Lawrence W. Masten

Development of cellular tolerance to lethality and analgesia concurrent with physical dependence following repeated oral administration of LAAM

Following the repeated oral administration of l-alpha-acetylmethadol (LAAM) employing a dose (28 mg/kg per day) shown to induce its own metabolism by three- to four-fold, mice exhibited a rapid development of cellular tolerance and physical dependence which correlated on a temporal basis with this self-induction of LAAM metabolism. Evidence of cellular tolerance included significant elevations in the LAAM oral LD50, LAAM ICV (intracerebroventricular) LD50, LAAM oral AD50 and the LAAM ICV AD50 following repeated administration of this narcotic. Since the ICV parameters and the morphine AD50 were elevated over water control values, cellular tolerance appeared to play an important role in explaining the elevations noted for the oral parameters because the former were not influenced by changes in the rate of LAAM metabolism. Moreover, SKF-525A, a microsomal enzyme inhibitor, had little effect on the LAAM oral LD50 and the LAAM oral AD50, further indicating a minor role for dispositional tolerance. It is concluded that the induction noted for repeated oral LAAM administration most likely is responsible for generating more potent metabolites which in turn act to produce the cellular tolerance and physical dependence in these mice through constant exposure of the CNS.

Clinical Experiences with Repeated Oral Administration of 1-Alpha-Acetylmethadol (LAAM) in the Rhesus Monkey

ABSTRACTFollowing the repeated oral administration of 1-alpha-acetyl-methadol (LAAM) to three adult male rhesus monkeys employing 2.0 mg/kg on Monday, Wednesday and Friday of the first week and 4.0 mg/kg on the same days of the second week, physiological parameters including heart rate, respiration rate and rectal temperature were monitored at regular intervals. Moreover, the plasma antipyrine half-life was also determined at intervals in these animals as an indicator of liver microsomal metabolism.A number of findings of clinical significance were derived from these studies. First, heart rate proved to be the best physiological indicator of CNS depression and impending crisis following repeated oral LAAM administration. In such a circumstance, naloxone, a potent, pure narcotic antagonist, was quite successful in reversing the LAAM induced effects within minutes of its intravenous administration. However, a significant degree of difficulty was encountered with naloxone treatment due to its short duration ...

Effect of repeated oral propoxyphene administration on analgesia, toxicity and microsomal metabolism in the mouse

Induction of hepatic propoxyphene N-demethylase and aniline hydroxylase activities resulted following repeated oral administration of 25, 50 and 100 mg d-propoxyphene hydrochloride per kg daily in the mouse over a six-day period. A significant elevation in both enzyme activities was noted after a single dose of propoxyphene (100 mg/kg). A dose-related response characterized the observed induction of each microsomal enzyme activity. Pentobarbital sleeping times (a measure of in vivo microsomal activity) also exhibited dose-related decrements in hypnosis with increasing doses of propoxyphene. These effects appeared to correlate with the development of tolerance to both the analgesic and lethal properties of propoxyphene. Pretreatment with SKF-525A, a potent microsomal enzyme inhibitor, abolished this tolerance in each case. Furthermore, a lack of central nervous system cellular tolerance was demonstrated by the finding that intracerebroventricular LD50 values for propoxyphene in propoxyphene- and water-treated mice were identical to the value derived from naive mice. Thus, the observed tolerance seems to be the result of dispositional (metabolic) and not central nervous system tolerance.

Additional metabolic correlates of l-α-acetylmethadol (LAAM)-induced cellular tolerance and physical dependence: The role of the hepatic microsomal electron transport system

The microsomal cytochromes P-450 and b5 and the enzymes of the hepatic microsomal electron transport system (HMETS) including NADPH-cytochrome c reductase and NADPH oxidase activities were monitored in male ICR mice (25-30 g) over a six-day period following the repeated oral administration of 7, 14 and 28 mg/kg per day of l-alpha-acetylmethadol hydrochloride (LAAM) or an equivalent volume of water. Cytochrome P-450 and the microsomal enzyme activity of NADPH oxidase were maximally elevated (three- to four-fold above control values) by the third day of LAAM administration (28 mg/kg per day). These elevations not only correlated on a dose and a temporal basis with previously reported microsomal activities including LAAM N-demethylase, but also with the reported development of cellular tolerance and physical dependence following an identical regimen of LAAM. In addition, NADPH-cytochrome c reductase and cytochrome b5 increased in activity and content, respectively, after the repeated administration of this narcotic. However, the enzyme activity was first significantly elevated after only a single dose of LAAM. Thereafter, it showed a pattern of induction similar to that of NADPH oxidase. In contrast, cytochrome b5 was only elevated after the last repeated dose. The significance of these findings is discussed in some detail relative to the generation of the two analgesically active metabolites of LAAM.

