reports.
hypertensionreporteda
By comparison, the increase in exercise capacity in our
study was modest compared with the effect reported in
idiopathic and collagen disease related pulmonary arterial
hypertension where 6MW distance increased by 112–225 m.
It is recognised that the natural history of HAPH differs from
that of idiopathic pulmonary arterial hypertension and
pulmonary hypertension associated with collagen lung
disease. It is evident, for example, that life expectancy is
substantially better in HAPH. In keeping with this, our study
population was older and the baseline 6MW distance was
greater (418–456 m) than in reports of sildenafil use in other
forms of pulmonary arterial hypertension studied to date
(226–376 m). Nonetheless, physical activity was limited in
our patient group, as demonstrated by the questionnaire, and
baseline PAP and PVR were raised, even at 760 m. Patients
from the same region who died of HAPH had remodelled
pulmonary arterioles. The increase in exercise distance and
improvement in physical activity score, together with a
reduction in PAP, is consistent with a beneficial response to
sildenafil.
This study extends our finding in an earlier report that
sildenafil attenuates the acute pressor response to hypoxia in
healthy Kyrgyz volunteers,12and a recent study in mountai-
neers and trekkers.24The latter, using echocardiography,
reported that sildenafil reduced systolic PAP during exercise
and increased maximum exercise performance. The effects of
PDE5 inhibition on pulmonary vessels are thought to be
mediated by cyclic GMP dependent factors, particularly NO
and natriuretic factors. The distribution of PDE5 in remo-
delled pulmonary arterioles shows that it is well placed to
influence the local actions of these factors. Studies in the rat
suggest that chronic hypoxia may upregulate PDE5 activity
and that this may contribute to the pathophysiology of
hypoxia induced pulmonary hypertension.25It was not
possible to quantify PDE5 activity in our lung specimens,
but increased expression of the enzyme is evident from its
aberrant distribution. PDE5 is expressed wherever there is
smooth muscle (both vascular and bronchial). Histological
examination of HAPH lungs showed typically increased
muscularisation including the presence of an extra long-
itudinally orientated layer in some vessels that is pathogno-
monic of chronic hypoxia,5and the increased muscularisation
demonstrated immunoreactive PDE5. By inhibiting this
enzyme, sildenafil would augment the local vasorelaxant
and antiproliferative actions of cyclic GMP dependent factors.
Given that NO levels are reduced in HAPH,8circulating
natriuretic peptide levels—raised as a result of pressure load
on the right ventricle—are likely to be the major stimulus of
cyclic GMP synthesis in this condition.26 27Work in animals
exposed to hypoxia (10% O2) has shown that dosing with
sildenafil for 3 weeks reduces pulmonary vascular remodel-
ling as well as PAP.12 15Clearly, it is more difficult to establish
whether the haemodynamic effects of sildenafil in humans
are also accompanied by changes in pulmonary vascular
remodelling.
Pulmonary hypertension is a recognised complication of
high altitude living—even in ethnic groups that have lived at
altitude for many centuries6—although accurate prevalence
data are difficult to obtain. While not the primary aim of this
study, our results suggest that about 18% of the adult Kyrgyz
population resident above 2500 m have a mean PAP above
25 mm Hg. This makes the assumption that the group
catheterised is representative of the community studied.
The ECG lacks sensitivity as a screening tool, but our figure
(18%) compares favourably with an earlier survey in which
20% of 136 Kyrgyz highlanders had a PAP of more than
25 mm Hg on direct measurement.1Others studies based on
ECG criteria have reported a prevalence of RVH of 17% in
indigenous Tibetans and 18% in Peruvian Indians resident at
Figure 1
pulmonary vessels in a lung from a Kyrgyz highlander with high altitude
pulmonary hypertension. PDE5 immunostaining is localised to
longitudinal intimal cells (A and B, white arrows) and muscularised distal
arterioles in alveolar ducts and alveolar walls (C and D). Endothelial cells
lack immunoreactivity (C, black arrows).
