Elimination profile of orally administered phenylethylamine
Stimulants have been banned as performance-enhancing drugs in sport since first anti-doping rules were introduced, and they are still frequently detected in doping controls.[1-3] Most of the stimulants on the World Anti-Doping Agency’s (WADA’s) Prohibited List, such as amphetamine, cocaine or ephedrine are either synthetic drugs or compounds of plant-origin. In addition, some (semi)synthetically derived substances are also naturally produced in the human body and involved in processes of the central nervous system.[4,5] This includes 2-phenylethylamine (PEA), which represents the core structure of several stimulants and psychoactive drugs and belongs to the so-called “trace amines”, present only at trace levels in the mammalian central nervous system.[6,7]. While these structurally related biogenic amines, also including tyramine and octopamine, have long been considered as simple metabolic by-products, the characterization of mammalian trace amine associated receptors (TAAR) has led to the understanding that trace amines act as potent neuromodulators, modifying the activity of coexisting neurotransmitters.[8-11] In physiological concentrations, PEA mainly affects dopamine-mediated neurotransmission, with elevated levels of PEA resulting in increased responsiveness to dopamine.[12,13] At high concentrations, PEA has welldocumented “amphetamine-like” effects on the release and reuptake of noradrenaline, dopamine and serotonin, thereby affecting appetite, mood and mental alertness.[5,14,15] Endogenous PEA is formed by enzymatic decarboxylation of phenylalanine. With an elimination half-life of only 0.4 minutes it is rapidly metabolized, mainly by monoamine oxidase (MAO) B, to phenylacetic acid.[11,16] Accordingly, urinary levels of PEA can be influenced by various factors, including nutrition, pharmacological interventions and mental diseases.[8,17-20] Being involved in the monoaminergic neurotransmission, PEA has been associated with major human disorders, including schizophrenia, Parkinson’s disease or depression.[21-23] In a study by Szabo et al. in 2001, it has further been linked to the antidepressant effects of sport, by showing substantial increases of phenylacetic acid in urine after aerobic exercise.
Two different studies were conducted to examine the excretion pattern of 2-phenylethylamine in urine.
a) Single dose (SD) administration study (100 mg): A single dose of 100 mg of PEA was administered to 14 healthy volunteers (7 male, 7 female). Urine samples were collected both before and up to 72 h following administration. Within the first 24 h, every urine sample was collected, afterwards only two urine samples approximately every 12 h.
b) Multiple dose (MD) administration study (5x100 mg): Five doses of 100 mg PEA each were administered on five consecutive days to 14 healthy volunteers (7 male, 7 female). Before and during drug administration, morning urine specimens were collected. Within 24 h after the last dose, every urine sample was collected, afterwards only two urine samples approximately every 12 h.
Dosing was facilitated by encapsulating an amount of the nutritional supplement corresponding to 100 mg of PEA. The collected urine specimens were frozen at -20 °C until analysis. The administration studies were approved by the local ethics committee (#107/2019) and all participants provided written consent.
Offering analytical strategies to detect the misuse of substances naturally produced in the human body remains crucial in the fight against doping in sport. Investigating the metabolism of prohibited substances in sport drug testing is a kay element, in order to find efficient markers for their detection, prolong detection windows or even enable the discrimination between the abuse of substances from the consumption of contaminated food or their natural occurences in hte human body. Witrhin this study, new insights could be provided concerning the renal elimination of 2-phenylethylamine and two of its respective metabolites on the basis of human administration studies. Given the rapied metabolism and thereby low impact of PEA ingestion on its urinary concentrations, threshold levels for PEA in doping control deemed inadequate. Evaluating the concentration ratio of M1 and PEA to detect potential misuse seemed promising at first, but respecting the large inter-individual variation within the calculated ratios, the establishment of a cut-off level for this marker proved difficult. However, assessing the urinary levels of M1 and a second major metabolity, PAG, combined by means of ligistic regression analysis presents a novel approach to distinguish endogenous PEA from a potential misuse. In regard to the natural presence of both metabolites in the human body and the large variability observed within urinary levels, further research into potential confirmation methods, i.e. isotope ratio mass spectrometry, seems advisable in order to clarify potential suspicions of PEA administration.