PDE5/phosphodiesterase 5
When 1 μM sildenafil was pretreated instead of serotonin stimulation alone, it improved phosphorylation of ERK1/ERK2, increased the percentage of cells in S phase, and promoted cell proliferation (P<0.05). The OD value increased dramatically to 0.33 after pretreatment with 1 μM sildenafil citrate and serotonin stimulation, which was significantly different from serotonin stimulation alone (P<0.05). The phosphorylation of ERK1/ERK2 elicited by serotonin was considerably increased by 1 μM sildenafil [2].

When 1 μM sildenafil was pretreated instead of serotonin stimulation alone, it improved phosphorylation of ERK1/ERK2, increased the percentage of cells in S phase, and promoted cell proliferation (P<0.05). The OD value increased dramatically to 0.33 after pretreatment with 1 μM sildenafil citrate and serotonin stimulation, which was significantly different from serotonin stimulation alone (P<0.05). The phosphorylation of ERK1/ERK2 elicited by serotonin was considerably increased by 1 μM sildenafil [2].

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Sildenafil (1 μmol/L) potentiated serotonin (10 μmol/L)-induced proliferation of porcine pulmonary artery smooth muscle cells (PASMCs), as shown by increased optical density in MTT assay, increased percentage of cells in S phase, and enhanced expression of proliferating cell nuclear antigen (PCNA).
Sildenafil enhanced serotonin-induced phosphorylation of ERK1/ERK2, which was prevented by pretreatment with the MEK inhibitor U0126 (10 μmol/L).
Sildenafil did not upregulate MKP-1 expression at 1 μmol/L, which may contribute to its lack of antiproliferative effect in this context.[2]

Sildenafil considerably raised ICP and ICP/BP in a canine erection model, but it had no discernible effect on blood pressure when compared to the vehicle [1]. At a dose of 10 mg/kg, sildenafil therapy dramatically decreased the number of TL+- cells. Eight days after pMCAo, sildenafil therapy (0.5 and/or 10 mg/kg dosages) dramatically decreased the amount of microglia/macrophages stained by Iba-1 [3]. According to preclinical animal models, sildenafil can lessen flap necrosis by secreting more growth factors (FGF and VEGF). It has also been demonstrated histologically to be beneficial against cavernous neural structures in rats [4].

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In a neonatal mouse model of permanent middle cerebral artery occlusion (pMCAo) at postnatal day 9 (P9), a single intraperitoneal dose of sildenafil (10 mg/kg) administered 5 minutes after ischemia did not significantly alter early (up to 90 min) cerebral blood flow velocities or cortical perfusion compared to PBS.
Sildenafil treatment dose-dependently reduced mean cortical tissue loss measured 8 days after pMCAo: 16.4 ± 4.8% with 0.5 mg/kg and 11.0 ± 4.8% with 10 mg/kg, compared to 23.8 ± 7.5% in PBS-treated animals.
At 72 hours after pMCAo, sildenafil (10 mg/kg) significantly decreased the density of total microglia/macrophages (tomato lectin-positive cells) and altered their distribution. It increased the number of COX-2+ (M1-like marker) cells in the penumbra but not in the core.
At 8 days after pMCAo, sildenafil significantly reduced the total number of Iba-1+ microglia/macrophages. It increased the proportion of Iba-1+Arg-1+ double-positive M2-like cells at 10 mg/kg and decreased Iba-1+COX-2+ double-positive M1-like cells at both 0.5 and 10 mg/kg.
Gene expression analysis at 8 days post-ischemia showed that sildenafil (10 mg/kg) upregulated M1-like markers (CD32, CD86) and several M2-like markers (CD206, Arg-1, Lgals3, IL1-Rn) in the ipsilateral cortex compared to PBS treatment.[3]

All experiments on ERK1/ERK2 activation, MKP-1, PCNA expression, as well as cell proliferation and cell cycle analysis were performed on cells that were three days in culture at passages 3-5. Thereafter, cells were serum starved for three days in RPMI-1640 containing 0.2% FBS and 1% antibiotics. Cells were then exposed to serotonin or sildenafil at 1 μmol/L then serotonin, as indicated. In some experiments, cells were pretreated with 10 μmol/L of U0126 for 30 minutes before sildenafil and subsequently exposed to serotonin, as indicated. In control groups, an equal volume of phosphate buffered saline (PBS) was substituted for the reagents.[2]

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Immunoblotting analysis of ERK1/ERK2 phosphorylation status[2]
Subconfluent serum-starved cells were treated with serotonin or 1 μmol/L sildenafil then serotonin stimulation with or without U0126, as indicated above. Protein was extracted at the indicated time as described above. Phosphorylation of ERK1/ERK2 protein was examined by Western blotting. Briefly, equal amounts of proteins (15-20 μg) were separated by SDS-PAGE, transferred to polyvinylidene difluoride membranes, probed with anti-phospho-ERKl/ERK2 antibody, and detected with horseradish peroxidase (HRP)-conjugated secondary antibody. To determine the expression of total ERK1/ERK2, the membrane was washed with stripping buffer at 50°C for 30 minutes, followed by blocking the membrane with 5% bovine serum albumin in PBST for 4 hours. Thereafter, the membrane was re-probed with specific ERK1/ERK2 antibody.

