Oxycodone Pharmacokinetics and Pharmacodynamics in the Rat in the Presence of the P-Glycoprotein Inhibitor PSC833

The Impact of P-Glycoprotein on CNS Drug Efflux and Variability in Response

Resistance against CNS drugs may arise from various mechanisms, with limited drug penetration across the blood-brain barrier (BBB) being a significant contributing factor. The BBB employs efflux transporters like P-glycoprotein (P-gp) to safeguard the brain by removing toxins and xenobiotics, however, P-gp also pumps out therapeutic drugs, and its upregulation in disease states can contribute to variability in drug response. While inhibiting P-gp to prevent drug efflux seems appealing, it could lead to toxicity since P-gp is also important for expulsion of toxins from the brain. This necessitates the incorporation of P-gp substrate liability assessment into early drug discovery stages using appropriate experimental approaches. Therefore, this review aims to draw interest in this crucial area by analyzing the existing research on P-gp's impact on brain distribution of major CNS drugs and exploring the detection methods for identifying P-gp substrates. By identifying confirmed P-gp substrates and evaluating effective detection methods, this work emphasizes the continued importance of monitoring P-gp-mediated CNS drug efflux out of the brain tissue. This knowledge can empower clinicians to anticipate potential treatment inefficacy and guide therapeutic decision-making, ultimately leading to improved patient treatment outcomes.

Reduced oxycodone brain delivery in rats due to lipopolysaccharide-induced inflammation: microdialysis insights into brain disposition and sex-specific pharmacokinetics

BACKGROUND

Oxycodone, a widely used opioid analgesic, has an unbound brain-to-plasma concentration ratio (Kp,uu) greater than unity, indicating active uptake across brain barriers associated with the putative proton-coupled organic cation (H+/OC) antiporter system. With this study, we aimed to elucidate oxycodone's CNS disposition during lipopolysaccharide (LPS)-induced systemic inflammation in Sprague-Dawley rats.

METHODS

Using brain microdialysis, we dynamically and simultaneously monitored unbound oxycodone concentrations in blood, striatum, lateral ventricle, and cisterna magna following intravenous administration of oxycodone post-LPS challenge.

RESULTS

Our results indicated a reduced, sex-independent brain net uptake of oxycodone across the blood-brain barrier (BBB) measured in the striatum. Notably, the LPS challenge has significantly altered the systemic pharmacokinetics (PK) of oxycodone, in a sex-specific manner, leading to lower clearance and higher blood concentrations in females compared to LPS-treated males and healthy rats of both sexes. Proteomic analysis using Olink Target 96 Mouse Exploratory assay confirmed the induction of systemic inflammation and neuroinflammation. The inflammation led to an increased paracellular transport, measured using 4 kDa dextran, while preserving net active uptake of oxycodone across both BBB and the blood-cerebrospinal fluid barrier (BCSFB), with Kp,uu values of 2.7 and 2.5, respectively. The extent of uptake was 1.6-fold lower (p < 0.0001) at the BBB and unchanged at the BCSFB after the LPS challenge compared to that in healthy rats. However, the mean exposure of unbound oxycodone in the brain following LPS was similar to that in healthy rats, primarily due to the LPS-induced changes in systemic exposure.

CONCLUSIONS

These findings highlight the dissimilar responses at blood-brain interfaces during LPS-induced inflammation. Advancing the knowledge of neuropharmacokinetic mechanisms, specifically those involving the H+/OC antiporter system, will enable the development of more effective therapeutic strategies during inflammation conditions.

Active CNS delivery of oxycodone in healthy and endotoxemic pigs

BACKGROUND

The primary objective of this study was to advance our understanding of active drug uptake at brain barriers in higher species than rodents, by examining oxycodone brain concentrations in pigs.

METHODS

This was investigated by a microdialysis study in healthy and endotoxemic conditions to increase the understanding of inter-species translation of putative proton-coupled organic cation (H+/OC) antiporter-mediated central nervous system (CNS) drug delivery in health and pathology, and facilitate the extrapolation to humans for improved CNS drug treatment in patients. Additionally, we sought to evaluate the efficacy of lumbar cerebrospinal fluid (CSF) exposure readout as a proxy for brain unbound interstitial fluid (ISF) concentrations. By simultaneously monitoring unbound concentrations in blood, the frontal cortical area, the lateral ventricle (LV), and the lumbar intrathecal space in healthy and lipopolysaccharide (LPS)-induced inflammation states within the same animal, we achieved exceptional spatiotemporal resolution in mapping oxycodone transport across CNS barriers.

RESULTS

Our findings provide novel evidence of higher unbound oxycodone concentrations in brain ISF compared to blood, yielding an unbound brain-to-plasma concentration ratio (Kp,uu,brain) of 2.5. This supports the hypothesis of the presence of the H+/OC antiporter system at the blood-brain barrier (BBB) in pigs. Despite significant physiological changes, reflected in pig Sequential Organ Failure Assessment, pSOFA scores, oxycodone blood concentrations and its active net uptake across the BBB remained nearly unchanged during three hours of i.v. infusion of 4 µg/kg/h LPS from Escherichia coli (O111:B4). Mean Kp,uu,LV values indicated active uptake also at the blood-CSF barrier in healthy and endotoxemic pigs. Lumbar CSF concentrations showed minimal inter-individual variability during the experiment, with a mean Kp,uu,lumbarCSF of 1.5. LPS challenge caused a slight decrease in Kp,uu,LV, while Kp,uu,lumbarCSF remained unaffected.

CONCLUSIONS

This study enhances our understanding of oxycodone pharmacokinetics and CNS drug delivery in both healthy and inflamed conditions, providing crucial insights for translating these findings to clinical settings.

Hydrocodone, Oxycodone, and Morphine Metabolism and Drug-Drug Interactions

Awareness of drug interactions involving opioids is critical for patient treatment as they are common therapeutics used in numerous care settings, including both chronic and disease-related pain. Not only do opioids have narrow therapeutic indexes and are extensively used, but they have the potential to cause severe toxicity. Opioids are the classical pain treatment for patients who suffer from moderate to severe pain. More importantly, opioids are often prescribed in combination with multiple other drugs, especially in patient populations who typically are prescribed a large drug regimen. This review focuses on the current knowledge of common opioid drug-drug interactions (DDIs), focusing specifically on hydrocodone, oxycodone, and morphine DDIs. The DDIs covered in this review include pharmacokinetic DDI arising from enzyme inhibition or induction, primarily due to inhibition of cytochrome p450 enzymes (CYPs). However, opioids such as morphine are metabolized by uridine-5'-diphosphoglucuronosyltransferases (UGTs), principally UGT2B7, and glucuronidation is another important pathway for opioid-drug interactions. This review also covers several pharmacodynamic DDI studies as well as the basics of CYP and UGT metabolism, including detailed opioid metabolism and the potential involvement of metabolizing enzyme gene variation in DDI. Based upon the current literature, further studies are needed to fully investigate and describe the DDI potential with opioids in pain and related disease settings to improve clinical outcomes for patients. SIGNIFICANCE STATEMENT: A review of the literature focusing on drug-drug interactions involving opioids is important because they can be toxic and potentially lethal, occurring through pharmacodynamic interactions as well as pharmacokinetic interactions occurring through inhibition or induction of drug metabolism.

