Effects of sildenafil on the relaxation of human corpus cavernosum tissue in vitro and on the activities of cyclic nucleotide phosphodiesterase isozymes

Sildenafil: efficacy, safety, tolerability and mechanism of action in treating erectile dysfunction

Sildenafil citrate is marketed under the trademark name Viagra and is widely used to treat male erectile dysfunction; therapeutic uses of this medication for other diseases related to vascular dysfunction are emerging. When used as recommended, the drug has a strong clinical efficacy and safety profile in a broad spectrum of the male population. Its widespread use and effects of long-term exposure to the drug due to particular treatment regimens or inappropriate use mandate an ongoing update of its molecular mechanism, pharmacological profile and associated safety issues. This review focuses on biochemical and pharmacological features of sildenafil, the active component in Viagra, interaction of sildenafil with phosphodiesterase 5, pharmacokinetic parameters, action in smooth muscle, side effects, safety profile and prospects for other uses.

Effects of sildenafil on blue-on-yellow and white-on-white Humphrey perimetry in 3 months regular use

PURPOSE

To assess the effects of sildenafil on blue-on-yellow (B/Y) and white-on-white (W/W) Humphrey perimetry in a group of men with erectile dysfunction in 3 months regular use.

METHODS

In this prospective study, 14 patients with erectile dysfunction received 50 mg doses of sildenafil (Viagra, Pfizer) two times per week regularly for 3 months. Patients underwent best-corrected visual acuity (BCVA), colour vision, anterior segment and fundus examination, and B/Y and W/W Humphrey perimetry in each eye before and after sildenafil treatment. Changes in mean deviations (MD) were compared separately for both eyes.

RESULTS

No significant changes from baseline were observed in BCVA and colour vision after sildenafil treatment. The anterior segment and fundus examinations revealed no abnormalities. There was no change on B/Y and W/W Humphrey visual field. MD values of B/Y and W/W Humphrey perimetry remained nonsignificant for both eyes after 3 months regular use of sildenafil (P=0.133, and 0.092, respectively, for right, and P=0.221 and 0.925, respectively, for left eyes).

CONCLUSION

Oral sildenafil used regularly for 3 months produced no effect on B/Y and W/W Humphrey perimetry.

The effects of sildenafil on ocular blood flow

PURPOSE

To investigate the effects of sildenafil, a popular new drug in the treatment of erectile dysfunction, on ocular blood flow.

METHODS

This study was designed as a prospective, double-blind, placebo-controlled study. Twenty participants with erectile dysfunction were given a single oral dose of 100 mg sildenafil, while 10 participants with erectile dysfunction were given placebo. All the participants underwent routine systemic and ophthalmological examinations. Intraocular pressure, systolic and diastolic blood pressure and ocular blood flow (ophthalmic, central retinal, short posterior ciliary arteries) were measured in both eyes before and 1 hour after the dose of sildenafil or placebo. Ocular blood flow measurements were performed using colour Doppler ultrasonography.

RESULTS

None of the parameters were significantly different between the groups before study drug intake. Although central retinal artery velocities were not changed, ophthalmic artery and short posterior ciliary artery peak systolic velocity, end-diastolic velocity, and mean velocity values were significantly increased 1 hour after drug intake in the sildenafil group compared to the placebo group (p < 0.05).

CONCLUSION

Sildenafil causes a significant increase in blood flow in these arteries. A possible role of inhibition of phosphodiesterase-5 in vascular smooth muscles by sildenafil is implicated. Further studies are needed to investigate the effects of sildenafil on ocular blood flow in patients with senile macular degeneration, diabetic retinopathy and glaucoma.

Acute Myocardial Infarction After Sildenafil Citrate Ingestion

OBJECTIVE

To report a case of acute myocardial infarction (MI) associated with the use of oral sildenafil in a nitrate-free patient.

CASE SUMMARY

A 45-year-old man was admitted to the hospital with acute left-sided chest pain, nausea, and vomiting that started approximately 30 minutes after taking sildenafil 100 mg before a sexual contact. The patient was diagnosed with an acute anterior MI, and therapy with aspirin, metoprolol, and unfractionated heparin was initiated. Early coronary reperfusion treatment with primary percutaneous transluminal coronary angioplasty was performed after initial evaluation. Balloon angioplasty followed by coronary stenting was performed successfully in the 80%-occluded left anterior descending artery. The patient was discharged one week after the coronary intervention without complication.

DISCUSSION

Sildenafil-associated MI is rarely seen in patients without documented coronary artery disease. By inhibiting phosphodiesterase type 5, sildenafil can cause an increase in cyclic guanosine monophosphate levels, which mediates the relaxation of vascular smooth muscle in the corpus cavernosum. Although sildenafil can cause a major decline in systemic arterial pressure in the existence of organic nitrates, physicians should be aware of its adverse cardiovascular effects even in nitrate-free patients. The Naranjo probability scale indicates that sildenafil was the possible cause of the MI.

CONCLUSIONS

Sildenafil may rarely be associated with MI in patients with no known cardiac history. Physicians should be aware of this rare and serious adverse reaction to sildenafil and counsel patients not to take sildenafil before undergoing a complete physical evaluation and further testing if warranted.

Erection capability is potentiated by long-term sildenafil treatment: role of blood flow-induced endothelial nitric-oxide synthase phosphorylation

Despite demonstrated clinical efficacy of sildenafil for the temporary treatment of erectile dysfunction, the possibility that sildenafil used long-term durably augments erectile ability remains unclear. We investigated whether continuous long-term administration of sildenafil at clinically relevant levels to aged rats "primes" the penis for improved erectile ability and involves nitric oxide (NO) or RhoA/Rho-kinase signaling pathways. In aged, but not young rats, sildenafil prolonged erection and increased the protein expressions of phosphorylated endothelial NO synthase (eNOS) at serine-1177 and phosphorylated Akt at serine-473 in penes. Only in the young rat penis, protein expressions of phosphodiesterase-5 and phosphomyosin phosphatase target subunit 1, a marker of Rho-kinase activity, were increased by sildenafil. Sildenafil inhibited phosphodiesterase-5 activity in penes of young and aged rats coincident with assayed free plasma levels of the drug equivalent to clinically therapeutic measurements. We conclude that erectile ability can be enhanced under preconditions of erectile impairment by long-term inhibition of phosphodiesterase-5 and that the effect is mediated by Akt-dependent eNOS phosphorylation. The lack of erectile ability enhancement in young rats by long-term phosphodiesterase-5 inhibition may relate to restrained NO signaling by phosphodiesterase-5 up-regulation, lack of incremental Akt and eNOS phosphorylation, and heightened Rho-kinase signaling in the penis.

High biochemical selectivity of tadalafil, sildenafil and vardenafil for human phosphodiesterase 5A1 (PDE5) over PDE11A4 suggests the absence of PDE11A4 cross-reaction in patients

The physiological role of phosphodiesterase (PDE)11 is unknown and its biochemical characteristics are poorly understood. We have expressed human His-tagged PDE11A4 and purified the enzyme to apparent homogeneity. PDE11A4 displays K(m) values of 0.97 microM for cGMP and 2.4 microM for cAMP, and maximal velocities were 4- to 10-fold higher for cAMP than for cGMP. Given the homology between PDE11 and PDE5, we have compared the biochemical potencies of tadalafil (Cialis, Lilly-ICOS), vardenafil (Levitra, Bayer-GSK), and sildenafil (Viagra, Pfizer Inc.) for PDE11A4 and PDE5A1. PDE5A1/PDE11A4 selectivities are 40-, 9300-, and 1000-fold for tadalafil, vardenafil, and sildenafil, respectively. This suggests that none of these three compounds is likely to crossreact with PDE11A4 in patients.

Synthesis and phosphodiesterase 5 inhibitory activity of novel pyrido[1,2-e]purin-4(3H)-one derivatives

Synthesis and primary SAR of a novel series of 2-phenylpyrido[1,2-e]purin-4(3H)-one derivatives with piperazinyl sulfonamide substituents were described herein. As potential PDE5 inhibitors for erectile dysfunction (ED) treatment, representative compounds exhibit improved selectivity versus PDE1 and PDE6. Meanwhile, compound 3e demonstrated functional efficacy on rabbit corpus cavernosum strip in vitro.

Inhibition of angiotensin-converting enzyme and phosphodiesterase type 5 improves endothelial function in heart failure

ACE (angiotensin-converting enzyme) inhibitors and PDE5 (phosphodiesterase type 5) inhibitors have each been reported to improve endothelial function in cardiovascular disease patients, but the comparative and combined effects of these two classes have not been studied previously. We sought to characterize the acute effects of ramipril alone, sildenafil alone, or their combination on endothelial function in patients with CHF (chronic heart failure). CHF subjects (n=64) were randomized to receive placebo, 10 mg of ramipril alone, 50 mg of sildenafil alone or a combination of ramipril and sildenafil in a double-blind manner. FMD (flow-mediated dilation) of the brachial artery was determined by high-resolution ultrasound imaging before and at 1, 2 and 4 h after administration of the study drug. Ramipril alone increased FMD at 4 h compared with placebo (+2.3+/-1.3%, P=0.02). Sildenafil alone increased FMD at 1, 2 and 4 h compared with placebo (+3.9+/-1.4, +4.6+/-1.8 and +3.7+/-1.3% respectively, all P<0.02). Sildenafil in combination with ramipril increased FMD at 1, 2 and 4 h when compared with placebo (+3.5+/-1.5, +4.5+/-1.8 and +4.8+/-1.3% respectively, all P<0.03). Ramipril and sildenafil both acutely improved FMD in patients with CHF, with additive effects evident at 4 h during combination therapy. Therefore further work to characterize chronic effects of combined ACE and PDE5 inhibition on endothelial function are warranted.

