Picogram level determination of medetomidine in dog serum by capillary gas chromatography with negative ion chemical ionization mass spectrometry

The dexmedetomidine concentration required after remifentanil anesthesia is three-fold higher than that after fentanyl anesthesia or that for general sedation in the ICU

Purpose

The general dexmedetomidine (DEX) concentration required for sedation of intensive care unit patients is considered to be approximately 0.7 ng/mL. However, higher DEX concentrations are considered to be required for sedation and/or pain management after major surgery using remifentanil. We determined the DEX concentration required after major surgery by using a target-controlled infusion (TCI) system for DEX.

Methods

Fourteen patients undergoing surgery for abdominal aortic aneurysms (AAA) were randomly, double-blindly assigned to two groups and underwent fentanyl- or remifentanil-based anesthetic management. DEX TCI was started at the time of closing the peritoneum and continued for 12 hours after stopping propofol administration (M0); DEX TCI was adjusted according to the sedation score and complaints of pain. The doses and concentrations of all anesthetics and postoperative conditions were investigated.

Results

Throughout the observation period, the predicted plasma concentration of DEX in the fentanyl group was stable at approximately 0.7 ng/mL. In contrast, the predicted plasma concentration of DEX in the remifentanil group rapidly increased and stabilized at approximately 2 ng/mL. The actual DEX concentration at 540 minutes after M0 showed a similar trend (0.54±0.14 [fentanyl] versus 1.57±0.39 ng/mL [remifentanil]). In the remifentanil group, the dopamine dose required and the duration of intubation decreased, and urine output increased; however, no other outcomes improved.

Conclusion

The DEX concentration required after AAA surgery with remifentanil was three-fold higher than that required after AAA surgery with fentanyl or the conventional DEX concentration for sedation. High DEX concentration after remifentanil affords some benefits in anesthetic management.

Maternal and preterm fetal sheep responses to dexmedetomidine

BACKGROUND

The α(2) adrenergic receptor agonist dexmedetomidine has some unique pharmacologic properties that could benefit pregnant patients (and their fetuses) when they require sedation, analgesia, and/or anesthesia during pregnancy. The purpose of the present study was to delineate maternal and fetal responses to an intravenous infusion of dexmedetomidine.

METHODS

This study was conducted on surgically-recovered preterm sheep instrumented for physiologic recording and blood sampling. Maternal and fetal cardiovascular and blood gas parameters and fetal cerebral oxygenation levels were recorded before, during, and after 3h of dexmedetomidine infusion to the ewe at a rate of 1 μg/kg/h.

RESULTS

Drug infusion produced overt sedation but no apparent respiratory depression as evidenced by stable maternal arterial blood gases; fetal blood gases were also stable. The one blood parameter to change was serum glucose, By the end of the 3-h infusion, glucose increased from 49±10 to 104±33mg/dL in the ewe and from 22±3 to 48±16mg/dL in the fetus; it declined post-drug exposure but remained elevated compared to the starting levels (maternal, 63±12mg/dL, P=0.0497; and fetal, 24±4mg/dL, P=0.012). With respect to cardiovascular status, dexmedetomidine produced a decrease in maternal blood pressure and heart rate with fluctuations in uterine blood flow but had no discernable effect on fetal heart rate or mean arterial pressure. Likewise, maternal drug infusion had no effect on fetal cerebral oxygenation, as measured by in utero near-infrared spectroscopy.

CONCLUSIONS

Using a clinically-relevant dosing regimen, intravenous infusion of dexmedetomidine produced significant maternal sedation without altering fetal physiologic status. Results from this initial acute assessment support the conduct of further studies to determine if dexmedetomidine has clinical utility for sedation and pain control during pregnancy.

Cardiopulmonary effects and pharmacokinetics of i.v. dexmedetomidine in ponies

REASONS FOR PERFORMING STUDY

Currently available sedatives depress cardiopulmonary function considerably; therefore, it is important to search for new, less depressive sedatives. The study was performed to assess duration and intensity of cardiopulmonary side effects of a new sedative, dexmedetomidine (DEX), in horses.

OBJECTIVES

To study pharmacokinetics and cardiopulmonary effects of i.v. DEX.

METHODS

Pharmacokinetics of 3.5 microg/kg bwt i.v. DEX were studied in a group of 8 mature (mean age 4.4 years) and 6 old ponies (mean age 20 years). Cardiopulmonary data were recorded in mature ponies before and 5, 10, 20, 30, 45 and 60 mins after administration of DEX 3.5 microg/kg bwt i.v. Data were analysed using ANOVA for repeated measures, and where appropriate Dunnett's t test was used to detect differences from resting values (P < 0.05).

RESULTS

Within 2 h after DEX administration, plasma levels were beyond limits of quantification (0.05 ng/ml). Mean values for kinetic parameters for mature and old ponies were: Cmax (ng/ml) 4.6 and 3.8, t 1/2 (min) 19.8 and 28.9 and AUC (ng.min/ml) 34.5 and 44.3, respectively. Heart rate, central venous pressure, pulmonary artery pressure and pulmonary capillary wedge pressure did not change significantly compared to presedation values throughout the 60 min observation period. Compared to presedation values, stroke volume and mixed venous PO2 were reduced for the first 5 mins, paralleled by an increase in systemic and pulmonary vascular resistance. Cardiac index was reduced for the first 10 mins, arterial blood pressures at 20, 30 and 45 mins and respiratory rate throughout the 60 min observation period, but no change in arterial PO2 or PCO2 occurred.

