Analgesic effectiveness of the narcotic agonist-antagonists

Progress in agonist therapy for substance use disorders: Lessons learned from methadone and buprenorphine

Substance use disorders (SUD) are serious public health problems worldwide. Although significant progress has been made in understanding the neurobiology of drug reward and the transition to addiction, effective pharmacotherapies for SUD remain limited and a majority of drug users relapse even after a period of treatment. The United States Food and Drug Administration (FDA) has approved several medications for opioid, nicotine, and alcohol use disorders, whereas none are approved for the treatment of cocaine or other psychostimulant use disorders. The medications approved by the FDA for the treatment of SUD can be divided into two major classes - agonist replacement therapies, such as methadone and buprenorphine for opioid use disorders (OUD), nicotine replacement therapy (NRT) and varenicline for nicotine use disorders (NUD), and antagonist therapies, such as naloxone for opioid overdose and naltrexone for promoting abstinence. In the present review, we primarily focus on the pharmacological rationale of agonist replacement strategies in treatment of opioid dependence, and the potential translation of this rationale to new therapies for cocaine use disorders. We begin by describing the neural mechanisms underlying opioid reward, followed by preclinical and clinical findings supporting the utility of agonist therapies in the treatment of OUD. We then discuss recent progress of agonist therapies for cocaine use disorders based on lessons learned from methadone and buprenorphine. We contend that future studies should identify agonist pharmacotherapies that can facilitate abstinence in patients who are motivated to quit their illicit drug use. Focusing on those that are able to achieve abstinence from cocaine will provide a platform to broaden the effectiveness of medication and psychosocial treatment strategies for this underserved population. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Buprenorphine prescription, misuse and service provision: a global perspective

Buprenorphine, a partial μ-opioid agonist and κ-opioid antagonist, is recommended as safe and effective maintenance treatment for opioid dependence. It offers the possibility of management in primary care settings. However, its prescription has led to diversion for illicit recreational use and resulted in medical complications and, rarely, fatal overdose in combination with other sedatives. The outcome of buprenorphine maintenance programmes varies from country to country and it is determined by the local therapeutic traditions, regulatory restrictions and existing service provision for opioid misusers. This article addresses the pharmacology of buprenorphine, the benefits and drawbacks of its prescription, service provision for opioid misuse around the world, policy recommendations, and prescribing training requirements.

The electroacupuncture-induced analgesic effect mediated by 5-HT1, 5-HT3 receptor and muscarinic cholinergic receptors in rat model of collagenase-induced osteoarthritis

BACKGROUND

Osteoarthritis (OA) is an degenerative disease characterized by chronic joint pain. Complementary and alternative treatment such as acupuncture have been utilized to alleviate pain. The objective of this study was to investigate the analgesic mechanisms of electroacupuncture (EA) in the collagenase-induced osteoarthritis (CIOA) rat model.

METHODS

Four weeks after inducing CIOA by injecting collagenase solution into the left knee of 5-week-old male Sprague-Dawley rats, 2 Hz and 100 Hz EA on Zusanli (ST 36) was performed. The analgesic effect of EA was evaluated by the tail flick latency (TFL) and paw pressure threshold (PPT) tests. To investigate the analgesic mechanism, serotonergic and muscarinic cholinergic receptor agonists and antagonists were injected 20 min prior to EA and the resultant changes were evaluated by the TFL and PPT tests.

RESULTS

EA on Zusanli (ST 36) demonstrated an analgesic effect in the CIOA rat model. The 2 Hz EA treatment showed a significantly greater analgesic effect than the 100 Hz treatment. The analgesic effect of 2 Hz EA was not strengthened by 5-HT1, 5-HT2, 5-HT3, and muscarinic cholinergic receptor agonist pretreatment, was blocked by 5-HT1, 5-HT3, and muscarinic cholinergic receptor antagonist pretreatment, but not blocked by 5-HT2 receptor antagonist pretreatment.

CONCLUSIONS

In the CIOA rat model, EA on Zusanli (ST 36) exhibited analgesic effects, and 2 Hz EA resulted in a significantly greater analgesic effect than 100 Hz EA. The analgesic effect of 2 Hz EA was reduced by pretreatment of 5-HT1 receptor, 5-HT3 receptor and muscarinic cholinergic receptor antagonists.

Pharmacogenetics of chronic pain management

OBJECTIVE

The experience of chronic pain is one of the commonest reasons individuals seek medical attention, making the management of chronic pain a major issue in clinical practice. Drug metabolism and responses are affected by many factors, with genetic variations offering only a partial explanation of an individual's response. There is a paucity of evidence for the benefits of pharmacogenetic testing in the context of pain management.

DESIGN AND METHODS

We reviewed the literature between 2000 and 2013, and references cited therein, using various keywords related to pain management, pharmacology and pharmacogenetics.

