Pharmacological characterization of noroxymorphone as a new opioid for spinal analgesia

Oxycodone, an opioid like the others?

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

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Oxycodone in the Opioid Epidemic: High 'Liking', 'Wanting', and Abuse Liability

It is estimated that nearly a third of people who abuse drugs started with prescription opioid medicines. Approximately, 11.5 million Americans used prescription drugs recreationally in 2016, and in 2018, 46,802 Americans died as the result of an opioid overdose, including prescription opioids, heroin, and illicitly manufactured fentanyl (National Institutes on Drug Abuse (2020) Opioid Overdose Crisis. https://www.drugabuse.gov/drugs-abuse/opioids/opioid-overdose-crisis . Accessed 06 June 2020). Yet physicians will continue to prescribe oral opioids for moderate-to-severe pain in the absence of alternative therapeutics, underscoring the importance in understanding how drug choice can influence detrimental outcomes. One of the opioid prescription medications that led to this crisis is oxycodone, where misuse of this drug has been rampant. Being one of the most highly prescribed opioid medications for treating moderate-to-severe pain as reflected in the skyrocketed increase in retail sales of 866% between 1997 and 2007, oxycodone was initially suggested to be less addictive than morphine. The false-claimed non-addictive formulation of oxycodone, OxyContin, further contributed to the opioid crisis. Abuse was often carried out by crushing the pills for immediate burst release, typically by nasal insufflation, or by liquefying the pills for intravenous injection. Here, we review oxycodone pharmacology and abuse liability as well as present the hypothesis that oxycodone may exhibit a unique pharmacology that contributes to its high likability and abuse susceptibility. We will discuss various mechanisms that likely contribute to the high abuse rate of oxycodone including clinical drug likability, pharmacokinetics, pharmacodynamics, differences in its actions within mesolimbic reward circuity compared to other opioids, and the possibility of differential molecular and cellular receptor interactions that contribute to its selective effects. We will also discuss marketing strategies and drug difference that likely contributes to the oxycodone opioid use disorders and addiction.

Electrochemical Detection of Oxycodone and Its Main Metabolites with Nafion-Coated Single-Walled Carbon Nanotube Electrodes

Oxycodone is a strong opioid frequently used as an analgesic. Although proven efficacious in the management of moderate to severe acute pain and cancer pain, use of oxycodone imposes a risk of adverse effects such as addiction, overdose, and death. Fast and accurate determination of oxycodone blood concentration would enable personalized dosing and monitoring of the analgesic as well as quick diagnostics of possible overdose in emergency care. However, in addition to the parent drug, several metabolites are always present in the blood after a dose of oxycodone, and to date, there is no electrochemical data available on any of these metabolites. In this paper, a single-walled carbon nanotube (SWCNT) electrode and a Nafion-coated SWCNT electrode were used, for the first time, to study the electrochemical behavior of oxycodone and its two main metabolites, noroxycodone and oxymorphone. Both electrode types could selectively detect oxycodone in the presence of noroxycodone and oxymorphone. However, we have previously shown that addition of a Nafion coating on top of the SWCNT electrode is essential for direct measurements in complex biological matrices. Thus, the Nafion/SWCNT electrode was further characterized and used for measuring clinically relevant concentrations of oxycodone in buffer solution. The limit of detection for oxycodone with the Nafion/SWCNT sensor was 85 nM, and the linear range was 0.5–10 μM in buffer solution. This study shows that the fabricated Nafion/SWCNT sensor has potential to be applied in clinical concentration measurements.

The impact of drug interactions on adverse effects of oral oxycodone in male geriatric patients

WHAT IS KNOWN AND OBJECTIVE

With increased opioid use, drug-drug interactions (DDIs) and associated adverse events are growing among geriatric patients. However, the clinical significance of potential metabolic DDIs associated with opioid use has not been fully evaluated among geriatric patients. Particularly, cytochrome (CYP) P450 enzymes are important in drug metabolism of oxycodone and a black box warning for oxycodone reveals serious risks associated with drug-oxycodone interactions. This study focused on the use of oxycodone in geriatric patients to evaluate its adverse drug reactions (ADRs) and DDIs associated with CYP P450 enzymes.