Pharmacogenetic Analysis Enables Optimization of Pain Therapy: A Case Report of Ineffective Oxycodone Therapy

Patients suffering from chronic pain may respond differently to analgesic medications. For some, pain relief is insufficient, while others experience side effects. Although pharmacogenetic testing is rarely performed in the context of analgesics, response to opiates, non-opioid analgesics, and antidepressants for the treatment of neuropathic pain can be affected by genetic variants. We describe a female patient who suffered from a complex chronic pain syndrome due to a disc hernia. Due to insufficient response to oxycodone, fentanyl, and morphine in addition to non-steroidal anti-inflammatory drug (NSAID)-induced side effects reported in the past, we performed panel-based pharmacogenotyping and compiled a medication recommendation. The ineffectiveness of opiates could be explained by a combined effect of the decreased activity in cytochrome P450 2D6 (CYP2D6), an increased activity in CYP3A, and an impaired drug response at the µ-opioid receptor. Decreased activity for CYP2C9 led to a slowed metabolism of ibuprofen and thus increased the risk for gastrointestinal side effects. Based on these findings we recommended hydromorphone and paracetamol, of which the metabolism was not affected by genetic variants. Our case report illustrates that an in-depth medication review including pharmacogenetic analysis can be helpful for patients with complex pain syndrome. Our approach highlights how genetic information could be applied to analyze a patient's history of medication ineffectiveness or poor tolerability and help to find better treatment options.

CYP2D in the brain impacts oral hydrocodone analgesia in vivo

Cytochrome P450 2D (CYP2D) metabolises many centrally-acting substrates including opioids. Hydrocodone, an opioid and CYP2D substrate, is metabolised to hydromorphone, an active metabolite. CYP2D in the brain is active in vivo and can alter drug response however, it is unknown whether metabolism by CYP2D in the brain alters oral hydrocodone induced analgesia. Propranolol, a selective CYP2D mechanism-based inhibitor, or vehicle, was administered into the right cerebral ventricle of male rats, (HAN Wistars, Envigo), 24 h before testing for analgesia from oral hydrocodone (or hydromorphone, a non-CYP2D substrate). Hydrocodone and its CYP2D-mediated metabolites were simultaneously quantified using a novel LC-MS/MS assay. After propranolol vs vehicle pretreatment, there was significantly higher analgesia from oral hydrocodone, and a significantly lower brain CYP2D metabolic ratio (an in vivo phenotype of brain CYP2D activity that was derived from the molar sum of hydromorphone and its metabolites divided by hydrocodone). The brain CYP2D metabolic ratio correlated significantly with analgesia. There was no pretreatment effect on plasma hydrocodone concentrations, elimination rates, or metabolic ratio (an in vivo phenotype for hepatic CYP2D activity). The liver CYP2D metabolic ratio did not correlate with analgesia. Propranolol pretreatment had no impact on analgesia from oral hydromorphone. These data suggest that inhibited CYP2D activity in brain, causing reduced metabolism of brain hydrocodone, resulted in higher analgesia from oral hydrocodone, despite hydrocodone having a lower μ-opioid receptor affinity than hydromorphone. Thus, variation in CYP2D in the brain may be an important source of interindividual differences in response to CYP2D substrates, including oral hydrocodone.

Differences in P-glycoprotein activity in human and rodent blood-brain barrier assessed by mechanistic modelling

Variation in the efficacy and safety of central nervous system drugs between humans and rodents can be explained by physiological differences between species. An important factor could be P-glycoprotein (Pgp) activity in the blood–brain barrier (BBB), as BBB expression of this drug efflux transporter is reportedly lower in humans compared to mouse and rat and subject to an age-dependent increase. This might complicate animal to human extrapolation of brain drug disposition and toxicity, especially in children. In this study, the potential species-specific effect of BBB Pgp activity on brain drug exposure was investigated. An age-dependent brain PBPK model was used to predict cerebrospinal fluid and brain mass concentrations of Pgp substrate drugs. For digoxin, verapamil and quinidine, in vitro kinetic data on their transport by Pgp were derived from literature and used to scale to in vivo parameters. In addition, age-specific digoxin transport was simulated for children with a postnatal age between 25 and 81 days. BBB Pgp activity in the model was optimized using measured CSF data for the Pgp substrates ivermectin, indinavir, vincristine, docetaxel, paclitaxel, olanzapine and citalopram, as no useful in vitro data were available. Inclusion of Pgp activity in the model resulted in optimized predictions of their brain concentration. Total brain-to-plasma AUC values (Kp,brain) in the simulations without Pgp were divided by the Kp,brain values with Pgp. Kp ratios ranged from 1 to 45 for the substrates investigated. Comparison of human with rodent Kp,brain ratios indicated ≥ twofold lower values in human for digoxin, verapamil, indinavir, paclitaxel and citalopram and ≥ twofold higher values for vincristine. In conclusion, BBB Pgp activity appears species-specific. An age-dependent PBPK model-based approach could be useful to extrapolate animal data to human adult and paediatric predictions by taking into account species-specific and developmental BBB Pgp expression.

Combining a Candidate Vaccine for Opioid Use Disorders with Extended-Release Naltrexone Increases Protection against Oxycodone-Induced Behavioral Effects and Toxicity

Opioid use disorders (OUDs) and opioid-related fatal overdoses are a significant public health concern in the United States and worldwide. To offer more effective medical interventions to treat or prevent OUD, antiopioid vaccines are in development that reduce the distribution of the targeted opioids to brain and subsequently reduce the associated behavioral and toxic effects. It is of critical importance that antiopioid vaccines do not interfere with medications that treat OUD. Hence, this study tested the preclinical proof of concept of combining a candidate oxycodone vaccine [oxycodone-keyhole limpet hemocyanin (OXY-KLH)] with an FDA-approved extended-release naltrexone (XR-NTX) depot formulation in rats. The effects of XR-NTX on oxycodone-induced motor activity and antinociception were first assessed in nonvaccinated naïve rats to establish a baseline for subsequent studies. Next, OXY-KLH and XR-NTX were coadministered to determine whether the combination would affect the efficacy of each individual treatment, and it was found that the combination of OXY-KLH and XR-NTX offered greater efficacy in reducing oxycodone-induced motor activity, thigmotaxis, antinociception, and respiratory depression over a range of repeated or escalating oxycodone doses in rats. These data support the feasibility of combining antibody-based therapies with opioid receptor antagonists to provide greater or prolonged protection against opioid-related toxicity or overdose. Combining antiopioid vaccines with XR-NTX may provide prophylactic measures to subjects at risk of relapse and accidental or deliberate exposure. Combination therapy may extend to other biologics (e.g., monoclonal antibodies) and medications against substance use disorders. SIGNIFICANCE STATEMENT: Opioid use disorders (OUDs) remain a major problem worldwide, and new therapies are needed. This study reports on the combination of an oxycodone vaccine [oxycodone-keyhole limpet hemocyanin (OXY-KLH)] with a currently approved OUD therapy, extended-release naltrexone (XR-NTX). Results demonstrated that XR-NTX did not interfere with OXY-KLH efficacy, and combination of low doses of XR-NTX with vaccine was more effective than each individual treatment alone to reduce behavioral and toxic effects of oxycodone, suggesting that combining OXY-KLH with XR-NTX may improve OUD outcomes.

