Pharmacokinetic-pharmacodynamic modeling of the antinociceptive effect of buprenorphine and fentanyl in rats: role of receptor equilibration kinetics

Development of opioid analgesic tolerance in rat to extended-release buprenorphine formulated for laboratory subjects

Buprenorphine in an extended-release formulation intended for use in laboratory subjects is frequently administered to rats to provide extended analgesia without repeated handling. While levels of buprenorphine may persist in serum once extended-release buprenorphine has been introduced, exposure to opioids can cause opioid tolerance or opioid-induced hypersensitivity. This work examined the analgesic duration and efficacy of a single administration of extended-release buprenorphine intended for use in laboratory subjects in models of inflammatory pain and post-operative pain and the development of opioid tolerance in rat. After subcutaneous administration of 1 mg/kg extended-release buprenorphine, analgesic efficacy did not persist for the expected 72 hours. No changes were observed in mechanical thresholds in the hindpaws that were contralateral to the injury, suggesting a lack of centrally mediated opioid-induced hypersensitivity. To determine whether opioid tolerance arose acutely after one exposure to extended-release buprenorphine, we conducted the warm water tail flick assay; on Day 1 we administered either saline or extended-release buprenorphine (1 mg/kg) and on Day 3 we quantified the standard buprenorphine dose-response curve (0.1-3 mg/kg). Rats previously given extended-release buprenorphine displayed decreased analgesic responses after administration of standard buprenorphine as compared to the robust efficacy of standard buprenorphine in control subjects. Males appeared to show evidence of acute opioid tolerance, while females previously exposed to opioid did not demonstrate a decreased response at the doses examined. Taken together, these results suggest that opioid tolerance arises quickly in male rats after exposure to the extended-release formulation of buprenorphine. This tolerance may account for the brief period of antinociception observed.

Clinical efficacy of buprenorphine after oral dosing in rats undergoing major surgery

Serum corticosterone, serum buprenorphine, body weight change, consumption of food and water and behaviour-based pain assessment were measured in catheterised and non-catheterised male Wistar rats undergoing myocardial infarct (MI) surgery under general anaesthesia following buprenorphine dosing by subcutaneous (Bup-SC, 0.05 mg/kg) and oral (Bup-O, 0.4 mg/kg) routes. Buprenorphine was dosed subcutaneously at half an hour before and 8, 16 and 24 hours after surgery (Bup-SC), orally at one hour before surgery (Bup-O1) or at one hour before and 12 hours after surgery (Bup-O2) in catheterised rats and at one hour before and 24 hours after surgery (Bup-O24) in non-catheterised rats. Serum corticosterone, body weight changes and food and water consumption were not significantly different between treatments in catheterised rats. Bup-SC resulted in rapidly decreasing serum concentrations below the clinically effective concentrations (1 ng/mL) already at two hours after the first dose. Bup-O provided significantly higher and slowly decreasing serum concentrations, at or above clinically effective concentrations, for 24 hours (Bup-O1) and 42 hours (Bup-O2) after surgery. In non-catheterised rats, body weight development and food consumption were significantly higher in Bup-O24 rats compared to Bup-SC rats. The results indicate that a SC buprenorphine dose of 0.05 mg/kg every eight hours provides long periods of serum concentrations below clinically effective levels, and that a higher dose and/or more frequent dosage are required to provide stable serum concentrations at or above clinically effective levels. A single oral buprenorphine dose of 0.4 mg/kg provides clinically effective and stable serum concentrations for 24 hours in rats after MI surgery.

Oxycodone, an opioid like the others?

The over-prescription of opioid analgesics is a growing problem in the field of addiction, which has reached epidemic-like proportions in North America. Over the past decade, oxycodone has gained attention as the leading opioid responsible for the North America opioid crisis. Oxycodone is the most incriminated drug in the early years of the epidemic of opioid use disorder in USA (roughly 1999-2016). The number of preclinical articles on oxycodone is rapidly increasing. Several publications have already compared oxycodone with other opioids, focusing mainly on their analgesic properties. The aim of this review is to focus on the genomic and epigenetic regulatory features of oxycodone compared with other opioid agonists. Our aim is to initiate a discussion of perceptible differences in the pharmacological response observed with these various opioids, particularly after repeated administration in preclinical models commonly used to study drug dependence potential.

A buprenorphine depot formulation provides effective sustained post-surgical analgesia for 72 h in mouse femoral fracture models

Adequate pain management is essential for ethical and scientific reasons in animal experiments and should completely cover the period of expected pain without the need for frequent re-application. However, current depot formulations of Buprenorphine are only available in the USA and have limited duration of action. Recently, a new microparticulate Buprenorphine formulation (BUP-Depot) for sustained release has been developed as a potential future alternative to standard formulations available in Europe. Pharmacokinetics indicate a possible effectiveness for about 72 h. Here, we investigated whether the administration of the BUP-Depot ensures continuous and sufficient analgesia in two mouse fracture models (femoral osteotomy) and could, therefore, serve as a potent alternative to the application of Tramadol via the drinking water. Both protocols were examined for analgesic effectiveness, side effects on experimental readout, and effects on fracture healing outcomes in male and female C57BL/6N mice. The BUP-Depot provided effective analgesia for 72 h, comparable to the effectiveness of Tramadol in the drinking water. Fracture healing outcome was not different between analgesic regimes. The availability of a Buprenorphine depot formulation for rodents in Europe would be a beneficial addition for extended pain relief in mice, thereby increasing animal welfare.

Pharmacodynamic model of slow reversible binding and its applications in pharmacokinetic/pharmacodynamic modeling: review and tutorial

Therapeutic responses of most drugs are initiated by the rate and degree of binding to their receptors or targets. The law of mass action describes the rate of drug-receptor complex association (k_on) and dissociation (k_off) where the ratio k_off/k_on is the equilibrium dissociation constant (K_d). Drugs with slow reversible binding (SRB) often demonstrate delayed onset and prolonged pharmacodynamic effects. This report reviews evidence for drugs with SRB features, describes previous pharmacokinetic/pharmacodynamic (PK/PD) modeling efforts of several such drugs, provides a tutorial on the mathematics and properties of SRB models, demonstrates applications of SRB models to additional compounds, and compares PK/PD fittings of SRB with other mechanistic models. We identified and summarized 52 drugs with in vitro-confirmed SRB from a PubMed literature search. Simulations with a SRB model and observed PK/PD profiles showed delayed and prolonged responses and that increasing doses/k_on or decreasing k_off led to greater expected maximum effects and a longer duration of effects. Recession slopes for return of responses to baseline after single doses were nearly linear with an inflection point that approaches a limiting value at larger doses. The SRB model newly captured literature data for the antihypertensive effects of candesartan and antiallergic effects of noberastine. Their PD profiles could also be fitted with indirect response and biophase models with minimal differences. The applicability of SRB models is probably commonplace, but underappreciated, owing to the need for in vitro confirmation of binding kinetics and the similarity of PK/PD profiles to models with other mechanistic determinants.