CONCLUSIONS

DEX administration i.v. causes similar cardiopulmonary changes to those caused by other alpha-2 adrenoceptor agonists, but of very short duration. DEX is redistributed particularly rapidly.

POTENTIAL RELEVANCE

DEX might be safer for sedation of horses because of its very short-lasting cardiopulmonary side effects. For long duration sedation, its kinetics favour its use as a continuous infusion.

Medetomidine as a candidate antifoulant: sublethal effects on juvenile turbot (Psetta maxima L.)

Medetomidine is proposed as a candidate antifouling compound proven effective against barnacles. It is routinely used as a sedative in veterinary medicine. It is therefore important to also investigate possible adverse effects on non-target organisms. Thus, sublethal effects on two different ages of juvenile turbot (Psetta maxima) exposed to a wide concentration range of medetomidine (0.063-420nM) were assessed after exposure under semistatic as well as flow-through conditions, for a maximum of 96h. Effects on respiration frequency and amount of oxygen consumed were assessed, as well as the ability of turbot to adapt to a dark background. A significant decrease was seen both in respiration frequency with a lowest observed effect concentration (LOEC) of 2.1nM as well as in amount of oxygen consumed (LOEC=420nM) and colour adaptation (LOEC=4.2nM). Colour adaptation was also evaluated in a short exposure experiment, 1h, where significant effects were observed (LOEC=2.1nM). Reversibility, when fish were incubated in clean seawater following exposure, was seen for all observed effects. Ecological relevance of the observed effects is discussed.

Effects of medetomidine, a novel antifouling agent, on the burrowing bivalve Abra nitida (Müller)

The effects of medetomidine, a novel antifouling candidate, on the burrowing bivalve Abra nitida were studied. The burrowing behaviour, sediment reworking activity and faeces production were assessed after 24 h exposure of A. nitida to sublethal concentrations of medetomidine. Medetomidine caused a significant decrease in the burrowing response and in the sediment reworking activity. The median effective concentrations (EC50) were 430 nM (86 microg/l) and 4.4 nM (0.9 microg/l), respectively. No effects on the faeces production were detected. Although significant effects of medetomidine on A. nitida were registered, a reversibility of the effects was observed when 24 h-exposed animals were incubated in clean seawater and sediment for 24 h. Considerations relating to the future commercialisation of medetomidine for antifouling purposes are discussed.

The Effects of Esmolol and Dexmedetomidine on Myocardial Oxygen Consumption During Sympathetic Stimulation in Dogs

OBJECTIVE

The purpose of this study was to compare the potential of the beta(1)-adrenergic receptor blocker esmolol and the alpha(2)-adrenergic receptor agonist dexmedetomidine to suppress the cardiovascular and neuroendocrine response to a sympathetic stimulus.

DESIGN

Experimental study.

SETTING

Laboratory of university.

PARTICIPANTS

Eleven anesthetized dogs.

INTERVENTIONS

Catheters for arterial and coronary venous blood sampling and calculation of myocardial oxygen consumption were inserted. Pressure sensors were placed in the aorta, left ventricle, and a carotid artery. Flow probes were placed around the aortic root and around the left anterior descending coronary artery. Esmolol was infused (loading dose of 1 mg/kg, infusion of 0.3 mg/kg/h), and the adequacy of beta-blockade was checked. Thirty minutes after stopping esmolol, dexmedetomidine infusion was started (loading dose of 1 microg/kg, infusion of 1.5 microg/kg/min). Occlusion of both carotid arteries was used as a sympathetic stimulus before and during infusion of esmolol and before and during infusion of dexmedetomidine.

MEASUREMENTS AND MAIN RESULTS

The variables were measured just before and during sympathetic stimulation, and changes were calculated. Both drugs suppressed the increase in dPdT(max). Dexmedetomidine suppressed the increase in plasma norepinephrine and the increase in systemic vascular resistance (dexmedetomidine 4% +/- 4% and esmolol 25% +/- 19% increase, p = 0.02). Esmolol attenuated the heart rate response (esmolol 2% +/- 2% and dexmedetomidine 20% +/- 18% increase, p = 0.02). However, dexmedetomidine decreased baseline heart rate more than esmolol; therefore, the absolute maximal heart rate during sympathetic stimulation was lower in the presence of dexmedetomidine (dexmedetomidine 119 +/- 14 and esmolol 141 +/- 15 beats/min, p = 0.01). Neither drug suppressed the increase in myocardial oxygen consumption.

CONCLUSIONS

Both esmolol and dexmedetomidine have the potential to suppress some of the cardiovascular and neuroendocrine changes to a sympathetic stimulus but neither drug abolished the increase in myocardial oxygen consumption.

Changes in the minimum alveolar concentration of isoflurane and some cardiopulmonary measurements during three continuous infusion rates of dexmedetomidine in dogs

OBJECTIVE

To measure the change in the minimum alveolar concentration of isoflurane associated with three constant rate infusions of dexmedetomidine.