RESULTS

Opioids continue to be the mainstay of chronic pain management. Several non-opioid based therapies, such as treatment with cannabinoids, gene therapy and epigenetic-based approaches are now available for these patients. Adjuvant therapies with antidepressants, benzodiazepines or anticonvulsants can also be useful in managing pain. Currently, laboratory monitoring of pain management patients, if performed, is largely through urine drug measurements.

CONCLUSIONS

Drug half-life calculations can be used as functional markers of the cumulative effect of pharmacogenetics and drug-drug interactions. Assessment of half-life and therapeutic effects may be more useful than genetic testing in preventing adverse drug reactions to pain medications, while ensuring effective analgesia. Definitive, mass spectrometry-based methods, capable of measuring parent drug and metabolite levels, are the most useful assays for this purpose. Urine drug measurements do not necessarily correlate with serum drug concentrations or therapeutic effects. Therefore, they are limited in their use in monitoring efficacy and toxicity.

75 years of opioid research: the exciting but vain quest for the Holy Grail

Over the 75-year lifetime of the British Pharmacological Society there has been an enormous expansion in our understanding of how opioid drugs act on the nervous system, with much of this effort aimed at developing powerful analgesic drugs devoid of the side effects associated with morphine--the Holy Grail of opioid research. At the molecular and cellular level multiple opioid receptors have been cloned and characterised, their potential for oligomerisation determined, a large family of endogenous opioid agonists has been discovered, multiple second messengers identified and our understanding of the adaptive changes to prolonged exposure to opioid drugs (tolerance and physical dependence) enhanced. In addition, we now have greater understanding of the processes by which opioids produce the euphoria that gives rise to the intense craving for these drugs in opioid addicts. In this article, we review the historical pathway of opioid research that has led to our current state of knowledge.

Opioid dependence treatment, including buprenorphine/naloxone

OBJECTIVE

To review opioid dependence (OD) and its treatment. Pharmacologic treatments, including the use of buprenorphine/naloxone, are presented. Pharmaceutical care functions for outpatient OD treatment are discussed.

DATA SOURCES

Primary and review articles were identified by MEDLINE and HEALTHSTAR searches (from 1966 to November 2000) and through secondary sources. Tertiary sources were also reviewed regarding general concepts of OD and its treatment.

STUDY SELECTION/DATA EXTRACTION

Relevant articles were reviewed after identification from published abstracts. Articles were selected based on the objectives for this article. Studies of the treatment of OD with buprenorphine were selected based on the topic (pharmacology, pharmacokinetics, adverse reactions) and study design (randomized, controlled clinical trials in patients with OD with active/placebo comparisons and/or comparisons of active OD treatments). Articles regarding pharmacists' activities in the treatment and prevention of OD were reviewed for the pharmaceutical care section.

DATA SYNTHESIS

OD is considered a medical disorder with costly adverse health outcomes. Although methadone maintenance treatment (MMT) is cost-effective for OD, only about 12% of individuals with OD receive this treatment. Psychological and pharmacologic modalities are used to treat OD, but patients often relapse. Drug therapy includes alpha 2-agonists for withdrawal symptoms, detoxification regimens with or without opioids, opioid antagonists, and opioid replacement including methadone, levomethadyl acetate, and buprenorphine. The Drug Addiction Treatment Act of 1999 allows for office-based opioid replacement therapies. Sublingual buprenorphine with naloxone can be used in this milieu. Buprenorphine with naloxone is currently under new drug application review with the Food and Drug Administration. Clinical research shows buprenorphine to be equal in effectiveness to methadone, but safer in overdose due to its ceiling effect on respiratory depression. It has lower abuse potential and fewer withdrawal symptoms when discontinued. Naloxone is included to decrease diversion and injection of the tablets. Pharmacists in outpatient settings who are familiar with OD have opportunities to provide pharmaceutical care to patients receiving this treatment. Pharmaceutical care functions for OD include ensuring appropriate drug administration, monitoring adverse effects, alleviating withdrawal symptoms, treating intercurrent illnesses, minimizing diversion, and preventing relapse.

CONCLUSIONS

OD is a critical unmet health problem in the US. Buprenorphine combined with naloxone represents an innovative treatment for OD in outpatient settings. This new treatment has advantages over MMT.

