Cyclopropylfentanyl is a fentanyl analog implicated in 78 deaths in Europe and over 100 deaths in the United States, but toxicological information including metabolism data about this drug is scarce. The aim of this study was to provide the exact structure of abundant and unique metabolites of cyclopropylfentanyl along with synthesis routes. In this study, metabolites were identified in 13 post-mortem urine samples using liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS). Samples were analyzed with and without enzymatic hydrolysis, and seven potential metabolites were synthesized in-house to provide the identity of major metabolites. Cyclopropylfentanyl was detected in all samples, and the most abundant metabolite was norcyclopropylfentanyl (M1) that was detected in 12 out of 13 samples. Reference materials were synthesized (synthesis routes provided) to identify the exact structure of the major metabolites 4-hydroxyphenethyl cyclopropylfentanyl (M8), 3,4-dihydroxyphenethyl cyclopropylfentanyl (M5) and 4-hydroxy-3-methoxyphenethyl cyclopropylfentanyl (M9). These metabolites are suitable urinary markers of cyclopropylfentanyl intake as they are unique and detected in a majority of hydrolyzed urine samples. Minor metabolites included two quinone metabolites (M6 and M7), not previously reported for fentanyl analogs. Interestingly, with the exception of norcyclopropylfentanyl (M1), the metabolites appeared to be between 40% and 90% conjugated in urine. In total, 11 metabolites of cyclopropylfentanyl were identified, including most metabolites previously reported after hepatocyte incubation.

Fentanyl analogs constitute a particularly dangerous group of new psychoactive compounds responsible for many deaths around the world. Little is known about their metabolism, and studies utilizing liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) analysis of hepatocyte incubations and/or authentic urine samples do not allow for determination of the exact metabolite structures, especially when it comes to hydroxylated metabolites. In this study, seven motifs (2-, 3-, 4- and β-OH as well as 3,4-diOH, 4-OH-3-OMe and 3-OH-4-OMe) of fentanyl and five fentanyl analogs, acetylfentanyl, acrylfentanyl, cyclopropylfentanyl, isobutyrylfentanyl and 4F-isobutyrylfentanyl were synthesized. The reference standards were analyzed by LC-QTOF-MS, which enabled identification of the major metabolites formed in hepatocyte incubations of the studied fentanyls. By comparison with our previous data sets, major urinary metabolites could tentatively be identified. For all analogs, β-OH, 4-OH and 4-OH-3-OMe were identified after hepatocyte incubation. β-OH was the major hydroxylated metabolite for all studied fentanyls, except for acetylfentanyl where 4-OH was more abundant. However, the ratio 4-OH/β-OH was higher in urine samples than in hepatocyte incubations for all studied fentanyls. Also, 3-OH-4-OMe was not detected in any hepatocyte samples, indicating a clear preference for the 4-OH-3-OMe, which was also found to be more abundant in urine compared to hepatocytes. The patterns appear to be consistent across all studied fentanyls and could serve as a starting point in the development of methods and synthesis of reference standards of novel fentanyl analogs where nothing is known about the metabolism.

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