Characteristics and comparative severity of respiratory response to toxic doses of fentanyl, methadone, morphine, and buprenorphine in rats.

Opioids are known to induce respiratory depression. We aimed to characterize in rats the effects of four opioids on arterial blood gases and plethysmography after intraperitoneal administration at 80% of their LD(50) in order to identify opioid molecule-specific patterns and classify response severity. Opioid-receptor (OR) antagonists, including intravenous 10 mg kg(-1)-naloxonazine at 5 min [mu-OR antagonist], subcutaneous 30 mg kg(-1)-naloxonazine at 24 h [mu1-OR antagonist], subcutaneous 3 mg kg(-1)-naltrindole at 45 min [delta-OR antagonist], and subcutaneous 5 mg kg(-1)-Nor-binaltorphimine at 6 h [kappa-OR antagonist] were pre-administered to test the role of each OR. Methadone, morphine, and fentanyl significantly decreased PaO(2) (P<0.001) and increased PaCO(2) (P<0.05), while buprenorphine only decreased PaO(2) (P<0.05). While all opioids significantly increased inspiratory time (T(I), P<0.001), methadone and fentanyl also increased expiratory time (T(E), P<0.05). Intravenous 10 mg kg(-1)-naloxonazine at 5 min completely reversed opioid-related effects on PaO(2) (P<0.05), PaCO(2) (P<0.001), T(I) (P<0.05), and T(E) (P<0.01) except in buprenorphine. Subcutaneous 30 mg kg(-1)-naloxonazine at 24 h completely reversed effects on PaCO(2) (P<0.01) and T(E) (P<0.001), partially reversed effects on T(I) (P<0.001), and did not reverse effects on PaO(2). Naltrindole reversed methadone-induced T(E) increases (P<0.01) but worsened fentanyl's effect on PaCO(2) (P<0.05) and T(I) (P<0.05). Nor-binaltorphimine reversed morphine- and buprenorphine-induced T(I) increases (P<0.001) but worsened methadone's effect on PaO(2) (P<0.05) and morphine (P<0.001) and buprenorphine's (P<0.01) effects on pH. In conclusion, opioid-related respiratory patterns are not uniform. Opioid-induced hypoxemia as well as increases in T(I) and T(E) are caused by mu-OR, while delta and kappa-OR roles appear limited, depending on the specific opioid. Regarding severity of opioid-induced respiratory effects at 80% of their LD(50), all drugs increased T(I). Methadone and fentanyl induced hypoxemia, hypercapnia, and T(E) increases, morphine caused both hypoxemia and hypercapnia while buprenorphine caused only hypoxemia.

[Preclinical management of accidental methadone intoxication of a 4-year-old girl. Antagonist or intubation?]. / Präklinisches Management der akzidentellen Methadonintoxikation bei einem 4-jährigen Mädchen. Antagonisierung oder Intubation?

We report on the preclinical management of a 4-year-old child who was found in a comatose condition with respiratory failure after accidental ingestion of methadone. Emergency airway management was carried out with endotracheal intubation instead of administering the antagonist naloxone. The child could be extubated 12 h later and was released from hospital after 3 days with no neurological symptoms. The authors attempt to formulate an algorithm for the preclinical management of opioid intoxication with reference to the literature and own experience. Endotracheal intubation seems to be superior to the use of the antagonist naloxone, especially in a critical situation. This is the only way to ensure a rapid oxygenation with adequate airway protection and with the simultaneous avoidance of the side-effects of naloxone. A restrictive and critical administration of the opioid antagonist naloxone is recommended when there is suspicion of opioid ingestion but no signs of intoxication.

Neuronal inhibitory effects of methadone are predominantly opioid receptor mediated in the rat spinal cord in vivo.

This study aims to assess whether the antinociceptive actions of methadone are mediated solely through opioid mechanisms, or whether its reported affinity for NMDA receptors has physiological relevance in vivo. Methadone is a mu-opioid receptor agonist reported to relieve pain unresponsive to other opioids. It is a racemic mixture comprising d- and l-optical isomers; the d-isomer has a lower affinity for opioid receptors, and both also exhibit NMDA receptor binding, likely to indicate antagonist activity. d -Methadone is antinociceptive in behavioural studies via non-opioid mechanisms, which could include functional NMDA receptor-blocking activity. Here we investigate the ability of d - and dl -methadone to inhibit noxious and innocuous electrically-evoked responses of dorsal horn neurones in the anaesthetized rat. Racemic methadone (5, 25, 50, 250 microg) applied spinally, dose-relatedly inhibited the C-fibre evoked response, input and wind-up of the neurones, with a profile resembling that of morphine. d-Methadone (5, 25, 50, 250, 500 microg) was also inhibitory, although less potent by a factor of between 13 and 48 depending on the neuronal measure; its profile of inhibition resembled that of the racemic mixture rather than an NMDA receptor antagonist. Both compounds had minimal effects on Abeta-fibre-evoked activity. The inhibitory effects of both d - and dl -methadone on noxious-evoked activity were naloxone reversible. The naloxone reversibility of d -methadone inhibitions is best interpreted as indicative of a purely opioid mechanism of action. However, the ability of naloxone to reverse the effects of d -methadone may also reflect a degree of synergy between weak NMDA antagonist and opioid agonist activity.