METHODS

A retrospective cohort study using patients treated at Korea Veterans Hospital was performed. Data from male patients aged 65 years and older who received oxycodone were analysed. Binomial variables describing patient-related characteristics, drug-related characteristics and CYP-mediating drugs were constructed. Associations between these variables and the frequency of ADRs were determined. The odds ratio (OR) and adjusted odds ratio (AOR) were calculated from univariable and multivariable analyses, respectively.

RESULTS AND DISCUSSION

Among 111 patients, 32.4% experienced at least one ADR. The most common ADR was gastrointestinal-related (n = 21), followed by dizziness and drowsiness (n = 8). Use of either CYP2D6 inhibitors or CYP3A4 inhibitors increased the rate of ADRs by 20.4 and 25.4 times, respectively. In the case of patients taking both inhibitors, the adjusted OR was 48.6, and the attributable risk was 97.9%.

WHAT IS NEW AND CONCLUSION

This study suggests that inappropriate combinations of oxycodone with CYP2D6 inhibitors and/or CYP3A4 inhibitors may warrant treatment modification to avoid ADRs in geriatric patients. Clinicians should monitor any signs of ADRs that may reflect DDIs while a geriatric patient is taking oxycodone.

Pharmacokinetics and Bioequivalence Evaluation of a New Oxycodone Tamper-Resistant Tablet Administered with an Opioid Antagonist in Patients with Chronic Pain

BACKGROUND AND OBJECTIVES

Oxycodone tamper resistant (OTR) is a new extended-release abuse-deterrent formulation providing improvements in the tamper resistant characteristics. This study aimed to investigate the pharmacokinetic properties of the new OTR tablets and evaluate the bioequivalence of oxycodone from OTR and the original extended release (ER) formulation tablets administered with an opioid antagonist in patients with chronic pain.

METHODS

In this open-label, randomized, cross-over study, the enrolled patients were randomised to receive a single dose of 40 mg OTR or 40 mg OXYCONTIN^® (OXY) tablet administered with naltrexone blockade under fasting conditions. Serial blood samples for pharmacokinetic analysis were collected. Plasma oxycodone was quantified by a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method. Tolerability was evaluated by monitoring adverse events, physical examinations, 12-lead ECG and laboratory tests.

RESULTS

A total of 38 patients were enrolled and 33 subjects completed the study. After a single dose of 40 mg tablets, pharmacokinetic results of the new OTR tablet were found to be similar to those of original extended-release oxycodone tablet. OTR 40 mg was bioequivalent to OXY 40 mg and was well tolerated in patients with chronic pain.

CONCLUSIONS

The new OTR formulation could provide a new choice in the treatment of chronic pain and reduce the potential for oxycodone abuse. Chictr.org identifier: ChiCTR1800017253.

Oxycodone concentrations in the central nervous system and cerebrospinal fluid after epidural administration to the pregnant ewe