Finding the right hazard function for time-to-event modeling: A tutorial and Shiny application

Parametric time-to-event analysis is an important pharmacometric method to predict the probability of an event up until a certain time as a function of covariates and/or drug exposure. Modeling is performed at the level of the hazard function describing the instantaneous rate of an event occurring at that timepoint. We give an overview of the parametric time-to-event analysis starting with graphical exploration by Kaplan-Meier plotting for the event data including censoring and nonparametric hazard estimators such as the kernel-based visual hazard comparison for the underlying hazard. The most common hazard functions including the exponential, Gompertz, Weibull, log-normal, log-logistic, and circadian functions are described in detail. A Shiny application was developed to graphically guide the modeler which of the most common hazard functions presents a similar shape compared to the data in order to guide which hazard functions to test in the parametric time-to-event analysis. For the chosen hazard function(s), the Shiny application can additionally be used to explore corresponding parameter values to inform on suitable initial estimates for parametric modeling as well as on possible covariate or treatment relationships to certain parameters. Moreover, it can be used for the dissemination of results as well as communication, training, and workshops on time-to-event analysis. By guiding the modeler on which functions and what parameter values to test and compare as well as to assist in dissemination, the Shiny application developed here greatly supports the modeler in complicated parametric time-to-event modeling.

The thermal antinociceptive effects of a high-concentration formulation of buprenorphine alone or followed by hydromorphone in conscious cats

OBJECTIVE

To evaluate the thermal antinociceptive effects of a high-concentration formulation of buprenorphine alone or followed by hydromorphone in conscious cats.

STUDY DESIGN

Randomized, blinded, placebo-controlled crossover study design.

ANIMALS

A total of six purpose-bred, adult female ovariohysterectomized Domestic Short Hair cats.

METHODS

Cats were allocated into three treatments each consisting of two injections, subcutaneous then intravenous (IV) administration, 2 hours apart: treatment SS, two injections of 0.9% saline; treatment BS, buprenorphine (0.24 mg kg^-1, 1.8 mg mL^-1) and saline; and treatment BH, buprenorphine (0.24 mg kg^-1) and hydromorphone (0.1 mg kg^-1). Skin temperature (ST) and thermal threshold (TT) were recorded before (baseline) and for 24 hours following first injection. TT data were analyzed using mixed linear models and a Benjamini-Hochberg sequential adjustment procedure (p < 0.05).

RESULTS

There were no significant differences among treatments for baseline ST and TT values, treatment SS over time and between treatments BS and BH. Compared with baseline, TT was significantly increased at all time points in treatments BH and BS except at 2 hours in treatment BS. TT was significantly higher than SS at 3-18 hours and 4-12 hours for treatments BS and BH, respectively. Maximal increases in TT were 47.5 °C at 2 hours, 53.9 °C at 3 hours and 52.4 °C at 6 hours in treatments SS, BS and BH, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of IV hydromorphone following high-concentration buprenorphine provided no additional antinociception and decreased the duration of effect when compared with high-concentration buprenorphine alone. Alternative analgesics should be considered if additional analgesia is required after administration of high-concentration buprenorphine.

Design and in vivo evaluation of a microparticulate depot formulation of buprenorphine for veterinary use

Buprenorphine is a frequently used analgetic agent in veterinary medicine. A major drawback, however, is the short duration of action requiring several daily administrations. We therefore designed a poly-lactic-co-glycolic acid (PLGA) based microparticulate drug formulation for sustained parenteral drug release. Particles were designed to allow for a fast onset of action and a duration of the analgesic effect of at least two days in laboratory mice. Microparticles were produced using a solvent evaporation technique. Release rate was dependent on polymer type and particle size. Spherical particles used for subsequent animal studies had a mean size of 50 µm and contained 4.5% of buprenorphine. Drug release was characterized by an initial burst release of 30% followed by complete release over seven days. In vivo pharmacokinetic experiments in female C57BL/6 J mice confirmed prolonged exposure in plasma and brain tissue and correlated with the pharmacological effect in the hot plate assay or after minor abdominal surgery. No adverse side effects with respect to food and water intake, body weight, local tolerability, or nesting behavior were observed. Our formulation is an attractive alternative to established immediate release formulations. A use for prolonged pain management in laboratory animals is proposed.

Higher naloxone dosing in a quantitative systems pharmacology model that predicts naloxone-fentanyl competition at the opioid mu receptor level

Rapid resuscitation of an opioid overdose with naloxone, an opioid antagonist, is critical. We developed an opioid receptor quantitative systems pharmacology (QSP) model for evaluation of naloxone dosing. In this model we examined three opioid exposure levels that have been reported in the literature (25 ng/ml, 50 ng/ml, and 75 ng/ml of fentanyl). The model predicted naloxone-fentanyl interaction at the mu opioid receptor over a range of three naloxone doses. For a 2 mg intramuscular (IM) dose of naloxone at lower fentanyl exposure levels (25 ng/ml and 50 ng/ml), the time to decreasing mu receptor occupancy by fentanyl to 50% was 3 and 10 minutes, respectively. However, at a higher fentanyl exposure level (75 ng/ml), a dose of 2 mg IM of the naloxone failed to reduce mu receptor occupancy by fentanyl to 50%. In contrast, naloxone doses of 5 mg and 10 mg IM reduced mu receptor occupancy by fentanyl to 50% in 5.5 and 4 minutes respectively. These results suggest that the current doses of naloxone (2 mg IM or 4 mg intranasal (IN)) may be inadequate for rapid reversal of toxicity due to fentanyl exposure and that increasing the dose of naloxone is likely to improve outcomes.

Pharmacokinetic-pharmacodynamic models that incorporate drug-target binding kinetics

Pharmacokinetic/pharmacodynamic (PK/PD) models predict the effect time course resulting from a drug dose. In this review, we summarize the development of mechanistic PK/PD models that explicitly integrate the kinetics of drug-target interactions into predictions of drug activity. Such mechanistic models are expected to have several advantages over approaches in which concentration and effect are linked using variations of the Hill equation, and where preclinical data are often used as a starting point for modeling drug activity. Instead, explicit use of the full kinetic scheme for drug binding enables time-dependent changes in target occupancy to be calculated using the kinetics of drug-target interactions and drug PK, providing a more precise picture of target engagement and drug action in the non-equilibrium environment of the human body. The mechanistic PK/PD models also generate target vulnerability functions that link target occupancy and effect, and inform on the sensitivity of a target to engagement by a drug. Key factors such as the rate of target turnover can also be integrated into the modeling which, together with target vulnerability, provide additional information on the PK profile required to achieve the desired pharmacological effect and on the utility of kinetic selectivity in developing drugs for specific targets.

