High lung PDE5: a strong basis for treating pulmonary hypertension with PDE5 inhibitors

Update on the clinical utility of sildenafil in the treatment of pulmonary arterial hypertension

Sildenafil is an orally administered phosphodiesterase type 5 inhibitor that is approved for the treatment of pulmonary arterial hypertension (PAH). The hemodynamic effects of sildenafil are mitigated primarily via potentiating the effects of endogenous nitric oxide, leading to smooth muscle cell relaxation and reductions in pulmonary arterial pressures and pulmonary vascular resistance. When added to standard background therapy in patients with idiopathic or associated PAH from congenital heart disease, anorexigen use, or connective tissue disease, sildenafil treatment results in improved exercise capacity as measured by 6 minute walk distance, improved hemodynamics, and favorable changes in quality of life. Sildenafil use is contraindicated with concomitant nitrate administration, and caution should be exercised when used in combination with antihypertensive agents due to risks of precipitating hypotension. Side effects are generally mild, and include flushing, headaches, and epistaxis. The combination of sildenafil with intravenous epoprostenol is safe and well tolerated, and further improves exercise capacity. Sildenafil is approved only for treatment of PAH, and although emerging data suggest a potential role in treating other types of pulmonary hypertension, larger trials are required to confirm these findings.

Brain natriuretic peptide in pulmonary arterial hypertension: biomarker and potential therapeutic agent

B-type natriuretic peptide (BNP) is a member of the natriuretic peptide family, a group of widely distributed, but evolutionarily conserved, polypeptide mediators that exert myriad cardiovascular effects. BNP is a potent vasodilator with mitogenic, hypertrophic and pro-inflammatory properties that is upregulated in pulmonary hypertensive diseases. Circulating levels of BNP correlate with mean pulmonary arterial pressure (mPAP) and pulmonary vascular resistance (PVR) in patients with pulmonary arterial hypertension (PAH). Elevated plasma BNP levels are associated with increased mortality in patients with PAH and a fall in BNP levels after therapy is associated with improved survival. These findings have important clinical implications in that a noninvasive blood test may be used to identify PAH patients at high-risk of decompensation and to guide pulmonary vasodilator therapy. BNP also has several biologic effects that could be beneficial to patients with PAH. However, lack of a convenient method for achieving sustained increases in circulating BNP levels has impeded the development of BNP as a therapy for treating pulmonary hypertension. New technologies that allow transdermal or oral administration of the natriuretic peptides have the potential to greatly accelerate research into therapeutic use of BNP for cor pulmonale and pulmonary vascular diseases. This review will examine the basic science and clinical research that has led to our understanding of the role of BNP in cardiovascular physiology, its use as a biomarker of right ventricular function and its therapeutic potential for managing patients with pulmonary vascular disease.

Tadalafil, a long-acting inhibitor of PDE5, improves pulmonary hemodynamics and survival rate of monocrotaline-induced pulmonary artery hypertension in rats

The aim of this study was to assess the effect of tadalafil (0.5, 2.5, and 10 mg/kg per day) on the progression of pulmonary arterial hypertension (PAH) in early treatment and on the survival rate in late treatment on the monocrotaline (MCT)-induced PAH rat model. Tadalafil was administered once daily to rats for 3 weeks from the day of MCT-injection or 21 days after the injection. With early treatment, tadalafil at 10 mg/kg per day prevented the development of PAH by maintaining mean pulmonary artery pressure within the normal range and attenuated right ventricular hypertrophy. With late treatment, tadalafil tended to increase the partial pressure of oxygen in arterial blood and dose-dependently improved the survival rate by 55%, 60%, and 70% at 0.5, 2.5, and 10 mg/kg per day, respectively, versus 40% in the MCT-control group. Both early and late treatments with tadalafil were associated with elevated lung cyclic guanosine monophosphate (cGMP). These results suggest that tadalafil relaxes pulmonary arteries by elevating cGMP in lungs and extend survival time by improving pulmonary hemodynamics even when treatment occurs in the late phase of PAH. Thus, it is expected that tadalafil may be an effective, once-daily treatment option in humans with PAH.

Phosphodiesterase type 5 inhibitors in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a rare disease characterized by vascular proliferation and remodeling, resulting in a progressive increase in pulmonary arterial resistance, right heart failure, and death. The pathogenesis of PAH is multifactorial, with endothelial cell dysfunction playing an integral role. This endothelial dysfunction is characterized by an overproduction of vasoconstrictors and proliferative factors, such as endothelin-1, and a reduction of vasodilators and antiproliferative factors, such prostacyclin and nitric oxide. Phosphodiesterase type 5 (PDE-5) is implicated in this process by inactivating cyclic guanosine monophosphate, the nitric oxide pathway second messenger. PDE-5 is abundantly expressed in lung tissue, and appears to be upregulated in PAH. Three oral PDE-5 inhibitors are available (sildenafil, tadalafil, and vardenafil) and are the recommended first-line treatment for erectile dysfunction. Experimental studies have shown the beneficial effects of PDE-5 inhibitors on pulmonary vascular remodeling and vasodilatation, justifying their investigation in PAH. Randomized clinical trials in monotherapy or combination therapy have been conducted in PAH with sildenafil and tadalafil, which are therefore currently the approved PDE-5 inhibitors in PAH treatment. Sildenafil and tadalafil significantly improve clinical status, exercise capacity, and hemodynamics of PAH patients. Combination therapy of PDE-5 inhibitors with prostacyclin analogs and endothelin receptor antagonists may be helpful in the management of PAH although further studies are needed in this area. The third PDE-5 inhibitor, vardenafil, is currently being investigated in PAH. Side effects are usually mild and transient and include headache, flushing, nasal congestion, digestive disorders, and myalgia. Mild and moderate renal or hepatic failure does not significantly affect the metabolism of PDE-5 inhibitors, whereas coadministration of bosentan decreases sildenafil and tadalafil plasma levels. Due to their clinical effectiveness, tolerance profile, and their oral administration, sildenafil and tadalafil are two of the recommended first-line therapies for PAH patients in World Health Organization functional classes II or III.