The main sites of the analgesic action of oxycodone are the brain and spinal cord. The present study describes the concentrations of oxycodone and its metabolites in the brain and spinal cord after epidural administration to the ewe. Twenty pregnant ewes undergoing laparotomy were randomized into two groups to receive epidural oxycodone: infusion group (n = 10, 0.1 mg·kg^-1 bolus followed by continuous infusion of 0.05 mg·kg^-1 ·h^-1 for five days) or repeated boluses group (n = 10, 0.2 + 2x0.1 mg·kg^-1 bolus followed by a 0.2 mg·kg^-1 bolus every 12 hours for five days). After five days of oxycodone administration, arterial blood samples were collected, the sheep were killed, and a CSF sample and tissue samples from the cortex, thalamus, cerebellum and spinal cord were obtained for the quantification of oxycodone and its main metabolites. The median plasma and CSF concentrations of oxycodone were 9.0 and 14.2 ng·mL^-1 after infusion and 0.4 and 1.1 ng·mL^-1 after repeated boluses. In the infusion group, the cortex, thalamus and cerebellum oxycodone concentrations were 4-8 times higher and in the spinal cord 1310 times higher than in plasma. In the repeated boluses group, brain tissue concentrations were similar in the three areas, and in the spinal cord were 720 times higher than in plasma. Oxymorphone was the main metabolite detected, which accumulated in the brain and spinal cord tissue. In conclusion, first, accumulation of oxycodone and oxymorphone in the CNS was observed, and second, high spinal cord concentrations suggest that epidural oxycodone may provide segmental analgesia.

Quality Assessment of Expired Naloxone Products from First-Responders' Supplies

Objective: Naloxone is an opioid receptor antagonist that reverses life-threatening effects of opioid overdose. Since the 1970s, naloxone products have been developed as injectable solutions, and more recently as nasal sprays. Naloxone products have saved many lives in emergency settings. These products are routinely carried by public safety first-responders including fire fighters (FF), law enforcement officers (LEO), and emergency medical services (EMS). Now, they are also distributed through community access programs to the public. While public safety medications are monitored, those publically distributed are not, so expired products can be possibly found on-hand in an emergency. This study analyzed the quality and stability of expired Naloxone HCl Solutions for Injection, to assess their remaining efficacies and potential risks. Methods: The samples were collected from EMS or law enforcement training supplies and expired returns, with expiration dates ranging from 1990 to 2018. Using standardized techniques, the remaining naloxone was quantified, and the main degradation products, nornaloxone (also known as noroxymorphone) and other possible species, were monitored and quantified systematically. Results: Most tested samples were found containing more than 90% of labeled naloxone, including those stored for nearly 30 years. The naloxone degradation was slow, but generally correlated with storage time length. There was no significant amount of degradation products detected across all samples. Nornaloxone was detected from some older samples, but all less than 1%. Therefore, although it is an opioid agonist, the risk caused by nornaloxone should be low. Conclusion: This quality assessment demonstrates that expired naloxone products may still meet USP standards, even after many years. Further pharmaceutical, clinical, and regulatory investigation should be conducted to confirm our findings, especially for new naloxone products with different formulations and routes of administration. Extending the shelf-life of naloxone products may have important financial and public health consequences in addressing future drug shortages and meeting the needs for this critical drug.

The analgesic efficacy and pharmacokinetics of epidural oxycodone after gynaecological laparotomy: a randomized, double-blind, double-dummy comparison with intravenous administration

AIM

The aim of the present study was to compare the analgesic efficacy of epidural and intravenous (i.v.) oxycodone at the same dose.

METHODS

In this randomized, double-blind, double-dummy clinical trial, 30 women, aged 24-67 years, undergoing elective gynaecological laparotomy, were administrated either i.v. saline and epidural oxycodone 0.1 mg·kg^-1 (EPI group; n = 15) or i.v. oxycodone 0.1 mg·kg^-1 and epidural saline (IV group; n = 15). For multimodal analgesia, patients received i.v. paracetamol and dexketoprofen, and a triple-mixture epidural infusion after the first 4 h postoperatively. The primary outcome was the total dose of i.v. fentanyl for rescue analgesia during the first 4 h postoperatively.

RESULTS

All patients required fentanyl during the first 4 h. The median number of fentanyl doses were three (quartiles 1, 8) in the EPI group and seven (6, 9) in the IV group (mean difference 3.1; 95% confidence interval 0.9, 5.2; P = 0.01). After the first 4 h, the two groups needed a similar total dose of epidural infusion. Patient satisfaction was similarly high in both groups, and both administration routes were well tolerated.

