In vitro-in vivo extrapolation predicts drug-drug interactions arising from inhibition of codeine glucuronidation by dextropropoxyphene, fluconazole, ketoconazole, and methadone in humans.

Because codeine (COD) is eliminated primarily via glucuronidation, factors that alter COD glucuronide formation potentially affect the proportion of the dose converted to the pharmacologically active metabolite morphine. Thus, in vitro-in vivo extrapolation approaches were used to identify potential drug-drug interactions arising from inhibition of COD glucuronidation in humans. Initial studies characterized the kinetics of COD-6-glucuronide (C6G) formation by human liver microsomes (HLM) and demonstrated an 88% reduction in the Michaelis constant (K(m)) (0.29 versus 2.32 mM) for incubations performed in the presence of 2% bovine serum albumin (BSA). Of 13 recombinant UDP-glucuronosyltransferase (UGT) enzymes screened for COD glucuronidation activity, only UGT2B4 and UGT2B7 exhibited activity. The respective S(50) values (0.32 and 0.27 mM) generated in the presence of BSA were comparable with the mean K(m) observed in HLM. Known inhibitors of UGT2B7 activity in vitro or in vivo and drugs marketed as compound formulations with COD were investigated for inhibition of C6G formation by HLM. Inhibition screening identified potential interactions with dextropropoxyphene, fluconazole, ketoconazole, and methadone. Inhibitor constant values generated for dextropropoxyphene (3.5 microM), fluconazole (202 microM), ketoconazole (0.66 microM), and methadone (0.32 microM) predicted 1.60- to 3.66-fold increases in the area under the drug plasma concentration-time curve ratio for COD in vivo. Whereas fluconazole and ketoconazole inhibited UGT2B4- and UGT2B7-catalyzed COD glucuronidation to a similar extent, inhibition by dextropropoxyphene and methadone resulted largely from an effect on UGT2B4. Interactions with dextropropoxyphene, fluconazole, ketoconazole, and methadone potentially affect the intensity and duration of COD analgesia.

Psychotropic drugs as behavioral teratogens.

Three psychotropic drugs were administered to pregnant rats and were then evaluated for their behavioral and reproductive effects in the offspring. Control rats received either saline or vitamin A. Prochlorperazine had the most disruptive effects on reproduction and growth, but had the least effect on behavior. Propoxyphene had no apparent effects on reproduction or growth, but produced a variety of behavioral changes. Fenfluramine was intermediate in its effects on reproduction and growth and had behavioral effects that were revealed in tests of preweaning development. The data suggest that systematic tests of behavior add important information to evaluations of reproductive toxicity that cannot, at present, be obtained by other means.

Screening of codeine, dextromethorphan & dextropropoxyphene for their genotoxicity in Swiss albino mice.

BACKGROUND & OBJECTIVE: It is mandatory for all new drugs to be tested for their potential genotoxicity in addition to general toxicity testing. Some old drugs have not been tested adequately for their genotoxic effects as these were in use before the regulations were enforced. The present study therefore aims to explore the genotoxic potential of some commonly used opioids like codeine, dextromethorphan and dextropropoxyphene in swiss albino mice. METHODS: Therapeutic equivalent doses of codeine, dextromethorphan and dextropropoxyphene were given orally. Single dose for acute study and multiple doses (repeated every 24 h for 7 times) in additional groups of mice (n=5 in each) for subacute study. Cyclophosphamide served as positive control while normal saline as negative control. About 0.5 ml of blood was collected by retroorbital sinus for comet assay and later the mice were sacrificed to aspirate the femoral bone marrow for micronucleus test. Percentage of micronucleated polychromatic erythrocytes (MnPCE) and comet tail length were calculated in micronucleus assay and comet assay respectively, which served as markers of genotoxicity. RESULTS: Significant (P<0.001) increase in comet tail length and % MnPCE was observed in both acute and subacute studies of cyclophosphamide group, whereas codeine, dextromethorphan and dextropropoxyphene treated groups did not show any significant changes. INTERPRETATION & CONCLUSION: The results indicated that codeine, dextromethorphan and dextropropoxyphene were devoid of genotoxicity in mice.

