Pharmacokinetic-pharmacodynamic modeling of the respiratory depressant effect of norbuprenorphine in rats

Brain PET imaging using 11C-flumazenil and 11C-buprenorphine does not support the hypothesis of a mutual interaction between buprenorphine and benzodiazepines at the neuroreceptor level

Among opioids, buprenorphine presents a favorable safety profile with a limited risk of respiratory depression. However, fatalities have been reported when buprenorphine is combined to a benzodiazepine. Potentiation of buprenorphine interaction with opioid receptors (ORs) with benzodiazepines, and/or vice versa, is hypothesized to explain this drug-drug interaction (DDI). The mutual DDI between buprenorphine and benzodiazepines was investigated at the neuroreceptor level in nonhuman primates (n = 4 individuals) using brain PET imaging and kinetic modelling. The binding potential (BPND) of benzodiazepine receptor (BzR) was assessed using 11C-flumazenil PET imaging before and after administration of buprenorphine (0.2 mg, i.v.). Moreover, the brain kinetics and receptor binding of buprenorphine were investigated in the same individuals using 11C-buprenorphine PET imaging before and after administration of diazepam (10 mg, i.v.). Outcome parameters were compared using a two-way ANOVA. Buprenorphine did not impact the plasma nor brain kinetics of 11C-flumazenil. 11C-flumazenil BPND was unchanged following buprenorphine exposure, in any brain region (p > 0.05). Similarly, diazepam did not impact the plasma or brain kinetics of 11C-buprenorphine. 11C-buprenorphine volume of distribution (VT) was unchanged following diazepam exposure, in any brain region (p > 0.05). To conclude, our PET imaging findings do not support a neuropharmacokinetic or neuroreceptor-related mechanism of the buprenorphine/benzodiazepine interaction.

Pharmacokinetics of buprenorphine and its metabolite norbuprenorphine in neutered male cats anesthetized with isoflurane

OBJECTIVE

To characterize the pharmacokinetics of buprenorphine and norbuprenorphine in isoflurane-anesthetized cats.

STUDY DESIGN

Prospective experimental study.

ANIMALS

A group of six healthy adult male neutered cats.

METHODS

Cats were anesthetized with isoflurane in oxygen. Catheters were placed in a jugular vein for blood sampling and in a medial saphenous vein for buprenorphine and lactated Ringer's solution administration. Buprenorphine hydrochloride (40 μg kg^-1 over 5 minutes) was administered intravenously. Blood samples were collected before buprenorphine administration and at various times up to 12 hours after administration. Plasma buprenorphine and norbuprenorphine concentrations were measured using liquid chromatography/tandem mass spectrometry. Compartment models were fitted to the time-concentration data using nonlinear mixed effect (population) modeling.

RESULTS

A five-compartment model (three compartments for buprenorphine and two compartments for norbuprenorphine) best fitted the data. Typical value (% interindividual variability) for the three buprenorphine volumes of distribution, and the metabolic clearance to norbuprenorphine, the remaining metabolic clearance and the two distribution clearances were 157 (33), 759 (34) and 1432 (43) mL kg^-1, and 5.3 (33), 16.4 (11), 58.7 (27) and 6.0 (not estimated) mL minute^-1 kg^-1, respectively. Typical values (% interindividual variability) for the two norbuprenorphine volumes of distribution, and the norbuprenorphine metabolic and distribution clearances were 1437 (30) and 8428 (not estimated) mL kg^-1 and 48.4 (68) and 235.9 (not estimated) mL minute^-1 kg^-1, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

The pharmacokinetics of buprenorphine in isoflurane-anesthetized cats were characterized by a medium clearance.