CONCLUSIONS

The data support the superiority of epidural oxycodone compared with that of i.v. administration in pain management after laparotomy.

Pain Therapy Guided by Purpose and Perspective in Light of the Opioid Epidemic

Prescription opioid misuse is an ongoing and escalating epidemic. Although these pharmacological agents are highly effective analgesics prescribed for different types of pain, opioids also induce euphoria, leading to increasing diversion and misuse. Opioid use and related mortalities have developed in spite of initial claims that OxyContin, one of the first opioids prescribed in the USA, was not addictive in the presence of pain. These claims allayed the fears of clinicians and contributed to an increase in the number of prescriptions, quantity of drugs manufactured, and the unforeseen diversion of these drugs for non-medical uses. Understanding the history of opioid drug development, the widespread marketing campaign for opioids, the immense financial incentive behind the treatment of pain, and vulnerable socioeconomic and physical demographics for opioid misuse give perspective on the current epidemic as an American-born problem that has expanded to global significance. In light of the current worldwide opioid epidemic, it is imperative that novel opioids are developed to treat pain without inducing the euphoria that fosters physical dependence and addiction. We describe insights from preclinical findings on the properties of opioid drugs that offer insights into improving abuse-deterrent formulations. One finding is that the ability of some agonists to activate one pathway over another, or agonist bias, can predict whether several novel opioid compounds bear promise in treating pain without causing reward among other off-target effects. In addition, we outline how the pharmacokinetic profile of each opioid contributes to their potential for misuse and discuss the emergence of mixed agonists as a promising pipeline of opioid-based analgesics. These insights from preclinical findings can be used to more effectively identify opioids that treat pain without causing physical dependence and subsequent opioid abuse.

Analgesic Plasma Concentrations of Oxycodone After Surgery for Breast Cancer-Which Factors Matter?

We investigated factors affecting analgesic oxycodone concentrations after breast cancer surgery in 1,000 women. Preoperatively, we studied heat and cold pain sensitivities and anxiety scores. Postoperatively, rest and motion pain intensities were measured and intravenous oxycodone was administered until satisfactory analgesia. At this point, the mean oxycodone concentration (variation coefficient) was 33.3 ng/mL (66%) and it was 21.7 ng/mL (69%) when the patient requested oxycodone again. At both time points, the concentrations varied >100-fold between individuals. The analgesic oxycodone concentration was increased by 21.3% per motion pain intensity score on a 0-10 scale and by 22.3% if axillary clearance was performed instead of sentinel node biopsy (P < 0.001). Forty-seven women who were older and less anxious than others (P < 0.01) required no oxycodone. Anxiety, age, chronic pain, or preoperative pain sensitivity were not independently associated with the analgesic oxycodone concentration. CYP2D6 and CYP3A genotypes did not affect analgesic concentration or duration of analgesia.

Perspectives on Intravenous Oxycodone for Control of Postoperative Pain

Intravenous (IV) analgesia has particular advantages in the immediate postoperative period. For example, IV administration results in a faster onset of pain relief and results in more predictable pharmacokinetics than does administration by other routes. It also allows for convenient dosing before or during surgery, permitting the initiation of effective analgesia in the early phase of the postoperative period. In addition, when patients are able to tolerate oral intake, they can be switched from IV to oral dosing based on maintaining the predictable analgesia established by the IV route. IV morphine is widely used for the control of postoperative pain, but there is a trend toward the use of oxycodone. Oxycodone (which may be mediated partly through kappa- as well as mu-opioid receptors) offers several potential advantages. Published studies comparing IV oxycodone to other IV opioids for postsurgical pain report that oxycodone is a safe and effective analgesic. Some studies show that IV oxycodone may be associated with greater pain control, fewer or less severe adverse events, and faster onset of action, although the results are not consistent across all studies. Oxycodone has been reported to be safe in the geriatric and other special populations when adequate clinical adjustments are made. Thus, the clinical reports and oxycodone's pharmacologic profile make intravenous oxycodone a potentially important "new" old drug for postoperative pain control.

