Naltrexone and human eating behavior: a dose-ranging inpatient trial in moderately obese men

To investigate the effects of the long-acting opiate antagonist naltrexone on spontaneous human eating behavior, eight moderately obese male paid volunteers were housed in a hospital metabolic unit for 28 days and offered palatable foods ad lib by a platter service method. Under double-blind conditions, equally divided doses of 100, 200 and 300 mg naltrexone, or an acetaminophen placebo, were administered twice daily in tablet form for 3-day periods each, according to a Latin Square design. The doses of naltrexone resulted in decreases of daily caloric intake from placebo level, but these reductions were neither statistically significant nor dose-related. When the averaged effects of the doses were compared to placebo, five subjects showed intake reductions but the overall intake reduction of 301.5 +/- 198.1 kcal/day (mean +/- SEM) was not statistically significant. Naltrexone administration failed to selectively alter intakes of individual meals and snacks or macronutrient consumption patterns. During active drug periods, subjects lost 0.62 +/- 0.22 lb over 3 days, while during the placebo period, subjects gained 0.46 +/- 0.68 lb. However, there was no reliable change of basal metabolic rate as a function of naltrexone administration. The present results, which indicate that naltrexone administration is relatively ineffective in reducing food intake and inducing body weight loss in obese humans, are thus in contrast with reports that administration of opiate antagonist agents promote significant reductions of food intake and attenuations of body weight gain in experimental animals.

d-Amphetamine raises cortisol levels in schizophrenic patients with and without chronic naltrexone pretreatment

Five drug-free schizophrenic patients received an infusion of 20 mg d-amphetamine and placebo on separate days in a pilot study. Plasma cortisol levels rose significantly at 1 and 2 hours after d-amphetamine compared to following placebo. A repeat infusion of d-amphetamine during naltrexone treatment led to a similar increase in cortisol concentrations.

Hyperthermia in the rat from handling stress blocked by naltrexone injected into the preoptic-anterior hypothalamus

Experimental handling and colonic temperature measurement have been shown to cause stress and induce a long-lasting rise in colonic temperature in the rat. This stress-induced hyperthermia was blocked by microinjection of the narcotic antagonist naltrexone into the preoptic-anterior hypothalamus (POAH) of the brain, but was not significantly affected by similar injections into areas of the brain above the POAH. Thus, the stress-induced hyperthermia may be caused by activation of the endogenous opioid mechanism in the POAH.

Naltrexone

Naltrexone (Trexan R), a long acting, orally affective narcotic antagonist was approved by the Food & Drug Administration in November, 1984 for use as an adjunct in the treatment of Opioid Addiction. This research capsule will explore what is known about naltrexone, and the most appropriate ways of using the drug.

Naltrexone disposition in man after subcutaneous administration

The metabolism, excretion, and pharmacokinetics of [15,16-3H2]naltrexone were studied in six human males after sc administration of the hydrochloride salt. Biological fluids were analyzed by a combination of high performance liquid chromatography with liquid scintillation measurement of radioactivity. After administration, naltrexone was rapidly absorbed into the systemic circulation. The mean absorption rate constant was 0.091 +/- 0.008 min-1 (half-life of 7.6 min). In general the metabolic, excretory, and pharmacokinetic patterns for naltrexone were similar to those observed after iv administration of naltrexone to man. The terminal phase plasma rate constant was 0.413 +/- 0.035 hr-1 (half-life of 1.68 hr) for parent drug and 0.0786 +/- 0.0090 hr-1 (half-life of 8.8 hr) for the major metabolite, 6 beta-naltrexone. An average of 76 +/- 6% (+/- SD) of the total radioactivity was recovered in the urine within 72 hr after administration. Naltrexone was found in the urine in both the free (3.4 +/- 0.8% of dose) and conjugated (6.8 +/- 2.1% of dose) form. 6 beta-Naltrexol was present in urine largely in the unconjugated form (28 +/- 7% of dose) but the conjugated form was also found (12 +/- 3% of dose).

Physiologic and morphologic changes and incidence of neoplasms in mice and rats fed naltrexone HCl for 24 months

Short-term (acute oral LD50 and 90-day oral subchronic) studies in mice and long-term (24 months) carcinogenesis bioassays were performed in B6C3F1 mice and Fischer 344 rats given naltrexone. The oral LD50 was approximately 1500 mg/kg; convulsions, hypopnea, and cardiac failure were dose-related. Naltrexone mixed with feed over 90 days did not evoke definitive signs of gross toxicity, and histopathology was unrelated to drug treatment. Similar drug/feed admixtures given for 24 months to mice or rats did not disturb behavior. In mice, naltrexone reduced growth rates 5-10% and food intake 9-19%, but survival rates were 70-82% for treated mice and controls. The frequency and location of predominant tumors were similar in treated and untreated mice. In the rat, the same dosages had little effect on growth or food intake. The majority of all sacrificed rats had neoplasms. Neither neoplasms nor nonneoplastic lesions in mice or rats were associated with drug treatment. It is concluded that naltrexone is not a carcinogen.