Immunohistochemical localisation of PDE5 in remodelled
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3000–4000 m.28The view that residence at high altitude over
many generations may be protective against the development
of HAPH is supported by the observation that ECG evidence
of RVH is more common in recently migrated Han Chinese in
Tibet (29%), immigrants from sea level areas to altitude in
Peru (28%), and school age children at 3093 m in North
America (30%).28
It is recognised that the need, for logistical reasons, to
conduct cardiac catheterisation in the main academic
medical center at 760 m is a limitation of our study as the
patients were removed from their hypoxic environment. It is
worth noting, however, that our studies in telemetred rats
indicate that, without treatment, PAP does not fall immedi-
ately but remains high for several days after removal from
chronic exposure to hypoxia15and vascular remodelling
persists for a very long time.6Secondly, only one in four
subjects with ECG evidence of RVH were willing or able to
travel the distance to the hospital for these studies and only
22 patients were able to repeat the hospital visits required for
the sildenafil component of the study. As a result, our study
lacks power to define the full potential of sildenafil in HAPH
and to explore the dose-response relationship.
Nonetheless, sildenafil is a promising treatment for the
management of HAPH, a condition for which there are few
treatment options. Calcium antagonists are used but have to
be given in relatively high doses, lack specificity for the
pulmonary vascular bed, and side effects such as ankle
oedema are frequent.29 30Sildenafil had little effect on
systemic blood pressure and was well tolerated. It is relatively
expensive for a country with poor health resources, and a
larger study of the long term effects of PDE5 inhibition
in HAPH is warranted before sildenafil can be widely
recommended.
ACKNOWLEDGEMENTS
The authors are grateful to Pfizer UK for supplies of sildenafil and
matching placebo and for the antibody (LIP1) to PDE5.
Authors’ affiliations
A A Aldashev, B K Kojonazarov, Institute of Molecular Biology and
Medicine, Bishkek, Kyrgyzstan
T A Amatov, T M Sooronbaev, M M Mirrakhimov, National Center of
Cardiology and Internal Medicine, Bishkek, Kyrgyzstan
N W Morrell, Addenbrooke’s Hospital, Cambridge, UK
J Wharton, M R Wilkins, Experimental Medicine & Toxicology, Imperial
College London, Hammersmith Hospital, London W12 0NN, UK
.....................
This study was supported by British Heart Foundation project grant PG/
02/024.
Competing interests: MRW has received support from Pfizer for research
projects and lectures.
REFERENCES
1 Aldashev AA, Sarybaev AS, Sydykov AS, et al. Characterization of high-
altitude pulmonary hypertension in the Kyrgyz: association with angiotensin-
converting enzyme genotype. Am J Respir Crit Care Med
2002;166:1396–402.
2 Moore LG, Niermeyer S, Zamudio S. Human adaptation to high altitude:
regional and life-cycle perspectives. Am J Phys Anthropol 1998;27:25–64.
3 Ward MP, Milledge JS, West J. High altitude medicine and physiology. New
York: Oxford University Press, 2000:434.
4 Heath D. Missing link from Tibet. Thorax 1989;44:981–3.
5 Heath D, Williams D, Rios-Dalenz J, et al. Small pulmonary arterial vessels of
Aymara Indians from the Bolivian Andes. Histopathology 1990;16:565–71.
6 Maggiorini M, Leon-Velarde F. High-altitude pulmonary hypertension: a
pathophysiological entity to different diseases. Eur Respir J
2003;22:1019–25.
7 Fagan KA, McMurtry I, Rodman DM. Nitric oxide synthase in pulmonary
hypertension: lessons from knockout mice. Physiol Res 2000;49:539–48.
8 Beall CM, Laskowski D, Strohl KP, et al. Pulmonary nitric oxide in mountain
dwellers. Nature 2001;414:411–2.
9 Anand IS, Prasad BA, Chugh SS, et al. Effects of inhaled nitric oxide and
oxygen in high-altitude pulmonary edema. Circulation 1998;98:2441–5.
10 Ignarro LJ, Cirino G, Casini A, et al. Nitric oxide as a signaling molecule in
the vascular system: an overview. J Cardiovasc Pharmacol 1999;34:879–86.
11 Thomas MK, Francis SH, Corbin JD. Characterisation of a purified bovine
lung cGMP-binding cGMP phosphodiesterase. J Biol Chem
1990;265:14964–70.
