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Sildenafil

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Sildenafil Basic information
Sildenafil Chemical Properties
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Sildenafil Usage And Synthesis
  • Uses

    Sildenafil, the first US FDA-approved, oral phosphodiesterase type-5 inhibitor, has revolutionized the treatment of erectile dysfunction sine its approval in 1998. Since sildenafil is a potent inhibitor of cyclic guanosine monophosphate in the corpus cavernosum and therefore increases the penile response to sexual stimulation.

  • Indications and UsageSildenafil Citrate is an oral drug used to treat male erectile dysfunction and is a 5-phosphodiesterase inhibitor developed by the American Pfizer Pharmaceuticals to originally treat cardiovascular disease that was later discovered to improve patients’ sex lives. Sildenafil Citrate treats erectile dysfunction and was the first clinical oral drug used to specifically target male erectile dysfunction.
  • Adverse ReactionsCommon adverse reactions include headache, dizziness, facial flushing, indigestion, nasal congestion, and sight abnormalities, which may be expressed as difficulty differentiation blue/green colors, light sensitivity, or blurry vision. More severe adverse effects include decreased supine blood pressure and decreased cardiac output. Additionally, clinical research shows that engaging in sexual activity after taking Sildenafil Citrate increases the risk of heart abnormalities, including angina pectoris, dizziness, nausea, and other symptoms, and it may also cause sudden cardiac death.
  • DescriptionSildenafil was launched as Viagra in the US for the treatment of organic orland psychological male erectile dysfunction (ED). Sildenafil can be obtained by functional rearrangement of the corresponding 5-aryl 1,3-dialkyl pyrazolo[4,3- d]pyrimidin-7-one, itself synthesized in a seven-step sequence from a pyrazole- 5-carboxylate. Sildenafil is a potent and selective inhibitor of type V cGMP phosphodiesterases (PDE5) ; IC50 = 3nM on isozymes from human corpus cavernosum tissue. This orally-active therapy is completely new and presents advantages over the classically recommended options such as vacuum constriction devices, drug injection or prosthesis implantation ; for these reasons, this first drug is likely to have a large acceptance fot the treatment of male ED. In experiments with incubated rabbit penile tissue, Sildenafil was shown to cause accumulation of cGMP. By inhibiting the enzyme PDE5, the predominant isozyme in the corpus cavernosum, Sildenafil induces an elevation of levels of second messager cGMP, which is involved in the regulation of vascular tone ; it was suggested that the specific elevation of cGMP due to Sildefanil would mediate an enhancement of nitric oxide-dependent relaxation of corpus cavernosal tissue. Several clinical studies using 10-100 mg Sildenafil have confirmed a good effectiveness and tolerability in healthy males. New trials in women with sexual disfunction have been initiated and positive results could enlarge the potential market of this drug.
  • Chemical PropertiesWhite Solid
  • OriginatorPfizer (UK)
  • Usesantiviral
  • UsesAn orally active selective type 5 cGMP phosphodiesterase inhibitor.
  • DefinitionChEBI: A pyrazolo[4,3-d]pyrimidin-7-one having a methyl substituent at the 1-position, a propyl substituent at the 3-position and a 2-ethoxy-5-[(4-methylpiperazin-1-yl)sulfonyl]phenyl group at the 5-position.
  • IndicationsSildenafil (Viagra) is a selective inhibitor of PD-5, an enzyme that inactivates cGMP. Vardenifil (Levitra) is a particularly effective inhibitor of PD-5. It has a shorter onset of action and can be used in smaller doses than sildenafil. Other drugs used in the treatment of ED exert their effects through other biochemical pathways, both central and peripheral.
  • IndicationsSildenafil (Viagra) was developed more than 10 years ago as an antihypertensive and antianginal drug. It proved ineffective in these applications but was shown to affect the smooth muscles of the penis.
  • Manufacturing ProcessA mixture of 3-n-propylpyrazole-5-carboxylic acid ethyl ester (24.1 g, 0.132 mol) (prepared by the method of Chem. Pharm. Bull., 1984, 32, 1568) and dimethyl sulfate (16.8 g, 0.133 mol) were heated to 90°C for 2.5 h. The mixture was dissolved in dichloromethane and the solution washed with sodium carbonate solution. The organic phase was separated, dried (MgSO4) and evaporated under vacuum to give a solid. Chromatography on silica gel (300 g), eluting with dichloromethane gave the 1-methyl-3-n-propylpyrazole- 5-carboxylic acid ethyl ester as a colourless oil (20.4 g, 79%).