Marked QRS complex abnormalities and sodium channel blockade by propoxyphene reversed with lidocaine.

The opiate analgesic propoxyphene produces cardiac toxicity when taken in overdose. We recently observed a patient with propoxyphene overdose in whom marked QRS widening was reversed by lidocaine. The reversal is apparently paradoxical as both agents block the inward sodium current (INa). We examined possible mechanisms of the reversal by measuring INa in rabbit atrial myocytes during exposure to propoxyphene and the combination of propoxyphene and lidocaine (60 and 80 microM, respectively). Propoxyphene caused use-dependent block of INa during pulse train stimulation. Block recovered slowly with time constants of 20.8 +/- 3.9 s. Block during lidocaine exposure recovered with time constants of 2-3 s. During exposure to the mixture, block recovered as a double exponential. The half time for recovery during exposure to the mixture was 1.6 +/- .9 s compared with a half-time of 14.3 +/- 2.9 s during exposure to propoxyphene alone. During pulse train stimulation, less steady-state block was observed during exposure to the mixture than during exposure to propoxyphene alone when the interval between pulses was greater than 0.95 s. Both drugs compete for a common receptor during the polarizing phase. The more rapid dissociation of lidocaine during the recovery period leads to less block during the mixture than during exposure to propoxyphene alone. The experiments suggest a mechanism for reversal of the cardiac toxicity of drugs which have slow unbinding kinetics.

Methadone block of K+ current in squid giant fiber lobe neurons.

Voltage-dependent ionic currents were recorded from squid giant fiber lobe neurons using the whole-cell patch-clamp technique. When applied to the bathing solution, methadone was found to block IK, I Na and I Ca. Both I Na and I Ca were reduced without apparent change in kinetics and exhibited IC(50)'s of 50-100 and 250-500 mu M, respectively, at +10 mV. In contrast, IK was reduced in a time-dependent manner that is well fit by a simple model of open channel block (K(D)= 32+/- or 2 mu M, +60 mV, 10 degrees Celsius). The mechanism of I(K) block was examined in detail and involves a direct action of methadone, a tertiary amine, on K channels rather than an opioid receptor-mediated pathway. The kinetics of I(K) block resemble those reported for internally applied long chain quaternary ammonium (QA) compounds; and recovery from I(K) block is QA-like in its slow time course and strong dependence on holding potential. A quaternary derivative of methadone (N-methyl-methadone) only reproduced the effects of methadone on I(K) when included in the pipette solution; this compound was without effect when applied externally. I(K) block thus appears to involve diffusion of methadone into the cytoplasm and occlusion of the open K channel at the internal QA blocking site by the protonated form of the drug. This proposed mode of action is supported by the pH and voltage dependence of block as well as by the observation that high external K+ speeds the rate of drug dissociation. In addition, the effect of methadone on I(K) evoked during prolonged (300 ms) depolarizations suggests that methadone block may interfere with endogenous K+ channel inactivation. The effects of temperature, methadone stereoisomers, and the methadone-like drugs propoxyphene and nor-propoxyphene on IK block were examined. Methadone was also found to block I(K) in GH3 cells and in chick myoblasts.

Therapeutic usefulness of propoxyphene napsylate in narcotic addiction.

The maximum doses of propoxyphene napsylate used to treat heroin addicts produce a degree of morphine-like activity equal to that produced by 20 to 25 mg/day of subcutaneously given morphine or 10 mg/day orally given methadone. This degree of activity wound be sufficient to ameliorate abstinence even in patients dependent on large doses of narcotics--an observation that supports the utility of propoxyphene napsylate in detoxification. On the other hand, only patients taking 10 mg/day or less of parenterally administered heroin could be maintained on maximum subtoxic levels of propoxyphene napsylate without abstinence signs or symptoms suggesting that propoxyphene napsylate would be less useful in maintenance therapy.

Drug- and mutagenesis-induced changes in the selectivity filter of a cardiac two-pore background K+ channel.