Opioids and their proper use as analgesics in the management of head and neck cancer patients

Opioid analgesics play an important part in the interdisciplinary management of head and neck cancer patients. Acute pain during the initial treatment period and chronic pain after treatment may be undertreated by head and neck surgeons because of unwarranted fear of the opioid side effects and toxicities. Properly used, opioid analgesics are among the safest and most effective medications in the armamentarium of the head and neck surgeon. Knowledge of opioid pharmacology can help surgeons treat pain aggressively in patients while managing side effects and toxicities. This understanding can also address the unique problems in pain management presented in head and neck cancer patients attributable to the involvement by the disease of the aerodigestive tract, which interferes with the oral route of administration of opioid analgesics. Significant difficulties in the use of opioid analgesics arise when the patient is a substance abuser. This article discusses the basic principles of opioid pharmacology, discusses opioid use in head and neck cancer patients, and provides guidelines on the use of opioids in the substance abuser.

14-methoxymetopon, a potent opioid, induces no respiratory depression, less sedation, and less bradycardia than sufentanil in the dog

Opioids of the mu-receptor type depress respiration and induce addiction. At 10-min intervals 14-methoxymetopon (HS-198), which is 20,000 times more potent than morphine in the acethylcholine-writhing test, was given in graded IV doses (3, 6, and 12 microg/kg) to awake, trained canines (n = 7). The following variables were derived: PaO(2), PaCO(2), heart rate (lead II of the electrocardiogram), mean arterial blood pressure, relative changes in the delta domain and the beta domain of the electroencephalogram, the somatosensory evoked potential, and the skin-twitch reflex to electrical stimuli. Thereafter, 20 microg/kg naltrexone was given for reversal. After a washout period, the same animals were exposed to similar doses of sufentanil (SUF) followed by naltrexone. Both opioids induced a dose-related bradycardia and hypotension. The maximal bradycardic effect was 19% after HS-198 and 42% after SUF (P < 0.005). The maximal hypotension was 6% after HS-198 and 20% after SUF (P < 0.01). In the electroencephalogram, power in the delta band increased by 288% after HS-198 and by 439% after SUF (P < 0.01); simultaneously, power in the beta band decreased by 71% and by 95.7%, respectively (P < 0.01). PaO(2) decreased by 41% after SUF and by 4% after HS-198, and PaCO(2) increased by 56.8% and 6.6% in SUF and HS-198, respectively (P < 0.001). Both opioids induced a dose-related depression in the somatosensory evoked potential and increased tolerance to skin-twitch. The maximal effect was 92.7% after SUF and 81.3% after HS-198 was not significant. Naltrexone reversed all changes back to control. Compared with SUF, HS-198 does not induce hypoxia and hypercarbia, induces less hypotension and bradycardia, and induces less sedative effects.

IMPLICATIONS

Compared with sufentanil, 14-methoxymetopone does not induce hypoxia and hypercarbia, induces less hypotension and bradycardia, and induces less sedative effects (electroencephalogram). Antinociception is similar to sufentanil (skin-twitch method, amplitude depression in the evoked potential). All effects are reversed by naltrexone. Interaction of kappa-receptor is suggested.

Transnasal butorphanol for the treatment of opioid-induced pruritus unresponsive to antihistamines

Pruritus is a common opioid side effect and can be so severe that opioid therapy must be modified or abandoned. Antihistamines, opioid antagonists, and propofol have been proposed as treatment options, but none is universally effective. The use of intranasal butorphanol, an opioid agonist-antagonist, for pruritus has not been described previously. Six patients complaining of severe opioid-induced pruritus unresponsive to diphenhydramine received 2 mg intranasal butorphanol every 4-6 hr. Scores for pruritus, pain, and sedation were recorded on separate visual analogue scales (VAS). All patients reported significant relief from pruritus 60 min after butorphanol administration (P < 0.001); five patients noted an improvement within 15 min (P < 0.08). Sedation and pain VAS scores were not significantly different from baseline at all time points. These preliminary data demonstrate a substantial effect of intranasal butorphanol on opioid-induced pruritus that has not responded to antihistamines. Prospective controlled studies are needed to validate these findings.

Opioid analgesics: comparative features and prescribing guidelines

The term 'opioid' is a generic term for naturally occurring, semisynthetic and synthetic drugs which combine with opioid receptors to produce physiological effects and which are stereospecifically antagonised by naloxone. For clinical purposes, opioids can be classified according to their receptor interactions (agonist, partial agonist, agonist-antagonist and antagonist), the pain intensity for which they are conventionally used (moderate or severe), and their half-life (short or long). Pure agonists conventionally used for moderate pain, short and long half-life pure agonists conventionally used for severe pain, mixed agonist-antagonists and partial agonist opioids are described in detail. The effective clinical use of opioid drugs requires familiarity with drug selection, routes of administration, dosage guidelines and potential adverse effects. Opioids are unequivocally indicated in the management of severe acute pain and moderate to severe pain associated with cancer. There is increasing acceptance of the role of opioids in the management of recurring acute pain, chronic nonmalignant pain of organic origin and severe neuropathic pain. The selection of opioids is influenced by pain intensity, pharmacokinetic and formulary considerations, previous adverse effects and the presence of coexisting disease. Some patients will require sequential trials of several different opioids before a drug which is effective and well tolerated is identified. Opioid agents should be administered by the most comfortable and convenient route that meets the specific needs of the patient. The regimen for opioid medications should generally provide around-the-clock analgesia with provision for rescue doses for the management of exacerbations of the pain not covered by the regular dosage. At all times, uncontrolled pain should be addressed by gradual increase in the opioid dose until either pain control is achieved or intolerable and unmanageable adverse effects supervene. The management of pain with opioid analgesics demands frequent patient assessment and a readiness to re-evaluate the therapeutic plan in the setting of either inadequate relief or adverse effects.

