l-a-Acetylmethadol, l-a-Acetyl-N-normethadol and l-a-Acetyl- N,N-dinormethadol: Comparisons with Morphine and Methadone in Suppression of the Opioid Withdrawal Syndrome in the Dog

l-a-Acetyl-N-normethadol (nor-LAAM) and l-a-acetyl-N,N-dinormethadol (dinor-LAAM) are active metabolites of the opiate l-a-acetylmethadol (LAAM), and they contribute to the prolonged actions of the parent compound. Single doses of nor-LAAM, dinor-LAAM, LAAM, methadone and morphine were given intravenously to the chronic spinal dog to determine acute, singledose effects and their ability to suppress withdrawal in morphinedependent dogs. These opioids produced dose-dependent antinociception, decreases in body temperature and pupillary constriction. For these measures, dinor-LAAM was 1.5 to 3 times and nor-LAAM 6 to 12 times as potent as LAAM. Five hours after the acute administration of LAAM or either of the metabolites, a 1-mg/kg dose of naltrexone given intravenously produced withdrawal, indicating the presence of acute physical dependence. In dogs physically dependent on a daily dose of 125 mg of morphine, nor-LAAM was 9 times as potent as either LAAM or dinor-LAAM in suppressing spontaneous withdrawal 40 hr after the last dose of morphine. The efficacies of LAAM and its demethylated metabolites in the dog for producing acute opiate effects were comparable with those of morphine and methadone. There was a trend, however, for LAAM to suppress the expression of abstinence more fully than either metabolite. The usefulness of LAAM as a treatment for opiate addiction is likely due in part to the equivalent efficacies and higher potencies of its nor and dinor metabolites. For more than 20 years, LAAM, a synthetic derivative of d-methadone (Pohland et al., 1949), has been proposed as an alternate to methadone in maintenance therapy. In July 1993, LAAM was approved in the United States for use in opioid substitution therapy (Federal Register, 1993). Four decades ago, Fraser and Isbell (1952) characterized the pharmacological actions and assessed the abuse liability of LAAM. They found the effects of a single oral dose of LAAM to be morphine-like. Furthermore, 30 to 60 mg of LAAM, given orally, relieved abstinence in morphine-dependent subjects (400 mg/day). A single, 40 to 60-mg dose of LAAM orally substituted for morphine (60 mg q.i.d.) and suppressed the abstinence syndrome for up to 72 hr. Morphine-like effects of LAAM required 4 to 6 hr to emerge after subcutaneous or intravenous injection and 1.5 hr to appear after oral administration, suggesting the formation of active metabolites. The initial preclinical pharmacological, metabolic and toxicological studies of LAAM and its metabolites included rodents, dogs and monkeys among the species tested (Archer, 1976). Metabolites of LAAM, particularly the N-demethylated compounds, nor-LAAM and dinor-LAAM, had more rapid onsets of action than LAAM, tended to be more potent than the parent compound (i.e., particularly nor-LAAM) and persisted in animals longer than the metabolites of methadone. For these reasons, the formation of nor metabolites was postulated to confer the long duration of action of LAAM (Archer, 1976; Wolven and Archer, 1976). Except for two analgesic assessments of nor-LAAM (Gruber and Babtisti, 1962; Houde et al., 1962), no other pharmacodynamic data exist for norand dinor-LAAM in humans; consequently, their activity profiles must be inferred from animal models. The chronic spinal dog is a validated model for assessing the agonistic actions and dependence-producing properties of morphine-like opioids (Martin and Jasinski, 1977). Using this model, studies were designed to compare the actions and relative potencies of nor-LAAM and dinor-LAAM to morphine, methadone and LAAM. Pharmacological profiles based on single-dose effects, suppression studies of morphine withdrawal and precipitated abstinence were used to define the opiate-like nature of nor-LAAM and dinor-LAAM. Received for publication February 25, 1997. ABBREVIATIONS: ANOVA, analysis of variance; AUC, area under time action curve; dinor-LAAM, l-a-acetyl-N,N-dinormethadol; LAAM, l-aacetylmethadol; nor-LAAM, l-a-acetyl-N-normethadol. 0022-3565/97/2832-0833$03.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 283, No. 2 Copyright © 1997 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. JPET 283:833–842, 1997 833 at A PE T Jornals on N ovem er 9, 2017 jpet.asjournals.org D ow nladed from Materials and Methods All animal procedures were approved by the Institutional Animal Care and Use Committee of the NIDA Addiction Research Center (Lexington, KY) and were in accordance with the “Guide for the Care and Use of Laboratory Animals” of the National Institutes of Health. Animals. The study included 12 beagle-type, T10 chronic spinal dogs ranging in age from 2 to 6 years and weighing between 7 and 12 kg. One group of six animals was used to evaluate acute opiate effects, and the second group of six came from a colony of morphinedependent animals and were used to assess the suppression of opiate withdrawal. All dogs had been used in previous studies of opiaterelated compounds but had different drug histories. Physiological and behavioral measurements. The following autonomic, reflex and behavioral measures were acquired using methods that have been previously published (Martin et al., 1974, 1976; Vaupel et al., 1986). The dogs, which were trained to the laboratory setting, were positioned on their right sides and loosely restrained at the neck and abdomen. Vertical pupil diameter and