Immunohistochemical localisation of PDE5 in rat lung during pre- and postnatal development

In mammalian lung, at the transition to extrauterine life, NO/cGMP signal transduction system is known to play crucial roles in the regulation of vascular resistance and is supposed to act in angiogenesis. PDE5, which is the most abundant cGMP metabolizing enzyme within the lung, is highly expressed in the perinatal period, but its localisation in the different pulmonary cells is still poorly known. In our research, PDE5 immunohistochemical distribution was investigated in foetal and neonatal rat lung. The highest expression of PDE5 was found in cells randomly located in the stroma; in newborns, in particular, many cells in the intersaccular walls were heavily labelled, while much lower staining levels were shown by smooth myocytes belonging to vessels and airways. On the basis of their immunoreactivity for alpha-SM actin and/or desmin, most of the heavily PDE5-positive cells were identified as interstitial myofibroblasts and transitional pericytes, while only a few were interpreted as interstitial lipofibroblasts.

B(2) kinin receptors mediate the Indian red scorpion venom-induced augmentation of visceral reflexes via the nitric oxide cyclic guanosine monophosphate pathway

AIM

This study was performed to delineate the kinin (receptor)-dependent pathways in the Indian red scorpion (Mesobuthus tamulus; MBT) venom-induced pulmonary oedema as well as the augmentation of cardio-pulmonary reflexes evoked by phenyldiguanide (PDG).

METHODS

In urethane-anaesthetized adult rats, the effect of venom on the PDG reflex responses (blood pressure, heart rate and respiration rate) and the pulmonary water content was ascertained using various antagonists(des- Arg, B(1) receptor antagonist; Hoe 140, B(2) receptor antagonist; N(omega)-nitro-l-arginine methyl ester (l-NAME), nitric oxide (NO) synthase inhibitor; methylene blue, soluble guanylate cyclase inhibitor; and glibenclamide, K(+)(ATP) channel blocker). The effect of phosphodiesterase V inhibitor (sildenafil citrate) on the reflex response and the pulmonary water content was also examined and compared with venom-induced responses.

RESULTS

Intravenous injection of PDG (10 microg kg(-1)) evoked apnoea, bradycardia and hypotension lasting >60 s. Exposure to MBT venom (100 microg kg(-1)) for 30 min augmented the PDG reflex responses by two times and increased the pulmonary water content, significantly. Hoe 140 blocked the venom-induced responses (augmentation of PDG reflex and increased pulmonary water content) whereas des-Arg did not. l-NAME, methylene blue or glibenclamide also blocked the venom-induced responses. Furthermore, sildenafil citrate (that increases cGMP levels) produced augmentation of PDG reflex response and increased the pulmonary water content as seen with venom.

CONCLUSION

The results indicate that venom-induced responses involve B(2) kinin receptors via the NO-dependent guanylate cyclase-cGMP pathway involving K(+)(ATP) channels.

Tadalafil therapy for pulmonary arterial hypertension

BACKGROUND

Treatment options for pulmonary arterial hypertension target the prostacyclin, endothelin, or nitric oxide pathways. Tadalafil, a phosphodiesterase type-5 inhibitor, increases cGMP, the final mediator in the nitric oxide pathway.

METHODS AND RESULTS

In this 16-week, double-blind, placebo-controlled study, 405 patients with pulmonary arterial hypertension (idiopathic or associated), either treatment-naive or on background therapy with the endothelin receptor antagonist bosentan, were randomized to placebo or tadalafil 2.5, 10, 20, or 40 mg orally once daily. The primary end point was the change from baseline to week 16 in the distance walked in 6 minutes. Changes in World Health Organization functional class, clinical worsening, and health-related quality of life were also assessed. Patients completing the 16-week study could enter a long-term extension study. Tadalafil increased the distance walked in 6 minutes in a dose-dependent manner; only the 40-mg dose met the prespecified level of statistical significance (P<0.01). Overall, the mean placebo-corrected treatment effect was 33 m (95% confidence interval, 15 to 50 m). In the bosentan-naive group, the treatment effect was 44 m (95% confidence interval, 20 to 69 m) compared with 23 m (95% confidence interval, -2 to 48 m) in patients on background bosentan therapy. Tadalafil 40 mg improved the time to clinical worsening (P=0.041), incidence of clinical worsening (68% relative risk reduction; P=0.038), and health-related quality of life. The changes in World Health Organization functional class were not statistically significant. The most common treatment-related adverse events reported with tadalafil were headache, myalgia, and flushing.

CONCLUSIONS

In patients with pulmonary arterial hypertension, tadalafil 40 mg was well tolerated and improved exercise capacity and quality of life measures and reduced clinical worsening.