Pharmacokinetics of Sublingual Buprenorphine and Naloxone in Subjects with Mild to Severe Hepatic Impairment (Child-Pugh Classes A, B, and C), in Hepatitis C Virus-Seropositive Subjects, and in Healthy Volunteers

BACKGROUND AND OBJECTIVES

Suboxone(®) is a sublingual tablet of buprenorphine/naloxone, approved for the treatment of opioid dependence. The objective of this study was to quantify the impact of hepatic impairment or hepatitis C virus infection on the pharmacokinetics of buprenorphine or naloxone and their major metabolites.

METHODS

Forty-three subjects received a single dose of a Suboxone 2.0/0.5-mg tablet. Blood samples were collected up to 168 h and pharmacokinetic parameters were calculated using non-compartmental analysis. Statistical analysis was performed using analysis of covariance.

RESULTS

Pharmacokinetic parameters were derived from 33 subjects. Compared with healthy subjects, for patients with severe hepatic impairment, total and peak exposures increased to 281.4 % [90 % confidence interval 187.1-423.3] and 171.8 % [117.9-250.2] for buprenorphine, 1401.9 % [707.6-2777.5] and 1129.8 % [577.2-2211.4] for naloxone. For moderate hepatic impaired subjects, naloxone total and peak exposure increased to 317.6 % [164.9-611.5] and 270.0  % [141.9-513.9]. For buprenorphine, only total exposure increased to 163.9 % [110.8-242.3]. Changes in maximum observed plasma concentration, area under the plasma concentration-time curve from time zero to time of the last quantifiable concentration, and area under the plasma concentration-time curve from time zero to infinity of buprenorphine or naloxone in subjects with mild hepatic impairment or with hepatitis C virus infection were within twofold of those of healthy subjects. Serious adverse events were not observed.

CONCLUSIONS

Severe and moderate hepatic impairment significantly increased exposure of naloxone and to a lesser extent of buprenorphine. Therefore, buprenorphine/naloxone combination products should generally be avoided in patients with severe hepatic impairment and may not be appropriate for patients with moderate hepatic impairment. However, buprenorphine/naloxone products may be used with caution for maintenance treatment in patients with moderate hepatic impairment who have initiated treatment on a buprenorphine product without naloxone [Registered at ClinicalTrials.gov as NCT01846455].

Cytochrome P450-mediated changes in oxycodone pharmacokinetics/pharmacodynamics and their clinical implications

In recent years the use of the opioid oxycodone has increased markedly and replacing morphine as the first-line choice of opioid in several countries. There are formulations for oral immediate, oral extended release and intravenous use. The bioavailability is higher than for morphine and less variable. Oxycodone is primarily metabolized in the liver by the cytochrome P450 (CYP) enzymes with CYP3A as the major metabolic pathway and CYP2D6 as the minor metabolic pathway to noroxycodone, oxymorphone and noroxymorphone. Oxycodone exerts its analgesic effect via the µ-opioid receptor. The metabolism of CYP2D6 substrates varies to a large degree between individuals as a result of allele functionality. Poor metabolizers (PM) have two non-functional alleles, extensive metabolizers (EM) are homozygous with two functional alleles or heterozygous with one functional allele and ultrarapid metabolizers (UM) have more than two functional alleles. There are pronounced interethnic differences in the allele distribution. On the basis of studies performed thus far, oxycodone concentrations in comparison with EM are similar in PM and reduced in UM. The pharmacokinetics in UM are insufficiently investigated. Simultaneous inhibition of both CYP3A and CYP2D6 results in increased oxycodone concentrations and such a combination should be avoided. A similar effect is to be expected with use of a CYP3A inhibitor in CYP2D6 PM. Concomitant use of enzyme inducers such as rifampicin, St John's wort and carbamazepine should be avoided because of the risk of subtherapeutic concentrations of oxycodone. When the dosage of morphine may result in unpredictable bioavailability, like in patients with severe hepatic cirrhosis, oxycodone might be beneficial because it has higher and less variability in bioavailability between patients than morphine.

