Xylazine – Another Dangerous Street Drug

Xylazine is the latest veterinary tranquilizer to be sold as a street drug. It has no approved human uses.  It is used as both a light and general anesthetic for horses depending on the extent of the surgery. Xylazine is a presynaptic alpha-2 adrenoceptor agonist inhibiting the release of norepinephrine from synaptic vesicles. This leads to decreased postsynaptic activation of adrenoceptors, inhibited sympathetic activity, leading to analgesia, sedation and anxiolysis.  This mechanism of action is also seen with clonidine and dexmedetomidine.  Xylazine has low potency and affinity for Alpha-2 receptor adrenergic receptors. It has been demonstrated by the use of a knock out genetic mouse model that the clinical effects are mediated through the alpha-2A receptor subtype (5).

Alpha-2 receptor adrenergic receptor (AR) profiles are complicated by the fact that there are 4 subtypes with central, peripheral and behavioral effects but very little seems to be written about the D subtype so I have not included it here.  The general associated mechanisms include a decrease in adenyl cyclase activity, suppressed voltage gated calcium currents, increased potassium currents and increased mitogen-activated protein kinase (MAP kinase) activity. At steady state the α-2A and α-2B receptor types are at the cell surface and the α-2C type is at the cell surface and intracellular.  Some drugs like clonidine and guanfacine promote α-2A internalization. The author (3) of the review suggests that this may account for the unique duration of signaling. α-2AR trafficking and signaling also undergoes complex regulation by a number of factors including protein kinases, G protein coupled receptors (GPCRs), and scaffolding proteins.  A table of receptor affinities for various drugs are listed below. These affinities are primarily from reference 2 and generally represent results for human cloned receptors of the averages of several experiments. Please note the very low affinities for xylazine. I have tried to corroborate these numbers from outside sources and have not been successful. If you have better affinities for xylazine please email me or post them here in the comments section.  

From a pharmacodynamic standpoint there are several relevant Alpha-2 AR polymorphisms that have been tentatively linked disease states like ADHD and hypertension. They have also been studied in heart rate, heart rate variability, blood pressure control, obesity and insulin resistance (4). As expected, these polymorphisms also effect drug response.  

Although Xylazine is approved only for veterinary uses, reports of human use and accidental or inadvertent overdoses began to appear in the 1980s.  A review of initial reports looking at the compound as an adulterant that was done in 2014 (7) and concluded that half of the human overdoses resulted in death.  

Central effects of alpha 2 agonists, results in decreased sympathetic output and resulting imbalances in the peripheral autonomic nervous system.  Decreased sympathetic output leads to the expected effects of bradycardia, hypotension, sedation and decreased level of consciousness. Unopposed vagal parasympathetic effects can lead to increasing heart block and arrhythmias.  

In addition to the central effects of α-2 agonists there are also peripheral effects.  A common α-1 and α-2 agonist used peripherally is oxymetazoline that is used as a topical nasal decongestant. It exhibits very high affinity for both receptors and the following K_is  α-2A (7.24 nM), α-2B (483.5 nM), α-2C (144.07 nM), α-1 (402.75 nM).  Peripheral α-2 adrenergic effects can lead to increased systemic vascular resistance due to effects at the level of arterioles. This is important from a toxicological perspective because it can cause hypertension and is probably the mechanism leading to soft tissue necrosis at injection sites.

The epidemiology of xylazine use is discussed in a few studies at this point (7,12,13). The original paper suggested it may have started in Puerto Rico and spread Philadelphia with the highest prevalence of overdoses in eastern states.  It is well described at this point both in terms of overdoses and as an adulterant when it is added to heroin, fentanyl, cocaine, methamphetamine, alcohol or combinations like heroin + cocaine. There are expected synergies with opioids including a depressed level of consciousness, and decreased respiratory drive. Synergies with stimulants would include increased likelihood of cardiac arrhythmias, hypertension, and tissue necrosis.

The CDC recently published a study of xylazine in Cook County, IL (Chicago area) in MMWR (12).  The study ran from January 2017 to October 2021.  Xylazine associated deaths were defined as positive post-mortem toxicology in any substance related death where the intent was unintentional, undetermined or pending. The authors identified 236 xylazine associated deaths that increased over the study period and are graphed below. The graph on the right is the percentage of fentanyl associated deaths involving xylazine by month. That graph peaks at 11.4% in October. Overall, fentanyl or its metabolites was present in 99.2% of xylazine associated deaths. The authors point out that naloxone does not reverse the effects of xylazine but it should be administered for any suspected opioid use in a polypharmacy toxidrome. They also state that better surveillance for this compound is probably indicated.  