OBJECTIVE: As compared with voltage-gated K(+) channels (Kv-type), our knowledge of the structure-function and pharmacology of two-pore background K(+) channels is still very limited. Here we have used a drug- and mutagenesis-based approach to study the effect of the antidepressant fluoxetine (FL) and analgesic D-norpropoxyphene (NORP) on the cardiac two-pore background K(+) channel. METHODS: Whole-cell currents of the cTBAK-1 channel expressed in Xenopus laevis oocytes were investigated using conventional two-microelectrode voltage-clamp recording method combined with functional mutagenesis of the channel protein. RESULTS: Both drugs inhibit cTBAK-1 current: FL proved to be a voltage-dependent pore-blocker, while NORP induced a change in the selectivity of cTBAK-1 giving rise to a shift in the reversal potential (E(rev)) toward more positive voltages due to an increased Na(+) permeability. Mutations were introduced into the selectivity filter of the first (Y105F) and the second (F211Y) pore to mimic the P-region of HERG (GFGN) and Kv1.1 (GYGD) channels. Point mutations in the channel resulted in two distinct phenotypes of cTBAK-1: the mutant Y105F channel lost its selectivity and was unaffected by NORP, in contrast to the F211Y mutant. CONCLUSION: FL and NORP block the current of cTBAK-1 channels differently, the latter modified the selectivity of the channel pore. Our mutagenesis study revealed that NORP interacts with the selectivity filter of cTBAK-1. The significant role of the GYGD motif in this type of K(+) channels is emphasized.

Norpropoxyphene-induced cardiotoxicity is associated with changes in ion-selectivity and gating of HERG currents.

OBJECTIVE: Norpropoxyphene (NP) is a major metabolite of propoxyphene (P), a relatively weak mu-opioid receptor agonist. Toxic blood concentrations ranging from 3 to 180 mumol/l have been reported and the accumulation of NP in cardiac tissue leads to naloxone-insensitive cardiotoxicity. Since several lines of evidence suggest that not only block of INa but also IK block may contribute to the non-opioid cardiotoxic effects of P and NP, we investigated the effects of P and NP on HERG channels. HERG presumably encodes IKr, the rapidly-activating delayed rectifier K+ current, which is known to have an important role in initiating repolarization of action potentials in cardiac myocytes. METHODS: Using the 2-microelectrode voltage clamp technique we investigated the interaction of P and NP with HERG channels, expressed in Xenopus oocytes. RESULTS: Our experiments show that low drug concentrations (5 mumol/l) facilitate HERG currents, while higher drug concentrations block HERG currents (IC50-values of approx. 40 mumol/l) and dramatically shift the reversal potential to a more positive value because of a 30-fold increased Na(+)-permeability. P and NP also alter gating of HERG channels by slowing down channel activation and accelerating channel deactivation kinetics. The mutant S631C nullifies the effect of P and NP on the channel's K(+)-selectivity. CONCLUSION: P and NP show a complex and unique drug-channel interaction, which includes altering ion-selectivity and gating. Site-directed mutagenesis suggests that an interaction with S631 contributes to the drug-induced disruption of K(+)-selectivity. No specific role of the minK subunit in the HERG block mechanism could be determined.

Analgesic activity of floctafenine after cholecystectomy.

In a double-blind, crossover study involving 40 post-cholecystectomy patients, the analgesic activity of a new oral nonnarcotic agent, floctafenine, was compared with that of oral meperidine and oral propoxyphene. A behavioral approach to the measurement of pain relief, i.e., the measurement of pain on movement or coughing, was used in addition to the accepted subjective method. Floctafenine 200 mg was found to have analgesic activity intermediate between that of meperidine 75 mg and propoxyphene 65 mg. Patients reported the fewest side effects after floctafenine. Initial pain level was found to affect the final pain score. Effects on pain relief due to age, sex, time of the day, and carryover effects were also examined and not found to be significant.

The cognitive and psychomotor effects of morphine in healthy subjects: a randomized controlled trial of repeated (four) oral doses of dextropropoxyphene, morphine, lorazepam and placebo.