Autonomic dysreflexia during sperm retrieval in spinal cord injury: influence of lesion level and sildenafil citrate

Autonomic dysreflexia (AD) can occur during penile vibratory stimulation in men with spinal cord injury, but this is variable, and the association with lesion level is unclear. The purpose of this study was to characterize the cardiovascular responses to penile vibratory stimulation in men with spinal cord injury. We hypothesized that those with cervical injuries would demonstrate a greater degree of AD compared with men with thoracic injuries. We also questioned whether the rise in blood pressure could be attenuated by sildenafil citrate. Participants were classified as having cervical (n = 8) or thoracic (n = 5) injuries. While in a supine position, subjects were instrumented with an ECG, and arterial blood pressure was determined beat by beat. Subjects reported to the laboratory twice and received an oral dose of sildenafil citrate (25-100 mg) or no medication. Penile vibratory stimulation was performed using a handheld vibrator to the point of ejaculation. At ejaculation during the nonmedicated trials, the cervical group had a significant decrease in heart rate (-5-10 beats/min) and increase in mean arterial blood pressure (+70-90 mmHg) relative to resting conditions, whereas the thoracic group had significant increases in both heart rate (+8-15 beats/min) and mean arterial pressure (+25-30 mmHg). Sildenafil citrate had no effect on the change in heart rate or mean arterial pressure in either group. In summary, men with cervical injuries had more pronounced AD during penile vibratory stimulation than men with thoracic injuries. Administration of sildenafil citrate had no effect on heart rate or blood pressure during penile vibratory stimulation in men with spinal cord injury.

Structural and functional features in human PDE5A1 regulatory domain that provide for allosteric cGMP binding, dimerization, and regulation

The cGMP-binding cGMP-specific phosphodiesterase (PDE5) contains a catalytic domain that hydrolyzes cGMP and a regulatory (R) domain that contains two GAFs (a and b; GAF is derived from the proteins mammalian cGMP-binding PDEs, Anabaena adenylyl cyclases, and Escherichia coli (FhlA)). The R domain binds cGMP allosterically, provides for dimerization, and is phosphorylated at a site regulated by allosteric cGMP binding. Quaternary structures and cGMP-binding properties of 10 human PDE5A1 constructs containing one or both GAFs were characterized. Results reveal that: 1) high affinity homo-dimerization occurs between GAF a modules (K(D) < 30 nM) and between GAF b modules (K(D) = 1-20 pM), and the sequence between the GAFs (Thr322-Asp403) contributes to dimer stability; 2) 176 amino acids (Val156-Gln331) in GAF a are adequate for cGMP binding; 3) GAF a has higher affinity for cGMP (K(D) < 40 nM) than does the isolated R domain (K(D) = 110 nM) or holoenzyme (K(D) = 200 nM), suggesting that the sequence containing GAF b and its flanking amino acids autoinhibits GAF a cGMP-binding affinity in intact R domain; 4) a mutant (Met1-Glu321) containing only GAF a has high affinity, biphasic cGMP-binding kinetics consistent with structural heterogeneity of GAF a, suggesting that the presence of GAF b is not required for biphasic cGMP-dissociation kinetics observed in holoenzyme or isolated R domain; 5) significant cGMP binding by GAF b was not detected; and 6) the sequence containing GAF b and its flanking amino acids is critical for cGMP stimulation of Ser102 phosphorylation by cyclic nucleotide-dependent protein kinases. Results yield new insights into PDE5 functions, further define boundaries that provide for allosteric cGMP binding, and identify regions that contribute to dimerization.

Stimulation of AQP2 membrane insertion in renal epithelial cells in vitro and in vivo by the cGMP phosphodiesterase inhibitor sildenafil citrate (Viagra)

Vasopressin-stimulated insertion of the aquaporin 2 (AQP2) water channel into the plasma membrane of kidney collecting duct principal cells is a key event in the urinary concentrating mechanism. The paradigm for vasopressin-receptor signaling involves cAMP-mediated protein kinase A activation, which results in the functionally critical phosphorylation of AQP2 on amino acid serine 256. We previously showed that a parallel cGMP-mediated signaling pathway also leads to AQP2 membrane insertion in AQP2-transfected LLC-PK1 (LLC-AQP2) cells and in outer medullary collecting duct principal cells in situ (Bouley R, Breton S, Sun T, McLaughlin M, Nsumu NN, Lin HY, Ausiello DA, and Brown D. J Clin Invest 106: 1115-1126, 2000). In the present report, we show by immunofluorescence microscopy, and Western blotting of plasma membrane fractions, that 45-min exposure of LLC-AQP2 cells to the cGMP phosphodiesterase type 5 (PDE5) inhibitors sildenafil citrate (Viagra) or 4-{[3',4'-methylene-dioxybenzyl]amino}-6-methoxyquinazoline elevates intracellular cGMP levels and results in the plasma membrane accumulation of AQP2; i.e., they mimic the vasopressin effect. Importantly, our data also show that acute exposure to PDE5 inhibitors for 60 min induces apical accumulation of AQP2 in kidney medullary collecting duct principal cells both in tissue slices incubated in vitro as well as in vivo after intravenous injection of Viagra into rats. These data suggest that AQP2 membrane insertion can be induced independently of vasopressin-receptor activation by activating a parallel cGMP-mediated signal transduction pathway with cGMP PDE inhibitors. These results provide proof-of-principle that pharmacological activation of vasopressin-independent, cGMP signaling pathways could aid in the treatment of those forms of nephrogenic diabetes insipidus that are due to vasopressin-2 receptor dysfunction.

The pharmacokinetics and hemodynamics of sildenafil citrate in male hemodialysis patients

BACKGROUND

Erectile dysfunction (ED) is highly prevalent in men with renal disease. The clearance of sildenafil citrate, a highly effective oral treatment for ED, is decreased in men with severe renal insufficiency, but the pharmacokinetic and hemodynamic profiles during maintenance hemodialysis in men with end-stage renal disease have not been studied.

METHODS

Fifteen men undergoing chronic outpatient maintenance hemodialysis received a single 50-mg oral dose of sildenafil on 2 occasions, once 2 hours before, and once 2 hours after hemodialysis, with randomized assignment to sequence. Blood and dialysate samples were collected, and hemodynamic measurements were made.

RESULTS

Hemodialysis did not significantly clear either sildenafil or its primary metabolite, UK-103,320. Administration after hemodialysis was associated with a 17% higher peak plasma concentration and earlier time to peak, which were not clinically meaningful, whereas the overall extent of absorption and the elimination half-life were not affected. The average extent of drug bound to plasma protein was approximately 96% in hemodialysis patients. Intradialytic hypotension was not observed more frequently when sildenafil was administered before hemodialysis. Systolic blood pressure tended to decrease less during hemodialysis when subjects were treated with sildenafil before dialysis.

CONCLUSION

The present study demonstrates that sildenafil is not cleared by hemodialysis, and the pharmacokinetic profile resembles more closely that observed in normal volunteers than that observed in patients with severe renal insufficiency. In addition, we found that sildenafil does not promote intradialytic hypotension.

ORIGINAL RESEARCH—BASIC SCIENCE: A Nitric Oxide‐Releasing PDE5 Inhibitor Relaxes Human Corpus Cavernosum in the Absence of Endogenous Nitric Oxide

INTRODUCTION

In conditions with severe deficiency of endogenous nitric oxide (NO), such as long-term diabetes and cavernosal nerve injury, phosphodiesterase type 5 (PDE5) inhibitors have reduced efficacy in the treatment of erectile dysfunction. NO-releasing PDE5 inhibitors could be an alternative therapeutic approach in such cases.

AIM

We therefore aimed to compare sildenafil and NO-releasing sildenafil (NCX-911) in relaxing human corpus cavernosum in the absence or presence of endogenous NO.

METHODS

The two compounds were compared in reducing the phenylephrine-induced tone of human corpus cavernosum in the presence or absence of an inhibitor of NO synthase (L-NAME; 500 microM) or an inhibitor of soluble guanylate cyclase (ODQ, 10 microM).

RESULTS

NCX-911 was as potent as sildenafil in control conditions (EC(50) = 733.1 +/- 94.4 nM and 800.7 +/- 155.8 nM, respectively). The potency of NCX-911 was not altered but that of sildenafil decreased significantly in the presence of L-NAME (EC(50) = 980.4 +/- 106.7 nM and 2446.7 +/- 256.8 nM, respectively; P < 0.001 for sildenafil vs. control). Both compounds below 1 microM failed to induce relaxation in the presence of ODQ (EC(50) = 6,578 +/- 1150 nM and 6,488 +/- 938 nM for NCX-911 and sildenafil, respectively).

CONCLUSION

These results show that the potency of NCX-911 was maintained unlike sildenafil in the absence of endogenous NO confirming the potential use of NO-releasing PDE5 inhibitors in NO-deficient conditions.