Respiratory depressant effects of fentanyl analogs are opioid receptor-mediated

Opioid-related fatalities involving synthetic opioids have reached unprecedented levels. This study evaluated the respiratory depressant effects of seven fentanyl analogs that have either emerged in the illicit drug supply or been identified in toxicological analyses following fatal or non-fatal intoxications. Adult male Swiss Webster mice were administered fentanyl analogs (isobutyrylfentanyl, crotonylfentanyl, para-methoxyfentanyl, para-methoxybutyrylfentanyl, 3-furanylfentanyl, thiophenefentanyl, and benzodioxolefentanyl) and their effects on minute volume as compared to mu-opioid receptor (MOR) agonist standards (fentanyl, morphine, and buprenorphine) were measured using whole body plethysmography (WBP). All drugs elicited significant (p ≤ 0.05) hypoventilation relative to vehicle for at least one dose tested: morphine (1, 3.2, 10, 32 mg/kg), buprenorphine, (0.032, 0.1, 0.32, 1, 3.2 mg/kg), fentanyl (0.0032, 0.01, 0.032, 0.1, 1, 32 mg/kg), isobutyrylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), crotonylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), para-methoxyfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), para-methoxybutyrylfentanyl (0.32, 1, 3.2, 10 mg/kg), 3-furanylfentanyl (0.1, 0.32, 1, 3.2, 10 mg/kg), thiophenefentanyl (1, 3.2, 10, 32, 100 mg/kg), and benzodioxolefentanyl (3.2, 10, 32, 100 mg/kg). The ED₅₀ values for hypoventilation showed a rank order of potency as follows: fentanyl (ED₅₀ = 0.96 mg/kg) > 3-furanylfentanyl (ED₅₀ = 2.60 mg/kg) > crotonylfentanyl (ED₅₀ = 2.72 mg/kg) > para-methoxyfentanyl (ED₅₀ = 3.31 mg/kg) > buprenorphine (ED₅₀ = 10.8 mg/kg) > isobutyrylfentanyl (ED₅₀ = 13.5 mg/kg) > para-methoxybutyrylfentanyl (ED₅₀ = 16.1 mg/kg) > thiophenefentanyl (ED₅₀ = 18.0 mg/kg) > morphine (ED₅₀ = 55.3 mg/kg) > benzodioxolefentanyl (ED₅₀ = 10,168 mg/kg). A naloxone pretreatment (10 mg/kg) attenuated the hypoventilatory effects of all drugs. These results establish that the respiratory depressant effects of these fentanyl analogs are at least in part mediated by the MOR.

Buprenorphine-Related Deaths in North Carolina from 2010-2018

Buprenorphine is a commonly prescribed medication for the treatment of opioid-use disorder. As prescriptions increase in North Carolina, buprenorphine is more frequently encountered statewide in routine postmortem casework. Between 2010 and 2018, there were 131 select cases investigated by the Office of the Chief Medical Examiner where buprenorphine was detected in peripheral blood and considered a primary cause of death, with no other opioids present and no other non-opioid substances found in the lethal range. The decedents ranged in age from 14 to 64 years, with 67% male. The mean/median peripheral blood concentrations were 4.1/2.1 ng/mL for buprenorphine and 7.8/3.4 ng/mL for the metabolite, norbuprenorphine. These postmortem blood concentrations overlap antemortem therapeutic concentrations in plasma reported in the literature for opioid-dependent subjects receiving sublingual maintenance therapy. The pathologist considered scene findings, prescription history, autopsy findings, toxicological analysis, and decedent behavior prior to death to conclude a drug-related cause of death. Many of the deaths were complicated by the presence of other central nervous system depressants along with contributory underlying cardiovascular and respiratory disease. The three most prevalent additive substances were alprazolam, ethanol, and gabapentin, found in 67, 36, and 32 cases out of 131, respectively. Interpreting buprenorphine involvement in a death is complex, and instances may be under-estimated in epidemiological data because of the lack of a defined toxic or lethal range in postmortem blood along with its good safety profile. As expansion to access of opioid-use disorder treatment becomes a priority, awareness of the challenges of postmortem interpretation is needed as increased use and diversion of buprenorphine are inevitable.