Transnasal butorphanol. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in acute pain management

Butorphanol is a synthetic opioid agonist-antagonist analgesic with a pharmacological and therapeutic profile that has been well established since its launch as a parenteral formulation in 1978. The introduction of a transnasal formulation of butorphanol represents a new and noninvasive presentation of an analgesic for moderate to severe pain. This route of administration bypasses the gastrointestinal tract, and this is an advantage for a drug such as butorphanol that undergoes significant first-pass metabolism after oral administration. The onset of action and systemic bioavailability of butorphanol following transnasal delivery are similar to those after parenteral administration. The analgesic efficacy of transnasal butorphanol was generally superior to that of placebo in clinical trials in patients with moderate to severe postoperative pain or migraine headache. Results from single trials indicate that transnasal butorphanol provides pain relief comparable to that of intramuscular pethidine (meperidine) in postsurgical pain and comparable to or greater than intramuscular methadone in migraine headache. Moderate to severe musculoskeletal pain also appears to be responsive to transnasal butorphanol on the basis of results from 1 small noncomparative study. Tolerability of transnasal butorphanol parallels that of the injectable form, with somnolence, dizziness, nausea and/or vomiting reported most frequently. Thus, transnasal butorphanol is a novel formulation of an established analgesic which appears suitable for the short term treatment of moderate to severe pain, especially in an ambulatory setting. Transnasal butorphanol is likely to provide an alternative to oral opioid analgesics, particularly in the presence of nausea or vomiting, or to parenteral opioids when the oral route of administration is not appropriate.

[Drug therapy for tumor pain I. Properties of non-opioids and opioids.]

Analgesic pharmacotherapy represents one of the major approaches to the treatment of cancer pain, since it is used in almost every patient. A thorough evaluation of the physical and mental status of the patient and of the pain is as necessary as a sound understanding of the pharmacokinetic and pharmacodynamic characteristics of the analgesics selected. The World Health Organization (WHO) has issued a basic 3 stage progression for the treatment of cancer pain, the "WHO Analgesic Ladder". Assignment to the stages depends mainly on the intensity of the pain rather than on its specific aetiology. Mild to moderate pain is treated with non-opioid drugs; moderate to severe pain, with a combination of a "weak" opioid and a non-opioid; and "strong" opioids should be used in combination with a non-opioid in the case of severe pain. Adjuvant drugs can be added if specifically indicated. Nonopioid analgesics include non-acidic compounds, e. g. paracetamol and metamizole, and acidic non-opioids, e. g. acetylsalicylic acid and newer non-steroidal anti-inflammatory drugs (NSAID). In contrast to most of the opioid analgesics, they have a ceiling effect for analgesia. Addiction and tolerance are extremely rare concerns. Opioids can be subgrouped into "weak" (e. g., codeine, dextropropoxyphene) and "strong" opioids (e. g., morphine) and also into drugs interacting with different opioid-receptor subtypes. Whereas pure agonists (e. g., morphine) produce increasingly intense analgesia with increasing dose, partial agonists and agonist-antagonists have a ceiling effect for analgesia and therefore have only a minor role in the treatment of chronic pain in cancer patients. Adverse effects occur in most patients in a dose-dependent manner. The most common of these is constipation; nausea, vomiting and sedation occur mostly at the start and can usually be treated effectively. The appropriate dosage, route of administration and dosage scheme of analgesics needs to be worked out for each individual patient in intensive work with the patient and a close follow-up, for years if necessary. Some analgesics may not be available in some countries, or only in specific preparations.