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Immunoblotting analysis of MKP-1, PCNA[2]
Subconfluent serum starved PASMCs were exposed to sildenafil, serotonin or U0126 for different periods of time as described above. At the end of the incubation period, protein was extracted and separated by SDS-PAGE with 12% gels. Then total protein was transferred to polyvinylidene difluoride membrane, and probed with PCNA and MKP-1 antibody (1:1000), glyceraldehydes phosphate dehydrogenase (GAPDH) antibody (1:2000) at 4°C overnight. After being washed, the appropriate secondary antibodies (1:5000) were added for one hour at room temperature. The blots were developed with a Super Signal enhanced chemiluminescence kit and visualized on Kodak-AR film. The bands were quantified by densitometry using image analysis software. The relative expression of protein was normalized to GAPDH.

MTT colorimetric assay[2]
Cells at approximately 90% confluence were harvested with 0.1% trypsin/0.01% ethylene diamine tetraacetic acid (EDTA) solution and seeded into a 96-well plate at a density of 2x104 cells/well and grown in RPMI-1640 containing 10% FBS for three days, followed by serum starvation for three days. Cells were then incubated for different time with various concentration of serotonin or 1 μmol/Lsildenafil followed by serotonin with or without U0126, as indicated. Control cells were treated in the same way except sterile PBS replaced the drug. After treatment, medium was changed to fresh medium, and cells were incubated with 5 g/L of MTT for four hours. MTT was then dissolved with 150 μl of 10% dimethylsulfoxide (DMSO) for 20 minutes. The optical densities (OD) in the 96-well plates were determined using a microplate reader at 570 nm.

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Flow cytometry analysis[2]
Cells at approximately 90% confluence were harvested with 0.1% trypsin/0.01% EDTA and seeded into 6-well plates at a density of 5x104 cells/well and grown in RPMI-1640 containing 10% FBS for 3 days, followed by serum starvation for 3 days. Then cells were incubated for 24 hours with serotonin or 1 μmol/L sildenafil followed by serotonin stimulation with or without U0126, as indicated. Cells were rinsed with PBS, trypsinized with 0.1% trypsin/0.01% EDTA solution, and collected by centrifugation at 1000 r/min at 20°C for five minutes. The cell pellets were fixed in 70% ethanol at 4°C for at least 24 hours. The fixed cells were washed twice with PBS, resuspended in PBS containing 50 g/L RNase A and 50 mg/L of propidium iodide (PI). The suspension was incubated at 37°C for 30 minutes, filtered through 200 μm nylon mesh, then analyzed by flow cytometer (FACS Calibur). The ModfitLT software was used for data analysis. The ratio of cells in S phase to all cells that are in G0G1+ S + G2M was calculated by the formula: S phase fraction (SPF) =S/(G0G1+S+G2M) x100%

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Porcine PASMCs were isolated by explant culture and used at passages 3–5. Cells were serum-starved for 3 days in RPMI-1640 with 0.2% FBS before treatment.
For proliferation assay, cells were seeded in 96-well plates and treated with serotonin (10 μmol/L) with or without sildenafil (1 μmol/L) pretreatment for 30 minutes. After 3 days, MTT assay was performed by incubating with MTT for 4 hours, dissolving in DMSO, and measuring absorbance at 570 nm.
For cell cycle analysis, cells were treated as above for 24 hours, fixed in ethanol, stained with propidium iodide and RNase A, and analyzed by flow cytometry to determine S phase fraction.
For protein expression, cells were lysed after treatment, and Western blotting was performed to detect phospho-ERK1/ERK2, total ERK1/ERK2, PCNA, and MKP-1 using specific antibodies.[2]

20 mg/kgSprague-Dawley rats In the first set of experiments, animals were randomly divided into five groups and treated with either PBS or a single dose of sildenafil citrate (0.5, 2.5, 10, and 15 mg/kg), given intraperitoneally (i.p.) 5 min after pMCAo. In the second set of experiments, animals were randomly divided into three groups and treated with either PBS or a single dose of sildenafil citrate (0.5 and 10 mg/kg, i.p.) 5 min after pMCAo (see Additional file 1: Figure S1 for an outline of the experimental procedure).[3]

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cGMP measurement[3]
Competitive enzyme immunoassay was used to quantify cGMP in the forebrain, according to the manufacturer’s instructions. Whole brains at P9 were harvested 1 and 3 h after the administration of sildenafil (0.5 and/or 10 mg/kg) and immediately frozen at −80 °C until measurements were performed.