Treating Chronic Pain: An Overview of Clinical Studies Centered on the Buprenorphine Option

The buprenorphine receptor binding profile is unique in that it binds to all three major opioid receptors (mu, kappa, delta), and also binds to the orphan-like receptor, the receptor for orphanin FQ/nociceptin, with lower affinity. Within the mu receptor group, buprenorphine analgesia in rodents is dependent on the recently discovered arylepoxamide receptor target in brain, which involves a truncated 6-transmembrane mu receptor gene protein, distinguishing itself from morphine and most other mu opioids. Although originally designed as an analgesic, buprenorphine has mainly been used for opioid maintenance therapy and only now is increasingly recognized as an effective analgesic with an improved therapeutic index relative to certain potent opioids. Albeit a second-, third-, or fourth-line analgesic, buprenorphine is a reasonable choice in certain clinical situations. Transdermal patches and buccal film formulations are now commercially available as analgesics. This review discusses buprenorphine pharmacodynamics and pharmacokinetics, use in certain populations, and provides a synopsis of systematic reviews and randomized analgesic trials. We briefly discuss postoperative management in patients receiving buprenorphine maintenance therapy, opioid equivalence to buprenorphine, rotations to buprenorphine from other opioids, and clinical relevance of buprenorphine-related QTc interval changes.

Modality-specific peripheral antinociceptive effects of μ-opioid agonists on heat and mechanical stimuli: Contribution of sigma-1 receptors

Morphine induces peripherally μ-opioid-mediated antinociception to heat but not to mechanical stimulation. Peripheral sigma-1 receptors tonically inhibit μ-opioid antinociception to mechanical stimuli, but it is unknown whether they modulate μ-opioid heat antinociception. We hypothesized that sigma-1 receptors might play a role in the modality-specific peripheral antinociceptive effects of morphine and other clinically relevant μ-opioid agonists. Mechanical nociception was assessed in mice with the paw pressure test (450 g), and heat nociception with the unilateral hot plate (55 °C) test. Local peripheral (intraplantar) administration of morphine, buprenorphine or oxycodone did not induce antinociception to mechanical stimulation but had dose-dependent antinociceptive effects on heat stimuli. Local sigma-1 antagonism unmasked peripheral antinociception by μ-opioid agonists to mechanical stimuli, but did not modify their effects on heat stimulation. TRPV1+ and IB4+ cells are segregated populations of small neurons in the dorsal root ganglia (DRG) and the density of sigma-1 receptors was higher in IB4+ cells than in the rest of small nociceptive neurons. The in vivo ablation of TRPV1-expressing neurons with resiniferatoxin did not alter IB4+ neurons in the DRG, mechanical nociception, or the effects of sigma-1 antagonism on local morphine antinociception in this type of stimulus. However, it impaired the responses to heat stimuli and the effect of local morphine on heat nociception. In conclusion, peripheral opioid antinociception to mechanical stimuli is limited by sigma-1 tonic inhibitory actions, whereas peripheral opioid antinociception to heat stimuli (produced in TRPV1-expressing neurons) is not. Therefore, sigma-1 receptors contribute to the modality-specific peripheral effects of opioid analgesics.

Modelling the delay between pharmacokinetics and EEG effects of morphine in rats: binding kinetic versus effect compartment models

Drug–target binding kinetics (as determined by association and dissociation rate constants, k_on and k_off) can be an important determinant of the kinetics of drug action. However, the effect compartment model is used most frequently instead of a target binding model to describe hysteresis. Here we investigate when the drug–target binding model should be used in lieu of the effect compartment model. The utility of the effect compartment (EC), the target binding kinetics (TB) and the combined effect compartment–target binding kinetics (EC–TB) model were tested on either plasma (EC_PL, TB_PL and EC–TB_PL) or brain extracellular fluid (ECF) (EC_ECF, TB_ECF and EC–TB_ECF) morphine concentrations and EEG amplitude in rats. It was also analyzed when a significant shift in the time to maximal target occupancy (Tmax_TO) with increasing dose, the discriminating feature between the TB and EC model, occurs in the TB model. All TB models assumed a linear relationship between target occupancy and drug effect on the EEG amplitude. All three model types performed similarly in describing the morphine pharmacodynamics data, although the EC model provided the best statistical result. The analysis of the shift in Tmax_TO (∆Tmax_TO) as a result of increasing dose revealed that ∆Tmax_TO is decreasing towards zero if the k_off is much smaller than the elimination rate constant or if the target concentration is larger than the initial morphine concentration. The results for the morphine PKPD modelling and the analysis of ∆Tmax_TO indicate that the EC and TB models do not necessarily lead to different drug effect versus time curves for different doses if a delay between drug concentrations and drug effect (hysteresis) is described. Drawing mechanistic conclusions from successfully fitting one of these two models should therefore be avoided. Since the TB model can be informed by in vitro measurements of k_on and k_off, a target binding model should be considered more often for mechanistic modelling purposes.

Evaluation of self-injurious behavior, food intake, fecal output, and thermal withdrawal latencies after injection of a high-concentration buprenorphine formulation in rats (Rattus norvegicus)

OBJECTIVE To evaluate effects of high-concentration buprenorphine (HCB) on self-injurious behavior, food intake, fecal output, and thermal withdrawal latencies in healthy rats.

ANIMALS 8 Sprague-Dawley rats.

PROCEDURES Rats received 4 SC treatments (HCB at 0.075, 0.15, or 0.30 mg/kg [HCB0.075, HCB0.15, and HCB0.30, respectively] or 5% dextrose solution [0.20 mL/kg]) in a randomized, crossover-design study. Self-injurious behavior was assessed for 8 hours after injection. Food intake and fecal output were assessed for predetermined periods before and after treatment and separated into 12-hour light and dark periods for further analysis. Withdrawal latencies were assessed before (time 0) and at predetermined times after injection. Data were compared among treatments and time points.

RESULTS Self-injurious behavior was observed up to 8 hours after injection for all HCB, but not dextrose, treatments. Preinjection food intake and fecal output amounts were similar among groups and higher during the dark period than during the light period. Food intake after all HCB treatments was higher during the light period and lower during the dark period, compared with preinjection results for the same treatments and with postinjection results for dextrose administration. Light-period fecal output was lower after HCB0.15 and HCB0.30 administration, compared with preinjection values for the same treatments and postinjection values for dextrose administration. Percentage change in withdrawal latency was significantly higher than that at time 0 (ie, 0%) for only 1 treatment (HCB0.30) at 1 time point (1 hour after injection).

CONCLUSIONS AND CLINICAL RELEVANCE Although HCB0.30 produced a degree of thermal hypoalgesia in healthy rats, self-injurious behavior and alterations in food intake and fecal output were detected, potentially affecting clinical utility of the treatment.