Propiram. A review of its pharmacodynamic and pharmacokinetic properties, and clinical use as an analgesic

Propiram is an orally administered opioid analgesic with partial morphine-like agonist and weak antagonist properties. Analgesic efficacy of propiram, usually 50 or 100mg, appears comparable to that of standard dosages of other oral opioid drugs [i.e. pentazocine, pethidine (meperidine)] in patients with acute pain of moderate to severe intensity arising from various gynaecological and surgical procedures, and may be superior to codeine in gynaecological and postoperative dental pain. Some evidence of a more rapid onset of action for propiram than for these opioid agents, and a longer duration of action for propiram than for codeine, is encouraging but remains to be substantiated in more extensive clinical use. The tolerability profile of propiram resembles those of others in its class, with drowsiness, nausea and vomiting, and dizziness experienced most frequently in controlled trials. The apparently low propensity of propiram for development of physical dependence and psychotomimetic effects requires confirmation with wider clinical experience. Available data thus indicate that propiram is an effective, orally administered opioid analgesic suitable for providing relief of acute moderate to severe pain arising from various surgical or gynaecological procedures, and that the drug is likely to become a useful alternative in such conditions where opioid analgesia is appropriate.

Opioid agonist-antagonist drugs in acute and chronic pain states

The agonist-antagonist opioid analgesics are a heterogeneous group of drugs with moderate to strong analgesic activity comparable to that of the pure agonist opioids such as codeine and morphine but with a limited effective dose range. The group includes drugs which act as an agonist or partial agonist at one receptor and an antagonist at another (pentazocine, butorphanol, nalbuphine, dezocine) and drugs acting as a partial agonist at a single receptor (buprenorphine). These drugs can be classified as nalorphine-like or morphine-like. Meptazinol does not fit into either classification and occupies a separate category. Pentazocine, butorphanol and nalbuphine are weak mu-antagonists and kappa-partial-agonists. All three drugs are strong analgesics when given by injection: pentazocine is one-sixth to one-third as potent as morphine, nalbuphine is slightly less potent than morphine, and butorphanol is 3.5 to 7 times as potent. The duration of analgesia is similar to that of morphine (3 to 4 hours). Oral pentazocine is closer in analgesic efficacy to aspirin and paracetamol (acetaminophen) than the weak opioid analgesics such as codeine. Neither nalbuphine nor butorphanol is available as an oral formulation. At usual therapeutic doses nalbuphine and butorphanol have respiratory depressant effects equivalent to that of morphine (though the duration of such effects with butorphanol may be longer). Unlike morphine there appears to be a ceiling to both the respiratory depression and the analgesic action. All of these 3 drugs have a lower abuse potential than the pure agonist opioid analgesics such as morphine. However, all have been subject to abuse and misuse, and pentazocine (but not the others) is subject to Controlled Drug restrictions. Buprenorphine is a potent partial agonist at the mu-receptor, and by intramuscular injection is 30 times as potent as morphine. A ceiling to the analgesic effect of buprenorphine has been demonstrated in animals and it is also claimed in humans. However, there are no reliable data available to define the maximal dose of buprenorphine in humans. A practical ceiling exists for sublingual use in that the only available formulation is a 2 micrograms tablet and few patients will accept more than 3 or 4 of these in a single dose. The duration of analgesia is longer than that of morphine, at 6 to 9 hours. There have been suggestions that buprenorphine causes less respiratory depression than morphine, but viewed overall it appears that in equianalgesic doses the 2 drugs have similar respiratory depressant effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Intravenous buprenorphine reduces pupil size and the light reflex in humans

The pupillary effects of intravenous buprenorphine were studied in eight nondependent male subjects who reported previous opiate use. Buprenorphine (0.3, 0.6, and 1.2 mg) decreased pupil size, the amplitude of the light reflex, and the velocities of constriction and dilation. Significant pupillary effects occurred within 15 min of the injection and persisted for 24 hr. At 48 hr most measures returned to baseline levels. Generally the magnitude of the effect was not dose related although recovery occurred sooner after the lower dose. The time course of the pupillary effects of buprenorphine exceeds duration of its analgesic and subjective effects. Previous studies have reported that pupillary measures are especially sensitive to the acute effects of full opiate agonists. The results of the present study indicate that buprenorphine, a partial opiate agonist, causes profound and persistent effects on pupillary size and dynamic measures.

Effects on opioid-induced rate reductions by doxepin and bupropion

Twelve pigeons key-pecked under a multiple variable interval 15-second, variable interval 150-second schedule of food reinforcement. Effects of two opioid drugs, buprenorphine and methadone, were determined alone and in combination with chronic daily administration of the antidepressants doxepin or bupropion. Methadone initially produced dose-dependent key-pecking rate reductions when administered acutely, prior to the session, while buprenorphine produced key-pecking rates that reached a plateau at 50-80% of baseline rate and were not reduced further by higher doses. Neither doxepin nor bupropion, given alone, had lasting effects on key-pecking rates. Chronic daily doxepin administration significantly attenuated methadone-induced response rate reductions. Bupropion reduced the effect of the highest methadone dose, but this effect was mitigated by the development of opioid tolerance. Unlike bupropion, doxepin interfered with the development of opioid tolerance. Neither antidepressant systematically altered effects of buprenorphine on key-pecking.