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Ultrasonographic brain imaging[3]
Thermoregulated mice (n = 6 per group) were subjected to ultrasound measurements under inhaled isoflurane anesthesia (0.8 % in air via a facemask) using an echograph equipped with a 14.5-MHz linear transducer (14L5 SP) [12]. Heart rate and time-average mean blood flow velocities (mBFVs) were measured in both intracranial carotid arteries (ICA) and the basilar trunk (BT) at baseline and 1 h after pMCAo and PBS and sildenafil (10 mg/kg) treatment.

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The study included a total of thirty adult Sprague-Dawley rats that were divided into three groups of ten rats each. In all rats, a crush injury was created by clamping the right sciatic nerve for one minute. One day before the procedure, rats in group 1 were started on a 28-day treatment consisting of a daily dose of 20 mg/kg body weight sildenafil citrate given orally via a nasogastric tube, while the rats in group 2 were started on an every-other-day dose of 10 mg/kg body weight sildenafil citrate. Rats from group 3 were not administered any drugs. Forty-two days after the nerve damage was created, functional and histopathological examination of both sciatic nerves and bone densitometric evaluation of the extremities were conducted.[4]

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Permanent middle cerebral artery occlusion (pMCAo) was performed on postnatal day 9 (P9) C57Bl/6 mouse pups (4.6 ± 0.6 g) under isoflurane anesthesia.
Sildenafil citrate was dissolved in PBS. A single intraperitoneal injection was administered 5 minutes after pMCAo. Animals were randomly assigned to treatment groups receiving PBS or sildenafil at doses of 0.5, 2.5, 10, or 15 mg/kg.
For hemodynamic assessment, cerebral blood flow was measured at baseline and at various time points up to 90 minutes post-pMCAo using color-coded pulsed Doppler ultrasound imaging of large cerebral arteries and laser speckle contrast imaging of cortical perfusion.
Animals were euthanized at 72 hours or 8 days after pMCAo for brain tissue collection. Lesion volume was determined on brain sections. Brain tissues were processed for RNA extraction (qRT-PCR), protein analysis (immunohistochemistry, immunofluorescence), and cGMP measurement (competitive enzyme immunoassay).[3]