Hemodynamic effects of phosphodiesterase 5 and angiotensin-converting enzyme inhibition alone or in combination in conscious SHR

The regional hemodynamic responses to continuous 4-day infusion of UK-357,903 [1-ethyl-4-{3-[3-ethyl-6,7-dihydro-7-oxo-2-(2-pyridylmethyl)-2H-pyrazolo[4,3-d]pyrimidin-5-yl]-2-(2-methoxyethoxy)-5-pyridylsulphonyl}piperazine] (266 microg kg(-1) h(-1)) alone and in combination with a low dose of enalapril (10 microg kg(-1) h(-1)) were measured in conscious spontaneously hypertensive rats to test the hypothesis that the renin-angiotensin system may influence the cardiovascular consequences of inhibition of phosphodiesterase 5 (PDE5) by UK-357,903 or vice versa. UK-357,903 alone caused a fall in mean blood pressure (-12.1 mm Hg) associated with vasodilatation in the mesenteric and hindquarters vascular beds. The only way in which the effects of enalapril given alone differed significantly from those of the vehicle was in causing mesenteric vasodilatation, which developed over the 4 days of infusion. UK-357,903 given in combination with enalapril caused hypotension (-17.8 mm Hg) and vasodilatation in the renal, mesenteric, and hindquarter vascular beds. There was evidence of a significant interaction between the effects of the two compounds on renal Doppler shift and vascular conductance with the combined action of the two compounds being greater than the sum of their individual effects. However, although there was a trend for the combination to have greater effects than either of the individual agents on blood pressure and mesenteric vascular conductance, there was no statistical evidence of an interaction. The results indicate that inhibition of the renin-angiotensin system uncovers additional renal vasodilator effects of UK-357,903, and/or inhibition of PDE5 enhances the renal vasodilator effects of angiotensin-converting enzyme inhibition.

Nitric oxide-cyclic GMP pathway with some emphasis on cavernosal contractility

Nitric oxide (NO) is formed from the conversion of L-arginine by nitric oxide synthase (NOS), which exists in three isoforms: neuronal (nNOS), endothelial (eNOS), and inducible (iNOS). nNOS is expressed in penile neurons innervating the corpus cavernosum, and eNOS protein expression has been identified primarily in both cavernosal smooth muscle and endothelium. NO is released from nerve endings and endothelial cells and stimulates the activity of soluble guanylate cyclase (sGC), leading to an increase in cyclic guanosine-3',5'-monophosphate (cGMP) and, finally, to calcium depletion from the cytosolic space and cavernous smooth muscle relaxation. The effects of cGMP are mediated by cGMP dependent protein kinases, cGMP-gated ion channels, and cGMP-regulated phosphodiesterases (PDE). Thus, cGMP effect depends on the expression of a cell-specific cGMP-receptor protein in a given cell type. Numerous systemic vasculature diseases that cause erectile dysfunction (ED) are highly associated with endothelial dysfunction, which has been shown to contribute to decreased erectile function in men and a number of animal models of penile erection. Based on the increasing knowledge of intracellular signal propagation in cavernous smooth muscle tone regulation, selective PDE inhibitors have recently been introduced in the treatment of ED. Phosphodiesterase 5 (PDE5) inactivates cGMP, which terminates NO-cGMP-mediated smooth muscle relaxation. Inhibition of PDE5 is expected to enhance penile erection by preventing cGMP degradation. Development of pharmacologic agents with this effect has closely paralleled the emerging science.

Effects of sildenafil on ocular perfusion demonstrated by color Doppler ultrasonography

The aim of this study was to investigate the effects of sildenafil on ocular hemodynamics by color Doppler ultrasonography (CDU). In all, 38 patients with erectile dysfunction diagnosed by International Index of Erectile Function (IIEF) and Sexual Health Inventory of Men (SHIM) scores were included into the study. After taking 100 mg of oral sildenafil citrate, all patients underwent CDU examination of central retinal artery at 60 and 75 min and CDU examination of cavernosal artery at 20, 60 and 75 min. All of the side effects during and after the test were also recorded. The mean cavernous artery peak systolic flow velocity increased significantly after sildenafil. However, no significant change was determined in central retinal artery flow parameters including peak systolic flow velocity, end-diastolic flow velocity, resistive index, pulsatility index, volume and diameter. Five patients experienced ocular side effects. No significant change was observed in retinal artery CDU measurements of patients having ocular side effects. Sildenafil has no effect on ocular hemodynamics on the basis of CDU. Ocular side effects may be the result of other changes in retinal photoreceptors rather than the ocular circulation.

Characteristics of photoreceptor PDE (PDE6): similarities and differences to PDE5

Phosphodiesterase 6 (PDE6) is highly concentrated in the retina. It is most abundant in the internal membranes of retinal photoreceptors, where it reduces cytoplasmic levels of cyclic guanosine monophosphate (cGMP) in rod and cone outer segments in response to light. The rod PDE6 holoenzyme comprises alpha and beta catalytic subunits and two identical inhibitory gamma subunits. Each catalytic subunit contains three distinct globular domains corresponding to the catalytic domain and two GAF domains (responsible for allosteric cGMP binding). The PDE6 catalytic subunits resemble PDE5 in amino-acid sequence as well as in three-dimensional structure of the catalytic dimer; preference for cGMP over cyclic adenosine monophosphate (cAMP) as a substrate; and the ability to bind cGMP at the regulatory GAF domains. Most PDE5 inhibitors inhibit PDE6 with similar potency, and electroretinogram studies show modest effects of PDE5 inhibitors on visual function-an observation potentially important in designing PDE5-specific therapeutic agents.

Expression of PDE5 splice variants during ontogenesis of chick dorsal root ganglia

Cyclic GMP (cGMP)-binding cGMP-specific phosphodiesterase (PDE5) activity was found in chick dorsal root ganglia (DRG). PDE5 expression was studied at different stages of development: in embryonic day 10 (E10) and E18 embryos and in 5-day post-hatching chick (P5). The presence of PDE5 was suggested by the ion exchange chromatography elution profile in E18 DRG extracts, where cGMP-specific hydrolytic calmodulin-independent activity was found; in other stages, this activity coeluted with the PDE1 calmodulin-stimulated isoform characterized previously. Inhibition studies supported the hypothesis that the newly identified PDE activity belongs to the PDE5 isoform. Western blot analysis using a PDE5-specific antibody was also carried out and revealed the presence of three specific immunoreactive bands with apparent molecular weights of 98, 93, and 86 kDa, corresponding to the three described splice variants (PDE5A1, PDE5A2, and PDE5A3). The expression in DRG of the three PDE5 isoforms was also confirmed by RT-PCR. Developmental regulation of PDE5 was revealed by the immunoblot analysis at different stages; expression was very low at E10 but an overall substantial increase occurred between E10-18 (about 12-fold, considering the three PDE5 isoforms together). Differences were revealed, however, when a single PDE5 isoform was considered. PDE5A1 and PDE5A3 showed an increase at all stages although more pronounced between E10-18, whereas PDE5A2 underwent a marked increase (about 38-fold) in the first period and remained nearly constant between E18 and P5. This is the first evidence of PDE5 in sensory neurons, and the distinct temporal expression patterns of enzyme isoforms may indicate different physiologic roles in developing and mature chick DRG.

Structural Basis for the Activity of Drugs that Inhibit Phosphodiesterases

Phosphodiesterases (PDEs) comprise a large family of enzymes that catalyze the hydrolysis of cAMP or cGMP and are implicated in various diseases. We describe the high-resolution crystal structures of the catalytic domains of PDE4B, PDE4D, and PDE5A with ten different inhibitors, including the drug candidates cilomilast and roflumilast, for respiratory diseases. These cocrystal structures reveal a common scheme of inhibitor binding to the PDEs: (i) a hydrophobic clamp formed by highly conserved hydrophobic residues that sandwich the inhibitor in the active site; (ii) hydrogen bonding to an invariant glutamine that controls the orientation of inhibitor binding. A scaffold can be readily identified for any given inhibitor based on the formation of these two types of conserved interactions. These structural insights will enable the design of isoform-selective inhibitors with improved binding affinity and should facilitate the discovery of more potent and selective PDE inhibitors for the treatment of a variety of diseases.

Regulation of intracellular Ca2+ release in corpus cavernosum smooth muscle: synergism between nitric oxide and cGMP

Tonic contraction of corpus cavernosum smooth muscle cells (SMCs) maintains the flaccid state of the penis, and relaxation is initiated by nitric oxide (NO), leading to erection. Our aim was to investigate the effect of NO on the smooth muscle cellular response to adrenergic stimulation in corpus cavernosum. Fura-2 fluorescence was used to record intracellular Ca(2+) concentration ([Ca(2+)](i)) from freshly isolated SMCs from rat and human. Phenylephrine (PE) transiently elevated [Ca(2+)](i) in the presence and absence of extracellular Ca(2+), indicating release from intracellular stores. Whereas the NO donor S-nitroso-N-acetylpenicillamine (SNAP) with sildenafil citrate (SIL) caused no change in basal [Ca(2+)](i), the PE-induced rise of [Ca(2+)](i) was reversibly inhibited by 27 +/- 7% (n = 21, P < 0.005) in rat and by 55 +/- 15% (n = 9, P < 0.01) in human SMCs. SNAP and SIL also reduced the contractile response to PE. To investigate the mechanism, we applied mediators alone or in combination. The soluble guanylyl cyclase inhibitor ODQ reduced the effect of SNAP and SIL. SIL, cGMP analogs, and NO donors without SIL did not reduce the PE-induced rise of [Ca(2+)](i). However, the combination of 8-bromo-cGMP with SNAP reduced the Ca(2+) peak by 42 +/- 9% (n = 22, P < 0.01). Our results demonstrate that NO and cGMP act synergistically to reduce Ca(2+) release from intracellular stores. Reduction of intracellular Ca(2+) release may contribute to relaxation of the corpus cavernosum, leading to erection.