Advances in blood-brain barrier modeling in microphysiological systems highlight critical differences in opioid transport due to cortisol exposure

Background

The United States faces a national crisis involving opioid medications, where currently more than 130 people die every day. To combat this epidemic, a better understanding is needed of how opioids penetrate into the central nervous system (CNS) to facilitate pain relief and, potentially, result in addiction and/or misuse. Animal models, however, are a poor predictor of blood–brain barrier (BBB) transport and CNS drug penetration in humans, and many traditional 2D cell culture models of the BBB and neurovascular unit have inadequate barrier function and weak or inappropriate efflux transporter expression. Here, we sought to better understand opioid transport mechanisms using a simplified microfluidic neurovascular unit (NVU) model consisting of human brain microvascular endothelial cells (BMECs) co-cultured with astrocytes.

Methods

Human primary and induced pluripotent stem cell (iPSC)-derived BMECs were incorporated into a microfluidic NVU model with several technical improvements over our previous design. Passive barrier function was assessed by permeability of fluorescent dextrans with varying sizes, and P-glycoprotein function was assessed by rhodamine permeability in the presence or absence of inhibitors; quantification was performed with a fluorescent plate reader. Loperamide, morphine, and oxycodone permeability was assessed in the presence or absence of P-glycoprotein inhibitors and cortisol; quantification was performed with mass spectrometry.

Results

We first report technical and methodological optimizations to our previously described microfluidic model using primary human BMECs, which results in accelerated barrier formation, decreased variability, and reduced passive permeability relative to Transwell models. We then demonstrate proper transport and efflux of loperamide, morphine, and oxycodone in the microfluidic NVU containing BMECs derived from human iPSCs. We further demonstrate that cortisol can alter permeability of loperamide and morphine in a divergent manner.

Conclusions

We reveal a novel role for the stress hormone cortisol in modulating the transport of opioids across the BBB, which could contribute to their abuse or overdose. Our updated BBB model represents a powerful tool available to researchers, clinicians, and drug manufacturers for understanding the mechanisms by which opioids access the CNS.

Human Abuse Potential of Oral NKTR-181 in Recreational Opioid Users: A Randomized, Double-Blind, Crossover Study

OBJECTIVE

To evaluate the human abuse potential, pharmacokinetics, pharmacodynamics, and safety of oral NKTR-181 (oxycodegol), a novel full mu-opioid receptor agonist, relative to oral oxycodone.

DESIGN

This double-blind, randomized, single-dose, crossover human abuse potential study was conducted in healthy, adult, non-physically dependent recreational opioid users.

SETTING

Inpatient clinical research site.

SUBJECTS

Seventy-one subjects randomized (95.7% male, 65.2% African American, mean age = 31.7 years).

METHODS

The primary objective was to compare two therapeutic doses of NKTR-181 (400 and 600 mg) with 40 and 60 mg of oxycodone and a supratherapeutic dose (1200 mg) of NKTR-181 with 60 mg of oxycodone using visual analog scale (VAS) ratings for Drug Liking "at this moment" (Drug Liking). Secondary objectives included VAS ratings for other subjective measures, and central nervous system (CNS) mu-opioid effects were assessed using pupillometry. Each subject received single oral doses of five treatments and matching placebo.

RESULTS

Compared with 40 and 60 mg of oxycodone, the maximum mean Drug Liking score at 400 and 600 mg NKTR-181 was significantly lower, and the rate of onset and extent of Drug Liking for all NKTR-181 doses in the first two hours postdose were also significantly lower. Delayed attenuated Drug Liking and pupillary miosis response following administration of NKTR-181 vs oxycodone were consistent with slower NKTR-181 CNS entry kinetics and mu-opioid receptor binding. No adverse events were rated as severe, and somnolence and dizziness occurred more frequently when subjects received oxycodone.

CONCLUSIONS

NKTR-181 at oral doses of 400 and 600 mg showed significantly fewer and less severe subjective effects accepted as representative of opioid abuse potential, such as lower peak Drug Liking in recreational opioid users, than 40 and 60 mg of oxycodone.

Rat brain CYP2D activity alters in vivo central oxycodone metabolism, levels and resulting analgesia

Oxycodone is metabolized by CYP2D to oxymorphone. Despite oxymorphone being a more potent opioid-receptor agonist, its contribution to oxycodone analgesia may be minor because of low peripheral production, low blood-brain barrier permeability and central nervous system efflux. CYP2D metabolism within the brain may contribute to variation in central oxycodone and oxymorphone levels, thereby affecting analgesia. Brain CYP2D expression and activity are subject to exogenous regulation; nicotine induces rat brain, but not liver, CYP2D consistent with higher brain CYP2D in smokers. We assessed the role of rat brain CYP2D in orally administered oxycodone metabolism (in vivo brain microdialysis) and analgesia (tail-flick test) by inhibiting brain CYP2D selectively with intracerebroventricular propranolol (mechanism-based inhibitor) and inducing brain CYP2D with nicotine. Inhibiting brain CYP2D increased brain oxycodone levels (1.8-fold; P < 0.03) and analgesia (1.5-fold AUC0-60 ; P < 0.001) after oxycodone, while inducing brain CYP2D increased brain oxymorphone levels (4.6-fold; P < 0.001) and decreased analgesia (0.8-fold; P < 0.02). Inhibiting the induced brain CYP2D reversed the change in oxycodone levels (1.2-fold; P > 0.1) and analgesia (1.1-fold; P > 0.3). Brain, but not plasma, metabolic ratios were affected by pre-treatments. Peak analgesia was inversely correlated with ex vivo brain (P < 0.003), but not hepatic (P > 0.9), CYP2D activity. Altering brain CYP2D did not affect analgesia from oral oxymorphone (P > 0.9 for AUC0-60 across all groups), which is not a CYP2D substrate. Thus, brain CYP2D metabolism alters local oxycodone levels and response, suggesting that people with increased brain CYP2D activity may have reduced oxycodone response. Factors that alter individual oxycodone response may be useful for optimizing treatment and minimizing abuse liability.

