Enhanced buprenorphine analgesia with the addition of ultra-low-dose naloxone in healthy subjects.

Animal studies have demonstrated that co-administration of an ultra-low-dose opioid antagonist with an opioid agonist may result in enhanced analgesia. Investigation of this effect in humans has been limited and produced inconsistent findings, with previous reports suggesting that dose ratio may be critical to analgesic potentiation. The aim of the current investigation was to determine whether buprenorphine analgesia could be enhanced with the addition of ultra-low-dose naloxone among healthy volunteers, using a range of dose ratios. Tolerance to cold pressor pain was significantly greater with the combination of buprenorphine and naloxone compared to buprenorphine alone, and this effect was dose ratio dependent. Importantly, this enhanced analgesia occurred without an increase in adverse effects; indeed at some ratios, respiratory depression was attenuated. These findings demonstrate that the addition of ultra-low-dose naloxone can enhance the analgesic effect of buprenorphine in humans without a concurrent increase in side effects.

Precipitated withdrawal following codeine administration is dependent on CYP genotype.

The role of metabolic polymorphism in the development of physical dependence to codeine was assessed in cytochrome P450 2D2 (CYP2D2) deficient Dark Agouti and CYP2D2 intact Sprague-Dawley rats by assessment of the severity of naloxone precipitated withdrawal after codeine and morphine administration. Plasma morphine concentrations after codeine were significantly higher (P<0.01) in Sprague-Dawley than in Dark Agouti rats with metabolic ratios of 0.71 +/- 0.27 and 0.07 +/- 0.04, respectively. Withdrawal after codeine resulted in significantly greater hypothermia (3.5-4 degrees C, P<0.0001) in Sprague-Dawley animals compared to the other groups. Body weight loss was similar for all groups ranging from 6.2 +/- 0.4 to 8.2 +/- 0.6 g. When strain and treatment data were combined, a relationship between body temperature and plasma morphine concentration could be described by the inverse Hill equation (r(2)=0.76, EC(50)=556 +/- 121 ng/ml, n=2.9 +/- 1.5). These data indicate that dependence and withdrawal after codeine administration are dependent on its bioconversion to morphine.

Hyperalgesic responses in methadone maintenance patients.

Opioid substitution treatment for dependence may alter sensitivity to pain. Previous studies on pain sensitivity in methadone maintenance patients have yielded contradictory results. This study compared nociceptive responses between 16 patients on stable, once daily, doses of methadone and 16 matched control subjects. Two types of nociceptive stimuli were used: (1) electrical stimulation; and (2) a cold pressor test. Two parameters were measured: detection for onset of pain, and pain tolerance. Methadone patients were tested over an inter-dosing period: at the time of trough plasma methadone concentration (0 h), and 3 h after their daily dose. Control subjects were tested twice 3 h apart. Blood samples were collected to determine plasma methadone concentration. In methadone patients, trough to peak increases in mean R-(-)- and S-(+)-methadone concentrations (118 and 138 ng/ml to 185 and 259 ng/ml, respectively) resulted in significant increases in pain detection and tolerance values for both nociceptive stimuli. Using electrical stimulation, methadone patients' pain tolerance values were lower than controls at 0 h, but higher than controls at 3 h; no significant differences in pain detection values were found. For the cold pressor test, methadone patients detected pain significantly earlier than controls at 0 h, and were also substantially less pain tolerant than controls at both 0 and 3 h. There were no significant differences in pain detection values between the two groups at 3 h. Pain tolerance to pain detection ratios for methadone patients were significantly lower than controls for the cold pressor test at 0 and 3 h, and for electrical stimulation at 0 h only. In summary, the relative pain sensitivity of methadone maintenance patients is determined by the nature of the nociceptive stimulus (e.g. cold pressor test versus electrical stimulation), the plasma methadone concentration (trough versus peak plasma concentration), and whether thresholds are determined for detection of pain or pain tolerance. Although responding to changes in plasma methadone concentration, maintenance patients are markedly hyperalgesic to pain induced by the cold pressor test.

The relationship between mood state and plasma methadone concentration in maintenance patients.

