Pharmacokinetics of ketamine and its metabolite, norketamine, after intravenous administration of a bolus of ketamine to isoflurane-anesthetized dogs

Stereoselective Pharmacokinetics of Ketamine Administered at a Low Dose in Awake Dogs

The present study aimed to examine the stereoselective pharmacokinetics of racemic ketamine in dogs at low doses. The secondary aims were to identify associated behavioural effects and propose a ketamine infusion rate. The study was conducted on nine intact male beagles, with each dog undergoing two treatments (BOL and INF). For treatment BOL, an intravenous bolus of 1 mg/kg was administered over 2 min. The treatment INF involved an initial bolus of 0.5 mg/kg given over 1 min, followed by an infusion at 0.01 mg/kg/min for 1 h. Blood samples were collected for pharmacokinetic analysis. The median R/S enantiomer ratio of ketamine remained close to 1 throughout the study. Levels of S-norketamine were significantly higher than those of R-norketamine across all time points. Based on the collected data, the infusion rate predicted to achieve a steady-state racemic ketamine plasma concentration of 150 ng/mL was 0.028 mg/kg/min. Higher scores for behavioural effects were observed within the first five minutes following bolus administration. The most common behaviours observed were disorientation, head movements and staring eyes. Furthermore, employing ROC curve analysis, a racemic ketamine plasma concentration of 102 ng/mL was defined as the cut-off value, correlating with the occurrence of undesirable behavioural patterns.

Plasma concentration, cardiorespiratory and analgesic effects of ketamine-fentanyl infusion in dogs submitted to mastectomy

Background

The analgesic and cardiorespiratory effects of ketamine, fentanyl, or ketamine-fentanyl constant rate infusion (CRI) in dogs undergoing mastectomy were evaluated. Seventeen female dogs received CRI of ketamine (GK [n = 6]: bolus 0.5 mg/kg; CRI 20 µg/kg/min in intra- and postoperative periods], fentanyl (GF [n = 5]: bolus 20 µg/kg; intraoperative CRI 5 20 µg/kg/hour and postoperative CRI 2 20 µg/kg/hour), or combination of ketamine-fentanyl (GKF [n = 6]: aforementioned doses) for 8 h. Cardiorespiratory, blood gas analyses, plasma drug concentrations, sedation score (SS), Pain Scores were evaluated.

Results

The heart rate decreased in the GF and GKF (p < 0.04); the mean arterial pressure was lower in the GKF than in the GK at 35 min (p < 0.001). Maximum plasma concentrations were observed 5 min after bolus in the GK (2847.06 ± 2903.03 ng/mL) and GKF (2811.20 ± 1931.76 ng/mL). Plasma concentration in intraoperative period of ketamine was of > 100 ng/mL in 5/5 and 2/5 animals in the GKF and GK, respectively; and > 1.1 ng/mL of fentanyl in 4/5 and 3/5 in GKF and GF, respectively.

Conclusion

Ketamine with/without fentanyl provided analgesia without significant cardiorespiratory and guaranteed the minimal plasma levels with analgesic potential during the 8 h.

Pharmacokinetics of ketamine following a short intravenous infusion to isoflurane-anesthetized New Zealand White rabbits (Oryctolagus cuniculus)

OBJECTIVE

To describe the pharmacokinetics of ketamine following a short intravenous (IV) infusion to isoflurane-anesthetized rabbits.

STUDY DESIGN

Prospective experimental study.

ANIMALS

A total of six adult healthy female New Zealand White rabbits.

METHODS

Anesthesia was induced with isoflurane in oxygen. Following determination of isoflurane minimum alveolar concentration (MAC), the isoflurane concentration was reduced to 0.75 MAC and ketamine hydrochloride (5 mg kg^-1) was administered IV over 5 minutes. Blood samples were collected before and at 2, 5, 6, 7, 8, 9, 13, 17, 21, 35, 65, 125, 215 and 305 minutes after initiating the ketamine infusion. Samples were processed immediately and the plasma separated and stored at -80 °C until analyzed for ketamine and norketamine concentrations using liquid chromatography-mass spectrometry. Compartment models were fitted to the concentration-time data for ketamine and for ketamine plus norketamine using nonlinear mixed-effects (population) modeling.