 

The toxidromes from these drug combinations can be complex so that on a clinical basis it will be hard to tell if the patient you are seeing has used xylazine. I was fortunate enough to attend a Hennepin County Medical Center Addiction Medicine Journal Club on 4/5/2022. In that presentation the pharmacology, clinical effects and toxicology of xylazine were discussed. The cases presented all had xylazine combined with other substances and severe necrosis of the lower extremities in two cases and hand and wrist in the other. In one case the patient no longer had venous access and was injecting into the area of necrosis.  All of these patients required skin grafting wanted to leave the hospital after the acute phase of intoxication had passed. In these cases, the transition to detoxification and maintenance medications is complicated because of the possible synergy between opioids and α-2 adrenergic agonists and the question of rebound or withdrawal from preadmission use of xylazine. The question of Takotsubo cardiomyopathy was discussed because some patients the literature were described as using xylazine. Rebound or withdrawal from xylazine and the associated rapid increase in catecholamines was discussed as a potential mechanism. A toxicologist attending the meeting also pointed out that with overdoses the α-2 adrenergic agonists can cause hypertension by peripheral effects and this has caused some acute cardiac problems. That toxicologist was also familiar with local testing for xylazine and it was not currently being done. He pointed out that a half life of 5 hours was determined in humans as contrasted with a few minutes in several animal species.   He suggested that in the case of a patient unresponsive to high dose naloxone, without hypercapnia via arterial blood gases, and normal brain imaging it would be reasonable to request xylazine toxicology.

In an interesting development, the FDA recently approved a dexmedetomidine sublingual film for the treatment of acute agitation in schizophrenia and bipolar disorder (14).  Dexmedetomidine has been available for intravenous use for 20 years with the indication “sedation of non-intubated patients prior to and/or during surgical and other procedures” (15).  It also has a place in critical care medicine – addressing all three aspects of the ICU triad of pain, agitation, and delirium (16). The film comes in 120 mcg and 180 mcg doses with a schedule in the package insert with dosing for adults and geriatric patients with and without varying degrees of hepatic impairment.  The clinical trials in the package insert describe the medication as effective for this indication. As a psychiatrist who spent most of his career in acute care there are fairly frequent situations where medications that are typically used to treat agitation (antipsychotics and benzodiazepines) do not work – even at high doses. It will be interesting to see if acute care psychiatrists find dexmedetomidine preparation useful. When I ran into that situation it was typically cases of severe mania with agitation or delirious mania with catatonia and the only available option was conscious sedation by anesthesiology. The other unknown at this point is how effective this medication will be over time.  The package insert specifies a maximum of two or three doses.  Clinicians will be on their own after that. It reminds me of how another α-2 adrenergic agonist – clonidine is currently used for anxiety, agitation, and insomnia. Many patients experience it as transiently effective until a more sustained preparation (typically a transdermal patch) is used.  

The appearance and gradual increase in xylazine as a street drug is not good news.  It is clearly used as an adulterant in both opioids and stimulants.  Its use can result in severe complications and death. The surveillance for this compound is not good at this time and clinicians have to have a high index of suspicion to request toxicology for it. People with substance use disorders need to be educated about this compound and its use as an adulterant and that deciding to use it with an opioid or other CNS depressants (including alcohol) is very dangerous and needs to be avoided. Using it with stimulants can also have significant negative effects.  At this point it is also an unknown danger because like fentanyl - it can be sold as anything.

 

George Dawson, MD, DFAPA

 

References:

 

1:  Törneke K, Bergström U, Neil A. Interactions of xylazine and detomidine with alpha2-adrenoceptors in brain tissue from cattle, swine and rats. J Vet Pharmacol Ther. 2003 Jun;26(3):205-11. doi: 10.1046/j.1365-2885.2003.00466.x. PMID: 12755905.

2:  PDSP Ki Database referenced as The Multiplicity of Serotonin Receptors: Uselessly diverse molecules or an embarrassment of riches? BL Roth, WK Kroeze, S Patel and E Lopez: The Neuroscientist, 6:252-262, 2000

3:  Wang Q.  α2-Adrenergic Receptors. In: Primer on the Autonomic Nervous System, Third Edition.  Robertson D, Biaggioni I, Burnstock G, Low PA, Paton JFR. 2012. Elsevier, Amsterdam. 55-58.