Although methadone maintenance is designed to stabilize opioid-dependent patients, some experience significant withdrawal in the latter part of the 24-hour interdosing interval. This study was designed to determine the mood changes that maybe associated with such withdrawal. Eighteen methadone patients, nine of whom experienced significant withdrawal, were tested over a single interdosing interval. During this time, 13 blood samples were collected to measure plasma racemic methadone concentrations, and the Profile of Mood States (POMS) was administered on 11 of these occasions. The POMS was also administered on 11 occasions over 24 hours to 10 drug-free healthy controls. In comparison with controls, methadone patients showed increased anger, depression, tension, confusion, and fatigue, as well as decreased vigor. For all scales, maximal differences from controls occurred at times of trough methadone concentration and minimal differences around the time of peak concentration. Changes in mood over the interdosing interval were more exaggerated in the nine patients who experienced significant withdrawal compared with those who did not. The composite Total Mood Disturbance (TMD) scores were calculated for each subject at each time point. The sigmoid Emax model was used to relate plasma concentrations to these data and to calculate the slope factor (N). This model could be fitted for 14 of the 18 patients with a mean +/- SEM slope factor of 2.2 +/- 0.5. TMD score was also shown to be inversely related to the rate of decline in methadone concentration from peak to trough. These results show that significant mood changes occur in response to changes in methadone concentration, and these are more pronounced in those who experience withdrawal. The concentration-effect relationships suggest that relatively small changes in plasma concentration will result in significant mood change. Differences in the degree of mood change between those who do and do not experience significant withdrawal may be explained by variation in the rate of decline in plasma concentration from peak to trough.

Steady-state pharmacokinetics of (R)- and (S)-methadone in methadone maintenance patients.

AIMS: To investigate the steady-state pharmacokinetics of (R)- and (S)-methadone in a methadone maintenance population. METHODS: Eighteen patients recruited from a public methadone maintenance program underwent an interdosing interval pharmacokinetic study. Plasma and urine samples were collected and analysed for methadone and its major metabolite (EDDP) using stereoselective h.p.l.c. Methadone plasma protein binding was examined using ultrafiltration, and plasma alpha1-acid glycoprotein concentrations were quantified by radial immunoassay. RESULTS: (R)-methadone had a significantly (P < 0.05) greater unbound fraction (mean 173%) and total renal clearance (182%) compared with (S)-methadone, while maximum measured plasma concentrations (83%) and apparent partial clearance of methadone to EDDP (76%) were significantly (P < 0.001) lower. When protein binding was considered (R)-methadone plasma clearance of the unbound fraction (59%) and apparent partial intrinsic clearance to EDDP (44%) were significantly (P < 0.01) lower than for (S)-methadone, while AUCtau_¿u¿ss (167%) was significantly (P < 0. 001) greater. There were no significant (P > 0.2) differences between the methadone enantiomers for AUCtauss, steady-state plasma clearance, trough plasma concentrations and unbound renal clearance. Patients excreted significantly (P < 0.0001) more (R)-methadone and (S)-EDDP than the corresponding enantiomers. Considerable interindividual variability was observed for the pharmacokinetic parameters, with coefficients of variation of up to 70%. CONCLUSIONS: Steady-state pharmacokinetics of unbound methadone are stereoselective, and there is large interindividual variability consistent with CYP3A4 mediated metabolism to the major metabolite EDDP; the variability did not obscure a significant dose-plasma concentration relationship. Stereoselective differences in the pharmacokinetics of methadone may have important implications for pharmacokinetic-pharmacodynamic modelling but is unlikely to be important for therapeutic drug monitoring of methadone, in the setting of opioid dependence.

Differential behavioural and neurochemical effects of para-methoxyamphetamine and 3,4-methylenedioxymethamphetamine in the rat.