Interaction of methadone with substrates of human hepatic cytochrome P450 3A4.

Methadone, a synthetic drug, is one of the most widely used drugs for opiate dependency treatment. This drug has been demonstrated to be extensively metabolized by cytochrome P450 3A4 in human liver microsomes. Thus, the aim of this in vitro study was to determine if methadone is an inhibitor of other P450s characterized by their specific catalytic activities. Enzymatic activities specific to P450 2E1, P450 1A, P450 2B and P450 2C were not inhibited by methadone. Conversely, nifedipine oxidation, mediated by cytochrome P450 3A4, was potently inhibited by methadone by a mixed-type inhibition mechanism with a Ki of 100 microM. Fluvoxamine, a new antidepressant, was shown to be a potent mixed-type inhibitor of methadone N-demethylation with a Ki of 7 microM. Finally, methadone appears to be a mixed-type inhibitor and not a suicide inhibitor of cytochrome P450 3A family. Accordingly, caution should be advised in the clinical use of methadone when other drugs are administered that are able to induce or inhibit P450 3A4, such as rifampicin or nifedipine, diazepam and fluvoxamine.

The involvement of cytochrome P450 3A4 in the N-demethylation of L-alpha-acetylmethadol (LAAM), norLAAM, and methadone.

The N-demethylation of LAAM, norLAAM, and methadone has been investigated in human liver microsomes and microsomes containing cDNA-expressed human P450s. Gas chromatography/mass spectrometry methods allowed detection of norLAAM and dinorLAAM formation from LAAM, dinorLAAM formation from norLAAM, and EDDP and EMDP formation from methadone. The rates of N-demethylation varied 4- to 7-fold in microsomes from four different donors with activities for LAAM and norLAAM consistently greater (5- to 14-fold) than for methadone. The N-demethylation of LAAM, norLAAM, and methadone were significantly inhibited by ketoconazole. IC50s could be determined for ketoconazole inhibition of LAAM and norLAAM N-demethylation of 1.6 and 1.1 microM, respectively. The ability of ketoconazole to reduce methadone N-demethylation below 40% varied in regard to liver donor. No other P450-selective inhibitors reduced the average activities more than 43%. cDNA-expressed P450 3A4 N-demethylated LAAM, norLAAM, and methadone at greater rates than the other cDNA-expressed P450s studied (1A2, 2C9, 2D6, or 2E1). P450 3A N-demethylation of LAAM, norLAAM, and methadone exceeded the next most active P450, respectively, by at least 2.5, 9.6, and 13.4 times when expressed per milligram protein and by 18.2, 6.0, and 6.1 times when expressed per nanomole P450. These results suggest that P450 3A4 is the primary site of N-demethylation of LAAM, norLAAM, and methadone in human liver. Although other enzymes may also be capable of N-demethylating these compounds, identification of specific enzymes, except P450 3A4, has yet to be established. Knowledge of these enzymatic pathways is essential for assessment of the impact of metabolic drug-drug interactions on therapeutic success and/or adverse events.

Opioids induce while nicotine suppresses apoptosis in human lung cancer cells.

Previously, we have shown that opioids acting via specific receptors inhibit the growth of human lung cancer cells while nicotine, acting through nicotinic acetylcholine receptors, reverses this inhibition. Therefore, we studied the role of apoptosis in these processes. Treatment of human lung cancer cells with 0.1-1 microM morphine or methadone resulted in morphological changes and cleavage of DNA into nucleosome-sized fragments characteristic of apoptosis. Quantitation of DNA fragmentation showed that a dose-dependent increase occurred within 2 h of opioid treatment and was blocked by the antagonist naloxone. The apoptotic effect of opioids was suppressed by nicotine, while the nicotinic acetylcholine receptor antagonists, hexamethonium and decamethonium, reversed this suppression. In contrast, sphingosine, a protein kinase C inhibitor, caused significant DNA fragmentation which was not suppressed by nicotine. Unexpectedly, the combination of hexamethonium and opioids or hexamethonium and nicotine stimulated apoptosis. We found that nicotine, like phorbol 12-myristate 13-acetate, increased total protein kinase C (PKC) activity, while morphine and sphingosine decreased PKC activity, and nicotine reversed morphine inhibition of PKC activity. In contrast, methadone unexpectedly increased PKC activity. These results indicate that engagement of opioid receptors in human lung cancer cells induces apoptosis, while engagement of nicotine receptors suppresses apoptosis, which in some cases appear to be working through a PKC pathway. They also suggest complexities in the system where blockade of C6 or C10 nicotinic receptors can lead to facilitation of apoptosis. These findings suggest new strategies for treatment and prevention of cancer using opioids or nicotine receptors antagonists and are consistent with the idea that nicotine functions as a tumor promoter.

