Phosphodiesterase type 5 (PDE5) inhibitors

Erectile Dysfunction: Treatments, Advances and New Therapeutic Strategies

Erectile dysfunction (ED) is the inability to get and maintain an adequate penile erection for satisfactory sexual intercourse. Due to its negative impacts on men's life quality and increase during aging (40% of men between 40 and 70 years), ED has always attracted researchers of different disciplines, from urology, andrology and neuropharmacology to regenerative medicine, and vascular and prosthesis implant surgery. Locally and/or centrally acting drugs are used to treat ED, e.g., phosphodiesterase 5 inhibitors (first in the list) given orally, and phentolamine, prostaglandin E1 and papaverine injected intracavernously. Preclinical data also show that dopamine D4 receptor agonists, oxytocin and α-MSH analogues may have a role in ED treatment. However, since pro-erectile drugs are given on demand and are not always efficacious, new strategies are being tested for long lasting cures of ED. These include regenerative therapies, e.g., stem cells, plasma-enriched platelets and extracorporeal shock wave treatments to cure damaged erectile tissues. Although fascinating, these therapies are laborious, expensive and not easily reproducible. This leaves old vacuum erection devices and penile prostheses as the only way to get an artificial erection and sexual intercourse with intractable ED, with penile prosthesis used only by accurately selected patients.

Erectile Function and Sexual Behavior: A Review of the Role of Nitric Oxide in the Central Nervous System

Nitric oxide (NO), the neuromodulator/neurotransmitter formed from l-arginine by neuronal, endothelial and inducible NO synthases, is involved in numerous functions across the body, from the control of arterial blood pressure to penile erection, and at central level from energy homeostasis regulation to memory, learning and sexual behavior. The aim of this work is to review earlier studies showing that NO plays a role in erectile function and sexual behavior in the hypothalamus and its paraventricular nucleus and the medial preoptic area, and integrate these findings with those of recent studies on this matter. This revisitation shows that NO influences erectile function and sexual behavior in males and females by acting not only in the paraventricular nucleus and medial preoptic area but also in extrahypothalamic brain areas, often with different mechanisms. Most importantly, since these areas are strictly interconnected with the paraventricular nucleus and medial preoptic area, send to and receive neural projections from the spinal cord, in which sexual communication between brain and genital apparatus takes place, this review reveals that central NO participates in concert with neurotransmitters/neuropeptides to a neural circuit controlling both the consummatory (penile erection, copulation, lordosis) and appetitive components (sexual motivation, arousal, reward) of sexual behavior.

In Silico Mapping of Essential Residues in the Catalytic Domain of PDE5 Responsible for Stabilization of Its Commercial Inhibitors

Phosphodiesterase type 5 (PDE5) is an important enzyme associated with the hydrolysis of cyclic guanosine monophosphate (cGMP) to guanosine monophosphate (GMP). Due to the relevant role of second messenger cGMP as a mediator in many physiological processes, efforts have been converged to find a safe pharmacological approach, seeking a specific, selective and potent inhibitor of the PDE5 enzyme. There are five commercial drugs with potential for clinical use: tadalafil, sildenafil, avanafil, udenafil and vardenafil. Here, we applied molecular modeling to obtain different profiles of protein–ligand interactions by adopting distinct PDE5 structures, specifically PDBid:1XOZ and two extracted from molecular dynamics (MD) simulations. The results generated by molecular docking showed several possibilities for inhibitor interactions with the catalytic pocket. Tadalafil, sildenafil and vardenafil were clearly stabilized by Gln817 via a well-oriented hydrogen bond. Another set of different interactions, such as polar, hydrophobic, π-stacking, metal–ligand and electrostatic, were responsible for accommodating avanafil and udenafil. All of the ligands are discussed in detail with consideration of the distinct protein structures, and a profile of the probability of residue–ligand contact is suggested, with the most frequently observed being: Tyr612, His613, Ser661, Thr723, Asp724, Asp764, Leu765, Val782 and Phe786. The molecular interactions displayed herein confirm findings achieved by previous authors and also present new contacts. In addition, the discussion can help researchers obtain a molecular basis for planning new selective PDE5 inhibitors, as well as explain an inhibitor’s experimental assays by considering the specific interactions occurring at the catalytic site.