A Validated HPLC-MS/MS Assay for 14-O-[(4,6-Diaminopyrimidine-2-yl)thioacetyl] Mutilin in Biological Samples and Its Pharmacokinetic, Distribution and Excretion via Urine and Feces in Rats

14-O-[(4,6-Diaminopyrimidine-2-yl)thioacetyl] mutilin (DPTM), a novel pleuromutilin candidate with a substituted pyrimidine moiety, has been confirmed to possess excellent antibacterial activity against Gram-positive bacteria. To illustrate the pharmacokinetic profile after intravenous (i.v.), intramuscular (i.m.) and oral (p.o.) administrations with DPTM, as well as tissue distribution and excretion via urine and feces in vivo, a specific, sensitive and robust HPLC-MS/MS method was first developed to determine DPTM in rat plasma, various tissues, urine and feces. The plasma, tissues, urine and feces samples were treated by protein precipitation with acetonitrile using tiamulin fumarate as an internal standard (IS). This method which was achieved on an HPLC system detector equipped with an ESI interface, was sensitive with 5 ng/mL as the lower limit of detection and exhibited good linearity (R² > 0.9900) in the range of 5⁻4000 ng/mL for plasma, various tissues, urine and feces, as well as intra-day precision, inter-day precision and accuracy. The matrix effects ranged from 94.2 to 109.7% with RSD ≤ 9.4% and the mean extraction recoveries ranged from 95.4 to 109.5% in plasma, tissue homogenates, urine and feces (RSD ≤ 9.9). After i.v., i.m. and p.o. administrations, DPTM was rapidly absorbed and metabolized in rats with the half-life (t1/2) of 1.70⁻1.86, 3.23⁻3.49 and 4.38⁻4.70 for 10, 25 and 75 mg/kg doses, respectively. The tissue distribution showed that DPTM was diffused into all the tested tissues, especially into the intestine and lung. Excretion via urine and feces studies demonstrated that DPTM was mainly excreted by feces after administration.

A new pleuromutilin candidate with potent antibacterial activity against Pasteurella multocida

This study assessed the antimicrobial activity of 14-O-[(4,6-Diaminopyrimidine-2-yl) thioacetyl] mutilin (DPTM), a novel pleuromutilin candidate with a substituted pyrimidine moiety, against Pasteurella multocida. Minimum Inhibitory Concentration (MIC), Oxford Cup assay, and time-kill experiments were used to measure the activity of DPTM against P. multocida serotype A in vitro. We observed that DPTM was potent against Pasteurella multocida serotype A with the MIC value of 0.781 μg/mL. The mean inhibition-zone diameters of DPTM (50, 25, and 12.5 μg/mL) were 29.4, 24.2 and 20.1 mm, respectively. Time-kill experiments showed that the drug caused a rapid decline in the number of bacteria compared with the initial inoculum at 4 h and killed 94.6% of the bacteria during 24 h. Furthermore, DPTM activity was also assessed in a lung infection model challenged with 4.0 × 109 CFU/mL P. multocida serotype A. The results showed that DPTM significantly reduced mortality rate and bacterial load, and alleviated the pathological changes of lung. The antibacterial effect of DPTM found in this study suggested that it was useful in the prevention or control of pneumonia caused by P. multocida.

Microdialysis in awake macaque monkeys for central nervous system pharmacokinetics

BACKGROUND

The brain bioavailability of novel small molecules developed to address central nervous system disease is classically documented through ex vivo or in vivo analyses conducted in rodent models. Data acquired in rodent models are, however, not easily transferrable to human as the pharmacokinetic and pharmacodynamics profiles of the species are quite different.

METHODS

Using drugs selected for their differential transport across the blood-brain barrier, we here demonstrate the feasibility of brain microdialysis in normal vigil macaque monkey by measuring brain extracellular fluid bioavailability of carbamazepine, digoxin, oxycodone, and quinidine.

RESULTS

All drugs, but digoxin, were found in dialysate samples. Drugs that are substrate of P-glycoprotein show a difference of bioavailability or brain pharmacokinetic parameters between rodents and primates.

CONCLUSION

Data suggest that brain microdialysis in vigil macaque monkey, the species of choice for classic pharmacokinetic/pharmacodynamics studies could help predicting human brain bioavailability of a small molecule depending on the protein involved in the efflux transport from the brain.

Modulation of Opioid Transport at the Blood-Brain Barrier by Altered ATP-Binding Cassette (ABC) Transporter Expression and Activity

Opioids are highly effective analgesics that have a serious potential for adverse drug reactions and for development of addiction and tolerance. Since the use of opioids has escalated in recent years, it is increasingly important to understand biological mechanisms that can increase the probability of opioid-associated adverse events occurring in patient populations. This is emphasized by the current opioid epidemic in the United States where opioid analgesics are frequently abused and misused. It has been established that the effectiveness of opioids is maximized when these drugs readily access opioid receptors in the central nervous system (CNS). Indeed, opioid delivery to the brain is significantly influenced by the blood-brain barrier (BBB). In particular, ATP-binding cassette (ABC) transporters that are endogenously expressed at the BBB are critical determinants of CNS opioid penetration. In this review, we will discuss current knowledge on the transport of opioid analgesic drugs by ABC transporters at the BBB. We will also examine how expression and trafficking of ABC transporters can be modified by pain and/or opioid pharmacotherapy, a novel mechanism that can promote opioid-associated adverse drug events and development of addiction and tolerance.

Opioids and the Blood-Brain Barrier: A Dynamic Interaction with Consequences on Drug Disposition in Brain

Background:

Opioids are widely used in pain management, acting via opioid receptors and/or Toll-like receptors (TLR) present at the central nervous system (CNS). At the blood-brain barrier (BBB), several influx and efflux transporters, such as the ATP-binding cassette (ABC)

P-glycoprotein (P-gp, ABCB1), Breast Cancer Resistance Protein (BCRP, ABCG2) and multidrug resistance-associated proteins (MRP, ABCC) transporters, and solute carrier transporters (SLC), are responsible for the transport of xenobiotics from the brain into the bloodstream or vice versa.

Objective:

ABC transporters export several clinically employed opioids, altering their neuro-

pharmacokinetics and CNS effects. In this review, we explore the interactions between opioids

and ABC transporters, and decipher the molecular mechanisms by which opioids can modify their expression at the BBB.

Results:

P-gp is largely implicated in the brain-to-blood efflux of opioids, namely morphine and oxycodone. Long-term ex-posure to morphine and oxycodone has proven to up-regulate the expression of ABC transporters, such as P-gp, BCRP and MRPs, at the BBB, which may lead to increased tolerance to the antinociceptive effects of such drugs. Recent studies uncov-er two mechanisms by which morphine may up-regulate P-gp and BCRP at the BBB: 1) via a glutamate, NMDA-receptor and COX-2 signaling cascade, and 2) via TLR4 activation, subsequent development of neuro-

inflammation, and activation of NF-κB, presumably via glial cells.

Conclusion:

The BBB-opioid interaction can culminate in bilateral consequences, since ABC transporters condition the brain disposition of opioids, while opioids also affect the expression of ABC transporters at the BBB, which may result in increased CNS drug pharmacoresistance.

The opioid epidemic: a central role for the blood brain barrier in opioid analgesia and abuse

Opioids are currently the primary treatment method used to manage both acute and chronic pain. In the past two to three decades, there has been a surge in the use, abuse and misuse of opioids. The mechanism by which opioids relieve pain and induce euphoria is dependent on the drug crossing the blood-brain barrier and accessing the central nervous system. This suggests the blood brain barrier plays a central role in both the benefits and risks of opioid use. The complex physiological responses to opioids that provide the benefits and drive the abuse also needs to be considered in the resolution of the opioid epidemic.