Effect of modafinil on pancreatic exocrine secretion in the rat. A comparison with methadone.

Modafinil is a recently developed drug which increases wakefulness in several animal species and in man, an effect involving, at least in part, central adrenoceptors. In the present experiments, the effect of modafinil was studied on a model of neurally stimulated secretion, pancreatic secretion induced by 2-deoxyglucose (2DG) in the rat, and compared with that of the mu-opiate methadone. Modafinil induced a dose-related inhibition of 2DG-stimulated pancreatic secretion, reaching more than 80% after 250 mg/kg i.p. The modafinil effect was suppressed by idazoxan or by large doses of prazosin but not by naloxone. In addition modafinil (250 mg/kg i.p.) did not change the pancreatic response to electrical vagal stimulation. Methadone also potently suppressed 2DG-stimulated pancreatic secretion, but in contrast to modafinil, the methadone effect was blocked by naloxone, but not by the adrenoceptor antagonists idazoxan, prazosin and propranolol. It is concluded that modafinil decreases centrally 2DG-stimulated pancreatic secretion through a pathway involving alpha 1- and alpha 2-adrenoceptors, without an interaction with opiate receptors.

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.

Butorphanol, levallorphan, nalbuphine and nalorphine as antagonists in the squirrel monkey.

The effects of several mixed-action opioid agonist/antagonists were examined alone and in combination with the mu-opioid agonist l-methadone and the kappa-opioid agonist, trans-3,4-dichloro-N-methyl-N[2-(1-pyrrolidinyl)cyclohexyl]benzeneace tam ide (U50,488). Effects were examined in squirrel monkeys responding under a schedule of shock titration. Under this procedure shock was scheduled to increase once every 15 sec from 0.01 to 3.7 mA in 30 steps. Five responses on a lever during the 15-sec shock period terminated the shock for 15 sec, after which the shock resumed at the next lower intensity. The intensity below which the monkeys maintained the shock 50% of the time (median shock level) was determined. l-Methadone and U50,488 produced dose-dependent increases in median shock level. Nalbuphine, butorphanol and levallorphan also increased median shock level, but these increases were much smaller than those observed with l-methadone or U50,488. Nalorphine and naltrexone did not increase median shock level. Butorphanol, levallorphan, nalbupine, nalorphine and naltrexone all produced parallel, rightward shifts in both the l-methadone and U50,488 dose-effect curves. The apparent pA2 values obtained for naltrexone in combination with l-methadone (7.7) were at least one log unit larger than those obtained for naltrexone in combination with U50,488 (6.5). Similar differences were revealed for nalbuphine in combination with l-methadone (6.1) and U50,488 (5.2) as well as for nalorphine in combination with l-methadone (6.0) and U50,488 (5.5).(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonism of the analgesic effects of mu and kappa opioid agonists in the squirrel monkey.

The effects of several mu and kappa opioid agonists were examined alone and in combination with the opioid antagonist quadazocine in squirrel monkeys responding under a schedule of shock titration. Under this procedure, shock was scheduled to increase once every 15 sec from 0.01 to 2.0 mA in 30 steps. Five responses on a lever during the 15-sec shock period terminated the shock for 15 sec, after which the shock resumed at the next lower intensity. The intensity below which the monkeys maintained the shock 50% of the time (median shock level) and the rate of responding in the presence of shock were determined under control conditions and after administration of the mu agonists, l-methadone and fentanyl and the kappa agonists, bremazocine, ethylketocyclazocine, ketocyclazocine and U50,488. When examined alone, intermediate doses of mu and kappa agonists increased median shock level. At the highest doses of these compounds responding was eliminated and shock rose to its peak intensity. When the mu and kappa agonists were examined in combination with quadazocine, dose-effect curves for median shock level and for rate of responding were shifted to the right in a dose-dependent fashion. A comparison of the pA2 values for quadazocine on median shock level and on rate of responding revealed similar values for the two measures; however, pA2 values differed depending on the agonist examined. That is, the pA2 values for quadazocine in combination with l-methadone and fentanyl on median shock level were 7.43 and 7.61, respectively; whereas the pA2 value for quadazocine in combination with bremazocine and U50,488 were 6.53 and 6.43, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Differentiating opioids by their pupillary effects in the rat.