Absorption, Distribution and Excretion
Sildenafil is known to be rapidly absorbed; in fasting patients, peak plasma concentrations are reached within 30–120 minutes after oral administration (median 60 minutes). Furthermore, the mean absolute bioavailability of sildenafil is approximately 41% (range 25–63%). Specifically, with three daily oral doses of sildenafil, within the recommended dose range of 25–100 mg, both AUC and Cmax increase with increasing dose. However, in patients with pulmonary hypertension, the average oral bioavailability of 80 mg of sildenafil three times daily is 43% higher than the lower dose. Finally, when sildenafil is taken with food, a decreased absorption rate is observed, with a mean time to peak concentration (Tmax) delayed by approximately 60 minutes and a mean peak concentration (Cmax) decreased by approximately 29%. Nevertheless, the extent of absorption is not significantly affected, as the recorded AUC decreases by only approximately 11%. Following oral or intravenous administration, sildenafil is primarily excreted as a metabolite in feces (approximately 80% of the oral dose) and a small amount in urine (approximately 13% of the oral dose). The mean steady-state volume of distribution of sildenafil is approximately 10⁵ liters, indicating its distribution into tissues. The total clearance of sildenafil is 41 liters per hour. After oral administration, sildenafil is rapidly and almost completely absorbed. A single oral dose of 20 mg is bioequivalent to a 10 mg/mL oral suspension. Although single-dose studies indicate that over 90% of the oral dose of sildenafil is absorbed from the gastrointestinal tract, the drug undergoes extensive metabolism in the gastrointestinal mucosa during absorption and during its first passage through the liver, with only approximately 40% of the dose entering systemic circulation unchanged. Within a single-dose range of 1.25–200 mg, the pharmacokinetics of the drug (measured as peak plasma concentration or area under the plasma concentration-time curve (AUC)) are dose-dependent. In fasting adults, peak plasma concentrations of sildenafil and its active metabolite N-desmethyl are reached within 30–120 minutes (median: 60 minutes). Sildenafil is widely distributed in the body, with a reported steady-state volume of distribution averaging 105 liters. It is currently unknown whether sildenafil is distributed into breast milk. The binding rate of sildenafil and its main circulating metabolite N-desmethyl to plasma proteins is approximately 96%; it has been reported that protein binding is independent of plasma concentration within the range of 0.01–10 μg/mL. The plasma protein binding rate is slightly higher in individuals over 65 years of age (97%) than in those under 45 years of age (96%). Following oral administration of sildenafil, its distribution in semen is limited; in healthy individuals, less than 0.001% of the single dose is present in semen within 90 minutes of administration. Such low concentrations are unlikely to have any effect on sexual partners exposed to semen. Sildenafil is primarily excreted in feces as metabolites. In healthy adults and patients with erectile dysfunction, approximately 80% of the oral dose is excreted in feces as metabolites, and 13% in urine. In volunteers with mild (creatinine clearance CL = 50–80 mL/min) and moderate (creatinine clearance CL = 30–49 mL/min) renal impairment, the pharmacokinetics of a single oral dose of 50 mg Viagra were not altered. In volunteers with severe renal impairment (creatinine clearance <30 mL/min), sildenafil clearance was reduced, resulting in approximately double the AUC and Cmax values compared to age-matched volunteers with normal renal function. For more complete data on the absorption, distribution, and excretion of sildenafil (10 items in total), please visit the HSDB record page. Metabolism/Metabolites Sildenafil metabolism is primarily catalyzed by hepatic microsomal CYP3A4 isoenzymes, with a small amount catalyzed by hepatic CYP2C9 isoenzymes. The major circulating metabolite is the N-demethylated form of sildenafil. This specific metabolite exhibits phosphodiesterase selectivity similar to the parent sildenafil molecule, with an in vitro PDE5 activity approximately 50% that of the parent drug. Furthermore, the plasma concentration of this metabolite is approximately 40% of that of sildenafil, contributing about 20% of its pharmacological action. This major N-demethylated metabolite of sildenafil is further metabolized, with a terminal half-life of approximately 4 hours. In patients with pulmonary hypertension, after three daily doses of 20 mg sildenafil, the plasma concentration of its major N-demethylated metabolite is approximately 72% of the parent sildenafil molecule; therefore, this metabolite contributes approximately 36% to the overall pharmacological action of sildenafil. Sildenafil is primarily cleared via the hepatic microsomal isoenzymes CYP3A4 (major pathway) and CYP2C9 (minor pathway). The major circulating metabolite is the N-demethylated form of sildenafil, which is also further metabolized. The phosphodiesterase (PDE) selectivity of this metabolite is similar to that of sildenafil, and its in vitro inhibitory potency against phosphodiesterase type 5 (PDE-5) is approximately 50% of that of the parent drug. The plasma concentration of this metabolite is approximately 40% of that of sildenafil, therefore it accounts for approximately 20% of the pharmacological action of sildenafil. Pharmacokinetics were studied in mice, rats, rabbits, dogs, and humans after single intravenous and/or oral administration of sildenafil or ¹⁴C-sildenafil (Viagra), respectively. …In all species, five major metabolic pathways were identified: piperazine N-demethylation, pyrazole N-demethylation, loss of two carbon segments on the piperazine ring (N,N'-deethylation), oxidation of the piperazine ring, and aliphatic hydroxylation. Other metabolites are produced by combinations of these pathways. The main component detected in human plasma was sildenafil. Following oral administration, the AUC(∞) of piperazine N-demethylated metabolites and piperazine N,N'-deethylated metabolites were 55% and 27% of the parent compound, respectively. Sildenafil is primarily excreted in feces as metabolites. In healthy adults and patients with erectile dysfunction, approximately 80% of the oral dose is excreted in feces as metabolites, and 13% in urine. In feces, N-dealkylated, hydroxylated, N-demethylated, and N-dealkylated/demethylated metabolites of sildenafil account for approximately 22%, 13%, 3%, and 3% of total fecal excretion, respectively. In healthy individuals, sildenafil is primarily excreted in urine as the hydroxylated metabolite, which accounts for approximately 41% of total urinary excretion. Sprague Dawley rats (n=10 per group, 10 males and 10 females) were administered sildenafil by gavage at doses of 10, 45, or 200 mg/kg/day for one month. /Sildenafil concentrations in the plasma of female rats were higher than those of male rats, while the concentrations of the metabolite UK-103,320 were higher in male rats than in female rats. Therefore, female rats were primarily exposed to the parent drug, while male rats were exposed to almost equal amounts of both the drug and its metabolites. These data suggest that N-demethylation of sildenafil to UK-103,320 is an important pathway for the biotransformation of sildenafil in male rats. Concentrations of UK-95,340 were typically below the limit of detection (30 ng/mL). ……/Excerpt from table/
Sildenafil appears to be completely metabolized in the liver to up to 16 metabolites, most of which constitute only a small fraction of the dose; the parent drug is barely detectable or completely undetectable in urine or feces after oral or intravenous administration. Sildenafil is primarily metabolized via hepatic cytochrome P-450 (CYP) microsomal isoenzymes 3A4 (major pathway) and 2C9 (minor pathway). Potent CYP3A4 inhibitors can significantly reduce sildenafil clearance. The hepatic metabolism of sildenafil is complex, typically involving N,N-deethylation (ring-opening) or N-demethylation of the piperazine ring and aliphatic hydroxylation; the drug and its metabolites appear not to undergo conjugation reactions. The N-demethylated metabolite is the major circulating metabolite, with phosphodiesterase selectivity similar to sildenafil, exhibiting approximately 50% of the PDE5 potency of the parent drug in vitro. The N-demethylated metabolite is further metabolized to the N-dealkylation (N,N-deethylation) metabolite. The drug also undergoes N-dealkylation and N-demethylation of the piperazine ring.

