Tesofensine: Phenyltropane Chemistry and Monoamine Transporter Structure

Tesofensine belongs to the phenyltropane series, constructed on the 8-azabicyclo[3.2.1]octane bicycle that also carries the natural tropane alkaloids. The full systematic name is (1R,2R,3S,5S)-3-(3,4-dichlorophenyl)-2-(ethoxymethyl)-8-methyl-8-azabicyclo[3.2.1]octane, describing an aryl substituent at carbon 3, an ether-linked ethoxymethyl arm at carbon 2, and a methylated bridgehead nitrogen.

The computed descriptor set explains most of the compound's physical behaviour. PubChem reports an XLogP of 4.5, a topological polar surface area of only 12.5 square angstroms, no hydrogen-bond donors, two acceptors and four rotatable bonds. A neutral molecule that greasy and that rigid partitions readily into lipid, which is consistent with the very large apparent volume of distribution recovered from population modelling and with the long terminal elimination phase discussed further below.

Stereochemistry is the part most easily lost in supplier documentation. Four defined stereocentres mean sixteen possible configurations, of which one is the compound in the published pharmacology, and the two-dimensional formula C17H23Cl2NO is identical across all of them. The Protein Data Bank assigns the modelled ligand its own chemical component identifier, A1EA1, and identity work therefore has to establish configuration as well as exact mass 327.11567 and InChIKey VCVWXKKWDOJNIT-ZOMKSWQUSA-N.

One structural detail separates this scaffold from cocaine, which shares the same bicycle. Cocaine carries a carbomethoxy ester at carbon 2; tesofensine carries an ether at the equivalent position. Ethers are not substrates for the carboxylesterases that clear cocaine, and consistent with that, the metabolites recovered from human urine are dealkylation and hydroxylation products rather than hydrolysis products.

Tesofensine inhibits all three monoamine transporters within roughly a sixfold window, placing it in the triple reuptake inhibitor class rather than among selective agents. Uptake assays in rat brain synaptosomes returned half-maximal inhibitory concentrations of 1.7 nM for noradrenaline, 6.5 nM for dopamine and 11 nM for serotonin, giving a rank order of NET, then DAT, then SERT.

A recombinant assay published in 2025 re-measured potency against purified human machinery. Concentration-inhibition curves for tritiated dopamine uptake, run across a range spanning 10 pM to 100 microMolar in three biologically independent experiments, gave a wild-type human DAT half-maximal inhibitory concentration of 9.12 nM, close to the older synaptosome figure for dopamine.

The same work used alanine substitutions to test which contacts carry the potency. Replacing phenylalanine 326 in transmembrane helix 6 shifted the inhibitory concentration from 9.12 nM to 816.5 nM, roughly ninetyfold, while replacing serine 422 in helix 8 shifted it to 204.2 nM. Both mutants retained measurable uptake activity, so the loss reflects inhibitor binding rather than a broken transporter.

Tesofensine was resolved inside the human dopamine transporter at 2.8 angstrom resolution in 2025, in a structure series that also captured four other triple reuptake inhibitors. The construct was an amino-terminally truncated human DAT expressed in HEK-293F cells, purified in lauryl maltose neopentyl glycol with cholesteryl hemisuccinate, and reconstituted into nanodiscs made from brain polar lipids and cholesterol.

The resolved density is described as cloverleaf-shaped and sits in the central cavity, cradled by transmembrane helices 1, 3, 6 and 8 about 15 angstroms below the extracellular membrane surface. Because that pocket remains open to the extracellular side, the complex represents an outward-facing transporter. The tropane bicycle sits between helices 1 and 6, surrounded by phenylalanine 76, aspartate 79 and phenylalanine 326, and its protonated bridgehead nitrogen forms a salt bridge to the aspartate 79 carboxylate.

The two substituents point in different directions. The ethoxymethyl arm projects toward the extracellular side near phenylalanine 155 and tyrosine 156, while the dichlorophenyl ring makes a T-shaped aromatic contact with tyrosine 156 and is enclosed by serine 149, valine 152 and serine 422. Three glycine residues, 323 in helix 6 and 425 and 426 in helix 8, contribute short side chains that make room for the ligand rather than contacting it directly.