Novel CNS drug discovery and development approach: model-based integration to predict neuro-pharmacokinetics and pharmacodynamics

INTRODUCTION

CNS drug development has been hampered by inadequate consideration of CNS pharmacokinetic (PK), pharmacodynamics (PD) and disease complexity (reductionist approach). Improvement is required via integrative model-based approaches. Areas covered: The authors summarize factors that have played a role in the high attrition rate of CNS compounds. Recent advances in CNS research and drug discovery are presented, especially with regard to assessment of relevant neuro-PK parameters. Suggestions for further improvements are also discussed. Expert opinion: Understanding time- and condition dependent interrelationships between neuro-PK and neuro-PD processes is key to predictions in different conditions. As a first screen, it is suggested to use in silico/in vitro derived molecular properties of candidate compounds and predict concentration-time profiles of compounds in multiple compartments of the human CNS, using time-course based physiology-based (PB) PK models. Then, for selected compounds, one can include in vitro drug-target binding kinetics to predict target occupancy (TO)-time profiles in humans. This will improve neuro-PD prediction. Furthermore, a pharmaco-omics approach is suggested, providing multilevel and paralleled data on systems processes from individuals in a systems-wide manner. Thus, clinical trials will be better informed, using fewer animals, while also, needing fewer individuals and samples per individual for proof of concept in humans.

Evaluation of buprenorphine hydrochloride Pluronic(®) gel formulation in male C57BL/6NCrl mice

Providing adequate analgesia while minimizing handling and stress post-surgery can be challenging. Recently, there have been commercial products made available for providing long acting analgesia in rodents. However, we find there are limitations for use in mice due to the viscosity of the product and the small dosing volumes needed. This project evaluated an in-house compounded formulation of buprenorphine easily made in the laboratory using pharmaceutical grade products. The release of buprenorphine was evaluated when compounded with two types of hydrogels (Pluronic(®) F-127 and F-68). Mice given buprenorphine in hydrogel (BP) demonstrated higher serum levels of buprenorphine for a longer period of time compared to mice given standard buprenorphine (Bup). However, the rate of decline in serum levels between the groups was similar; thus, it is more likely that the higher buprenorphine concentration seen in the BP group is due to the higher dose of buprenorphine given, rather than a slower release of product. Feed consumption was decreased in both groups one day after dosing; however, there was no difference in body weights. Increased activity in the open field was observed with both buprenorphine formulations, and lipemia was observed in mice given BP which persisted to at least 96 h. Based on our results, we conclude that this formulation did not sustain the release of buprenorphine or eliminate the increased activity commonly seen in mice given buprenorphine. In addition, the lipemia may confound research parameters, especially in cardiac studies and lipid metabolism studies. Therefore, we cannot recommend this formulation for use.

Pharmacokinetic-pharmacodynamic modelling of drug-induced QTc interval prolongation in man: prediction from in vitro human ether-à-go-go-related gene binding and functional inhibition assays and conscious dog studies

BACKGROUND AND PURPOSE

Functional measures of human ether-à-go-go-related gene (hERG; Kv 11.1) channel inhibition have been prioritized as an in vitro screening tool for candidate molecules. However, it is unclear how these results can be translated to humans. Here, we explore how data on drug binding and functional inhibition in vitro relate to QT prolongation in vivo. Using cisapride, sotalol and moxifloxacin as paradigm compounds, we assessed the relationship between drug concentrations, binding, functional measures and in vivo effects in preclinical species and humans.

EXPERIMENTAL APPROACH

Pharmacokinetic-pharmacodynamic modelling was used to characterize the drug effects in hERG functional patch clamp, hERG radio-labelled dofetilide displacement experiments and QT interval in conscious dogs. Data were analysed in parallel to identify potential correlations between pharmacological activity in vitro and in vivo.

KEY RESULTS

An Emax model could not be used due to large variability in the functional patch clamp assay. Dofetilide displacement revealed that binding curves are unrelated to the in vivo potency estimates for QTc prolongation in dogs and humans. Mean in vitro potency estimates ranged from 99.9 nM for cisapride to 1030 μM for moxifloxacin.

CONCLUSIONS AND IMPLICATIONS

The lack of standardized protocols for in vitro assays leads to significant differences in experimental conditions, making the assessment of in vitro-in vivo correlations unreliable. Identification of an accurate safety window during the screening of candidate molecules requires a quantitative framework that disentangles system- from drug-specific properties under physiological conditions, enabling translation of the results to humans. Similar considerations will be relevant for the comprehensive in vitro pro-arrhythmia assay initiative.

Electromyographic activation reveals cortical and sub-cortical dissociation during emergence from general anesthesia

During emergence from anesthesia patients regain their muscle tone (EMG). In a typical population of surgical patients the actual volatile gas anesthetic concentrations in the brain (C_eMAC) at which EMG activation occurs remains unknown, as is whether EMG activation at higher C_eMACs is correlated with subsequent severe pain, or with cortical activation. Electroencephalographic (EEG) and EMG activity was recorded from the forehead of 273 patients emerging from general anesthesia following surgery. We determined C_eMAC at time of EMG activation and at return of consciousness. Pain was assessed immediately after return of consciousness using an 11 point numerical rating scale. The onset of EMG activation during emergence was associated with neither discernible muscle movement nor with the presence of exogenous stimulation in half the patients. EMG activation could be modelled as two distinct processes; termed high- and low-C_eMAC (occurring higher or lower than 0.07 C_eMAC). Low-C_eMAC activation was typically associated with simultaneous EMG activation and consciousness, and the presence of a laryngeal mask. In contrast, high-C_eMAC EMG activation occurred independently of return of consciousness, and was not associated with severe post-operative pain, but was more common in the presence of an endotracheal tube. Patients emerging from general anesthesia with an endotracheal tube in place are more likely to have an EMG activation at higher C_eMAC concentrations. These activations are not associated with subsequent high-pain, nor with cortical arousal, as evidenced by continuing delta waves in the EEG. Conversely, patients emerging from general anesthesia with a laryngeal mask demonstrate marked neural inertia-EMG activation occurs at a low C_eMAC, and is closely temporally associated with return of consciousness.

Pharmacokinetic-pharmacodynamic modelling in anaesthesia

Anaesthesiologists adjust drug dosing, administration system and kind of drug to the characteristics of the patient. They then observe the expected response and adjust dosing to the specific requirements according to the difference between observed response, expected response and the context of the surgery and the patient. The approach above can be achieved because on one hand quantification technology has made significant advances allowing the anaesthesiologist to measure almost any effect by using noninvasive, continuous measuring systems. On the other the knowledge on the relations between dosing, concentration, biophase dynamics and effect as well as detection of variability sources has been achieved as being the benchmark specialty for pharmacokinetic-pharmacodynamic (PKPD) modelling. The aim of the review is to revisit the most common PKPD models applied in the field of anaesthesia (i.e. effect compartmental, turnover, drug-receptor binding and drug interaction models) through representative examples. The effect compartmental model has been widely used in this field and there are multiple applications and examples. The use of turnover models has been limited mainly to describe respiratory effects. Similarly, cases in which the dissociation process of the drug-receptor complex is slow compared with other processes relevant to the time course of the anaesthetic effect are not frequent in anaesthesia, where in addition to a rapid onset, a fast offset of the response is required. With respect to the characterization of PD drug interactions different response surface models are discussed. Relevant applications that have changed the way modern anaesthesia is practiced are also provided.