T-0156, a novel phosphodiesterase type 5 inhibitor, and sildenafil have different pharmacological effects on penile tumescence and electroretinogram in dogs

T-0156 (2-(2-methylpyridin-4-yl)methyl-4-(3,4,5-trimethoxyphenyl)-8-(pyrimidin-2-yl)methoxy-1,2-dihydro-1-oxo-2,7-naphthyridine-3-carboxylic acid methyl ester hydrochloride) is a newly synthesized phosphodiesterase type 5 inhibitor, and its potency and selectivity are higher than those of sildenafil in an enzyme assay. In the present study with anesthetized dogs, we examined the effects of intravenous T-0156 or sildenafil on the pelvic nerve stimulation-induced penile tumescence and light-adapted flicker stimulation-induced electroretinogram, parameters of which are reported to be indicators for inhibition of phosphodiesterase type 5 and type 6, respectively. Both compounds potentiated the penile tumescence in a dose-dependent manner. T-0156 at 10 microg/kg and sildenafil at 100 microg/kg showed almost the same potentiating percentage (181.5+/-31.1% and 190.0+/-37.9%) in spite of the plasma concentration of T-0156 being about five times lower than that of sildenafil (16.7+/-1.6 and 78.8+/-5.3 ng/ml), indicating that the effect of T-0156 on tumescence is more potent than that of sildenafil. While the high dose of T-0156 (1000 microg/kg) reduced the amplitude and increased the latency of the electroretinogram positive wave, the effects of T-0156 were conversely weaker than those of sildenafil (reduction of amplitude; T-0156: 41.1+/-8.0%, sildenafil: 71.7+/-3.9%, increase of latency; T-0156: 3.9+/-0.6%, sildenafil: 14.5+/-1.4%, at 1000 microg/kg). These results clearly showed the difference in the properties of T-0156 and sildenafil in pharmacological studies with anesthetized dogs, and the difference appeared to correspond with their inhibitory potencies for phosphodiesterase type 5 and type 6. It was concluded that T-0156 would be a useful pharmacological tool as a potent and highly selective phosphodiesterase type 5 inhibitor.

Pharmacotherapy for Erectile Dysfunction

INTRODUCTION

Advances in understanding of the biochemistry and physiology of penile erection have led to breakthroughs in pharmacotherapy of erectile dysfunction.

AIM

To provide recommendations/guidelines concerning state-of-the-art knowledge for the putative molecular and cellular mechanisms of action of centrally and peripherally acting drugs currently utilized in pharmacotherapy of erectile dysfunction.

METHODS

An international consultation in collaboration with the major urology and sexual medicine associations assembled over 200 multidisciplinary experts from 60 countries into 17 committees. Committee members established specific objectives and scopes for various male and female sexual medicine topics. The recommendations concerning state-of-the-art knowledge in the respective sexual medicine topic represent the opinion of experts from five continents developed in a process over a two-year period. Concerning the Pharmacotherapy for Erectile Dysfunction Committee there were 25 experts from 10 countries.

MAIN OUTCOME MEASURE

Expert opinion was based on grading of evidence-based medical literature, widespread internal committee discussion, public presentation and debate.

RESULTS

Selective and potent oral PDE5 inhibitors have significantly more affinity than cGMP and form broader molecular interactions with multiple amino acids, thereby blocking access to cGMP in the catalytic sites of the PDE5 enzyme. PDE5 inhibitors, which vary as to biochemical potency, selectivity and pharmacokinetics, lead to cGMP elevation and relaxation facilitation of penile corpus cavernosum smooth muscle cells following sexual stimulation. Various centrally acting drugs influence sexual behaviour. In particular, the dopaminergic substance apomorphine is a central enhancer that acts in the paraventricular nucleus of the hypothalamus as a dopamine (D2) receptor agonist, induces and increases penile erection responses via disinhibition, following sexual stimulation.

CONCLUSIONS

There is a need for more research in the pharmacotherapeutic development of central and peripheral agents for safe and effective erectile dysfunction treatment.

Differential inhibitor sensitivity between human recombinant and native photoreceptor cGMP-phosphodiesterases (PDE6s)

Human photoreceptor cGMP-phosphodiesterases (PDE6s) are important reagents in PDE inhibitor discovery. However, recombinant human PDE6s have not been expressed, and isolation of native human PDE6s is highly difficult. In this study, the catalytic subunit(s) of human rod and cone PDE6s (PDE6alphabeta and PDE6alpha', respectively) were co-expressed or expressed separately as catalytically active enzymes. Sildenafil inhibited both the recombinant PDE6s in a dose-dependent manner with Ki values of 94 and 98 nM, respectively. These Ki values were four-fold higher than that (25 nM) of a human native PDE6 preparation. Similarly, 3-isobutyl-1-methylxanthine (IBMX)'s Ki values for the recombinant PDE6s were five- to eight-fold higher than that of the native enzyme. However, E4021 and zaprinast exhibited much (30-80-fold) lower potencies for the recombinant PDE6s than for the native enzyme. Additional PDE5 inhibitors representing other structural classes and possessing different selectivity against native PDE6 also showed different potencies against the recombinant and native PDE6s. In particular, one class of xanthine analogues exhibited significantly (5-15-fold) higher potencies for the recombinant PDE6s than for the native enzyme. Our data demonstrates that the recombinant and native PDE6s exhibit differential sensitivity to inhibitors, and cautions the use of recombinant catalytic subunits of PDE6 in drug discovery or in structural/functional studies.

Sildenafil alters calcium signaling and vascular tone in pulmonary arteries from chronically hypoxic rats

Sildenafil, a potent type 5 nucleotide-dependent phosphodiesterase (PDE) inhibitor, has been recently proposed as a therapeutic tool to treat or prevent pulmonary artery hypertension (PAHT). We thus studied the effect of sildenafil on both the calcium signaling of isolated pulmonary artery smooth muscle cells (PASMCs) and the reactivity of pulmonary artery (PA) obtained from chronic hypoxia (CH)-induced pulmonary hypertensive rats compared with control (normoxic) rats. CH rats were maintained in an hypobaric chamber (50.5 kPa) for 3 wk leading to full development of PAHT. Intracellular calcium concentration ([Ca2+]i) was measured in PASMCs loaded with the calcium fluorophore indo 1. Unlike in control rats, sildenafil (10–100 nM) decreased the resting [Ca2+]ivalue in PASMCs obtained from CH rats. In PASMCs from both control and CH rats, sildenafil concentration dependently inhibited the [Ca2+]iresponse induced by G-coupled membrane receptor agonists such as angiotensin II and phenylephrine but had no effect on the amplitude of the [Ca2+]iresponse induced by caffeine. Sildenafil (0.1 nM–1 μM) concentration dependently reduced basal PA tone that is present in CH rats and relaxed PA rings precontracted with phenylephrine in both control and CH rats. These data show that sildenafil is a potent pulmonary artery relaxant in CH rats and that it normalizes CH-induced increases in resting [Ca2+]iand basal tone. Consequently, pharmacological inhibition of sildenafil-sensitive PDE5 downregulates the Ca2+signaling pathway involved in this model of pulmonary hypertension.

Effect of combination endothelial nitric oxide synthase gene therapy and sildenafil on erectile function in diabetic rats

Erectile dysfunction associated with diabetes mellitus is caused in part by disordered endothelial smooth muscle relaxation, neuropathy, and a decrease in cavernosal nitric oxide synthase (NOS) activity. The purpose of this study was to determine whether a combination of sildenafil and adenoviral gene transfer of endothelial NOS (eNOS) could enhance the erectile response in diabetic rats. Five groups of animals were utilized: (1) age-matched control rats, (2) streptozotocin (STZ)-induced diabetic rats (60 mg/kg i.p.), (3) STZ-rats + sildenafil (2 mg/kg i.v.), (4) STZ-rats transfected with AdCMVbetagal or AdCMVeNOS, and (5) STZ-rats transfected with AdCMVeNOS +sildenafil (2 mg/kg i.v.). At 2 months after i.p. injection of STZ, groups 4 and 5 were transfected with the adenoviruses and 1-2 days after transfection, all animals underwent cavernosal nerve stimulation (CNS) to assess erectile function. Cyclic 3',5'-guanosine monophosphate (cGMP) levels were assessed in the cavernosal tissue. STZ-diabetic rats had a significant decrease in erectile function as determined by the peak intracavernosal pressure (ICP) and total ICP (area under the erectile curve; AUC) after CNS when compared to control rats. STZ-diabetic rats+AdCMVeNOS had a peak ICP and AUC, which were similar to control animals. STZ-diabetic rats administered sildenafil demonstrated a significant increase in peak ICP at the 5 and 7.5 V settings, while the AUC was significantly increased at all voltage (V) settings. The increase in both ICP and AUC of STZ-diabetic rats transfected with AdCMVeNOS at all V settings was greater than STZ-diabetic rats transfected with AdCMVbetagal. STZ-diabetic rats transfected with AdCMVeNOS and administered sildenafil had a significant increase in total ICP that was greater than eNOS gene therapy alone. Cavernosal cGMP levels were significantly decreased in STZ-diabetic rats, but were increased after transfection with AdCMVeNOS to values greater than control animals. In conclusion, overexpression of eNOS and cGMP in combination with sildenafil significantly increased both the peak ICP and total ICP to CNS in the STZ-diabetic rat, which was similar to the response observed in control rats. Moreover, the total erectile response was greater in STZ-diabetic rats receiving eNOS gene therapy plus sildenafil than STZ-rats receiving sildenafil or eNOS gene therapy alone.