STUDY DESIGN

Prospective, randomized, and blinded experimental trial. Animals Six healthy 6-year-old Beagles weighing between 13.0 and 17.7 kg.

METHODS

The dogs received each of four treatments; saline or dexmedetomidine at 0.1, 0.5 or 3 microg kg(-1) loading dose given intravenously (IV) over 6 minutes followed by infusions at 0.1, 0.5 or 3 microg kg(-1) hour(-1), respectively. There were 2 weeks between treatments. The dogs were mask-induced with and maintained on isoflurane in oxygen. Acetated Ringer's (5 mL kg(-1) hour(-1)) and saline or dexmedetomidine (each at 0.5 mL kg(-1) hour(-1)) were given IV. Pulse rate, blood pressure, samples for the measurement of blood gases, pH, lactate, packed cell volume (PCV), total protein (TP) and dexmedetomidine concentrations were obtained from an arterial catheter. Sixty minutes after induction minimum alveolar concentration (MAC) was determined by intermittently applying supramaximal electrical stimuli to the thoracic and pelvic limbs. Cardiopulmonary measurements and arterial blood samples were collected before each set of stimuli. Statistical analyses were conducted with analysis of variance or mixed models according to the experimental design.

RESULTS

There was a significant decrease in the MAC of isoflurane associated with 0.5 and 3 microg kg(-1) hour(-1) but not with 0.1 mg kg(-1)hour(-1). Serum concentrations of dexmedetomidine were not measurable at the 0.1 mg kg(-1) hour(-1) and averaged 0.198 +/- 0.081 and 1.903 +/-0.621 ng mL(-1) for the 0.5 and 3 microg kg(-1) hour(-1) infusion rates, respectively. Heart rate decreased with increasing doses of dexmedetomidine while blood pressure increased. Packed cell volume increased at 3 microg kg(-1) hour(-1) but not with other doses.

CONCLUSIONS AND CLINICAL RELEVANCE

Dexmedetomidine infusions decrease the intra-operative requirement for isoflurane and may be useful in managing dogs undergoing surgery, where the provision of analgesia and limitation of the stress response is desirable.

Sublethal effects of a new antifouling candidate on lumpfish (Cyclopterus lumpus L.) and Atlantic cod (Gadus morhua L.) larvae

Sublethal effects of medetomidine, a new generation antifouling compound, on lumpfish (Cyclopterus lumpus L.) and cod (Gadus morhua L.) larvae were examined. The effects on respiration rate and on colour adaptation of newly hatched larvae were assessed after 24-96 h exposure. Exposure of lumpfish larvae to the experimental concentrations resulted in a significant decrease in respiration rate (Lowest Observed Effect Concentration (LOEC) = 5-10 nM) and in the percentage of dark larvae (LOEC = 4 nM). However, no effects on respiration rate of cod larvae were detected. In addition to lumpfish larvae being affected at low concentrations of medetomidine, a reversibility of the effects was observed when 96 h-exposed larvae were incubated in clean seawater for 24-48 h. Considerations relating to the future commercialisation of medetomidine for antifouling purposes are discussed.

Bioavailability of dexmedetomidine after extravascular doses in healthy subjects

AIM

To determine the absolute bioavailability of extravascularly administered dexmedetomidine, a novel a2-adrenoceptor agonist, in healthy subjects.

METHODS

Single 2 microg x kg-1 doses of dexmedetomidine were given intravenously, intramuscularly, perorally and buccally (where the solution is not swallowed) to 12 healthy male subjects. The drug concentration-time data were analysed using linear one-compartment (buccal and peroral data), or two-compartment modelling (intravenous data), or noncompartmental methods (intramuscular data).

RESULTS

Mean (95% CI) absolute bioavailability after peroral, buccal and intramuscular administration was 16% (12-20%), 82% (73-92%) and 104% (96-112%), respectively.

CONCLUSION

Dexmedetomidine is well absorbed systemically through the oral mucosa, and therefore buccal dosing may provide an effective, noninvasive route to administer the drug.

Pharmacokinetics and sedative effects of intramuscular medetomidine in domestic sheep

Intramuscular (i.m.) administration of medetomidine (MED) may avoid the severe pressor effects caused by peripheral actions of MED associated with intravenous (i.v.) dosing. The purpose of this study was to determine the pharmacokinetics, the time course of sedation and occurence of hypoxaemia after i.m. administration of MED in domestic sheep. The MED was injected i.m. at a dose of 30 micro g/kg in nine domestic sheep. Blood was sampled at 0, 5, 10, 20, 30, 40, 60, 120, 180, 240, 360 and 600 min after MED. Sedation was assessed and arterial blood samples were taken before and 35 min after MED application. Mean (SD) pharmacokinetic parameters of i.m. MED were: absorption half-life: 13.2 (7.5) min; terminal half-life: 32.7 (14.9) min; time to peak concentration: 29.2 (8.9) min; peak concentration: 4.98 (1.89) ng/mL; volume of distribution: 3.9 (2.4) l/kg; total body clearance: 81.0 (21.5) mL/(min kg). Peak sedation occurred between 30 and 40 min after injection of MED. The degree of sedation correlated with individual plasma concentrations (rS: 0.926). One animal became hypoxaemic (PaO2 = 54.1 mmHg).