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Biological Half-Life
The terminal half-life of sildenafil is approximately 3 to 5 hours.

After oral administration, plasma sildenafil concentrations exhibit a biphasic decline, with a terminal elimination half-life of approximately 4 hours (range: 3-5 hours).
High clearance rates in rodents are the main determinant of its short elimination half-life (0.4-1.3 hours), while moderate clearance rates in dogs and humans result in longer half-lives (6.1 hours and 3.7 hours, respectively).

Toxicity Summary
Identification and Uses: Sildenafil is a white to off-white crystalline powder available in film-coated tablets, oral suspensions, and injections. Sildenafil is a phosphodiesterase-5 (PDE-5) inhibitor. It is used to treat erectile dysfunction and pulmonary arterial hypertension (PAH) in adults to improve exercise capacity and delay clinical deterioration. Human Exposure and Toxicity: Overdose of sildenafil can generally exacerbate common adverse reactions. In studies of single doses of up to 800 mg sildenafil in healthy individuals, the types of adverse events observed (e.g., decreased blood pressure, syncope, and prolonged erection) were similar to those observed at lower doses, but at an increased incidence. Serious adverse reactions have also been reported at therapeutic doses, including sudden hearing loss or impairment, sudden loss of vision in one or both eyes, and erections lasting longer than 4 hours or priapism (painful erections lasting longer than 6 hours). Post-marketing reports indicate a time-related correlation between sildenafil treatment for erectile dysfunction and serious cardiovascular, cerebrovascular, and vascular events, including myocardial infarction, sudden death, ventricular arrhythmias, cerebral hemorrhage, transient ischemic attack, hypertension, subarachnoid hemorrhage, and pulmonary hemorrhage. Most (but not all) of these patients had pre-existing cardiovascular risk factors. Therefore, it is impossible to determine whether these events are directly related to sildenafil, sexual activity, underlying cardiovascular disease, a combination of these factors, or other factors. Sildenafil is not recommended for children. In a long-term trial in children with pulmonary arterial hypertension (PAH), increased sildenafil doses were observed to lead to increased mortality. Pulmonary vasodilators (such as sildenafil) may significantly worsen cardiovascular conditions in patients with pulmonary venous occlusive disease. Sildenafil significantly enhances the vasodilatory effects of organic nitrates and nitrites. This drug did not show chromosome breakage in vitro in human lymphocyte assays. Animal studies: Death occurred in rats after oral administration of 1000 mg/kg and 500 mg/kg doses, and in mice after oral administration of 1000 mg/kg dose. Female rats were more affected than male rats. Acute sildenafil treatment stimulated testosterone production in adult male rats. No fertility impairment was observed in female rats administered up to 60 mg/kg daily for 36 consecutive days and in male rats for 102 consecutive days. However, in another study, male rats were given sildenafil citrate (0.06 mg/0.05 mL) by gavage and allowed to mate. Fertilization rate and embryo number were assessed after treatment. In mating of males given sildenafil citrate, the fertilization rate was significantly reduced on day 1 (approximately 33%). On days 2–4, the number of embryos developing in the treatment group was significantly lower than in the control group. The cleavage rate of these embryos also showed a decreasing trend, but this did not reach statistical significance. During organogenesis, rats and rabbits receiving doses up to 200 mg/kg/day showed no evidence of teratogenicity, embryotoxicity, or fetal toxicity. In another study, adult male rabbits were administered sildenafil up to 9 mg/kg/day for four consecutive weeks to investigate testicular histological changes caused by drug overdose. Abnormalities in the seminiferous tubule germinal epithelium included spermatocyte nuclear pyknosis, spermatocyte degeneration and shedding, spermatocyte giant cells, and spermatogenesis arrest. Furthermore, increased interstitial cells, tubular degeneration, and interstitial thickening were observed. These histological findings suggest that long-term overdose of sildenafil leads to significant morphological and histological changes in the testes, potentially leading to complete spermatogenesis arrest. Oral administration of sildenafil to rats and mice for up to two years showed no carcinogenicity. Sildenafil did not show mutagenicity in in vitro bacterial and Chinese hamster ovary cell assays. The drug also did not show chromosomal breakage in in vivo mouse micronucleus assays. Sildenafil is primarily cleared via hepatic microsomal isoenzymes CYP3A4 (major pathway) and CYP2C9 (minor pathway). Its main circulating metabolite is the N-demethylated form of sildenafil, which itself is further metabolized. The phosphodiesterase (PDE) selectivity of this metabolite is similar to that of sildenafil, exhibiting approximately 50% of the in vitro inhibitory potency against phosphodiesterase type 5 (PDE-5) compared to the parent drug. The plasma concentration of this metabolite is approximately 40% of that of sildenafil, thus accounting for approximately 20% of the pharmacological action of sildenafil. Hepatotoxicity
At least five cases of acute liver injury have been reported related to sildenafil use, but no cases of acute liver failure have been reported. Due to the intermittent and sometimes undocumented use of sildenafil, the latency period in most reports is unclear, but appears to be between 1 and 8 weeks. Elevated serum enzyme patterns range from hepatocellular to cholestatic, and sometimes evolve from one type to another. The most compelling case was mild cholestatic or “mixed” hepatitis occurring within 1 to 3 months of starting sildenafil. No immune hypersensitivity features or autoantibodies were observed. Cases with acute onset and elevated serum transaminase levels following sildenafil use have been reported, exhibiting some characteristics of ischemic injury. In other cases, the injury pattern suggested use of anabolic steroids. In two cases, relapse did not occur upon re-exposure to sildenafil. Therefore, the hepatotoxicity of sildenafil is not entirely convincing, and even if it occurs, it must be extremely rare. Probability Score: C (likely a rare cause of clinically significant liver injury). Pregnancy and Lactation Effects ◉ Overview of Use During Lactation Limited data suggest that the amount of sildenafil and its active metabolites excreted in breast milk is minimal. The amount ingested by infants is far below the doses used to treat infants, and no adverse effects are expected on breastfed infants.
◉ Effects on breastfed infants
A 23-year-old woman with congenital heart disease and pulmonary hypertension received sildenafil and bosentan during pregnancy at unknown doses. She continued to take these medications and warfarin postpartum. Her infant was delivered by cesarean section at 30 weeks of gestation, weighing 1.41 kg. According to the authors, she breastfed in the neonatal intensive care unit for 11 weeks with “good results”, but the infant died at 26 weeks due to respiratory syncytial virus infection. [3]
A breastfeeding woman whose 21-month-old infant had pulmonary hypertension was treated with 20 mg sildenafil three times a day and 125 mg bosentan twice a day for pulmonary hypertension. These medications were started more than 6 months postpartum. The mother did not report any possible adverse reactions, serious health problems or hospitalizations in the infant from birth to 651 days postpartum (the infant was partially breastfed). [2]
◉ Effects on lactation and breast milk
No relevant published information was found as of the revision date.

