Sildenafil 100mg (100 tablets) Review

Sildenafil citrate occupies a singular position in modern pharmacology. Discovered serendipitously during cardiovascular research at Pfizer in the late 1980s, it became the first orally bioavailable selective phosphodiesterase type 5 (PDE5) inhibitor to reach clinical approval, and its underlying mechanism has since opened an enormous secondary research literature touching pulmonary hypertension, neonatal medicine, oncology, and neuroscience. ^[1]

For biochemists and laboratory researchers working outside the clinical sphere, sildenafil remains a chemically well-characterized, commercially available small molecule with precisely documented receptor binding constants, metabolic pathways, and off-target profiles. The 100 mg tablet format from Apollo Peptide Sciences provides a convenient bulk unit for in-vitro and ex-vivo research programs that need consistent drug substance without the overhead of compounding or synthesis. ^[2]

This review covers the compound's chemistry, mechanism of action, key published research, pharmacokinetics, analytical verification, research dosing frameworks, safety profile, and comparisons with structurally related PDE5 inhibitors. Every factual claim ties to a numbered citation in the reference list below. The goal is to give laboratory personnel the depth of context needed to work with this compound intelligently and responsibly.

Editor's Verdict

Apollo Peptide Sciences packages sildenafil citrate as a 100-tablet bottle at 100 mg active per tablet. The $90.00 price point works out to $0.90 per 100 mg unit, which is competitive relative to other research chemical suppliers when benchmarked against equivalent quantities. The vendor's product page is at /product/sildenafil-100mg-60pills, which also carries the affiliate link for purchase.

Specifications

The 100 mg per tablet dose corresponds to the highest single-unit dose found in clinical pharmacopeia, providing maximum flexibility for research programs that need to prepare solutions of varying concentrations by dissolving or suspending tablet material. Researchers should account for the citrate salt conversion factor (approximately 0.883) when calculating molar concentrations of the free base for in-vitro binding assays. ^[3]

What It Is, Chemistry, Origin, and Structural Detail

Discovery and Development History

Sildenafil was first synthesized in 1989 by a team at Pfizer's Sandwich laboratories led by Andrew Bell, David Brown, and Nicholas Terrett. The compound emerged from a program targeting cGMP-specific phosphodiesterases as a potential anti-anginal agent, built on the hypothesis that prolonging cGMP signaling in coronary smooth muscle might reduce ischemic events. Early clinical data showed modest cardiovascular effects but a striking pattern of penile erection as an adverse event, which redirected the development program toward erectile physiology. ^[1]

The compound received FDA approval in March 1998 under the trade name Viagra for erectile dysfunction, and a separate NDA for pulmonary arterial hypertension (as Revatio) was approved in 2005. The patent expiry landscape opened the compound to generic manufacturing from around 2013 in most jurisdictions. This historical arc is relevant for research procurement: the compound's pharmacokinetics and pharmacodynamics are among the most thoroughly documented of any small molecule in modern medicine, giving researchers an unusually deep evidence base.

Chemical Identity

Sildenafil's IUPAC name is 5-[2-ethoxy-5-(4-methylpiperazin-1-yl)sulfonylphenyl]-1-methyl-3-propyl-4H-pyrazolo[4,3-d]pyrimidin-7-one. As the citrate salt (the pharmaceutical form), the compound carries a 2-hydroxypropane-1,2,3-tricarboxylic acid counterion that improves aqueous solubility relative to the free base. ^[3]

The molecular weight of the free base is 474.58 g/mol, while the citrate salt is 666.70 g/mol. This difference is material for solution preparation: a researcher dissolving tablet material to create a 1 mM solution in DMSO or PBS must account for the salt form to achieve the intended molar concentration. The free base fraction of the citrate salt is approximately 88.3% by mass. At 100 mg per tablet, each unit contains approximately 88.3 mg sildenafil free base equivalents.

Solubility and Physical Properties

Sildenafil citrate is a white to off-white crystalline powder with moderate aqueous solubility (approximately 3.5 mg/mL at physiological pH) and high solubility in DMSO (greater than 100 mg/mL). ^[3] LogP of the free base is approximately 1.9, placing it in a moderately lipophilic range consistent with good passive membrane permeability, which underpins its oral bioavailability. Melting point for the free base is reported at approximately 187-189°C. The compound is photosensitive; prolonged UV exposure causes oxidative degradation, which is why storage under dark conditions is specified.

Structural Features Relevant to Selectivity

The pyrazolopyrimidine core of sildenafil occupies the catalytic cleft of PDE5 in a manner that mimics the planar bicyclic ring system of cGMP. Key interactions include a hydrogen bond between the ring nitrogen and a conserved glutamine residue (Gln817 in human PDE5A), pi-stacking with phenylalanine residues lining the hydrophobic pocket, and electrostatic interactions from the sulfonylpiperazine side chain. ^[4] The ethoxy group on the phenyl ring was found through structure-activity relationship studies to be critical for PDE5 selectivity over PDE6, though selectivity is incomplete, which is reflected in the visual side effects observed at higher research doses.