Treatment of cancer pain of the head and neck by continuous intravenous infusion of high-dose morphine: report of two cases

Two pain-ridden patients in the terminal stage of a malignancy of the head and neck were treated with high-dose morphine which was administered as a continuous IV infusion along with a hyperalimentation solution using a pump. Good pain relief and a lessening of anxiety were achieved after failure to control pain in spite of the use of the analgesic ladder recommended by the WHO.

Buprenorphine-induced pupillary effects in human volunteers

The pupillary effects of the partial opiate agonist buprenorphine were studied in 16 male subjects who were heroin dependent at the time of admission to the study. Sublingual buprenorphine (8 mg) was administered daily for 18 days and continued either daily or on alternate days from study days 19 through 36. On days 37 through 56, all subjects received buprenorphine placebo. Compared to placebo, buprenorphine decreased pupil size and diminished the constriction and dilation velocities of the light reflex. These effects occurred within 5 hours of buprenorphine administration. Following placebo administration during alternate-day dosing of buprenorphine, pupil size increased and constriction and dilation velocities of the light reflex were significantly greater than after buprenorphine administration in the same subjects. This pattern of effects was observed after buprenorphine was discontinued (day 37). The results indicated that buprenorphine has pupillary effects like those of full opiate agonists. The time course of these effects was similar to previously-reported effects of buprenorphine on the electroencephalogram but not to the time course of subjective effects.

Dezocine. A preliminary review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy

Dezocine is an analgesic agent with opioid agonist and antagonist activity. After parenteral administration of therapeutic doses it is approximately equipotent with morphine, and has proved at least as effective an analgesic as morphine, pethidine (meperidine) and butorphanol in moderate to severe postoperative pain. However, preliminary pharmacodynamic data indicate that the ceiling of analgesic activity of dezocine occurs at a higher level of analgesia than that of reference agonist/antagonist agents. Also, the drug exhibited a morphine-like degree of anaesthetic-sparing activity in animals. Although long term data are very limited, single doses of dezocine are well tolerated, with mild and transient sedation and gastrointestinal upset the principal adverse effects. As with some other agonist/antagonist analgesics, a 'ceiling' effect to dezocine-induced respiratory depression occurs with increasing dosage, beyond which further depression has not been observed. In single analgesic doses, however, dezocine is a slightly more potent respiratory depressant than morphine. Clinically important haemodynamic changes have not been observed with usual analgesic doses of dezocine. As an agonist/antagonist opioid, the dependence liability of dezocine would be expected to be lower than that of pure agonist opioids, but extended clinical use is required before more definitive conclusions can be drawn in this regard. Unlike older drugs of its type, dezocine produced opiate-like subjective effects and was identified as morphine-like by drug abusers. Thus, provided the promising conclusions of currently available clinical studies are confirmed with its wider use, dezocine should be a useful additional agent for the treatment of moderate to severe postoperative pain.

Narcotic analgesics, their detection and pain measurement in the horse: a review

Narcotic analgesics produce pharmacological effects by interacting with specific opiate receptors. At least five major types of opiate receptors have been recognised. These include mu (morphine) and kappa (ethylketazocine) receptor types. Narcotic analgesics which interact with mu receptors produce locomotor and autonomic stimulation at doses that produce little or no analgesia. Therefore, use of these drugs as analgesics in equine medicine has not been very satisfactory. Theoretical considerations suggested that the role of kappa agonists in equine analgesia be investigated. Using a pure kappa agonist, U-50, 488H, good analgesia was produced in the horse with little or no locomotor stimulation or autonomic effects. These data suggest that kappa agonists may be superior analgesics for clinical use in the horse. On the other hand, the locomotor stimulant effects of mu agonist analgesics enable their use as illegal medications. Specifically, these agents produce a good running response, signs of central nervous stimulation and analgesia, all potentially useful effects in a racehorse. Regulatory control of most narcotic analgesics can be obtained by high performance thin layer chromatographic screening. However, effective screening for the fentanyls and small doses of etorphine can only be achieved by use of immunoassay.

Pharmacokinetics and bioavailability of hydromorphone: effect of various routes of administration

The pharmacokinetics and bioavailability of hydromorphone following various routes of administration, i.e., intravenous, oral, intranasal, and transdermal, were investigated in rabbits. Hydromorphone plasma concentrations were determined by reverse-phase high-performance liquid chromatography (HPLC). Comparison of area under the concentration versus time curve (AUC) between intravenous and oral administrations showed a low bioavailability of hydromorphone after oral administration. The nasal absorption of hydromorphone was studied by the in situ nasal recirculation technique, and the results showed that hydromorphone is well absorbed from the nasal mucosa. The transdermal permeation of hydromorphone was also evaluated for 24 hr and a steady-state plasma concentration (0.135 micrograms/ml) was achieved during the 6- to 24-hr periods following the application of a transdermal patch on the inner pinna of the rabbit's ear.

