Tolazoline HCl 200 mg/mL, Injectable Solution, 30mL
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Tolazoline (tolazoline HCl) is part of the synthetic group of α-adrenergic blocking agents known as imidazoline derivatives. It has been approved by the U.S. Food and Drug Administration (FDA) and indicated for the reversal of effects associated with xylazine in horses, but it is used extra-label in numerous species. It also reverses the analgesic effects of α-agonists. Tolazoline also antagonizes the effects of detomidine, speeds recovery and longer-lasting than atipamezole.1
As a vasodilator, tolazoline directly acts on blood vessels and increases cardiac output. It can interact to some degree with histamine, adrenergic, and cholinergic receptors, but the mechanisms of its therapeutic effects are not clear. Its mechanism of action is by means of a direct effect on peripheral vascular smooth muscle and indirect effects produced, in part, by release of endogenous histamine. Tolazoline usually reduces pulmonary arterial pressure and vascular resistance. In humans, it is used in treatment of persistent pulmonary hypertension of newborns.2,3
Tolazoline is fast-acting (usually within 5 minutes of IV administration), but may not fully reverse the effects of sedation or heart rate/rhythm. Its duration of action is relatively short, sometimes requiring repeat doses.2 In horses that received detomidine and then tolazoline one hour later, tolazoline’s effect on detomidine-induced changes in chin-to-ground distance were minimal; detomidine-induced changes in heart rate and rhythm only persisted for 15 to 20 minutes.4
Following tolazoline doses of 4 mg/kg IV in horses, the average heart rate decreased beginning 2 minutes post-injection, with heart rate nadir at 45 minutes. No apparent effect on chin-to-ground distance was noted. Packed cell volume significantly increased in all horses throughout the times sampled, with the maximal change at 15 minutes. Serum glucose concentrations increased significantly in all horses at the first time point measured through 1.5 hours postinjection.5
A 2013 study investigated the pharmacokinetics and pharmacodynamic effects of tolazoline when administered as a sole agent. Nine healthy adult horses were administered tolazoline (4 mg/kg IV) and blood samples were collected at time 0 and at various times up to 72 hours post drug administration. Plasma samples were analyzed using liquid chromatography–mass spectrometry and resulting data was analyzed using compartmental analysis.6
The study found that systemic clearance, steady state volume of distribution and terminal elimination half-life were 0.820 ± 0.182 L/h/kg, 1.68 ± 0.379 L/kg and 2.69 ± 0.212 h, respectively. Tolazoline administration had no effect on chin to ground distance, but the heart rate decreased (relative to baseline) and the percentage of atrial–ventricular block increased in all horses within 2 min of administration. Packed cell volume and glucose concentrations were also increased throughout the sampling period.6
Cautions and Warnings
According to Plumb’s Veterinary Drugs, the following drug interactions have either been reported or are theoretical in humans or animals receiving tolazoline:
- Alcohol: Accumulation of acetaldehyde can occur if tolazoline and alcohol are given simultaneously.
- Dopamine: Concurrent use may cause severe hypotension.
- Epinephrine, Norepinephrine: If large doses of tolazoline are given with either norepinephrine or epinephrine, a paradoxical drop in blood pressure can occur followed by a precipitous increase in blood pressure.3
- In horses given tolazoline alone (and no previous xylazine), doses 5 times the recommended dose resulted in GI hypermotility with resultant flatulence and defecation or attempt to defecate. Some horses exhibited mild colic and transient diarrhea. Intraventricular conduction may be slowed when horses are overdosed, with a prolongation of the QRS-complex noted.3
Tolazoline dosages per Plumb’s Veterinary Drugs are as follows:
a) Reversal of xylazine effects (labeled dose; FDA-approved): 4 mg/kg slow IV (4 mL/220 lb of body weight); administration rate should approximate 1 mL/second. (Package Insert; Tolazine®)
b) Horses: Reversal of xylazine effects (extra-label): 0.5 – 2 mg/kg IM (preferred) or slow IV; if administering IV, may wish to divide dose and give multiple smaller doses.
c) Horses: Reversal of low-dose α-agonist effects after dental procedures (extra-label): Following sedation with 1.1 – 2.2 mg/kg IM xylazine, reverse with tolazoline 0.04 mg/kg slow IV; may repeat 0.02 mg/kg slow IV 20-30 minutes later if needed.
NOTE: The Association of Racing Commissioners International Uniform Classification Guidelines for Foreign Substances has designated tolazoline a CLASS 3 DRUG.
Donkeys: Reversal of detomidine (extra-label): tolazoline 4 mg/kg IV given 15 minutes after detomidine 0.04 mg/kg IV reduced the degree and duration of sedation, ataxia, and analgesia.3
Camelids: Reversal of alpha-2 agonists (extra-label):
a) 1 – 2 mg/kg IM or SC. IV administration (especially the labeled dose of 4 mg/kg) should be avoided, as adverse reactions have occurred.
b) 0.5 – 1.5 mg/kg IV. These lower doses are now recommended for use in all ruminants, including camelids.3
Swine: Reversal of alpha-2 agonists (extra-label): 2 – 4 mg/kg IV.
Deer: Reversal of xylazine effects (extra-label):
a) 2 – 4 mg/kg slow IV; titrate to effect. Slaughter withdrawal: 30 days.
b) 2 – 4 mg/kg; 50% of dose given IV, 50% given IM.
NOTE: Tolazoline is not FDA-approved in the U.S. for use in food animals.
Where to buy Tolazoline
Tolazoline is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. TOLAZOLINE HCL 200 MG/ML INJECTABLE SOLUTION, 30ML by NexGen Pharmaceuticals is an excellent xylazine reversing agent and antagonist for intravenous use, with minimal adverse effects. Depending on the depth and duration of sedation, onset of arousal is generally apparent within 5 minutes of tolazoline administration.
This product carries numerous potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.
FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.
1Zeiler GE. A review of clinical approaches to antagonism of alpha(2)-adrenoreceptor agonists in the horse. Equine Veterinary Education. 2015;27(1):48-54.
3Plumb’s Veterinary Drugs.
4Knych HK, Stanley SD. Effects of three antagonists on selected pharmacodynamic effects of sublingually administered detomidine in the horse. Vet Anaesth Analg. 2014;41(1):36-47.
5Casbeer C, Knych HK. Pharmacokinetics and pharmacodynamic effects of tolazoline following intravenous administration to horses. Vet J. 2013;196(3):504-509.
6Casbeera, H.C., Knychab, H.K. Pharmacokinetics and pharmacodynamic effects of tolazoline following intravenous administration to horses. The Veterinary Journal, Volume 196, Issue 3, June 2013, Pages 504-509.