A chemical proteomics based enrichment technique targeting the interactome of the PDE5 inhibitor PF-4540124

The starting point for the discovery and development of new drugs is the design of molecules that bind to their target proteins with high specificity. Here we describe a systematic chemical proteomics based approach, whereby we use a novel PDE5 inhibitor as bait in mice lung tissue. The compound N-(6-aminohexyl)-3-(1-ethyl-3-methyl-7-oxo-6,7-dihydro-1H-pyrazolo[4,3-d]pyrimidin-5-yl)-4-propoxybenzenesulfonamide (or PF-4540124), which binds to phosphodiesterase-5 (PDE5) with high affinity, was therefore immobilized on an affinity support. Initial affinity enrichment revealed the binding of hundreds of proteins to this immobilized PDE5 inhibitor. Therefore, selective pre-clearing and elution protocols were designed and used in combination with differential stable-isotope labeling to discriminate between the specific binding of low abundant proteins and less specific binding of high abundant proteins. The optimized method allowed us to selectively analyze the "interactome" of the PDE5 inhibitor PF-4540124 and enabled us to identify different isoforms of PDE5 present in mouse lung. Additionally, we enriched for the prenyl binding protein PrBP, which is also known as PDE6delta. Further analysis, applying in vitro binding assays allowed us to verify PrBP as a novel interactor of PF-4540124. The presented method provides a generic highly-specific chemical proteomics based enrichment technique for analyzing drug-protein interactions in mammalian tissue lysates.

Sildenafil monotherapy in portopulmonary hypertension can facilitate liver transplantation

Portopulmonary hypertension (POPH), or pulmonary arterial hypertension associated with cirrhosis, carries a high mortality and often precludes liver transplantation. Many POPH patients have preserved or increased cardiac output; therefore, decreasing pulmonary artery pressure rather than improving cardiac output is more important in reducing liver transplant risk, and this makes sildenafil an attractive therapeutic option. We assessed the clinical response of patients with POPH treated with sildenafil monotherapy. We retrospectively reviewed the charts of 10 patients with POPH and sildenafil monotherapy. Eight of 10 patients had hepatitis C virus infection. Patients took 31 +/- 14 mg (mean +/- standard deviation) thrice daily and were followed for 21 +/- 16 months. The World Health Organization functional class improved from 3.0 +/- 0.0 to 2.3 +/- 0.5 at 1 year (P < 0.05). Four of 8 patients increased the 6-minute walk distance at 1 year by 30 m or more. Three patients became transplant-eligible, 1 of whom underwent successful transplantation, and 3 patients have been stable without progression of liver disease or POPH. The remainder were not transplant candidates because of refractory POPH (n = 2) or other comorbidities (n = 2). We conclude that sildenafil may be an effective therapy for POPH that can stabilize or improve hemodynamics in patients with POPH and thereby facilitate liver transplantation.

cGMP in the vasculature

Cyclic guanosine 3', 5'-monophosphate (cGMP) plays an integral role in the control of vascular function. Generated from guanylate cyclases in response to the endogenous ligands, nitric oxide (NO) and natriuretic peptides (NPs), cGMP influences a number of vascular cell types and regulates vasomotor tone, endothelial permeability, cell growth and differentiation, as well as platelet and blood cell interactions. Reciprocal regulation of the NO-cGMP and NP-cGMP pathways is evident in the vasculature such that one cGMP generating system may compensate for the dysfunction of the other. Indeed, aberrant cGMP production and/or signalling accompanies many vascular disorders such as hypertension, atherosclerosis, coronary artery disease and diabetic complications. This chapter highlights the main vascular functions of cGMP, its role in disease and the resulting current and potential therapeutic applications. With respect to pulmonary hypertension, heart failure and erectile dysfunction, as well as cGMP signal transduction, the reader is specifically referred to other dedicated chapters.

Acute effects of the combination of sildenafil and inhaled treprostinil on haemodynamics and gas exchange in pulmonary hypertension

BACKGROUND

Inhaled treprostinil was recently developed for the treatment of pulmonary arterial hypertension (PAH). We investigated the safety and acute haemodynamic effects of the combination oral sildenafil and inhaled treprostinil in an open label study in patients with precapillary pulmonary hypertension.

METHODS AND PATIENTS

Inhaled nitric oxide (20ppm; n=50), sildenafil (50mg; n=50) and inhaled treprostinil (15microg; n=25 or 30microg; n=25) were applied in subsequent order during right heart catheter investigation to consecutive patients with pulmonary arterial hypertension (PAH; n=28), non-operable chronic thromboembolic pulmonary hypertension (CTEPH; n=17) and pulmonary fibrosis associated pulmonary hypertension (n=5).

RESULTS

Inhaled nitric oxide reduced pulmonary vascular resistance (PVR) to 87.3+/-5.1% of baseline values, reduced mean pulmonary arterial pressure (PAP) to 89.7+/-3.5% and increased cardiac output (CO) to 102.4+/-2.9%. Sildenafil reduced PVR to 80.1+/-5.0%, mPAP to 86.5+/-2.9% and increased CO to 103.8+/-3.2%. Treprostinil, inhaled 1h after sildenafil, reduced PVR to 66.3+/-3.8%, mPAP to 77.8+/-3.3%, and increased CO to 107.1+/-3.3% (mean+/-95% confidence interval). Subgroup analysis showed similar acute haemodynamic effects in PAH and CTEPH patients. Ventilation/perfusion distribution measurement in six patients with pre-existing gas exchange limitations was not changed by sildenafil and treprostinil. Relevant side effects were not observed.

CONCLUSION

The combination of sildenafil and inhaled treprostinil was well tolerated and induced additive, pulmonary selective vasodilatation in pulmonary hypertension patients. This could be of relevance also for long-term treatment of PAH and CTEPH patients.