RESULTS

A three- and five-compartment model best fitted the plasma concentration-time data for ketamine and for ketamine plus norketamine, respectively. For the ketamine only model, the volume of distribution at steady state (Vss) was 3217 mL kg^-1, metabolic clearance was 88 mL minute^-1 kg^-1 and the terminal half-life was 59 minutes. For the model including both ketamine and norketamine, Vss were 3224 and 2073 mL kg^-1, total metabolic clearance was 107 and 52 mL minute^-1 kg^-1 and terminal half-lives were 52 and 55 minutes for the parent drug and its metabolite, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

This study characterized the pharmacokinetics of ketamine and norketamine in isoflurane-anesthetized New Zealand White rabbits following short IV infusion. The results obtained herein will be useful to determine ketamine infusion regimens in isoflurane-anesthetized rabbits.

Pharmacokinetics, absolute bioavailability and tolerability of ketamine after intranasal administration to dexmedetomidine sedated dogs

Intranasal ketamine has recently gained interest in human medicine, not only for its sedative, anaesthetic or analgesic properties, but also in the management of treatment resistant depression, where it has been shown to be an effective, fast acting alternative treatment. Since several similarities are reported between human psychiatric disorders and canine anxiety disorders, intranasal ketamine could serve as an alternative treatment for anxiety disordered dogs. However, to the authors knowledge, intranasal administration of ketamine and its pharmacokinetics have never been described in dogs. Therefore, this study aimed to examine the pharmacokinetics, absolute bioavailability and tolerability of intranasal ketamine administration compared with intravenous administration. Seven healthy, adult laboratory Beagle dogs were included in this randomized crossover study. The dogs received 2 mg/kg body weight ketamine intravenously (IV) or intranasally (IN), with a two-week wash-out period. Prior to ketamine administration, dogs were sedated intramuscularly with dexmedetomidine. Venous blood samples were collected at fixed times until 480 min post-administration and ketamine plasma concentrations were determined by liquid chromatography-tandem mass spectrometry. Cardiovascular parameters and sedation scores were recorded at the same time points. Non-compartmental pharmacokinetic analysis revealed a rapid (Tmax = 0.25 ± 0.14 h) and complete IN bioavailability (F = 147.65 ± 49.97%). Elimination half-life was similar between both administration routes (T1/2el IV = 1.47 ± 0.24 h, T1/2el IN = 1.50 ± 0.97 h). Heart rate and sedation scores were significantly higher at 5 and 10 min following IV administration compared to IN administration, but not at the later time-points.

Serum concentration of ketamine and antinociceptive effects of ketamine and ketamine-lidocaine infusions in conscious dogs

Background

Central sensitization is a potential severe consequence of invasive surgical procedures. It results in postoperative and potentially chronic pain enhancement. It results in postoperative pain enhancement; clinically manifested as hyperalgesia and allodynia. N-methyl-D-aspartate (NMDA) receptor plays a crucial role in the mechanism of central sensitisation. Ketamine is most commonly used NMDA-antagonist in human and veterinary practice. However, the antinociceptive serum concentration of ketamine is not yet properly established in dogs. Six dogs were used in a crossover design, with one week washout period. Treatments consisted of: 1) 0.5 mg/kg ketamine followed by continuous rate infusion (CRI) of 30 μg/kg/min; 2) 0.5 mg/kg ketamine followed by CRI of 30 μg/kg/min and lidocaine (2 mg/kg followed by CRI of 100 μg/kg/min); and 3) 0.5 mg/kg ketamine followed by CRI of 50 μg/kg/min. The infusion was administered up to 120 min. Nociceptive thresholds and ketamine serum concentrations were measured before drug administration, and at 5, 10, 20, 40, 60, 90, 120, 140 and 160 min after the start of infusion.

Results

Maximum concentration recorded was 435.34 ± 26.18 ng/mL, 582.34 ± 227.46 ng/mL and 733.77 ± 133.6 ng/mL for K30, KL30 and K50, respectively. The concentration at 120 min was 250.87 ± 39.87, 221.73 ± 91.03 and 343.67 ± 63.21 ng/mL at 120 min in K30, KL30 and K50, respectively. All the three infusion regimes maintained serum concentrations above 200 ng/mL. The thresholds returned towards baseline values within 20 min, after cessation of infusion.