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.

Cancer cachexia raises the plasma concentration of oxymorphone through the reduction of CYP3A but not CYP2D6 in oxycodone-treated patients

This study evaluated the plasma concentrations of oxycodone and its demethylates and opioid-induced adverse effects based on cachexia stage in cancer patients receiving oxycodone. Seventy patients receiving oxycodone for cancer pain were enrolled. Cachexia was evaluated using the Glasgow Prognostic Score (GPS). Predose plasma concentrations of oxycodone, oxymorphone, and noroxycodone were determined at the titration dose. Opioid-induced adverse effects were monitored for 2 weeks after the titration. Plasma concentrations of oxycodone and oxymorphone but not noroxycodone in patients with a GPS of 2 were significantly higher than that with a GPS of 0. The metabolic ratios of noroxycodone but not oxymorphone to oxycodone in patients with a GPS of 1 and 2 were significantly lower than in those with a GPS of 0. A higher GPS was associated with a higher incidence of somnolence, while the GPS did not affect the incidence of vomiting. Plasma concentrations of oxycodone and oxymorphone were not associated with the incidence of adverse effects. In conclusion, cancer cachexia raised the plasma exposures of oxycodone and oxymorphone through the reduction of CYP3A but not CYP2D6. Although the cachexia elevated the incidence of somnolence, alterations in their pharmacokinetics were not associated with the incidence.

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.

Pharmacogenomic considerations in opioid analgesia

Translating pharmacogenetics to clinical practice has been particularly challenging in the context of pain, due to the complexity of this multifaceted phenotype and the overall subjective nature of pain perception and response to analgesia. Overall, numerous genes involved with the pharmacokinetics and dynamics of opioids response are candidate genes in the context of opioid analgesia. The clinical relevance of CYP2D6 genotyping to predict analgesic outcomes is still relatively unknown; the two extremes in CYP2D6 genotype (ultrarapid and poor metabolism) seem to predict pain response and/or adverse effects. Overall, the level of evidence linking genetic variability (CYP2D6 and CYP3A4) to oxycodone response and phenotype (altered biotransformation of oxycodone into oxymorphone and overall clearance of oxycodone and oxymorphone) is strong; however, there has been no randomized clinical trial on the benefits of genetic testing prior to oxycodone therapy. On the other hand, predicting the analgesic response to morphine based on pharmacogenetic testing is more complex; though there was hope that simple genetic testing would allow tailoring morphine doses to provide optimal analgesia, this is unlikely to occur. A variety of polymorphisms clearly influence pain perception and behavior in response to pain. However, the response to analgesics also differs depending on the pain modality and the potential for repeated noxious stimuli, the opioid prescribed, and even its route of administration.

Is oxycodone efficacy reflected in serum concentrations? A multicenter, cross-sectional study in 456 adult cancer patients

CONTEXT

The relationship between oxycodone and metabolite serum concentrations and clinical effects has not previously been investigated in cancer pain patients.

OBJECTIVES

The aim of this study was to assess whether there is a relationship between oxycodone concentrations and pain intensity, cognitive functioning, nausea, or tiredness in cancer patients. Also, oxymorphone and noroxymorphone contributions to analgesia and the adverse effects of oxycodone were assessed.

METHODS

Four hundred fifty-six cancer patients receiving oxycodone for cancer pain were included. Pain was assessed using the Brief Pain Inventory. The European Organisation for Research and Treatment of Cancer Quality-of-Life Questionnaire-C30 was used to assess the symptoms of tiredness, nausea, constipation, and depression. Cognitive function was assessed using the Mini-Mental State Examination. Associations were examined by multiple linear or ordinal logistic regressions. Whether patients classified as being a "treatment success" or a "treatment failure" had different serum concentrations of oxycodone or metabolites was assessed using Mann-Whitney U-tests.