12 Zhao L, Mason NA, Morrell NW, et al. Sildenafil inhibits hypoxia-induced
pulmonary hypertension. Circulation 2001;104:424–8.
13 Schermuly RT, Kreisselmeier KP, Ghofrani HA, et al. Chronic sildenafil
treatment inhibits monocrotaline-induced pulmonary hypertension in rats.
Am J Respir Crit Care Med 2004;169:39–45.
14 Itoh T, Nagaya N, Fujii T, et al. A combination of oral sildenafil and beraprost
ameliorates pulmonary hypertension in rats. Am J Respir Crit Care Med
2004;169:34–8.
15 Sebkhi A, Strange JW, Phillips SC, et al. Phosphodiesterase type 5 as a target
for the treatment of hypoxia-induced pulmonary hypertension. Circulation
2003;107:3230–5.
16 Green CP, Porter CB, Bresnahan DR, et al. Development and evaluation of the
Kansas City Cardiomyopathy Questionnaire: a new health status measure for
heart failure. J Am Coll Cardiol 2000;35:1245–55.
17 Cattoretti G, Pileri S, Parravicini C, et al. Antigen unmasking on formalin-
fixed, paraffin-embedded tissue sections. J Pathol 1993;171:83–98.
18 Stacey P, Rulten S, Dapling A, et al. Molecular cloning and expression of
human cGMP-binding cGMP-specific phosphodiesterase (PDE5). Biochem
Biophys Res Commun 1999;247:249–54.
19 Lin CS, Lau A, Tu R, et al. Expression of three isoforms of cGMP-binding
cGMP-specific phosphodiesterase (PDE5) in human penile cavernosum.
Biochem Biophys Res Commun 2000;268:628–35.
20 Mikhail GW, Prasad SK, Li W, et al. Clinical and haemodynamic effects of
sildenafil in pulmonary hypertension: acute and mid-term effects. Eur Heart J
2004;25:431–6.
21 Michelakis ED, Tymchak W, Noga M, et al. Long-term treatment with oral
sildenafil is safe and improves functional capacity and hemodynamics in
patients with pulmonary arterial hypertension. Circulation 2003;108:2066–9.
22 Sastry BK, Narasimhan C, Reddy NK, et al. A study of clinical efficacy of
sildenafil in patients with primary pulmonary hypertension. Indian Heart J
2002;54:410–4.
23 Ghofrani HA, Schermuly RT, Rose F, et al. Sildenafil for long-term treatment of
nonoperable chronic thromboembolic pulmonary hypertension. Am J Respir
Crit Care Med 2003;167:1139–41.
24 Ghofrani HA, Reichenberger F, Kohstall MG, et al. Sildenafil increased
exercise capacity during hypoxia at low altitudes and at Mount Everest base
camp: a randomized, double-blind, placebo-controlled crossover trial. Ann
Intern Med 2004;141:169–77.
25 MacLean MR, Johnston ED, McCulloch KM, et al. Phosphodiesterase isoforms
in the pulmonary arterial circulation of the rat: changes in pulmonary
hypertension. J Pharmacol Exp Ther 1997;283:619–24.
26 Muramatsu M, Tyler RC, Gutkowska J, et al. Atrial natriuretic peptide
accounts for increased cGMP in hypoxia-induced hypertensive rat lungs.
Am J Physiol 1997;272:L1126–32.
27 Soodanbekova GK, Batyraliev TA, Niyazova ZA, et al. Atrial natriuretic factor
in high-altitude pulmonary hypertension: the influence of acute hypoxia on
plasma atrial natriuretic factor, renin, and aldosterone concentrations in
highlanders with initially normal or elevated pulmonary artery pressure and
without evidence of right ventricular hypertrophy. Angiology 1995;46:833–7.
28 Halperin BD, Sun S, Zhuang J, et al. ECG observations in Tibetan and Han
residents of Lhasa. J Electrocardiol 1998;31:237–43.
29 Hackett PH, Roach RC. High-altitude illness. N Engl J Med
2001;345:107–14.
30 Rich S, Kaufmann E, Levy PS. The effect of high doses of calcium-channel
blockers on survival in primary pulmomary hypertension. N Engl J Med
1992;327:76–81.
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