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Protein binding
Sildenafil and its major circulating metabolite N-demethylated metabolite are generally observed to bind to plasma proteins at an estimated rate of approximately 96%. However, protein binding of sildenafil has been determined to be independent of total drug concentration.

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Interactions
Sildenafil and other phosphodiesterase type 5 (PDE) inhibitors (e.g., tadalafil, vardenafil) can significantly enhance the vasodilatory effects of organic nitrates and nitrites (e.g., nitroglycerin, isosorbide dinitrate) (e.g., sildenafil can reduce systolic blood pressure by more than 25 mmHg) and may lead to life-threatening hypotension and/or hemodynamic disturbances. Nitrates and nitrites promote the formation of cyclic guanosine monophosphate (cGMP) by stimulating guanylate cyclase, while phosphodiesterase type 5 (PDE) inhibitors (such as sildenafil, tadalafil, and vardenafil) reduce cGMP degradation by inhibiting PDE, leading to increased cGMP accumulation and more pronounced smooth muscle relaxation and vasodilatory effects compared to the use of PDE inhibitors or nitrates/nitrites alone. This interaction can occur with any organic nitrate, nitrite, or nitric oxide donor (such as sodium nitroprusside), regardless of their primary hemodynamic site of action. PDE type 5 inhibitors (including sildenafil) may also enhance the hypotensive effects of inhaled nitrites (such as pentyl nitrite or butyl nitrite, sometimes referred to as "poppers"), which may be abused during sexual activity (recreational use) to enhance the sexual experience. Because these medications are often used for recreational purposes, patients may not be aware of their pharmacological effects and potential risks, and may not report their use to their clinicians. Concomitant use of PDE5 inhibitors with "poppers" (a fast-acting, vasodilator) can cause a sudden and significant drop in blood pressure, potentially leading to serious or even fatal consequences. Interactions with organic nitrates and nitrites may be more pronounced in patients taking certain HIV protease inhibitors. Male homosexuals may face higher risks due to their higher likelihood of nitrite inhalation and antiretroviral therapy. Combination antiretroviral therapy typically includes one or more HIV protease inhibitors, such as CYP3A4 and/or CYP2C9 inhibitors. The clearance of sildenafil may be affected by these medications, increasing the risk of sildenafil-related adverse reactions such as headache, flushing, visual changes, priapism, and even hypotension and syncope. Patients receiving antiretroviral therapy should inform their doctor.
Sildenafil enhances the hypotensive effect of nitrates (including nitroglycerin); concomitant use is contraindicated.
For more complete data on interactions of sildenafil (23 in total), please visit the HSDB record page.

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In P9 mice, administration of a high dose of sildenafil (15 mg/kg, single intraperitoneal injection) 5 minutes after pMCAo significantly increased the mean cortical infarct volume at 72 hours (21.6 ± 4.6%) compared to the PBS group (12.5 ± 3.0%), and also increased mortality (3 out of 10 animals died), indicating that this dose is toxic. [3]

[1]. The Selectivity and Potency of the New PDE5 Inhibitor TPN729MA. J Sex Med. 2013 Nov;10(11):2790-7.

