Therapeutic Review
Alfaxalone

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Action

Alfaxalone has a rapid onset of action, provides satisfactory muscle relaxation, is not associated with accumulation after repeat dosing, allows for rapid anesthetic recovery, and causes only minimal respiratory depression.3, 4 These properties allow for alfaxalone to be titrated to effect and used for anesthetic maintenance, without prolonging recovery.5, 6

In domestic cats, it is recommended to use a constant-rate infusion of alfaxalone if anesthesia is desired for longer than approximately 5

Pharmacologic Effects

Alfaxalone has nonlinear pharmacokinetics. The absorption, distribution, metabolism, or elimination of alfaxalone may be variable with each administration. There was an unexplained rebound effect of plasma concentrations in a 2008 study that evaluated the pharmacokinetic and pharmacodynamic effects of a clinical 5 mg/kg dosage against a supraclinical 25 mg/kg dosage of alfaxalone in felines.6 This property should make it difficult to predict the onset and duration of effects; however, at standard

Use in Neonates

Alfaxalone has been shown to be safe for use in bitches undergoing cesarean section and also for primary use in dogs and cats less than 12 weeks of age. In a study, 48 pregnant bitches were induced with alfaxalone and 26 with propofol.16 Induction, anesthesia, and recovery were smooth, and no bitch fatalities were observed in either group with live puppy survival rates being 96% (alfaxalone) and 95% (propofol), 24 hours after birth.16 When compared to isoflurane, alfaxalone used as a

Use in Exotic Mammals

When darting wallabies, the maximum volume of alfaxalone that can be safely administered is 5 ml. This translates to 50 mg of alfaxalone, when administered as the sole anesthetic agent. This dosage does not produce sufficient immobilization, so alfaxalone is recommended as an adjunct rather than a single anesthetic agent. When 4 mg/kg alfaxalone is combined with 0.1 mg/kg medetomidine, wallabies achieved a suitable depth for minor procedures and recovered uneventfully after a dose of atipamezole.20

Use in Birds

Alfaxalone has been used as an anesthetic agent in birds since the 1970s. When 2 mg/kg alfaxalone was given IV in combination with isoflurane, heart and respiratory rates remained more stable than with the use of isoflurane alone.29 Occurrence of breath-holding was reduced, facilitating more rapid induction. Also, a lower flow rate of isoflurane was required when alfaxalone was used as an adjunct, and overall fewer intraoperative and postoperative side effects were observed in the avian patients.

Use in Reptiles

In a study performed on 13 healthy adult green iguanas, an endoctracheal tube could be inserted within 2 minutes, and a surgical plane of anesthesia was reached within 2 to 9 minutes of 5 mg/kg IV administration.30 The iguanas returned to full activity 20 minutes following the dosing of the animals with alfaxalone.30 The iguanas׳ heart rate, respiratory rate, pulse oxygen level, and end-tidal CO2 were monitored once probes could be attached. Alfaxalone was not found to cause significant

Use in Amphibians

Frogs are typically anesthetized with MS-222; however, they require higher doses than fish, take longer to reach an anesthetic plane, and have prolonged recoveries. Although alfaxalone given via immersion at 2 g/L for 30 minutes does not cause immobilization or anesthesia, IM administration between 10 and 17.5 mg/kg results in muscle relaxation and immobility.35

Amphibians weighing <100 g are usually anesthetized using immersion, as their skeletal muscles are too small to inject necessary volumes

Use in Fish

The only FDA-approved anesthetic agent for fish in the United States is MS-222 (tricaine methanesulfonate); however, other drugs have been tested with variable efficacy.39 MS-222 is not commonly stocked in veterinary hospitals, so finding efficacy in more available agents would facilitate procedures in aquatic species.38, 40, 41

Water immersion provides rapid and predictable anesthesthetic induction in koi carp at a dose of 1 to 10 mg/L of alfaxalone. It may also be used for maintenance at a dose

Analgesic Effects

It is critical that clinicians and researchers understand the limitations of alfaxalone including that it does not provide analgesia for the patient. Adjunctive medications are necessary if alfaxalone is used for any painful procedures.7, 10, 14 Further, alfaxalone has been shown to not enhance the antinociceptive effects of opioids. It does not appear to have any action on those receptors, unlike other neurosteroids (e.g., alphadolone), which potentiates the effect of opioids.44

Use in Practice

Alfaxalone is becoming increasingly popular in companion exotic animal practice, and more information on its use in these animal species is being published on a regular basis. When used at proper dosages, this synthetic neuroactive steroid can rapidly induce anesthesia without causing significant respiratory or cardiovascular depression or prolong recovery times. Aflaxalone will likely continue to grow in popularity as an effective anesthetic induction agent for pet exotic animal species.

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