Pharmacokinetics and analgesic effects of intravenous, intramuscular or subcutaneous buprenorphine in dogs undergoing ovariohysterectomy: a randomized, prospective, masked, clinical trial

BACKGROUND

Buprenorphine is used for canine postoperative pain management. This study aimed to describe the pharmacokinetics and evaluate the analgesic efficacy of buprenorphine (Simbadol, 1.8 mg/mL) administered by different routes in dogs undergoing ovariohysterectomy. Twenty-four dogs were included in a randomized, prospective, masked, clinical trial. Buprenorphine (0.02 mg/kg) was administered intravenously (IV), intramuscularly (IM) or subcutaneously (SC) (n = 8/group) 0.5 h before general anesthesia with propofol-isoflurane. Carprofen (4.4 mg/kg SC) was administered after anesthetic induction and before ovariohysterectomy. Pain was scored using the short-form Glasgow composite pain scale for dogs (SF-GCPS). Dogs were administered morphine (0.25 mg/kg IV) when SF-GCPS scores were ≥ 5/20. Blood sampling was performed up to 720 min after drug administration. Plasma buprenorphine and norbuprenorphine concentrations were analyzed using liquid chromatography mass spectrometry. Pharmacokinetics of buprenorphine was described using a non-compartmental model (PK Solver 2.0). Statistical analysis was performed using linear mixed models and Fisher's exact test (p < 0.05).

RESULTS

Pain scores were significantly higher than baseline after IV (0.5-2 h), IM (0.5-3 h) and SC (0.5-4 h) but not among groups. Prevalence of rescue analgesia was significantly higher in SC (7/8 dogs) than IV (2/8) but not different between IV and IM (3/8) or IM and SC. The frequency of rescue analgesia was not significantly different among groups (IV = 2, IM = 5 and SC = 9). Norbuprenorphine was not detected. For IV, IM and SC administration, clearance was 1.29, 1.65 and 1.40 L/hour/kg, volume of distribution was 6.8, 14.2 and 40.1 L/kg, the elimination half-life was 3.7, 5.7, 22 h, and the area under the plasma concentration-time curved extrapolated to infinity was 15.7, 12.4 and 16.4 ng/mL/hour, respectively. Bioavailability for IM and SC was 62.6 and 40%, respectively. Maximum plasma concentrations of buprenorphine were 6.2 and 1.3 ng/mL at 0.14 and 0.33 h after IM and SC administration, respectively.

CONCLUSIONS

The route of administration influences the analgesic efficacy of buprenorphine in dogs. SC administration of buprenorphine failed to provide clinical analgesia due to erratic drug absorption. At the doses administered, the IV and IM routes are preferred for postoperative analgesia.

In Utero Exposure to Norbuprenorphine, a Major Metabolite of Buprenorphine, Induces Fetal Opioid Dependence and Leads to Neonatal Opioid Withdrawal Syndrome

Buprenorphine is the preferred treatment of opioid use disorder during pregnancy but can cause fetal opioid dependence and neonatal opioid withdrawal syndrome (NOWS). Notably, withdrawal severity is independent of maternal buprenorphine dose, suggesting that interindividual variance in pharmacokinetics may influence risk and severity of NOWS. Using a rat model of NOWS, we tested the hypothesis that clinically relevant doses of the active metabolite norbuprenorphine (NorBUP) can induce in utero opioid dependence, manifested as naltrexone-precipitated withdrawal signs in the neonate. Pregnant Long-Evans rats were implanted with 14-day osmotic minipumps containing vehicle, morphine (positive control), or NorBUP (0.3-10 mg/kg per day) on gestation day 9. By 12 hours post-delivery, an intraperitoneal injection of the opioid antagonist naltrexone (1 or 10 mg/kg) or saline was administered to pups. Precipitated withdrawal signs were graded by raters blinded to treatment conditions. In a separate group, NorBUP concentrations in maternal and fetal blood and brain on gestation day 20 were determined by liquid chromatography-tandem mass spectrometry. Steady-state maternal blood concentrations of NorBUP in dams infused with 1 or 3 mg/kg per day were comparable to values reported in pregnant humans treated with buprenorphine (1.0 and 9.6 ng/ml, respectively), suggesting a clinically relevant dosing regimen. At these doses, NorBUP increased withdrawal severity in the neonate as shown by an evaluation of 10 withdrawal indicators. These findings support the possibility that NorBUP contributes to fetal opioid dependence and NOWS following maternal buprenorphine treatment during pregnancy.

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.