Bioequivalence, dose-proportionality, and pharmacokinetics of naltrexone after oral administration

Healthy male volunteers (N = 24) participated in a four-way crossover study to compare the rate and extent of absorption of naltrexone after administration of 50 mg tablets as 50, 100, and 200 mg doses and a 10 mg/ml reference syrup. A high-performance liquid chromatographic method was employed to measure naltrexone and 6-beta-naltrexol in plasma and urine. Compared to the syrup, the 50 mg tablets were absorbed more slowly but equally well. There was excellent linearity between the administered dose and the area under the plasma concentration-time profile, as well as total urinary recovery of both drug and metabolite. The mean half-lives for naltrexone and beta-naltrexol were approximately 4 and 12 hours, respectively. The fraction of drug reaching the systemic circulation was estimated to be 5% of the administered dose because of extensive first-pass metabolism. Less than 1% of the dose was excreted in the urine as naltrexone after 48 hours, while 25% was recovered as unconjugated beta-naltrexol. The renal clearance of naltrexone and beta-naltrexol was approximately 127 ml/min and 283 ml/min, respectively. The total systemic clearance for naltrexone was approximately 94 L/hr.

Review of naltrexone, a long-acting opiate antagonist

The basis for using narcotic antagonists for the treatment of opiate addiction is discussed briefly, and the chemistry, pharmacology, pharmacokinetics, clinical efficacy, adverse effects, and dosage of naltrexone hydrochloride, an opiate antagonist drug, are reviewed. Naltrexone is rapidly and completely absorbed after oral administration but undergoes substantial first-pass extraction and metabolism by the liver. Naltrexone has a half-life of 3.9-10.3 hours and a slow terminal elimination-phase half-life of 96 hours. The major metabolite of naltrexone is 6-beta-naltrexol, which is present in plasma in greater concentrations than the parent drug. Problems in study design and patient adherence to treatment have made assessment of naltrexone's clinical efficacy difficult; however, studies have consistently demonstrated that patients who remained off opiates longer were those who took naltrexone longer. Factors associated with successful treatment outcomes include sustained therapy with naltrexone, participation in multidisciplinary programs of behavioral therapy and psychotherapy, and good family and social support systems. Gastrointestinal irritation and, in some studies, clinically insignificant increases in blood pressure, have been the most common adverse effects. Currently available data suggest that naltrexone is a very safe drug. The usual dosage of naltrexone hydrochloride is 50 mg orally once daily or 350 mg orally per week in three divided doses. Patients should be detoxified and opiate free for two to five days before initiation of naltrexone therapy. Naltrexone appears to be a useful adjunct to therapy in opiate addicts who are well motivated and who have strong psychological support systems.

The sequential use of clonidine and naltrexone in the treatment of opiate addicts

The efficacy of clonidine in the management of opiate withdrawal states has improved and refined the medical approach to this condition. In addition, the use of clonidine for opiate detoxification paves the way for naltrexone maintenance. Naltrexone, by providing chronic opiate receptor blockade, prevents opiate intoxication and subsequent readdiction in recovered addicts. The sequential use of clonidine and naltrexone, in conjunction with drug rehabilitation, appears to represent a viable and effective treatment for opiate addiction in motivated patients. The development of clonidine and naltrexone as treatment agents for opiate addiction also demonstrates that neurobiological advances can be translated into new and effective clinical approaches. This paper summarizes some of our experiences with the clonidine/naltrexone approach in motivated opiate addicts.

Ventromedial and lateral hypothalamic injections of naloxone or naltrexone suppress the acute food intake of food-deprived rats

Experiments were conducted to determine the anorexigenic effects of ventromedial (VMH) and lateral hypothalamic (LH) injections of the mu-opiate receptor antagonists, naloxone and naltrexone, on food-deprived (20 h) rats. Lever pressing to obtain food pellets was measured in groups of hungry, male Sprague-Dawley rats following VMH, LH or subcutaneous (SC) injections of saline, naloxone or naltrexone. VMH injections of either narcotic antagonist (5 and 10 micrograms/microliter) and LH injections of naloxone (5 and 10 micrograms/microliter) decreased the total 90-min food intake, compared to saline controls, due to suppressed feeding especially during the initial 30-min interval. Rats given SC injections of naloxone (10 mg/kg) also decreased their food intake compared to amounts eaten after SC saline was given. Decrements in food consumption relative to saline controls were similar following VMH or LH administration of naloxone. Moreover, the anorexia observed following VMH naloxone administration was similar to that found after VMH injections of equal doses of naltrexone.