    1-Methyl-3-n-propylpyrazole-5-carboxylic acid ethyl ester (20.2 g, 0.10 mol) was suspended in 6 N aqueous sodium hydroxide solution (50 ml, 0.30 mol). The mixture was heated to 80°C for 2 h then diluted with water (50 ml) and acidified with concentrated hydrochloric acid (25 ml). Filtration gave the 1- methyl-3-n-propylpyrazole-5-carboxylic acid as pale brown crystals (12.3 g, 71%), melting point 150°-154°C.
    1-Methyl-3-n-propylpyrazole-5-carboxylic acid (12.1 g, 0.072 mol) was added portionwise to a mixture of oleum (13 ml) and fuming nitric acid (11 ml), keeping the temperature below 60°C. After the addition, the mixture was heated at 60°C overnight and then cooled to room temperature before being poured onto ice. Filtration of the precipitate gave the 1-methyl-4-nitro-3-npropylpyrazole- 5-carboxylic acid as a white solid (11.5 g, 75%), melting point 124°-127°C.
    1-Methyl-4-nitro-3-n-propylpyrazole-5-carboxylic acid (11.3 g, 0.053 mol) was added to thionyl chloride (50 ml) and the resulting mixture heated under reflux for 3 h. The reaction mixture was then cooled and excess thionyl chloride removed by evaporation under vacuum. The oily residue was dissolved in acetone (50 ml) and the solution cautiously added to a mixture of ice (50 g) and concentrated aqueous ammonium hydroxide solution (50 ml). The precipitate was collected by filtration to provide the 1-methyl-4-nitro-3-npropylpyrazole- 5-carboxamide as a pale yellow solid (8.77 g, 78%), melting point 141°-143°C.
    1-Methyl-4-nito-3-n-propylpyrazole-5-carboxamide (3.45 g, 16.2 mmol) and stannous chloride dihydrate (18.4 g, 81 mmol) were suspended in ethanol and the mixture heated under reflux for 2 h. The resulting solution was cooled to room temperature, basified to pH 9 by the addition of 2 N aqueous sodium hydroxide solution and extracted with dichloromethane (3 x 150 ml). The organic extracts were combined, dried (MgSO4) and evaporated under vacuum. Trituration of the residue with ether gave the 4-amino-1-methyl-3-npropylpyrazole- 5-carboxamide as an off-white solid (2.77 g, 94%), melting point 98°-101°C.
    A solution of 2-ethoxybenzoyl chloride (6.1 g, 33.0 mmol) in dichloromethane (50 ml) was added to a stirred solution of 4-amino-1-methyl-3-npropylpyrazole- 5-carboxamide (3.0 g, 16.4 mmol), 4-dimethylaminopyridine (0.02 g, 0.1 64 mmol) and triethylamine (3.34 g, 33.0 mmol) in dichloromethane (50 ml) at 0°C. The resulting mixture was allowed to warm to room temperature and stirred for a further 2 h. The solvent was evaporated under vacuum, the residue dissolved in a 19:1 mixture of dichloromethane and methanol (250 ml), and then the solution washed with 1 N hydrochloric acid (100 ml), dried (MgSO4) and evaporated under vacuum. The crude material was chromatographed on silica gel (200 g), eluting with a 97:3 mixture of dichloromethane and methanol, to give a pink solid; crystallisation from ethyl acetate-hexane gave the 4-(2-ethoxybenzamido)-1-methyl-3-npropylpyrazole- 5-carboxamide as a pale pink solid (2.2 g, 40%), melting point 153°-155°C.
    4-(2-Ethoxybenzamido)-1-methyl-3-n-propylpyrazole-5-carboxamide (223 g, 0.676 mol) was added portionwise to a solution of sodium hydroxide (54 g, 1.35 mol) and 30% hydrogen peroxide solution (224 ml) in water (2000 ml). Ethanol (700 ml) was added and the resulting mixture heated under reflux for 2.5 h, cooled, then evaporated under vacuum. The resulting solid was treated with 2 N hydrochloric acid (380 ml), with external cooling, and the mixture was extracted with dichloromethane (1 x 700 ml, 3 x 200 ml). The combined organic extracts were washed successively with saturated aqueous sodium carbonate solution (3 x 400 ml) and brine (300 ml), then dried (Na2SO4) and evaporated under vacuum. Chromatography of the residue on silica gel (1000 g), using a methanol in dichloromethane elution gradient (0-1%), followed by trituration of the crude product with ether (300 ml), gave the 5-(2- ethoxyphenyl)-1-methyl-3-n-propyl-l,6-dihydro-7H-pyrazolo[4,3-d]pyrimidin- 7-one as a colourless solid (152.2 g, 72%), melting point 143°-146°C.