Advances in the Treatment of Erectile Dysfunction: A Focus on Tadalafil

Erectile dysfunction (ED) affects more than 150 million men and their partners throughout the world. Sildenafil, the first oral phosphodiesterase-type 5 (PDE-5) inhibitor approved for the treatment of ED, has led to the discovery of 2 more products in this class, with tadalafil being the latest. Tadalafil has proven to be well tolerated in various clinical studies and has demonstrated effectiveness in men with mild to severe ED, regardless of severity, etiology, or age. One major characteristic that may distinguish tadalafil from other oral PDE-5 inhibitors is its extensive duration of effect. Tadalafil has a therapeutic duration of up to 36 hours that may allow patients to engage in sexual activity at their own convenience rather than in a limited time period. This, along with other clinical and practical advantages, should make tadalafil an attractive alternative to currently available treatments options. This article summarizes the pharmacology, pharmacokinetics, adverse effects, and clinical efficacy of tadalafil.

Phosphodiesterases in the vascular system

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers involved in the intracellular signal transduction of a variety of extracellular stimuli in several tissues. In the vascular system, these nucleotides play important roles in the regulation of vascular tone and in the maintenance of the mature contractile phenotype in smooth muscle cells. Given that cyclic nucleotide signaling regulates a wide variety of cellular functions, it is not surprising that cyclic nucleotide phosphodiesterases (PDEs). In paticular, the accumulating data showing that there are a large number of different PDE isozymes have triggered an equally large increase in interest about these enzymes. At least 11 different gene families of PDEs are currently known to exist in mammalian tissues. Most families contain several distinct genes, and many of these genes are expressed in different tissues as functionally unique alternative splice variants. This article reviews many of the important aspects about the structure, cellular localization, and regulation of each family of PDEs. Particular emphasis is placed on new information obtained in the last few years about vascular disease. The development of novel methods to deliver more potent and selective PDE inhibitors to individual cell types and subcellular locations will lead to new therapeutic uses for this class of drugs in diseases of the vascular system.

Relaxant action of sildenafil citrate (Viagra) on human myometrium of pregnancy

OBJECTIVE

The purpose of this study was to investigate the effect of the cyclic guanosine monophosphate phosphodiesterase 5-specific inhibitor, sildenafil citrate, on the contractions of isolated pregnant human myometrium.

STUDY DESIGN

Myometrial samples were obtained from women who underwent elective cesarean delivery. Myometrial contractions that were recorded in response to sildenafil in the absence and presence of the potassium channel blocker, tetraethylammonium or the guanylate cyclase inhibitor, methylene blue (10 micromol/L) were studied. One-way analysis of variance with post-hoc analysis was used to test differences among groups.

RESULTS

Sildenafil caused relaxation of myometrium in a concentration-dependent manner. The log(10) EC(50) value for this relaxation in the presence of 20 mmol/L tetraethylammonium was significantly different (P<.01) than values that were obtained with sildenafil alone or sildenafil in the presence of either methylene blue or 5 and 10 mmol/L tetraethylammonium.

CONCLUSION

Myometrial relaxation that is evoked by the direct application of sildenafil occurs independently of cyclic guanosine monophosphate. Potassium channels appear to be the likely candidates in mediating this response.

Binding of tritiated sildenafil, tadalafil, or vardenafil to the phosphodiesterase-5 catalytic site displays potency, specificity, heterogeneity, and cGMP stimulation

Sildenafil, tadalafil, and vardenafil each competitively inhibit cGMP hydrolysis by phosphodiesterase-5 (PDE5), thereby fostering cGMP accumulation and relaxation of vascular smooth muscle. Biochemical potencies (affinities) of these compounds for PDE5 determined by IC(50), K(D) (isotherm), K(D) (dissociation rate), and K(D) ((1/2) EC(50)), respectively, were the following: sildenafil (3.7 +/- 1.4, 4.8 +/- 0.80, 3.7 +/- 0.29, and 11.7 +/- 0.70 nM), tadalafil (1.8 +/- 0.40, 2.4 +/- 0.60, 1.9 +/- 0.37, and 2.7 +/- 0.25 nM); and vardenafil (0.091 +/- 0.031, 0.38 +/- 0.07, 0.27 +/- 0.01, and 0.42 +/- 0.10 nM). Thus, absolute potency values were similar for each inhibitor, and relative potencies were vardenafil >> tadalafil > sildenafil. Binding of each (3)H inhibitor to PDE5 was specific as determined by effects of unlabeled compounds. (3)H Inhibitors did not bind to isolated PDE5 regulatory domain. Close correlation of EC(50) values using all three (3)H inhibitors competing against one another indicated that each occupies the same site on PDE5. Studies of sildenafil and vardenafil analogs demonstrated that higher potency of vardenafil is caused by differences in its double ring. Exchange-dissociation studies revealed two binding components for each inhibitor. Excess unlabeled inhibitor did not significantly affect (3)H inhibitor dissociation after infinite dilution, suggesting the absence of subunit-subunit cooperativity. cGMP addition increased binding affinity of [(3)H]tadalafil or [(3)H]vardenafil, an effect presumably mediated by cGMP binding to PDE5 allosteric sites, implying that either inhibitor potentiates its own binding to PDE5 in intact cells by elevating cGMP. Without inhibitor present, cGMP accumulation would stimulate cGMP degradation, but with inhibitor present, this negative feedback process would be blocked.

Erectile dysfunction in hypertensive rats results from impairment of the relaxation evoked by neurogenic carbon monoxide and nitric oxide

Erectile dysfunction (ED) with aging and diabetes mellitus is caused by impairment of the relaxation evoked by nitric oxide (NO) of penile cavernous smooth muscles and arterioles. However, the mechanism of ED in hypertension is unknown. Carbon monoxide (CO), which is produced by heme oxygenase (HO)-2 in the neuronal system is a neurotransmitter and a vasodilator. We examined the neurogenic role of CO in penile erection and the neurogenic mechanisms of ED in hypertension, using spontaneously hypertensive rats (SHR) or Wistar-Kyoto rats (WKY). The isometric tension of corpus cavernosum tissues from both strains was recorded after guanethidine and atropine treatment. Relaxation in response to electrical field stimulation (EFS) in WKY was suppressed dose-dependently by HO inhibitors both in the absence and presence of an NO synthase (NOS) inhibitor. Reverse transcription-polymerase chain reaction (RT-PCR) showed that the HO-2 gene was expressed in the corpus cavernosum. CO-saturated solution induced a concentration-dependent relaxation in WKY. The neurogenic relaxation to EFS in SHR was impaired as compared with that in WKY after the age of 5 weeks, when blood pressure began to be elevated, due to the attenuated relaxation in response to neurogenic NO and CO. In the corpus cavernosum of SHR, expression of the HO-2 and nNOS genes was similar, and NOx levels after EFS were similar to those of WKY. cGMP levels after EFS and the relaxation evoked by the NO donor was lower in SHR than WKY. Thiobarbituric acid-reacting substance (TBARS) levels were increased, and superoxide dismutase (SOD) activity was suppressed in SHR, as compared with those in WKY, suggesting that the increasing oxidative stress partially causes the impairment of NO-dependent relaxation. These findings suggest that CO regulates the relaxation evoked by EFS in the rat corpus cavernosum, and that ED in hypertension in rats results from an impairment of the relaxation induced by neurogenic CO and NO.

Effect of acute in vivo sildenafil citrate and in vitro 8-bromo-cGMP treatments on semen parameters and sperm function

OBJECTIVE

To determine the effect of acute in vivo sildenafil citrate (VIAGRA) and in vitro 8-Bromo-cGMP treatments on semen parameters and sperm function.

DESIGN

Prospective double-blind, placebo-controlled, crossover, two-period clinical investigation.

SETTING

Healthy volunteers in an academic research environment.

PATIENT(S)

Twenty male volunteers with normal erectile function and normal electrocardiogram were recruited.

INTERVENTION(S)

In vivo: 50 mg of sildenafil citrate (VIAGRA) or placebo was administered p.o., and semen samples were collected 1 hour after administration of the test drug. In vitro: 8-Bromo-cGMP (20 microM; 60 minutes) was added to semen samples.

MAIN OUTCOME MEASURE(S)

Macroscopic and microscopic seminal parameters were measured and motility studies performed. Various acrosome reaction studies and sperm-zona pellucida binding studies were also done.

RESULT(S)

Either sildenafil citrate (VIAGRA) or 8-Bromo-cGMP treatments had no effect on both macroscopic and microscopic seminal parameters as well as the acrosome reaction. Sperm-zona pellucida binding results were however increased to 148.75% and 134%, respectively, by these treatments. Various kinematical parameters increased after treatment with the most significant increase detected in the population of rapid cells.

CONCLUSION(S)

VIAGRA can be used successfully to enhance sperm motility and binding to the oocyte especially during fertility treatments.

Biochemical and functional assessment of equine lymphocyte phosphodiesterases and protein kinase C

Lymphocytes play an important role in allergic inflammation and have been implicated in the pathogenesis of equine allergic skin and respiratory disease. Targeting intracellular signalling pathways in human lymphocytes has demonstrated a role for both phosphodiesterase and protein kinase C in cell activation. The aim of this study was to measure total cyclic nucleotide hydrolysing phosphodiesterase activity and to identify the phosphodiesterase and protein kinase C isoenzymes present in equine lymphocytes. The functional significance of these isoenzymes was then investigated by examining their role in peripheral blood mononuclear cell proliferation using isoenzyme selective inhibitors. Total cyclic adenosine monophosphate hydrolysing phosphodiesterase activity was double that of cyclic guanosine monophosphate (30+/-2 pmol/min mg versus 16+/-3 pmol/min mg for cyclic adenosine and cyclic guanosine monophosphate phosphodiesterase activity, respectively). Evidence for the presence of PDE1, 3, 4 and 5 was obtained and PKCalpha, beta, delta, eta, iota, theta and zeta were identified. Selective inhibitors of PDE4, PKCdelta and conventional PKCs alpha and beta caused significant inhibition of mitogen-induced peripheral blood mononuclear cell proliferation. This study demonstrates a functional role for specific signalling isoenzymes and suggests that, in the context of allergic inflammation, targeting inflammatory cells involved in disease pathogenesis with relevant isoenzyme inhibitors may have therapeutic potential.

