The anesthetic properties of steroid hormones were first identified in 1941, leading to the development of neurosteroids as clinical anesthetics. CT-1341 was developed in the early 1970’s, featuring a combination of two neurosteroids (alfaxalone and alphadolone) solubilized in Cremophor EL®, a polyethylated castor oil derivative that allows hydrophobic compounds to be carried in aqueous solution as micelles. Though also possessing anesthetic properties, alphadolone was included principally to improve the solubility of alfaxalone.
CT-1341, marketed as Althesin® and Saffan®, was characterized by smooth anesthetic induction and recovery in many species, a wide therapeutic range, and no cumulative effects with repeated administration. Its cardiorespiratory effects in humans and cats were generally mild. However, it induced severe hypersensitivity reactions in dogs, with similar reactions occasionally occurring in cats and humans. The hypersensitivity reactions associated with this formulation were linked to Cremophor EL®, leading to the discontinuation of Althesin® and some other Cremophor®-containing anesthetics.
More recently, alternate vehicles for hydrophobic drugs have been developed, including cyclodextrins. Cyclodextrins are rings of a-1,4-linked glucose subunits that feature a hydrophilic exterior and hydrophobic interior that allows them to complex with appropriately sized hydrophobic drugs. One such molecule, hydroxy-ß-cyclodextrin, has a very wide therapeutic index, increases the solubility of alfaxalone by over 8,000 times, and has been combined with alfaxalone as Alfaxan-CD RTU®.
The objectives of the described studies were to administer clinical and supraclinical doses of Alfaxan-CD RTU®, to assess the resultant quality of anesthesia, and to assess anesthetic effects on hemodynamic, respiratory, pH, and blood gas variables. We hypothesized that Alfaxan-CD RTU® would produce general anesthesia of rapid onset and brief duration in healthy dogs and cats, that minimal cardiorespiratory depression would result from clinical doses, and that no histaminoid reactions would occur.
Eight healthy adult cats and dogs were included in the study. Each cat was sequentially administered placebo, 5 mg/kg, 15 mg/kg, and 50 mg/kg of Alfaxan-CD RTU®, with washout periods of 1, 3, and 24 hours between treatments, respectively. Monitored parameters included respiratory rate, tidal volume, minute volume, lead II electrocardiography, heart rate, arterial blood gas (pH, PaO2, PaCO2, and HCO3), systemic (systolic, mean, and diastolic) and pulmonary arterial pressures, and body temperature. Rate pressure product and systemic vascular resistance were calculated.
The qualities of induction, anesthetic maintenance, and recovery and the response to a noxious stimulus were assessed as categorical variables.
Alfaxan-CD RTU® doses of 2 mg/kg, 6 mg/kg, and 20 mg/kg were administered to each dog as blinded treatments in random order. Each treatment was followed by a 3 hour washout period. Monitored and calculated parameters were the same as for cats, with the addition of left ventricular end-diastolic pressure.
In cats, dose-dependent decreases in heart rate, blood pressure, systemic vascular resistance, and rate pressure product were noted. These effects were most pronounced at the higher doses, with significant decreases in systolic arterial blood pressure and pulmonary arterial pressure after the 50 mg/kg doses. Hemodynamic changes were mild at the clinically recommended dose (5 mg/kg). Dose-dependent decreases in PaO2 occurred at all doses, but lasted for 30-60 minutes after the higher doses. The majority of cats at all doses required supplementation with 100% oxygen and mechanical ventilation after PaO2 dropped below 60 mmHg, including all cats at the highest dose. Apnea occurred in 1 and 2 cats at the 5 and 15 mg/kg doses, respectively, but occurred in all cats at the 50 mg/kg dose. Decreased tidal volume, pH, and HCO3, and increased PaCO2 also occurred at the 50 mg/kg dose. The qualities of induction and maintenance were excellent at all doses. The quality of recovery was excellent at lower doses, but poor in 5/7 cats at the 50 mg/kg dose due to prolonged recovery periods.
In dogs, hemodynamic changes included dose-dependent decreases in systemic and pulmonary arterial pressures, rate pressure product, and systemic vascular resistance. Changes were generally mild and were most pronounced at the 6 and 20 mg/kg doses. Dose-dependent decreases in respiratory rate, minute volume, and PaO2 and decreases in PaCO2 and pH were also noted, but lasted for only 15 minutes and were most pronounced at the 20 mg/kg dose. Apnea was more frequent at higher doses. The quality of induction, maintenance, and recovery were good to excellent at all doses. The duration of anesthesia was dose-dependent.
In conclusion, our studies indicate that Alfaxan-CD RTU® produced good to excellent anesthesia in cats, characterized by rapid induction to anesthesia, excellent
muscle relaxation, unresponsiveness to noxious stimuli, and smooth, uneventful recovery from anesthesia. Hypoventilation and apnea were uncommon at clinically relevant doses, but became the most important adverse effects when larger doses were administered rapidly IV. Arterial blood pressure remained stable following the IV administration of clinically relevant doses (<5 mg/kg) of Alfaxan® CD-RTU, but caution is advised with larger doses or with administration to animals with cardiovascular compromise.
In dogs, Alfaxan®-CD RTU produced safe and effective anesthesia. Induction to anesthesia was rapid and uneventful. The maintenance and recovery periods were characterized by good to excellent muscle relaxation and analgesia. Cardiovascular status was well maintained when dosages of up to 6 mg/kg IV were administered. Respiratory depression and apnea were the only notable disadvantages, and then only when larger dosages (>6 mg/kg IV) are administered. Alfaxan®-CD RTU should be an excellent alternative to currently available injectable anesthetics when administered for short surgical procedures or as induction to inhalant anesthesia.