Table 3.

In vitro and in vivo metabolism of synthetic opioids.

Compound	Study type	Matrix (species)	Total # phase I metabolites	Major metabolites (decreasing order of relative intensity)	Phase II metabolites	Recommended target analytes in urine	References
Acetyl Fentanyl	In vivo	Urine (humans)	6	– Hydroxylated metabolite at phenylethyl ring – Hydroxy-methoxy metabolite at phenylethyl ring	Glucuronide of hydroxylated metabolites	– Hydroxylated metabolite at phenylethyl ring – Hydroxy-methoxy metabolite at phenylethyl ring – Acetyl fentanyl	Melent'ev et al., 2015
	In vitro	Pool human liver hepatocytes	7	– N-dealkylated metabolite at the piperidine moiety – Hydroxylated metabolites at the ethyl linker – Dihydroxylation at phenylethyl ring			Watanabe et al., 2017
	In vivo	Urine (human)	24	– Hydroxy-methoxy metabolite at phenylethyl ring – Hydroxy metabolite at the ethyl linker – N-dealkylated metabolite at the piperidine moiety	Glucuronides and sulfates of hydroxy-metabolites	– Hydroxy metabolite at the ethyl linker – Hydroxy-methoxy metabolite at phenylethyl ring – Acetyl fentanyl
	In vitro	Pluripotent stem cell-derived hepatocytes	6	– N-dealkylated metabolite at the piperidine moiety – Hydroxylated metabolite at phenylethyl ring – Hydroxylated metabolites at the ethyl linker			Kanamori et al., 2018
Acrylfentanyl	In vitro	Pool human liver hepatocytes	8	– N-dealkylated metabolite at the piperidine moiety – Hydroxylated metabolite at the piperidine moiety – Hydroxylated metabolite at the ethyl linker			Watanabe et al., 2017
	In vivo	Urine (human)	12	– N-dealkylated metabolite the piperidine moiety – Hydroxylated at the ethyl linker – Dihydroxylated metabolite at the piperidine and at the ethyl linker – Hydroxy-methoxy metabolite at phenylethyl ring	Glucuronides of hydroxy-metabolites	– Hydroxylated at the ethyl linker – Dihydroxylated metabolite at the piperidine and at the ethyl linker – Acrylfentanyl
Butyrfentanyl	In vitro	Human liver microsomes	36	– N-dealkylated metabolite – Hydroxy-metabolite at butanamide chain – Dihydroxy-metabolite at phenylethyl ring			Steuer et al., 2017
	In vivo	Urine (human)		– Carboxy-metabolite at butanamide chain – Hydroxy-metabolite at butanamide chain – Carboxy at butanamide chain and hydroxy at phenylethyl ring metabolite	Glucuronides of hydroxy-metabolites	– N-dealkylated metabolite – Hydroxy-metabolite at butanamide chain – Carboxy-metabolite at butanamide chain
		Blood (human)		– Carboxy-metabolite at butanamide chain
Carfentanil	In vitro	Pool human liver hepatocytes	11	– Monohydroxylated metabolite at of piperidine ring N-dealkylated metabolite	Glucuronide of hydroxylated metabolite	– Monohydroxylated metabolite at of piperidine ring	Feasel et al., 2016
Furanylfentanyl (Fu-F)	In vitro	Human hepatocytes Pooled human hepatocytes	13	– Amide hydrolysis – N-dealkylated metabolite – Dihydrodiol metabolite at furan group			Watanabe et al., 2017
	In vivo	Urine (human)	9	– Amide hydrolysis – Dihydrodiol metabolite at furan group – Dihydrodiol at furan group and hydroxy at ethyl linker metabolite	Glucuronide and sulfate of hydroxylated metabolites	– Dihydrodiol metabolite at furan group
	In vivo	Urine (human)		– Amide hydrolysis – Dihydrodiol metabolite at furan group	Sulfate metabolite of amide hydrolysis metabolite	– Sulfate metabolite of amide hydrolysis metabolite – Dihydrodiol metabolite at furan group	Goggin et al., 2017
	In vitro	Human liver microsomes	17	– Despropnionyl fentanyl – Monohydroxylated metabolite – N-dealkylated metabolite			Gaulier et al., 2017
