Enzyme-inducing Anticonvulsants Increase Plasma Clearance of Dexmedetomidine

Population Modelling of Dexmedetomidine Pharmacokinetics and Haemodynamic Effects After Intravenous and Subcutaneous Administration

Background and Objective

Dexmedetomidine is a potent agonist of α₂-adrenoceptors causing dose-dependent sedation in humans. Intravenous dexmedetomidine is commonly used perioperatively, but an extravascular route of administration would be favoured in palliative care. Subcutaneous infusions provide desired therapeutic plasma concentrations with fewer unwanted effects as compared with intravenous dosing. We aimed to develop semi-mechanistic population models for predicting pharmacokinetic and pharmacodynamic profiles of dexmedetomidine after intravenous and subcutaneous dosing.

Methods

Non-linear mixed-effects modelling was performed using previously collected concentration and haemodynamic effects data from ten (eight in the intravenous phase) healthy human subjects, aged 19–27 years, receiving 1 µg/kg of intravenous or subcutaneous dexmedetomidine during a 10-min infusion.

Results

The absorption of dexmedetomidine from the subcutaneous injection site, and distribution to local subcutaneous fat tissue was modelled using a semi-physiological approach consisting of a depot and fat compartment, while a two-compartment mammillary model explained further disposition. Dexmedetomidine-induced reductions in plasma norepinephrine concentrations were accurately described by an indirect response model. For blood pressure models, the net effect was specified as hyper- and hypotensive effects of dexmedetomidine due to vasoconstriction on peripheral arteries and sympatholysis mediated via the central nervous system, respectively. A heart rate model combined the dexmedetomidine-induced sympatholytic effect, and input from the central nervous system, predicted from arterial blood pressure levels. Internal evaluation confirmed the predictive performance of the final models, as well as the accuracy of the parameter estimates with narrow confidence intervals.

Conclusions

Our final model precisely describes dexmedetomidine pharmacokinetics and accurately predicts dexmedetomidine-induced sympatholysis and other pharmacodynamic effects. After subcutaneous dosing, dexmedetomidine is taken up into subcutaneous fat tissue, but our simulations indicate that accumulation of dexmedetomidine in this compartment is insignificant.

ClinicalTrials.org

NCT02724098 and EudraCT 2015-004698-34

Electronic supplementary material

The online version of this article (10.1007/s40262-020-00900-3) contains supplementary material, which is available to authorized users.

Electroencephalographic Arousal Patterns Under Dexmedetomidine Sedation

BACKGROUND

The depth of dexmedetomidine-induced sedation is difficult to assess without arousing the patient. We evaluated frontal electroencephalogram (EEG) as an objective measure of dexmedetomidine-induced sedation. Our aims were to characterize the response patterns of EEG during a wide range of dexmedetomidine-induced sedation and to determine which spectral power best correlated with assessed levels of dexmedetomidine-induced sedation.

METHODS

Sedline EEG sensor was positioned on the forehead of 16 volunteers. Frontal EEG data were collected at 250 Hz using the Sedline monitor. A computer-controlled infusion pump was used to infuse dexmedetomidine to four 15-minute target plasma concentrations of 0.3, 0.6, 1.2, and 2.4 ng/mL. Arterial blood samples for dexmedetomidine plasma concentration and sedation (self-reported numerical rating scale) and arousal were measured at baseline and at the end of each infusion step. The EEG signal was used to estimate spectral power in sequential 4-second data segments with 75% overlap for 3 power bands: delta = 0.5-1.5 Hz, alpha = 9-14 Hz, beta = 15-24 Hz. We quantified the relationships among the plasma concentrations of dexmedetomidine, level of sedation, and various EEG parameters.

RESULTS

EEG data at the end of the dexmedetomidine infusion steps show progressive loss of high frequencies (beta) and increase in alpha and delta powers, with increasing dexmedetomidine concentrations. Beta prearousal spectral power was best in predicting dexmedetomidine-induced level of sedation (R = -0.60, 95% CI, -0.43 to -0.75). The respective values for delta and alpha powers were R = 0.28 (95% CI, 0.03-0.45) and R = 0.16 (95% CI, -0.09 to 0.38). When the beta power has dropped below -16 dB or the delta power is above 15 dB, the subjects show moderate to deep levels of sedation. When awakening the subject, there is a reduction in power in the delta and alpha bands at the 0.6, 1.2, and 2.4 ng/mL dexmedetomidine target levels (P < .001 for all). In beta band, there is a rapid awakening-induced increase in power (P < .001) followed by a slow return toward baseline values. After arousing the subjects, the EEG powers returned toward baseline values significantly slower than our clinical observation of the subjects' wakefulness would have suggested.

CONCLUSIONS

Using a wide range of dexmedetomidine doses, we found that frontal EEG beta power of less than -16 dB and/or a delta power of over 15 dB was associated with a state of moderate to deep sedation and that poststimulus return of EEG powers toward baseline values took significantly longer than expected from observation of the arousal response. It is unclear whether these observations are robust enough for clinical applicability.