Pharmacological and molecular dynamics analyses of differences in inhibitor binding to human and nematode PDE4: Implications for management of parasitic nematodes

Novel chemical controls are needed that selectively target human, animal, and plant parasitic nematodes with reduced adverse effects on the host or the environment. We hypothesize that the phosphodiesterase (PDE) enzyme family represents a potential target for development of novel nematicides and anthelmintics. To test this, we identified six PDE families present in the nematode phylum that are orthologous to six of the eleven human PDE families. We characterized the binding interactions of family-selective PDE inhibitors with human and C. elegans PDE4 in conjunction with molecular dynamics (MD) simulations to evaluate differences in binding interactions of these inhibitors within the PDE4 catalytic domain. We observed that roflumilast (human PDE4-selective inhibitor) and zardaverine (selective for human PDE3 and PDE4) were 159- and 77-fold less potent, respectively, in inhibiting C. elegans PDE4. The pan-specific PDE inhibitor isobutyl methyl xanthine (IBMX) had similar affinity for nematode and human PDE4. Of 32 residues within 5 Å of the ligand binding site, five revealed significant differences in non-bonded interaction energies (van der Waals and electrostatic interaction energies) that could account for the differential binding affinities of roflumilast and zardaverine. One site (Phe506 in the human PDE4D3 amino acid sequence corresponding to Tyr253 in C. elegans PDE4) is predicted to alter the binding conformation of roflumilast and zardaverine (but not IBMX) into a less energetically favorable state for the nematode enzyme. The pharmacological differences in sensitivity to PDE4 inhibitors in conjunction with differences in the amino acids comprising the inhibitor binding sites of human and C. elegans PDE4 catalytic domains together support the feasibility of designing the next generation of anthelmintics/nematicides that could selectively bind to nematode PDEs.

Targeting the purinome

Purines are critical cofactors in the enzymatic reactions that create and maintain living organisms. In humans, there are approximately 3,266 proteins that utilize purine cofactors and these proteins constitute the so-called purinome. The human purinome encompasses a wide-ranging functional repertoire and many of these proteins are attractive drug targets. For example, it is estimated that 30% of modern drug discovery projects target protein kinases and that modulators of small G-proteins comprise more than 50% of currently marketed drugs. Given the importance of purine-binding proteins to drug discovery, the following review will discuss the forces that mediate protein:purine recognition, the factors that determine druggability of a protein target, and the process of structure-based drug design. A review of purine recognition in representatives of the various purine-binding protein families, as well as the challenges faced in targeting members of the purinome in drug discovery campaigns will also be given.

Pharmacokinetic and pharmacodynamic consequences of inhibition of terazosin metabolism via CYP3A1 and/or 3A2 by DA-8159, an erectogenic, in rats

BACKGROUND AND PURPOSE

Recently, orthostatic hypotension was observed in patients with benign prostatic hyperplasia who are taking vardenafil (a PDE 5 inhibitor) and terazosin (a long acting alpha blocker). Therefore, this study was performed with DA-8159 (a long acting PDE 5 inhibitor) and terazosin in rats to find whether or not pharmacokinetic and pharmacodynamic interactions between the two drugs were observed.

EXPERIMENTAL APPROACH

Pharmacokinetic and pharmacodynamic (changes in blood pressure) interactions between DA-8159 and terazosin were evaluated after simultaneous i.v. and p.o. administration of DA-8159 (30 mg kg(-1)) and terazosin (5 mg kg(-1)) to male Sprague-Dawley rats.

KEY RESULTS

After simultaneous i.v. and p.o. administration of terazosin and DA-8159, the total area under the plasma concentration-time curve from time zero to time infinity (AUC) of terazosin became significantly greater (57.4 and 75.4% increase for i.v. and p.o. administration, respectively) than those of without DA-8159. The blood pressure dropping effect was considerable after simultaneous p.o. administration of DA-8159 and terazosin compared with each drug alone.

CONCLUSIONS AND IMPLICATIONS

The significantly greater AUC of terazosin after both simultaneous i.v. and p.o. administration of both drugs could be due to the hepatic (both i.v. and p.o.) and intestinal (p.o.) inhibition of the metabolism of terazosin via CYP3A1 and/or 3A2 by DA-8159, since both DA-8159 and terazosin are metabolized via CYP3A1 and/or 3A2 in rats. The blood pressure lowering effect after simultaneous p.o. administration of both drugs could be due to significant increase in plasma concentrations of terazosin.