Mode of potentiating action of cocaine in morphine analgesia

The mechanism of antinociceptive interactions among morphine, cocaine and alcohol was studied in mice, guinea pigs and rabbits. In the tail-pressure test in mice, cocaine and alcohol alone showed almost no antinociceptive effects at doses up to 8 mg/kg, s.c., and 4 g/kg, respectively. Alcohol at 2 g/kg, i.g., also did not influence the effect of morphine, while cocaine at 4 mg/kg, s.c., significantly potentiated the antinociceptive effects of not only morphine but also pentazocine. In an analysis of serum and brain concentration levels of morphine in mice, when morphine and cocaine were simultaneously administered at 2 mg/kg, s.c., and 4 mg/kg, s.c., respectively, both serum and brain levels of morphine showed neither increase nor decrease in comparison with the levels in mice administered morphine alone. In myenteric plexus-longitudinal muscle preparations of isolated guinea pig ileum, 1 microM cocaine enhanced the agonistic effects of both pentazocine and ethylketocyclazocine. Furthermore, cocaine as well as ethylketocyclazocine showed naloxone-reversible agonistic effects in isolated rabbit vas deferens. These results indicate that cocaine may potentiate the antinociceptive effects of morphine and pentazocine by acting on the kappa-opioid receptors as an agonist.

Progressive ratio performance maintained by buprenorphine, heroin and methadone in Macaque monkeys

The relative reinforcing efficacy of buprenorphine (0.01-0.10 mg/kg/inj.), heroin (0.01-0.10 mg/kg/inj.), methadone (0.03-0.25 mg/kg/inj.) and saline were compared in Macaque male monkeys using progressive ratio procedures. Responding for drugs and food (1 g banana pellet) was maintained on a second order FR 4 (VR 16:S) schedule of reinforcement that required an average of 64 responses for each injection or food pellet. After 40 sessions (10 days) of stable performance, the number of responses for each drug injection was systematically increased until monkeys reached a breakpoint defined by 2 consecutive days of no drug injections. Progressive ratio breakpoints for heroin at intermediate and high doses (0.05 and 0.10 mg/kg/inj.) were two to three times higher than for any other drug (P less than 0.05). Breakpoints for buprenophine (0.01-0.10 mg/kg/inj.) were higher than for a low dose of methadone (0.03 mg/kg/inj.) but were equivalent to an intermediate methadone dose (0.10 mg/kg/inj.). Breakpoints for both low (0.01 mg/kg/inj.) and high (0.10 mg/kg/inj.) buprenorphine doses were greater than for intermediate doses (0.03 and 0.05 mg/kg/inj.) but these differences were not statistically significant. Linear regression and area under the curve analyses were used to quantify the rate at which drug maintained responding decreased by 30% of control levels as response requirements were increased. Extrapolated breakpoints derived from linear regression analyses were significantly correlated (P less than 0.05-0.01) with actual breakpoints for buprenorphine, heroin and methadone. The concordance between 'extrapolated' and traditional breakpoint ranking of these drugs suggests that regression analysis techniques can be used to predict traditional breakpoints.

Opioid modulation of ingestive behaviors in the spiny mouse (Acomys cahirinus)

Feeding and drinking behavior were studied in deprived or sated spiny mice (Acomys cahirinus) at various time intervals following peripheral administration of naloxone hydrochloride and butorphanol tartrate. Naloxone attenuated both food and water intake, but not latency to respond, indicating existence of functional opioid-sensitive feeding and drinking systems in this species. Butorphanol tartrate, a mixed opioid agonist/antagonist produced a dose-related enhancement or suppression of feeding, the former naloxone reversible, but had no measureable effect on drinking.

The effectiveness of nalbuphine and hydroxyzine for the emergency treatment of severe headache

The present treatment for acute attacks of headache is empiric. Intramuscular nalbuphine (Nubain) and hydroxyzine (Vistaril) were assessed for pain relief in a prospective, double-blind clinical trial. Ninety-four patients were assigned randomly to treatment groups receiving nalbuphine 10 mg, nalbuphine 10 mg plus hydroxyzine 50 mg, hydroxyzine 50 mg, or placebo. The treatment groups were found to be adequately homogenous with regard to age, sex, type and duration of headaches, and history of prior narcotic use. All data were analyzed by one-way analysis of variance. Patients who had headaches diagnosed as other than classic migraine had significantly greater pain relief with nalbuphine compared to placebo (P less than .01). The combination of nalbuphine and hydroxyzine was not significantly more effective than other treatment groups. In 20 patients with classic migraine, none of the treatment regimens significantly outperformed placebo. There were no clinically significant adverse effects attributed to the study drugs. These findings are similar to others that showed a lack of efficacy of kappa receptor agonists in classic migraineurs. Nalbuphine appears to be clinically useful in other types of severe headache. This study does not support the routine addition of hydroxyzine for presumed synergistic effect.