1. This study was prompted by recent deaths that have occurred after recreational administration of the substituted amphetamine para-methoxyamphetamine (PMA). Because relatively little is known regarding its mechanism(s) of action, its effects on physiological, behavioural and neurochemical parameters were compared with the well known effects of 3,4-methylenedioxymethamphetamine (MDMA). 2. Equivalent doses of PMA (5-20 mg/kg) produced greater hypothermia than MDMA at an ambient temperature of 20 degrees C. At 30 degrees C, PMA continued to evoke hypothermia except the highest dose where hyperthermia ensued. MDMA altered body temperature only at the highest dose where hyperthermia also resulted. 3. At both 20 and 30 degrees C, MDMA stimulated locomotor activity whereas PMA had modest effects and then, only at high doses. 4. In vivo chronoamperometry was used to measure the effect of MDMA and PMA on release, and inhibition of uptake, of serotonin (5-HT) and dopamine (DA) in the dorsal striatum of anaesthetised rats. As expected, MDMA evoked release of DA and inhibited uptake of both DA and 5-HT. By contrast, PMA was a relatively weak releasing agent and did not inhibit DA uptake. However, PMA potently inhibited uptake of 5-HT. 5. Taken together these data suggest that the acute adverse effects of PMA are more likely to be associated with alterations in serotonergic rather than dopaminergic neurotransmission.

Stereoselective quantification of methadone and its major oxidative metabolite, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine, in human urine using high-performance liquid chromatography.

A stereoselective HPLC assay was developed for the quantification of the enantiomers of methadone and its major oxidative metabolite, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) in human urine. The compounds were quantified in a single assay following liquid-liquid extraction and stereoselective HPLC with UV detection. Calibration curve concentrations ranged from 0.125 to 12.5 microM for each enantiomer. Assay performance was assessed using quality control samples, and the inter- and intra-assay bias (<10%) and precision (<15%) were acceptable for all compounds. The assay was successfully used to quantitate the enantiomers of methadone and EDDP in urine samples obtained from subjects receiving methadone maintenance therapy.

Flunitrazepam oxidative metabolism in human liver microsomes: involvement of CYP2C19 and CYP3A4.

The aims were to examine the kinetics of the oxidative metabolism of flunitrazepam in vitro when flunitrazepam was dissolved in dimethylformamide and acetonitrile, and to determine which cytochrome P450 isoform(s) are involved. The kinetics of the formations of 3'-hydroxyflunitrazepam and desmethyl-flunitrazepam were non-linear and best estimated using the Hill equation. Inhibition of their formation was studied using specific chemical inhibitors, expressed enzyme systems and specific antibodies. Ks, Vmax, Clmax and n (slope factor) for the formation of 3'-hydroxyflunitrazepam and desmethylflunitrazepam had ranges of 165-338 and 179 391 microM, 22-81 and 3-10 nmol x mg protein(-1) x h(-1), 6-17 and 0.9-1.9 microl x mg protein(-1) x h(-1), and 2.3-3.6 and 1.6-2.6 respectively when dimethylformamide was the organic solvent. When acetonitrile was the solvent, Ks, Vmax, Clmax and n (slope factor) for the formation of 3'-hydroxyflunitrazepam and desmethylflunitrazepam had ranges of 173-231 and 74-597 microM, 35-198 and 2.7-48 nmol.mg protein(-1) x h(-1), 1347 and 0.7-6.3 microl.mg protein(-1) x h(-1), and 1.5-3.6 and 1.1-2.7 respectively. CYP2C19, CYP3A4 and CYP1A2 mediated the formation of both 3'-hydroxyflunitrazepam and desmethylflunitrazepam. Investigators need carefully to consider the choice of organic solvent to avoid false CYP identification.

Simultaneous quantification of amoxycillin and metronidazole in plasma using high-performance liquid chromatography with photodiode array detection.

A simple, robust HPLC method was developed to measure simultaneously the plasma concentrations of amoxycillin and metronidazole in order to assess their disposition in the eradication of Helicobacter pylori. Plasma samples were protein precipitated, pH adjusted and the supernatant injected onto the HPLC system which used a C18 column, paired-ion aqueous mobile phase and photodiode array detection of amoxycillin at 230 nm and metronidazole at 313 nm. Intra- and inter-day precision and inaccuracy were less than 10% for concentrations between 5 and 20 mg/l. The limit of quantification was 1 mg/l. Samples were stable on the HPLC injector for 48 h at room temperature and multiple freeze-thaw cycles led to no decomposition.