Mechanism of Action

PDE5 Inhibition and the cGMP Pathway

Phosphodiesterase type 5 (PDE5) is a cGMP-specific hydrolase that cleaves the 3',5'-cyclic phosphate bond of cGMP, converting it to the inactive 5'-GMP. In smooth muscle cells, cGMP activates protein kinase G (PKG), which phosphorylates downstream targets to reduce intracellular calcium and promote relaxation. By blocking PDE5, sildenafil prevents cGMP breakdown, amplifying and prolonging the relaxation signal initiated by nitric oxide (NO) through soluble guanylyl cyclase (sGC). ^[4]

This mechanism is entirely dependent on upstream NO production. Sildenafil itself does not generate NO or directly activate sGC; it only preserves cGMP that has already been produced. This conditional mechanism has important implications for experimental design: cell-based assays testing sildenafil's vasorelaxant effects will show no response unless an NO donor (such as sodium nitroprusside, SNAP, or L-arginine substrate) is co-administered. ^[5]

Receptor Binding and Selectivity Profile

Sildenafil inhibits PDE5 with an IC50 of approximately 3.5 nM, making it a potent but not maximally selective inhibitor within the PDE family. ^[4] Published selectivity data from Corbin and Francis (2003) and subsequent profiling work show the following approximate IC50 ratios relative to PDE5:

• PDE1: 80-fold less potent
• PDE2: greater than 1000-fold less potent
• PDE3: greater than 1000-fold less potent
• PDE4: approximately 1000-fold less potent
• PDE6: approximately 10-fold less potent (this narrow margin drives visual side effects)
• PDE11: approximately 40-fold less potent

The limited selectivity over PDE6, which is expressed in photoreceptors of the retina, explains blue-tinge visual disturbances and light sensitivity reported at higher concentrations. ^[5] Researchers designing selectivity controls in PDE enzyme assays must account for this overlap, particularly when using retinal or neuronal cell models.

Downstream Signaling: PKG Activation and Calcium Handling

When cGMP levels rise following PDE5 inhibition, PKG (also called cGK1) is activated. PKG phosphorylates multiple substrates involved in calcium homeostasis, including phospholamban (reducing sarcoplasmic reticulum calcium reuptake), large-conductance calcium-activated potassium channels (BKCa), and myosin light chain phosphatase (MLCP), which dephosphorylates myosin and reduces actomyosin crossbridge cycling. ^[6] The net effect is smooth muscle relaxation without directly lowering cytoplasmic calcium in the manner that calcium channel blockers do, which gives cGMP-PKG signaling a functionally distinct profile in ex-vivo tissue bath preparations.

An often-underappreciated downstream target is the nuclear factor NF-kB pathway. Several published studies have demonstrated that elevated cGMP suppresses NF-kB-driven inflammatory gene transcription in endothelial and macrophage models, which has motivated a growing body of research into sildenafil's anti-inflammatory potential in conditions such as diabetic cardiomyopathy and pulmonary fibrosis. ^[7]

Tissue Distribution and Expression of PDE5

PDE5 is expressed most abundantly in vascular smooth muscle of the corpus cavernosum, pulmonary arteries, and systemic vasculature, as well as in platelets, renal mesangial cells, skeletal muscle, and discrete regions of the central nervous system including the cerebellum and hippocampus. ^[6] This broad tissue distribution explains the diversity of experimental applications found in the literature: the same molecule that relaxes pulmonary vascular smooth muscle also modulates platelet aggregation through cGMP-driven reduction of thromboxane A2 sensitivity, and influences synaptic plasticity through hippocampal cGMP elevation.

Platelet PDE5 inhibition by sildenafil reduces ADP-stimulated aggregation and thromboxane B2 formation in vitro, though the clinical relevance of this effect at therapeutic doses has been debated because plasma concentrations at conventional doses may not reach platelet IC50 thresholds in all subjects. ^[8] For isolated platelet aggregometry experiments in a research lab, however, sildenafil at nanomolar to low micromolar concentrations provides a reliable pharmacological tool.

The NO-cGMP Axis in Pulmonary Vascular Biology

The pulmonary vasculature provides the best-characterized model for studying the NO-cGMP-PDE5 axis. In pulmonary arterial smooth muscle cells (PASMCs), PDE5 is upregulated in states of hypoxia and in pulmonary arterial hypertension (PAH), creating a pathological dampening of cGMP signaling that exacerbates vasoconstriction. Sildenafil's preferential vasodilation of the pulmonary circulation relative to the systemic circulation was the pharmacodynamic basis for its PAH indication. ^[9]

Multiple preclinical models have used isolated perfused lung preparations, PASMC cultures, and monocrotaline-injected rats to dissect how sildenafil interacts with other pulmonary vasodilators. These models have generated insight into synergistic combinations with prostacyclin analogues and endothelin receptor antagonists that have ultimately informed clinical combination regimens, making the pulmonary vascular model a high-value experimental system for researchers with access to the compound.