Pregnancy Increases Norbuprenorphine Clearance in Mice by Induction of Hepatic Glucuronidation

Norbuprenorphine (NBUP) is the major active metabolite of buprenorphine (BUP) that is commonly used to treat opiate addiction during pregnancy; it possesses 25% of BUP's analgesic activity and 10 times BUP's respiratory depression effect. To optimize BUP's dosing regimen during pregnancy with better efficacy and safety, it is important to understand how pregnancy affects NBUP disposition. In this study, we examined the pharmacokinetics of NBUP in pregnant and nonpregnant mice by administering the same amount of NBUP through retro-orbital injection. We demonstrated that the systemic clearance (CL) of NBUP in pregnant mice increased ∼2.5-fold compared with nonpregnant mice. Intrinsic CL of NBUP by glucuronidation in mouse liver microsomes from pregnant mice was ∼2 times greater than that from nonpregnant mice. Targeted liquid chromatography tandem-mass spectrometry proteomics quantification revealed that hepatic Ugt1a1 and Ugt2b1 protein levels in the same amount of total liver membrane proteins were significantly increased by ∼50% in pregnant mice versus nonpregnant mice. After scaling to the whole liver with consideration of the increase in liver protein content and liver weight, we found that the amounts of Ugt1a1, Ugt1a10, Ugt2b1, and Ugt2b35 protein in the whole liver of pregnant mice were significantly increased ∼2-fold compared with nonpregnant mice. These data suggest that the increased systemic CL of NBUP in pregnant mice is likely caused by an induction of hepatic Ugt expression and activity. The data provide a basis for further mechanistic analysis of pregnancy-induced changes in the disposition of NBUP and drugs that are predominately and extensively metabolized by Ugts.

P-gp/ABCB1 exerts differential impacts on brain and fetal exposure to norbuprenorphine

Norbuprenorphine is the major active metabolite of buprenorphine which is commonly used to treat opiate addiction during pregnancy. Norbuprenorphine produces marked respiratory depression and was 10 times more potent than buprenorphine. Therefore, it is important to understand the mechanism that controls fetal exposure to norbuprenorphine, as exposure to this compound may pose a significant risk to the developing fetus. P-gp/ABCB1 and BCRP/ABCG2 are two major efflux transporters regulating tissue distribution of drugs. Previous studies have shown that norbuprenorphine, but not buprenorphine, is a P-gp substrate. In this study, we systematically examined and compared the roles of P-gp and BCRP in determining maternal brain and fetal distribution of norbuprenorphine using transporter knockout mouse models. We administered 1mg/kg norbuprenorphine by retro-orbital injection to pregnant FVB wild-type, Abcb1a-/-/1b-/-, and Abcb1a-/-/1b-/-/Abcg2-/- mice on gestation day 15. The fetal AUC of norbuprenorphine was ∼64% of the maternal plasma AUC in wild-type mice, suggesting substantial fetal exposure to norbuprenorphine. The maternal plasma AUCs of norbuprenorphine in Abcb1a-/-/1b-/- and Abcb1a-/-/1b-/-/Abcg2-/- mice were ∼2 times greater than that in wild-type mice. Fetal AUCs in Abcb1a-/-/1b-/- and Abcb1a-/-/1b-/-/Abcg2-/- mice were also increased compared to wild-type mice; however, the fetal-to-maternal plasma AUC ratio remained relatively unchanged by the knockout of Abcb1a/1b or Abcb1a/1b/Abcg2. In contrast, the maternal brain-to-maternal plasma AUC ratio in Abcb1a-/-/1b-/- or Abcb1a-/-/1b-/-/Abcg2-/- mice was increased ∼30-fold compared to wild-type mice. Protein quantification by LC-MS/MS proteomics revealed significantly higher amounts of P-gp protein in the wild-type mice brain than that in the placenta. These results indicate that fetal exposure to norbuprenorphine is substantial and that P-gp has a minor impact on fetal exposure to norbuprenorphine, but plays a significant role in restricting its brain distribution. The differential impacts of P-gp on norbuprenorphine distribution into the brain and fetus are likely, at least in part, due to the differences in amounts of P-gp protein expressed in the blood-brain and blood-placental barriers. BCRP is not as important as P-gp in determining both the systemic and tissue exposure to norbuprenorphine. Finally, fetal AUCs of the metabolite norbuprenorphine-β-d-glucuronide were 3-7 times greater than maternal plasma AUCs, while the maternal brain AUCs were <50% of maternal plasma AUCs, suggesting that a reversible pool of conjugated metabolite in the fetus may contribute to the high fetal exposure to norbuprenorphine.