Pulmonary embolism after tadalafil ingestion

Tadalafil is a phosphodiesterase 5 (PDE5) inhibitor that is used to treat male erectile dysfunction. It is currently undergoing clinical trials for the treatment of pulmonary hypertension. Although at present there is no evidence of hypercoagulation states being associated with the use of PDE5 inhibitors, several studies suggest that PDE5 inhibitors may, in rare cases, cause arterial and venous thrombotic or embolic events in the heart, eye, brain, and ear. We report a 54-year-old male patient with reduced plasma levels of protein C who developed pulmonary embolism (PE) just after he started taking tadalafil. The Naranjo probability scale indicates tadalafil as the possible cause of a PE in our patient. To the best of our knowledge, this is the first case report of tadalafil-induced PE in the literature.

Control of pulmonary vascular tone during exercise in health and pulmonary hypertension

Despite the importance of the pulmonary circulation as a determinant of exercise capacity in health and disease, studies into the regulation of pulmonary vascular tone in the healthy lung during exercise are scarce. This review describes the current knowledge of the role of various endogenous vasoactive mechanisms in the control of pulmonary vascular tone at rest and during exercise. Recent studies demonstrate an important role for endothelial factors (NO and endothelin) and neurohumoral factors (noradrenaline, acetylcholine). Moreover, there is evidence that natriuretic peptides, reactive oxygen species and phosphodiesterase activity can influence resting pulmonary vascular tone, but their role in the control of pulmonary vascular tone during exercise remains to be determined. K-channels are purported end-effectors in control of pulmonary vascular tone. However, K(ATP) channels do not contribute to regulation of pulmonary vascular tone, while the role of K(V) and K(Ca) channels at rest and during exercise remains to be determined. Pulmonary hypertension is associated with alterations in pulmonary vascular function and structure, resulting in blunted pulmonary vasodilatation during exercise and impaired exercise capacity. Although there is a paucity of studies pertaining to the regulation of pulmonary vascular tone during exercise in idiopathic pulmonary hypertension, the few studies that have been performed in models of pulmonary hypertension secondary to left ventricular dysfunction suggest altered control of pulmonary vascular tone during exercise. Since the increased pulmonary vascular tone during exercise limits exercise capacity, future studies are needed to investigate the vasomotor mechanisms that are responsible for the blunted exercise-induced pulmonary vasodilatation in pulmonary hypertension.

Phosphorylation increases affinity of the phosphodiesterase-5 catalytic site for tadalafil

Phosphodiesterase-5 (PDE5) is phosphorylated at a single serine residue by cyclic nucleotide-dependent protein kinases. To test for a direct effect of phosphorylation on the PDE5 catalytic site, independent of cGMP binding to the allosteric sites of the enzyme, binding of the catalytic site-specific substrate analog [(3)H]tadalafil to PDE5 was measured. Phosphorylation increased [(3)H]tadalafil binding 3-fold, whereas cGMP caused a 1.6-fold increase. Combination of both treatments caused more than 4-fold increase in [(3)H]tadalafil binding, and effects were additive only at submaximal stimulation. Consistent with the increase in affinity, phosphorylation slowed the [(3)H]tadalafil exchange-dissociation rate from PDE5 more than 6-fold. Finally, phosphorylation increased affinity for hydrolysis of a catalytic site-specific cGMP analog, 2'-O-anthraniloyl-cGMP, by approximately 3-fold. The combined results showed that phosphorylation activates PDE5 catalytic site independently of cGMP binding to the allosteric sites. The results suggested that phosphorylation acts in concert with allosteric cGMP binding to stimulate the PDE5 catalytic site, which should promote negative feedback regulation of the cGMP pathway in intact cells. By increasing the affinity of the catalytic site, phosphorylation should also consequently increase the potency and duration of PDE5 inhibitor action.

Vardenafil protects isolated rat hearts at reperfusion dependent on GC and PKG

BACKGROUND AND PURPOSE

The type-5 PDE inhibitor vardenafil reduces myocardial infarct size in situ, following ischemia/reperfusion, when applied at reperfusion in animal models. Little is known about the underlying protective signaling. Here, we test whether vardenafil is protective in rat isolated hearts and in a cell model of calcium stress.

EXPERIMENTAL APPROACH

Infarct size in rat isolated hearts was measured after a 30 min regional ischemia and 120 min reperfusion. Vardenafil (1 nM-1 microM) was infused during reperfusion. HL-1 cardiomyocytes were loaded with tetramethylrhodamine ethyl ester (TMRE), a fluorescent marker of mitochondrial membrane potential (psi m).

KEY RESULTS

Vardenafil at reperfusion reduced infarct size as percentage of the ischemic zone from 45.8+/-2.0% in control hearts to 26.2+/-2.7% (P<0.001) only at 10 nM, whereas higher or lower dosages failed to protect. This protective effect was blocked by co-administration of either the GC inhibitor, 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), or the PKG inhibitor, KT-5823. HL-1 cardiomyocytes, loaded with TMRE, were treated for 80 min with the calcium ionophore, calcimycin, to induce calcium stress. This reduced the mean cell fluorescence to 63.3 +/- 3.8% of baseline values and vardenafil protected against this fall (78.6 +/- 3.6%, P<0.01). The vardenafil-induced protection of HL-1 cells was blocked by ODQ, KT-5823 or the PKG-inhibiting peptides DT-2 and DT-3, confirming a role for GC and PKG.

CONCLUSIONS AND IMPLICATIONS

These results further support the hypothesis that PDE-5 inhibitors are protective in ischemic hearts, in addition to their known clinical effects in the treatment of erectile dysfunction in men.

Safety, tolerability and pharmacokinetics of udenafil, a novel PDE-5 inhibitor, in healthy young Korean subjects

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

The phosphodiesterase (PDE) type 5 inhibitor is a widely used agent that facilitates penile erection. Udenafil is newly developed as a PDE-5 inhibitor.