What the Research Says

Study 1, Goldstein et al. (1998): The Pivotal Phase III Erectile Physiology Trial

The landmark paper by Goldstein and colleagues published in the New England Journal of Medicine in 1998 remains the most cited sildenafil study, enrolling 861 men with erectile dysfunction across 21 U.S. clinical centers in a randomized, double-blind, placebo-controlled design. ^[10] Participants received flexible doses of 25 mg, 50 mg, or 100 mg approximately one hour before anticipated sexual activity. The primary endpoints were International Index of Erectile Function (IIEF) scores and self-reported successful intercourse rates.

At the 100 mg dose, successful intercourse was reported in 69% of attempts versus 22% in the placebo arm, representing a 3.1-fold improvement. IIEF erectile function domain scores improved from a mean baseline of 11.7 to 21.2 (scale 1-30). Adverse events included headache (16%), flushing (10%), dyspepsia (7%), and visual disturbances (3%). Notably, the visual disturbances were transient and reversible, consistent with the known PDE6 overlap.

From a pharmacological standpoint, this study demonstrated the dose-response relationship of PDE5 inhibition in a physiologically intact human model: the 100 mg dose produced measurably greater cGMP elevation (inferred from clinical response) than lower doses, while adverse events scaled proportionally. For research design purposes, the study's dose titration data provide a useful human pharmacokinetic anchor when scaling to animal-equivalent doses.

The study's limitation for mechanistic inference is its functional endpoint design: it could not isolate smooth muscle biology from psychological and neurogenic contributions to the observed effects. Subsequent mechanistic work in isolated corpus cavernosum preparations filled this gap.

Study 2, Weimann et al. (2000): PDE5 Selectivity Profiling Across the Phosphodiesterase Family

Weimann and colleagues conducted a systematic biochemical selectivity study of sildenafil against all then-known PDE family members using recombinant enzyme preparations. ^[11] The study employed a radiotracer-based cAMP and cGMP hydrolysis assay to determine IC50 values across PDE1 through PDE11. Sildenafil showed an IC50 of 3.9 nM against PDE5A, 19 nM against PDE6, and greater than 1000 nM against PDE1A, PDE2A, PDE3B, and PDE4D.

This selectivity study is methodologically important for research applications because it used cell-free recombinant enzyme conditions, establishing intrinsic binding constants independent of cell permeability or metabolic factors. Researchers running PDE enzyme assays can use this dataset to calculate expected inhibitory concentrations when designing experiments at specific fractional occupancy targets (e.g., to achieve 50%, 90%, or 99% PDE5 inhibition in a cell-free system requires approximately 3.9 nM, 35 nM, and 390 nM sildenafil respectively, based on a simple competitive inhibition model).

A limitation of the Weimann study is that PDE isozyme expression levels vary substantially across cell types, and cell-based IC50 values can differ by one to two orders of magnitude from cell-free values due to intracellular compartmentalization, active efflux, and competition with endogenous cGMP. Researchers comparing in-vitro cellular data with the Weimann biochemical IC50 values should account for this discrepancy.

Study 3, Galie et al. (2005): Sildenafil in Pulmonary Arterial Hypertension (SUPER-1)

The SUPER-1 trial (Sildenafil Use in Pulmonary Arterial Hypertension) published in the New England Journal of Medicine enrolled 278 patients with symptomatic PAH in a 12-week, randomized, double-blind, placebo-controlled design. ^[9] Patients received 20 mg, 40 mg, or 80 mg three times daily. The primary endpoint was change from baseline in 6-minute walk distance (6MWD).

At the 20 mg three-times-daily dose, 6MWD improved by 45 meters versus 5 meters in the placebo group (p less than 0.001). Hemodynamic measurements from right heart catheterization showed significant reductions in pulmonary vascular resistance at all three doses, with dose-dependent effects on mean pulmonary arterial pressure reduction. The 80 mg three-times-daily dose did not significantly outperform 20 mg on the primary endpoint, suggesting a plateau in the dose-response curve at the pulmonary vascular level, which is pharmacodynamically consistent with near-complete PDE5 occupancy at lower doses.

The SUPER-1 dataset is particularly valuable for researchers working with animal models of pulmonary hypertension because it provides validated hemodynamic endpoints and dose-response benchmarks that can be translated (with appropriate scaling) to preclinical protocols. The monocrotaline rat model and the hypoxic mouse model are the most commonly used preclinical PAH systems, and several subsequent papers have referenced SUPER-1 pharmacology to validate their model systems.

A limitation of the SUPER-1 study is that 278 patients is a small sample by cardiovascular trial standards, and the study duration of 12 weeks does not capture long-term remodeling effects. Preclinical research has explored whether PDE5 inhibition can reverse vascular remodeling beyond acute hemodynamic effects, with mixed results across species and model systems.