Ten healthy subjects (four male) of mean age 31 years (range 25-40) took part in a randomized double-blind four-way crossover study to examine the cognitive and psychomotor effects of repeated oral doses of dextropropoxyphene and morphine. Four treatments were compared: dextropropoxyphene napsylate 100 mg, morphine sulphate 10 mg, lorazepam 0.5 mg and placebo. Four doses of each drug were given at 4-h intervals to each subject on four separate study days at least 1 week apart. Cognitive function was assessed using choice reaction time, number vigilance, memory scanning, immediate and delayed word recall, word recognition, picture recognition, critical flicker fusion threshold (CFFT) and subjective measures of alertness, calmness and contentment. Lorazepam impaired the speed of responding on all tasks in which speed was recorded (except digit vigilance) and increased subjective ratings of calmness. Morphine had one major effect, which was to increase the accuracy of responding on the choice reaction time task, at every assessment. Morphine produced some sporadic effects in other tests and an increase in subjective calmness. Dextropropoxyphene impaired performance on choice reaction time and picture recognition. These data show that oral morphine may enhance performance in some measures of cognitive function, whereas dextropropoxyphene (in usual therapeutic doses) seems more likely to cause impairment. Neither opioid has substantial effects on cognition and psychomotor function compared with lorazepam.

Further evidence for a motility substitution test as a tool to detect the narcotic character of new drugs in rats.

Time effects of three doses of codeine, d-propoxyphene, meperidine and dextromethorphan upon locomotor activity have been investigated in naive rats as well as in animals treated chronically with 20 mg/kg of morphine daily. The first three drugs exhibited a motility pattern qualitatively similar to that of morphine; in chronically-treated rats cross-tolerance occurred to acute depressant effects and cross-sensitization to acute excitatory effects. In the case of meperidine however cross-sensitization appeared to be small. Dextromethorphan presented effects similar to morphine only in a very large dose. The present data, together with those of previous work, demonstrate that a locomotor activity substitution test may predict the morphine-like addictive properties of a drug.

Effects of narcotic analgesics and antagonists on the in vivo release of acetylcholine from the cerebral cortex of the cat.

1. In cats under light allobarbitone anaesthesia, the effects of intravenous injections of narcotic and non-narcotic analgesics, of a general depressant, and of narcotic antagonists were investigated on the spontaneous release of acetylcholine (ACh) from the surface of the sensorimotor cortex.2. The narcotic analgesics morphine (0.1, 1.0 and 5 mg/kg), meperidine (1.0 and 2.0 mg/kg), methadone (1.0 mg/kg) and codeine (5.0 and 10.0 mg/kg) greatly reduced ACh release.3. The non-narcotic analgesics pentazocine (1.0 and 2.0 mg/kg) and propoxyphene (5.0 and 10.0 mg/kg) as well as the depressant chlorpromazine (0.25, 0.5 and 1.0 mg/kg) also greatly reduced ACh release.4. Two of the three narcotic antagonists examined, levallorphan (0.1, 1.0 and 5 mg/kg) and nalorphine (1.0 mg/kg) had the property of reducing ACh release. They were thus partial agonists. With levallorphan the greatest reduction occurred with the smallest dose injected and the effect was regularly obtained, whereas with nalorphine a reduction was obtained in some experiments only. The third, naloxone, was a specific narcotic antagonist and did not reduce the ACh release in any dose (0.01, 0.1, 0.5 and 1.0 mg/kg) examined. In a dose of 1.0 mg/kg it actually produced a small increase in Ach release.5. Naloxone (0.1-1.0 mg/kg) restored the reduction in ACh release produced by the narcotic analgesics and by the partial agonist levallorphan. It partially restored the reduction produced by the non-narcotic analgesics and by nalorphine, but had no effect on the reduction produced by chlorpromazine.6. The relevance of these results with regard to analgesia and to the narcotic abstinence syndrome is discussed.

The effect of narcotic analgesics on the uptake of 5-hydroxytryptamine and (-)-metaraminol by blood platelets.

