Analysis of naltrexone and its metabolite 6-beta-naltrexol in serum with high-performance liquid chromatography

BACKGROUND

Naltrexone has been proven to be an effective treatment option for the treatment of alcohol dependency. In this article we introduce a reliable and simple method developed for the simultaneous determination of naltrexone and 6-β-naltrexol in human serum by using high-performance liquid chromatography (HPLC).

FINDINGS

Liquid-liquid extraction with butyl acetate from basic solutions (pH 9) was chosen for extraction with nalorphine as an internal standard (IS). Analytes were back-extracted from organic solvent into perchloric acid. The acid extract was chromatographed by HPLC with a reverse-phase ODS-column and electrochemical detector. The mobile phase was a NaH(2)PO(4)-solution with acetonitrile as an organic modifier and octanesulphonic acid and tetraethylammonium hydrogen sulphate as ion-pair reagents. The recovery of the extraction method was 48% for naltrexone and 75% for 6-β-naltrexol. The limit of quantification was 5.0 ng/ml for naltrexone and 1.0 ng/ml for 6-β-naltrexol. The analysed concentrations of naltrexone differed from the theoretic concentrations by 0.7 to 2.3% and those of 6-β-naltrexol by 2.6%. The relative standard deviation of within-day assay was from 0.9 to 5.7% for naltrexone and from 0.8 to 4.2% for 6-β-naltrexol; for the between-day assay it was 5.7% and 4.2%, respectively.

CONCLUSIONS

Our results indicate that the developed method is suitable for determination of naltrexone and 6-β-naltrexol in human serum.

Comparative effects of analgesics on pain threshold, respiratory frequency and gastrointestinal propulsion

In the rat, the ratio of the analgesic to the respiratory depressant potency was the same for morphine, codeine, diamorphine, methadone, dipipanone, piperidylisomethadone, phenadoxone, dextromoramide, and propoxyphene. The relative respiratory depressant activity of pethidine tended to be less, but the difference was not significant. The ratio of the analgesic dose to the dose preventing transport of a charcoal meal in the rat was about the same for morphine, codeine, pethidine, methadone, phenadoxone, dimethylthiambutene, and propoxyphene; the relative activities of these compounds in inhibiting the peristaltic reflex of the isolated guinea-pig ileum were also similar. However, because of differences in the slopes of regression lines in the charcoal meal test, some compounds (for example, morphine) had a greater effect on gastrointestinal propulsion than others (for example, pethidine) when given at moderate analgesic dose levels.In studies of the effects of intracisternal morphine in the rat, effects on the spinal reflex of the tail were to some extent dissociated from effects on the threshold for a squeak response. Further, the delaying of transport of a charcoal meal paralleled depression of respiratory rate, and this is evidence for the participation of a central as well as a peripheral action in the effect of morphine on the gastrointestinal tract. The delay in propulsion was reduced by nalorphine and increased by atropine and two general anaesthetic substances, but was unaffected by a number of other pharmacological agents.

Similarly between receptors responsible for the production of analgesia and lenticular opacity

Evidence is presented that the receptors responsible for the mediation of analgesia by morphine-like drugs are similar to those which are involved in the production of a reversible lenticular opacity. The activity of a number of compounds in mice on the lens was closely correlated with analgesic potency in this species. Stereospecificity for isomers with D configuration was demonstrated for both effects. Nalorphine only antagonized the lenticular opacity activity of those drugs the analgesic action of which it abolished.

Multiple toe-pinch method for testing analgesic drugs

In guinea-pigs and rats, an immediate squeak was one of the most consistent and readily observed responses to application of a light artery clip to the base of a toe. Morphine and related drugs suppressed this response. Squeak-responses from each toe of an experimental animal formed the basis of a technique for measuring activity of analgesic drugs. A statistical method was developed to analyse the correlated quantal observations obtained. It provided an estimate of the increase of information from several toes compared with one. Testing all toes of each animal yielded a substantial increase of information, because the correlation between responses of different toes was low. Among drugs having an analgesic action in man, 1-(beta-diethyl-aminoethyl)-2-(p-ethoxybenzyl)-5-nitrobenzimidazole, methadone, morphine, pethidine and codeine (in descending order of potency) were active in this test in guinea-pigs. Acetylsalicylic acid, amidopyrine, amphetamine, chlorpromazine, 4-hydroxyisophthalic acid, lysergic acid diethylamide, mephenesin, nalorphine, pentetrazole, phenobarbitone, phencyclidine, phenytoin, salicylamide, strychnine and troxidone showed little or no activity. The time-courses of active drugs were estimated, and morphine had the longest action.

INHIBITION OF BACTERIAL GROWTH BY DRUGS OF THE MORPHINE SERIES

The growth of Escherichia coli is reversibly inhibited by drugs of the morphine series. The order of inhibitory effectiveness for the drugs tested was levallorphan > levorphanol > dextrorphan > nalorphine > morphine. The synthetic analgesic, levorphanol, was studied in greater detail. Its effectiveness was found to be strongly dependent on the pH of the medium. Raising the pH of the medium provides a higher concentration of the neutral free base which is thought to diffuse across cell membranes more readily. However, considerations other than the rate of entry of drug into the cells must be of importance since an already established growth inhibition is promptly reversed upon lowering the pH of the medium. Two mutants of Escherichia coli with altered sensitivity to levorphanol were isolated.