Ion density in the same map is informative. Sodium was resolved at the Na2 site, coordinated by the side chains of aspartate 421 and serine 422 together with backbone carbonyl oxygens from glycine 75, valine 78 and leucine 418, and chloride at its canonical site coordinated by asparagine 82, tyrosine 102, glutamine 317, serine 321 and serine 357. No density appeared at the Na1 site, which the authors attribute to the bound inhibitor disrupting that coordination.

Companion structures answered whether the pose generalises. Human NET and human SERT complexes were solved at 2.9 and 3.2 angstroms, and in all three transporters the compound occupies the same central cavity formed by helices 1, 3, 6 and 8 and stabilises an outward-facing state. Equivalents of aspartate 79, tyrosine 156, phenylalanine 320 and phenylalanine 326 adopt near-identical side-chain rotamers, even though DAT phenylalanine 76 becomes phenylalanine 72 in NET and tyrosine 95 in SERT, and DAT phenylalanine 155 becomes tyrosine 151 and tyrosine 175 respectively.

Only the ethoxymethyl arm moves between the three. It faces the extracellular side in DAT, rotates toward the intracellular side and helix 8 in SERT, and was modelled in both orientations in NET, which the authors read as two coexisting binding modes arising from the absence of strong contacts to that arm. The tropane ring and dichlorophenyl group hold consistent contacts throughout, and are therefore the anchoring elements. Coordinates are deposited as 9J6S, 9VWR and 9VWS with maps EMD-61181, EMD-65400 and EMD-65401.

Tesofensine and cocaine are both tropane alkaloid-family molecules that occupy the same central pocket of the dopamine transporter, and the 2024 human DAT cocaine structure provides the direct comparison. Superposing the two shows the bicyclic tropane rings landing in almost the same position, so divergence comes from the carbon 2 substituent and how far the aryl ring reaches.

In cocaine the phenyl group is attached through an ester linkage, which lets it extend deeper into the cavity between helices 3 and 8 than the dichlorophenyl group of tesofensine manages. That difference weakens the aromatic contact between the phenyl ring and tyrosine 156, and the authors of the 2025 series propose it as a partial explanation for cocaine binding the dopamine transporter roughly fiftyfold more weakly than tesofensine.

The shared scaffold has a practical consequence in imaging chemistry. Radioligands used to visualise the dopamine transporter, including iodine-123 labelled FP-CIT and carbon-11 labelled beta-CIT-FE, are themselves phenyltropanes competing for the same pocket, so any occupancy by tesofensine reduces radioligand binding. A 2023 systematic review of interference in dopamine transporter imaging names the compound among agents shown to block striatal binding in vivo.

Tesofensine occupancy of the striatal dopamine transporter has been quantified directly in humans by positron emission tomography using carbon-11 labelled beta-CIT-FE. Across steady-state exposures, mean striatal occupancy varied between 18 percent and 77 percent, a sigmoid maximum-effect model fitted the plasma-concentration relationship, the estimated ceiling was about 80 percent, and half that ceiling fell near 4 ng/mL.

Rodent microdialysis has traced the neurochemical consequence of that blockade. Work in diet-manipulated rats reported reduced baseline extracellular dopamine in the nucleus accumbens and prefrontal cortex relative to standard-chow controls, an increase in accumbal dopamine after acute exposure to 2.0 mg/kg tesofensine in the diet-manipulated group but not in controls, and discrete changes in striatal dopamine D2 receptor expression and transporter binding.

Circuit-level recording has localised one downstream target population. In vivo ensemble electrophysiology sampled 343 lateral hypothalamic neurons in chow-fed rats and 361 in high-fat-diet rats, and paired recordings in Vgat-ChR2 and Vgat-IRES-cre transgenic mice, using optogenetic tagging at 50 Hz with 10 ms pulses, identified a subset of GABAergic lateral hypothalamic neurons that tesofensine inhibited. Chemogenetic silencing of the same population using a Gi-coupled designer receptor reproduced part of the effect.