Pharmacokinetic and pharmacodynamic modelling after subcutaneous, intravenous and buccal administration of a high-concentration formulation of buprenorphine in conscious cats

Background

The aim of this study was to describe the joint pharmacokinetic-pharmacodynamic model and evaluate thermal antinociception of a high-concentration formulation of buprenorphine (Simbadol^™) in cats.

Methods

Six healthy cats (4.9 ± 0.7 kg) were included in a prospective, randomized, blinded, crossover study. Simbadol^™ (1.8 mg mL^-1) was administered by the subcutaneous (SC; 0.24 mg kg^-1), intravenous (IV; 0.12 mg kg^-1) or buccal (OTM; 0.12 mg kg^-1) route of administration and thermal thresholds (TT) were compared with a saline group (SAL). Thermal threshold testing and blood sampling were performed at predetermined time points up to 72 hours including a placebo group. Plasma buprenorphine and norbuprenorphine concentrations were measured using liquid chromatography mass spectrometry. A bespoke bicompartmental pharmacokinetic model simultaneously fitted data from two analytes/three routes of administration. Temporal changes in TT were analyzed using one-way ANOVA followed by Dunnett’s test and treatment comparisons using two-way ANOVA with Bonferroni’s correction (P < 0.05).

Results

Thermal thresholds were significantly increased after SC, IV and OTM from 1–24 hours (except 2 hours), 0.5–8 hours (except 6 hours), and 1–8 hours (except 6 hours), respectively, when compared with baseline. Thermal thresholds were significantly increased after SC (1–30 hours), IV (1–8 hours) and OTM (1–12 hours) when compared with SAL, but not different among buprenorphine-treated cats. The absolute buprenorphine clearance was 0.98 L kg^-1 hour^-1, volume of distribution at steady state was 7.9 L kg^-1 and the elimination-half-life was 12.3 hours. Bioavailability for SC and OTM was 94% and 24%, respectively. Subcutaneous absorption was biphasic. An initial peak (0.08 hours) was followed by a slow (half-life 11.2 hours) and progressive (peak acceleration at 2.8 hours) uptake.

Conclusion

The SC administration of Simbadol^™ was characterized by prolonged absorption half-life and sustained plasma concentrations yielding long-lasting antinociception (≥ 24 hours) when compared with the IV and OTM routes.

Opioid-induced constipation: rationale for the role of norbuprenorphine in buprenorphine-treated individuals

Buprenorphine and buprenorphine–naloxone fixed combinations are effective for managing patients with opioid dependence, but constipation is one of the most common side effects. Evidence indicates that the rate of constipation is lower when patients are switched from sublingual buprenorphine–naloxone tablets or films to a bilayered bioerodible mucoadhesive buccal film formulation, and while the bilayered buccal film promotes unidirectional drug flow across the buccal mucosa, the mechanism for the reduced constipation is unclear. Pharmacokinetic simulations indicate that chronic dosing of sublingually administered buprenorphine may expose patients to higher concentrations of norbuprenorphine than buprenorphine, while chronic dosing of the buccal formulation results in higher buprenorphine concentrations than norbuprenorphine. Because norbuprenorphine is a potent full agonist at mu-opioid receptors, the differences in norbuprenorphine exposure may explain the observed differences in treatment-emergent constipation between the sublingual formulation and the buccal film formulation of buprenorphine–naloxone. To facilitate the understanding and management of opioid-dependent patients at risk of developing opioid-induced constipation, the clinical profiles of these formulations of buprenorphine and buprenorphine-naloxone are summarized, and the incidence of treatment-emergent constipation in clinical trials is reviewed. These data are used to propose a potential role for exposure to norbuprenorphine, an active metabolite of buprenorphine, in the pathophysiology of opioid-induced constipation.

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.