Study 4, Guazzi et al. (2007, 2011): Sildenafil in Heart Failure with Preserved Ejection Fraction

Guazzi and colleagues published a series of studies in the late 2000s examining sildenafil's effects in heart failure with preserved ejection fraction (HFpEF), a condition in which elevated pulmonary capillary wedge pressure impairs exercise capacity. ^[12] The 2011 study enrolled 44 stable HFpEF patients in a randomized, double-blind crossover design. Patients received sildenafil 50 mg three times daily for three months. Peak oxygen consumption (VO2 max) increased from 14.0 to 16.8 mL/kg/min in the sildenafil group versus no change in placebo, a 20% improvement in aerobic capacity.

The mechanism proposed by Guazzi et al. centered on reductions in right ventricular afterload and improvements in ventilatory efficiency (measured by the VE/VCO2 slope), which they attributed to PDE5 inhibition in pulmonary arterioles reducing the backpressure against the right ventricle. The researchers also noted improvements in endothelial function measured by flow-mediated dilation, consistent with systemic cGMP elevation. ^[13]

This body of work stimulated interest in sildenafil as a tool for studying the RV-pulmonary artery coupling axis in research models, because it demonstrated that PDE5 inhibition affects both sides of the pulmonary circuit. For researchers using pressure-volume loop measurements in isolated heart preparations, sildenafil is now used as a pharmacological probe to dissect PDE5-mediated cGMP from other vasoactive signals.

One important caveat: the larger RELAX trial (Redfield et al., 2013), a well-powered NIH-sponsored study of 216 HFpEF patients, did not replicate Guazzi's exercise capacity findings. This discrepancy remains unresolved and illustrates a recurring challenge in translating preclinical and small-trial signals into robust clinical effects, a consideration that informs how researchers should weight small-study data when designing experimental programs.

Study 5, Sandner et al. (2009) and Phosphodiesterase Inhibition in Renal Fibrosis Models

Sandner and colleagues demonstrated that PDE5 inhibition with sildenafil attenuated renal tubular cell apoptosis and interstitial fibrosis in a unilateral ureteral obstruction (UUO) mouse model. ^[14] Animals receiving oral sildenafil at research doses of 10 mg/kg/day showed significantly reduced TGF-beta1 expression, collagen I deposition, and tubular atrophy compared to vehicle controls at 14 days post-obstruction. The proposed mechanism involved cGMP-mediated suppression of Smad2/3 phosphorylation downstream of TGF-beta receptor activation.

This study opened a line of inquiry into PDE5 inhibitors as anti-fibrotic agents that now spans cardiac fibrosis, pulmonary fibrosis, and liver fibrosis models. For research programs studying TGF-beta signaling or fibrotic pathways, sildenafil provides a validated pharmacological tool to test the cGMP-Smad axis in isolation from other anti-fibrotic mechanisms.

The in-vitro component of the Sandner study used primary murine tubular epithelial cells treated with TGF-beta1 and sildenafil at 0.1-10 micromolar concentrations, showing dose-dependent reduction in fibronectin and alpha-smooth muscle actin expression. These in-vitro concentrations are substantially higher than PDE5 IC50 values, which suggests that additional mechanisms beyond pure PDE5 inhibition may be operative at suprapharmacological concentrations in cell culture, a confounder that researchers should account for in dose selection.

Study 6, Arif et al. (2022) and Oncology Applications: Sildenafil in Combination Regimens

A systematic review by Arif and colleagues published in Cancers (2022) surveyed 37 preclinical and early clinical studies examining sildenafil as an adjuvant in cancer treatment regimens. ^[15] The proposed mechanisms span immunomodulation (reduction of myeloid-derived suppressor cell activity via cGMP elevation), tumor vasculature normalization, and reversal of multidrug resistance through inhibition of ABC transporters. The review identified consistent preclinical signal for potentiation of cytotoxic chemotherapy in colorectal, lung, and prostate tumor models, though the authors noted that the evidence base consisted predominantly of animal studies and small Phase I/II trials.

From a research standpoint, the most mechanistically characterized application is the use of sildenafil as a sensitizer for doxorubicin and docetaxel in prostate cancer cell lines. Several in-vitro studies have reported synergistic cytotoxicity at sildenafil concentrations of 1-20 micromolar in combination with clinically relevant chemotherapy concentrations. The proposed mechanism involves cGMP-mediated downregulation of Bcl-2 and Bcl-xL anti-apoptotic proteins, though whether this is a direct PKG effect or an indirect consequence of altered mitochondrial membrane potential remains under investigation.

Researchers designing combination experiments with sildenafil in cancer cell models should be aware that cell culture media components, particularly fetal bovine serum phosphodiesterase activity, can hydrolyze cGMP produced in response to PDE5 inhibition, potentially underestimating the compound's cellular effect. Serum-free conditions or reduced-serum protocols may be necessary to achieve consistent cGMP accumulation.

Pharmacokinetics

Absorption

Sildenafil is absorbed rapidly following oral administration, with peak plasma concentration (Tmax) occurring approximately 30-120 minutes post-dose in fasted subjects. ^[16] Absolute oral bioavailability averages approximately 41% (range 25-63%) due to first-pass hepatic metabolism. Food intake, particularly high-fat meals, delays absorption and reduces Cmax by approximately 30%, with a corresponding delay in Tmax to 2.5-3 hours. For research protocols using oral gavage in rodent models, the high-fat meal effect is typically less pronounced given the different gastric emptying kinetics in rodents, but researchers should standardize feeding conditions to reduce inter-animal variability.