Pupillary effects of several opioids were examined as part of a broader in vivo study of multiple opioid receptors in the rat. Agonist activity, stereospecificity, and naloxone sensitivity were determined by methadone (Me), ethylketocyclazocine (EK), and N-allylnormetazocine (SKF 10,047), selected for their purportedly predominant actions at mu, kappa, and sigma receptors, respectively. After an acute, subcutaneous injection of each drug, pupil area and fluctuations in pupil size were measured by means of an infrared video pupillometer on line with a microcomputer data processing and storage system. Despite differences in the magnitude of the response, each opioid tested produced an increase in pupil size which was stereospecific, independent of behavioral responses to the drugs and, for 1-Me and 1-SKF 10,047, dose-related. Other differences among the opioids were found in their ability to induce fluctuations (1-Me and 1-EK) and a pendular nystagmus (1-SKF 10,047 only), and in their sensitivity to naloxone. Although 1.0 mg/kg naloxone completely reversed 1-Me-induced mydriasis, 10 mg/kg was needed to reverse 1-EK, and this dose only partially antagonized 1-SKF 10,047. These characteristic patterns of pupillary responses to opioids in terms of agonist activities and naloxone sensitivities indicate that the different opioid receptor types subserve different functions with respect to pupillary control.

The interference of naloxone hydrochloride in the teratogenic activity of opiates.

Diamorphine hydrochloride, methadone hydrochloride, and the synthetic enkephalin analogue FK 33-824 are potent teratogens for the central nervous system in mouse embryos. They induce the "neurotropic syndrome of malformations," which is restricted to the central nervous system if administered during the critical period of neural tube closure. Pretreatment with corresponding equimolecular doses of the antagonist naloxone hydrochloride applied 30 minutes before treatment with the opiate agonists abolishes the major severe malformations, i.e., exencephaly, craniorachischisis, and brachyury, and reduces the number of cases of kinking of the spinal cord. Dilation of the fourth brain ventricle remains unaffected. It is suggested that the mechanism of interference in the teratogenicity of the opiates by naloxone hydrochloride reported here is based on competition for opiate receptors. In general, these observations are regarded as evidence that the pharmacological affinity of opiate agonists to receptors in the central nervous system is responsible for the malformations caused by them in this system.

Cimetidine inhibits the in vitro N-demethylation of methadone.

The effect of cimetidine on the metabolism of methadone was investigated in hepatic microsomes isolated from male Sprague-Dawley rats. N-demethylation of methadone was determined by formation of formaldehyde. Cimetidine inhibited methadone N-demethylation in a noncompetitive manner with an IC50 of 5.3 X 10(-4) M. This observation is consistent with previous reports of microsomal enzyme inhibition by cimetidine and suggests that caution must be exercised when methadone and cimetidine are co-administered to patients in order to avoid excessive sedation or respiratory depression.

Endocrine effects of methadone in rats; acute effects in adults.

The effects of methadone (METH) on serum levels of prolactin (PRL), growth hormone (GH), corticosterone (CS), TSH and T4 were determined in male and female rats. METH initially (15-45 min after injection) decreased TSH and increased PRL, GH and CS in both male and female rats. These changes were followed by a rebound decrease in CS and increase in TSH 4 h after METH administration. METH decreased GH at 0.1 mg/kg, but increased secretion with doses of 0.5 mg/kg and higher. Changes in PRL, CS and TSH were only seen with 1 mg/kg and higher. Naloxone blocked increases in serum GH, PRL and CS and the decrease in TSH caused by METH. Serum PRL increased significantly more in females than in males, although other endocrine actions of METH were the same in males and females. Pretreatment of animals with METH 4 h before a second challenge dose did not affect TSH, PRL or GH responses to METH. However, peak CS levels were lower in pretreated animals than in controls. These findings demonstrate that METH elicits a complex profile of endocrine response which probably reflects actions at multiple receptor and/or anatomical sites, and suggest that reported sex differences in METH metabolism are not limiting in determining the hormonal response to this drug. These results also suggest that single-dose tolerance develops to the effects of METH on CS but not on other endocrine parameters.

The role of naloxone infusions in the treatment of overdoses of long half-life narcotic agonists: application to nor-methadone.

