Trace level identifications of fentanyl and eutylone in equine plasma, pharmacological significance and probable origins – a case report and analysis

Summary: Consistent with recently increased street availability and recreational use of the potent synthetic opioid fentanyl, there has been a parallel increase in trace level plasma identifications of fentanyl in racing horses, at times in association with trace level amounts of other human recreational substances such as the synthetic cathinone eutylone, currently classified as a designer drug. Fentanyl has three basic effects in the horse, i.e., an analgesic effect, a locomotor stimulation response and a potential endurance effect. Fentanyl is considered relatively straightforward to synthesize following a four-step procedure. Eutylone is the most frequently identified cathinone-related substance identified in the US and is considered a synthetic stimulant. Eutylone is inexpensive to produce and mimics the effects of cocaine, methamphetamine and 3,4-methylenedioxymethamphetamine, commonly known as ecstasy, and is, like fentanyl, a street marketed human recreational substance. Both substances are listed in horseracing as Class 1 with Penalty Class A substances, therefore having the highest penalties for identifications in horses given their stimulant properties, according to the ARCI (Association of Racing Commissioners International). Given that racing authorities recognize the potential for substances of “human use and addiction” to inadvertently transfer to racing horses, we were asked to develop an Irrelevant Plasma Concentration (IPC) for fentanyl in horses. Additionally, the finding of a trace level of eutylone along with a trace level of fentanyl increases the likelihood that these paired trace level identifications were caused by inadvertent transfer from a human using a combination of recreational substances. With regard to fentanyl, review of the published pharmacology of fentanyl suggested that locomotor responses disappear below plasma concentrations of 5 ng/mL, with the locomotor response peaking at 50 ng/mL. Similarly, the antinociceptive effects of fentanyl require concentrations above 6.5 ng/mL. An effective plasma concentration (EPC) of 25 ng/mL was therefore decided on. Dividing this EPC by 500, the conservative Toutain & Lassourd safety factor (SF) gives a 50 pg/ml IPC for fentanyl in the horse. This value was not exceeded by any of the low fentanyl concentrations identified in 125,000 post-race samples in the 2018–2022 time range. Plasma concentrations of fentanyl in the sub-40 pg/mL range are therefore pharmacologically irrelevant with a significant likelihood of transfer from recreational users. This IPC value is consistent with a number of recent trace level plasma fentanyl identifications in equine samples and judgements in these matters by regulatory authorities that the likely source of these trace level plasma fentanyl identifications was inadvertent transfer from human recreational users to the horses in question. The first detections of fentanyl detections in racehorses occurred in the period 1978–1983 corresponding to the introduction of sensitive radioimmunoassay screening methods. This contrasts with the more recent uptick in 2018–2022, apparently due to exposure of horses to inadvertent trace level transfers from recreational users of fentanyl. Eutylone continues to be a concern, but more information on its equine pharmacokinetics is necessary before similar development of EPC and IPC can be determined.