Atypical beta-adrenoceptor subtypes mediate relaxations of rabbit corpus cavernosum

This study was performed to characterize the beta-adrenoceptor population in rabbit isolated corpus cavernosum (RbCC) by using nonselective and selective beta-adrenoceptor agonists and antagonists in functional assays. Metaproterenol, ritodrine, fenoterol, and 8-hydroxy-5-[(1R)-1-hydroxy-2-[N-[(1R)-2-(rho-methoxyphenyl)-1-methylethyl]amino]ethyl]carbostyril (TA 2005) (3-100 nmol each) dose dependently relaxed the RbCC preparations. These relaxations were markedly reduced by N(omega)-nitro-L-arginine methyl ester (L-NAME; 10 microM) and 1H-[1,2,4]-oxadiazolo-[4,3,-a]quinoxalin-1-one (ODQ) (10 microM), whereas the adenylyl cyclase inhibitor SQ 22,536 [9-(2-tetrahydrofuryl) adenine] (10 microM) had no effect. In contrast, neither L-NAME nor ODQ affected the isoproterenol-induced RbCC relaxations, but SQ 22,536 abolished this response. Sildenafil (1 microM) significantly potentiated the relaxations induced by beta(2)-agonists without affecting the isoproterenol-evoked relaxations. Rolipram (10 microM) enhanced the relaxations elicited by isoproterenol but had no effect on those induced by the selective beta(2) agonists. Propranolol and (+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride (ICI 118,551) determined a rightward shift in the concentration-response curves to isoproterenol in a noncompetitive manner with a reduction of maximum response at the highest antagonist concentration, with the slope values significantly different from unity. Propranolol and ICI 118,551 had no effect on the relaxations elicited by fenoterol, TA 2005, metaproterenol, and ritodrine. Atenolol and 1-[2-((3-carbamoyl-4-hydroxy)phenoxy) ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl)-phenoxy]-2-propanol methanesulfonate (CGP 20712A) (0.1-10 microM) failed to affect the relaxations induced by all tested beta-adrenoceptor agonists. Our study revealed the existence of two atypical beta-adrenoceptors in the rabbit erectile tissue. Isoproterenol relaxes the rabbit cavernosal tissue by activating atypical beta-adrenoceptors coupled to adenylyl cyclase pathway, whereas the selective beta(2)-adrenoceptor agonists relax the RbCC tissue through another atypical beta-adrenoceptor subtype coupled to nitric oxide release from the sinusoidal endothelium.

Visual short-term effects of Viagra: double-blind study in healthy young subjects

PURPOSE

To investigate short-term visual effects of a single 100-mg dose of Viagra (sildenafil citrate) in healthy men.

DESIGN

Randomized, double-blind, placebo-controlled clinical trial of drug effects on normal volunteers conducted by a single center.

METHODS

Twenty men, aged 20 to 40 years, were treated with either a placebo or 100 mg sildenafil. Visual function tests included electroretinogram (ERG) recordings, on-/off- and 3.3 Hz-flicker-ERG recordings, anomaloscope matches, and measurements of cone contrast sensitivities and transient tritanopia.

RESULTS

Most visual tests did not differ between the sildenafil and placebo groups. However, statistically significant increases in sensitivity during transient tritanopia were observed as well as significant prolongations in the implicit times of scotopic a-wave, photopic b-wave, and 3.3 Hz-flicker a-wave and b-wave ERG recordings. The magnitude of the differences correlated with peak sidenafil plasma concentration. Although rod amplitudes of the ERG recordings tended to be higher and cone amplitudes lower in the sildenafil group after drug ingestion, the differences were nonsignificant. There were no reports of visual side effects, and all electrophysiologic and psychophysical measurements returned to the normal range within 24 hours.

CONCLUSIONS

A single oral dose of 100-mg sildenafil given to healthy young men led to small but statistically significant transient changes of outer and inner retinal function, as detected by ERG and psychophysical methods. Although the acute effects were fully reversible within 24 hours, it would be worthwhile to compare them with those induced by other PDE5 and PDE6 inhibitors.

Effects of combined use of sildenafil citrate (Viagra) and 17beta-estradiol on ovine coronary and uterine hemodynamics

OBJECTIVE

This study was undertaken to determine whether Viagra (Pfizer, New York, NY), a type 5 phosphodiesterase (PDE-5) inhibitor, increases baseline coronary and uterine blood flow or potentiates estrogen-induced increases in blood flow.

STUDY DESIGN

Seven nonpregnant ovariectomized ewes were chronically instrumented to measure blood pressure, heart rate, cardiac output, and coronary and uterine blood flow. Estradiol-17beta (E2, 0.3 microg/kg) (Sigma Chemical Co, St Louis, Mo) and Viagra (50 mg) were intravenously administered separately or together and hemodynamic responses monitored.

RESULTS

Viagra alone rapidly decreased mean arterial pressure and cardiac output while increasing heart rate and uterine blood flow. Viagra tended to decrease coronary blood flow but not significantly. E2 by itself increased coronary and uterine blood flow over a 2-hour time course, whereas mean arterial pressure, cardiac output, and heart rate remained unchanged. Administration of E2 to animals pretreated with Viagra reversed Viagra-related decrease in cardiac output and coronary blood flow, resulting in a time-related increase in both parameters over baseline values. Viagra significantly augmented estradiol-related increases in uterine blood flow.

CONCLUSION

In our surgically menopausal ovine model, Viagra alone has a tendency to decrease baseline coronary blood flow, but the reduction does not reach statistical significance. The presence of estradiol reverses this Viagra-related negative trend and results in a net increase in coronary blood flow and cardiac output. Furthermore, Viagra significantly augments baseline and estrogen-induced increases in uterine blood flow suggesting the presence of PDE-5 in the ovine uterine vasculature.

Vardenafil: a review of its use in erectile dysfunction

Vardenafil (Levitra) is a potent and highly selective oral phosphodiesterase type 5 (PDE5) inhibitor. Vardenafil improved erectile function in men with mild to severe erectile dysfunction (ED) of varying aetiology in two randomised, double-blind, multicentre, fixed-dose studies of 12 or 26 weeks' duration. Men receiving vardenafil 10 or 20 mg had significantly greater improvements in International Index of Erectile Function (IIEF) questionnaire erectile function domain scores than placebo recipients. Moreover, improvements in penetration and maintenance of erection (assessed using IIEF or Sexual Encounter Profile [SEP] questions) were significantly greater with vardenafil 5-20 mg than with placebo. Improvements in IIEF intercourse satisfaction and orgasmic function domain scores were significantly greater with vardenafil 10 or 20 mg than with placebo and the proportion of patients with a positive response to a Global Assessment Question (GAQ) concerning improvement in erections after 12 or 26 weeks' therapy was significantly higher with vardenafil 5-20 mg than with placebo. Vardenafil improved erectile function in men with ED associated with diabetes mellitus or ED following unilateral or bilateral nerve-sparing radical retropubic prostatectomy in two randomised, double-blind, multicentre, fixed-dose, 3-month studies. In both studies, improvements from baseline in the erectile function domain score of the IIEF and in positive responses to SEP questions were significantly greater with vardenafil 10 or 20 mg than with placebo. In addition, a significantly higher proportion of vardenafil 10 or 20 mg recipients than placebo recipients had positive GAQ responses. Vardenafil was generally well tolerated in men with ED; treatment-emergent adverse events were of mild to moderate intensity and transient in nature. The most commonly reported adverse events (typical of those seen with PDE5 inhibitors) in vardenafil 5-20 mg recipients included headache, flushing, rhinitis, dyspepsia and sinusitis. There were no reports of abnormal colour vision in men with ED taking vardenafil at clinically recommended doses (5-20 mg).

CONCLUSION

Vardenafil is a potent and highly selective oral PDE5 inhibitor. It is effective and generally well tolerated in men with mild to severe ED of varying aetiology, as well as in men with ED associated with diabetes mellitus or ED after radical prostatectomy. Vardenafil should be considered a first-line treatment option in men with ED who are suitable candidates for oral PDE5 inhibitor therapy.

