Therapeutic Applications of Sildenafil Citrate in the Management of Paediatric Pulmonary Hypertension

Perioperative sildenafil therapy in pulmonary hypertension associated with congenital cardiac disease: An updated meta-analysis

To provide an updated meta-analysis to evaluate the efficacy and safety of sildenafil on pediatric patients with pulmonary hypertension (PH) associated with congenital heart disease (CHD). To assess the efficacy and safety of sildenafil, five outcomes, time duration of post-operative need for mechanical ventilation, time duration of post-operative ICU stay, length of hospitalization (LOH), the incidence of mortalities and pulmonary arterial pressure to aortic pressure ratio (PAP/AoP) were regarded as primary efficacy outcomes. Standardized mean difference (SMD) was calculated for continuous data. In comparison to the control group (CG), there was a significant decrease in the time duration of ICU stay in the sildenafil group (SG) (SMD = -0.61 [95% CI -1.17, 0.04]; P < 0.01, I2 = 85%). Length of hospitalization was assessed in the sildenafil and control groups (SMD = -0.18 [95% CI -0.67, 0.31] P = 0.05, I2 = 62%). However, there was no significant difference seen in mortality rates between the SG and CG (SMD = 0.53 [ 95% CI 0.13, 2.17] p = 0.61, I2 = 0%), in the time duration of postoperative mechanical ventilation between the SG and CG (SMD = -0.23 [95% CI -0.49, 0.03] p = 0.29, I2 = 19%), or PAP/AoP ratio between the SG and CG (SMD = -0.42 [95% CI -1.35, 0.51] P < 0.01, I2 = 90%). Based on our analysis, sildenafil has little to no effect in reducing postoperative morbidity and mortality due to PH in infants and children with CHD.

Intra-amniotic sildenafil administration in rabbits: Safety, pharmacokinetics, organ distribution and histologic evaluation

BACKGROUND

The effectiveness of sildenafil in the management of pulmonary hypertension in congenital diaphragmatic hernia (CDH) has been reported but has not been systematically evaluated. Our studies have also demonstrated that intra-amniotic (IA) sildenafil administration improves pulmonary hypertension in CDH.

METHODS

We evaluated the pharmacokinetics of sildenafil after IA administration in pregnant rabbits. Following maternal laparotomy, fetuses received IA injection of 0.8 mg of sildenafil. Maternal blood, amniotic fluid, and fetal tissues were collected at various time points. The concentrations of sildenafil and its major metabolite in samples were analyzed by liquid chromatography-mass spectrometry. To assess organ toxicity, 7 days after IA sildenafil administration, fetal organs were examined histologically.

RESULTS

After IA dosing, sildenafil was absorbed quickly with an absorption half-life of 0.03-0.07 h into the fetal organs. All the organs showed a maximum concentration within 1 h and the disposition half-life ranged from 0.56 to 0.73 h. Most of the sildenafil was eliminated from both mothers and fetuses within 24 h after a single dose. There was no histological evidence of organ toxicity in the fetuses after a single dose of IA administration of sildenafil.

CONCLUSION

IA sildenafil is rapidly absorbed into the fetus, distributes into the mother and is eliminated by the mother without accumulation or fetal organ toxicity. This study confirms the feasibility and the safety of IA administration of sildenafil and enables future applications in the treatment of CDH fetuses.

A Canadian, retrospective, multicenter experience with selexipag for a heterogeneous group of pediatric pulmonary hypertension patients

Introduction

Selexipag, an oral nonprostanoid prostaglandin receptor agonist, has led to reduced morbidity and mortality in adults with pulmonary arterial hypertension (PAH). While the adult literature has been extrapolated to suggest selexipag as an oral treatment for severe pediatric pulmonary hypertension (PH), longitudinal, multicenter data on the benefits of selexipag in this population are lacking. The purpose of this study is to present a longitudinal, multicentre experience with selexipag in a relatively large cohort of pediatric PH patients and add to the existing selexipag literature.

Materials and methods

We performed a retrospective, multicenter review describing the clinical outcomes of pediatric PH patients receiving selexipag in addition to standard oral pulmonary vasodilator therapy across three Canadian centers between January 2005 and June 2021.

