Tolazoline hydrochloride
Applications in Veterinary Medicine and Wildlife Management
General Information and History
Tolazoline hydrochloride is a combination alpha-one and alpha-two antagonist developed to reverse the sedation and analgesia effects of xylazine hydrochloride. Tolazoline hydrochloride was developed and marketed for use in horses by Lloyd Laboratories in the 1990s. It was approved under the trade name Tolazine in 1996. Prior to this tolazoline was available as the human formulation trade marked Priscoline at a concentration of 25 mg/ml from Novartis.
Use in Wildlife Management
In field applications, as a general rule, tolazoline is more effective in antagonism of an alpha-two agonist in the non-domestic members of the family Bovidae. It appears to be equal to yohimbine in Cervidae although veterinarians in the field have reported seizures in some of Asian species of deer when tolazoline was used to reverse xylazine (Bush, R. 1999. Personal communication). Some of the key literature regarding tolazoline use in non-domestic species is as follows:
- White-tailed deer (Kreeger et al. 1986. J Wildl Dis 22:407-412 re: efficacy of
tolazoline to reverse xylazine HCl ) - Elephant (Raath, J.P. 1993. In Capture and Care Manual, pp. 484-511)
(Kock et al. 1993. In Zoo and Wildlife Medicine Current Therapy 3,
pp. 436-441) - Bighorn sheep and other ruminants. Kreeger (Handbook of Wildlife Chemical
Immobilization, 200l) states that tolazoline appears to provide a “more consistent”
antagonism than yohimbine in reversing xylazine sedation. (Allen, J. L. 1989.
Proc. AAZV, pp. 3-4) - Non-domestic bovids (Klein, L. and Klide, A. 1989. J Zoo Wildl Med 20:138-153
re: bovids appearing to respond “more reliably” to tolazoline than yohimbine)
Use in Domestic Species
Tolazoline HCl is used in horses to reverse the effects of sedation and analgesia from
xylazine HCl. Studies conducted by Lloyd Laboratories have demonstrated efficacy
of tolazoline in reversing xylazine sedation in cattle. (Powell et al. 1998. JAVMA
212(1):90-92)
Pharmacology and Chemistry
Tolazoline hydrochloride belongs to the synthetic group of alpha-adrenergic blocking
agents known as imidazole derivatives. It is a mixed alpha-1 and alpha-2 adrenergic
receptor antagonist that competitively inhibits alpha-adrenoreceptors. Tolazoline is
also a direct peripheral vasodilator that decreases peripheral resistance and increases
venous capacitance. (Tranquilli et al. 1984. JAVMA 184:1400-1402)
Chemically, tolazoline hydrochloride is 1H- imidazole, 4, 5-dihydro-2-(phenylmethyl)-
monohydrochlroide.
Pharmacokinetics
Xylazine is an alpha-2 adrenergic agonist with sedative and analgesic properties related to central nervous system depression. Administration of tolazoline hydrochloride reverses xylazine’s depressant effects resulting in rapid recovery from sedation. The competitive blocking of the alpha-2 adrenergic receptor by tolazoline hydrochloride displaces xylazine from these sites, rapidly canceling the effect of the xylazine.
CONTRAINDICATIONS AND PRECAUTIONS
Heart rate may briefly increase immediately after tolazoline HCl injection at the recom mended dose with a return to pre-treatment rate within 5-10 minutes. In safety studies, the degree and duration of tachycardia increased with higher doses, although the increased rate usually lasted less than 60 minutes.
WARNING:
Tolazoline hydrochloride is not for use in food-producing animals.
Caution
Tolazoline hydrochloride should not be administered to animals exhibiting signs of
stress, debilitation, cardiac disease, sympathetic blockage, hypovolemia or shock.
ADVERSE REACTION
When administered without prior xylazine administration in horses, tolazoline hydrochloride caused gastrointestinal hypermotility. Some horses exhibited mild colic and transient diarrhea.
RECOMMENDED DOSE RANGE
The tolazoline hydrochloride dose is 4.0 mg/kg body weight or 1.8 mg/lb (4ml/100kg or 4ml/220 lb) to reverse the sedative effects of xylazine hydrochloride in horses.
In studies with cervids, a dose of 2mg/kg of tolazoline to reverse xylazine in deer,
resulted in a reduced time to standing.
ADMINISTRATION
Tolazoline hydrochloride should be administered slowly by intravenous injection at a rate of approximately 1 ml per second. Onset of arousal is usually apparent within 5 minutes of tolazoline hydrochloride administration, depending on depth and duration of xylazine-induced sedation.
FDA Approved Presentation for Tolazoline hydrochloride
Tolazoline hydrochloride was approved by FDA in 1996 in a 100mg/ml concentration.
Compounded Forms Possible
Tolazoline hydrochloride is routinely compounded and available from ZooPharm in a 200mg/ml concentration in 30 ml multiple use vials.
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