[2]. Sildenafil potentiates the proliferative effect of porcine pulmonary artery smooth muscle cells induced by serotonin in vitro. Chin Med J (Engl). 2011 Sep;124(17):2733-40.

[3]. Sildenafil, a cyclic GMP phosphodiesterase inhibitor, induces microglial modulation after focal ischemia in the neonatal mouse brain. J Neuroinflammation. 2016 Apr 28;13(1):95.

[4]. The Effect of Sildenafil on Recuperation from Sciatic Nerve Injury in Rats. Balkan Med J. 2016 Mar;33(2):204-11.

Therapeutic Uses
Phosphodiesterase 5 inhibitor; urological medication; vasodilator. Viagra is indicated for the treatment of erectile dysfunction. /Included in the US product label/ Revastatin is indicated for the treatment of pulmonary hypertension in adults to improve exercise capacity and delay clinical progression. /Included in the US product label/ The role of sildenafil in the treatment of female sexual dysfunction (if any) remains to be determined. /Not included in the US product label/ For more complete data on the therapeutic uses of sildenafil (of 9 types), please visit the HSDB record page. Drug Warnings Viagra should not be taken concurrently with any form of nitric oxide donor (such as organic nitrates or organic nitrites). Consistent with known effects on the nitric oxide/cGMP pathway, Viagra has been shown to enhance the hypotensive effects of nitrates. Post-marketing reports indicate a temporal association between Viagra use and serious cardiovascular, cerebrovascular, and vascular events, including myocardial infarction, sudden death, ventricular arrhythmias, cerebral hemorrhage, transient ischemic attack, hypertension, subarachnoid hemorrhage, and pulmonary hemorrhage. Most (but not all) of these patients had pre-existing cardiovascular risk factors. Many of these events have been reported during or shortly after sexual activity, while a minority have been reported shortly after taking Viagra without sexual activity. Other events have been reported hours to days after taking Viagra and engaging in sexual activity. It is currently impossible to determine whether these events are directly related to Viagra, sexual activity, the patient's underlying cardiovascular disease, a combination of these factors, or other factors. Since Viagra's market launch, reports of erections lasting longer than 4 hours and priapism (painful erections lasting longer than 6 hours) have been uncommon. If an erection lasts longer than 4 hours, the patient should seek immediate medical attention. Untreated priapism can lead to penile tissue damage and permanent erectile dysfunction. In controlled clinical trials and post-marketing surveillance, less than 2% of patients with erectile dysfunction treated with sildenafil experienced other adverse reactions, such as angina, atrioventricular block, tachycardia, palpitations, myocardial ischemia and infarction, sudden death, chest pain, cerebral thrombosis, cerebrovascular hemorrhage (e.g., subarachnoid hemorrhage, cerebral hemorrhage), transient ischemic attack, stroke (e.g., hemorrhagic stroke or brainstem stroke), cardiac or cardiopulmonary arrest, coronary artery disease, heart failure, electrocardiographic abnormalities (including ventricular arrhythmias (e.g., tachycardia, premature beats) or Q wave abnormalities (without myocardial infarction)), hypertension, edema (including facial and peripheral edema), shock, and cardiomyopathy. These have not been directly attributed to the drug. The incidence of myocardial infarction or stroke was similar in patients with erectile dysfunction treated with sildenafil compared to those treated with placebo, with most cases occurring within hours to days after administration of sildenafil or placebo. Most patients experiencing serious cardiovascular adverse reactions have pre-existing cardiovascular risk factors, and many of these adverse reactions have been reported to occur shortly after taking sildenafil, regardless of sexual activity. At least one patient with hypertrophic cardiomyopathy experienced a decrease in blood pressure, significant reduction in ventricular size, increased resting ejection fraction and subaortic pressure gradient, premature ventricular contractions, and non-sustained ventricular tachycardia after taking sildenafil for erectile dysfunction. For more complete data on sildenafil (32 total), please visit the HSDB records page. Pharmacodynamics: In vitro studies have shown that sildenafil is selective for phosphodiesterase-5 (PDE5). Its effect on PDE5 is stronger than on other known phosphodiesterases. In particular, its selectivity for PDE6, which is involved in the retinal phototransduction pathway, is 10-fold higher. Sildenafil exhibits 80 times greater selectivity for PDE1 than for PDE2, and over 700 times greater selectivity for PDE2, 3, 4, 7, 8, 9, 10, and 11 than for PDE3. Furthermore, sildenafil demonstrates over 4000 times greater selectivity for PDE5 (a cAMP-specific phosphodiesterase subtype involved in the regulation of cardiac contractility) than for PDE3. In eight double-blind, placebo-controlled crossover studies in patients with organic or psychogenic erectile dysfunction, sexual stimulation improved erectile function after sildenafil administration compared to placebo, as confirmed by objective measurements of erectile rigidity and duration (using RigiScan®). Most studies assessed the efficacy of sildenafil approximately 60 minutes after administration. Erectile responses assessed by RigiScan® generally increase with increasing sildenafil dose and plasma concentration. One study examined the time course of drug action, showing that its effects could last up to 4 hours, but the efficacy was diminished compared to 2 hours. Sildenafil can cause a mild and transient decrease in systemic blood pressure, but in most cases, this decrease does not translate into clinical symptoms. In patients with systemic hypertension, long-term administration of 80 mg three times daily resulted in a mean decrease in systolic and diastolic blood pressure of 9.4 mmHg and 9.1 mmHg, respectively, compared to baseline. In patients with pulmonary hypertension, long-term administration of 80 mg three times daily resulted in a smaller decrease in blood pressure (a decrease of 2 mmHg in both systolic and diastolic blood pressure). At the recommended dose of 20 mg three times daily, no decrease in systolic or diastolic blood pressure was observed. A single oral dose of up to 100 mg of sildenafil in healthy volunteers did not have a clinically significant effect on electrocardiogram (ECG). Long-term administration of 80 mg three times daily in patients with pulmonary hypertension did not report any clinically significant effect on ECG. In a study of 14 patients with severe coronary artery disease (CAD) (at least one coronary artery with >70% stenosis), a single oral dose of 100 mg of sildenafil showed hemodynamic effects, with a mean decrease in resting systolic and diastolic blood pressure of 7% and 6%, respectively, compared to baseline. Mean pulmonary artery systolic blood pressure decreased by 9%. Sildenafil had no effect on cardiac output and did not impair blood flow in narrowed coronary arteries. One hour after administration of 100 mg sildenafil, some subjects showed a slight and transient difference in color discrimination (blue/green) using the Farnsworth-Munsell 100 hue test; no significant effect was observed after two hours. It is speculated that this altered color discrimination ability is related to the inhibition of PDE6, which is involved in the phototransduction cascade of the retina. Sildenafil had no effect on visual acuity or contrast sensitivity. In a small, placebo-controlled study (n = 9) of patients diagnosed with early age-related macular degeneration, no significant changes were observed in any visual tests (including visual acuity, Amsler grid, simulated traffic light color discrimination, Humphrey perimeter, and photostress test) after a single dose of 100 mg sildenafil. Sildenafil is a selective PDE5 inhibitor that increases intracellular cGMP levels.
In this neonatal stroke model, the neuroprotective effect of sildenafil in reducing late lesion expansion appears to be independent of early hemodynamic changes, but rather involves the modulation of the post-ischemic inflammatory response.
This study shows that sildenafil promotes the transformation of microglia/macrophage polarization to the neuroprotective M2-like phenotype in the late post-ischemic period.
Sildenafil is suitable for idiopathic pulmonary hypertension and could be explored as a potential immunomodulatory therapy for neonatal ischemic stroke. [3]