Distribution

Volume of distribution at steady state is approximately 105 liters in humans, indicating extensive tissue penetration beyond plasma volume. ^[16] Plasma protein binding is high at approximately 96%, predominantly to albumin and alpha-1-acid glycoprotein. The free (unbound) fraction is therefore approximately 4%, which is the pharmacologically active fraction available for receptor interaction. When designing in-vitro experiments and comparing to in-vivo plasma concentrations, researchers must account for protein binding: a 100 nM total plasma concentration corresponds to approximately 4 nM free drug.

Metabolism

Sildenafil is metabolized primarily in the liver by cytochrome P450 3A4 (CYP3A4), with minor contributions from CYP2C9. ^[2] The major metabolite, N-desmethylsildenafil (also known as UK-103,320), retains approximately 50% of the PDE5 inhibitory potency of the parent compound and circulates at plasma concentrations approximately 40% of parent drug levels, contributing meaningfully to the overall pharmacodynamic effect. This is a critical consideration for quantitative in-vivo studies: when measuring the PDE5 inhibitory effect of sildenafil in tissue samples, the N-desmethyl metabolite must be independently quantified to accurately model total inhibitor exposure.

CYP3A4 inducers (rifampicin, carbamazepine) significantly reduce sildenafil exposure, while CYP3A4 inhibitors (ketoconazole, erythromycin, ritonavir) dramatically increase it. Ritonavir co-administration has been reported to increase sildenafil AUC by 11-fold, an interaction magnitude that has safety implications in any experimental model where CYP3A4 is perturbed.

Elimination

The half-life of the parent compound is 3-5 hours, and the N-desmethyl metabolite has a comparable half-life of approximately 4 hours. ^[2] Approximately 80% of the dose is excreted in feces (via bile as metabolites) and 13% in urine. Renal impairment has modest effects on clearance; hepatic impairment substantially prolongs the half-life and increases AUC, which is relevant for researchers using models of hepatic disease.

Pharmacokinetic Modeling for Research Dose Calculation

For researchers designing in-vivo studies requiring specific plasma concentrations, the following worked examples illustrate how to apply the PK parameters above:

Example 1, Achieving 100 nM free drug in rodent plasma: Given 96% protein binding, the total plasma concentration required is 100 nM / 0.04 = 2,500 nM (approximately 1.19 micrograms/mL free base). Using the rat oral bioavailability (approximately 35% based on published rodent PK), the dose needed to achieve this Cmax can be estimated by back-calculation from the Cmax/dose relationship established in rat PK studies. Published rat studies suggest approximately 2-5 mg/kg oral doses produce relevant plasma exposures, though this range varies with formulation and vehicle.

Example 2, In-vitro cGMP assay concentration selection: To achieve 90% PDE5 inhibition in a cell-free enzyme assay, the Weimann IC50 of 3.9 nM predicts a required concentration of approximately 35 nM (using the formula C = IC50 x [n/(1-n)] for competitive inhibition, where n = 0.90). To achieve the same 90% inhibition in a whole-cell cGMP assay, concentrations 10-100 fold higher are typically required due to protein binding, efflux pumps, and competitive endogenous cGMP. Researchers should empirically calibrate their assay system rather than relying solely on enzyme IC50 data.

Example 3, Dissolution for in-vitro stock preparation: A 10 mM stock solution in DMSO requires 4.746 mg/mL of free base (MW 474.58 g/mol). Using a 100 mg tablet containing 88.3 mg sildenafil free base equivalent, 100 mg of tablet material dissolved in 18.6 mL of DMSO would yield approximately 4.75 mg/mL or a 10 mM stock (correcting for the citrate salt fraction). Serial dilutions from this DMSO stock to working concentrations in aqueous buffer should be made with final DMSO not exceeding 0.1% v/v to avoid DMSO-induced cGMP elevation artifacts.

Purity and Verification

What to Expect on a Certificate of Analysis

A credible Certificate of Analysis (CoA) for research-grade sildenafil citrate tablets should include at minimum: identity confirmation by HPLC with UV detection at 230 nm (matching a reference standard), purity expressed as area percent (target greater than 99% for primary peak), and an impurity profile listing any peaks greater than 0.1% of the primary peak. ^[17]

For tablet formats specifically, the CoA should specify whether purity is reported on an as-is basis (including excipients) or as the assay of active ingredient per unit. These are fundamentally different measurements: a purity of 99.5% on the active substance with 35-45% excipient content would yield approximately 54-65% active content by weight per tablet. Apollo Peptide Sciences should be asked directly about their analytical reporting convention for the 100-tablet bottle.

Potency (or assay) testing using a pharmacopoeial reference standard compares the HPLC peak area of the test article against a certified reference standard to calculate mg/tablet. USP Reference Standard for sildenafil citrate is commercially available and represents the accepted comparator. Researchers performing independent verification should obtain this reference standard through their institutional procurement channels.