Dexmedetomidine decreases perioperative myocardial lactate release in dogs

The sympatholytic effect of the alpha(2)-adrenergic agonist dexmedetomidine may decrease emergence-related myocardial ischemic load in patients. However, a direct measure of myocardial ischemia, such as myocardial lactate release, is difficult to obtain in patients. Therefore, we studied mongrel dogs and measured myocardial lactate release, myocardial oxygen supply, hemodynamic variables, and neurohumoral indices of the stress response. After the induction of a standardized degree of borderline myocardial ischemia, either dexmedetomidine (dexmed group, n = 9) or normal saline (control group, n = 9) was infused. Measurements were repeated at the end of the anesthetic period and every 10 min during the 90-min emergence period. In the dexmed group, the cumulative emergence-related lactate release was 46% less than in the control group (95% confidence interval, 20%-80%; P = 0.02). Simultaneously, dexmedetomidine increased the endo-/epicardial blood flow ratio by 35% (control group, 0.4 +/- 0.1; dexmed group, 0.6 +/- 0.1; P = 0.03). These antiischemic effects of dexmedetomidine were accompanied by reduced plasma concentrations of norepinephrine (126 versus 577 pg/mL) and epinephrine (158 versus 1909 pg/mL) and a slower heart rate (123 +/- 6 versus 160 +/- 10 bpm, dexmed versus control). The antiischemic effect of dexmedetomidine started before emergence, as evidenced by a decreased prevalence of myocardial lactate release at that time (zero of eight dogs in the dexmed group and four of seven dogs in the control group had lactate release before emergence; P = 0.03).

IMPLICATIONS

Dexmedetomidine decreases plasma catecholamines and heart rate during emergence from anesthesia. In dogs with a coronary stenosis, these sympatholytic effects decrease myocardial lactate release and, therefore, minimize emergence-related myocardial ischemia.

Effect of synthesis parameters of the sol-gel-processed spray-dried silica gel microparticles on the release rate of dexmedetomidine

The objective of this study was to evaluate the possibilities to control the release rate of dexmedetomidine (DMED) from different spray-dried silica gel microparticle formulations. Microparticles were prepared by spray drying a silica sol polymer solution containing the drug. Drug release was investigated both in vitro and in vivo. The influence of sol-gel synthesis parameters, like pH and the water/alkoxide ratio (r) of the sol, on the release behaviour of the drug was studied. Silica gel microparticles had a smooth surface. Microparticles prepared from diluted sol, however, were more aggregated and clustered. The drug release conformed to zero order release from microparticles prepared near the isoelectric point of silica (pH 2.3 and pH 3) and to the square root of time kinetics from microparticles prepared at pH 1 and pH 5. The release also showed a dual-phasic profile with an initial burst and after that a slower release period. The dexmedetomidine release conformed to zero order kinetics from microparticles prepared at water/ alkoxide ratios between r = 6 and r = 35 (at pH 2.3). The release rate was the slowest from microparticles prepared with water/ alkoxide ratio 35. The bioavailability of dexmedetomidine in dogs showed that the release was sustained from silica gel microparticles as compared with a subcutaneously administered reference dose of 0.1 mg.

Some factors influencing the level of clinical sedation induced by medetomidine in rabbits

Rabbits (n=23) received intravenous bolus medetomidine at 100 mug/kg. Prior to medetomidine administration, heart and respiratory rates were measured, arterial blood was collected and analysed for plasma cortisol, glucose and albumin concentrations. Fifteen minutes after medetomidine administration, heart and respiratory rates were measured again and sedation was scored. The rabbit was afterwards anaesthetized with 20 mg/kg ketamine administered intravenously to enable spinal tap and heart puncture. Cerebrospinal fluid (CSF) was collected (this occurred 20 min post medetomidine administration) and analysed for medetomidine concentration. Blood was collected by heart puncture immediately after the spinal tap and analysed for serum medetomidine concentration. Cerebrospinal fluid medetomidine concentration correlated negatively with sedation. Serum medetomidine correlated positively with CSF medetomidine concentration. Cerebro-spinal fluid medetomidine was 17 +/- 13% of serum medetomidine concentration. Plasma cortisol and glucose concentrations correlated negatively with serum medetomidine. We conclude that after an intravenous bolus administration of a low sedative dose of medetomidine to rabbits; CSF concentration of the drug correlate negatively with sedation and that this may be because of the fact that only the free and unbound medetomidine may be available for detection in the CSF, the concentration of medetomidine detected in the CSF was much lower than that in blood and a positive correlation exists between CSF and serum medetomidine concentrations. Stress may have some effect on the distribution or metabolism of medetomidine in rabbits.