1. The effects of narcotic analgesic and related drugs were studied on the uptake of 5-hydroxytryptamine (5-HT) and (-)-metaraminol by blood platelets.2. The most potent drug in inhibiting the uptake of 5-HT (10 muM) by human platelets was methadone, followed by pentazocine>piminodine approximately pethidine approximately anileridine approximately cyclazocine approximately thebaine > dextropropoxyphene. Alphaprodine, papaverine, apomorphine, nalorphine, codeine, and morphine were almost without effect. Methadone was slightly less active than desipramine, and had 10% of the activity of imipramine under similar conditions. Naloxone did not antagonize the effect of methadone on 5-HT uptake.3. The most potent inhibitor of metaraminol (3 muM) uptake by human platelets was piminodine, followed by pentazocine>/=anileridine>cyclazocine=methadone > dextropropoxyphene approximately thebaine >/= papaverine approximately alphaprodine >pethidine>morphine. The activity of morphine was 1% of that of piminodine. Piminodine was more potent than desipramine and protriptyline under similar conditions. The order of potency of drugs studied in inhibiting the uptake of metaraminol by rabbit platelets was similar to that obtained with human platelets.4. The effects of the analgesics studied on inhibiting uptake of monoamines did not correlate with their pain-relieving properties.

Simple analgesics.

A number of drugs are available that act fairly specifically as "mild" analgesics, although this description by no means implies that their clinical effectiveness is limited to the relief of slight pain and trivial disability. They are effective by mouth and their action is mediated peripherally. Among the possible mechanisms of action, the inhibition of prostaglandin synthesis is currently regarded as most likely to be relevant. Some centrally acting drugs of the narcotic analgesic type, such as codeine and dextropropoxyphene are effective orally; they are usable in the same way as other mild analgesics and may be preferable for some types of pain. Many problems arise in the assessment and comparison of mild analgesics, both experimentally and clinically. Subjective assessments may be made on a pain scale by the patient himself, or by a trained observer. Individual variations are all-important, and the limitations of controlled trials need to be remembered. Alternative drugs and mixtures have little advantage over aspirin, but specific drug tolerance, in the long term, varies from patient to patient. Gastric irritation is most likely to occur with aspirin in the presence of chronic dyspepsia or acute precipitating causes such as alchoholic gastritis. Allergy also occurs in some susceptible individuals. The risk of renal damage with phenacetin is increasingly appreciated, and the possibility of hepatic damage from paracetamol is now recognised. Other side-effects and interactions are summarized in the review, and some notes are given on therapeutic and non-therapeutic use.

Opioid analgesics as noncompetitive N-methyl-D-aspartate (NMDA) antagonists.

Much evidence points to the involvement of N-methyl-D-aspartate (NMDA) receptors in the development and maintainance of neuropathic pain. In neuropathic pain, there is generally involved a presumed opioid-insensitive component, which apparently can be blocked by NMDA receptor antagonists. However, in order to obtain complete analgesia, a combination of an NMDA receptor antagonist and an opioid receptor agonist is needed. Recent in vitro data have demonstrated that methadone, ketobemidone, and dextropropoxyphene, in addition to being opioid receptor agonists, also are weak noncompetitive NMDA receptor antagonists. Clinical anecdotes suggest that the NMDA receptor antagonism of these opioids may play a significant role in the pharmacological action of these compounds; however, no clinical studies have been conducted to support this issue. In the present commentary, we discuss evidence for the NMDA receptor antagonism of these compounds and its relevance for clinical pain treatment; an overview of structure-activity relationships for the relevant opioids as noncompetitive NMDA receptor antagonists also is given. It is concluded that although the finding that some opioids are weak noncompetitive NMDA receptor antagonists in vitro has created much attention among clinicians, no clinical studies have been conducted to evaluate the applicability of these compounds in the treatment of neuropathic pain conditions.

Methadone: a potent inhibitor of rat liver aldehyde oxidase.