The effects of intrathecal morphine and naltrexone on autotomy in sciatic nerve sectioned rats

Rats were implanted with an intrathecal catheter aimed at the lumbar enlargement (LE). Morphine hydrochloride (240 micrograms/day) was infused continuously on the spinal cord for 14 days with an osmotic minipump delivering 0.5 microliter/h solution or a bolus dose of naltrexone (37.5, 75 or 150 micrograms) was injected intrathecally. Intrathecally infused morphine delivered on the dorsum of the LE induced analgesia, as tested on the hot plate, whereas normal saline was without effect. Naltrexone caused hyperalgesia revealed as decreased threshold for vocalization to electrical stimulation of the tail. Rats with unilaterally sectioned sciatic nerves that were continuously infused with morphine on the dorsum of the LE autotomized significantly less than saline controls. Nerve sectioned rats injected with naltrexone had an overall level of autotomy similar to saline controls. However, autotomy had a somewhat earlier onset and was more severe with naltrexone than with saline. It is therefore concluded that intrathecal infusion of opiates specifically reduces autotomy, a behavior that may occur as a result of chronic discomfort or pain following nerve injury. Furthermore, the endogenous opiate system at the spinal level may be involved in the control of autotomy.

Effect of naltrexone on food intake and hoarding in white-footed mice (Peromyscus)

Increases in food consumption and hoarding in mammals have been shown to be immediate and preparative adjustments to the energetic stresses of temperate winters. The sensitivity of these behaviors to the opiate antagonist naltrexone was tested in non-deprived white-footed mice (Peromyscus leucopus). Mice received naltrexone hydrochloride intraperitoneally (10 mg/kg in saline vehicle) once daily for four consecutive days subsequent to an equivalent period of injection with saline as control. Daily food intake was significantly (p less than 0.05) lower after naltrexone treatment. Hoarding, as assessed by logarithmic hoarding scores and the weight of cached food, was not affected by administration of this drug at this dosage. These results suggest that hoarding, a complex behavioral pattern that does not immediately affect internal energy stores, may not fall within the opiate regulatory scheme.

The behavioural depression of hippocampal kindled rats is attenuated by subcutaneous and intracerebroventricular naltrexone

Two questions were asked: Does naltrexone attenuate the behavioural depression (BD) in other models of limbic epilepsy besides amygdala kindling? Does intracerebroventricular (ICV) administration produce the same effects as subcutaneous injection, i.e., attenuation of the BD. Male wistar rats with bipolar electrodes implanted bilaterally in the dorsal hippocampus and a metal cannula in the lateral ventricle were kindled through 1 electrode and EEG recorded through the contralateral electrode. Subcutaneous (sc) and ICV naltrexone administration attenuated the BD of hippocampal kindled rats. These results further implicate the brain opioid system in the postictal phase of kindling and possibly epilepsy.

Pharmacokinetic quantitation of naltrexone controlled release from a copolymer delivery system

Naltrexone release rates from a controlled release delivery system have been quantitated over a time period greater than one month in the monkey. The method requires calibration of the pharmacokinetic parameters of each monkey utilizing an intravenous bolus dose and assay of unchanged naltrexone levels in plasma as a function of time after dosing. Also required are periodic plasma levels of unchanged naltrexone obtained subsequent to administration of the delivery system. Release rates are then calculated as well as the total amount released. Application of the methodology to a biodegradable copolymer naltrexone delivery system in three monkeys showed an initial release rate of 3-8% of the dose per day over the first 3-5 days followed by a slow, rather constant release rate of 1-3% per day from day 5 to the time of the last measurable plasma sample (36-43 days). Comparison of alternative calculation methods using both experimental and simulated plasma naltrexone data verified the accuracy of the release rate calculations. The sum of the calculated total amount of naltrexone released plus the assayed amount remaining in the delivery system after removal from the animal accounted for 91-94% of the administered dose in the two monkeys in which complete data were obtained.

Naltrexone metabolism and sustained release following administration of an insoluble complex to rhesus monkeys and guinea-pigs

Both the release and the metabolism of naltrexone have been evaluated after intramuscular administration of a sustained release [15,16-3H2]naltrexone aluminium tannate complex in guinea-pigs and rhesus monkeys. In both species, measurable excretion of radioactivity was obtained for greater than 50 days and complete recovery of the dose was obtained in the guinea-pig. The radioactivity excretion rate-time profile differed in the two species with guinea-pig yielding a continuously declining rate and monkey yielding a peak at 5 days. In selected monkey urine samples (days 4, 17-20 and 49-52) subjected to t.l.c., evidence was obtained for the presence of naltrexone, beta-naltrexol and 2-hydroxy-3-O-methyl-beta-naltrexol, mostly as glucuronide and/or sulphate conjugates. The t.l.c. data also suggest that in monkey a naltrexone metabolite builds up relative to naltrexone over the 52 day release period.

A case of episodic flushing and organic psychosis: reversal by opiate antagonists

A woman had episodic attacks of flushing associated with severe skin, bone, and abdominal pain, accompanied by mood alterations and anxiety, and followed by an organic psychosis. These symptoms and signs could be induced by small doses of clonidine, L-5-hydroxytryptophan, pentagastrin, insulin, epinephrine, compound 48/80, methacholine, morphine, histamine, or d-tubocurarine. Skin testing showed abnormally sensitive mast cells and an exaggerated axon flare. Cimetidine initially prevented the attacks but became less effective after 18 months. Thyrotropin-releasing hormone stopped the skin pain whereas neurotensin reproduced the burning sensation in the skin without inducing the attack. Both the flush and the organic psychosis were reversed entirely by naloxone or naltrexone, and the hallucinosis could be reversed by vasodilators.