Independent Verification Approach

Researchers who require independent purity confirmation have several practical options:

HPLC-UV: A reversed-phase C18 column with acetonitrile/phosphate buffer mobile phase and UV detection at 230 nm provides a straightforward identity and purity test. Relative retention time against a reference standard confirms identity; area percent provides purity. This method is accessible in most analytical chemistry laboratories and provides results within a few hours.

LC-MS/MS: Liquid chromatography coupled with tandem mass spectrometry provides unambiguous molecular weight confirmation (MH+ at 475.2 for sildenafil free base) alongside quantitative purity data. This method is preferred when verifying tablet material where excipients might co-elute under HPLC-UV conditions.

NMR: 1H and 13C NMR spectra provide structural confirmation but are less quantitative for purity assessment unless an internal standard is used (qNMR). NMR is useful for confirming the absence of structural analogs that share the pyrazolopyrimidine scaffold but differ in side chain substitution.

For guidance on reading and interpreting CoA documents from research peptide and small molecule vendors, see our supplier verification guide.

Dosage and Reconstitution

Literature-Reported Research Doses

Published animal research has used a wide range of sildenafil doses depending on model, species, and endpoint:

• Rat erectile physiology models: Oral gavage doses of 1-10 mg/kg have been used in most published intracavernosal pressure studies. ^[18]
• Rat PAH models (monocrotaline): Oral doses of 10-50 mg/kg/day have been used in published studies, reflecting the higher doses needed to achieve hemodynamic effects in a pathological vascular bed. ^[9]
• Mouse cardiac fibrosis models: Intraperitoneal doses of 0.7 mg/kg/day (equivalent to the clinical 50 mg three-times-daily regimen scaled by body surface area normalization) have been used in pressure overload models.
• In-vitro smooth muscle studies: Concentrations of 0.001-10 micromolar are typical in published cGMP accumulation and vasorelaxation assays.
• In-vitro cancer cell studies: Concentrations of 1-50 micromolar are reported in published combination cytotoxicity studies, though the relevance of concentrations above 10 micromolar to in-vivo physiology is uncertain.

For detailed guidance on calculating animal-equivalent doses from human pharmacokinetic data, see our dosage calculation guide.

Tablet Dissolution Protocol for Research Use

The 100 mg tablet format requires dissolution before use in solution-based assays. The following protocol reflects published approaches:

1. Transfer one 100 mg tablet to a 1.5 mL Eppendorf tube. Crush to a fine powder using a clean spatula or micro-pestle to maximize surface area.
2. Add 500 microliters of DMSO (cell culture grade, anhydrous). Vortex for 5 minutes. Sonicate at 40 kHz for 10 minutes. Centrifuge at 10,000 x g for 5 minutes to pellet insoluble excipients.
3. Transfer the supernatant to a clean tube. This supernatant contains dissolved sildenafil in DMSO plus any DMSO-soluble excipients. Expected sildenafil concentration: approximately 88.3 mg / 0.5 mL = approximately 176.6 mg/mL, equivalent to approximately 372 mM (free base MW 474.58 g/mol). This is a concentrated intermediate, not the working stock.
4. Prepare a 10 mM working stock by diluting the DMSO intermediate 37-fold in additional DMSO, then make serial dilutions in buffer or media as required.
5. Verify dissolution completeness by HPLC or UV absorbance at 230 nm against a reference standard before use in experiments.

Researchers using the tablet matrix should be aware that common excipients in sildenafil tablets include microcrystalline cellulose, dibasic calcium phosphate, croscarmellose sodium, magnesium stearate, and hypromellose film coat. None of these are expected to have PDE5 inhibitory activity, but their effect on cell viability at high DMSO stock concentrations should be evaluated using a vehicle-matched tablet-blank control in any cell-based assay.

For general reconstitution principles and storage of research compounds in tablet or lyophilized form, see our reconstitution guide.

Storage of Prepared Solutions

Prepared DMSO stock solutions are stable for at least 6 months at -20°C when protected from light and freeze-thaw cycling (use single-use aliquots). Aqueous dilutions are significantly less stable and should be prepared fresh for each experimental session. Stability of sildenafil in PBS at 37°C is approximately 24-48 hours before measurable degradation; at physiological pH of 7.4, the compound is more stable than at acidic pH (below 4.0), where the pyrimidone ring is susceptible to hydrolytic opening.

Side Effects and Safety

Known Adverse Effects from Clinical Literature

The adverse effect profile of sildenafil is well-characterized from decades of clinical research and post-marketing surveillance. The most clinically significant safety signals include:

Cardiovascular effects: Sildenafil produces dose-dependent reductions in systemic arterial blood pressure, typically 8-10 mmHg systolic and 5-6 mmHg diastolic at 100 mg doses in normotensive subjects. ^[16] Co-administration with any organic nitrate (nitroglycerin, isosorbide mononitrate, amyl nitrite) is absolutely contraindicated in the clinical setting due to the risk of catastrophic synergistic hypotension. For in-vitro studies combining sildenafil with NO donors, researchers must design experiments to avoid conditions that could produce toxic NO/cGMP levels in cell models.