The effect of hematin on the development of microsomal enzyme induction and physical dependence in mice following repeated oral propoxyphene administration

Male ICR mice (25 - 30 g) were pretreated with bovine hematin (20, 40 or 50 mumoles per kg per day, i.p.) for three days. During the next six-day period, animals received either 50 mg per kg per day propoxyphene-HCl or saline, p.o., in addition to the daily hematin injections. Only the highest hematin regimen depressed the induction of propoxyphene-N-demethylase activity significantly in the drug-treated animals. A similar depression below control levels was noted in the animals receiving only saline (p.o.) and hematin (i.p.). While hematin treatment abolished the metabolic tolerance to propoxyphene analgesia such treatment failed to generate any appreciable degree of physical dependence to propoxyphene as assessed by a challenge with naloxone. These findings may be helpful in assessing the risk factors associated with the widespread use of propoxyphene.

The role of the hepatic microsomal electron-transport system in the development of metabolic tolerance from repeated oral methadone administration in mice

The hepatic microsomal cytochromes P-450 and b5, as well as the enzymes of the hepatic microsomal electron-transport system (HMETS), including NADPH oxidase and NAPDH cytochrome c reductase, were monitored in male ICR mice (25 - 30 g) over a six-day period following repeated oral administration of methadone hydrochloride 12.5, 25, or 50 mg/kg per day, or an equivalent volume of water. Cytochrome P-450 content, when expressed per milligram of microsomal protein, was elevated as early as day 1 of administration. This increase in cytochrome P-450, which lasted throughout the period of administration, appeared to correlate with the previously reported increase in the hepatic microsomal enzyme methadone N-demethylase and tolerance to methadone lethality. The activities of the enzymes NADPH cytochrome c reductase and NADPH oxidase were both elevated significantly by day 2 of administration. However, these increases returned to control levels by day 6 of treatment. The only other cytochrome in the HMETS, cytochrome b5, showed no significant change following repeated oral methadone administration. Further, methadone administration depressed the hepatic microsomal protein content following two days of treatment and no elevation above control values was noted. The significance of these findings with respect to the role of the HMETS in the development of tolerance is discussed in some detail for methadone, as well as the findings previously reported by this laboratory for its acetylated congener, l-alpha-acetylmethadol.

Effect of the catecholamine-depleting agent 1-phenyl-3-(2-thiazolyl)-2-thiourea (U-14,624) on drug metabolism in the rat☆

Abstract Male rats injected with 1-phenyl-3-(2-thiazolyl)-2-thiourea (U-14,624) (25 mg/kg/day i.p.) for 3 days prior to induction of anesthesia with pentobarbital (40 mg/kg, i.p.) slept significantly (P N -demethylase, aniline hydroxylase and p -nitroanisole O -demethylase enzymes in vitro ; this effect could not be demonstrated at lower doses. Single doses of U-14,624 (100–200 mg/kg, i.p.) also suppressed the activities of the three oxidative enzymes. The suppression was positively correlated with reduced levels of hepatic microsomal cytochrome P-450. Levels of cytochrome b 5 and NADPH-cytochrome c reductase activity were not affected consistently by acute dosage with U-14,624. The inhibitory effects of single doses (100–400 mg/kg, i.p.) on all enzymatic systems were reversible, and recovery was complete within 48 hr. Whereas all three oxidative drug-metabolizing enzymes were inhibited in a mixed manner by in vitro exposure to U-14,624 (10 −5 –10 −2 M), neotetrazolium diaphorase was not inhibited by U-14,624 at concentrations as high as 5 mM. Inhibition of oxidative drug metabolism by U-14,624 is mechanistically related to depletion of cytochrome P-450, but inhibition of these enzymes in vitro indicates that a second inhibitory mechanism may also be operative.