Effects of beta-chlornaltrexamine on separation distress in chicks

Intraventricular B-chlornaltrexamine (2 micrograms) increased distress vocalizations (DVs) in chicks, and reduced the ability of intraventricular morphine (.1-.5 micrograms), to inhibit DVs. Object imprinting was not blocked by central CNA, but systemic naloxone (10 mg/kg) did attenuate imprinting to a green but not a red object.

Clonidine and naltrexone. A safe, effective, and rapid treatment of abrupt withdrawal from methadone therapy

Clonidine hydrochloride and naltrexone hydrochloride, given in combination, were found to provide a safe, effective, and extremely rapid treatment of abrupt withdrawal from methadone hydrochloride therapy. Under controlled inpatient conditions established to assess dosage guidelines and to examine specific signs and symptoms of withdrawal, ten (91%) of 11 patients were able to withdraw completely from methadone therapy by the end of a six-day period. Six days of clonidine hydrochloride treatment, with a peak mean dose of 2.9 mg/day on treatment day 2, attenuated the withdrawal-inducing effects of naltrexone. Naltrexone hydrochloride was gradually increased from an initial 1-mg dose on treatment day 2 to 50-mg maintenance dose on treatment day 5 without an associated increase in withdrawal symptoms. Clonidine significantly decreased BP without producing clinical problems. The withdrawal symptoms of anxiety, restlessness, and muscular aching were most resistant to treatment, but at discharge most patients were completely asymptomatic.

Naltrexone-insensitive facilitation and naltrexone-sensitive inhibition of passive avoidance behavior of the ACTH-(4-9) analog (ORG 2766) are located in two different parts of the molecule

Subcutaneous injection of the ACTH-(4-9) analog (OG 2766) in ng amounts prior to the retention test facilitated, while microgram doses attenuated passive avoidance behavior. The inhibitory effect could easily be overcome by treatment with ACTH-(1-10) either before or after ORG 2766 administration. Thus, inhibition of passive avoidance behavior by ORG 2766 probably was not due to competition with ACTH-like peptides or a functional antagonistic influence on brain structures sensitive to ACTH-like peptides. Intracerebroventricular administration of ACTH-(4-10) in a wide dose range (0.5-10.0 micrograms) and of ORG 2766 in low doses (0.5-1.0 ng) facilitated passive avoidance behavior, whereas 'high' doses of ORG 2766 (5.0 and 10.0 ng) and graded doses of COOH terminal tripeptide of ORG 2766 (Phe-D-Lys-Phe; PDLP; 0.5-10.0 ng) attenuated passive avoidance behavior. The NH2 terminal tetrapeptide of ORG 2766 (H-Met/O2/-Glu-His-Phe) facilitated passive avoidance behavior, whereas the NH2 terminal tripeptide (H-Met/O2/-Glu-His) was ineffective. Naltrexone pretreatment antagonized the attenuating effect of ORG 2766 and PDLP. Following pretreatment with this opiate antagonist both 'low' and 'high' doses of ORG 2766 and the NH2 terminal tetrapeptide of ORG 2766 induced facilitation of passive avoidance behavior, while PDLP was ineffective in the presence of naltrexone. Thus, ORG 2766 exerts a dual effect on passive avoidance behavior. The facilitating effect of ORG 2766 resides in the NH2 terminal part and is unrelated to naltrexone-sensitive brain opiate receptor sites, whereas the inhibiting influence is located in the COOH terminal part of the peptide and depends on naltrexone-sensitive brain opiate receptor sites.

Naltrexone improves survival rate and cardiovascular function in canine hemorrhagic shock

The possible involvement of opiate receptors in the cardiovascular depression associated with hypovolemic shock was investigated. Opiate receptor blockade with naltrexone increased mean arterial pressure, cardiac output, stroke volume and left ventricular contractility in dogs bled to a mean arterial pressure of 45 mm Hg. Naltrexone also increased survival rate. At high doses, naltrexone adversely affected cardiac performance which may outweigh its advantages of greater potency and putatively longer action than naloxone, at least in the dog. Similar actions with another opiate antagonist gives further proof for endogenous opiate involvement in the cardiovascular pathophysiology of hypovolemic shock.

Heroin and cocaine intravenous self-administration in rats: mediation by separate neural systems

The hypothesis that separate neural systems mediate the reinforcing properties of opiate and psychomotor stimulant drugs was tested in rats trained to lever-press of IV injections of either cocaine or heroin during daily 3-h sessions. Pretreatment with the opiate receptor antagonist drug naltrexone produced dose-dependent increases in heroin self-administration, but had no effect on the rate or pattern of cocaine self-administration. Similarly, pretreatment with low doses of the dopamine antagonist drug alpha-flupenthixol produced dose-dependent increases in cocaine but not heroin self-administration. High doses of alpha-flupenthixol eliminated all responding for cocaine and slightly reduced heroin self-administration. The specificity with which the two antagonist drugs exerted their behavioral effects strongly suggests that independent neural substrates are responsible for the reinforcing actions of heroin and cocaine.