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

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

The Effects of Tamsulosin and Sildenafil in Separate and Combined Regimens on Detailed Hemodynamics in Patients With Benign Prostatic Enlargement

PURPOSE

We measured the detailed hemodynamic effects of tamsulosin and sildenafil separately and together in patients with benign prostatic enlargement.

MATERIALS AND METHODS

The supine effects of and responses to passive orthostasis (60 degrees for 8 minutes) were measured in 16 patients with benign prostatic enlargement with the finger blood pressure method and whole-body impedance cardiography. The medications, 100 mg sildenafil (single doses) and 0.4 mg tamsulosin (once daily for up to 14 days), were administered in a randomized, double-blind, crossover fashion.

RESULTS

Supine systolic arterial pressure decreased with sildenafil (mean +/- SEM -11 +/- 2 mm Hg) and sildenafil plus tamsulosin (-14 +/- 2 mm Hg) more than with placebo (-2 +/- 4 mm Hg, p <0.05). In comparison to placebo sildenafil plus tamsulosin decreased the systemic vascular resistance index (328 +/- 129 vs -241 +/- 134 dyn.sec/cm.m, p = 0.01). Tamsulosin alone did not cause any significant changes in comparison to placebo. Heart rate, diastolic arterial pressure, stroke index, cardiac index and arterial pulse wave velocity were not affected to a statistically significant degree by any of the treatments compared to placebo. Upon head-up tilt the drugs caused only 1 significant change in that diastolic arterial pressure was significantly higher (-2.7 vs -8.0 mm Hg, p = 0.04) in the placebo group than in the tamsulosin plus sildenafil group.

CONCLUSIONS

Tamsulosin does not disturb hemodynamics in patients with benign prostatic enlargement. Sildenafil decreases blood pressure with the patient supine but not during head-up tilt. The combination treatment also decreases the systemic vascular resistance index in the supine position.

Hemodynamic interaction study between the alpha1-blocker alfuzosin and the phosphodiesterase-5 inhibitor tadalafil in middle-aged healthy male subjects

OBJECTIVES

To examine the hemodynamic interactions of the phosphodiesterase type 5 (PDE-5) inhibitor tadalafil with the uroselective alpha1-blocker alfuzosin (10 mg daily), commonly prescribed for benign prostatic hyperplasia-related lower urinary tract symptoms. Erectile dysfunction is commonly associated with lower urinary tract symptoms. PDE-5 inhibitors are the first-line treatment of choice for erectile dysfunction. When co-administered with alpha1-blockers, PDE-5 inhibitors could induce orthostatic hypotension.

METHODS

During each of the two periods of a randomized, double-blind, placebo-controlled, crossover study, 18 healthy middle-aged men received alfuzosin 10 mg daily for 7 days and either a single 20-mg dose of tadalafil or placebo on day 7. The blood pressure and heart rate were monitored before and for 24 hours after tadalafil or placebo.

RESULTS

The combination of tadalafil 20 mg with alfuzosin 10 mg daily elicited a maximal decrease in standing systolic blood pressure that was not significantly different from that after placebo (mean difference 4.35 mm Hg, P = nonsignificant). Analysis of the blood pressure outliers showed that only 1 subject had an asymptomatic standing systolic blood pressure of less than 85 mm Hg. No vasodilatory adverse events were observed with the combined medication.

CONCLUSIONS

In healthy, middle-aged men, tadalafil 20 mg showed no clinically relevant hemodynamic interactions with alfuzosin 10 mg daily.

Effect of sildenafil on gastric emptying in healthy adults

BACKGROUND AND AIM

Phosphodiesterase type 5 hydrolyzes and inactivates cyclic guanosine monophosphate produced by the nitric oxide-stimulated guanylate cyclase. Sildenafil is a potent, reversible, and highly selective inhibitor of this phosphodiesterase. It causes smooth muscle relaxation by increasing intracellular concentrations of cyclic guanosine monophosphate. The aim of this study was to test the hypothesis that sildenafil alters gastric emptying and the intragastric distribution of food in healthy adults.