Several drugs with structural similarities to SKF-525A were tested for their ability to inhibit rat liver aldehyde oxidase using the experimental antitumour agent N-[(2'-dimethylamino)ethyl]acridine-4-carboxamide (AC; NSC 601316; acridine carboxamide) as substrate. The antihistamine D-chlorpheniramine, and the antiarrhythmics disopyramide, procainamide and lignocaine were ineffective in inhibiting this reaction. The antihistamines diphenhydramine, pheniramine, doxylamine, orphenadrine, methapyrilene and pyrilamine, gave IC50 values of 100-500 microM. The narcotic analgesics D-propoxyphene and, in particular, methadone were potent inhibitors of acridine formation with IC50 values of 15.5 and 0.31 microM, respectively. Further analysis indicates mixed non-competitive type inhibition by methadone with inhibition constants (Kis and Kii, respectively) of 0.03 +/- 0.01 (SE) and 0.57 +/- 0.12 microM.

Characterisation of theophylline metabolism by human liver microsomes. Inhibition and immunochemical studies.

Anti-human NADPH-cytochrome P-450 reductase inhibited all theophylline metabolic pathways confirming the involvement of cytochrome P-450 isozymes in the metabolism of theophylline. Tolbutamide, debrisoquine, mephenytoin, theobromine, phenylbutazone, sulphaphenazole and sulphinpyrazone did not inhibit theophylline metabolism by human liver microsomes. Verapamil and dextropropoxyphene were non-selective competitive inhibitors of theophylline metabolism. Cimetidine and caffeine selectively inhibited the two demethylations as Ki values for these two pathways were lower than for the 8-hydroxylation pathway. The effects of nifedipine, propranolol and alpha-naphthoflavone were atypical. The degree of inhibition by propranolol reached a plateau, which was greater for the two demethylations than for the 8-hydroxylation. Alpha-naphthoflavone (ANF) at low concentrations inhibited the demethylations to a greater extent than the 8-hydroxylation. At higher concentrations ANF activated all pathways, with this effect being most marked for the 8-hydroxylation. Nifedipine inhibited the theophylline demethylations but not the 8-hydroxylation. In some livers the 8-hydroxylation was markedly activated. The results confirm that there are at least two distinct cytochrome P-450 isozymes involved in theophylline metabolism, one isozyme being involved with the demethylations and a different isozyme involved in the 8-hydroxylation pathway. Preliminary correlation studies suggest that the human orthologue to the rabbit polycyclic hydrocarbon inducible P-450 Form 4 may be involved in the N-demethylations of theophylline.

Naloxone-reversible analgesic action of SKF 525-A in mice.

SKF 525-A, given i.p. to mice at doses from 50 to 100 mg/kg, had analgesic activity approximately 40% the analgesic potency of d-propoxyphene HCl, a chemically similar narcotic. While the analgesia produced by propoxyphene was totally antagonized by naloxone, however, that produced by SKF 525-A was only partly reversed. We suspect that SKF 525-A may exert its antinociceptive actions partly by an interaction with CNS narcotic receptors and partly by a nonspecific sedating action.

Propoxyphene napsylate compared to methadone for opiate dependence.

When high single doses of propoxyphene napsylate (PN) were given to patients on a methadone maintenance program, results indicated that, to avoid undesirable side effects, the dose should not exceed 600 mg. However, when PN was given in divided doses (800 mg/day in two equal doses), no significant adverse reactions were noted. In the double-blind comparison of 800 mg PN in two divided doses versus 20 mg methadone, 10 mg methadone, or placebo methadone, it was found that PN (1) did not alleviate withdrawal symptoms in patients previously maintained on 20 mg methadone, (2) produced a slightly overmedicated effect in the detoxified group of ex-methadone patients, and (3) compared favorably to 10 mg methadone in suppressing withdrawal symptoms without producing evidence of overmedication in those patients previously stabilized on a methadone maintenance dose of 10 mg. It is concluded that on a mg for mg basis, PN at a dose of 80-times that of methadone will relieve withdrawal symptoms in the treatment of mildly addicted patients requiring 10 mg methadone or less per day.

Enhancement of propoxyphene-induced analgesia by doxepin.

The activity of the centrally acting analgesic, propoxyphene, either alone or combined with the tricyclic antidepressant, doxepin, has been studied. Doses of doxepin, in themselves lacking any analgesic effect, remarkably enhanced the analgesic activity of propoxyphene, either by the oral or intraperitoneal route. On the other hand, oral toxicity data prove that doxepin does not alter significantly propoxyphene acute toxicity.