A fatal mono-intoxication with 4-fluoroisobutyrylfentanyl: Case report with postmortem concentrations

We report on a case of a 35-year-old man who died suddenly and unexpectedly due to a 4-fluoroisobutyrylfentanyl (4-FIBF) mono-intoxication. Pathological, toxicological and chemical investigations were conducted at the Netherlands Forensic Institute. A full three-cavity forensic pathological examination was performed according to international guidelines. Biological samples obtained during autopsy were comprehensively investigated for the presence of toxic substances using headspace gas chromatography (GC) with flame ionization detection, liquid chromatography-time-of-flight mass spectrometry (LC-TOF-MS), GC-MS, high-performance LC with diode array detection and LC-tandem MS (LC-MS-MS). The seized crystalline substance found next to the body was investigated using a presumptive color test, GC-MS, Fourier-transform infrared spectroscopy and nuclear magnetic resonance. Pathological investigation identified minor lymphocytic infiltrates in the heart, considered irrelevant for the cause of death. Toxicological analysis of the victims' blood indicated the presence of a fluorobutyrylfentanyl (FBF) isomer, with no other compounds detected. The FBF isomer was identified in the seized crystalline substance as 4-FIBF. 4-FIBF concentrations were quantified in femoral blood (0.030 mg/L), heart blood (0.12 mg/L), vitreous humor (0.067 mg/L), brain tissue (>0.081 mg/kg), liver tissue (0.44 mg/kg) and urine (approximately 0.01 mg/L). Based on the outcomes of the pathological, toxicological and chemical investigations, the cause of death of the deceased was attributed to a fatal 4-FIBF mono-intoxication. The presented case underlines the added value of a combined bioanalytical and chemical investigative approach to identify and subsequently quantify fentanyl isomers in postmortem cases. Furthermore, it demonstrates the importance of investigating the postmortem redistribution of novel fentanyl analogs to establish reference values and to subsequently allow for correct interpretation of cause of death analysis in future casework.