Metabolism and disposition of naltrexone in man after oral and intravenous administration

The metabolism and elimination of [15, 16,-3H2]naltrexone was studied in man after oral and intravenous administration. The same metabolites, although in varying proportions, were observed in both cases; conjugated naltrexone and conjugated and unconjugated 6 beta-naltrexol were the major metabolites observed in plasma, urine, and feces. 2-Hydroxy-3-O-methyl-6 beta-naltrexol was found in minor quantities. Naltrexone was almost completely absorbed after oral administration. After oral and intravenous administration of naltrexone, about 60% of the dose was recovered in the urine in 48 and 72 hr, respectively. The route of administration did not significantly affect urinary clearance values obtained for unconjugated or conjugated naltrexone and 6 beta-naltrexol. The route of administration significantly affected terminal plasma half-life values obtained for unconjugated naltrexone (2.7 hr, iv; 8.9 hr, oral), but had little effect on comparable values obtained for total drug, conjugated naltrexone, and unconjugated and conjugated 6 beta-naltrexol. Combined gas chromatography-mass spectrometry was used to validate the presence of naltrexone, 6 beta-naltrexol, and 2-hydroxy-3-O-methyl-6 beta-naltrexol in urine.

Use of synthetic polypeptides in the preparation of biodegradable delivery systems for narcotic antagonists

The goal of this program was the development of biocompatible sustained-release systems that would release naltrexone at a rate of 20 to 25 microgram/hr for 30 days, and that would biodegrade within 90 days. The focus was on the use of macrocapsules prepared from synthetic polypeptides, specifically copolymers of glutamic acid and ethyl glutamate (i.e., Glu/EGlu copolymers). Tubular capsules prepared from 18/82 Glu/EGlu were the most promising systems developed. Capsules 1 cm in length, 0.19 cm in outside diameter, and 0.005 cm in wall thickness released naltrexone in mice at rates in the range of 20 to 40 microgram/hr for 18 days. The rates then decreased during the next 12 days as the capsules became exhausted of drug. These capsules were biocompatible and they appeared to biodegrade within 90 days. In general, the Glu/EGlu copolymers exhibit permeation and degradation rates that increase as the glutamic acid content is increased. Radiotracing studies revealed that the ultimate degradation product was carbon dioxide, which appeared in the expired air. This result is consistent with a polypeptide degradation process that involves hydrolysis of the ethyl esters followed by hydrolysis of the peptide bonds to produce glutamic acid, which enters the metabolic pool.

A review of parenteral sustained-release naltrexone systems

The ideal naltrexone sustained-release delivery system should be easy to inject or implant, not cause adverse tissue reaction, release the drug at a relatively constant rate for at least 30 days, and biodegrade within a short time afterwards. Mechanisms which can be used for sustaining drug release include reducing solubility and surface area, coating, encapsulation and microencapsulation, complexation, binding and hydrophilic gelation. Drug release from such systems is controlled by diffusion through a barrier/film, diffusion from a monolithic device, erosion of the surface, hydrolysis, ion exchange, biodegradation, or a combination of these. Injectable systems would seem to be ultimately preferred because of the ease of administration and handling, while the implantable devices may find first use in man since they are easily removable, should that be necessary. Maintaining particulate-free products and sterilization methods are two problems with all parenteral dosage forms. Production must be particularly well controlled and validated.

Suppressant effects of naltrexone on water intake in rats

Parenteral administration of naltrexone to water-deprived female rats suppressed water intake when injected 4, but not 12 hours prior to the drinking session. Intraperitoneal injection 5 mg/kg naltrexone twice daily or oral self-administration of naltrexone in doses sufficient to block morphine-induced analgesia failed to reduce water intake or to alter body weight in non-deprived animals. These findings suggest that the suppressant effects of naltrexone on appetitive behavior are mediated by a different population of opiate receptors than those mediating morphine-induced analgesia.

Plasma naltrexone kinetics after intravenous bolus administration in dogs and monkeys

This investigation generated data characterize a specific electron-capture GLC assay reported previously for naltrexone and applied the method to a determination of naltrexone pharmacokinetics. Extraction efficiencies are reported for the assay, and mass spectral evidence indicates that naltrexone forms a triester when derivatized for electron-capture GLC with pentafluoropropionic anhydride and a base catalyst. Plasma level-time data for intravenous naltrexone at two dose levels in monkeys yielded no evidence of dose-dependent kinetics. A two-compartment open pharmacokinetic model was fitted to plasma level-time data for naltrexone in two dogs and yielded a total body clearance of 51-55 ml/min/kg. Urine collected for 0-24 hr contained 36% of the dose as naltrexone conjugates with less than 1% as unchanged naltrexone. Plasma level-time data for intravenous naltrexone in six monkeys yielded an average terminal half-life of 7.8 hr and a total body clearance of 64 ml/min/kg. The total body clearance for naltrexone was greater than the hepatic plasma or blood flow in both dogs and monkeys. This finding, together with the extremely low renal excretion of naltrexone, suggests the existence of elimination mechanisms besides liver metabolism and renal excretion.