RESULTS

Serum concentrations of oxycodone and metabolites were not associated with pain intensity, nausea, tiredness, or cognitive function, with the exception that increased pain intensity was associated with higher oxymorphone concentrations. Patients with poor pain control and side effects had higher serum concentrations of the oxycodone metabolites, noroxycodone and noroxymorphone, compared with those with good pain relief and without side effects.

CONCLUSION

This study of patients receiving oxycodone for cancer pain confirms previous observations that there is most likely no association between serum concentrations of opioid analgesics and clinical effects.

The metabolism of opioid agents and the clinical impact of their active metabolites

BACKGROUND

The metabolism of opioids is critical to consider for multiple reasons. The most commonly prescribed opioid agents often have metabolites that are active and are the source of both analgesic activity and an increased incidence of adverse events. Many opioids are metabolized by cytochrome P450 enzymes. Polymorphisms in cytochrome P450 genes and inhibition or induction of cytochrome P450 enzymes by coadministered drugs may significantly impact the systemic concentration of opioids and their metabolites and the associated efficacy or adverse events.

METHODS

This is a narrative review of the metabolism of various opioids that will highlight the impact of their active metabolites, and the potential impact of cytochrome P450 activity on analgesic activity.

RESULTS

An understanding of "opioid metabolic machinery," cytochrome P450 activity, and drug-drug interactions in the context of opioid selection may benefit clinicians and patients alike.

CONCLUSIONS

A greater appreciation of the metabolism of commonly prescribed opioid analgesics and the impact of their active metabolites on efficacy and safety may aid prescribers in tailoring care for optimal outcomes.

Grapefruit juice enhances the exposure to oral oxycodone

Grapefruit juice alters the concentrations of many CYP3A substrates. The objective of this study was to examine the effect of grapefruit juice on the pharmacokinetics and pharmacodynamics of oral oxycodone in a randomized cross-over study with two phases at an interval of 4 weeks. Twelve healthy volunteers ingested 200 ml of grapefruit juice or water t.i.d. for 5 days. An oral dose of oxycodone hydrochloride 10 mg was administered on day 4. Oxycodone, noroxycodone, oxymorphone and noroxymorphone concentrations were analysed from the plasma samples for 48 hr and behavioural and analgesic effects were recorded for 12 hr. Grapefruit juice increased the mean area under the oxycodone concentration-time curve (AUC(0-∞) ) by 1.7-fold (p<0.001), the peak plasma concentration by 1.5-fold (p<0.001) and the half-life of oxycodone by 1.2-fold (p<0.001) as compared to the water. The metabolite-to-parent AUC(0-∞) ratios (AUC(m)/AUC(p) ) of noroxycodone and noroxymorphone decreased by 44% (p<0.001) and 45% (p<0.001), respectively. Oxymorphone AUC(0-∞) increased by 1.6-fold (p<0.01) after grapefruit juice, but the AUC(m)/AUC(p) remained unchanged. Pharmacodynamic changes were modest and only self-reported performance significantly impaired after grapefruit juice. Analgesic effects were not influenced. Grapefruit juice inhibited the CYP3A4-mediated first-pass metabolism of oxycodone, decreased the formation of noroxycodone and noroxymorphone and increased that of oxymorphone. We conclude that dietary consumption of grapefruit products may increase the concentrations and effects of oxycodone in clinical use.

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 combinations for pain relief

No single analgesic drug provides the perfect therapeutic/adverse effect profile for every pain condition. In addition to convenience and possibly improved compliance, a combination of analgesic drugs offers the potential, requiring verification, of providing greater pain relief and/or reduced adverse effects than the constituent drugs when used individually. We review here analgesic combinations containing oxycodone. We found surprisingly little preclinical information about the analgesic or adverse effect profiles of the combinations (with acetaminophen, paracetamol, nonsteroidal anti-inflammatory drugs, morphine, gabapentin or pregabalin). Clinical experience and studies suggest that the combinations are safe and effective and may offer certain advantages. As with all combinations, the profile of adverse effects must also be determined in order to provide the clinician with the overall benefit/risk assessment.