Acute toxicity of 3,4-methylenedioxymethamphetamine (MDMA) in Sprague-Dawley and Dark Agouti rats.

Ingestion of MDMA ("ecstasy") by humans can cause acute toxicity manifested by hyperthermia and death. Demethylenation of MDMA is catalyzed by cytochrome P-450 2D6 (CYP2D6) and cytochrome P-450 2D1 (CYP2D1) in humans and rats, respectively, and is polymorphically expressed. It has been proposed that CYP2D6 deficiency may account for the unexplained toxicity of MDMA. The female Dark Agouti rat is deficient in CYP2D1, and serves as a model for the human poor metabolizer. We investigated thermogenic and locomotor actions of MDMA in adult female Sprague-Dawley (CYP2D1 replete) and Dark Agouti rats. MDMA (2, 5, and 10 mg/kg) and saline were injected subcutaneously at ambient temperatures of 22 and 31 degrees C. There was no difference in core temperature responses between the two rat strains. Hypothermia occurred in the first 30 min and temperature elevation thereafter. MDMA increased locomotor activity in Sprague-Dawley but not in Dark Agouti rats. However, MDMA had pronounced lethal effects at 31 degrees C ambient in the Dark Agouti rats only. We conclude that the poor metaboliser phenotype may predispose to lethality, but the mechanism is as yet unknown.

Comparison of (S)-mephenytoin and proguanil oxidation in vitro: contribution of several CYP isoforms.

AIMS: To compare the oxidative metabolism of (S)-mephenytoin and proguanil in vitro and to determine the involvement of various cytochrome P450 isoforms. METHODS: The kinetics of the formation of 4'-hydroxymephenytoin and cycloguanil in human liver microsomes from 10 liver samples were determined, and inhibition of formation was studied using specific chemical inhibitors and monoclonal antibodies directed towards specific CYP450 isoforms. Expressed CYP450 enzymes were used to characterize further CYP isoform contribution in vitro. Livers were genotyped for CYP2C19 using PCR amplification of genomic DNA followed by restriction endonuclease digestion. RESULTS: All livers were wildtype with respect to CYP2C19, except HLS#5 whose genotype was CYP2C19*1/CYP2C19*2. The Km, Vmax and CLint values for the formation of 4'-hydroxymephenytoin from (S)-mephenytoin and the formation of cycloguanil from proguanil ranged from 50.8 to 51.6 and 43-380 microm, 1.0-13.9 and 0.5-2.5 nmol mg-1 h-1, and 20.2-273.8 and 2.7-38.9 microl h-1 mg-1, respectively. There was a significant association between the Vmax values of cycloguanil and 4'-hydroxymephenytoin formation (rs=0.95, P=0.0004). Cycloguanil formation was inhibited significantly by omeprazole (CYP2C19/3A), troleandomycin (CYP3A), diethyldithiocarbamate (CYP2E1/3A), furafylline (CYP1A2), and (S)-mephenytoin. 4'-Hydroxymephenytoin formation was inhibited significantly by omeprazole, diethyldithiocarbamate, proguanil, furafylline, diazepam, troleandomycin, and sulphaphenazole (CYP2C9). Human CYP2E1 and CYP3A4 monoclonal antibodies did not inhibit the formation of cycloguanil or 4'-hydroxymephenytoin, and cycloguanil was formed by expressed CYP3A4 and CYP2C19 supersomes. However, only expressed CYP2C19 and CYP2C19 supersomes formed 4'-hydroxymephenytoin. CONCLUSIONS: The oxidative metabolism of (S)-mephenytoin and proguanil in vitro is catalysed by CYPs 2C19 and 1A2, with the significant association between Vmax values suggesting that the predominant enzymes involved in both reactions are similar. However the degree of selectively of both drugs for CYP isoforms needs further investigation, particularly the involvement of CYP3A4 in the metabolism of proguanil. We assert that proguanil may not be a suitable alternative to (S)-mephenytoin as a probe drug for the CYP2C19 genetic polymorphism.

Steady-state pharmacokinetics and pharmacodynamics in methadone maintenance patients: comparison of those who do and do not experience withdrawal and concentration-effect relationships.