METHODS

Nine normal subjects (mean age 28 years, range 25-33) were given a placebo or a tablet of sildenafil (50 mg) at different times along with radio-opaque markers. A gastric emptying scan was used to calculate the t(1/2) for gastric emptying (the time taken for the initial radioactivity to fall by 50%). Intragastric food distribution was also assessed using the gastric emptying scan to calculate proximal gastric emptying t(1/2), the proximal volume (the highest activity value in the proximal stomach at any time point in the study), and the distal volume (the highest activity value in the distal stomach at any time point in the study. Gastric emptying of an indigestible solid meal was assessed by calculating gastric clearance of radio-opaque markers.

RESULTS

Sildenafil did not change total gastric emptying or gastric clearance of radio-opaque markers. It shortened the proximal T(1/2), decreased proximal volume, and significantly increased distal volume.

CONCLUSION

Sildenafil alters the intragastric distribution of food rather than causing gastric stasis.

Phosphodiesterase 5 (PDE 5) inhibitors for the treatment of male erectile disorder: Attaining selectivity versus PDE6

The role of phosphodiesterase type 5 (PDE5) in the mechanism of male erection has been well understood, and several drugs inhibiting this enzyme are being used for the treatment of erectile dysfunction (ED). Discovery of inhibitors with improved selectivity versus other PDE isozymes could lead to drugs with improved safety profile. Achievement of selectivity versus PDE6, co-inhibition of which results in disturbances of color perception, remains the most challenging aspect of current drug discovery programs. The present review describes several case studies, where significant (>100 fold) selectivity versus PDE6 has been attained via investigation of structure-activity relationships (SAR). Special attention is given to the chemical routes leading to novel chemotypes and allowing efficient exploration of their SAR's. Strategies for attaining inhibitor selectivity discussed below may be applicable for other drug discovery programs.

Optimization of purine based PDE1/PDE5 inhibitors to a potent and selective PDE5 inhibitor for the treatment of male ED

In search of a PDE5 inhibitor for erectile dysfunction, an SAR was developed from a PDE1/PDE5 purine series of leads, which had modest PDE5 potency and poor isozyme selectivity. A compound (41) with PDE5 inhibition and in vivo activity similar to sildenafil was discovered from this effort. In addition, purine 41 demonstrated superior overall PDE isozyme selectivity when compared to the approved PDE5 inhibitors sildenafil, vardenafil, and tadalafil, which may result in a more favorable side-effect profile.

INTERACTION BETWEEN THE PHOSPHODIESTERASE 5 INHIBITOR, TADALAFIL AND 2 α-BLOCKERS, DOXAZOSIN AND TAMSULOSIN IN HEALTHY NORMOTENSIVE MEN

PURPOSE

Tadalafil, a phosphodiesterase type 5 inhibitor, is effective therapy for erectile dysfunction (ED). Men with ED have a high incidence of comorbid conditions including cardiovascular disease, diabetes mellitus and benign prostatic hyperplasia. Although phosphodiesterase type 5 inhibitors are safe when administered with most medications, sildenafil given with doxazosin and vardenafil given with terazosin evoke orthostatic hypotension in some patients. We examined the hemodynamic interactions of tadalafil with the alpha-blockers doxazosin and tamsulosin.

MATERIALS AND METHODS

In separate double-blind, placebo controlled, randomized crossover studies (18 patients in each study) we evaluated the hemodynamic effects of doxazosin 8 mg with tadalafil 20 mg, and tamsulosin 0.4 mg with tadalafil 10 and 20 mg. Blood pressure (BP) and heart rate were recorded before dosing and for 24 hours after dosing.

RESULTS

Tadalafil 20 mg augmented the hypotensive effect of doxazosin by producing a mean maximal decrease in standing systolic BP (SBP) that was significantly greater than placebo (a mean difference of 9.8 mm Hg). Analysis of BP outliers showed that the number of subjects with a standing SBP of less than 85 mm Hg was greater after doxazosin plus tadalafil (28%) versus doxazosin plus placebo (6%). In subjects on tamsulosin, tadalafil 10 and 20 mg produced mean maximal decreases in standing SBP that were similar to placebo (mean difference of 1.7 and 2.3 mm Hg, respectively). No subject taking tamsulosin had a decrease in standing SBP less than 85 mm Hg.

CONCLUSIONS

Tadalafil augmented the hypotensive effects of doxazosin but had little hemodynamic interaction with tamsulosin. In patients taking tadalafil for ED, tamsulosin 0.4 mg may be given for the treatment of benign prostatic hyperplasia.