C22H30N6O4S

474.5764

474.204

C, 55.68; H, 6.37; N, 17.71; O, 13.48; S, 6.76

139755-83-2

Sildenafil citrate;171599-83-0;Sildenafil-d3;1126745-90-1;Sildenafil-d8;951385-68-5;Sildenafil Mesylate;1308285-21-3;Sildenafil-d3N-1;1126745-87-6

135398744

White to off-white solid powder

189-190 °C
187-189 °C

1.5

1

8

7

33

838

0

S(C1C([H])=C([H])C(=C(C2=NC3C(C([H])([H])C([H])([H])C([H])([H])[H])=NN(C([H])([H])[H])C=3C(N2[H])=O)C=1[H])OC([H])([H])C([H])([H])[H])(N1C([H])([H])C([H])([H])N(C([H])([H])[H])C([H])([H])C1([H])[H])(=O)=O

BNRNXUUZRGQAQC-UHFFFAOYSA-N

InChI=1S/C22H30N6O4S/c1-5-7-17-19-20(27(4)25-17)22(29)24-21(23-19)16-14-15(8-9-18(16)32-6-2)33(30,31)28-12-10-26(3)11-13-28/h8-9,14H,5-7,10-13H2,1-4H3,(H,23,24,29)

Piperazine, 1-((3-(4,7-dihydro-1-methyl-7-oxo-3-propyl-1H-pyrazolo(4,3-d)pyrimidin-5-yl)-4-ethoxyphenyl)sulfonyl)-4-methyl-

UK-92480; UK 92480-10; UK 92480; UK92480 citrate; UK-92,480-10. Trade names: Revatio.

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

Solubility in Formulation 1: ≥ 2.5 mg/mL (5.27 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (5.27 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 3: 30% PEG400+0.5% Tween80+5% propylene glycol:30 mg/mL

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Solubility in Formulation 4: ≥ 10 mg/mL (21.07 mM) (saturation unknown) in 50% PEG300 50% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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 (Please use freshly prepared in vivo formulations for optimal results.)