OBJECTIVE: To determine plasma racemic methadone concentration-effect relationships for subjective and objective responses and whether pharmacokinetic and/or pharmacodynamic factors influence withdrawal severity. METHODS: Eighteen patients enrolled in a public methadone maintenance program, nine of whom experienced significant withdrawal, received constant doses of methadone once daily for at least 2 months. During an interdosing interval, 13 blood samples were collected to measure plasma racemic methadone concentrations (patients); subjective (withdrawal severity, direct opioid effects, and pain threshold) and objective (pupil diameter and respiratory rate) opioid effects were quantified on 11 occasions (all participants). The sigmoid Emax model was used to relate plasma concentrations and effects and to calculate the slope factor (N). The rate of decline in plasma concentration during each hour from the peak to the trough concentration was calculated. RESULTS: There was an inverse relationship between plasma concentrations and withdrawal severity and pupil diameter, as well as a direct relationship with subjective opioid effects and pain threshold. The mean N values were 5.4+/-0.9 for withdrawal severity, 5.1+/-1.1 for subjective opioid effects, 1.2+/-0.1 for pupil diameter, and 2.8+/-0.7 for pain threshold. Withdrawal severity correlated with the maximum rate of decrease in plasma concentration (P < .01). There were no differences between those who reported significant withdrawal and those who did not with respect to mean area under the plasma concentration versus time curve and predose plasma concentration, but maximal rate of decline was greater in the former group (74.5 versus 42.1 ng/mL/h). CONCLUSIONS: In this group of long-term methadone-maintained recipients, opioid responses were strongly correlated with changes in plasma racemic methadone concentrations. For the subjective responses, notably withdrawal, small changes in plasma concentrations translate into relatively large changes in effect; therefore, clinically important withdrawal is a consequence of more rapid decline in methadone concentration.

Methadone N-demethylation in human liver microsomes: lack of stereoselectivity and involvement of CYP3A4.

AIMS: To investigate the kinetics of CYP-mediated N-demethylation of methadone in human liver microsomes, and examine the role of stereoselectivity and CYP isoforms involved. METHODS: The kinetics of 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) formation via N-demethylation of rac-, (R)- and (S)-methadone in human liver microsomes prepared from six liver samples were determined by h.p.l.c., and inhibition of metabolic function was studied using isoform-specific chemical inhibitors and monoclonal antibodies. Microsomes containing expressed CYP3A4, CYP2D6 and CYP2C19 were also used to examine the formation of EDDP. RESULTS: The V max, Km, and CLint values for the formation of EDDP from rac-, (R)- and (S)-methadone were in the ranges of 20-77 nmol mg-1 protein h-1, 125-252 microm, and 91-494 ml h-1 g-1 protein. Km and CLint values for (R)- and (S)-methadone were not statistically significantly different (P >0.05), while V max values for (S)-methadone were 15% (P=0.045) lower than for (R)-methadone. Expressed CYP3A4 and CYP2C19 showed similar reaction rates for both (R)- and (S)-methadone, while CYP2D6 did not catalyse this reaction. Selective chemical inhibitors of CYP3A (troleandomycin, ketoconazole) and monoclonal human CYP3A4 antibodies significantly inhibited (P<0.05) the formation of EDDP in a concentration dependent manner by up to 80%. Sulphaphenazole (CYP2C9) also significantly inhibited (P<0.05) EDDP formation (range 14-25%). There were no statistically significant differences in the inhibition observed between the three substrates. Selective inhibitors of CYP1A2 (furafylline), CYP2A6 (coumarin), CYP2C19 ((S)-mephenytoin), CYP2D6 (quinidine) and CYP2E1 (diethyldithiocarbamic acid sodium salt and monoclonal human CYP2E1 antibodies) had no significant (P >0.05) effect. CONCLUSIONS: The N-demethylation of methadone in human liver microsomes is not markedly stereoselective, and is mediated mainly by CYP3A4 with the possible involvement of CYP2C9 and CYP2C19. Thus, the large interindividual variation reported for methadone pharmacokinetics may be due to variability in the expression of these CYP isoforms, and the reported stereoselectivity in the systemic clearance of methadone in vivo is not due to stereoselectivity in N-demethylation.