This report describes the use of a naloxone infusion at a rate of 300 micrograms/h to provide constant antagonism of the respiratory depressant effects of a near fatal nor-methadone dose in a 2 year old boy. The terminal half-life for nor-methadone was calculated to be 13.5 h. The results indicate that a single dose of a narcotic antagonist with a short half-life (e.g. naloxone) will probably be inadequate to provide long lasting reversal of the effects of overdoses of long half-life narcotic agonists, and that continuous infusion may be the preferable to repeated bolus doses.

Calcium antagonism of an opiate drug effect on an excitable cell membrane.

When frog sartorius muscles are exposed to methadone (2-4 X 10(-4) M) the action potential recorded intracellularly is depressed and eliminated in 3 to 4 hr. This is due to a decrease in the sodium conductance as measured by the maximum rate of rise of the action potential. When the calcium concentration in frog Ringer's solution (normally 1.08 mM) was either lowered to 0.54 mM or raised to 2 mM, the effect of methadone on excitability was unchanged. However, increasing the extracellular calcium to 4 mM decreased the action of methadone. As this would also increase the intracellular calcium concentration, in other experiments the free intracellular Ca+ concentration was raised by adding 0.4 mM caffeine to the Ringer's solution. This also antagonized the depressant action of methadone. The antagonistic action of caffeine was not due to a direct effect of caffeine on sodium conductance because the administration of caffeine by itself caused a 10% depression of the action potential maximum rate of rise and caffeine did not antagonize the action of tetrodotoxin, which is a specific sodium channel blocking agent. Finally, it was shown that the calcium ionophore, A23187, in low concentrations antagonized the depressant actions of methadone and meperidine on action potential production. It was concluded that increasing the intracellular free calcium concentration antagonized the depressant effect on excitability produced by opiate drugs acting on an intracellularly oriented opiate drug receptor.

Lack of effect of diazepam on methadone metabolism in methadone-maintained addicts.

Four methadone-maintained subjects were given diazepam (0.3 mg/kg) for 9 days. During the dual drug period, the effects and kinetics of methadone and of its major pyrrolidine metabolite were not altered. These findings indicate that, unlike its effects in rodents, diazepam does not inhibit the metabolism of methadone in man.

The effects of levo-alpha-acetylmethadol and its metabolites on schedule-controlled behavior in the pigeon.

The effects of orally administered levo-alpha-acetylmethadol (LAAM) on the schedule-controlled behavior of the pigeon were compared with those of methadone. Both LAAM and methadone decreased rates of responding under a multiple fixed-interval 5-min, fixed-ratio 30-response schedule of food presentation. Although LAAM had a longer duration of action than methadone, both drugs were similar in the onset and potency of their behavioral effects. The chronic administration of either LAAM or methadone produced tolerance to the respective behavioral effects of each drug. In one set of experiments, the behavioral effects of intramuscularly administered LAAM were compared with those of its active metabolites, levo-alpha-noracetylmethadol and levo-alpha-dinoracetylmethadol. All three drugs decreased rates of responding under both components of the multiple schedule. The onset of these rate-decreasing effects was rapid and the order of potency for the production of these rate-decreases was levo-alpha-noracetylmethadol greater than levo-alpha-dinoracetylmethadol greater than or equal to LAAM. The rate-decreasing effects of LAAM and its metabolites were typical narcotic effects as defined by their reversal by naloxone.

Blockade of ovulation by methadone in the rat: a central nervous system-mediated acute effect.

Morphine blocks ovulation in the rat and is associated with irregular menstrual cycles in humans. Methadone, the synthetic narcotic used to treat heroin addiction, was tested in rats for its ability to block ovulation. Equal doses of methadone were administered subcutaneously at 13:00, 14:00 and 15:00 hr on proestrus. Three different total (cumulative) doses were tested: 6, 9 and 15 mg/kg, in Wistar and Sprague-Dawley rats. The 6 mg/kg dose was ineffective, 9 mg/kg affected by Sprague-Dawley, whereas the 15 mg/kg dose blocked ovulation in both strains. Injection of saline at 13:00, 14:00 and 15:00 hr, or of the narcotic antagonist naltrexone at 12:00 had no effect on ovulation. Naltrexone prevented the blockade of ovulation by methadone, implicating narcotic receptors in the mediation of this action of methadone. Electrochemical stimulation of the medial preoptic area of the brain, or administration of exogenous luteinizing hormone overcame methadone's blockade of ovulation suggesting that the drug is acting via the central nervous system. The results of these studies suggest that menstrual dysfunction may not be a major problem in the clinical application of methadone.