WHAT THIS STUDY ADDS

This is the first study to determine the safety, tolerability and pharmacokinetics of udenafil in healthy subjects. Udenafil was safe and well tolerated in healthy Korean subjects. The AUC and C(max) of udenafil increased supraproportionally with increasing dose upon single administration, but there was no significant drug accumulation upon multiple administrations.

AIM

To evaluate the safety, tolerability and pharmacokinetics (PK) of udenafil, a novel phosphodiesterase type 5 inhibitor.

METHODS

A double-blind, randomized, placebo-controlled, dose-rising, parallel-group, single- and multiple-dose study was conducted in healthy Korean subjects. The subjects were allocated to single-dose groups of 25, 50, 100, 200 or 300 mg (eight subjects in each dose group, including two placebos), or to multiple-dose groups of 100 or 200 mg (once-daily dosing for 7 days; nine subjects in each dose group, including three placebos). Serial samples of blood and urine were collected after oral administration and the drug concentrations in plasma and urine were determined by high-performance liquid chromatography. Safety and tolerability were evaluated by monitoring clinical laboratory parameters and adverse events.

RESULTS

Udenafil reached peak plasma concentrations at 0.8-1.3 h, and then declined mono-exponentially with a terminal half-life of 7.3-12.1 h in the single-dose study. The area under the time-concentration curves (AUC) and maximum plasma concentrations (C(max)) increased supraproportionally with increasing dose in the single-dose study. During multiple dosing, a steady state was reached at 5 days and little accumulation occurred after repeated dosing for 7 days. Udenafil was generally well tolerated in these healthy subjects, and no serious adverse events occurred.

CONCLUSIONS

Udenafil was safe and well tolerated in healthy volunteers. The AUC and C(max) of udenafil increased supraproportionally with increasing dose upon single administration, but there was no significant drug accumulation upon multiple administrations.

Phosphodiesterase type 5: expanding roles in cardiovascular regulation

Phosphodiesterase type 5A (PDE5A) selectively hydrolyzes cyclic GMP. Inhibitors of PDE5A such as sildenafil are widely used to treat erectile dysfunction, but growing evidence supports important roles for the enzyme in both the vasculature and heart. In disorders such as cardiac failure, PDE5A upregulation may contribute to a decline in cGMP and protein kinase G signaling, exacerbating dysfunction. PDE5A plays an important role in the pulmonary vasculature where its inhibition benefits patients with pulmonary hypertension. In the heart, PDE5A signaling appears compartmentalized, and its inhibition is cardioprotective against ischemia-reperfusion and antracycline toxicity, blunts acute adrenergic contractile stimulation, and can suppress chronic hypertrophy and dysfunction attributable to pressure-overload. In this review, we discuss the molecular biology, pharmacology, and physiology of PDE5A, mechanisms of vascular and cardiac regulation, and recent evidence supporting the utility of selective PDE5A inhibition for the treatment of cardiovascular disorders.

Daily Administration of Phosphodiesterase Type 5 Inhibitors for Urological and Nonurological Indications

OBJECTIVES

Although the discovery of phosphodiesterases (PDEs) was made soon after the identification of cyclic adenosine monophosphate nearly half a century ago, their true importance in medicine has taken many decades to be realised. The recognition of the important role PDE enzymes play and the impact of altering intracellular cyclic nucleotide levels became significant for most urologists and clinicians in the early 1990s with the discovery of sildenafil, a PDE5 inhibitor (PDE5-I). Once approved around the world, on-demand use of PDE5-Is became the gold standard. Recently, the potential beneficial effects of PDE5-Is on the pulmonary, vascular, and other systems has led to examination of alternative dosing regimens. In this review, we have synthesised the available published peer-reviewed literature to provide a critical contemporary view of evolving indications for PDE5-Is and how alternative dosing regimens may impact on sexual and other functions.

METHODS

MEDLINE search of all peer-reviewed English literature for the period 1990-2007.

RESULTS

The plethora of articles detailing potential uses of PDE5-I in multiple fields of medicine was uncovered. Use of alternative dosing regimens shows great promise across a number of clinical indications, including post-radical retropubic prostatectomy, pulmonary hypertension, endothelial dysfunction, and salvage of on-demand PDE5-I nonresponders.

CONCLUSIONS

Use of PDE5-I on a daily basis may evolve into a major form of drug administration both for men with erectile dysfunction and for those with a myriad of other conditions shown to benefit from this approach.

Approach to patients with heart failure and pulmonary hypertension

Pulmonary hypertension (PH), defined as a mean pulmonary artery pressure greater than 25 mm Hg, is not a diagnosis, but rather the physiologic consequence of the interaction between pulmonary blood flow, pulmonary vascular impedance, and downstream pulmonary venous pressure. The diagnosis and appropriate treatment of PH in patients with or without heart failure (HF) requires an understanding of the underlying pathogenesis, whether it be due to increased pulmonary venous pressure, increased pulmonary vascular resistance (PVR), increased pulmonary blood flow, or a combination thereof. Furthermore, an explanation for the underlying cause must also be sought. For example, a rise in pulmonary venous pressure may relate primarily to an increase in left ventricular end-diastolic pressure in a patient with a known cardiomyopathy; however, it may be complicated by severe mitral regurgitation. Similarly, an increased PVR may reflect reactive changes in the pulmonary vasculature due to long-standing pulmonary venous hypertension, concomitant hypoxemia/hypercapnia, or it may be the harbinger of chronic thromboembolic disease. It is imperative that reversible causes of PH be considered. Although most often diagnosed by Doppler echocardiography, full hemodynamic characterization of PH requires right heart catheterization to measure biventricular filling pressures and PVR. Integration of invasive pulmonary hemodynamics with an assessment of right ventricular function is essential to appreciate the clinical and prognostic significance of PH of an individual patient. Right heart catheterization is not practically feasible in all patients with HF and PH; however, at a minimum it should be performed in patients with a Doppler-estimated pulmonary artery pressure greater than 60 mm Hg, those who present clinically with predominant right HF, significant mitral valve disease, and in particular, patients with impaired right ventricular function.