Mechanistic models enable the rational use of in vitro drug-target binding kinetics for better drug effects in patients

INTRODUCTION

Drug-target binding kinetics are major determinants of the time course of drug action for several drugs, as clearly described for the irreversible binders omeprazole and aspirin. This supports the increasing interest to incorporate newly developed high-throughput assays for drug-target binding kinetics in drug discovery. A meaningful application of in vitro drug-target binding kinetics in drug discovery requires insight into the relation between in vivo drug effect and in vitro measured drug-target binding kinetics.

AREAS COVERED

In this review, the authors discuss both the relation between in vitro and in vivo measured binding kinetics and the relation between in vivo binding kinetics, target occupancy and effect profiles.

EXPERT OPINION

More scientific evidence is required for the rational selection and development of drug-candidates on the basis of in vitro estimates of drug-target binding kinetics. To elucidate the value of in vitro binding kinetics measurements, it is necessary to obtain information on system-specific properties which influence the kinetics of target occupancy and drug effect. Mathematical integration of this information enables the identification of drug-specific properties which lead to optimal target occupancy and drug effect in patients.

A Systems Pharmacology Model for Predicting Effects of Factor Xa Inhibitors in Healthy Subjects: Assessment of Pharmacokinetics and Binding Kinetics

Factor Xa (FXa) emerged as a promising target for effective anticoagulation and several FXa inhibitors are now available for the prevention of venous thromboembolism. However, in previously reported pharmacokinetic/pharmacodynamic (PK/PD) models, the complex coagulation processes and detailed information of drug action are usually unclear, which makes it difficult to predict clinical outcome at the drug discovery stage. In this study, a large‐scale systems pharmacology model was developed based on several published models and clinical data. It takes into account all pathways of the coagulation network, and captures drug‐specific features: plasma pharmacokinetics and drug‐target binding kinetics (BKs). We aimed to predict the anticoagulation effects of FXa inhibitors in healthy subjects, and to use this model to compare the effects of compounds with different binding properties. Our model predicts the clotting time and anti‐FXa effects and could thus serve as a predictive tool for the anticoagulant potential of a new compound.

Striatal dopamine D1 and D2 receptors are differentially regulated following buprenorphine or methadone treatment

RATIONALE

Chronic administration of morphine induces adaptations in neurotransmission system such as the dopamine pathway, and these modifications could be influenced by the drug administration pattern. Methadone and buprenorphine are the two main opioid substitution therapies, and despite their protracted use in humans, no study has investigated their ability to regulate dopamine system after chronic exposure/withdrawal.

OBJECTIVES

We evaluated the consequences of two administration patterns of methadone and buprenorphine on striatal dopamine D1 (D1R) and D2 (D2R) receptor levels.

METHODS

Mice were treated with escalating doses of methadone or buprenorphine for 5 days either once daily (binge) or three times a day (TTD). D1R and D2R density in striatum was measured by autoradiography using [(3)H]-SCH23390 and [(3)H]-raclopride, respectively, at 1 (WD1), 14 (WD14), and 35 (WD35) days after the last opioid injection.

RESULTS

A downregulation of D1R was observed upon TTD administration of buprenorphine and binge methadone treatment while an increase of those receptor levels was detected both with binge buprenorphine and TTD methadone treatments. Concerning the D2R, we rather measured an early or late downregulation with both agonists and administration patterns.

CONCLUSIONS

Our results demonstrated that methadone and buprenorphine were able to differentially regulate dopamine receptor density depending on the withdrawal period and the administration pattern.

Preclinical to clinical translation of CNS transporter occupancy of TD-9855, a novel norepinephrine and serotonin reuptake inhibitor

BACKGROUND

Monoamine reuptake inhibitors exhibit unique clinical profiles that reflect distinct engagement of the central nervous system (CNS) transporters.

METHODS

We used a translational strategy, including rodent pharmacokinetic/pharmacodynamic modeling and positron emission tomography (PET) imaging in humans, to establish the transporter profile of TD-9855, a novel norepinephrine and serotonin reuptake inhibitor.

RESULTS

TD-9855 was a potent inhibitor of norepinephrine (NE) and serotonin 5-HT uptake in vitro with an inhibitory selectivity of 4- to 10-fold for NE at human and rat transporters. TD-9855 engaged norepinephrine transporters (NET) and serotonin transporters (SERT) in rat spinal cord, with a plasma EC50 of 11.7 ng/mL and 50.8 ng/mL, respectively, consistent with modest selectivity for NET in vivo. Accounting for species differences in protein binding, the projected human NET and SERT plasma EC50 values were 5.5 ng/mL and 23.9 ng/mL, respectively. A single-dose, open-label PET study (4-20mg TD-9855, oral) was conducted in eight healthy males using the radiotracers [(11)C]-3-amino-4- [2-[(di(methyl)amino)methyl]phenyl]sulfanylbenzonitrile for SERT and [(11)C]-(S,S)-methylreboxetine for NET. The long pharmacokinetic half-life (30-40 h) of TD-9855 allowed for sequential assessment of SERT and NET occupancy in the same subject. The plasma EC50 for NET was estimated to be 1.21 ng/mL, and at doses of greater than 4 mg the projected steady-state NET occupancy is high (>75%). After a single oral dose of 20mg, SERT occupancy was 25 (±8)% at a plasma level of 6.35 ng/mL.

CONCLUSIONS

These data establish the CNS penetration and transporter profile of TD-9855 and inform the selection of potential doses for future clinical evaluation.

Rewarding or aversive effects of buprenorphine/naloxone combination (Suboxone) depend on conditioning trial duration

Buprenorphine is used as a sublingual medication in the treatment of opioid dependence. However, its misuse by i.v. injection may limit its acceptability and dissemination. A buprenorphine/naloxone (ratio 4:1) combination has been developed to reduce diversion and abuse. So far, the relevance of this combination has not been investigated in the animal models traditionally used to study the reinforcing effects of drugs of abuse. The aim of this study was to compare the rewarding effects, assessed by conditioned place preference (CPP), of buprenorphine and buprenorphine/naloxone combination following i.v. administration in mice. Animals were treated with different doses of buprenorphine or buprenorphine/naloxone combination (ratio 4:1), and CPP conditioning trial duration was 5 or 30 min. At the longest trial duration, a bell-shaped dose-response curve was obtained with buprenorphine, which was shifted significantly to the right with naloxone combination. At the shortest trial duration, an aversive effect was observed with the buprenorphine/naloxone combination in animals, involving opioid receptor-like 1 (ORL1). These findings may explain the discrepancies reported in the literature as some authors have shown a reduced buprenorphine/naloxone misuse compared to buprenorphine in opioid abusers, while others have not.