lateral nictitating membrane width of the left eye were measured from photographs taken with a Polaroid close-up camera, and rectal body temperature was recorded continuously. Heart rate and respiration were counted by auscultation and visual observation, respectively. For these five autonomic measures, changes for a selected time period (e.g., 300 min) were based on AUCs: change in response 5 (AUCtreatment 2 AUCpretreatment baseline control)/minutesAUC. Spinal and supraspinal antinociception was determined using two nociceptive reflexes: the flexor reflex (reflex arc restricted to the spinal cord below the level of transection), and the skin twitch reflex (reflex arc to supraspinal levels), respectively. The left hindlimb flexor reflex was evoked at 1-min intervals by a programmed pneumatic toe squeezer delivering stimuli of three strengths (4.5, 9 and 18 p.s.i.) in a random order to the superior toe of the ipsilateral limb, which was placed in a freely moving, suspended sling connected to a chart recorder. The flexor reflex, evoked by a 3-sec compression of the toe, and any spontaneous movements or drug-induced (i.e., stepping reflex) reflexes were continuously recorded in an isotonic manner. For the flexor reflex, antinociceptive effects of opiates are measured by the extent to which the reflex amplitude is depressed and the maximally depressed reflex exhibits no response to the toe pinch stimulus. Results were expressed as the percentage of maximum antinociception using the percentage of maximum possible effect method. Individual reflex responses were first normalized by converting their amplitude (in mm) to a percentage of the mean of the nine predrug control values obtained for each stimulus pressure and calculating 5-hr AUCs: Maximum (%) antinociceptionflexor reflex 5 (AUC treatment /AUCmaximally depressed reflex) 3 100. The skin twitch reflex was evoked from a dermatome above the level of spinal cord transection using thermal stimulation (i.e., a heat lamp) directed at an India ink-blackened area of shaved skin on the left shoulder. The intensity of the heat lamp was adjusted for each dog to provide a control reaction time of 3 to 5 sec, and a 10-sec cutoff was used. Skin twitch reflex antinociceptive responses were calculated as follows: Maximum (%) antinociceptionskin twitch reflex 5 [(AUCtreatment 2 AUCpretreatment baseline control)/(AUCmaximum latency 2 AUCpretreatment baseline control)] 3 100. Naltrexone-precipitated withdrawal in dogs acutely treated with opiates and suppression of withdrawal abstinence in chronically maintained morphine-dependent spinal dogs were quantified using additional physiological symptoms and behaviors and the revised scoring system of Martin et al. (1976). These measures, with their weighting factors shown in parentheses, were as follows: yawning (4), lacrimation (6), rhinorrhea (8), salivation (4), tremor (4), restlessness (5), whining (16), gnawing (4), head tossing (14), barking (40), retching (38), urination (5), panting (3), fragmentary hindlimb stepping movements (2), continuous hindlimb stepping movements (i.e., the stepping reflex) (7), a 0.1°C change in temperature (24), a 1 beat/min change in heart rate (0.9) and a 1-mm change in pupillary diameter (4). Abstinence or suppression was quantified by multiplying the changes in incidence or magnitude of the response, relative to baseline values, by the weighting factor and summing the values over the three observation periods. According to this system, abstinence scores are positive and suppression scores are negative. Single-dose studies and precipitated abstinence. Subjects were six nondependent chronic spinal dogs. Their past experience included use in opiate-related studies, including dose-ranging experiments for the present study, but their drug histories were not identical. In the present study, the acute, single-dose effects of the three methadols were compared with those of morphine and methadone in two series of crossover experiments in which all dogs received all treatments using the following design. In the first series, the effects of morphine (0.5 and 2.0 mg/kg), LAAM (0.25 and 1.0 mg/kg), nor-LAAM (0.05 and 0.2 mg/kg), dinorLAAM (0.1 and 0.4 mg/kg), methadone (0.125 and 0.5 mg/kg) and a double-distilled water vehicle control (administered twice) were studied over a 12-week period using a randomized block design. The design incorporated six blocks, and each block was 2 weeks in length. Within each block, dogs received a pair of different drug treatments (one treatment per week). Treatment pairs were randomly assigned to dogs across the blocks to counterbalance for time. When a preliminary analysis indicated that higher doses of the methadols were needed to produce more complete dose-response curves, a second, 6-week series of experiments was added. The six treatments [LAAM (4.0 mg/kg), nor-LAAM (0.8 mg/kg), dinor-LAAM (1.6 mg/kg), morphine (0.125 and 2.0 mg/kg) and double distilled water] were tested using a 6 3 6 (dogs 3 doses) crossover design. In retrospect, a higher dose of methadone should have been included in the second series. However, in making a preliminary assessment of the data, we overestimated the efficacy of methadone. This error limited our data interpretation with respect to the efficacy and relative potency of methadone and its comparison with the LAAM compounds. An additional component to the second series of experiments was the use of a precipitated withdrawal paradigm to assess the development of acute physical dependence 5 hr after drug ad-