Results

Twenty-four pediatric patients (fifteen female) with a mean age of 9.7 (range 2.0-15.5) years were included. Of this cohort, eighteen (75.0%) were in group 1, one (4.2%) was in group 2, four (16.7%) were in group 3, and one (4.2%) was in group 4. Twenty-two (91.7%) patients were on dual PH therapy after six months. Dosing was targeted to achieve 20-30 mcg/kg/dose orally every twelve hours. Median dose after twelve months was 30 mcg/kg/dose. Twelve months following selexipag initiation, median decreases of 0.2 cm in tricuspid annular plane systolic excursion, 3.5 mmHg in right-ventricular systolic pressure, and 6.1 mmHg in mean pulmonary arterial pressure were observed; none of these changes were statistically significant. Three patients died, one clinically deteriorated and required admission to a pediatric intensive care unit, ten had gastrointestinal symptoms, and three had flushing.

Conclusion

Selexipag appears to be a safe and effective adjunctive therapy for pediatric PH patients and has a tolerable adverse effect profile aside from gastrointestinal disturbances. Additional prospective studies of changes in hemodynamics and functional classification over a longer period and with a larger sample are needed. Future research should aim to identify subgroups that stand to benefit from the addition of selexipag as well as optimal timing and dosing for the pediatric population.

Sildenafil Exposure in the Neonatal Intensive Care Unit

OBJECTIVE

Pulmonary hypertension causes substantial morbidity and mortality in infants. Although Food and Drug Administration approved to treat pulmonary arterial hypertension in adults, sildenafil is not approved for infants. We sought to describe sildenafil exposure and associated diagnoses and outcomes in infants.

STUDY DESIGN

Retrospective cohort of neonates discharged from more than 300 neonatal intensive care units from 2001 to 2016.

RESULTS

Sildenafil was administered to 1,336/1,161,808 infants (0.11%; 1.1 per 1,000 infants); 0/35,977 received sildenafil in 2001 versus 151/90,544 (0.17%; 1.7 per 1,000 infants) in 2016. Among infants <32 weeks' gestational age (GA) with enough data to determine respiratory outcome, 666/704 (95%) had bronchopulmonary dysplasia (BPD). Among infants ≥32 weeks GA, 248/455 (55%) had BPD and 76/552 (14%) were diagnosed with meconium aspiration. Overall, 209/921 (23%) died prior to discharge.

CONCLUSION

The use of sildenafil has increased since 2001. Exposed infants were commonly diagnosed with BPD. Further studies evaluating dosing, safety, and efficacy of sildenafil are needed.

Synthesis, Antimicrobial and Antioxidant Activity of Pyrazole Based Sulfonamide Derivatives

A series of new sulfonamides have been synthesized from Ampyrone with different benzene sulfonyl chlorides to yield the N-1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl) benzenesulfonamides (4a-e). All synthesized compounds were characterized on the basis of FTIR, 1H NMR, and 13C NMR, and also by the aid of mass spectral data. Further, all synthesized compounds have studied for their in vitro antimicrobial activities against selected bacterial as well as fungal strains by the agar well diffusion method. Free radical scavenging activity has been investigated by using DPPH method. Among all the synthesized compounds, 4b, 4d, and 4e exhibited significant antimicrobial and antioxidant activities.

The role of preoperative sildenafil therapy in controlling of postoperative pulmonary hypertension in children with ventricular septal defects

Introduction: Most of the ventricular septal defects (VSD) are complicated with pulmonary arterial hypertension (PAH) which is the major cause of pulmonary hypertensive crisis and right ventricular failure.

Methods: We reviewed clinical outcomes of 63 infants who underwent cardiac surgery and were divided into three groups. Control group (n=20) did not received sildenafil while group A (n = 22) received drug (0.3 mg/kg) before and after surgery. Group B (n=21) received drug at the initiation of surgery. Demographic data, preoperative and postoperative variables were compared among the patients.

Results: Patients in the group A had lower preoperative pulmonary arterial pressure (PAP) compared to other groups (P < 0.001). Also, patients in control group had longer cardiopulmonary bypass time (P < 0.05). Postoperative PAP in patients of group A and B decreased significantly compared to control group (P < 0.001). Also, pre- and postoperative PVR (pulmonary vascular resistance) showed a significant decrease in group A compared with control and group B (P < 0.001). The intubation time in patients of the control group was significantly more prolonged compared with patients of group A and B (P < 0.001). Moreover, the length of ICU stay was significantly longer in patients of control group compared with group A and B (P < 0.001).