The nitric oxide/cGMP signaling pathway in pulmonary hypertension

This article briefly reviews the background of endothelium-dependent vasorelaxation, describes the nitric oxide/cGMP/protein kinase pathway and its role in modulating pulmonary vascular tone and remodeling, and describes three approaches that target the nitric oxide/cGMP pathway in the treatment of patients with pulmonary arterial hypertension.

Acute Phosphodiesterase 5 Inhibition Mimics Hemodynamic Effects of B-Type Natriuretic Peptide and Potentiates B-Type Natriuretic Peptide Effects in Failing But Not Normal Canine Heart

OBJECTIVES

The aim of this work was to test whether acute phosphodiesterase 5 (PDE5) inhibition via sildenafil (SIL) mimics and/or potentiates cardiorenal effects of exogenous natriuretic peptide (NP) infusion.

BACKGROUND

Heart failure (HF) is often accompanied by elevated NP secretion yet blunted responsiveness. Such NP resistance may, in part, relate to increased cyclic guanosine monophosphate (cGMP) catabolism by PDE5.

METHODS

Dogs (n = 7) were studied before and after tachypacing-induced HF. Animals received 30-min infusion of B-type natriuretic peptide (BNP) (2 mug/kg bolus, 0.02 mug/kg/min), and on a separate day SIL (1 mg/kg, intravenous), followed by BNP (SIL + BNP). Phosphodiesterase 5 activity was measured in lung, vasculature, and kidney.

RESULTS

At baseline (non-failing), BNP lowered central venous, pulmonary capillary wedge, diastolic, mean pulmonary artery, and mean arterial pressure. Sildenafil had no effects, and SIL + BNP was similar to BNP alone. In contrast, SIL lowered these pressures similarly to BNP in dogs with HF, and SIL + BNP was additive in further reducing pulmonary pressures over BNP alone. Plasma cGMP/plasma BNP ratio was markedly reduced with HF, indicating NP resistance. Sildenafil plus BNP increased this ratio in HF, but had no effect in non-failing animals. Sildenafil had no independent diuretic/natriuretic effects nor did it enhance BNP effects under baseline or HF conditions. In HF, PDE5 activity was significantly increased in the systemic and pulmonary vasculature and in the kidney.

CONCLUSIONS

The PDE5 activity in systemic and pulmonary vasculature increases in HF rendering hemodynamic responses to PDE5 inhibition identical to those from BNP infusion. Natriuretic peptide desensitization in HF relates, in part, to increased PDE5 activity, supporting a therapeutic role for PDE5 inhibition.

Roles of accumulated endogenous nitric oxide synthase inhibitors, enhanced arginase activity, and attenuated nitric oxide synthase activity in endothelial cells for pulmonary hypertension in rats

Nitric oxide (NO) has been suggested to play a key role in the pathogenesis of pulmonary hypertension (PH). To determine which mechanism exists to affect NO production, we examined the concentration of endogenous nitric oxide synthase (NOS) inhibitors and their catabolizing enzyme dimethylarginine dimethylaminohydrolase (DDAH) activity and protein expression (DDAH1 and DDAH2) in pulmonary artery endothelial cells (PAECs) of rats given monocrotaline (MCT). We also measured NOS and arginase activities and NOS protein expression. Twenty-four days after MCT administration, PH and right ventricle (RV) hypertrophy were established. Endothelium-dependent, but not endothelium-independent, relaxation and cGMP production were significantly impaired in pulmonary artery specimens of MCT group. The constitutive NOS activity and protein expression in PAECs were significantly reduced in MCT group, whereas the arginase, which shares l-arginine as a common substrate with NOS, activity was significantly enhanced in PAECs of MCT group. The contents of monomethylarginine (MMA) and asymmetric dimethylarginine (ADMA), but not symmetric dimethylarginine (SDMA), were increased in PAECs of MCT group. The DDAH activity and DDAH1, but not DDAH2, protein expression were significantly reduced in PAECs of MCT group. These results suggest that the impairment of cGMP production as a marker of NO production is possibly due to the blunted endothelial NOS activity resulting from the downregulation of endothelial NOS protein, accumulation of endogenous NOS inhibitors, and accelerated arginase activity in PAECs of PH rats. The decreased overall DDAH activity accompanied by the downregulation of DDAH1 would bring about the accumulation of endogenous NOS inhibitors.