Prolonged therapy with the anticonvulsant carbamazepine leads to increased plasma clearance of fentanyl

OBJECTIVES

Fentanyl is a potent analgesic that accounts for an increasing number of overdose deaths in the United States. This study tested whether altered pharmacokinetics plays a pivotal role in the increased fentanyl dose requirements in patients receiving the enzyme-inducing anticonvulsant, carbamazepine.

METHODS

Neurosurgical patients receiving carbamazepine for >6 weeks (N = 11) or no carbamazepine (N = 6, controls) received a single bolus dose of fentanyl (200 μg) intravenously. Plasma was collected before and for up to 9 h after the bolus. Fentanyl concentrations were measured using liquid chromatography-mass spectrometry. Pharmacokinetic variables were derived from plasma concentration-time curves best fitted to a two-compartment model.

KEY FINDINGS

Fentanyl clearance was significantly higher in the carbamazepine group compared to controls (mean ± SD: 20.1 ± 6.8 vs 13.2 ± 4.8 ml/min per kg, P < 0.05), and area under the plasma concentration curve (AUC) was significantly lower (150 ± 65 vs 233 ± 70 ng/ml × min, P < 0.02). Volume of distribution was larger in the carbamazepine group, but the difference was not statistically significant (5.4 ± 3.1 vs 3.6 ± 1.2 l/kg, P > 0.15). The terminal elimination half-life did not differ between the two groups.

CONCLUSIONS

Chronic carbamazepine therapy leads to increased fentanyl clearance and decreased AUC, which may result in decreased duration of therapeutic plasma concentrations of fentanyl and an increased dose requirement. Assuming that carbamazepine does not change fentanyl pharmacodynamics, patients on chronic carbamazepine therapy may require more frequent or higher fentanyl doses to maintain therapeutic plasma concentrations.

Clinical Pharmacokinetics and Pharmacodynamics of Dexmedetomidine

Dexmedetomidine is an α₂-adrenoceptor agonist with sedative, anxiolytic, sympatholytic, and analgesic-sparing effects, and minimal depression of respiratory function. It is potent and highly selective for α₂-receptors with an α₂:α₁ ratio of 1620:1. Hemodynamic effects, which include transient hypertension, bradycardia, and hypotension, result from the drug’s peripheral vasoconstrictive and sympatholytic properties. Dexmedetomidine exerts its hypnotic action through activation of central pre- and postsynaptic α₂-receptors in the locus coeruleus, thereby inducting a state of unconsciousness similar to natural sleep, with the unique aspect that patients remain easily rousable and cooperative. Dexmedetomidine is rapidly distributed and is mainly hepatically metabolized into inactive metabolites by glucuronidation and hydroxylation. A high inter-individual variability in dexmedetomidine pharmacokinetics has been described, especially in the intensive care unit population. In recent years, multiple pharmacokinetic non-compartmental analyses as well as population pharmacokinetic studies have been performed. Body size, hepatic impairment, and presumably plasma albumin and cardiac output have a significant impact on dexmedetomidine pharmacokinetics. Results regarding other covariates remain inconclusive and warrant further research. Although initially approved for intravenous use for up to 24 h in the adult intensive care unit population only, applications of dexmedetomidine in clinical practice have been widened over the past few years. Procedural sedation with dexmedetomidine was additionally approved by the US Food and Drug Administration in 2003 and dexmedetomidine has appeared useful in multiple off-label applications such as pediatric sedation, intranasal or buccal administration, and use as an adjuvant to local analgesia techniques.

Sedative Plasma Concentrations and Delirium Risk in Critical Illness

BACKGROUND

The relationship between plasma concentration of sedatives and delirium is unknown.

OBJECTIVE

We hypothesized that higher plasma concentrations of lorazepam are associated with increased delirium risk, whereas higher plasma concentrations of dexmedetomidine are associated with reduced delirium risk.

METHODS

This prospective cohort study was embedded in a double-blind randomized clinical trial, where ventilated patients received infusions of lorazepam and dexmedetomidine. Plasma concentrations of these drugs and delirium assessments were measured at least daily. A multivariable logistic regression model accounting for repeated measures was used to analyze associations between same-day plasma concentrations of lorazepam and dexmedetomidine (exposures) and the likelihood of next-day delirium (outcome), adjusting for same-day mental status (delirium, coma, or normal) and same-day fentanyl doses.

RESULTS

This critically ill cohort (n = 103) had a median age of 60 years (IQR: 48-66) with APACHE II score of 28 (interquartile range [IQR] = 24-32), where randomization resulted in assignment to lorazepam (n = 51) or dexmedetomidine (n = 52). After adjusting for same-day fentanyl dose and mental status, higher plasma concentrations of lorazepam were associated with increased probability of next-day delirium (comparing 500 vs 0 ng/mL; odds ratio [OR] = 13.2; 95% CI = 1.4-120.1; P = 0.02). Plasma concentrations of dexmedetomidine were not associated with next-day delirium (comparing 1 vs 0 ng/mL; OR = 1.1; 95% CI = 0.9-1.3; P = 0.45).

CONCLUSIONS

In critically ill patients, higher lorazepam plasma concentrations were associated with delirium, whereas dexmedetomidine plasma concentrations were not. This implies that the reduced delirium risk seen in patients sedated with dexmedetomidine may be a result of avoidance of benzodiazepines, rather than a dose-dependent protective effect of dexmedetomidine.