Interactions between cGMP- and cAMP-pathways are involved in the regulation of penile smooth muscle tone

Nitric oxide (NO)/cyclic GMP (cGMP)-mediated mechanisms have a pivotal function in reducing the tone of the penile smooth musculature during normal erectile responses. The cyclic AMP (cAMP) signaling pathway is also involved in the adjustment of smooth muscle contractility, and suggestions for interactions between cGMP- and cAMP-mediated mechanisms have been presented. Using activators of the cGMP- or the cAMP-pathway, as well as inhibitors of protein kinase A (PKA; cAMP-dependent kinase) and protein kinase G (PKG; cGMP-dependent kinase), the present study was undertaken to further delineate the functional relation between these pathways in the penis. In addition, the distribution of PKA and some cAMP-binding phosphodiesterases (cAMP-PDEs) were investigated in human erectile tissue. Functional experiments were performed on isolated human corpus cavernosum (HCC). The effects of an inhibitor of the PKA, Rp-8CPT-cAMPS (10 microM), or the PKG, Rp-8-pCPT-cGMPS (10 microM), on relaxation induced by the cumulative administration of sodium nitroprusside (SNP), forskolin, sildenafil or tadalafil (IC351) were studied in preparations of HCC precontracted with 1 microM norepinephrine (NE). Using immunohistochemical procedures, the presence of immunoreactivity for cAMP-PDEs PDE3, PDE4, and PDE4A, as well as for PKA was investigated in specimens of HCC from which preparations were also used in the functional experiments. Forskolin, SNP, sildenafil, and IC 351 dose-dependently reversed NE-induced tension of isolated HCC preparations. The relaxing effects of SNP were significantly attenuated by Rp-8-pCPT-cGMPS, but not by Rp-8CPT-cAMPS. In contrast, relaxation induced by forskolin, sildenafil and tadalafil were significantly reversed by both Rp-8-pCPT-cGMPS and Rp-8CPT-cAMPS. Abundant immunoreactivity for PDE3 and PKA was observed in the corpus cavernosum smooth muscle cells. Immunoreactivity for PDE4 was also detected in the smooth musculature and in the cytoplasm of endothelial cells lining the cavernous sinusoids, as well as in nerve fibres interspersing the trabecular stroma. The present results support the hypothesis of interactions between cGMP- and cAMP-mediated signals in the HCC, and suggest that the effects of inhibitors of PDE5 on isolated erectile tissue may also partly or indirectly include actions of the cAMP second messenger system. The exact mechanism by which such an interaction occurs is not clear, but it may involve altered activity of the cGMP-inhibited PDE3 brought about by a change in the intracellular levels of cGMP by the inhibition of PDE5. This will in turn lead to increasing levels of cAMP, facilitating the interaction of cAMP with the PKA. The immunoreactivity specific for PDE3, PDE4, PDE4A and PKA registered in HCC section is also in support of an important role for the cAMP/PKA-system for penile smooth muscle function.

Erectile dysfunction: etiology, evaluation, and treatment options

The evaluation and treatment of male sexual dysfunction has evolved into a more extensive evaluation. This new evaluation should now include evaluation of hypogonadism, ejaculatory function, lower urinary tract symptoms, and depression. The evaluation may be readily accomplished with the use of questionnaires. The management of these entities is discussed, including the novel phosphodiesterase-5 for male erectile dysfunction. Inclusion of the partner in the evaluation and management scheme will provide added benefit and may produce a better outcome.

Molecular mechanisms and pharmacokinetics of phosphodiesterase-5 antagonists

The prominence of phosphodiesterase-5 (PDE-5) inhibitors in the treatment of male erectile dysfunction and other diseases related to vascular dysfunction mandates a comprehensive understanding of the properties and effects of these compounds. Three potent and selective PDE-5 inhibitors (sildenafil, tadalafil, and vardenafil) have been approved for clinical use. The clinical efficacy and safety profiles of these medications are related to their molecular mode of action, the selectivity for PDE-5, and the pharmacokinetic properties (absorption, bioavailability, time to onset of action, distribution, metabolism, and elimination). These PDE-5 inhibitors share some common properties with regard to mechanisms of action and selectivities for PDE-5. They also have distinctive characteristics that may impact their clinical use. This article focuses on the basic biochemistry of cyclic guanosine monophosphate signaling and the pharmacokinetic parameters that describe characteristics of drug action of these PDE-5 inhibitors in facilitating smooth muscle relaxation, leading to improved penile erectile response or causing side effects.

Histopathological effects of sildenafil citrate on rat corpus cavernosum

Sildenafil citrate (Viagra) is widely used for the treatment of erectile dysfunction with various etiologies. The aim of the present study was the investigation of histopathological effects of sildenafil citrate on rat corpus cavernosum using tight and electron microscopical techniques. Twenty male rats were divided into two groups. The first group (n = 10) was used as a control and the second group (n = 10) was treated with sildenafil citrate. Penile tissue was collected, fixed with formalin and embedded in paraffin for light microscopy, or fixed with gluteraldehyde and osmium tetroxide and embedded in Epon for electron microscopy. Light microscopical analysis showed that the corpus cavernosum was elongated and the number of blood vessels was increased. The amount of connective tissue in the penis was increased and dense collagen and smooth muscle fibers were observed in treated rats. Electron microscopical analysis showed that stromal structures of the corpus cavernosum (collagen fibers and number of cellular elements) were increased in treated rats. Fibroblasts showed signs of activation and the number of other stromal cells was increased. Immature newly synthesized collagen fibers were observed and penetrated endothelial basement membranes. In addition, endothelial cells also showed signs of activation such as cytoplasmic granules in treated rats, whereas the surface area of blood vessels was increased and basement membranes were thickened. These histopathological changes due to treatment with sildenafil citrate indicate that prolonged use of sildenafil citrate may increase the risk of fibrosis in the penis.

Efficacy and safety of sildenafil citrate in men with erectile dysfunction and stable coronary artery disease

This was a double-blind, placebo-controlled, flexible-dose study of the efficacy and safety of sildenafil in men with erectile dysfunction (ED) and clinically stable coronary artery disease (CAD). Patients were randomized to receive sildenafil or placebo for 12 weeks. Primary outcomes were questions 3 and 4 of the International Index of Erectile Function (IIEF). Secondary outcomes included the other IIEF questions and functional domains, the Life Satisfaction Checklist, the Erectile Dysfunction Inventory of Treatment Satisfaction, 2 global efficacy assessment questions, and intercourse success rate. By week 12, sildenafil-treated patients (n = 70) showed significant improvements on questions 3 and 4 compared with placebo-treated patients (n = 72; p <0.01). Larger percentages of sildenafil-treated patients reported improved erections (64%) and improved intercourse (65%) compared with placebo-treated patients (21% and 19%, respectively). Sildenafil-treated patients were highly satisfied with treatment and their sexual life compared with placebo-treated patients. Forty-seven percent of sildenafil- and 32% of placebo-treated patients experienced adverse events, including transient headache, hypertension, flushing, and dyspepsia. There were no serious drug-related cardiovascular effects. Thus, sildenafil is an effective and well-tolerated treatment for ED in men with CAD. Sildenafil was not associated with additional safety risks in this patient population.

The presence and function of phosphodiesterase type 5 in the rat myometrium

OBJECTIVE

Cyclic nucleotide phosphodiesterases (PDEs) are a diverse enzyme group with multiple regulatory properties and wide tissue distribution. Such activity includes cyclic adenosine (cAMP) and guanosine monophosphate (cGMP) breakdown. The type 5 isoform (PDE-5, cGMP specific) is the target of specific antagonists (ie, sildenafil, Viagra). We tested the hypothesis that PDE-5 is present in rat myometrium and modulates myometrial activity.

STUDY DESIGN

Full-thickness uterine wall was collected from nonpregnant (n=3) and pregnant Sprague-Dawley rats on days 10 (n=4), 17 (n=6), 22 nonlabor (n=5), and 22 during term labor (TL, n=4). Preterm labor (PTL, n=3) was induced in some animals on day 16 with 15 mg/kg mifepristone (RU 486). Tissue samples were prepared for Western blotting using a monoclonal antibody against rodent PDE-5. In a second series, cumulative doses of sildenafil (0.005, 0.05, 0.5, 5 mg/kg, intraperitoneal) were administered and the effect on uterine contractility recorded in vivo during term (TL, n=7) and preterm labor (PTL, n=6). Saline solution-injected rats provided temporal control. Uterine contractility was estimated from intrauterine pressure (IP) measured electronically with a sensor tip pressure catheter. Heart rate was recorded simultaneously using electrodes attached to the chest and connected to the same data acquisition system.

RESULTS

PDE-5 immunoreactivity was present in the nonpregnant rat uterus and at all gestational times studied, although the expression was unaffected by either pregnancy or the state of labor (preterm or term). A dominant antibody-specific band was identified at 86 kd in the uterine samples, contrasting with lung where the 100-kd PDE-5 isoform was most abundant. Two additional lower molecular weight (55 and 32 kd) bands were also identified as antibody specific. Despite the lack of change in PDE-5 during pregnancy, sildenafil reduced IP during TL and PTL beginning at 0.5 mg/kg. The highest dose of sildenafil reduced IP during both TL and PTL by 45% and 59% of baseline, respectively (two-way analysis of variance, P<.01). This effect was not accompanied by changes in heart rate.

CONCLUSION

PDE-5 is constitutively present in the rat uterine wall. There was no observed change in the PDE-5 protein expression throughout pregnancy. In contrast to the lung, the uterus expresses an 80-kd PDE-5 isoform. Sildenafil in pharmacologic doses inhibits mechanical uterine activity and might be of benefit if selectively used for treatment of preterm labor.

Flavonoids from Achyrocline satureioides with relaxant effects on the smooth muscle of Guinea pig corpus cavernosum

Ethanol extract of the aerial parts of Achyrocline satureioides (Lam.) DC. (Asteraceae) showed a significant, dose dependent, relaxant effect on the smooth muscle of corpus cavernosum strips, obtained from Guinea pig (65.5 +/- 4.1% of relaxation at the dose of 25.0 mg/ml). Bioassay guided fractionation of this extract furnished two flavonoids, quercetin and quercetin 3-methyl ether, with important vasorelaxing effects on the corpus cavernosum strips (79.8 +/- 8.4 and 66.0 +/- 4.8% of relaxation respectively at the dose of 0.075 mg/ml). Two methyl derivatives of quercetin obtained by synthesis, quercetin 3,7,3',4'-tetramethylether and quercetin 3,5,7,3',4'-pentamethylether, showed similar relaxant effects at the dose of 0.075 mg/ml (86.4 +/- 8.5 and 67.31 +/- 1.4% of relaxation respectively). The results show that the ethanol extract of A. satureioides and the assayed compounds exhibit significant vasorelaxing properties. Additionally, it is shown that the number of methyl groups in the quercetin nucleus has no significant influence on the effectiveness of these compounds.

