Tadalafil is a small-molecule inhibitor of cGMP-specific phosphodiesterase type 5 (PDE5), built on a tetrahydro-beta-carboline scaffold fused to a diketopiperazine ring. It is a reference compound for PDE5 in enzyme kinetics, phosphodiesterase selectivity panels and cell-based cGMP signalling assays, and it is often run alongside sildenafil and vardenafil as a structurally distinct comparator. NuVion supplies tadalafil as a laboratory chemical for in vitro research use only.
Key facts
| Type | Small molecule, PDE5 inhibitor (not a peptide) |
| Molecular formula | C22H19N3O4 |
| Molecular weight | 389.4 g/mol |
| CAS number | 171596-29-5 |
| Stereochemistry | (6R,12aR), two defined stereocentres |
| Synonyms | IC351; (6R,12aR)-6-(1,3-benzodioxol-5-yl)-2-methyl-2,3,6,7,12,12a-hexahydropyrazino[1′,2′:1,6]pyrido[3,4-b]indole-1,4-dione |
| Supplied form | Dry powder in a sealed vial |
Tadalafil
$35 AUD
View productResearch use only. Not for human or veterinary use.
Structure and chemistry
Tadalafil is not a peptide, although it is made from an amino acid. The core is a tetrahydro-beta-carboline, the ring system that forms when the indole of tryptophan condenses with an aldehyde. In tadalafil the aldehyde partner is piperonal, which supplies the 1,3-benzodioxol-5-yl group at C6. The tryptophan carboxylate is then closed into a 2,5-diketopiperazine with sarcosine, giving the N-methyl piperazinedione that fuses to the carboline. The synthesis runs from D-tryptophan methyl ester through a Pictet-Spengler condensation, N-acylation with chloroacetyl chloride and ring closure with methylamine. The D-tryptophan sets the configuration at C12a, and the cis product of the Pictet-Spengler step supplies the C6 stereocentre.
Stereochemistry matters for activity. The cis (6R,12aR) diastereomer is the PDE5 inhibitor; the trans diastereomers and the enantiomer are far weaker, so chiral purity is a meaningful specification. The C12a proton is alpha to a carbonyl and can epimerise under strongly basic conditions, which is one reason stock solutions are kept away from base. The molecule has no ionisable group across the working pH range, so it is neutral and moderately lipophilic, practically insoluble in water (of the order of micrograms per millilitre) and freely soluble in DMSO. The indole chromophore absorbs near 285 nm, which is the wavelength used for HPLC detection. As a dry solid it is chemically stable when kept sealed and away from light.
Mechanism of action
PDE5A is a homodimeric enzyme in which each subunit carries two N-terminal GAF domains and a C-terminal catalytic domain. The catalytic domain holds two divalent metal ions (zinc and magnesium) that activate a water molecule to hydrolyse the 3′-phosphodiester bond of cGMP, producing 5′-GMP. GAF-A binds cGMP allosterically, and phosphorylation of Ser102 by cGMP-dependent protein kinase (PKG) increases catalytic activity, forming a feedback loop that limits cGMP signals in cells that express the enzyme.
Tadalafil is a competitive inhibitor at the catalytic site. Crystal structures of the PDE5 catalytic domain show the planar carboline and diketopiperazine rings clamped between hydrophobic residues, in a pi-pi interaction with Phe820 and forming a hydrogen bond with Gln817, the invariant glutamine that recognises the guanine ring of cGMP, while the benzodioxole group occupies an adjacent hydrophobic pocket. Inhibition constants are in the low nanomolar range. Selectivity over PDE6, the photoreceptor enzyme with the most similar catalytic site, is roughly 700-fold, and selectivity over PDE1 to PDE4 and PDE7 to PDE10 exceeds 10,000-fold. Selectivity over PDE11 is only about an order of magnitude, which distinguishes tadalafil from sildenafil and vardenafil and is used experimentally to flag PDE11-dependent responses.
In cells, tadalafil raises cGMP only where a guanylyl cyclase is producing it: soluble guanylyl cyclase activated by nitric oxide, or particulate guanylyl cyclases activated by natriuretic peptides. The higher cGMP concentration activates PKG I, which phosphorylates VASP at Ser239, RhoA at Ser188, the myosin phosphatase subunit MYPT1, IRAG, phospholamban and large-conductance calcium-activated potassium channels. In vascular smooth muscle cells the sum of these events is lower cytosolic calcium, less myosin light chain phosphorylation and reduced actomyosin contractility. cGMP also cross-talks with the cAMP pathway by inhibiting PDE3 and activating PDE2. Because basal cGMP is low in most cell lines, cell assays pair tadalafil with a nitric oxide donor such as DETA-NONOate, or with a natriuretic peptide, to produce a measurable signal.