    5-(2-Ethoxyphenyl)-1-methyl-3-n-propyl-l,6-dihydro-7H-pyrazolo[4,3- d]pyrimidin-7-one (10.0 g, 32.1 mmol) was added portionwise to chlorosulfonic acid (20 ml) at 0°C under a nitrogen atmosphere. After being stirred overnight, the reaction solution was cautiously added to ice-water (150ml) and the aqueous mixture extracted with a 9:1 mixture of dichloromethane and methanol (4 x 100 ml). The combined extracts were dried (Na2SO4) and evaporated under vacuum to give the required 5-(5-chlorosulphonyl-2- ethoxyphenyl)-1-methyl-3-n-propyl-1,6-dihydro-7H-pyrazolo[4,3-d]pyrimidin- 7-one as a white solid (12.8 g, 97%), melting point 179°-181°C.
    4-Methylpiperidine was added to a stirred suspension of 5-(5-chlorosulphonyl- 2-ethoxyphenyl)-1-methyl-3-n-propyl-1,6-dihydro-7H-pyrazolo[4,3- d]pyrimidin-7-one in ethanol at room temperature. The resulting mixture was stirred for 4 days before removing the solvent by evaporation under vacuum. The residue was dissolved in a 9:1 mixture of dichloromethane and methanol and the solution washed with saturated aqueous sodium carbonate solution. The aqueous phase was further extracted with dichloromethane-methanol mixtures (3 x 100 ml) and all the organic fractions were combined, dried (MgSO4) and evaporated under vacuum to give a solid. Crystallisation from a mixture of methanol-dimethylformamide gave the 5-[2-ethoxy-5-(4- methylpiperidinylsulphonyl)phenyl]-1-methyl-3-n-propyl-1,6-dihydro-7Hpyrazolo[ 4,3-d]-pyrimidin-7-one as an off-white solid, melting point 187°- 189°C.
    After addition of citric acid to the 5-[2-ethoxy-5-(4- methylpiperidinylsulphonyl)phenyl]-1-methyl-3-n-propyl-1,6-dihydro-7Hpyrazolo[ 4,3-d]-pyrimidin-7-one (sildenafil) the it's salt is obtained, namely sildenafil citrate.
  • brand nameViagra (Pfizer);Segurex.
  • Therapeutic FunctionVasodilator
  • Mechanism of actionSildenafil is readily absorbed after oral administration and reaches peak plasma levels after about an hour. It undergoes hepatic metabolism and has a terminal half-life of about 4 hours.An initial dose of 50 mg is taken about an hour prior to sexual activity to induce penile erection.
  • Clinical UseSildenafil is a selective inhibitor of cGMP-specific PD-5 and therefore inhibits the degradation of cGMP. PD-5, the predominant type in the corpus cavernosum, also is present in other tissues (e.g., lungs, platelets, and eye). The selective inhibition of this enzyme facilitates the release of nitric oxide and smooth muscle relaxation of the corpus cavernosa. Sildenafil enhances erection by augmenting nitric oxide–mediated relaxation pathways. It has been suggested that sildenafil’s mechanism of action is due to cross-talk between cGMP- and cAMPdependent transduction pathways within the cavernous muscles.
  • Side effectsOrally administered sildenafil is an effective and well-tolerated treatment for men with ED, including those with diabetes mellitus. It has also been used for so-called salvage therapy in men who do not respond to intracorporeal injections of other agents. Headache is a common side effect, as are flushing and rhinitis.More serious side effects include definite or suspected myocardial infarctions and cardiac arrest.
  • Enzyme inhibitorSildenafil is rapidly absorbed and peaks in concentration (127–560 ng/mL) after 0.5 to 2.0 hours, displaying a half-life of 3 to 4 hours for the full therapeutic dose (25–100 mg). It is 96% bound to plasma proteins and is metabolized by the liver CYP3A4. The metabolite N-desmethylsildenafil possesses approximately 50% of the activity of the parent molecule.
  • Metabolism In vitro metabolism studies for sildenafil have shown that the primary metabolite, N-desmethylsildenafil, and the minor metabolite, oxidative opening of the piperazine ring, are mediated by CYP3A4, CYP2C9, CYP2C19, and CYP2D6. The estimated relative contributions to clearance were 79% for CYP3A4, 20% for CYP2C9, and less than 2% for CYP2C19 and CYP2D6. These results demonstrate that CYP3A4 is the primary cytochrome mediating N-demethylation and that drugs inhibiting CYP3A4 likely impair sildenafil biotransformation and clearance. The pharmacokinetics of radiolabeled sildenafil were consistent with rapid absorption, first-pass metabolism, and primarily fecal elimination of N-demethylated metabolites. The absorption of sildenafil following oral administration was rapid (~92%), whereas the oral bioavailability was approximately 38% as a result of first-pass metabolism.
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