Sildenafil citrate

Description

Sildenafil is a potent inhibitor of phosphodiesterase 5 (PDE5) with IC₅₀ values of 3.6 and 3 nM for PDE5 activity in isolated rabbit platelets and human corpus cavernosum, respectively. It is selective for PDE5 over PDE1 and PDE3 (IC₅₀s = 0.26 and 65 μM, respectively). Sildenafil reverses glucose-induced decreases in angiopoietin 1 (ANG1) expression and reduction of capillary-like tube formation by mouse dermal endothelial cells in vitro and increases the number of functional blood vessels and regional blood flow in the sciatic nerve in a db/db mouse model of diabetic peripheral neuropathy. It increases the ratio of maximum intracavernosal pressure to mean arterial blood pressure (ICP/MAP), a measure of erectile function, in castrated rats when administered at a dose of 20 mg/kg per day. Sildenafil (0.5 mg/kg) also reduces cardiac arrest and resuscitation-induced increases in angiotensin II , angiotensin converting enzyme (ACE), ACE2, and various angiotensin receptors and increases survival in a porcine model of ischemia/reperfusion injury. Formulations containing sildenafil have been used in the treatment of erectile dysfunction, pulmonary arterial hypertension, and high-altitude pulmonary edema associated with altitude sickness.

Chemical properties

Sildenafil citrate is a white to off-white crystalline powder soluble in DMF, acetic acid and slightly soluble in methanol. Solubility of sildenafil citrate is pH dependent and it decreases with increase of pH. pH ranges between 3.7 and 3.8 and the pKa from 8.2 to 9.6.

Originator

Alsigra,Alembic Ltd.,India

The Uses of Sildenafil citrate

Sildenafil citrate is an orally active selective type 5 cgmp phosphodiesterase inhibitor that is used in the treatment of erectile dysfunction and primary pulmonary hypertension.

Definition

ChEBI: Sildenafil citrate is the citrate salt of sildenafil. It has a role as a vasodilator agent and an EC 3.1.4.35 (3',5'-cyclic-GMP phosphodiesterase) inhibitor. It contains a sildenafil.

Manufacturing Process

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 name

Viagra (Pfizer).

Therapeutic Function

Vasodilator

General Description

Sildenafil Citrate is the citrate salt form of sildenafil, an orally bioavailable pyrazolopyrimidinone derivative structurally related to zaprinast, with vasodilating and potential anti-inflammatory activities. Upon oral administration, sildenafil selectively targets and inhibits cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5), thereby inhibiting the PDE5-mediated degradation of cGMP found in smooth muscle and increasing cGMP availability. This results in prolonged smooth muscle relaxation in the corpus cavernosum of the penis, thereby causing vasodilation, blood engorgement and a prolonged penile erection.  In the smooth muscle of the pulmonary vasculature, the increase in cGMP results in smooth muscle relaxation, vasodilation of the pulmonary vascular bed, relieving pulmonary hypertension and increasing blood flow in the lungs. In addition, sildenafil may reduce airway inflammation and mucus production.

Biological Activity

Orally active, potent inhibitor of phosphodiesterase 5 (PDE5) (IC 50 = 4 nM). Enhances nitric oxide-dependent relaxation of human corpus cavernosum in vitro .

Biochem/physiol Actions

Sildenafil is a potent, selective inhibitor of cGMP specific phosphodiesterase type 5 (PDE5). Sildenafil is used to treat erectile dysfunction and pulmonary arterial hypertension. NO activates guanylate cyclase, which results in increased levels of cGMP, producing smooth muscle relaxation. Sildenafil enhances the effect of NO by inhibiting PDE5, which is responsible for degradation of cGMP.

Pharmacokinetics

Analysis of Middle Eastern data (mean ± SD) revealed Cmax = 398.9 ± 107.7 ng/ml; Tmax = 1.84 ± 0.22 h; t1/2 = 2.66 ± 0.97 h; AUC0–24 = 1475 ± 515.3 ng.h/ml; AUC0-∞ = 1556 ± 567.58 ng.h/ml.

Side Effects

The more common side effects of sildenafil citrate include: headache, skin flushing, indigestion, abnormal vision, nasal congestion, back pain, nausea, dizziness, rash, etc.

Safety Profile

A poison by ingestion. Human systemic effects. When heated to decomposition it emits toxic vapors of NOx and SOx.

Synthesis

The synthesis of sildenafil citrate was first reported in the Bioorganic & Medicinal Chemistry Letters, Vol 6, pp. 1819, 1824, 1996. The reaction scheme is reproduced below. Sildenafil was reported in this journal as "a potent and selective inhibitor of type 5 PDE with utility for the treatment of male erectile dysfunction".

The first step of the synthesis is the reaction of a diketoester (1) and hydrazine to give the pyrazole ring. The regioselective N-methylation of the pyrazole and hydrolysis gives a carboxylic acid (3). Compound (3) is then reacted with HNO3 and H2SO4 to give a nitrated product.
This is then followed by a carboxamide formation and the reduction of the nitro group. The compound (4) is then acylated under basic conditions and this produces the pyrazolopyrimidinone (6). (6) is then chlorosulphonylated selectively on the 5'-position of the phenyl ring. This can then couple with an amine to give sildenafil (7).
www.ch.ic.ac.uk/local/projects/p_hazel/synthesis2.html

Veterinary Drugs and Treatments

Sildenafil may be of benefit in the adjunctive treatment of pulmonary hypertension in small animals.
In humans, sildenafil is indicated for erectile dysfunction or pulmonary hypertension.

storage

Desiccate at RT

Side Effects

During taking Sildenafil citrate, dizziness, headache, flushing, or stomach upset may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green colors apart may also occur.

structure and hydrogen bonding

Sildenafil citrate (SC) has been widely used for the treatment of erectile disorder. A detailed study concerning solid-state structure of this compound is very important for understanding enzyme (PDE5)-inhibitor (sildenafil) interaction. It is also of interest to determine sildenafil’s protonation sites, as they may be responsible for its binding to the phosphodiesterase acidic amino acids.
Sildenafil citrate (Viagra) and sildenafil base in pure form were characterized by 1H, 13C, 15N NMR spectroscopy in solution, solid-state, and pharmaceutical dosage forms.42 The analysis of chemical shifts showed that: (i) N6-H forms intramolecular hydrogen bonds, (ii) N25 is protonated in the salt, and (iii) intermolecular OH. . .N hydrogen bonds involving N2 and N4 are present in the solid sildenafil citrate. The 13C CPMAS spectra of the tablets containing different amounts of sildenafil citrate were recorded and showed that chemical shifts of sildenafil citrate in pure form and in pharmaceutical dosage forms are the same. SC is easily detected in the pharmaceutical dosage forms since only two of its carbon resonances (OCH2 and quaternary carbon of the citrate anion) fall into carbohydrate-type region of the excipient.
Solid-state 13C and 15N MAS NMR have recently been used to investigate how water interacts with SC.43 When the humidity is altered, the water concentration in the solid compound changes but does not reach a stoichiometric (e.g., 1:1) ratio to form a true hydrate. Only one set of 15N and 13C signals was observed for each humidity level indicating that water incorporated into the crystal lattice of SC is very mobile and exchanges rapidly between various sites. The 13C data showed the formation of a hydrogen bond between water molecule and one carbonyl of the citrate anion. The spectra also show that the water content affects the conformation of the propyl group. Additionally, 15N dipolar dephasing (DD) experiments confirmed that the sildenafil molecule is only protonated in the piperazine ring.