B-298 Quantitation and Abundance of Xylazine and Xylazine Metabolites in Human Urine by LC-MS/MS
Authors: Lin Y , Farnsworth C W , Crews B O 
Published: October 2024

Abstract

Background
The most common cause of drug overdose fatality in the US involves synthetic opioids such as fentanyl. Recently the Director of the White House Office of National Drug Control Policy designated fentanyl associated with xylazine as an emerging threat to the US, and a national response plan will include xylazine testing. Xylazine, a clonidine analog is not FDA-approved for use in humans but has been increasingly detected in unregulated illicit drugs supplies. Our understanding of xylazine detection, metabolism, and clinical impact on patients, however, is limited. The objectives of this study are to develop a liquid chromatography tandem mass spectrometry (LC-MS/MS) method to investigate the presence and abundance of xylazine metabolites in remnant urine samples submitted for clinical testing.

Methods
Remnant urine samples were identified when tested positive for xylazine during routine clinical LC-MS/MS testing. All experiments were performed using an Acquity HPLC coupled to a Waters TQS-micro triple quadrupole mass spectrometer. Urines were diluted with xylazine-D6 internal standard, hydrolyzed using β-glucuronidase and separated on a 100mm x 2.1mm, 2.6μm Kinetex C18 column (Phenomenex) with a 4 minute step gradient from 2% to 98% mobile phase B (0.1% formic acid and 2mM ammonium acetate in A: water; B: methanol) with a flow rate of 400 μl/min at 50°C. Xylazine and 4-OH-xylazine were detected in positive mode using multiple reaction monitoring, with transitions optimized from standards (Cayman Chemical and/or Sigma Aldrich), resulting in the following transitions: xylazine: 221&gt;90, 221&gt;164; 4-OH xylazine: 237&gt;90, 237&gt;137. Xylazine and 4-OH-xylazine were quantified using calibration curves prepared from 1 - 5,000 ng/mL in negative urine. Product ion scanning (PIS) was used to detect four additional xylazine metabolites, based on spectral matching to previously published full scan high resolution product ion spectra. The highest abundance fragment ions and identified retention times from PIS experiments were used to perform targeted detection for sulfone-xylazine (253&gt;181, 253&gt;147), oxo-xylazine (235&gt;122, 235&gt;114), OH-sulfone-xylazine (269&gt;197, 269&gt;163), and OH-oxo-xylazine (251&gt;197, 251&gt;148).

Results
Xylazine was detected at &gt;=1 ng/mL in 38% (54/138) of fentanyl positive urines. A subset (n= 362) of urines that screened positive for opiates, amphetamines, or cocaine metabolite, were also tested but xylazine was detected in only 1 urine that did not contain fentanyl.Xylazine concentrations quantified in 61 remnant urines ranged from &lt;1 to &gt;5,000 ng/mL. 4-OH-xylazine concentrations ranged from &lt;1 to 509 ng/mL. We observed five previously reported xylazine metabolites: 4-OH-xylazine, oxo-xylazine, sulfone-xylazine, OH-oxo-xylazine, OH-sulfone-xylazine. The mean concentration ratio of 4-OH-xylazine-to-xylazine was 0.11. The mean peak area ratios (metabolite-to-xylazine) for sulfone-xylazine, OH-oxo-xylazine, OH-sulfone-xylazine, and oxo-xylazine were 3.5, 3.8, 2.1, and &lt;0.1, respectively. The proposed sulfone-xylazine and OH-oxo-xylazine metabolites were observed to have the most abundant signal of all detected metabolites, although there is currently no commercially available standard to facilitate quantification.

Conclusions
Xylazine and 4-OH-xylazine were quantitated in 61 patient urines. Four additional xylazine metabolites were observed and relative abundance to xylazine was estimated using peak areas. Future studies include quantitation of xylazine and xylazine metabolites in plasma and association of xylazine detection with clinical presentation and patient outcomes.