Buprenorphine for Cancer Pain

Buprenorphine (BUP) is a semisynthetic derivative of the opium alkaloid thebaine found in the poppy Papaver somniferum. Its chemical structure contains the morphine structure but differs by having a cyclopropylmethyl group. Buprenorphine is a potent µ opioid agonist. Buprenorphine undergoes extensive first-pass metabolism in the liver and gut. The development of a transdermal BUP formulation in 2001 led to its evaluation in cancer pain. This article provides the practitioner with an update on the current role of BUP in cancer care. It highlights data suggesting effectiveness in various types of cancer pain. The article reviews pharmacology, routes of administration, adverse effects, drug interactions, and cost considerations.

Modulation of peripheral μ-opioid analgesia by σ1 receptors

We evaluated the effects of σ1-receptor inhibition on μ-opioid-induced mechanical antinociception and constipation. σ1-Knockout mice exhibited marked mechanical antinociception in response to several μ-opioid analgesics (fentanyl, oxycodone, morphine, buprenorphine, and tramadol) at systemic (subcutaneous) doses that were inactive in wild-type mice and even unmasked the antinociceptive effects of the peripheral μ-opioid agonist loperamide. Likewise, systemic (subcutaneous) or local (intraplantar) treatment of wild-type mice with the selective σ1 antagonists BD-1063 [1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine dihydrochloride] or S1RA [4-[2-[[5-methyl-1-(2-naphthalenyl)1H-pyrazol-3-yl]oxy]ethyl] morpholine hydrochloride] potentiated μ-opioid antinociception; these effects were fully reversed by the σ1 agonist PRE-084 [2-(4-morpholinethyl)1-phenylcyclohexanecarboxylate) hydrochloride], showing the selectivity of the pharmacological approach. The μ-opioid antinociception potentiated by σ1 inhibition (by σ1-receptor knockout or σ1-pharmacological antagonism) was more sensitive to the peripherally restricted opioid antagonist naloxone methiodide than opioid antinociception under normal conditions, indicating a key role for peripheral opioid receptors in the enhanced antinociception. Direct interaction between the opioid drugs and σ1 receptor cannot account for our results, since the former lacked affinity for σ1 receptors (labeled with [(3)H](+)-pentazocine). A peripheral role for σ1 receptors was also supported by their higher density (Western blot results) in peripheral nervous tissue (dorsal root ganglia) than in several central areas involved in opioid antinociception (dorsal spinal cord, basolateral amygdala, periaqueductal gray, and rostroventral medulla). In contrast to its effects on nociception, σ1-receptor inhibition did not alter fentanyl- or loperamide-induced constipation, a peripherally mediated nonanalgesic opioid effect. Therefore, σ1-receptor inhibition may be used as a systemic or local adjuvant to enhance peripheral μ-opioid analgesia without affecting opioid-induced constipation.

Buprenorphine—Clinically useful but often misunderstood

Background

There are a number of false myths about buprenorphine based on unconfirmed animal data, even from isolated animal organs, and early clinical research. These myths came into textbooks on pharmacology and pain about 30 years ago and have been difficult to eradicate. Animal models of pain and pain relief are notoriously unreliable as predictors of human clinical effects. The fact is that in clinical practice there is NO bell-shaped dose-response curve, there is NO plateau on the dose-response curve, and there is NO antagonist effect from buprenorphine on other mu-opioid agonists.

Methods

This narrative, topical review of relevant research publications evaluates new knowledge on the pharmacodynamics and pharmacokinetics of buprenorphine of importance in clinical practice.

Results

Buprenorphine is a potent opioid analgesic acting on all four opioid receptors: it is an agonist on the mu-, the delta, and the ORL-1 receptors. It is an antagonist at the kappa-receptor. Buprenorphine has a number of active metabolites with different effects on the four opioid receptors; all except the norbup-3-glu are analgesic. Buprenorphine itself is not a respiratory depressant or sedative, but some of its active metabolites are. Buprenorphine and its active metabolites are not excreted by the kidney. Therefore buprenorphine may be used in patients with advanced renal failure.

Buprenorphine has a slow onset and a long offset. These properties are advantageous, except sometimes when treating severe acute pain. Its agonist effect on the ORL-1 receptor reduces reward-effects and slows the development of tolerance to the analgesic effects.

Buprenorphine inhibits voltage-gated sodium-channels and enhances and prolongs peripheral nerve blocks. Its ORL-1 -effect at the spinal cord may do the same.

Buprenorphine is well suited for treatment of chronic pain, especially chronic neuropathic pain and cancer pain. The beneficial effects as a co-medication during treatment of the opioid-abuse disease are due to its slow onset (less “kick-effect”). Its prolonged offset-time reduces the likelihood of acute withdrawal problems and reduces the “craving” of opioids.

Adverse effects

Buprenorphine, being a mu-agonist, may induce or maintain opioid addiction. Illegally obtained high-dose transmucosal buprenorphine, intended for treatment of addiction, is dissolved and injected by opioid abusers. This is an increasing problem in some countries.

Conclusions

Buprenorphine’s unusual pharmacodynamics and pharmacokinetics make it an ideal opioid for treatment of most chronic pain conditions where opioid therapy is indicated.

Implications

Buprenorphine is a well studied and often misunderstood analgesic opioid drug. The evidence base predicts that it will be an increasingly important alternative for treatment of chronic pain conditions caused by cancer and non-cancer diseases. It will continue to be an attractive alternative to methadone for opioid abuse rehabilitation.

A predictive pharmacokinetic/pharmacodynamic model of fentanyl for analgesia/sedation in neonates based on a semi-physiologic approach

BACKGROUND AND OBJECTIVES

Fentanyl is a synthetic opioid commonly used as an anesthetic and also increasingly popular as a sedative agent in neonates. Initial dosage regimens in this population are often empirically derived from adults on a body weight basis. However, ontogenic maturation processes related to drug disposition are not necessarily always body weight correlates. We developed a predictive pharmacokinetic/pharmacodynamic model that includes growth and maturation physiologic changes for fentanyl in neonatal care.