Ethanol Reversal of Tolerance to the Antinociceptive Effects of Oxycodone and Hydrocodone

This study compared the development of tolerance to two orally bioavailable prescription opioids, oxycodone and hydrocodone, to that of morphine, and the reversal of this tolerance by ethanol. Oxycodone (s.c.) was significantly more potent in the mouse tail-withdrawal assay than either morphine or hydrocodone. Oxycodone was also significantly more potent in this assay than hydrocodone when administered orally. Tolerance was seen following chronic subcutaneous administration of each of the three drugs and by the chronic administration of oral oxycodone, but not following the chronic oral administration of hydrocodone. Ethanol (1 g/kg i.p.) significantly reversed the tolerance to the subcutaneous administration of each of the three opioids that developed when given 30 minutes prior to challenge doses. It took twice as much ethanol, when given orally, to reverse the tolerance to oxycodone. We investigated whether the observed tolerance to oxycodone and its reversal by ethanol were due to biodispositional changes or reflected a true neuronal tolerance. As expected, a relationship between brain oxycodone concentrations and activity in the tail-immersion test existed following administration of acute oral oxycodone. Following chronic treatment, brain oxycodone concentrations were significantly lower than acute concentrations. Oral ethanol (2 g/kg) reversed the tolerance to chronic oxycodone, but did not alter brain concentrations of either acute or chronic oxycodone. These studies show that there is a metabolic component of tolerance to oxycodone; however, the reversal of that tolerance by ethanol is not due to an alteration of the biodisposition of oxycodone, but rather is neuronal in nature.

Revisiting Oxycodone Analgesia: A Review and Hypothesis

Oxycodone, a semisynthetic opioid analgesic, is widely used in clinical practice. Oxycodone and morphine seem to be equally effective and equipotent; however, morphine is 10 times more potent than oxycodone when given epidurally. This article provides an updated review of the basic pharmacology of oxycodone with a special focus on pharmacokinetic/pharmacodynamics properties. The controversy regarding oxycodone-mediated effects for visceral pain via agonism and the possible role of peripheral opioid analgesia are discussed in the present investigation in an attempt to propose a plausible explanation to the perplexing question of oxycodone analgesia.

Do Diuretics have Antinociceptive Actions: Studies of Spironolactone, Eplerenone, Furosemide and Chlorothiazide, Individually and with Oxycodone and Morphine

Spironolactone, eplerenone, chlorothiazide and furosemide are diuretics that have been suggested to have antinociceptive properties, for example via mineralocorticoid receptor antagonism. In co-administration, diuretics might enhance the antinociceptive effect of opioids via pharmacodynamic and pharmacokinetic mechanisms. Effects of spironolactone (100 mg/kg, i.p.), eplerenone (100 mg/kg, i.p.), chlorothiazide (50 mg/kg, i.p.) and furosemide (100 mg/kg, i.p.) were studied on acute oxycodone (0.75 mg/kg, s.c.)- and morphine (3 mg/kg, s.c.)-induced antinociception using tail-flick and hot plate tests in male Sprague Dawley rats. The diuretics were administered 30 min. before the opioids, and behavioural tests were performed 30 and 90 min. after the opioids. Concentrations of oxycodone, morphine and their major metabolites in plasma and brain were quantified by mass spectrometry. In the hot plate test at 30 and 90 min., spironolactone significantly enhanced the antinociceptive effect (% of maximum possible effect) of oxycodone from 10% to 78% and from 0% to 50%, respectively, and that of morphine from 12% to 73% and from 4% to 83%, respectively. The brain oxycodone and morphine concentrations were significantly increased at 30 min. (oxycodone, 46%) and at 90 min. (morphine, 190%). We did not detect any independent antinociceptive effects with the diuretics. Eplerenone and chlorothiazide did not enhance the antinociceptive effect of either opioid. The results suggest that spironolactone enhances the antinociceptive effect of both oxycodone and morphine by increasing their concentrations in the central nervous system.

Opioid Concentrations in Oral Fluid and Plasma in Cancer Patients With Pain

CONTEXT

Measuring opioid concentrations in pain treatment is warranted in situations where optimal opioid analgesia is difficult to reach.

OBJECTIVES

To assess the usefulness of oral fluid (OFL) as an alternative to plasma in opioid concentration monitoring in cancer patients on chronic opioid therapy.

METHODS

We collected OFL and plasma samples from 64 cancer patients on controlled-release (CR) oral morphine, CR oral oxycodone, or transdermal (TD) fentanyl for pain. Samples were obtained on up to five separate days.

RESULTS

A total of 213 OFL and plasma samples were evaluable. All patients had detectable amounts of the CR or TD opioid in both plasma and OFL samples. The plasma concentrations of oxycodone and fentanyl (determination coefficient R(2) = 0.628 and 0.700, respectively), but not morphine (R(2) = 0.292), were moderately well correlated to the daily opioid doses. In contrast to morphine and fentanyl (mean OFL/plasma ratio 2.0 and 3.0, respectively), the OFL oxycodone concentrations were significantly higher than the respective plasma concentrations (mean OFL/plasma ratio 14.9). An active transporter could explain the much higher OFL vs. plasma concentrations of oxycodone compared with morphine and fentanyl.

CONCLUSION

OFL analysis is well suited for detecting the studied opioids. For morphine and fentanyl, an approximation of the plasma opioid concentrations is obtainable, whereas for oxycodone, the OFL/plasma concentration relationship is too variable for reliable approximation results.

Transporter-Mediated Disposition of Opioids: Implications for Clinical Drug Interactions

Opioid-related deaths, abuse, and drug interactions are growing epidemic problems that have medical, social, and economic implications. Drug transporters play a major role in the disposition of many drugs, including opioids; hence they can modulate their pharmacokinetics, pharmacodynamics and their associated drug-drug interactions (DDIs). Our understanding of the interaction of transporters with many therapeutic agents is improving; however, investigating such interactions with opioids is progressing relatively slowly despite the alarming number of opioids-mediated DDIs that may be related to transporters. This review presents a comprehensive report of the current literature relating to opioids and their drug transporter interactions. Additionally, it highlights the emergence of transporters that are yet to be fully identified but may play prominent roles in the disposition of opioids, the growing interest in transporter genomics for opioids, and the potential implications of opioid-drug transporter interactions for cancer treatments. A better understanding of drug transporters interactions with opioids will provide greater insight into potential clinical DDIs and could help improve opioids safety and efficacy.