Research applications
- Reference inhibitor in recombinant PDE5A catalytic-domain assays (scintillation proximity, fluorescence polarisation, IMAP and luminescence-based cGMP depletion formats) for IC50 determination and Ki calculation.
- Comparator in phosphodiesterase selectivity panels across PDE1 to PDE11, particularly as the PDE11-cross-reactive control.
- cGMP accumulation (HTRF or ELISA) and VASP Ser239 phosphorylation readouts in vascular smooth muscle cells, platelets and HEK293 cells co-expressing soluble guanylyl cyclase.
- Live-cell imaging with FRET-based cGMP biosensors such as cGi500 to resolve cGMP dynamics after guanylyl cyclase stimulation.
- Structural and biophysical work on the PDE5 catalytic domain, including co-crystallisation, surface plasmon resonance and isothermal titration calorimetry.
- Analytical method development: RP-HPLC with UV detection near 285 nm, chiral separation of the four stereoisomers, and LC-MS identity work.
Tadalafil is listed with NuVion’s research peptides as one of the small-molecule tool compounds in the range.
Handling in the laboratory
Tadalafil differs from the peptides in the range in one practical respect: its aqueous solubility is far too low for a water-based stock. The usual approach is a 10 mM stock in DMSO, stored in single-use aliquots, diluted into assay buffer or medium immediately before use so that the final DMSO concentration stays at or below 0.1% and matches the vehicle control. Bacteriostatic water on its own will not dissolve the compound at stock concentrations. The reconstitution calculator still handles the mass-to-concentration arithmetic once a solvent volume has been chosen, using the molecular weight of 389.4 g/mol. Dilutions above a few micromolar in aqueous buffer can precipitate, so serial dilutions are checked for turbidity.
The sealed vial is kept dry, away from light and stored as directed on the product documentation. Solutions are kept refrigerated or frozen, protected from light, and used within the period given on the documentation, avoiding repeated freeze-thaw cycles. The compound is characterised by RP-HPLC for purity and by mass spectrometry for identity.
Testing and supply from NuVion
NuVion’s tadalafil is manufactured at a GMP-audited facility and independently tested by Janoshik Analytical, with purity determined by RP-HPLC and identity confirmed by mass spectrometry. Most batches are tested, and the Certificate of Analysis for a tested batch is published on the product page and in the COA library. The compound is supplied in sealed vials and dispatched from within Australia.
Related compounds
Other signalling tool compounds in the range include PT-141, a cyclic melanocortin receptor agonist, and kisspeptin, the RFamide agonist of KISS1R, both of which act through G protein-coupled receptors upstream of second messenger pathways.
Frequently asked questions
What is tadalafil used for in research?
It is a reference inhibitor of PDE5. Laboratories use it to calibrate phosphodiesterase enzyme assays, to build selectivity panels across the PDE families, and to raise cGMP in cell-based assays where the NO-sGC-cGMP-PKG pathway is being studied. Its cross-reactivity with PDE11 makes it a useful comparator against sildenafil and vardenafil.
Is tadalafil a peptide?
No. It is a small molecule of 389.4 g/mol with no peptide bonds in the usual sense, although its carboline core derives from tryptophan and its diketopiperazine ring is formed from a second amino acid, sarcosine. It is handled like other small-molecule inhibitors, with a DMSO stock and vehicle-matched controls.
Is tadalafil a therapeutic good in Australia?
No. NuVion’s tadalafil is a laboratory chemical for in vitro research. It is not included in the Australian Register of Therapeutic Goods, has not been assessed by the Therapeutic Goods Administration, and is not for human or veterinary use.
How is tadalafil dissolved and stored?
The powder is dissolved in DMSO to make a concentrated stock, which is aliquoted and frozen. Working dilutions are made fresh in buffer or medium. The sealed vial is kept dry and away from light as directed on the product documentation.
Research use only. This product is a laboratory chemical supplied for in vitro research. It is not included in the Australian Register of Therapeutic Goods and has not been assessed by the Therapeutic Goods Administration for quality, safety or efficacy. It is not for human or veterinary use, and nothing on this page is a representation about therapeutic use.