The effects of the acute administration of buprenorphine hydrochloride on the release of anterior pituitary hormones in the rat: evidence for the involvement of multiple opiate receptors

Multiple opiate receptor agonists and antagonists have been found to produce different patterns of anterior pituitary hormone release. The present studies examined the pattern of anterior pituitary hormone release produced by buprenorphine. The effects of the kappa agonist ethylketocyclazocine on thyroid stimulating hormone release were also examined. Following buprenorphine, serum levels of corticosterone and luteinizing hormone were not changed while growth hormone release was stimulated in a dose-dependent manner. Prolactin release was stimulated after the lowest dose of buprenorphine while the highest dose induced a fall in serum prolactin. Similar biphasic effects on thyroid stimulating hormone were seen after either buprenorphine or ethylketocyclazocine. The results provide support for the role of multiple opiate receptors in opiate-induced changes in anterior pituitary hormone release.

The treatment of cancer pain

Pain is one of the most feared consequences of cancer. Control of pain from cancer should be possible with the approaches discussed above. Changing attitudes toward the effective use of narcotic analgesics, the development of novel routes and methods of administration, and a clinical approach based on scientific principles and humane care offer the promise of improved management of pain in patients with cancer.

Pharmacological advances in addiction treatment

In the past 20 years significant advances in the pharmacological treatment of opioid dependence have been made, and research in this area is continuing. Therapeutic applications and current research in the use of pharmacological agents in maintenance therapy, treatment with narcotic antagonists, and narcotic detoxification are discussed. In addition, an overview is presented of recent developments in opioid pharmacology and of recently developed novel pharmacological agents which may prove useful in the future treatment and/or prevention of opioid dependence.

Disposition in the rat of buprenorphine administered parenterally and as a subcutaneous implant

Disposition of [15, 16(n)-3H]buprenorphine in the rat has been investigated after a single 0.2 mg/kg i.v. bolus dose and continuous administration via a s.c. implantable long-acting delivery system. After the i.v. injection, the tri-exponential decay of drug from brain occurred with t1/2 values of 0.6, 2.3 and 7.2 h, respectively (plasma t1/2 0.5, 1.4 h, third phase not estimated due to sustained concn.) Decay of drug from another high-affinity binding site in brain occurred with t1/2 values of 1.1 and 68.7 h, respectively. Fat and lung had higher concn. than other tissues and plasma. No metabolites of drug were detected in brain. Unmetabolized drug excreted in urine and faeces one week after i.v. injection were 1.9 and 22.4% of dose, respectively, and 92% of the dose was accounted for in one week. Urinary metabolites (%) were: conjugated buprenorphine 0.9; norbuprenorphine (free 9.4, conjugated 5.2); tentative 6-O-desmethylnorbuprenorphine (free 5.4, conjugated 15.9). Peak plasma concn. of buprenorphine occurred four weeks after s.c. implantation of a long-acting 10 mg 3H-buprenorphine pellet, and apparent dissociation half-lives of drug from low- and high-affinity binding sites in brain were 4.6 and 6.8 weeks, respectively. Fat, spleen and skeletal muscle had higher concn. than other tissues and plasma. No significant difference in brain morphine concn. was observed in placebo and nonlabelled buprenorphine-pelleted animals after a 2 mg/kg i.v. challenge dose of 3H-morphine. This study emphasizes the importance of high-affinity binding of buprenorphine in brain and subsequent slow dissociation as a prime factor in its prolonged agonist/antagonist effects and higher potency than other narcotic agonists.

Buprenorphine

Buprenorphine is a mixed agonist-antagonist with high affinity at both mu and kappa opiate receptors. Its pharmacological profile is determined primarily by partial agonism at mu-receptors and unusually slow kinetics at these receptors. Its intrinsic activity is such that in nearly all clinical situations it is as effective an analgesic as morphine with considerably longer duration and much more favourable acute safety. In long-term dosing studies in rodents and primates buprenorphine did not produce the manifestations of physical dependence when treatment was stopped. In self-administration studies in the same species only limited levels of reinforcing efficacy were demonstrated when compared with the opiates. In human former opiate addicts the limited potential of buprenorphine to produce psychological dependence was confirmed as was the favourable physical dependence profile. Some misuse of buprenorphine has been reported in 3 of the 29 countries in which buprenorphine is marketed despite its wide clinical acceptance, particularly as the sublingual formulation.