Validation of a P-Glycoprotein (P-gp) Humanized Mouse Model by Integrating Selective Absolute Quantification of Human MDR1, Mouse Mdr1a and Mdr1b Protein Expressions with In Vivo Functional Analysis for Blood-Brain Barrier Transport

It is essential to establish a useful validation method for newly generated humanized mouse models. The novel approach of combining our established species-specific protein quantification method combined with in vivo functional studies is evaluated to validate a humanized mouse model of P-gp/MDR1 efflux transporter. The P-gp substrates digoxin, verapamil and docetaxel were administered to male FVB Mdr1a/1b(+/+) (FVB WT), FVB Mdr1a/1b(-/-) (Mdr1a/1b(-/-)), C57BL/6 Mdr1a/1b(+/+) (C57BL/6 WT) and humanized C57BL (hMDR1) mice. Brain-to-plasma total concentration ratios (Kp) were measured. Quantitative targeted absolute proteomic (QTAP) analysis was used to selectively quantify the protein expression levels of hMDR1, Mdr1a and Mdr1b in the isolated brain capillaries. The protein expressions of other transporters, receptors and claudin-5 were also quantified. The Kp for digoxin, verapamil, and docetaxel were 20, 30 and 4 times higher in the Mdr1a/1b(-/-) mice than in the FVB WT controls, as expected. The Kp for digoxin, verapamil and docetaxel were 2, 16 and 2-times higher in the hMDR1 compared to the C57BL/6 WT mice. The hMDR1 mice had 63- and 9.1-fold lower expressions of the hMDR1 and Mdr1a proteins than the corresponding expression of Mdr1a in C57BL/6 WT mice, respectively. The protein expression levels of other molecules were almost consistent between C57BL/6 WT and hMDR1 mice. The P-gp function at the BBB in the hMDR1 mice was smaller than that in WT mice due to lower protein expression levels of hMDR1 and Mdr1a. The combination of QTAP and in vivo functional analyses was successfully applied to validate the humanized animal model and evaluates its suitability for further studies.

[Effect of repeated oral treatment with etoposide on the expression of intestinal P-glycoprotein and oral morphine analgesia]

Currently, the World Health Organization recommends oral administration of opioid analgesics for patients with cancer to treat cancer-related pain from the initial stage of treatment. Furthermore, many anticancer drugs have been newly-developed and approved as oral form. Because of this trend, the chances of drug-drug interactions between anticancer drugs and opioid analgesics during absorption process from the intestine are likely to increase. To investigate these possible drug-drug interactions, we have focused on intestinal P-glycoprotein (P-gp) which regulates the absorption of various substrate drugs administered orally. Previously, we have found that repeated oral treatment with etoposide (ETP), an anticancer drug, attenuates analgesia of oral morphine, a substrate drug for P-gp, by increasing the expression and activity of intestinal P-gp. However, the mechanism by which ETP treatment increases the intestinal P-gp expression and decreases oral morphine analgesia remains unclear. RhoA, a small G-protein, and ROCK, an effector of RhoA, pathway has been attracted attention with regard to their involvement in the regulatory mechanism of the expression and activity of P-gp. Interestingly, this pathway is activated in response to various signaling induced by some anticancer drugs. Furthermore, it has been reported that ezrin/radixin/moesin (ERM) play a key role in the plasma membrane localization of P-gp, and that RhoA/ROCK pathway regulates the activation process of ERM. This review article introduces the result of our previous research as well as recent findings on the involvement of ERM via activation of RhoA/ROCK in the increased expression of intestinal P-gp and decreased oral morphine analgesia induced by repeated oral treatment with ETP.

Prediction of Metabolic Interactions With Oxycodone via CYP2D6 and CYP3A Inhibition Using a Physiologically Based Pharmacokinetic Model

Evaluation of a potential risk of metabolic drug–drug interactions (DDI) is of high importance in the clinical setting. In this study, a physiologically based pharmacokinetic (PBPK) model was developed for oxycodone and its two primary metabolites, oxymorphone and noroxycodone, in order to assess different DDI scenarios using published in vitro and in vivo data. Once developed and refined, the model was able to simulate pharmacokinetics of the three compounds and the DDI extent in case of coadministration with an inhibitor, as well as the oxymorphone concentration variation between CYP2D6 extensive metabolizers (EM) and poor metabolizers (PM). The reliability of the model was tested against published clinical studies monitoring different inhibitors and dose regimens, and all predicted area under the concentration–time curve (AUC) ratios were within the twofold acceptance range. This approach represents a strategy to evaluate the impact of coadministration of different CYP inhibitors using mechanistic incorporation of drug-dependent and system-dependent available in vitro and in vivo data.

Differences between opioids: pharmacological, experimental, clinical and economical perspectives

Clinical studies comparing the response and side effects of various opioids have not been able to show robust differences between drugs. Hence, recommendations of the regulatory authorities have been driven by costs with a general tendency in many countries to restrict physician's use of opioids to morphine. Although this approach is recognized as cost-effective in most cases there is solid evidence that, on an individual patient basis, opioids are not all equal. Therefore it is important to have an armamentarium of strong analgesics in clinical practice to ensure a personalized approach in patients who do not respond to standard treatment. In this review we highlight differences between opioids in human studies from a pharmacological, experimental, clinical and health economics point of view. We provide evidence that individuals respond differently to opioids, and that general differences between classes of opioids exist. We recommend that this recognition is used to individualize treatment in difficult cases allowing physicians to have a wide range of treatment options. In the end this will reduce pain and side effects, leading to improved quality of life for the patient and reduce the exploding pain related costs.

Does the pharmacology of oxycodone justify its increasing use as an analgesic?

Oxycodone is a semisynthetic opioid analgesic that is increasingly used for the treatment of acute, cancer, and chronic non-malignant pain. Oxycodone was synthesized in 1917 but its pharmacological properties were not thoroughly studied until recently. Oxycodone is a fairly selective μ-opioid receptor agonist, but there is a striking discrepancy between the relatively low binding potential and G protein activation by oxycodone and its analgesic efficacy. It has been claimed that this is because of active metabolites and enhanced passage to the central nervous system by active transport. We critically review studies on the basic pharmacology of oxycodone and on its pharmacokinetics and pharmacodynamics in humans. In particular, the role of pharmacogenomics and population pharmacokinetics in understanding the properties of oxycodone is discussed in detail. We compare oxycodone with morphine, the standard opioid in clinical use.

Elucidation of mu-Opioid Gene Structure: How Genetics Can Help Predict Responses to Opioids

Opioid drugs are among the most commonly used and effective human analgesics. To date, the clinical benefits of opioid analgesics have not been fully realized due to substantial individual variations in the responses to opioids, insufficient drug dosing, and a high rate (up to 66%) of adverse events. As such, there is a substantial need to identify the genetic and molecular biological mechanisms that mediate individual responses to opioid therapy. Recent discoveries show that genetic variations in the μ-opioid receptor (OPRM1) gene locus play an essential role in inter-individual responses. The majority of genetic association studies have focused on the A118G polymorphism, which codes for a non-synonymous change in OPRM1 exon 1. In addition to the A118G polymorphism, another functional SNP (rs563649), which is located within an alternatively-spliced OPRM1 isoform (MOR-1K), has been identified. The MOR-1k isoform codes for 6TM OPRM1 isoforms that display excitatory rather than the inhibitory cellular effects, which are characteristic of the canonical 7TM isoforms. Thus, stimulation of the 6TM isoforms may engage the molecular mechanisms mediating opioid-dependent hyperalgesia, tolerance and dependence. Future clinical and basic studies that seek to identify the functional genetic variants within OPRM1 locus, and associated molecular mechanisms, will result in a better understanding of individual responses to opioid therapy and ultimately to the development new pharmacotherapeutics and diagnostic tools.