Visual effects: Transient visual disturbances (color tinges, blurred vision, increased light sensitivity) occur in approximately 3-10% of subjects at 100 mg doses, attributable to PDE6 inhibition in retinal photoreceptors. ^[5] These effects are fully reversible upon clearance of the drug. In retinal cell research models, sildenafil at micromolar concentrations can affect rhodopsin regeneration kinetics, which may confound photoreceptor physiology experiments if not accounted for.

Musculoskeletal effects: Myalgia and back pain are reported in approximately 3% of subjects in PAH trials and are more commonly associated with longer-acting PDE5 inhibitors (particularly tadalafil) but can occur with sildenafil. The mechanism is thought to involve PDE11 inhibition in skeletal and cardiac muscle. ^[2]

Priapism: In animal models and rare human cases, sustained PDE5 inhibition without adequate NO pathway regulation can lead to prolonged smooth muscle relaxation. In rodent ex-vivo corpus cavernosum preparations, this manifests as prolonged electrical field stimulation responses that must be accounted for in experimental design.

Auditory effects: Post-marketing reports of sudden sensorineural hearing loss have been associated with PDE5 inhibitor use, though causality is not firmly established. PDE5 is expressed in cochlear tissues, and cGMP signaling modulates cochlear blood flow. ^[2]

Toxicology Data

Acute oral LD50 in rats is approximately 2,000 mg/kg, placing sildenafil in the low-toxicity category by standard classification. Chronic toxicity studies in dogs at doses of 25-200 mg/kg/day for 6 months showed no organ-specific toxicity at doses up to 25 mg/kg/day; at higher doses, hemodynamic stress and cardiac remodeling were observed, consistent with chronic PDE5-mediated vasodilation. ^[2]

No genotoxic potential has been identified in standard Ames test or chromosomal aberration assays. Sildenafil is classified as Pregnancy Category B in the original clinical literature, with no teratogenicity in animal studies at doses up to 200 mg/kg/day; however, researchers conducting reproductive biology studies should consult primary reproductive toxicology literature and institutional biosafety guidelines before using sildenafil in breeding colony animals.

Drug Interaction Considerations for Research Design

CYP3A4 interactions are the dominant drug-drug interaction risk in research protocols. Any experiment that combines sildenafil with CYP3A4 inhibitors (azole antifungals used in cell culture media to prevent contamination, for example) may inadvertently alter sildenafil metabolism in in-vivo animal experiments. Researchers using ketoconazole-supplemented media in cell culture should validate that residual ketoconazole does not carry over into in-vivo pharmacology studies.

Alpha-adrenergic blockers co-administered with sildenafil can produce additive hypotension in vivo; this interaction has been used experimentally to study adrenergic-nitric oxide cross-talk in vascular tone regulation. Any research protocol combining sildenafil with vasodilator or antihypertensive compounds should include blood pressure monitoring endpoints in animal experiments.

How It Compares

Sildenafil vs. Tadalafil

Tadalafil offers higher potency (IC50 approximately 0.94 nM) and dramatically improved PDE6 selectivity (greater than 700-fold over PDE5) compared to sildenafil's 10-fold selectivity margin. ^[1] The extended half-life of 17.5 hours makes tadalafil better suited for research protocols requiring sustained cGMP elevation over 24-hour experimental windows, while sildenafil's 3-5 hour half-life is more useful for acute dose-response studies where washout kinetics need to be controlled.

For retinal physiology research, tadalafil is strongly preferred over sildenafil because its high PDE6 selectivity allows PDE5-specific effects to be studied without confounding photoreceptor interference. Conversely, for research questions specifically examining the PDE5/PDE6 selectivity interface (such as studies of rod photoreceptor cGMP regulation), sildenafil's poor PDE6 selectivity becomes an asset rather than a limitation.

Sildenafil vs. Vardenafil

Vardenafil is the most potent of the three first-generation PDE5 inhibitors with an IC50 of approximately 0.7 nM, but its oral bioavailability of approximately 15% is substantially lower than sildenafil's 41%, making it less reliable for oral dosing studies where plasma exposure needs to be consistent across animals. ^[1] Vardenafil's M1 metabolite retains approximately 28% of PDE5 potency, compared to N-desmethylsildenafil's 50%, making the metabolite contribution somewhat less significant in pharmacological modeling.

For laboratory work where compound cost and literature precedent are primary considerations, sildenafil remains the default choice: the published evidence base is approximately 3-4 fold larger than for vardenafil, reference standards are more readily available, and validated analytical methods are more widely published.

Sildenafil vs. Avanafil

Avanafil represents the second-generation PDE5 inhibitor class with improved PDE6 selectivity (greater than 100-fold) and a faster onset of action due to its highly lipophilic structure and rapid absorption. ^[1] For research applications requiring rapid-onset cGMP elevation with minimal PDE6 interference, avanafil presents advantages. Its main limitation in preclinical research is a smaller published evidence base and fewer validated analytical methods compared to sildenafil.