Some observations on the opiate peptides and schizophrenia

With the discovery of the opiate peptides, several major avenues of research became apparent. These peptides produced a great deal of focused attention on their anatomy, biochemistry, and physiology. In this article, we present an overview of some of the main research issues and recent findings in the field of opiate peptides. The possible relationship of the opiate peptide neuronal systems to schizophrenia is discussed in light of attempts to alter schizophrenic symptoms with opiate antagonists, beta-endorphin, and dialysis. It is hypothesized that if the opiate peptides are involved in schizophrenia, then their involvement with dopamine systems and/or with stress responses may be critical.

Effects of repeated injections of naltrexone on antagonism of rate decreases by morphine in the pigeon

Responding of three pigeons was maintained under a multiple schedule of food presentation in which key-pecks produced access to grain under a fixed-interval schedule in the presence of one stimulus and a fixed-ratio schedule in the presence of another stimulus. Repeated daily injections of 1 mg/kg naltrexone had no systematic effect on overall response rate during either schedule component, and the naltrexone continued to antagonize rate decreases of periodic single injections of 10 mg/kg morphine for seven weeks. Tolerance did not occur to the antagonistic effects of naltrexone on rate decreases generally produced by morphine.

The effects of naltrexone on the development of physical dependence on morphine

A single i.p. injection of naltrexone (20 mg/kg) partially inhibited the development of physical dependence upon morphine in mice rendered dependent on morphine by implantation of a pellet containing 75 mg of morphine free base for three days. This was evidenced by an increase in the dose of naloxone (ED50) required to precipitate withdrawal jumping response. The increase in naloxone ED50 was much more pronounced when naltrexone was given prior to and during the course of pellet implantation. Inhibition was also observed when naltrexone was administered one day after the morphine pellet implantation, i.e., after some dependence had already developed. Naltrexone administration prior to and during the development of dependence also inhibited, but only partially, the loss of body weight and hypothermic response observed during abrupt withdrawal of morphine in morphine-dependent mice. The inhibitory effect of naltrexone on morphine dependence development was not associated with changes in brain morphine concentration.

Elimination of radioactivity following administration of [15,16-3H]naltrexone to rats and guinea pigs

The elimination of radioactivity after [15,16-3H]naltrexone administration was studied in rats and guinea pigs. An average of 42% of the dose was eliminated in urine and 55% in feces following administration of 1 mg/kg iv to each of three rats. Analysis of radioactivity in the excreta of one rat that received the same dose im yielded similar results. On the other hand, four guinea pigs that received 1 mg/kg iv excreted only 14% of the dose in feces and 84% in urine. Similar results were obtained following im administration to guinea pigs at 1 and 20 mg/kg doses. In guinea pig excreta, an average of 64% of the dose corresponded to naltrexone and conjugates, 19% to beta-naltrexol and conjugates, and 2% to alpha-naltrexol and conjugates. In urine, the radioactivity corresponding to alpha-naltrexol and naltrexone was present mainly in conjugated form, whereas apparent beta-naltrexol was mainly unconjugated. The radioactivity in feces corresponded principally to unconjugated naltrexone and beta-naltrexol.

Plasma corticosteroid levels in rats maintained on a long-acting naltrexone delivery system

A long-acting delivery system for naltrexone has been described, which blocked the antinociceptive action of 10 mg kg-1 s.c. dose of morphine in rats for a period of 2 to 3 months. Male Wistar rats implanted s.c. with such a delivery system showed highly significant depression of plasma corticosteroid levels (40.2% in one week and 22.4 to 27.2% in 3 months) as compared to placebo pellet-implanted animals. Morphine-dependent male rats implanted with 75 mg morphine pellets showed a small (17.5%) but significant increase in plasma corticosteroid levels as compared to the placebo controls 72 hr. after pellet implantation.

Continuous intravenous naltrexone effects on morphine self-administration in rhesus monkeys

Rhesus monkeys, surgically prepared with intravenous catheters, were given opportunities to self-administer morphine for 3 days, methamphetamine for 2 days and saline for 2 days in a constantly repeating cycle. Access to drugs was limited to a 15-minute period every 4 hours. After stable base-line self-administration rates, saline or various concentrations of naltrexone were infused continuously through the catheter. In the first phase of the study each concentration of naltrexone was infused for 4 weeks (separated by 3 weeks of saline) while the dose of morphine available for self-administration was held constant at 8 microgram/kg/injection. Stable naltrexone dose-related suppression of morphine self-administration occurred throughout each 4-week infusion. In the second phase of the study, various doses of morphine were made available for self-administration during 6- to 8-week continuous infusions of saline or various concentrations of naltrexone. The dose-effect curve relating self-administration rate to morphine dose per injection shifted to the right and decreased in maximum as the rate of infusion of naltrexone increased. Methamphetamine and saline self-administration rates were unaffected by naltrexone.