METHODS

Key pharmacokinetic variables and principles (protein binding, clearance, distribution) as related to fentanyl pharmacokinetic/pharmacodynamic behavior in adults (tricompartmental model) and to neonatal physiologic data (organ weights and blood flows, body composition, renal and hepatic function, etc.) were used to guide the building of a semi-physiologic ontogenic model. The model applies to a normal-term neonate without any other intervention. Then, extension to a pharmacokinetic/pharmacodynamic link model for fentanyl was made. The final model was evaluated by predicting the time course of plasma concentrations and the effect of a standard regimen of 10.5 μg/kg over a 1-h period followed by 1.5 μg/kg/h for 48 h.

RESULTS

Hepatic clearance was linked to ontogeny of unbound fraction and of α1-acid glycoprotein. All parameters were reduced in the neonate compared to adults but in a differing proportion due to qualitative changes in physiology that are analyzed and accounted for. Systemic clearance (CLS), volume of the central compartment (V1) and steady-state volume of distribution predicted by the model were 0.028 L/min, 1.26 L, and 22.04 L, respectively. Weight-corrected parameters generally decreased in adults compared with neonates, but differentially, e.g., CLS = 0.0093 versus 0.0088 L/min/kg, while V1 = 0.42 versus 0.18 L/kg (neonates vs. adults). Under such complexity a pharmacokinetic/pharmacodynamic model is the appropriate method for rational efficacy targeting. Fentanyl pharmacodynamics in neonates were considered to be similar to those in adults except for the equilibrium rate constant, which was also scaled on an ontogenic basis. The model adequately predicted the reported mean expected concentration-time profiles for the standard regimen.

CONCLUSIONS

Integrated pharmacokinetic/pharmacodynamic modeling showed that the usually prescribed dosage regimens of fentanyl in neonates may not always provide the optimum degree of sedation. The model could be used in optimal design of clinical trials for this vulnerable population. Prospective clinical testing is the reasonable next step.

Oxymorphone active uptake at the blood-brain barrier and population modeling of its pharmacokinetic-pharmacodynamic relationship

The aim of this study was to characterize the blood-brain barrier (BBB) transport and pharmacokinetics-pharmacodynamics (PKPD) relationship of oxymorphone and to further elucidate its possible contribution to oxycodone analgesia. The BBB transport of oxymorphone was studied using microdialysis in male Sprague-Dawley rats. Samples from microdialysis blood and brain probes, brain tissue, and plasma were analyzed by liquid chromatography with tandem mass spectrometry. The effect was measured as tail-flick latency. The study consisted of a PKPD experiment with combined microdialysis and antinociceptive measurements (n = 8), and another antinociceptive effect experiment (n = 9) using a 10 times lower dose. The combined data were analyzed with an integrated PKPD model in nonlinear mixed effect modeling utilizing a specific method (M3) for handling missing PD observations. The concentration of unbound oxymorphone was higher in brain than in blood, with a ratio of 1.9 (RSE, 9.7%), indicating active uptake at the BBB. The integrated PKPD model described the oxymorphone BBB transport and PKPD relationship successfully, with an EC50 in the brain of 63 ng/mL, and the M3 method was able to address the issue of censored observations. Oxymorphone has active uptake transport at the BBB in rats, with moderate uptake clearance to the brain. Its contribution to analgesia after oxycodone administration is not significant.

Safety and efficacy of buprenorphine for analgesia in laboratory mice and rats

Buprenorphine is a long-acting opiate with a high therapeutic index. The authors review the pharmacology, toxicity, analgesic effects and delivery of buprenorphine for use in laboratory mice and rats. Buprenorphine-based analgesic therapy has a substantial record of safety, and there is growing evidence of its effectiveness for treating post-operative pain. Nonetheless, more research is needed to determine optimal delivery systems and analgesic regimens for pain therapy in laboratory animals.

The pharmacokinetic-pharmacodynamic model of azithromycin for lipopolysaccharide-induced depressive-like behavior in mice

A mechanism-based model was developed to describe the time course of lipopolysaccharide-induced depressive-like behavior and azithromycin pharmacodynamics in mice. The lipopolysaccharide-induced disease progression was monitored by lipopolysaccharide, proinflammatory cytokines, and kynrenine concentration in plasma. The depressive-like behavior was investigated by forced swimming test and tail suspension test. Azithromycin was selected to inhibit the surge of proinflammatory cytokines induced by lipopolysaccharide. Disease progression model and azithromycin pharmacodynamics were constructed from transduction and indirect response models. A delay in the onset of increased proinflammatory cytokines, kynrenine, and behavior test compared to lipopolysaccharide was successfully characterized by series transduction models. The inhibition of azithromycin on proinflammatory cytokines was described by an indirect response model. After lipopolysaccharide challenging, the proinflammatory cytokines, kynrenine and behavior tests would peak approximately at 3, 12, and 24 h respectively, and then the time courses slowly declined toward a baseline state after peak response. During azithromycin administration, the peak levels of proinflammatory cytokines, kynrenine and behavior indexes decreased. Model parameters indicated that azithromycin significantly inhibited the proinflammatory cytokines level in plasma and improved the depressive-like behavior induced by inflammation. The integrated model for disease progression and drug intervention captures turnovers of proinflammatory cytokines, kynrenine and the behavior results in the different time phases and conditions.

Different patterns of administration modulate propensity of methadone and buprenorphine to promote locomotor sensitization in mice

Following chronic administration of morphine a locomotor sensitization is usually observed. This behavioral sensitization is influenced by numerous factors including the administration pattern. While methadone and buprenorphine are the two main treatments for opioid dependence, literature about their ability to promote long-term sensitization after chronic exposure/withdrawal are scarce considering the long-term treatments used in patients. In this study we investigated the propensity of these two opioids to promote behavioral sensitization using different patterns of administration. Mice were treated with escalating doses of methadone or buprenorphine during 5 days given either once (binge) or three times a day (TTD). After 1-, 7-, 14- and 35-days washout period, animals were challenged with either morphine or the opioid used for the chronic treatment to evaluate heterologous or homologous sensitization, respectively. While methadone produced homologous and heterologous sensitization either with the binge or the TTD regimen over the whole period studied, buprenorphine induced a transient heterologous locomotor sensitization on day 14 only with the binge administration. Our data evidenced a different ability of methadone and buprenorphine to produce behavioral sensitization and that the pattern of drug exposure determined the expression of this phenomenon.

Translational PK-PD modeling in pain

The current gap between animal research and clinical development of analgesic drugs presents a challenge for the application of translational PK-PD modeling and simulation. First, animal pain models lack predictive and construct validity to accurately reflect human pain etiologies and, secondly, clinical pain is a multidimensional sensory experience that can't always be captured by objective and robust measures. These challenges complicate the use of translational PK-PD modeling to project PK-PD data generated in preclinical species to a plausible range of clinical doses. To date only a few drug targets identified in animal studies have shown to be successful in the clinic. PK-PD modeling of biomarkers collected during the early phase of clinical development can bridge animal and clinical pain research. For drugs with novel mechanism of actions understanding of the target pharmacology is essential in order to increase the success of clinical development. There is a specific interest in the application of human pain models that can mimic different aspects of acute/chronic pain symptoms and serves as link between animal and clinical pain research. In early clinical development the main objective of PK-PD modeling is to characterize the relationship between target site binding and downstream biomarkers that have a potential link to the clinical endpoint (e.g. readouts from the human pain models) so as to facilitate the selection of doses for proof of concept studies. In patient studies, the role of PK-PD modeling and simulation is to characterize and confirm patient populations in terms of responder profiles with the aim to find the right dose for the right patient.