Targeting blood-brain barrier changes during inflammatory pain: an opportunity for optimizing CNS drug delivery

The blood-brain barrier (BBB) is the most significant obstacle to effective CNS drug delivery. It possesses structural and biochemical features (i.e., tight-junction protein complexes and, influx and efflux transporters) that restrict xenobiotic permeation. Pathophysiological stressors (i.e., peripheral inflammatory pain) can alter BBB tight junctions and transporters, which leads to drug-permeation changes. This is especially critical for opioids, which require precise CNS concentrations to be safe and effective analgesics. Recent studies have identified molecular targets (i.e., endogenous transporters and intracellular signaling systems) that can be exploited for optimization of CNS drug delivery. This article summarizes current knowledge in this area and emphasizes those targets that present the greatest opportunity for controlling drug permeation and/or drug transport across the BBB in an effort to achieve optimal CNS opioid delivery.

Exposure to oral oxycodone is increased by concomitant inhibition of CYP2D6 and 3A4 pathways, but not by inhibition of CYP2D6 alone

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Oxycodone is an opioid analgesic that is metabolized mainly in the liver by cytochrome P450 (CYP) 2D6 and 3A4 enzymes. So far, the effects of CYP2D6 or CYP3A4 inhibitors on the pharmacokinetics of oxycodone in humans have not been systematically studied.

WHAT THIS STUDY ADDS

Drug interactions arising from CYP2D6 inhibition most likely have minor clinical importance for oral oxycodone. When both of CYP2D6 and CYP3A4 pathways are inhibited, the exposure to oral oxycodone is increased substantially.

AIM

The aim of this study was to find out whether the inhibition of cytochrome P450 2D6 (CYP2D6) with paroxetine or concomitant inhibition of CYP2D6 and CYP3A4 with paroxetine and itraconazole, altered the pharmacokinetics and pharmacological response of orally administered oxycodone.

METHODS

A randomized placebo-controlled cross-over study design with three phases was used. Eleven healthy subjects ingested 10 mg of oral immediate release oxycodone on the fourth day of pre-treatment with either placebo, paroxetine (20 mg once daily) or paroxetine (20 mg once daily) and itraconazole (200 mg once daily) for 5 days. The plasma concentrations of oxycodone and its oxidative metabolites were measured for 48 h, and pharmacological (analgesic and behavioural) effects were evaluated.

RESULTS

Paroxetine alone reduced the area under concentration-time curve (AUC(0,0-48 h)) of the CYP2D6 dependent metabolite oxymorphone by 44% (P < 0.05), but had no significant effects on the plasma concentrations of oxycodone or its pharmacological effects when compared with the placebo phase. When both oxidative pathways of the metabolism of oxycodone were inhibited with paroxetine and itraconazole, the mean AUC(0,infinity) of oxycodone increased by 2.9-fold (P < 0.001), and its C(max) by 1.8-fold (P < 0.001). Visual analogue scores for subjective drug effects, drowsiness and deterioration of performance were slightly increased (P < 0.05) after paroxetine + itraconazole pre-treatment when compared with placebo.

CONCLUSIONS

Drug interactions arising from CYP2D6 inhibition most likely have minor clinical importance for oral oxycodone if the function of the CYP3A4 pathway is normal. When both CYP2D6 and CYP3A4 pathways are inhibited, the exposure to oral oxycodone is increased substantially.

CYP3A5*3 affects plasma disposition of noroxycodone and dose escalation in cancer patients receiving oxycodone

The aim of this study was to evaluate the plasma dispositions of oxycodone and its demethylates and dose escalation based on genetic polymorphisms of CYP2D6, CYP3A5, ABCB1, and OPRM1 in cancer patients receiving oxycodone. Sixty-two Japanese cancer patients receiving oxycodone extended-release tablets were enrolled. Predose plasma concentrations (C(12)) of oxycodone, noroxycodone, and oxymorphone were determined at the titrated dose. Daily oxycodone escalation rate was evaluated as the opioid escalation index (OEI). Genetic variants did not significantly alter oxycodone C(12). Oxymorphone C(12) and its ratio to oxycodone C(12) were significantly higher in CYP2D6 extensive metabolizers than in intermediate metabolizers but did not affect dose escalation. In contrast, noroxycodone C(12) and its ratio to oxycodone C(12) were significantly higher in the CYP3A5*1 carrier group than in the *3/*3 group. The OEI was significantly higher in the CYP3A5*3/*3 group than in the *1 carrier group. No significant difference was observed in the OEI in the other genetic variants. Noroxycodone C(12) was higher in the dose escalation group as compared to the nonescalation group and significantly affected the incidence of dose escalation. In conclusion, CYP3A5*3 altered the plasma disposition of noroxycodone, which was inversely affecting the dose escalation in cancer patients receiving oxycodone.

Oxycodone concentrations are greatly increased by the concomitant use of ritonavir or lopinavir/ritonavir

PURPOSE

this study aimed to investigate the effect of antivirals ritonavir and lopinavir/ritonavir on the pharmacokinetics and pharmacodynamics of oral oxycodone, a widely used opioid receptor agonist used in the treatment of moderate to severe pain.

METHODS

a randomized crossover study design with three phases at intervals of 4 weeks was conducted in 12 healthy volunteers. Ritonavir 300 mg, lopinavir/ritonavir 400/100 mg, or placebo b.i.d. for 4 days was given to the subjects. On day 3, 10 mg oxycodone hydrochloride was administered orally. Plasma concentrations of oxycodone, noroxycodone, oxymorphone, and noroxymorphone were determined for 48 h. Pharmacokinetic parameters were calculated with standard noncompartmental methods. Behavioral effects and experimental cold pain analgesia were assessed for 12 h. ANOVA for repeated measures was used for statistical analysis.

RESULTS

ritonavir and lopinavir/ritonavir increased the area under the plasma concentration-time curve of oral oxycodone by 3.0-fold (range 1.9- to 4.3-fold; P <0.001) and 2.6-fold (range 1.9- to 3.3-fold; P <0.001). The mean (± SD) elimination half-life increased after ritonavir and lopinavir/ritonavir from 3.6 ± 0.6 to 5.6 ± 0.9 h (P <0.001) and 5.7 ± 0.9 h (P <0.001), respectively. Both ritonavir (P <0.001) and lopinavir/ritonavir (P <0.05) increased the self-reported drug effect of oxycodone.

CONCLUSIONS

ritonavir and lopinavir/ritonavir greatly increase the plasma concentrations of oral oxycodone in healthy volunteers and enhance its effect. When oxycodone is used clinically in patients during ritonavir and lopinavir/ritonavir treatment, reductions in oxycodone dose may be needed to avoid opioid-related adverse effects.