Controlled clinical study of naltrexone side effects comparing first-day doses and maintenance regimens

In a controlled double-blind clinical study, 42 patients reported side effects and severity of side effects to naltrexone on three different first-day doses and maintenance dosage regimens. Initiating doses of 25, 100, and 150 mg were administered. The maintenance regimens involved 350 mg of naltrexone per week for 4 weeks with drug administration in Group A, five times weekly; in Group B, three times weekly; and in Group C, twice weekly. All three groups received identical doses for the last dosage administered each week. The first-day doses produced no significant quantitative difference in side effects. Overall, the three groups reported little difference in side effects. Nonetheless, the regimen with the least number of patients reporting side effects daily was that of Group B. In no case, regardless of dose or dosage regimen, did any patient have side effects of such a nature as to require termination of their participation in the study.

Naltrexone and cyclazocine. A controlled treatment study

The induction side effects of cyclazocine and naltrexone were compared in double-blind placebo-controlled studies involving 40 patients (20 for each drug). These studies were carried out with a twice-a-day dosage regimen. Naltrexone produced fewer side effects than cyclazocine. Naltrexone side effects fell to levels indistinguishable from those of placebo in the "induction after placebo" phase. In contrast, cyclazocine "induction after placebo" produced an even higher level of side effects than found in its induction. In no case was naltrexone discontinued because of side effects. On the other hand, three of 20 cyclazocine-treated patients discontinued the drug because of distressing side effects. No toxicity was noted with either agent. The controlled data reported supports the clinical impression that naltrexone produces fewer induction side effects than cyclazocine.

Prospects, problems, side effects, and safety of narcotic antagonists

The use of the narcotic antagonists naltrexone, naloxone, and cyclazocine is reviewed with attention being given to their prospects, problems, side effects, and safety.

Methadone maintenance and narcotic blocking drugs. Appendix

The research history of norcodeine as a narcotic blocking drug is presented. Because of the limitations found for norcodeine usage, naloxone, cyclazocine, and naltrexone have recently received attention, and the results of research with them are detailed.

Estimation of the systemic availability and other pharmacokinetic parameters of naltrexone in man after acute and chronic oral administration

First pass metabolism, metabolic clearance, volume of distribution and steady state plasma levels were estimated in man following acute and chronic 100 mg oral doses of naltrexone. Essentially no statistical differences was observed in these values between the acute and chronic physiologic state. The values for the first pass effect were 79.6 +/- 4.6% and 78.0 +/- 3.0% for acute and chronic treatment respectively. From our pharmacokinetic data an apparent chronic release rate (ACRR) for a sustained release preparation of naltrexone was calculated as 11.8 microgram/kg/hr. In practice a release rate of one half the ACRR should be sufficient to provide continuous antagonism of 25 mg i.v. heroin. In conclusion our data clearly indicate that naltrexone is an effective and safe narcotic antagonist in man.

Psychopathology and mood during heroin use: acute vs chronic effects

In the context of evaluating the effects of a narcotic antagonist on opiate acquisition, 14 detoxified addicts self-administered increasing doses of unblocked heroin intravenously over a ten-day period. Early in the addiction cycle, subjects experienced tension relief and euphoria but this was followed shortly by a shift in the direction of increasing dysphoria and psychopathology. Nonetheless, individual injections of the drug continued to induce brief episodes of positive mood, an effect enhanced by frequent injection. Heroin self-administration was sharply reduced when subjects were blocked with naltrexone, a narcotic antagonist, and the negative effects observed during unblocked drug use were not observed.

Short-term effects of naltrexone in 155 heroin ex-addicts

The narcotic antagonist naltrexone was administered for periods of up to 8 months to a total of 155 patients at a dose of 40-200 mg per day. The antagonistic effect of naltrexone was tested by injections of heroin. Eighty milligrams of natrexone was effective for 48hr. The antagonistic effect decreased at 72 hr after the administration of 120-200 mg of naltrexone. Laboratory tests indicated no signs of toxicity. Naltrexone may elicit an increase in blood pressure and opigastric pain. Neither of these side effects appear clinically important. No signs of dependence on naltrexone were detected. These results suggest that naltrexone may be useful for clinical treatment of opiate dependence.