	In vitro	HepaRG cell Line	17	– Despropnionyl fentanyl – N-dealkylated metabolite – Dihydrodiol metabolite (at furan group)	Glucuronide hydroxylated metabolite
4-Fluoro-isobutyrylfentanyl	In vitro	Pooled human hepatocytes	9	– N-dealkylated metabolite of the piperidine moiety – Monohydroxy metabolite at the piperidine ring or at the ethyl linker – N-oxidation at the piperidine ring			Watanabe et al., 2017
	In vivo	Urine (human)	13	– N-dealkylated metabolite of the piperidine moiety – Monohydroxy metabolite at the piperidine ring or at the ethyl linker – Hydroxymethoxy metabolite at phenylethyl ring	Glucuronide hydroxylated metabolites	– Monohydroxy metabolite at the piperidine ring or at the ethyl linker – Hydroxymethoxy metabolite at phenylethyl ring
Isofentanyl	In vitro	Urine (rats)	11	– N-dealkylation followed by hydroxylation of the alkyl and aryl moiety – Hydroxylation of the propanamide side chain followed by oxidation to the carboxylic acid – Hydroxylation of the benzyl moiety followed by methylation – N-oxidation	Glucuronides of hydroxy metabolites	– N-dealkylated metabolite	Meyer et al., 2012
3-methylfentanyl	In vivo	Urine (rats)	9 /5	– N-dealkylation followed by hydroxylation of the alkyl and aryl moiety – Hydroxylation of the propanamide side chain followed by oxidation to the carboxylic acid – Hydroxylation of the benzyl moiety followed by methylation	Glucuronides of hydroxy metabolites	– N-dealkylated metabolite	Meyer et al., 2012
Ocfentanil (OcF)	In vitro	Human liver microsomes	3	– O-desmethyl metabolite – Monohydroxylated metabolite at phenylethyl ring – O-desmethyl metabolite hydroxylated at phenylethyl ring	Glucuronide of O-desmethylated metabolite		Allibe et al., 2018
	In vivo	– Blood (human, n = 1) – Bile (human, n = 1)	3	– O-desmethyl metabolite – Monohydroxylated metabolite at phenylethyl ring – O-desmethyl metabolite hydroxylated at phenylethyl ring		– O-desmethylated-metabolite
AH-7921	In vitro	Human hepatocytes	11	– N-demethyl metabolite – N-dis-demethyl metabolite – N-demethyl metabolite hydroxylated at cyclohexyl	Glucuronide demethylated metabolite		Wohlfarth et al., 2016
	In vivo	Urine (human)	10	– N-demethylation – N-dis-demethyl metabolite	Glucuronide demethylated metabolite	– N-demethylation – N-dis-demethyl metabolite
MT-45	In vitro	Rat hepatocytes	10	– Hydroxy metabolite – Dihydroxy metabolite – 1-cyclohexyl-piperazine	Glucuronides of hydroxy metabolites		Montesano et al., 2017
	In vivo	Urine (rat)	10	– Hydroxy metabolite – Dihydroxy metabolite – 1-cyclohexyl-piperazine – OH-1-cyclohexyl-piperazine	Glucuronides of hydroxy metabolites	– Hydroxy metabolite – Dihydroxy metabolite
U-47700	In vitro	Human liver microsomes	4	– N-desmethyl-U-47700 – N,N-didesmethyl-U-47700 – N-desmethyl-hydroxy-U-47700 – N,N-didesmethyl-hydroxy-U-47700			Krotulski et al., 2018a
	In vivo	Urine (human, n = 5)	5	– N-desmethyl-U-47700 – N,N-didesmethyl-U-47700 – N-desmethyl-hydroxy-U-47700 – N,N-didesmethyl-hydroxy-U-47700 – N,N-didesmethyl-N-desmethyl-U-47700		– N-desmethyl-U-47700 – N,N-didesmethyl-U-47700
U-49900	In vitro	Human liver microsomes	5	– N-desethyl-U-49900 – N,N-didesethyl-U-49900; – N,N-didesethyl-N-desmethyl-U-49900 – N-desethyl-hydroxyo-U-49900 – N-desethyl-N-desmethyl-U-49900			Krotulski et al., 2018a
	In vivo	Urine (human, n = 5)	5	– N-desethyl-U-49900 – N,N-didesethyl-U-49900 – N,N-didesethyl-N-desmethyl-U-49900 – N-desethyl-hydroxyo-U-49900 – N-desethyl-N-desmethyl-U-49900		– N,N-didesethyl-N-desmethyl-U-49900