Transdifferentiation of pulmonary arteriolar endothelial cells into smooth muscle-like cells regulated by myocardin involved in hypoxia-induced pulmonary vascular remodelling

Myocardin gene has been identified as a master regulator of smooth muscle cell differentiation. Smooth muscle cells play a critical role in the pathogenesis of hypoxia-induced pulmonary hypertension (PH) and pulmonary vascular remodelling (PVR). The purpose of this study was to investigate the change of myocardin gene expression in the pulmonary vessels of hypoxia-induced PH affected by Sildenafil treatment and the involvement of endothelial cells transdifferentiation into smooth muscle cells in the process of hypoxia-induced PH and PVR. Myocardin and relative markers were investigated in animal models and cultured endothelial cells. Mean pulmonary artery pressure (mPAP) was measured. Immunohistochemistry and immunofluorescence were used to show the expression of smooth muscle alpha-actin (SMA), in situ hybridization (ISH) and reverse transcription polymerase chain reaction (RT-PCR) were performed respectively to detect the myocardin and SMA expression at mRNA levels. Small interfering RNA (siRNA) induced suppression of myocardin in cultured cells. We confirmed that hypoxia induced the PH and PVR in rats. Sildenafil could attenuate the hypoxia-induced PH. We found that myocardin mRNA expression is upregulated significantly in the hypoxic pulmonary vessels and cultured cells but downregulated in PH with Sildenafil treatment. The porcine pulmonary artery endothelial cells (PAECs) transdifferentiate into smooth muscle-like cells in hypoxic culture while the transdifferentiation did not occur when SiRNA of myocardin was applied. Our results suggest that myocardin gene, as a marker of smooth muscle cell differentiation, was expressed in the pulmonary vessels in hypoxia-induced PH rats, which could be downregulated by Sildenafil treatment, as well as in hypoxic cultured endothelial cells. Hypoxia induced the transdifferentiation of endothelial cells of vessels into smooth muscle-like cells which was regulated by myocardin.

Induction of TIMP-1 and HSP47 synthesis in primary keloid fibroblasts by exogenous nitric oxide

BACKGROUND

The excessive accumulation of extracellular matrix is a hallmark of many fibrotic diseases, including the hypertrophic scar and keloid. Recent reports from this research team had shown that exogenous nitric oxide (NO) participates in the keloid formation; however, its role on the synthesis of fibrotic factor (TGF-beta1, TIMP-1 and HSP47) in the keloid fibroblasts (KF) remained unclear.

OBJECTIVE

In this study, to better define the potential effect of exogenous NO on the expression of fibrotic factors in KF, the enhancing effect of exogenous NO, released from a NO donor, on the synthesis of fibrotic factors in KF was investigated.

METHODS

The seven primary KF cultures were set up to measure the effect of exogenous NO on enhancing the expression of fibrotic factor.

RESULTS

Elevation of cellular cGMP levels was observed to be induced by NO or blocked by the hydrolysis activity of phosphodiesterase (PDE) by the PDE inhibitor. The elevated levels of cellular cGMP were noted to enhance the expression of TIMP-1 and HSP47 in KF. Exogenous NO was found to significantly accelerate the production of TIMP-1 and HSP47 in the seven primary KFs with a corresponding increase in the production of TGF-beta1.

CONCLUSION

The results have led to a conclusion, that is: the excess collagen formations in the keloid lesion may be attributed to the NO/cGMP signal pathway by initiating a rapid increase in the expression of TGF-beta1, TIMP-1 and HSP47 in the KF cells.

Synthesis, radiolabeling and in vivo evaluation of [11C]RAL-01, a potential phosphodiesterase 5 radioligand

Very few tracers are available for imaging studies of second messenger systems. We developed a radiosynthesis for the phosphodiesterase (PDE) 5 inhibitor [(11)C]RAL-01. Whole body distribution studies using positron emission tomography (PET) revealed a time-dependant passage through the liver and accumulation of radioactivity in the bile of the Landrace pig. Displaceable binding was readily discerned in the myocardium, and traces of binding were seen in pulmonary tissue, consistent with the use of this class of drug in the treatment of pulmonary hypertension and heart failure. [(11)C]RAL-01 readily entered the brain and obtained an equilibrium distribution volume of 4-5 ml g(-1). Mean parametric images suggested the presence of a small displaceable binding component, but this binding was not significant in the present group of three pigs. Thus, [(11)C]RAL-01 shows considerable promise for PET studies of biliary elimination and of PDE5 binding in the cardiovascular system. However, analogues of higher affinity may be required for investigations of central nervous system binding sites.

Exogenous nitric oxide stimulated collagen type I expression and TGF-beta1 production in keloid fibroblasts by a cGMP-dependent manner

Keloids arise from the aberrant wound healing process and nitric oxide (NO) plays an important role in the inflammation stage of wound healing. In order to better define the potential effect of NO/cGMP signal pathway in the keloid pathogenesis, the enhancing effect of exogenous NO (released from NO donor) on collagen expression in the keloid fibroblast (KF) as well as on the induction of collagen type I protein and TGF-beta1 expression in the KF were studied in this investigation. The DETA NONOate, an NO donor, was added to the KF, as the exogenous NO, to release NO in the culture medium. The expression of collagens was then determined by assaying the total soluble collagens and collagen type I in the KF. The cellular concentration of cGMP was measured by EIA in the KF. Exogenous NO was found to enhance the expression of collagens and elevate the cellular levels of cGMP. Moreover, to evaluate the effect of the elevated cellular cGMP levels on the expression of collagen and TGF-beta1, both cGMP and TGF-beta1 were measured by ELISA. The inhibitors for phosphodiesterase (PDE), such as IBMX (3-isobutyl-1-methylxanthine), Vinpocetine, EHNA, Milrinone and Zapriast, which have been reported to reduce the ability of PDE and subsequently produce an increase of cellular cGMP, induce the production of autocrine TGF-beta1 as well as the synthesis of collagen in the KF. In this investigation, the inhibition of the PDE enzyme activity was observed to enhance the effect on the collagen synthesis, and was induced by exogenous NO. Taken together, these results have suggested that the NO/cGMP pathway could positively influence the progression of keloid formation, via the TGF-beta1 expression in the KF.