4:  Matušková L, Javorka M. Adrenergic receptors gene polymorphisms and autonomic nervous control of heart and vascular tone. Physiol Res. 2021 Dec 30;70(Suppl4):S495-S510. doi: 10.33549/physiolres.934799. PMID: 35199539.

5:  Kitano T, Kobayashi T, Yamaguchi S, Otsuguro K. The α_2A -adrenoceptor subtype plays a key role in the analgesic and sedative effects of xylazine. J Vet Pharmacol Ther. 2019 Mar;42(2):243-247. doi: 10.1111/jvp.12724. Epub 2018 Nov 11. PMID: 30417462.

6:  Weerink MAS, Struys MMRF, Hannivoort LN, Barends CRM, Absalom AR, Colin P. Clinical Pharmacokinetics and Pharmacodynamics of Dexmedetomidine. Clin Pharmacokinet. 2017 Aug;56(8):893-913. doi: 10.1007/s40262-017-0507-7. PMID: 28105598; PMCID: PMC5511603.

7:  Ruiz-Colón K, Chavez-Arias C, Díaz-Alcalá JE, Martínez MA. Xylazine intoxication in humans and its importance as an emerging adulterant in abused drugs: A comprehensive review of the literature. Forensic Sci Int. 2014 Jul;240:1-8. doi: 10.1016/j.forsciint.2014.03.015. Epub 2014 Mar 26. PMID: 24769343.

8:  Sinclair MD. A review of the physiological effects of alpha 2-agonists related to the clinical use of medetomidine in small animal practice. Can Vet J. 2003 Nov;44(11):885-97. PMID: 14664351; PMCID: PMC385445.

9:  Giovannitti JA Jr, Thoms SM, Crawford JJ. Alpha-2 adrenergic receptor agonists: a review of current clinical applications. Anesth Prog. 2015 Spring;62(1):31-9. doi: 10.2344/0003-3006-62.1.31. PMID: 25849473; PMCID: PMC4389556.

10:  Kanagy NL. Alpha(2)-adrenergic receptor signalling in hypertension. Clin Sci (Lond). 2005 Nov;109(5):431-7. doi: 10.1042/CS20050101. PMID: 16232127.

Activation of alpha(2A)-ARs in cardiovascular control centres of the brain lowers blood pressure and decreases plasma noradrenaline (norepinephrine), activation of peripheral alpha(2B)-ARs causes sodium retention and vasoconstriction, whereas activation of peripheral alpha(2C)-ARs causes cold-induced vasoconstriction

11:  Talke P, Lobo E, Brown R. Systemically administered alpha2-agonist-induced peripheral vasoconstriction in humans. Anesthesiology. 2003 Jul;99(1):65-70. doi: 10.1097/00000542-200307000-00014. PMID: 12826844.

12:  Chhabra N, Mir M, Hua MJ, et al. Notes From the Field: Xylazine-Related Deaths — Cook County, Illinois, 2017–2021. MMWR Morb Mortal Wkly Rep 2022;71:503–504. DOI: http://dx.doi.org/10.15585/mmwr.mm7113a3

13:  Friedman J, Montero F, Bourgois P, Wahbi R, Dye D, Goodman-Meza D, Shover C. Xylazine spreads across the US: A growing component of the increasingly synthetic and polysubstance overdose crisis. Drug Alcohol Depend. 2022 Apr 1;233:109380. doi: 10.1016/j.drugalcdep.2022.109380. Epub 2022 Feb 26. PMID: 35247724.

14:  FDA Package Insert. IGALMITM (dexmedetomidine) sublingual film, for sublingual or buccal use.  April 5, 2022.  https://www.igalmihcp.com/igalmi-pi.pdf

15:  FDA Package Insert.  Dexmedetomidine hydrochloride injection. 1999. https://www.accessdata.fda.gov/drugsatfda_docs/label/2015/206628s000lbl.pdf

16:  Reade MC, Finfer S. Sedation and delirium in the intensive care unit. N Engl J Med. 2014 Jan 30;370(5):444-54. doi: 10.1056/NEJMra1208705. PMID: 24476433.