Conclusion: Preoperative sildenafil therapy seems to be effective and safe to prevent postoperative PAH and pulmonary hypertensive crisis in children with ventricular septal defects and has a positive impact on postoperative care.

Aberrant cGMP signaling persists during recovery in mice with oxygen-induced pulmonary hypertension

Bronchopulmonary dysplasia (BPD), a common complication of preterm birth, is associated with pulmonary hypertension (PH) in 25% of infants with moderate to severe BPD. Neonatal mice exposed to hyperoxia for 14d develop lung disease similar to BPD, with evidence of associated PH. The cyclic guanosine monophosphate (cGMP) signaling pathway has not been well studied in BPD-associated PH. In addition, there is little data about the natural history of hyperoxia-induced PH in mice or the utility of phosphodiesterase-5 (PDE5) inhibition in established disease. C57BL/6 mice were placed in room air or 75% O₂ within 24h of birth for 14d, followed by recovery in room air for an additional 7 days (21d). Additional pups were treated with either vehicle or sildenafil for 7d during room air recovery. Mean alveolar area, pulmonary artery (PA) medial wall thickness (MWT), RVH, and vessel density were evaluated at 21d. PA protein from 21d animals was analyzed for soluble guanylate cyclase (sGC) activity, PDE5 activity, and cGMP levels. Neonatal hyperoxia exposure results in persistent alveolar simplification, RVH, decreased vessel density, increased MWT, and disrupted cGMP signaling despite a period of room air recovery. Delayed treatment with sildenafil during room air recovery is associated with improved RVH and decreased PA PDE5 activity, but does not have significant effects on alveolar simplification, PA remodeling, or vessel density. These data are consistent with clinical studies suggesting inconsistent effects of sildenafil treatment in infants with BPD-associated PH.

Evaluating the safety of intermittent intravenous sildenafil in infants with pulmonary hypertension

OBJECTIVE

To compare the occurrence of hypotension following administration of intermittent intravenous (IV) and enteral sildenafil for treatment of pulmonary hypertension (PH) in infants. We hypothesized there may be more adverse effects associated with intermittent IV sildenafil compared with enteral sildenafil.

METHODS

This was a retrospective matched-cohort study conducted in a tertiary care children's hospital. Patients were included if they were less than 1 year of age and received intermittent sildenafil for PH. Exclusion criteria consisted of concurrent extracorporeal membrane oxygenation during the initiation of sildenafil, the utilization of sildenafil as a one-time dose, continuation of home-dosing regimen, or inclusion in the other cohort. A total of 40 patients were matched 1:1 based on postmenstrual age and primary diagnosis.

RESULTS

There was no statistically significant difference in the primary outcome, as 30% (6/20) of patients receiving IV sildenafil required a hypotension intervention compared with 10% (2/20) in the enteral cohort (P = 0.24). The majority of interventions occurred within 24 hr of the initiation of sildenafil with 4/6 patients (67%) in the IV group and 2/2 patients (100%) in the enteral group, respectively. Baseline mean arterial pressure was significantly lower in the IV patients that required an intervention compared with those that did not (44 ± 6.3 vs. 65 ± 13.4 mmHg, P = 0.0024).

CONCLUSIONS

There were no statistically significant differences in safety outcomes between intermittent IV and enteral sildenafil in infants with PH. Hemodynamic parameters should be monitored closely upon sildenafil initiation. Limitations include the retrospective nature and small sample size. Pediatr Pulmonol. 2017;52:232-237. © 2016 Wiley Periodicals, Inc.

Patterns of Off-Label Prescribing in the Pediatric Intensive Care Unit and Prioritizing Future Research

OBJECTIVES

To characterize off-label prescribing among US pediatric intensive care units (PICUs), determine characteristics associated with off-label use, and identify medications in highest need for additional study.

METHODS

Medications prescribed for ≥1% PICU patients (age < 18 years) in 2010 were identified from 39 children's hospitals. Use in a patient younger than the Food and Drug Administration (FDA)-approved age for any indication was considered off-label. Hierarchical multivariable modeling was used to identify characteristics associated with off-label use, accounting for center effects. Highest-impact drugs were defined by: 1) high off-label use (off-label use in at least 5% of the PICU cohort), 2) high risk medication, and 3) high priority status by the FDA or Best Pharmaceuticals for Children Act (BPCA).