A review of pharmacological interactions between HIV or hepatitis C virus medications and opioid agonist therapy: implications and management for clinical practice

Global access to opioid agonist therapy and HIV/hepatitis C virus (HCV) treatment is expanding but when used concurrently, problematic pharmacokinetic and pharmacodynamic interactions may occur. Articles published from 1966 to 2012 in Medline were reviewed using the following keywords: HIV, AIDS, HIV therapy, HCV, HCV therapy, antiretroviral therapy, highly active antiretroviral therapy, drug interactions, methadone and buprenorphine. In addition, a review of abstracts from national and international meetings and conference proceedings was conducted; selected reports were reviewed as well. The metabolism of both opioid and antiretroviral therapies, description of their known interactions and clinical implications and management of these interactions were reviewed. Important pharmacokinetic and pharmacodynamic drug interactions affecting either methadone or HIV medications have been demonstrated within each class of antiretroviral agents. Drug interactions between methadone, buprenorphine and HIV medications are known and may have important clinical consequences. Clinicians must be alert to these interactions and have a basic knowledge regarding their management.

High-sensitivity analysis of buprenorphine, norbuprenorphine, buprenorphine glucuronide, and norbuprenorphine glucuronide in plasma and urine by liquid chromatography-mass spectrometry

A new method using ultra-fast liquid chromatography and tandem mass spectrometry (UFLC-MS/MS) was developed for the simultaneous determination of buprenorphine and the metabolites norbuprenorphine, buprenorphine-3β-glucuronide, and norbuprenorphine-3β-glucuronide in plasma and urine. Sample handling, sample preparation and solid-phase extraction procedures were optimized for maximum analyte recovery. All four analytes of interest were quantified by positive ion electrospray ionization tandem mass spectrometry after solid-phase microextraction. The lower limits of quantification in plasma were 1pg/mL for buprenorphine and buprenorphine glucuronide, and 10pg/mL for norbuprenorphine and norbuprenorphine glucuronide. The lower limits of quantitation in urine were 10pg/mL for buprenorphine, norbuprenorphine and their glucuronides. Overall extraction recoveries ranged from 68-100% in both matrices. Interassay precision and accuracy was within 10% for all four analytes in plasma and within 15% in urine. The method was applicable to pharmacokinetic studies of low-dose buprenorphine.

Buprenorphine may not be as safe as you think: a pediatric fatality from unintentional exposure

Buprenorphine is a partial μ-opioid receptor agonist that is approved for the treatment of opioid dependency. It is generally believed to be safer than methadone because of its ceiling effect on respiratory depression. As more adults in US households use buprenorphine, an increasing number of children are being exposed. We report a fatal exposure to buprenorphine in a small child that occurred after ingestion of a caretaker's buprenorphine/naloxone. Postmortem toxicology analysis showed free serum concentrations of 52 ng/mL and 39 ng/mL for buprenorphine and norbuprenorphine, respectively. No other drugs were detected. Autopsy did not find signs of injury or trauma. The theoretical safety provided by the ceiling effect in respiratory depression from buprenorphine may not apply to children, and buprenorphine may cause dose-dependent respiratory depression.

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.

Buprenorphine: the basic pharmacology revisited

The historical background leading to the current use of buprenorphine as an analgesic and its role in the management of opioid dependence is summarized. The popular description of buprenorphine as a "partial agonist" is discussed in relation to efficacy in animal models of antinociception and clinical analgesia. The latest information on the respiratory depressant effects of buprenorphine and its N-dealkylated metabolite (norbuprenorphine) is presented. New data on the buprenorphine withdrawal syndrome in rats are described.

P-glycoprotein is a major determinant of norbuprenorphine brain exposure and antinociception