Sildenafil: from angina to erectile dysfunction to pulmonary hypertension and beyond

In less than 20 years, the first selective type 5 phosphodiesterase inhibitor, sildenafil, has evolved from a potential anti-angina drug to an on-demand oral treatment for erectile dysfunction (Viagra), and more recently to a new orally active treatment for pulmonary hypertension (Revatio). Here we describe the key milestones in the development of sildenafil for these diverse medical conditions, discuss the advances in science and clinical medicine that have accompanied this journey and consider possible future indications for this versatile drug.

Maximizing the renal cyclic 3'-5'-guanosine monophosphate system with type V phosphodiesterase inhibition and exogenous natriuretic peptide: a novel strategy to improve renal function in experimental overt heart failure

Type V phosphodiesterase (PDE V) metabolizes cyclic guanosine monophosphate (cGMP) and is abundant in the kidney and vasculature and was found recently in the heart. Sildenafil is a PDE V inhibitor that is used clinically for erectile dysfunction. Brain natriuretic peptide (BNP) is a cardiac peptide with vasodilating, lusitropic, and natriuretic properties that are mediated via cGMP. It was hypothesized that chronic inhibition of PDE V (PDE VI) will enhance the renal actions of exogenous BNP by potentiating the renal cGMP. The cardiorenal and humoral function was determined at baseline in two groups of dogs with pacing-induced overt chronic heart failure (CHF; 240 bpm for 10 d): Group 1 (n = 6) received Sildenafil 50 mg orally three times daily during the 10 d of pacing, and group 2 (n = 5) received no PDE V inhibitor. The response to acute subcutaneous BNP (5 microg/kg) administration also was compared in both groups on day 11. The GFR was assessed by inulin clearance (P < 0.05). There was no improvement of renal function in group 1 after 10 d of PDE VI as compared with group 2, despite having higher cardiac output (P < 0.05). Group 1 had significantly higher plasma (44 +/- 2 versus 21 +/- 3 pmol/ml; P < 0.05) and urinary cGMP (4219 +/- 900 versus 1954 +/- 300 pmol/min; P < 0.05) as compared with group 2. With acute subcutaneous BNP administration, group 1 had a natriuretic and diuretic response that was associated with an increase in GFR (30 +/- 6 to 45 +/- 6 ml/min; P < 0.05) and that was not observed in group 2 (25 +/- 6 to 29 +/- 4 ml/min). Plasma BNP increased to a similar extent in both groups with subcutaneous BNP. In contrast, group 1 had a much greater urinary cGMP excretion (4219 +/- 900 to 8600 +/- 1600 pmol/min; P < 0.05) as compared with group 2 (1954 +/- 300 to 3580 +/- 351 pmol/min; P < 0.05). In experimental overt CHF, chronic administration of PDE V inhibitor did not enhance renal function despite an improvement in cardiac output. However, chronic PDE VI significantly enhanced the renal hemodynamic and excretory responses to exogenous BNP. This study supports a role for PDE V as contributing to renal maladaptation in a model of experimental overt CHF and the strategy of maximizing the renal cGMP system by combined PDE VI and natriuretic peptides in CHF to improve renal function.

Analysis of the cGMP/cAMP interactome using a chemical proteomics approach in mammalian heart tissue validates sphingosine kinase type 1-interacting protein as a genuine and highly abundant AKAP

The cyclic nucleotide monophosphates cAMP and cGMP play an essential role in many signaling pathways. To analyze which proteins do interact with these second messenger molecules, we developed a chemical proteomics approach using cAMP and cGMP immobilized onto agarose beads, via flexible linkers in the 2- and 8-position of the nucleotide. Optimization of the affinity pull-down procedures in lysates of HEK293 cells revealed that a large variety of proteins could be pulled down specifically. Identification of these proteins by mass spectrometry showed that many of these proteins were indeed genuine cAMP or cGMP binding proteins. However, additionally many of the pulled-down proteins were more abundant AMP/ADP/ATP, GMP/GDP/GTP, or general DNA/RNA binding proteins. Therefore, a sequential elution protocol was developed, eluting proteins from the beads using solutions containing ADP, GDP, cGMP, and/or cAMP, respectively. Using this protocol, we were able to sequentially and selectively elute ADP, GDP, and DNA binding proteins. The fraction left on the beads was further enriched, for cAMP/cGMP binding proteins. Transferring this protocol to the analysis of the cGMP/cAMP "interactome" in rat heart ventricular tissue enabled the specific pull-down of known cAMP/cGMP binding proteins such as cAMP and cGMP dependent protein kinases PKA and PKG, several phosphodiesterases and 6 AKAPs, that interact with PKA. Among the latter class of proteins was the highly abundant sphingosine kinase type1-interating protein (SKIP), recently proposed to be a potential AKAP. Further bioinformatics analysis endorses that SKIP is indeed a genuine PKA interacting protein, which is highly abundant in heart ventricular tissue.

Immunohistochemical localization of phosphodiesterase 10A in multiple mammalian species

A monoclonal antibody directed against the amino terminal of rat phosphodiesterase 10A (PDE10A) was used to localize PDE10A in multiple central nervous system (CNS) and peripheral tissues from mouse, rat, dog, cynomolgus macaque, and human. PDE10A immunoreactivity is strongly expressed in the CNS of these species with limited expression in peripheral tissues. Within the brain, strong immunoreactivity is present in both neuronal cell bodies and neuropil of the striatum, in striatonigral and striatopallidal white matter tracks, and in the substantia nigra and globus pallidus. Outside the brain, PDE10A immunoreactivity is less intense, and distribution is limited to few tissues such as the testis, epididymal sperm, and enteric ganglia. These data demonstrate that PDE10A is an evolutionarily conserved phosphodiesterase highly expressed in the brain but with restricted distribution in the periphery in multiple mammalian species.