QUANTITATIVE STUDIES OF THE ANTAGONISM BY NALORPHINE OF SOME OF THE ACTIONS OF MORPHINE-LIKE ANALGESIC DRUGS

A quantitative investigation has been made of the antagonism by nalorphine of the analgesia and lenticular opacity produced in mice by a number of compounds. ED50 values have been obtained for each drug in the absence and in the presence of increasing doses of nalorphine, and from these, appropriate dose-ratios have been calculated. It has been possible to derive the equivalent of a pA(2) value for each drug with nalorphine and, since these are almost identical, it may be concluded that all the drugs combine with similar receptors. Nalorphine antagonizes both actions by competing for the receptors. It was not possible to antagonize quantitatively the analgesic action of pethidine with nalorphine, although the lenticular effect could be abolished. The effect of nalorphine on the change in skin temperature in mice induced by some of the analgesic drugs was also investigated.

THE ACTION OF MORPHINE-LIKE DRUGS ON IMPULSE TRANSMISSION IN MAMMALIAN NERVE FIBRES

Experiments on nerves in situ and on isolated nerves provide no evidence that morphine interferes with impulse transmission in myelinated or nonmyelinated nerve fibres. The concentrations used in experiments on isolated nerves were 10- to 100-times as high as those required to depress transmission at autonomic nerve-effector cell junctions. Examination of the resting membrane potential, the action potential and the positive after-potential, the conduction velocity, the time courses of the recovery of the size of the action potential and of the excitability after a conditioning stimulus, the ability of the axons to sustain repetitive activity and the posttetanic hyperpolarization gave no indication that morphine affects either the mechanisms involved in the initiation of the propagated impulse or those leading to restoration of the resting state after activity. Analgesic drugs, such as pethidine and methadone which have a local anaesthetic action, may cause a reversible decrease in the size of the compound action potential and in the conduction velocity of A-B and C fibres.

Opioid agonist/antagonists in general anaesthesia

Pain is a symptom that is universally recognized yet poorly treated. Almost every patient undergoing surgery will experience pain postoperatively. There is increased interest in the control of this pain and many new analgesics have been introduced. One such group of analgesics, the agonist/antagonist opioids, has evoked much interest among anaesthetists.

[The additive index--a new preclinical indicator of the addictive potential of analgesic agents]

A comparative study of analgesic and secondary-reinforcing properties of morphine, nalorphine, pentazocine and phencyclidine was carried out on mice. Different rations of their analgesic and reinforcement potencies were revealed. Based on these results, a new indicator--"addictive index" is proposed for characterization of the addictive potential of analgesics.

Analgesic effectiveness of the narcotic agonist-antagonists

1 Two fundamentally different types of narcotic-antogonists have been found to be very effective analgesics with relatively low dependence-producing potentials.

2 These two drug classes can be distinguished as being either morphine-like or nalorphine-like on the basis of their subjective and objective effects after single doses and on chronic administration, and by the character of their abstinence syndromes on abrupt withdrawal or on precipitation by other antagonists.

3 To explain differences in side effects associated with their analgesic actions, the existence of three types of receptors has been postulated: a μ receptor which is believed to be associated with euphoria and other typical morphine-like effects and a kappa (χ) and a sigma (σ) receptor which are believed to be associated with the sedative and psychotomimetic effects, respectively, of the nalorphine-like drugs.

4 The antagonist-analgesics of the morphine-type have the characteristics of being agonists of low intrinsic activity but with high affinity for the μ receptor. Representative analgesics of this type are profadol, propiram and buprenorphine.

5 The antagonist-analgesics of the nalorphine-type are drugs which are believed to have varying degrees of affinity and intrinsic activity at all three receptors, but characteristically seem to act merely as competitive antagonists with no intrinsic activity at the μ receptor. Representative analgesics of this type are pentazocine, nalbuphine and butorphanol.

6 There are considerable differences among the individual drugs of each type in terms of their analgesic and narcotic-antagonistic potencies. However, clear differences in analgesic efficacy among any of the antagonist-analgesics remain to be proved. All give evidence of being capable of relieving pain in nondependent patients in situations in which doses of morphine (or its surrogates) usually used would be effective.

7 The major advantages of the partial agonists of the morphine-type over the nalorphine-type drugs are that they have not been found to produce psychotomimetic reactions, and they seem to have fewer potentially deleterious effects in cardiac patients.

[Effect of morphine and its derivatives on the electric conductivity of artificial phospholipid membranes]

It has been shown that morphine, codeine, dionine and nalorphine do not change significantly the electroconductivity of bilayer lipid membranes (BLM) in the presence of potassium ions in the medium. Changes in BLM phospholipid composition, KC1 and opiate concentration in the medium, as well as the ratio between ionized and unionized forms of the narcotics do not affect the conductivity of lipid bilayers. A conclusion is made that morphine derivatives do not produce immediate effect on membrane potassium permeability.

Human pharmacology of narcotic antagonists

1 Human studies at the Addiction Research Center enable narcotic antagonists to be classified into three subgroups: (1) nalorphine-like agents; (2) pure antagonists; and (3) morphine-like agents. 2 Six narcotic antagonists (pentazocine, nalbuphine, cyclazocine, butorphanol, propiram and buprenorphine) developed in recent years seem to have a lesser abuse potential than codeine or propoxyphene. 3 When adjusted for relative availability of the agents, epidemiological data shows that pentazocine is abused less than codeine or propoxyphene in the US. 4 Recent studies with buprenorphine indicate that this agent would find application both as an analgesic of low abuse potential and as a new type of drug for the treatment of addiction.

The opiate receptor: a model explaining structure-activity relationships of opiate agonists and antagonists

A model of the opiate receptor is proposed which explains structure-activity relationships of opiate drugs, including (i) the unique potency of certain opiates such as etonitazene, fentanyl, phenazocine, and oripavines; (ii) the role of N-allyl substituents in conferring antagonist properties; and (iii) chemical features that afford "pure" antagonists. The model indicates mlecular mechanisms for interconversion of the opiate receptor between respective states that bind agonists or antagonists with high affinity.