RESULTS

A total of 66,896 patients received ≥1 medication of interest (n = 162) during their PICU stay. A median of 3 (interquartile range, 2-6) unique drugs per patient were used off-label. Those who received ≥1 drug off-label (85% of the cohort) had longer median PICU (2 days vs 1 day) and hospital (6 days vs 3 days) lengths of stay and higher mortality (3.6% vs 0.7%), p < 0.001. Factors independently associated with off-label drug use included: age 1 to 5 years, chronic conditions, acute organ failures, mechanical ventilation, arterial or venous catheters, dialysis, and blood products. Half of prescribed medications (n = 84) had been used off-label: 26 with significant off-label use, 30 high-risk medications, and 47 with high FDA/BPCA priority. The highest impact medications identified were: dexmedetomidine, dopamine, hydromorphone, ketamine, lorazepam, methadone, milrinone, and oxycodone.

CONCLUSIONS

Most PICU patients are exposed to off-label medication use, with uncertain evidence. Future medication research in this population should focus on medications with high impact potential.

Development and stability assessment of liquid paediatric formulations containing sildenafil citrate

The purpose of this study is to develop and improve oral liquids formulations of sildenafil citrate for paediatric use. Four different formulations were developed, which are as follows: two aqueous solutions of sildenafil citrate (2.5 mg/mL), with or without preservatives, and two other solutions of sildenafil in simple syrup (1.25 mg/mL), with or without preservatives. All of the formulations were physically, chemically and microbiologically stable for three months. The results of the stability studies allowed for the optimisation of formulations without preservatives due to their simplicity and their similar stable conditions when compared to the formulations containing antimicrobials. The shelf life of both formulations was three months; however, upon opening, aqueous solutions should be used within 10 days and kept refrigerated, and syrup solutions should be used within 14 days in a hospital setting.

Persistent pulmonary hypertension of the newborn: recent advances in pathophysiology and treatment

OBJECTIVES

Although recognized for decades, little is known about the etiology, physiopathology, and prevention of persistent pulmonary hypertension of the newborn (PPHN), and its treatment remains a major challenge for neonatologists. In this review, the clinical features and physiopathology of the syndrome will be addressed, as well as its general and specific treatments.

DATA SOURCE

A review was carried out in PubMed, Cochrane Library, and MRei consult databases, searching for articles related to the syndrome and published between 1995 and 2011.

DATA SYNTHESIS

Risk factors and the physiopathological mechanisms of the syndrome are discussed. The clinical presentation depends on the different factors involved. These are related to the etiology and physiopathology of the different forms of the disease. In addition to the measures used to allow for the decrease in pulmonary vascular resistance after birth, in some instances pulmonary vasodilators will be required. Although inhaled nitric oxide has proved effective, other vasodilators have been recently used, but clinical evidence is still lacking to demonstrate their benefits in the treatment of PPHN.

CONCLUSIONS

Despite recent technological advances and new physiopathological knowledge of this disease, mortality associated with PPHN remains at 10%. More clinical research and evidence-based experimental results are needed to prevent, treat, and reduce the morbidity/mortality associated with this neonatal syndrome.

Reversal of pulmonary vascular remodeling in pulmonary hypertensive rats

Pulmonary hypertension is responsible for significant mortality and morbidity among newborns and infants. The pathology is characterized by pulmonary vascular remodeling with medial hypertrophy and adventitial thickening, leading to decreased gas exchange. Since it is unknown if these abnormalities are reversible, we analyzed these vascular changes in pulmonary hypertensive rats. Exposure of rats to hypobaric hypoxia for 4 weeks induced clinical signs of pulmonary hypertension, such as increased right ventricular systolic pressure, increased right ventricular weight and considerable pulmonary vascular remodeling. The vascular changes were associated with the expression of Non -Muscle Myosin Heavy Chain B in the pre-acinar vessels and an increased expression of alpha Smooth Muscle Actin, Smooth Muscle Myosin Heavy Chain 2 and Calponin in the intra-acinar vessels. The right ventricular systolic pressure and right ventricular weight gradually decreased after specific periods of recovery in normoxia, although this reversal did not reach baseline levels after six weeks at normoxia. However, the cellular changes in the pulmonary vasculature were completely reversed. Development of pulmonary hypertension is associated with an increase of synthetic perivascular cells in the pre-acinar arteries and an aberrant differentiation of perivascular cells in the smallest intra-acinar arteries. These cellular and structural changes in the pulmonary vasculature are completely reversible after recovery in normoxia.