Conclusion

Serum concentration to produce mechanical antinociceptive effects in dogs is between 100 and 200 ng/mL. All the three infusion regimes in this study provided antinociceptive effects throughout the infusions. In this study, we found that the serum concentration of ketamine to produce mechanical antinociceptive effects in dogs is above 200 ng/mL. All three infusion regimes provided antinociceptive effects throughout the infusions without causing harmful effects. Further studies are recommended in a clinical setting.

Effects of medetomidine and its active enantiomer dexmedetomidine on N-demethylation of ketamine in canines determined in vitro using enantioselective capillary electrophoresis

Cytochrome P450 (CYP) enzymes catalyze the metabolism of both, the analgesic and anesthetic drug ketamine and the α₂ -adrenergic receptor-agonist medetomidine that is used for sedation and analgesia. As racemic medetomidine or its active enantiomer dexmedetomidine are often coadministered with racemic or S-ketamine in animals and dexmedetomidine together with S- or racemic ketamine in humans, drug-drug interactions are likely to occur and have to be characterized. Enantioselective CE with highly sulfated γ-cyclodextrin as chiral selector was employed for analyzing in vitro (i) the kinetics of the N-demethylation of ketamine mediated by canine CYP3A12 and (ii) interactions occurring with racemic medetomidine and dexmedetomidine during coincubation with ketamine and canine liver microsomes (CLM), canine CYP3A12, human liver microsomes (HLM), and human CYP3A4. For CYP3A12 without an inhibitor, Michaelis-Menten kinetics was determined for the single enantiomers of ketamine and substrate inhibition kinetics for racemic ketamine. Racemic medetomidine and dexmedetomidine showed an inhibition of the N-demethylation reaction in the studied canine enzyme systems. Racemic medetomidine is the stronger inhibitor for CLM, whereas there is no difference for CYP3A12. For CLM and CYP3A12, the inhibition of dexmedetomidine is stronger for the R- compared to the S-enantiomer of ketamine, a stereoselectivity that is not observed for CYP3A4. Induction is observed at a low dexmedetomidine concentration with CYP3A4 but not with CYP3A12, CLM, and HLM. Based on these results, S-ketamine combined with dexmedetomidine should be the best option for canines. The enantioselective CE assay with highly sulfated γ-cyclodextrin as chiral selector is an effective tool for determining kinetic and inhibition parameters of metabolic pathways.

A comparison of epidural analgesia provided by bupivacaine alone, bupivacaine + morphine, or bupivacaine + dexmedetomidine for pelvic orthopedic surgery in dogs

OBJECTIVE

To compare the analgesic efficacy of bupivacaine, bupivacaine + morphine, or bupivacaine + dexmedetomidine administered epidurally in dogs undergoing pelvic limb orthopedic surgery.

STUDY DESIGN

Prospective, randomized, double blinded clinical trial.

ANIMALS

Sixty dogs weighing (mean ± SD) 35 ± 15.7 kg, aged 5 ± 3 years.

METHODS

Dogs were assigned to receive a lumbosacral epidural containing bupivacaine (B) 0.5%, 1 mg kg(-1) ; B, bupivacaine 0.5%, 1 mg kg(-1)  + morphine 1%, 0.1 mg kg(-1) ; B + M, or bupivacaine 0.5%, 1 mg kg(-1)  + dexmedetomidine 0.05%, 4 μg kg(-1) ; B + D. The anesthetic protocol was standardized. The median expired isoflurane concentration (E'Iso) and requirement for additional induction agent preventing purposeful movement were recorded. Pain was scored using visual analog (VAS) and modified University of Melbourne (UMPS) pain scales. Sedation was assessed using a 0-4 scale. All parameters were recorded preoperatively, and at extubation (t = 0), then at 1, 2, 4, 8, 12, 16, and 20-24 hours. Hydromorphone was administered postoperatively to patients with a VAS ≥ 35 and/or UMPS ≥ 9. Time to first voluntary urination and first motor activity were recorded.

RESULTS

Postoperatively, B + D had a lower UMPS pain score than B at t = 1 hour (p = 0.013), but not compared to B + M. The B + D group had a shorter time to urination (p = 0.0131) and a longer time for return of motor function (p = 0.0068). There were no other differences between the treatments.