Norbuprenorphine is a major metabolite of buprenorphine and potent agonist of μ, δ, and κ opioid receptors. Compared with buprenorphine, norbuprenorphine causes minimal antinociception but greater respiratory depression. It is unknown whether the limited antinociception is caused by low efficacy or limited brain exposure. Norbuprenorphine is an in vitro substrate of the efflux transporter P-glycoprotein (Mdr1), but the role of P-glycoprotein in norbuprenorphine transport in vivo is unknown. This investigation tested the hypothesis that limited norbuprenorphine antinociception results from P-glycoprotein-mediated efflux and limited brain access. Human P-glycoprotein-mediated transport in vitro of buprenorphine, norbuprenorphine, and their respective glucuronide conjugates was assessed by using transfected cells. P-glycoprotein-mediated norbuprenorphine transport and consequences in vivo were assessed by using mdr1a(+/+) and mdr1a(-/-) mice. Antinociception was determined by hot-water tail-flick assay, and respiratory effects were determined by unrestrained whole-body plethysmography. Brain and plasma norbuprenorphine and norbuprenorphine-3-glucuronide were quantified by mass spectrometry. In vitro, the net P-glycoprotein-mediated efflux ratio for norbuprenorphine was nine, indicating significant efflux. In contrast, the efflux ratio for buprenorphine and the two glucuronide conjugates was unity, indicating absent transport. The norbuprenorphine brain/plasma concentration ratio was significantly greater in mdr1a(-/-) than mdr1a(+/+) mice. The magnitude and duration of norbuprenorphine antinociception were significantly increased in mdr1a(-/-) compared with mdr1a(+/+) mice, whereas the reduction in respiratory rate was similar. Results show that norbuprenorphine is an in vitro and in vivo substrate of P-glycoprotein. P-glycoprotein-mediated efflux influences brain access and antinociceptive, but not the respiratory, effects of norbuprenorphine.

Buprenorphine metabolites, buprenorphine-3-glucuronide and norbuprenorphine-3-glucuronide, are biologically active

BACKGROUND

The long-lasting high-affinity opioid buprenorphine has complex pharmacology, including ceiling effects with respect to analgesia and respiratory depression. Plasma concentrations of the major buprenorphine metabolites norbuprenorphine, buprenorphine-3-glucuronide, and norbuprenorphine-3-glucuronide approximate or exceed those of the parent drug. Buprenorphine glucuronide metabolites pharmacology is undefined. This investigation determined binding and pharmacologic activity of the two glucuronide metabolites, and in comparison with buprenorphine and norbuprenorphine.

METHODS

Competitive inhibition of radioligand binding to human μ, κ, and δ opioid and nociceptin receptors was used to determine glucuronide binding affinities for these receptors. Common opiate effects were assessed in vivo in SwissWebster mice. Antinociception was assessed using a tail-flick assay, respiratory effects were measured using unrestrained whole-body plethysmography, and sedation was assessed by inhibition of locomotion measured by open-field testing.

RESULTS

Buprenorphine-3-glucuronide had high affinity for human μ (Ki [inhibition constant] = 4.9 ± 2.7 pM), δ (Ki = 270 ± 0.4 nM), and nociceptin (Ki = 36 ± 0.3 μM) but not κ receptors. Norbuprenorphine-3-glucuronide had affinity for human κ (Ki = 300 ± 0.5 nM) and nociceptin (Ki = 18 ± 0.2 μM) but not μ or δ receptors. At the dose tested, buprenorphine-3-glucuronide had a small antinociceptive effect. Neither glucuronide had significant effects on respiratory rate, but norbuprenorphine-3-glucuronide decreased tidal volume. Norbuprenorphine-3-glucuronide also caused sedation.

CONCLUSIONS

Both glucuronide metabolites of buprenorphine are biologically active at doses relevant to metabolite exposures, which occur after buprenorphine. Activity of the glucuronides may contribute to the overall pharmacology of buprenorphine.

Mechanism-based pharmacodynamic modeling

Pharmacodynamic modeling is based on a quantitative integration of pharmacokinetics, pharmacological systems, and (patho-) physiological processes for understanding the intensity and time-course of drug effects on the body. Application of such models to the analysis of meaningful experimental data allows for the quantification and prediction of drug-system interactions for both therapeutic and adverse drug responses. In this chapter, commonly used mechanistic pharmacodynamic models are presented with respect to their important features, operable equations, and signature profiles. In addition, literature examples showcasing the utility of these models to adverse drug events are highlighted. Common model types that are covered include simple direct effects, biophase distribution, indirect effects, signal transduction, and irreversible effects.

Toddlers requiring pediatric intensive care unit admission following at-home exposure to buprenorphine/naloxone

BACKGROUND

Sublingual buprenorphine is an alternative to methadone for office-based treatment of opioid dependence. Recent reports have examined a growing number of unintentional buprenorphine exposures in children resulting in significant toxicity, even after a single lick or taste of a sublingual tablet. Here, we report a series of unintentional buprenorphine exposures in toddlers over a 2.5-yr period that led to admission to the pediatric intensive care unit.