Vascular abnormalities in human newborns with pulmonary hypertension

Pulmonary vascular disease embodies all congenital or acquired pathologies that affect the pulmonary vasculature. One of them is pulmonary hypertension of the newborn (PHN), which is clinically characterized by a persistent high pulmonary vascular resistance postnatally and an abnormal vascular response. Morphologically, the vascular walls of the small pulmonary arteries become thickened, leading to increased resistance of these vessels and thus a worsening of gas exchange. PHN occurs as a primary disease or in association with abnormal lung development, for example as in congenital diaphragmatic hernia, and is a critical determinant of morbidity and mortality. Here we review the current knowledge about vascular abnormalities in PHN and discuss the vascular abnormalities in different conditions associated with pulmonary hypertension in human newborns in relation to recent findings from molecular biology.

Management of pulmonary arterial hypertension in children

In this review we discuss the new anti- Pulmonary Arterial Hypertension [PAH] drugs and the available data on their use in paediatric PAH. Treatment of patients with PAH, children and adults, is aimed at a reduction of symptoms, survival and improvement of haemodynamics as well as exercise capacity. PAH may reflect significant different disease conditions in infants and children when compared to PAH in adults. In contrast to adult PAH, characterized mainly by idiopathic PAH and PAH associated with connective tissue disease, more than half of the cases of PAH in children are associated with congenital heart disease. Therefore, efficacy of PAH drugs in these diseases can not be extrapolated from that in adults with PAH.

PPHN: is sildenafil the new nitric? A review of the literature

Persistent pulmonary hypertension of the newborn (PPHN) is a life-threatening neonatal pathology resulting from poor hemodynamic and respiratory transition to extrauterine life. Inhaled nitric oxide (iNO) is a current, commonly used treatment of PPHN. However, some infants with PPHN do not respond to iNO therapy. Because of the significant morbidity and mortality associated with PPHN, it is useful to look at other possible therapies. This article explores the use of sildenafil in the treatment of PPHN, either as a supplement to iNO or as an alternative to iNO. Current research, including a case study, is reviewed. An emphasis is placed on the administration and efficacy of sildenafil in synergy with, and in lieu of, current iNO therapy in the treatment of PPHN.

Sildenafil for the treatment of pulmonary hypertension in pediatric patients

Sildenafil is a phosphodiesterase 5 inhibitor widely used for the treatment of pulmonary hypertension in children. Despite limited available safety and efficacy evidence, use of sildenafil continues to increase. To date, sildenafil use for pediatric pulmonary hypertension has been characterized for 193 children through 16 studies and 28 case series and reports. The primary efficacy data suggest that sildenafil is beneficial for facilitating the weaning of inhaled nitric oxide in children after cardiac surgery. Compiled safety data suggest that sildenafil is well tolerated among children with idiopathic pulmonary arterial hypertension and pulmonary arterial hypertension associated with congenital heart disease. This review summarizes the available data describing the use, safety, and efficacy of sildenafil for children with pulmonary hypertension.

Treatment of pediatric pulmonary hypertension

Pulmonary hypertension was once thought to be a rare condition and only managed in specialized centers. Now however, with the advent of echocardiography, it is found in many clinical scenarios, in the neonate with chronic lung disease, in the acute setting in the intensive care unit, in connective tissue disease and in cardiology pre- and postoperatively. We have a better understanding of the pathological process and have a range of medication which is starting to be able to palliate this previously fatal condition. This review describes the areas that are known in this condition and those that are less familiar. The basic physiology behind pulmonary hypertension and pulmonary vascular disease is explained. The histopathologic process and the various diagnostic tools are described and are followed by the current and future therapy at our disposal.

Sildenafil attenuates pulmonary inflammation and fibrin deposition, mortality and right ventricular hypertrophy in neonatal hyperoxic lung injury

Background

Phosphodiesterase-5 inhibition with sildenafil has been used to treat severe pulmonary hypertension and bronchopulmonary dysplasia (BPD), a chronic lung disease in very preterm infants who were mechanically ventilated for respiratory distress syndrome.

Methods

Sildenafil treatment was investigated in 2 models of experimental BPD: a lethal neonatal model, in which rat pups were continuously exposed to hyperoxia and treated daily with sildenafil (50–150 mg/kg body weight/day; injected subcutaneously) and a neonatal lung injury-recovery model in which rat pups were exposed to hyperoxia for 9 days, followed by 9 days of recovery in room air and started sildenafil treatment on day 6 of hyperoxia exposure. Parameters investigated include survival, histopathology, fibrin deposition, alveolar vascular leakage, right ventricular hypertrophy, and differential mRNA expression in lung and heart tissue.