Effect of medetomidine on electroencephalography and use of a quantitative electroencephalograph for evaluating sedation levels in dogs

This study was designed to characterize the effect of medetomidine (Med) on canine electroencephalography (EEG), to evaluate the use of quantitative EEG for assessing sedation levels and to explore the correlation between the serum concentration of Med and the quantitative EEG. Four groups of dogs were given Med at doses of 20, 40, 80 and 160 microg/kg (Med-20, Med-40, Med-80 and Med-160 groups). Following Med administration, there was synchrony between each unipolar EEG lead. On EEG power spectrum analysis of the bipolar leads, all groups showed a significant depression of the 14-30 Hz components. The power of the 1-3 Hz component in the Med-80 and Med-160 groups was significantly increased, although there were few changes in the other groups. Similar results were obtained from raw data analysis. As a result of quantitative EEG analysis, spectrum edge frequency 90 analysis (SEP90) showed that the frequency was significantly reduced in all groups after Med administration. A dose-response effect was observed in all groups except for the Med-160 group. Both of these EEG analyses were significantly correlated with the serum concentration of Med. However, the result of the SPF90 analysis sugested a stronger correlation than that for median edge frequency analysis. In conclusion, care must be taken in veterinary clinical diagnoses when Med is used during EEG recording, as Med may cause increased activity in the low frequency band and a decrease in high frequency band activity. In addition, quantitative EEG analysis may be useful in assessing the depth of sedation and in further studies on Med administration.

The pharmacokinetics of dexmedetomidine in volunteers with severe renal impairment

Dexmedetomidine, an alpha2-adrenergic agonist with sedative and analgesic properties, is mainly cleared by hepatic metabolism. Because the pharmacokinetics of dexmedetomidine have not been determined in humans with impaired renal function, we studied them in volunteers with severe renal disease and in control volunteers. Six volunteers with severe renal disease and six matched volunteers with normal renal function received dexmedetomidine, 0.6 microg/kg, over 10 min. Venous blood samples for the measurement of plasma dexmedetomidine concentrations were drawn before, during, and up to 12 h after the infusion. Two-compartmental pharmacokinetic models were fit to the drug concentration versus time data. We also determined its hemodynamic, respiratory, and sedative effects. There was no difference between Renal Disease and Control groups in either volume of distribution at steady state (1.81 +/- 0.55 and 1.54 +/- 0.08 L/kg, respectively; mean +/- SD) or elimination clearance (12.5 +/- 4.6 and 8.9 +/- 0.7 mL x min(-1) x kg(-1), respectively). However, elimination half-life was shortened in the Renal Disease group (113.4 +/- 11.3 vs 136.5 +/- 13.0 min; P < 0.05). A mild reduction in blood pressure occurred in most volunteers. Although most volunteers were sedated by dexmedetomidine, renal disease volunteers were sedated for a longer period of time.

IMPLICATIONS

The pharmacokinetics of dexmedetomidine in volunteers with severe renal impairment differed little from those in volunteers with normal renal function. In addition, there were no clinically significant differences in the hemodynamic responses to dexmedetomidine. However, dexmedetomidine resulted in more prolonged sedation in subjects with renal disease.

Alkyl-substituted silica gel as a carrier in the controlled release of dexmedetomidine

The effect of alkyl substitution of the silica xerogel matrix on the release rate of dexmedetomidine was evaluated. Silica sol was processed by either casting or spray drying. When the reaction precursor tetraethylorthosilicate (TEOS) was partially substituted with tri- or dialkoxysilane, the release of dexmedetomidine and degradation of the matrix were decreased compared with 100% TEOS-based gel. Increasing the number or length of the organic groups attached to silicon, modified the silica gel structure and reduced the release rate of dexmedetomidine from monoliths. The release of dexmedetomidine from alkyl-substituted silica gel microparticles, however, showed a burst in drug release. Subcutaneously administered silica xerogel matrices (manufactured by casting, containing 25 mol% dimethyldiethoxysilane at two different doses of dexmedetomidine) were studied in dogs. Sustained delivery of dexmedetomidine was obtained for at least 48 h.

The effect of intravenous dexmedetomidine premedication on the dose requirement of propofol to induce loss of consciousness in patients receiving alfentanil

Dexmedetomidine reduces the dose requirements for opioids and anaesthetic agents. We conducted a single-centre, open-label, noncomparative phase II study of the effect of intravenous dexmedetomidine on the dose requirement of propofol to induce loss of consciousness in 49 ASA I and II patients. The initial dexmedetomidine infusion scheme was reduced twice because of adverse events. Forty patients who received the final infusion scheme were randomly allocated to receive one of five stepped propofol infusions; loss of consciousness was assessed after 21 min. The ED50 for the final infusion rate of propofol to suppress consciousness was 3.45 mg x kg(-1) x h(-1) (95% CL 2.7-4.2): ED95 was 6.68 mg x kg(-1) x h(-1) (95% CL 5.1-19.1), EC50 was 1.69 microg x ml(-1) (95% CL 0.95-2.5) and EC95 was 5.7 microg x ml(-1) (95% CL 3.2 to > 10). Our final dose of dexmedetomidine of 0.63 microg x kg(-1) caused a reduction in the overall concentration and dose of propofol required to produce loss of consciousness, but no significant shift in the dose-response curve compared with other studies.

Sedative, analgesic, and cardiovascular effects of levomedetomidine alone and in combination with dexmedetomidine in dogs

OBJECTIVE

To determine whether a high dose of levomedetomidine had any pharmacologic activity or would antagonize the sedative and analgesic effects of dexmedetomidine in dogs.

ANIMALS

6 healthy Beagles.

