Phenethylamine and alkylamine analogues are regularly detected in nutritional supplements aimed at sport performance enhancement and weight loss. Whereas some of these analogues are known to have sympathomimetic effects (e.g. increased heart rate and blood pressure) by activating adrenergic and trace-amine associated receptors, for many of the compounds their mechanism of action remains unknown and their use may thus pose serious health risks for consumers. Therefore, the aim of this study is to pharmacologically characterize a selection of frequently detected phenethylamine and alkylamine analogues in nutritional supplements, by investigating their activating properties of adrenergic and trace-amine associated receptors in vitro.

Cell lines overexpressing human adrenergic receptors α_1A, α_1B, α_1D and β₁ (ADRα_1A/B/D, ADRβ₁), and trace amine associated receptor 1 (TAAR1) were used to study the potency (EC₅₀) and efficacy (E_max) of the selected compounds for these receptors by quantification of the resulting Ca²⁺ influx or cAMP accumulation upon receptor activation. Concentration-response relationships are presented as percentages of the maximal signal obtained by full ADRα_1A/B/D/ADRβ₁ agonist epinephrine or full TAAR1 agonist tyramine.

Halostachine (EC₅₀=1.02-6.60 µM, E_max=55-75%), p-synephrine (EC₅₀=0.751-2.09 µM, E_max=79-88%) and octopamine (EC₅₀=1.14-8.90 µM, E_max=80.0-94.9%) were all found to (partially) activate ADRα_1A/B/D. Hordenine only activated ADRα_1B and _1D (EC₅₀=5.72, 12.9 µM, E_max=36.6, 32.1%). Additionally, isopropyloctopamine and higenamine acted as full ADRβ₁ agonists (EC₅₀=106 nM, E_max=101% and EC₅₀=25.1 nM, E_max=101%), whereas methylsynephrine (EC₅₀=16.5 µM, E_max=63.7%), p-synephrine (EC₅₀=21.7 µM, E_max=59.1%) and octopamine (EC₅₀=5.72 µM, E_max=87.5%) only partially activated ADRβ₁. The studied alkylamines did not activate ADRα_1A/B/D and ADRβ₁. Experiments with TAAR1-overexpressing cells are in progress.

Our results indicate that phenethylamines detected in supplements can directly activate the sympathetic nervous system by activating adrenergic receptors. The use of nutritional supplements containing these compounds may therefore potentially lead to adverse health effects. More research is needed to further assess the potential risks of these supplements.