Results

Prophylactic treatment with an optimal dose of sildenafil (2 × 50 mg/kg/day) significantly increased lung cGMP levels, prolonged median survival, reduced fibrin deposition, total protein content in bronchoalveolar lavage fluid, inflammation and septum thickness. Treatment with sildenafil partially corrected the differential mRNA expression of amphiregulin, plasminogen activator inhibitor-1, fibroblast growth factor receptor-4 and vascular endothelial growth factor receptor-2 in the lung and of brain and c-type natriuretic peptides and the natriuretic peptide receptors NPR-A, -B, and -C in the right ventricle. In the lethal and injury-recovery model we demonstrated improved alveolarization and angiogenesis by attenuating mean linear intercept and arteriolar wall thickness and increasing pulmonary blood vessel density, and right ventricular hypertrophy (RVH).

Conclusion

Sildenafil treatment, started simultaneously with exposure to hyperoxia after birth, prolongs survival, increases pulmonary cGMP levels, reduces the pulmonary inflammatory response, fibrin deposition and RVH, and stimulates alveolarization. Initiation of sildenafil treatment after hyperoxic lung injury and continued during room air recovery improves alveolarization and restores pulmonary angiogenesis and RVH in experimental BPD.

Familial interstitial disease with I73T mutation: A mid- and long-term study

OBJECTIVES

To describe the long-term course and the management in children of chronic interstitial lung disease associated with I73T mutation.

MATERIALS AND METHODS

Clinical, radiological, and histological data from one family including five children and two adults were analyzed retrospectively for three patients and prospectively for the others.

RESULTS

Mean age of onset of respiratory symptoms for children was 6 months (2-15 months). The follow up was 14 months to 15 years (mean 55 months). The children were treated by intravenous high dose methylprednisolone pulses (6-15, mean 12). Four received oral prednisolone (mean 16 months) and hydroxychloroquine, one of these had additional mycophenolate mofetil. One adult with mild respiratory symptoms in infancy and another who was symptom free were also diagnosed. Both of them received no treatment. BAL fluids were obtained in all children: pro-SPC and SPB were positive in all. Lung biopsies were performed in two children respectively at 7 months, showing interstitial pneumonia features with endoluminal macrophage and type II alveolar cells hyperplasia, and at 33 months, showing subpleural microbullae, areas of interstitial pneumonia and type II alveolar cells hyperplasia. Immunohistochemistry showed for both an increased SPB and TTF1 staining in type II cells nuclei and a faint staining for pro-SPC and for ABCA3. Genetic diagnosis obviated the need for biopsy in other cases. The clinical status progressively improved and oxygen supplementation could be stopped after 3-14 months (mean 9 months). The CT scans initially showed ground glass opacities, then reduction in the ground glass pattern associated with clinical improvement and development of cysts.

CONCLUSION

This kindred illustrates the variability of respiratory involvement and prognosis. It confirms the value of genetic screening for surfactant protein genes mutations.

The effects of sildenafil citrate (Viagra) on uterine blood flow and well being in the intrauterine growth-restricted fetus

OBJECTIVE

This study examined whether the type-5 phosphodiesterase inhibitor sildenafil citrate (Viagra; Pfizer, New York, NY) could increase uterine blood flow in intrauterine growth restriction (IUGR), thereby improving fetal oxygenation and well being.

STUDY DESIGN

In fetal sheep, we induced IUGR at 105-110 days (0.7 gestation) using single umbilical artery ligation (SUAL). In SUAL and control animals, we measured uterine blood flow (UBF) and blood gases before and after sildenafil administration.

RESULTS

SUAL fetuses were hypoxemic compared with controls. Following sildenafil, UBF was significantly decreased in both SUAL and control ewes for approximately 40 minutes. In response to sildenafil, pO(2) was decreased in SUAL and control fetuses and both groups displayed significant hypotension and tachycardia. At postmortem SUAL fetal body weight was significantly reduced by 23% compared with controls.

CONCLUSION

Sildenafil does not improve UBF or fetal well being in SUAL-induced IUGR pregnancies and should be used with caution in IUGR and healthy pregnancies because of its detrimental effects on uteroplacental perfusion and on the fetus.

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.