Impact of the CYP2D6 genotype on post-operative intravenous oxycodone analgesia

BACKGROUND

Oxycodone is a semi-synthetic opioid with a mu-receptor agonist-mediated effect in several pain conditions, including post-operative pain. Oxycodone is metabolized to its active metabolite oxymorphone by O-demethylation via the polymorphic CYP2D6. The aim of this study was to investigate whether CYP2D6 poor metabolizers (PMs) yield the same analgesia post-operatively from intravenous oxycodone as extensive metabolizers (EMs).

METHODS

Two hundred and seventy patients undergoing primarily thyroid surgery or hysterectomy were included and followed for 24 h post-operatively. The CYP2D6 genotype was blinded until study procedures had been completed for all patients. All patients received intravenous oxycodone as pain treatment for 24 h post-operatively and morphine 5 mg was used as escape medication. A responder was characterized as a patient without the need for escape medication and a positive evaluation in a questionnaire 24 h post-operatively.

RESULTS

Twenty-four patients were PM (8.9%) and 246 were EM (91.1%). One PM (4.17%, CI=0.1-21.1) was a non-responder and 42 EM (17.07%, CI=12.6-22.4) were non-responders. The non-responder rate did not differ between the two genotypes (P=0.14). There was no difference in the total consumption of oxycodone between the two genotypes (EM=14.7 mg, CI=13.0-16.4 and PM=13.0 mg, CI=8.9-17.0, P=0.42). The mean oxymorphone/oxycodone ratios were 0.0031 and 0.00081 in the EMs and PMs, respectively (P<0.0001).

CONCLUSION

This study showed for the first time in patients that the oxymorphone formation depends on CYP2D6, but we found no difference in the post-operative analgesic effect of intravenous oxycodone between the two CYP2D6 genotypes.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The hypoalgesic effect of oxycodone in human experimental pain models in relation to the CYP2D6 oxidation polymorphism

Oxycodone is O-demethylated by CYP2D6 to oxymorphone which is a potent micro-receptor agonist. The CYP2D6 oxidation polymorphism divides the Caucasian population in two phenotypes: approximately 8% with no enzyme activity, poor metabolizers (PM) and the remainder with preserved CYP2D6 activity, extensive metabolizers (EM). The objective of the study was to determine if the analgesic effect of oxycodone in human experimental pain depends on its metabolism to oxymorphone. The analgesic effect of oxycodone was evaluated in a randomized, placebo-controlled, double-blinded, crossover experiment including 33 (16 EM and 17 PM) healthy volunteers. Pain tests were performed before and 1, 2, 3 and 4 hr after medication and included pain detection and tolerance thresholds to single electrical sural nerve stimulation, pain summation threshold to repetitive electrical sural nerve stimulation and the cold pressor test with rating of discomfort and pain-time area under curve (AUC(0-2 min.)). For single sural nerve stimulation, there was a less pronounced increase in thresholds on oxycodone in pain detection (9% vs. 20%, P = 0.02, a difference of 11%, CI: 2%-20%) and pain tolerance thresholds (15% vs. 26%, P = 0.037, a difference of 10%, CI: 1%-20%) for PM compared with EM. In the cold pressor test, there was less reduction in pain AUC on oxycodone for PM compared with EM (14% vs. 26%, P = 0.012, a difference of 12%, CI: 3%-22%). The plasma oxymorphone/oxycodone ratio was significantly lower in PM compared with EM (P < 0.001). Oxycodone analgesia seems to depend both on oxycodone itself and its metabolite oxymorphone.