Longer exposure alters transcription in hippocampus. Fourteen days of tesofensine in rats raised brain-derived neurotrophic factor messenger RNA in region CA3 by 35 percent and activity-regulated cytoskeleton-associated protein messenger RNA in CA1 by 65 percent, with parallel increases in Ki-67 and NeuroD immunoreactive cells; five days produced neither change. A 2025 rat study using DeepLabCut pose tracking characterised motor output, reporting head-weaving in females only and to a lesser degree than other dopaminergic comparators, with orolingual dyskinesia the most frequent motor signature.

Tesofensine is cleared principally by cytochrome P450 3A4, which removes an alkyl group to give the desalkyl metabolite designated M1 or NS2360, historically the only metabolite detectable in human plasma. Population modelling drew on 320 subjects contributing 1,969 parent and 1,714 metabolite concentrations, resolving apparent volumes of distribution of 653 and about 502 litres respectively.

Those volumes, combined with low apparent clearance, produce unusually long terminal phases: 234 hours with a standard deviation of 89 hours for the parent and 374 hours with a standard deviation of 98 hours for the metabolite. The bioanalysis behind that model was high-performance liquid chromatography with tandem mass spectrometry and deuterated internal standards, calibrated linearly from 0.1 to 50 ng/mL with inaccuracy under 6 percent and imprecision under 11 percent.

A 2026 study extended the metabolite map using urine. In vitro metabolism experiments were combined with urine collected for up to 600 hours from six volunteers after 483 micrograms of tesofensine taken as a supplement product; samples were prepared by solid-phase extraction and analysed by liquid chromatography with high-resolution mass spectrometry. Four principal metabolites were characterised from their tandem mass spectrometric dissociation patterns: three dealkylated species, M1 to M3, and one hydroxylated and glucuronidated species, M4.

The validated urinary method reached a limit of detection of 0.01 ng/mL with 34 percent recovery and 8 percent interday imprecision. Interindividual variability was marked: peak concentrations of 1 to 4 ng/mL appeared anywhere from 4 to 46 hours, and detection windows reached 500 hours. Tesofensine is listed by the World Anti-Doping Agency under section S6, stimulants, prohibited in competition only, and appears in the 2026 annual review of analytical approaches in sports drug testing.

Tesofensine free base is a neutral, markedly lipophilic amine with an XLogP of 4.5, a polar surface area of 12.5 square angstroms and no hydrogen-bond donors, so aqueous solubility is poor and organic solvents are the usual route into solution. Formulation work generally uses the citrate salt, C23H31Cl2NO8, molecular mass 520.4 g/mol, in which the bridgehead nitrogen is protonated.

Published formulation chemistry gives a usable picture of solid-state compatibility. A granted European application covering tesofensine compositions places the active at 0.30 to 0.70 percent by weight of the tablet and identifies povidone, crospovidone, copovidone, macrogol 400 and colloidal silicon dioxide as incompatible, along with titanium dioxide in the tablet core. Hydroxypropylcellulose, lactose monohydrate, microcrystalline cellulose, croscarmellose sodium and magnesium stearate were the compatible set carried into the final composition.

Stability figures from that same document are specific. Tablets held in PVC/PVDC blisters below 30 degrees Celsius met specification for up to 36 months, and in high-density polyethylene bottles for 24 months. Two degradation products were detected across all storage conditions but stayed below identification and qualification thresholds, against a total degradation specification of not more than 2.0 percent by weight. Protection from light and low-temperature storage of stock solutions are the ordinary precautions for solutions of the free base.

Identity confirmation rests on more than the molecular formula, because four stereocentres share it. Exact mass 327.11567 for the free base, InChIKey VCVWXKKWDOJNIT-ZOMKSWQUSA-N and a chiral separation together establish the configuration modelled in the deposited structures, and the analytical chemistry developed for urine, solid-phase extraction followed by liquid chromatography with high-resolution mass spectrometry, transfers directly to characterising bulk material and its dealkylated and glucuronidated related substances.