Effects of quinidine on antinociception and pharmacokinetics of morphine in rats

Aim

The aim of this study was to investigate the effect of quinidine, a P-glycoprotein inhibitor, on the pharmacokinetics and pharmacodynamics of morphine in rats.

Methods

Rats were given morphine (30 mg/kg p.o. or 30 mg/kg over 10 min i.v.) 30 min after pretreatment with quinidine (30 mg/kg p.o.). Antinociceptive effects were determined using the tail immersion test. Concentrations of morphine in plasma and brain were also determined.

Key findings

The antinociception of morphine was significantly enhanced by oral administration of quinidine, with a 3.1-fold greater area under the effect–time curve than that in vehicle-treated rats. Morphine concentrations in plasma and brain were significantly increased by quinidine. The area under the plasma concentration–time curve after oral or intravenous administration of morphine was increased 5.2- and 1.7-fold, respectively, in quinidine-pretreated rats compared with vehicle-pretreated rats. Quinidine caused a 40% decrease in the total clearance of morphine and increased the concentration of morphine in the brain, although the brain-to-plasma concentration ratio was not changed.

Conclusions

Oral administration of quinidine increases the absorption of morphine from the gastrointestinal tract and subsequently enhances the concentration in the brain and its antinociceptive effect. Enhanced intestinal absorption of morphine may be due largely to inhibition of intestinal P-glycoprotein by quinidine.

Does co-administration of paroxetine change oxycodone analgesia: An interaction study in chronic pain patients

Oxycodone is a strong opioid and it is increasingly used in the management of acute and chronic pain. The pharmacodynamic effects of oxycodone are mainly mediated by the μ-opioid receptor. However, its affinity for the μ-opioid receptor is significantly lower compared with that of morphine and it has been suggested that active metabolites may play a role in oxycodone analgesia. Oxycodone is mainly metabolized by hepatic cytochrome (CYP) enzymes 2D6 and 3A4. Oxycodone is metabolized to oxymorphone, a potent μ-opioid receptor agonist by CYP2D6. However, CYP3A4 is quantitatively a more important metabolic pathway. Chronic pain patients often use multiple medications. Therefore it is important to understand how blocking or inducing these metabolic pathways may affect oxycodone induced analgesia. The aim of this study was to find out whether blocking CYP2D6 would decrease oxycodone induced analgesia in chronic pain patients.

The effects of the antidepressant paroxetine, a potent inhibitor of CYP2D6, on the analgesic effects and pharmacokinetics of oral oxycodone were studied in 20 chronic pain patients using a randomized, double-blind, placebo-controlled cross-over study design. Pain intensity and rescue analgesics were recorded daily, and the pharmacokinetics and pharmacodynamics of oxycodone were studied on the 7th day of concomitant paroxetine (20 mg/day) or placebo administration. The patients were genotyped for CYP2D6, 3A4, 3A5 and ABCB1.

Paroxetine had significant effects on the metabolism of oxycodone but it had no statistically significant effect on oxycodone analgesia or use of morphine for rescue analgesia. Paroxetine increased the dose-adjusted mean AUC0–12h of oxycodone by 19% (−23 to 113%; P = 0.003), and that of noroxycodone by 100% (5–280%; P < 0.0001) but decreased the AUC0–12 h of oxymorphone by 67% (−100 to −22%; P < 0.0001) and that of noroxymorphone by 68% (−100 to −16%; P < 0.0001).

Adverse effects were also recorded in a pain diary for both 7-day periods (placebo/paroxetine). The most common adverse effects were drowsiness and nausea/vomiting. One patient out of four reported dizziness and headache during paroxetine co-administration, whereas no patient reported these during placebo administration (P = 0.0471) indicating that these adverse effects were due to paroxetine.

No statistically significant associations of the CYP2D6 or CYP3A4/5 genotype of the patients and the pharmacokinetics of oxycodone or its metabolites, extent of paroxetine–oxycodone interaction, or analgesic effects were observed probably due to the limited number of patients studied.

The results of this study strongly suggest that CYP2D6 inhibition does not significantly change oxycodone analgesia in chronic pain patients and that the analgesic activity of oxycodone is mainly due to the parent compound and that metabolites, e.g. oxymorphone, play an insignificant role. The clinical implication of these results is that induction of the metabolism of oxycodone may lead to inadequate analgesia while increased drug effects can be expected after addition of potent CYP3A4/5 inhibitors particularly if combined with CYP2D6 inhibitors or when administered to poor metabolizers of CYP2D6.

The challenge of pain management in patients with myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disorder of the neuromuscular junction. The complexity of the disease and its treatments make MG patients particularly susceptible to adverse effects of drugs. MG is not a painful condition; however, as pain management armamentarium includes drugs from diverse pharmacological groups and with potential for drug-drug interactions, managing pain in patients with MG can be challenging. The underlying disease and the concomitant medications of each patient must be considered and the analgesic treatment individualized. This review presents an update on the various aspects of pain pharmacotherapy in patients with MG, focusing primarily on medications used to treat chronic pain. Drugs discussed are opioids, nonsteroidal anti-inflammatory drugs, antidepressants, anticonvulsants, muscle relaxants, benzodiazepines, intravenous magnesium, and local anesthetics. Drug interactions with agents used for MG treatment (acethylcholinesterase inhibitors, corticosteroids, immunosuppressants) and plasmapheresis are discussed. The clinical usefulness and limitations of each of the drug classes and agents are described.

CNS penetration of small molecules following local inflammation, widespread systemic inflammation or direct injury to the nervous system

AIMS

We sought to investigate effects of local and systemic inflammation on CNS permeability of small molecules and compare these to effects of direct injury to the nervous system.

MAIN METHODS

Evans blue was used to determine the integrity of the blood-brain barrier (BBB) following local inflammation, systemic inflammation, injury to the L5 spinal nerve or transient occlusion of the middle cerebral artery. In addition, three compounds having low, medium and high brain permeability (atenolol, morphine and oxycodone, respectively) were used. Following model establishment (4-hr post-carrageenan, 24-hr post-FCA, 2-, 4- and 24-hr post-LPS, 21 days post-nerve injury) compounds were administered and 30 min later the brain, spinal cord and blood removed. The plasma and tissue concentrations of compounds were quantified by LC/MS/MS.

KEY FINDINGS

Localized inflammation did not affect Evans blue penetration into the CNS but significantly increased morphine penetration into the spinal cord. Systemic inflammation increased the quantity of Evans blue in the CNS but also decreased the penetration of atenolol, morphine and oxycodone into the brain 4-hr post-insult. Nerve injury had no effect on Evans blue or compound penetration, while middle cerebral artery occlusion resulted in a large but short lived increase in Evans blue penetration into both the cortex and striatum.

SIGNIFICANCE

The presence of inflammation may affect the CNS penetration of some compounds but is unlikely to lead to a large non-selective BBB breakdown. As a result, it is appropriate to test for side-effects, and conduct brain pharmacokinetic determinations, in naïve rats.