CONCLUSION AND CLINICAL RELEVANCE

Epidurally administered B, B + M, or B + D in dogs all provided acceptable analgesia to manage post-operative orthopedic pelvic limb pain. Epidural administration of B + D is an effective alternative to the analgesia provided by B or B + M, but is associated with increased time to return of motor function. The direct neurotoxic effects of epidural dexmedetomidine have not been fully tested.

Chronic ketamine impairs fear conditioning and produces long-lasting reductions in auditory evoked potentials

Ketamine is an NMDA receptor antagonist with a variety of uses, ranging from recreational drug to pediatric anesthetic and chronic pain reliever. Despite its value in the clinical setting, little is known about the immediate and long-lasting effects of repeated ketamine treatment. We assessed the effects of chronic administration of a subanesthetic dose of ketamine on contextual fear conditioning, detection of pitch deviants and auditory gating. After four, but not two, weeks of daily ketamine injections, mice exhibited decreased freezing in the fear conditioning paradigm. Gating of the P80 component of auditory evoked potentials was also significantly altered by treatment condition, as ketamine caused a significant decrease in S1 amplitude. Additionally, P20 latency was significantly increased as a result of ketamine treatment. Though no interactions were found involving test week, stimulus and treatment condition, these results suggest that repeated ketamine administration impairs fear memory and has lasting effects on encoding of sensory stimuli.

Plasma levels of a low-dose constant-rate-infusion of ketamine and its effect on single and repeated nociceptive stimuli in conscious dogs

This study quantitatively investigated the analgesic action of a low-dose constant-rate-infusion (CRI) of racemic ketamine (as a 0.5 mg kg(-1) bolus and at a dose rate of 10 microg kg(-1) min(-1)) in conscious dogs using a nociceptive withdrawal reflex (NWR) and with enantioselective measurement of plasma levels of ketamine and norketamine. Withdrawal reflexes evoked by transcutaneous single and repeated electrical stimulation (10 pulses, 5 Hz) of the digital plantar nerve were recorded from the biceps femoris muscle using surface electromyography. Ketamine did not affect NWR thresholds or the recruitment curves after a single nociceptive stimulation. Temporal summation (as evaluated by repeated stimuli) and the evoked behavioural response scores were however reduced compared to baseline demonstrating the antinociceptive activity of ketamine correlated with the peak plasma concentrations. Thereafter the plasma levels at pseudo-steady-state did not modulate temporal summation. Based on these experimental findings low-dose ketamine CRI cannot be recommended for use as a sole analgesic in the dog.

Ketamine disposition in children presenting for procedural sedation and analgesia in a children's emergency department

BACKGROUND

The aim of this study was to describe ketamine pharmacokinetics in children to simulate time-concentration profiles to predict duration of concentrations associated with anesthesia, arousal and analgesia.

METHODS

Children presenting for painful procedures in the Emergency Dept were given ketamine 1-1.5 mgxkg(-1) i.v. Blood was assayed for ketamine on 3-6 occasions (median 3) over the subsequent 14-152 min (median 28.5). A population pharmacokinetic analysis was undertaken by using nonlinear mixed effects models (NONMEM). Simulation was used to predict time-concentration profiles in this cohort

RESULTS

There were 188 observations from 54 children (age 8.3 sd 3.5 years, weight 32.5 sd 15.6 kg). A two-compartment (central, peripheral) linear disposition model fitted data better than a one-compartment model. Population parameter estimates and their between subject variability (BSV), standardized to a 70-kg person using allometric models, were central volume (V1) 38.7 (BSV 64%) l.70 kg(-1), peripheral volume of distribution (V2) 102 (51.7%) l.70 kg(-1), clearance (CL) 90 (38.1%) l.h(-1) 70 kg(-1) and intercompartment clearance (Q) 215 (19%) l.h(-1) 70 kg(-1). At 10 min half of the children given 1 mgxkg(-1) will have a serum concentration below 0.75 mgxl(-1). This is a concentration associated with 'awakening' in adults. However, almost all the children will still have a serum concentration above 0.1 mgxl(-1), a level associated with analgesia in adults.

CONCLUSIONS

Ketamine 1 mgxkg(-1) i.v. provides satisfactory serum concentrations for children undergoing sedation for painful procedures of <5-min duration and produces concentrations associated with analgesic effect for more than 10 min. Clearance increases with decreasing age in children. The relationship between serum concentration and effect is poorly defined in children.