Efficacy of DA-8159, a new PDE5 inhibitor, for inducing penile erection in rabbits with acute spinal cord injury

DA-8159 is a pyrazolopyrimidinone derivative which exhibits potent and selective phosphodiesterase type 5 (PDE5) inhibition. The aim of this study was to investigate the effects of DA-8159 on inducing a penile erection in rabbits with an acute spinal cord injury (ASCI). DA-8159 was given either orally (1, 3, or 10 mg/kg) or intravenously (0.1 or 0.3 mg/kg) to conscious male albino rabbits with a surgical transection of the spinal cord at the L2-L4 lumbar vertebra or ischemic-reperfusion SCI rabbits. Erection was evaluated in a time-course manner by measuring the length of the uncovered penile mucosa. DA-8159 induced a dose-dependent erection in both transection and ischemic-reperfusion ASCI rabbits. The efficacy of DA-8159 was potentiated by an intravenous injection of sodium nitroprusside, a nitric oxide donor. Potentiation of the effect by nitric oxide donor implies that DA-8159 can enhance the erectile activity during sexual arousal. These results suggest that DA-8159 may be useful for treating erectile dysfunction in patients with an SCI.

Haemodynamic effects of the selective phosphodiesterase 5 inhibitor, UK-357,903, in conscious SHR

1. Regional haemodynamic responses to a continuous, 4-day infusion of the selective phosphodiesterase type 5 inhibitor, UK-357,903 (0.133 or 1.33 mg x kg(-1) h(-1)) were measured in conscious spontaneously hypertensive rats, and compared with those of enalapril (1 mg x kg(-1) h(-1)). 2. Both doses of UK-357,903 caused modest reductions in mean blood pressure that were not dose-dependent and only significantly different from the vehicle effects on Day 1 of the study (mean -11.8 and -15.3 mmHg for low and high doses, respectively). UK-357,903 had mesenteric and hindquarters vasodilator effects, which were, again, similar for both dose levels and only significantly different from vehicle on Day 1. Neither dose of UK-357,903 affected renal vascular conductance or heart rate. 3. Although the haemodynamic effects of UK-357,903 were not clearly dose-related and some appeared to wane with time, geometric mean plasma levels of UK-357,903 increased in proportion to dose, and were sustained throughout the infusion period. Furthermore, plasma cyclic guanosine monophosphate, a biomarker of phosphodiesterase 5 inhibition, was persistently elevated, and increased with increasing dose. 4. Enalapril caused a fall in mean blood pressure on day 1 (-14.1 mmHg) that was associated with dilatation in renal, mesenteric and hindquarters vascular beds. The haemodynamic effects of enalapril were sustained or increased over the 4-day infusion, although plasma free drug levels were stable. 5. In conclusion, we have shown regional and temporal changes in the haemodynamic effects of UK-357,903, which may be due to activation of compensatory mechanisms, but there were no signs of functional compensation to the cardiovascular effects of enalapril.

The L-arginine/nitric oxide/cyclic-GMP pathway apparently mediates the peripheral antihyperalgesic action of fentanyl in rats

There are only a few studies on the molecular mechanisms underlying the peripheral antihyperalgesic effect of opioids. The aim of this study was to investigate the molecular bases of the peripheral antihyperalgesic effect of fentanyl in a model of prostaglandin-induced chemical hyperalgesia. Prostaglandin E2 (1.4 nmol) injected into one hind paw of male Wistar rats (200-250 g, N = 6 in each experimental or control group) pretreated with indomethacin (2.5 mg/kg) potentiated the nocifensive response to formalin (1%) injection made 60 min later. Drugs applied locally 30 min after prostaglandin E2 induced the following effects: fentanyl (0.1-1.0 nmol) caused a dose-dependent reversal of the hyperalgesic state, naloxone (2 nmol) co-injected with fentanyl (1 nmol) completely reversed the antihyperalgesic effect, Nomega-nitro-L-arginine (NOARG, 0.05-0.2 mol) in combination with fentanyl (1.0 nmol) caused a dose-dependent inhibition of the antihyperalgesic effect of fentanyl, co-administration of L-arginine (0.5 mol) with NOARG (0.2 mol) plus fentanyl (1.0 nmol) fully restored the antihyperalgesic effect, and the cyclic-GMP phosphodiesterase inhibitor UK-114,542-27 (5-[2-ethoxy-5-(morpholinylacetyl) phenyl]-1,6-dihydro-1-methyl-3-propyl-7H-pyrazolo [4,3-d]-pyrimidin-7-one methanesulfonate monohydrate; 0.5-2.0 mol) potentiated a subeffective dose of fentanyl (0.1 nmol) in a dose-dependent manner. However, UK-114,542-27 (2.0 mol) injected alone did not produce this antihyperalgesic effect. Systemically administered fentanyl (1.0 nmol, sc) did not cause antinociception. Taken together, these results support the view that fentanyl reverses prostaglandin E2-induced hyperalgesia, probably by activating an opioid receptor at the periphery, and furthermore the L-arginine/nitric oxide/cyclic-GMP pathway may mediate this peripheral effect of fentanyl.

Effect of phosphodiesterase-5 inhibitor, sildenafil (Viagra), in animal models of airways disease

Phosphodiesterase (PDE)-5 degrades guanosine 3',5'cyclic monophosphate (cGMP) and its inhibitor sildenafil citrate (Viagra) treats erectile dysfunction by smooth muscle relaxation through elevated cGMP. Sildenafil was examined in two guinea pig models of airways disease: guinea pigs exposed to LPS or sensitized guinea pigs with atopy exposed to ovalbumen. Ovalbumen exposure caused early- and late-phase bronchoconstrictor responses, measured in conscious animals by whole-body plethysmography. Twenty-four hours after ovalbumen exposure there was airway hyperreactivity (AHR) to inhaled histamine and significantly elevated macrophages, eosinophils, and nitric oxide (NO) metabolites in bronchoalveolar lavage fluid. Sildenafil treatment (1 mg/kg, intraperitoneally) failed to affect the early and late responses but significantly reduced AHR, leukocyte influx, and elevated NO. LPS exposure (30 microg/ml) caused AHR to histamine at 1 hour and macrophage, eosinophil, and neutrophil influx at 24 hours with raised NO. Sildenafil pretreatment inhibited LPS-induced AHR, leukocyte influx, and NO generation. The effectiveness of sildenafil was not dependent on endogenous NO because inhibition of NO synthase with Nomega-nitro-L-arginine methyl ester did not prevent its action. Inhibition of PDE5 by sildenafil was confirmed by elevated S-nitroso-N-acetylpenicillamine-induced cGMP generation in isolated lungs. These antiinflammatory actions of sildenafil in guinea pig models suggest that PDE5 inhibitors may have potential in treating airways disease.

Argentinian plant extracts with relaxant effect on the smooth muscle of the corpus cavernosum of guinea pig

Extracts of different polarity from Baccharis trimera, Haplopappus rigidus Huperzia saururus, Maytenus ilicifolia, Satureja parvifolia and Senecio eriophyton were tested for their relaxant activity on smooth muscle using L-phenylephrine precontracted strips of corpus cavernosum obtained from Guinea pigs. Highly significant and dose dependent results were obtained with the dichloromethane extracts of H. saururus (87% of relaxation at the dose of 10 mg/ml), S. parvifolia (95% of relaxation at 2.5 mg/ml) and S. eriophyton (94% of relaxation at 5 mg/ml). Similar effects were observed with the methanol extracts of H. saururus (88% of relaxation at 10 mg/ml) and S. parvifolia (84% of relaxation at 10 mg/ml). These results were comparable to those obtained with the dichloromethane and methanol extracts of the well known Mexican species Turnera diffusa. Moreover, the aqueous extract of H. rigidus and the aqueous and methanol extracts of S. eriophyton were highly effective in a dose dependent manner (more than 90% of relaxation at the dose of 10 mg/ml). Significant results, but with a lower overall relaxant activity (about 70% of relaxation at 10 mg/ml), could also be obtained with the aqueous extract of S. parvifolia and with the dichlormethane and methanol extracts of B. trimera and M. ilicifolia. The positive controls with Sildenafil citrate at doses ranging from 0.35 to 35 microg/ml yielded moderate effects (up to 46% of relaxation at 35 microg/ml). The effects observed in the present study seem to validate the folk medicinal use of the tested plants and open new ways in the search for natural products with vasodilatory effects.

Single step isolation of sildenafil from commercially available Viagra™ tablets

Sildenafil, the active ingredient in Viagra, has been purified from commercially available tablets. The purification, using Sephadex G25 chromatography under conditions of low ionic strength, is simple and inexpensive. Sildenafil purified according to this protocol has been characterized with respect to its IC50 for PDE5, its ultraviolet absorption profile, and by collision-induced dissociation fingerprinting, positive ion nanospray, and MALDI mass spectrometry. Tritated sildenafil (6 Ci/mmol) was prepared commercially using the sildenafil purified by this protocol and was verified to retain the potency of unlabeled sildenafil. This protocol and similar procedures will allow investigators to easily isolate sufficient amounts of sildenafil or other PDE5 inhibitors for conducting biochemical and in vitro studies of drug action.