The urinary excretion profiles of naltrexone in man, monkey, rabbit, and rat

A gas-chromatographic method has been developed for the simultaneous determination of naltrexone, alpha-naltrexol, and beta-naltrexol as trimethylsiyl derivatives. Analysis of urine from rabbit, monkey, and rat demonstrated that, like man, these species reduce naltrexone primarily to beta-naltrexol. In naltrexone maintenance patients receiving 125 mg po three times per week, an average of 37% of the dose was recovered in 48-hr urine as free naltrexone (0.8%), conjugated naltrexone (7.6%), free beta-naltrexol (16.8%), and conjugated beta-naltrexol (11.8%). Thirty-four percent of the dose appeared in 0-24 hr and 3% during 24-48 hr. The ratio of beta-naltrexol to naltrexone rose from 2 at 0-4 hr to 34-48 hr. Monkeys receiving a daily dose of 12 mg/kg po, chronically, excreted very little free beta-naltrexol and exhibited an apparent sex-related difference in excretion patterns, with females excreting more than twice as much total base as males. Rabbits given a dose of 30 mg/kg ip for 4 days excreted conjugated naltrexone as the predominant urinary metabolite, accounting for 80% of total base recovered in 24 hr. In rats receiving 100 mg/kg po, less than 1% of the administered dose could be accounted for in the 24-hr urine, indicating that although the beta-naltrexol is produced as a urinary metabolite, other means of disposition of the drug must exist. Thus, in man and the monkey, beta-naltrexol is the predominant and persistent urinary metabolite. Urinary excretion profiles of naltrexone differ greatly between species commonly examined for chronic toxicity studies.

Interim report on clinic intake and safety data collected from 17 NIDA-funded naltrexone studies

In 17 studies of naltrexone, a totoal of 1,536 patients had been logged in as potential study subjects as of February 29, 1976. Of these, 883 had been started on study medication, including 107 on placebo as controls. A relatively high rate of attrition was seen in all studies over the first two months of study medication; this attrition rate tended to flatten out at about the fourth month. Of the 883 subjects beginning study medication, 47 (5.3%) were subsequently terminated for medical reasons. The data available on 45 of these subjects indicate equivalent percentages, both with respect to the total number of dropouts in the two study medication groups (naltrexone: 39 of 676, or 5.0%; placebo: 6 of 107, or 5.6%) and to the number of dropouts which the clinic reported as "possibly drug-related" (naltrexone: 6 out of 676, or 0.9%; placebo: 1 of 107, or 0.9%). However, one of the "possibly drug-related" dropouts developed idiopathic thrombocytopenic purpura after the administration of naltrexone for approximately 13 months during four separate treatment admissions. Statistical review of the data and subsequent analyses of the five double-blind, placebo-controlled studies administered by the National Academy of Sciences revealed no significant medication-group differences with respect to the physical/psychiatric or laboratory data. A review of the symptom data and analyses indicates that the frequency of occurrences of certain of the gastrointestinal tract symptoms recorded was somewhat higher in those subjects treated with naltrexone. Specific symptoms involved included "Loss of Appetite", "Abdominal Pain or Cramps", "Nausea or Vomiting", and "Constipation". However, the relative severity of these symptoms for all subjects experiencing any symptomatology was not statistically differentiable with respect to study medication group.

Naltrexone: disposition, metabolism, and effects after acute and chronic dosing

The disposition of naltrexone during acute and chronic administration of 100-mg oral dose was studied in 4 subjects. Following an acute dose the mean (X) peak naltrexone plasma level was 43.6 +/- 29.9 ng/ml at 1 hr and for the major biotransformation product, beta-naltrexol, was 87.2 +/- 25.0 ng/ml at 2 hr. Twenty-four hours after the dose the X levels of naltrexone and beta-naltrexol declined to 2.1 +/- 0.47 and 17.6 +/- 5.0 ng/ml, respectively. Following chronic administration and X peak plasma levels of naltrexone and beta-naltrexol rose to 46.4 +/- 18.5 and 158.4 +/- 89.9 ng/ml at 1 hr, but by 24 hr both compounds declined to levels of the same order as in the acute state at 24 hr. Plasma levels of naltrexone and beta-naltrexol measured 24 hr after the daily doses of naltrexone throughout the study indicated that steady-state equilibrium was rapidly attained and that there was no accumulation of naltrexone and beta naltrexol in the plasma after chronic treatment on 100 mg oral doses. Biexponential kinetics were observed for naltrexone and beta-naltrexol in the first 24 hr. The half-life of naltrexone and beta-naltrexol decreased slightly from the acute to thechronic study from 10.3 +/- 3.3 to 9.7 +/- 1.1 hr and from 12.7 +/- 2.6 to 11.4 +/- 2.0 hr. The plasma levels of naltrexone declined slowly from 24 through 72 hr from 2.4 to 1.7 ng/ml, with an apparent half-life of 96 hr. The renal clearance data indicate that naltrexone is partially reabsorbed while beta naltrexol is actively secreted by the kidney. During acute and chronic naltrexone administration the mean fecal excretion was 2.1% and 3.6% while urinary excretion was 38% and 70% of the dose in a 24-hr period. Opiate antagonism to 25 mg heroin challenges was nearly complete through 48 hr after naltrexone. At 72 hr the objective responses reappeared to a greater extent than the subjective ones. Correlation coefficient (r) between naltrexone plasma levels and opiate antagonism was 0.91 and between individual half-life of naltrexone and opiate antagonism it was 0.99.