A physiologically-based recirculatory meta-model for nasal fentanyl in man

Pharmacokinetic (PK) and pharmacodynamic (PD) data were available from a study of a nasal delivery system for the opioid analgesic fentanyl, together with data on the kinetics of fentanyl in arterial blood in man, and in the lung and brain of sheep. Our aim was to reconcile these data using a physiologically-based population recirculatory PK-PD model, with emphasis on achieving a meta-model that could simultaneously account for the arterial and venous (arm) concentrations of fentanyl, could relate PD effects (pain scores) to the CNS concentrations of fentanyl, and could account for the effect of body size and age on fentanyl kinetics. Data on the concentration gradients of fentanyl across brain, lung and muscle were used to develop sub-models of fentanyl kinetics in these organs. The sub-models were incorporated into a "whole body" recirculatory model by adding additional sub-models for a venous mixing compartment, the liver and gut, the kidney and the "rest of the body" with blood flows and organ volumes based on values for a Standard Man. Inter-individual variability was achieved by allometric scaling of organ size and blood flows, evidence-based assumptions about the effect of weight and age on cardiac output, and inter-individual variability in the free fraction in plasma and hepatic extraction of fentanyl. Post-operative pain scores were found to be temporally related to the predicted brain concentrations of fentanyl. We conclude that a physiologically-based meta-modelling approach was able to describe clinical PK-PD studies of fentanyl whilst providing a mechanistic interpretation of key aspects of its disposition.

Pontine μ-opioid receptors mediate bradypnea caused by intravenous remifentanil infusions at clinically relevant concentrations in dogs

Life-threatening side effects such as profound bradypnea or apnea and variable upper airway obstruction limit the use of opioids for analgesia. It is yet unclear which sites containing μ-opioid receptors (μORs) within the intact in vivo mammalian respiratory control network are responsible. The purpose of this study was 1) to define the pontine region in which μOR agonists produce bradypnea and 2) to determine whether antagonism of those μORs reverses bradypnea produced by intravenous remifentanil (remi; 0.1-1.0 μg·kg(-1)·min(-1)). The effects of microinjections of agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO; 100 μM) and antagonist naloxone (NAL; 100 μM) into the dorsal rostral pons on the phrenic neurogram were studied in a decerebrate, vagotomized, ventilated, paralyzed canine preparation during hyperoxia. A 1-mm grid pattern of microinjections was used. The DAMGO-sensitive region extended from 5 to 7 mm lateral of midline and from 0 to 2 mm caudal of the inferior colliculus at a depth of 3-4 mm. During remi-induced bradypnea (~72% reduction in fictive breathing rate) NAL microinjections (~500 nl each) within the region defined by the DAMGO protocol were able to reverse bradypnea by 47% (SD 48.0%) per microinjection, with 13 of 84 microinjections producing complete reversal. Histological examination of fluorescent microsphere injections shows that the sensitive region corresponds to the parabrachial/Kölliker-Fuse complex.

Pharmacokinetic-pharmacodynamic modeling of the D₂ and 5-HT (2A) receptor occupancy of risperidone and paliperidone in rats

PURPOSE

A pharmacokinetic-pharmacodynamic (PK-PD) model was developed to describe the time course of brain concentration and dopamine D₂ and serotonin 5-HT(2A) receptor occupancy (RO) of the atypical antipsychotic drugs risperidone and paliperidone in rats.

METHODS

A population approach was utilized to describe the PK-PD of risperidone and paliperidone using plasma and brain concentrations and D₂ and 5-HT(2A) RO data. A previously published physiology- and mechanism-based (PBPKPD) model describing brain concentrations and D₂ receptor binding in the striatum was expanded to include metabolite kinetics, active efflux from brain, and binding to 5-HT(2A) receptors in the frontal cortex.

RESULTS

A two-compartment model best fit to the plasma PK profile of risperidone and paliperidone. The expanded PBPKPD model described brain concentrations and D₂ and 5-HT(2A) RO well. Inclusion of binding to 5-HT(2A) receptors was necessary to describe observed brain-to-plasma ratios accurately. Simulations showed that receptor affinity strongly influences brain-to-plasma ratio pattern.

CONCLUSION

Binding to both D₂ and 5-HT(2A) receptors influences brain distribution of risperidone and paliperidone. This may stem from their high affinity for D₂ and 5-HT(2A) receptors. Receptor affinities and brain-to-plasma ratios may need to be considered before choosing the best PK-PD model for centrally active drugs.

Relative contributions of peripheral versus supraspinal or spinal opioid receptors to the antinociception of systemic opioids

The contribution of supraspinal, spinal or peripheral mu-opioid receptors (MORs) to the overall antinociception of systemic centrally penetrating versus peripherally restricted opioids has not been thoroughly investigated. Therefore, we examined paw pressure thresholds in Wistar rats with complete Freund's adjuvant hindpaw inflammation following different doses of intraplantar (i.pl.) as well as intravenous (i.v.) fentanyl (6.25-50 μg/kg), morphine (1-7.5 mg/kg) or loperamide (1-7.5 mg/kg). Antagonism of the i.v. mu-opioid agonists by intracerebroventricular (i.c.v.), intrathecal (i.t.) or i.pl. naloxone-methiodide (NLXM) revealed the relative contributions of supraspinal, spinal and peripheral MOR to the overall antinociceptive effects. In parallel, the MOR density at these three levels of pain transmission was assessed by radioligand binding. Antinociceptive effects of i.v. fentanyl and morphine, but not of the peripherally restricted loperamide were two- to threefold greater and longer lasting compared with their i.pl. administration. I.c.v. but not i.pl. NLXM significantly antagonized fentanyl's and morphine's antinociception by 70-80%, whereas i.t. NLXM reduced it by 20-30%. In contrast, antinociception of i.v. loperamide was abolished by i.pl. but not by i.c.v. or i.t. NLXM. In parallel, a respective 32- and sixfold higher MOR density in supraspinal and spinal versus peripheral sensory neurons was detected. In conclusion, in comparison with supraspinal and spinal opioid receptors, peripheral opioid receptors do not significantly contribute to the antinociception of systemic fentanyl and morphine during inflammatory pain. Antinociception of their i.v. administration was superior over both i.v and i.pl. loperamide, acting exclusively via peripheral MOR. These findings may guide the future development of novel peripherally restricted opioids.