PROCEDURE

Each dog received the following treatments on separate days: a low dose of levomedetomidine (10 microg/kg), IV, as a bolus, followed by continuous infusion at a dose of 25 microg/kg/h; a high dose of levomedetomidine (80 microg/kg), IV, as a bolus, followed by continuous infusion at a dose of 200 microg/kg/h; and a dose of isotonic saline (0.9% NaCl) solution, IV, as a bolus, followed by continuous infusion (control). For all 3 treatments, the infusion was continued for 120 minutes. After 60 minutes, a single dose of dexmedetomidine (10 microg/kg) was administered IV. Sedation and analgesia were scored subjectively, and heart rate, blood pressure, respiratory rate, arterial blood gas partial pressures, and rectal temperatures were monitored.

RESULTS

Administration of levomedetomidine did not cause any behavioral changes. However, administration of the higher dose of levomedetomidine enhanced the bradycardia and reduced the sedative and analgesic effects associated with administration of dexmedetomidine.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that administration of dexmedetomidine alone may have some cardiovascular benefits over administration of medetomidine, which contains both dexmedetomidine and levomedetomidine. Further studies are needed to confirm the clinical importance of the effects of levomedetomidine in dogs.

Correlation between serum concentrations following continuous intravenous infusion of dexmedetomidine or medetomidine in cats and their sedative and analgesic effects

Dexmedetomidine (DEX) may have some therapeutic advantages over the racemate medetomidine (MED). Here we have examined how serum concentrations of DEX correlate with some of its anaesthetic effects. Cats (n = 6) were administered with a continuous stepwise intravenous (i.v.) infusion of DEX or MED on different occasions in a cross-over design. Maintenance infusion rates (mg/kg/min) used were: DEX = 0.25 (MED = 0.50); DEX = 1 (MED = 2) and DEX = 4 (MED = 8) for infusion steps 1, 2 and 3, respectively. Each maintenance infusion lasted at least 50 min and was preceded with a loading dose. There was no significant difference between serum DEX and 0.5 serum MED concentrations at any dose level nor was there a significant difference between serum DEX and the (entire) serum MED concentrations. There was no significant difference between DEX and MED for sedation, analgesia, muscular relaxation and heart and respiratory rates. For both DEX and MED, serum drug concentration and analgesia were dose-dependent and sedation increased until the end of infusion step 2 (dose level 2) and decreased at the end of step 3 (dose level 3). Muscular relaxation was not dose-dependent. We conclude that increasing the blood concentration of DEX or MED beyond a certain level decreases the level of sedation instead of increasing it even though analgesia increases. The rate at which DEX and MED are metabolized in cats may not be the same.

Clinical effects and pharmacokinetics of medetomidine and its enantiomers in dogs

The clinical effects and pharmacokinetics of medetomidine (MED) and its enanti-omers, dexmedetomidine (DEX) and levomedetomidine (LEVO) were compared in a group of six beagle dogs. The dogs received intravenously (i.v.) a bolus of MED (40 microg/kg), DEX (20 and 10 microg/kg), LEVO (20 and 10 microg/kg), and saline placebo in a blinded, randomized block study in six separate sessions. Sedation and analgesia were scored subjectively, and the dogs were monitored for heart rate, ECG lead II, direct blood pressure, respiratory rate, arterial blood gases, and rectal body temperature. Blood samples for drug analysis were taken. Peak sedative and analgesic effects were observed at mean (+/- SD) plasma levels of 18.5 +/- 4.7 ng/mL for MED40, 14.0 +/- 4.5 ng/mL for DEX20, and 5.5 +/- 1.3 ng/mL for DEX10. The overall level of sedation and cardiorespiratory effects did not differ between MED40, DEX20 and DEX10 during the first hour, apparently due to a ceiling effect. However, the analgesic effect of DEX20 lasted longer than the effect of the corresponding dose of racemic medetomidine, suggesting greater potency for dexmedetomidine in dogs. Levomedetomidine had no effect on cardio-vascular parameters and caused no apparent sedation or analgesia. The pharmacokinetics of dexmedetomidine and racemic medetomidine were similar, but clearance of levomedetomidine was more rapid (4.07 +/- 0.69 L/h/kg for LEVO20 and 3.52 +/- 1.03 for LEVO10) than of the other drugs (1.26 +/- 0.44 L/h/kg for MED40, 1.24 +/- 0.48 for DEX20, and 0.97 +/- 0.33 for DEX10).

Effect of dexmedetomidine on the minimum alveolar concentration (MAC) of sevoflurane in adults age 55 to 70 years

STUDY OBJECTIVE

To determine the effect of two target dexmedetomidine infusions (0.3 ng/ml and 0.6 ng/ml) on the minimal alveolar concentration (MAC) of sevoflurane in adults age 55 to 70 years.

DESIGN

Prospective, randomized, placebo-controlled study.

SETTING

University-affiliated hospital.

PATIENTS

45 ASA physical status I and II adults, age 55 to 70 years, undergoing elective surgery with at least a 3 inch skin incision.

INTERVENTIONS

Patients were given a dexmedetomidine or placebo infusion for at least 15 minutes before anesthetic induction with sevoflurane in oxygen by face mask. After tracheal intubation, a target sevoflurane concentration was maintained for 15 minutes while the dexmedetomidine or placebo infusion continued to run.