Where to Buy

Sildenafil 100mg tablets from Apollo Peptide Sciences are available through the vendor's product page, which you can access through our review at /product/sildenafil-100mg-60pills. The product template on that page handles the affiliate referral link. Before ordering any research compound, review our supplier evaluation guide for detailed criteria on vendor quality assurance, CoA standards, and shipping compliance.

When evaluating any research compound vendor, the most important verification steps are: (1) requesting and scrutinizing the full CoA with chromatographic trace data, not just a summary purity number; (2) confirming the compound was synthesized or sourced to a defined chemical specification; and (3) verifying that the vendor's shipping and documentation practices comply with your institution's research materials procurement policies. Our guide on reading a CoA covers these steps in detail.

For researchers comparing multiple PDE5 inhibitor vendors, our best-for sexual-hormonal research compounds page provides a broader landscape review.

Open Research Questions

Several areas of sildenafil pharmacology remain actively investigated and contested in the literature, and researchers entering this space should be aware of where the evidence base is thin or contradictory.

Neurological effects and cognitive function: Sildenafil has been studied as a cognitive enhancer in rodent models due to its ability to elevate hippocampal cGMP, which has been associated with improved long-term potentiation (LTP). ^[7] However, translation to human cognition has produced inconsistent results across small trials, and the mechanism remains unclear: whether PDE5 inhibition in hippocampal neurons directly enhances synaptic plasticity, or whether the observed effects are secondary to improved cerebrovascular blood flow, has not been resolved. This is an active and productive area for translational researchers with access to rodent behavioral pharmacology facilities.

Anti-cancer mechanisms: The oncology applications discussed in Section 5 (Study 6) rest on a foundation of in-vitro and animal data that has not yet been validated in adequately powered human trials. The mechanisms proposed (MDSC reduction, ABC transporter inhibition, Bcl-2 downregulation) are plausible but have not been shown to operate simultaneously at physiologically achievable sildenafil concentrations. Researchers should treat oncology applications as exploratory, with in-vitro data requiring validation at concentrations that correspond to achievable in-vivo exposures.

Cardioprotection in ischemia-reperfusion: A body of preclinical literature, including work by Salloum and colleagues, suggests that PDE5 inhibition reduces infarct size in myocardial ischemia-reperfusion models through PKG-mediated opening of mitochondrial ATP-sensitive potassium channels. ^[12] Clinical translation has been attempted in small studies but has not been pursued in large-scale trials, leaving this mechanism incompletely characterized at the human level.

Sex differences in PDE5 expression and sildenafil response: Most published preclinical sildenafil research has used male animals. Emerging data suggests sex differences in PDE5 expression and cGMP signaling, particularly in cardiac and pulmonary tissue, that may produce different pharmacological responses in female models. This is an under-studied area with implications for experimental design, particularly in protocols using mixed-sex animal cohorts.

Pharmacological Context

The NO-cGMP-PDE5 axis is one of the most fundamentally conserved signaling pathways in vertebrate smooth muscle biology, and sildenafil's mechanism of action sits at a well-characterized node in this cascade. The discovery that nitric oxide (NO) activates soluble guanylyl cyclase (sGC) to produce cGMP won Furchgott, Ignarro, and Murad the Nobel Prize in Physiology or Medicine in 1998, the same year sildenafil received FDA approval. This confluence was not coincidental: sildenafil's development was directly enabled by the mechanistic understanding of NO-cGMP signaling emerging from academic laboratories in the late 1980s. ^[4]

PDE5 itself has undergone structural revision since sildenafil's development. Crystal structures of the PDE5 catalytic domain in complex with sildenafil (Zhang et al., 2004, PDB: 1TBF) revealed that the compound's binding mode involves displacement of the conserved water molecule in the catalytic magnesium coordination sphere, a feature that explains both its potency and its selectivity relative to earlier, less-specific PDE inhibitors like theophylline. This structural knowledge has since enabled structure-guided design of third-generation PDE5 inhibitors with improved isoform selectivity profiles.

From an evolutionary perspective, PDE5 is expressed across virtually all mammalian tissues with smooth muscle or vascular endothelial component, and its inhibition by sildenafil produces tissue-specific effects that reflect local cGMP concentrations, PDE5 expression levels, and the activity of upstream NO-producing enzymes (eNOS, nNOS, iNOS). This context-dependence makes sildenafil one of the more complex pharmacological tools in the research arsenal: the same compound can produce vasodilation, antiplatelet effects, anti-inflammatory signaling, or cytoprotection depending entirely on the cellular context in which it is administered.

The availability of well-validated transgenic mouse lines with tissue-specific PDE5 deletion or overexpression has greatly advanced the mechanistic dissection of sildenafil's effects. Researchers with access to these genetic tools can now use sildenafil as a pharmacological comparator to validate whether observed effects are mediated through PDE5 rather than off-target mechanisms, providing a level of mechanistic confidence not available for most research peptides or less-characterized small molecules.

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