OBJECTIVES

The goals of this study were to determine: 1) the prevalence of symptomatic buprenorphine exposure in children <3 yrs of age; 2) the severity of toxicity associated with such exposures; and 3) effective clinical interventions.

METHODS AND MAIN RESULTS

A retrospective case review was performed on records from the pediatric intensive care unit at an academic medical center located in the northeastern United States. Unintentional buprenorphine/naloxone exposure (n = 9) accounted for the largest single fraction of toxic ingestions among patients younger than 3 yrs within the study period (9/33, 27%). All exposures occurred at the child's place of residence (n = 9, 100%). Clinical signs of opioid toxicity were evident in all nine cases, with the most common symptom being drowsiness or lethargy (n = 9, 100%), followed by miosis (n = 6, 67%) and respiratory depression (n = 5, 56%). Six patients were effectively treated with naloxone (n = 6, 67%).

CONCLUSIONS

The increased use and similarity to candy of the current formulation of buprenorphine pose a special risk to children, especially toddlers. Buprenorphine exposure in children <3 yrs old can cause significant opioid toxidrome. Naloxone is an effective agent for reversal of symptoms; however, given buprenorphine's high affinity and long action, higher doses or continuous infusion may be required. Adults on buprenorphine should be educated on the risks posed to young children in their household and the appropriate storage of medication.

Interaction of drugs of abuse and maintenance treatments with human P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2)

Drug interaction with P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) may influence its tissue disposition including blood-brain barrier transport and result in potent drug-drug interactions. The limited data obtained using in-vitro models indicate that methadone, buprenorphine, and cannabinoids may interact with human P-gp; but almost nothing is known about drugs of abuse and BCRP. We used in vitro P-gp and BCRP inhibition flow cytometric assays with hMDR1- and hBCRP-transfected HEK293 cells to test 14 compounds or metabolites frequently involved in addiction, including buprenorphine, norbuprenorphine, methadone, ibogaine, cocaine, cocaethylene, amphetamine, N-methyl-3,4-methylenedioxyamphetamine, 3,4-methylenedioxyamphetamine, nicotine, ketamine, Delta9-tetrahydrocannabinol (THC), naloxone, and morphine. Drugs that in vitro inhibited P-gp or BCRP were tested in hMDR1- and hBCRP-MDCKII bidirectional transport studies. Human P-gp was significantly inhibited in a concentration-dependent manner by norbuprenorphine>buprenorphine>methadone>ibogaine and THC. Similarly, BCRP was inhibited by buprenorphine>norbuprenorphine>ibogaine and THC. None of the other tested compounds inhibited either transporter, even at high concentration (100 microm). Norbuprenorphine (transport efflux ratio approoximately 11) and methadone (transport efflux ratio approoximately 1.9) transport was P-gp-mediated; however, with no significant stereo-selectivity regarding methadone enantiomers. BCRP did not transport any of the tested compounds. However, the clinical significance of the interaction of norbuprenorphine with P-gp remains to be evaluated.

Incorporating receptor theory in mechanism-based pharmacokinetic-pharmacodynamic (PK-PD) modeling

Pharmacokinetic-Pharmacodynamic (PK-PD) modeling helps to better understand drug efficacy and safety and has, therefore, become a powerful tool in the learning-confirming cycles of drug-development. In translational drug research, mechanism-based PK-PD modeling has been recognized as a tool for bringing forward early insights in drug efficacy and safety into the clinical development. These models differ from descriptive PK-PD models in that they quantitatively characterize specific processes in the causal chain between drug administration and effect. This includes target site distribution, binding and activation, pharmacodynamic interactions, transduction and homeostatic feedback mechanisms. Compared to descriptive models mechanism-based PK-PD models that utilize receptor theory concepts for characterization of target binding and target activation processes have improved properties for extrapolation and prediction. In this respect, receptor theory constitutes the basis for 1) prediction of in vivo drug concentration-effect relationships and 2) characterization of target association-dissociation kinetics as determinants of hysteresis in the time course of the drug effect. This approach intrinsically distinguishes drug- and system specific parameters explicitly, allowing accurate extrapolation from in vitro to in vivo and across species. This review provides an overview of recent developments in incorporating receptor theory in PK-PD modeling with a specific focus on the identifiability of these models.