MEASUREMENTS AND MAIN RESULTS

At the time of skin incision, two observers independently determined movement or nonmovement to the incision. Blood samples for dexmedetomidine were taken before the infusion and at the time of skin incision. The dexmedetomidine plasma concentrations were 0 before infusion with all treatment groups. At the time of incision, they were 0 in the placebo group, 0.39 +/- 0.13 ng/ml in the 0.3 ng/ml target group, and 0.7 +/- 0.13 ng/ml in the 0.6 ng/ml target group. The MAC of sevoflurane was 1.83% in the placebo group, 1.78% in the 0.3 ng/ml target dexmedetomidine group, and 1.51% in the 0.6 ng/ml target dexmedetomidine group.

CONCLUSIONS

Dexmedetomidine 0.7 ng/ml decreased the MAC of sevoflurane by 17%, whereas there was no difference between the placebo and the dexmedetomidine 0.39 ng/ml group.

Determination of dexmedetomidine in rat plasma by a sensitive [3H]clonidine radioreceptor assay

This paper describes the development and implementation of a sensitive radioreceptor assay (RRA) for determining concentrations of dexmedetomidine, an alpha-2 adrenergic agonist with anesthetic properties, in rat plasma. Calf retina membranes were selected as the alpha-2 adrenergic receptor source, and the alpha-2 antagonist [3H]RX821002 and the alpha-2 agonist [3H]clonidine were evaluated as radioligands. We optimized the binding conditions for both radioligands and chose a radioligand for implementation in the RRA based on the characteristics of the inhibition binding curves with dexmedetomidine. The final method is based on competition between the radioligand [3H]clonidine and dexmedetomidine for high-affinity binding sites present in calf retina membranes. The assay has a coefficient of variation of 8% in the range 23.7-592 pg for 0.2 mL of plasma. This assay can be applied to pharmacokinetic-pharmacodynamic studies of dexmedetomidine.

Analytical method development for the simultaneous quantitation of dexmedetomidine and three potential metabolites in plasma

Dexmedetomidine, a novel alpha 2-adrenergic receptor agonist, is being developed as an anesthetic adjunct for perioperative use. An assay method has been developed for the sensitive quantitation of dexmedetomidine and three metabolites in plasma: MPV-1305, MPV-1306 and MPV-1709. The method involves solid-phase extraction (C18 cartridge) of dexmedetomidine and metabolites followed by a two-step derivatization. The first step utilized BF3-MeOH to simultaneously mask a primary alcohol in MPV-1305 and a carboxylic acid in MPV-1306. The second step applied PFB-Cl to derivatize the imidazole ring for sensitive detection of these compounds by GC-negative chemical ionization MS at pg/ml concentrations. MPV-1709 was not derivatized in the process and was detected by GC-positive chemical ionization MS. Optimization of extraction and derivatization is discussed. The method is suitable for quantitation of dexmedetomidine, MPV-1305, MPV-1306 and MPV-1709 over concentration ranges of 0.1, 40, 0.5-100, 0.5-100 and 1.0-500 ng/ml, respectively. The method showed excellent specificity, linearity and sensitivity and is useful for profiling the pharmacokinetic disposition of these compounds.

Buccal delivery of an alpha 2-adrenergic receptor antagonist, atipamezole, in humans

OBJECTIVE

To evaluate the pharmacokinetics, systemic effects and clinical applicability of buccally administered atipamezole in healthy volunteers.

METHODS

The study was carried out in two parts. In the first part, spray preparations of atipamezole hydrochloride in water/alcohol (50/50) solution were applied on buccal mucosa of six volunteers. Single doses of 5, 10, 20, and 40 mg atipamezole hydrochloride were administered in ascending order during separate sessions. In the second part, nine subjects received single 20 mg doses as buccal spray, intravenous infusion, or oral solution in randomized order.

RESULTS

Values for area under the concentration-time curve for atipamezole (mean +/- SD) ranged from 26 +/- 4 ng x hr/ml after 5 mg to 112 +/- 21 ng x hr/ml after 40 mg and peak concentrations ranged from 11 +/- 3 ng/ml after 5 mg to 38 +/- 9 ng/ml after 40 mg. Individual peak concentrations were mainly measured at 30 and 60 minutes after administration. Mean elimination half-lives were approximately 1 1/2 hours after every treatment. In part two, a mean bioavailability of 33% was calculated for buccal administration (compared with intravenous), whereas systemic availability after an oral dose was < 2%. After intravenous administration the mean total clearance, apparent volume of distribution, and elimination half-life were 1.2 L/hr/kg, 2.9 L/kg, and 1.8 hours, respectively. The intravenous administration of 20 mg atipamezole hydrochloride produced a fivefold elevation in mean plasma norepinephrine concentration, a slight and short-lasting elevation in blood pressure and, in most subjects, increased tension, alertness and restlessness, and sweating. After buccal administration, some subjects reported short-lasting restlessness or tension after the 20 and 40 mg doses. No significant changes in heart rate, blood pressure, or plasma catecholamines were observed. No effects were observed after swallowing of 20 mg atipamezole hydrochloride. The spray caused local reactions at buccal mucosa. Superficial white spots or areas were observed for several hours; these disappeared gradually. Subjects also reported transient numbness at the application site.

CONCLUSION

Atipamezole hydrochloride is well absorbed systemically through oral mucosa. The oral bio-availability of atipamezole is negligible, probably because of extensive first-pass metabolism.

Atipamezole increases medetomidine clearance in the dog: an agonist-antagonist interaction

Medetomidine, an alpha 2-adrenoceptor agonist, is a potent sedative and analgesic agent in the dog. When necessary, its action can be effectively antagonized by atipamezole. The present work was designed to study the effects of these drugs on each others' pharmacokinetics when a single intramuscular dose of medetomidine (50 micrograms kg-1) was followed by a dose of atipamezole (250 micrograms kg-1). Three different treatments were used: medetomidine alone, atipamezole alone, and atipamezole after medetomidine. Drug concentrations in plasma were measured by GC-MS. Statistical analysis of the results (ANOVA) revealed significant differences between treatments in the kinetic parameters of medetomidine. Atipamezole decreased the AUC of medetomidine from 41.3 to 28.6 ng h ml-1 (P = 0.005), t1/2 from 1.44 to 0.87 h (P = 0.015), and increased Cl from 21 to 31 ml min-1 kg-1 (P = 0.017). Differences in Vz did not reach statistical significance. The only statistically significant effects of medetomidine on the pharmacokinetics of atipamezole in this study were the slight decrease of Cl and Cmax as well as the increase of AUC. It is suggested that the large dose of medetomidine used caused haemodynamic changes, resulting in decreased hepatic circulation and slower drug metabolism. Antagonism by atipamezole restored the hepatic blood flow and, consequently, increased the elimination of medetomidine by biotransformation.

Radioreceptor assay for determination of xylazine and medetomidine in sheep plasma

A radioreceptor assay technique is described for the measurement of xylazine and medetomidine in sheep plasma. The assay was based on the displacement of tritiated clonidine from a2-adrenoceptors in a rat brain homogenate by xylazine or medetomidine extracted from plasma. Plasma samples from sheep which had been given xylazine and medetomidine were treated with alumina to remove endogenous catecholamines which would otherwise have bound to alpha 2-adrenoceptors and interfered with the assay. The drugs were then extracted using chloroform, reconstituted in buffer and used to displace [3H]clonidine. The concentration of alpha 2-agonist was calculated by reference to standard curves. The method had a detection limit of 2.5 ng/mL for xylazine and 0.24 ng/mL for medetomidine. The assay could also be used to detect metabolites capable of binding to alpha 2-receptors.

Implementation of a radioreceptor assay for dexmedetomidine

We have implemented a radioreceptor assay for dexmedetomidine, a novel alpha 2-adrenoceptor agonist. Receptor-bearing membranes were prepared from rat cerebral cortex and 3H-clonidine, 4 nM, was used as the labeled ligand. Dexmedetomidine displaced 3H-clonidine in a linear fashion over a concentration of 2 x 10(-10) to 2 x 10(-8)M. The detection limit of dexmedetomidine (i.e. 10% of radiolabeled ligand displaced) in this assay was 50 pg.ml-1 which is comparable to that seen with the reference method which utilizes gas chromotography with mass spectrometer (GC/MS) in series (Vuorilehto et al. 1989). Endogenous catecholamines, which can displace the radiolabeled ligand from its binding site, could easily be eliminated with a one-step extraction procedure. A comparison was made with the reference method (GC/MS) in 47 human plasma samples; the correlation coefficient (r2) was 0.61 (P < 0.001). The radioreceptor assay was also successfully applied for determining dexmedetomidine concentration in rabbit samples. These data indicate that the radioreceptor assay can be utilized for characterizing the pharmacokinetics of novel alpha 2 agonists which are now being introduced into the clinical practice of anaesthesia.

Systemic absorption and systemic effects of ocularly administered dexmedetomidine in rabbits

Dexmedetomidine is a selective alpha 2-adrenoceptor agonist which has previously been shown to reduce the ocular pressure of normotensive rabbits as well as those with pressures artificially elevated by laser irradiation. In this study instillation of an equivalent hypotensive dose (12.5 micrograms) did not cause changes in heart rate, blood pressure, blood glucose or plasma catecholamine content even though dexmedetomidine could be detected in plasma. However, this dose given intravenously (i.v.) was also without effect. Higher ocular doses resulted in equivalent bradycardia and changes in blood glucose levels as when the dose was given i.v. These two parameters proved to be most sensitive indicators of systemic alpha 2-agonism, blood pressure did not change and plasma catecholamine levels were too low to be reliably assayed. It is concluded that when hypotensive doses of dexmedetomidine are instilled into the eye, intraocular concentrations are sufficiently high to exert pharmacological effects. As it is absorbed into the general circulation, it is diluted such that its systemic effects are minimal.

Liquid chromatography-mass spectrometry for the determination of medetomidine and other anaesthetics in plasma

A liquid chromatographic-atmospheric pressure chemical ionization mass spectrometric method is presented for the simultaneous determination of medetomidine and other anaesthetic drugs in solutions and dog plasma. The drugs examined were flumazenil, butorphanol, atropine, ketamine, xylazine, medetomidine, atipamezole and midazolam. The separation was carried out on a reversed-phase column using methanol-0.1 M ammonium acetate (3:2) as eluent.