Quantification of flunitrazepam's oxidative metabolites, 3-hydroxyflunitrazepam and desmethylflunitrazepam, in hepatic microsomal incubations by high-performance liquid chromatography.

A high-performance liquid chromatographic assay for the quantification of the oxidative metabolites of flunitrazepam, 3-hydroxyflunitrazepam and desmethylflunitrazepam, in human liver microsomal incubations was developed. Both metabolites were quantifiable in a single assay following a solvent extraction and reversed-phase high-performance liquid chromatography with UV detection. Standard curve concentrations for both metabolites ranged from 0.2 to 10 microM. Assay performance was determined using quality control samples and the intra- and inter-day accuracy and precision as determined by the coefficient of variations which were less than 15% (0.5-6 microM) for both metabolites. This method provides good precision and accuracy for use in kinetic studies of the oxidative metabolism of flunitrazepam in human liver microsomes.

ADP, adrenaline and serotonin stimulate inositol 1,4,5-trisphosphate production in human platelets.

Although adenosine diphosphate (ADP) is a well-known stimulus of platelet aggregation, it is not the generally accepted view that ADP stimulates phosphatidylinositolbisphosphate (PtdIns(4,5)P2) hydrolysis. Using a very sensitive competitive receptor binding assay for inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), we have detected Ins(1,4,5)P3 production at early ( < 10 s) time points after stimulation of human platelets by the weak agonists ADP, adrenaline and serotonin (5-hydroxytryptamine, 5-HT). When adrenaline or 5-HT was combined with ADP in the presence of aspirin, there was a significant potentiation of ADP-induced platelet aggregation, but there was no potentiation of Ins(1,4,5)P3 generation. Also, the increases in intracellular calcium (Ca2+) concentrations stimulated by ADP were not potentiated by adrenaline in the presence of aspirin. Therefore, the synergism between the purinergic and adrenergic pathways of platelet activation occurs downstream from PtdIns(4,5)P2 hydrolysis and intracellular Ca2+ mobilization, although prior to platelet aggregation.

Association between CYP2C19 genotype and proguanil oxidative polymorphism.

AIMS: To examine the relationship between proguanil metabolism and the number of mutations in CYP2C19 by comparing the CYP2C19 genotype and proguanil phenotype of 10 subjects. METHODS: Partial clearance and urinary metabolic ratio data were obtained from a previous study of 10 subjects [5]. Analysis of CYP2C19 genotypes was performed using PCR amplification followed by restriction endonuclease digestion of genomic DNA from a blood sample. RESULTS: The intrinsic partial clearance of PG to CG ranged from 0.41-10.11 h-1, and was related to the number of functional CYP2C19 alleles present. Genotypic PMs had metabolic ratios > 13, while genotypic heterozygote EMs had metabolic ratios < 9. CONCLUSIONS: Proguanil may be a suitable phenotyping probe for the CYP2C19 genetic polymorphism, however the exact antimode of the urinary metabolic ratio chosen to separate poor and extensive metabolisers needs further investigation.

Approaches to rationing drugs in hospitals. An Australian perspective.

The increasing need to consider rationing strategies within the healthcare environment is being driven by pressures such as the development of new medical technologies, the aging population, patient awareness and expectations, healthcare professionals' appetite for new treatments, and government restrictions on healthcare costs. Solutions to these pressures will need to be sought urgently to avoid a situation in which quality of healthcare is affordable only for the wealthy. The fundamental principles of equity and fairness will need to be applied universally if the solutions are to be accepted by the community, patients and practitioners. There are several measures that a hospital must have in place before the concept of drug rationing can be contemplated. The approach essentially involves ensuring rational drug approval processes based on critical review of the available data, coupled with ongoing education and audit. Thus, accurate information and clinical budgeting systems, processes which encourage and ensure structural and technical efficiencies within the drug use sequence and an effective Drug and Therapeutics committee are required to facilitate this approach. To assist with its overriding goals of the quality use of medicines and optimal patient care, the Drug and Therapeutics committee needs to implement an effective formulary system, obtain detailed guidelines governing drug use within the institution, conduct an ongoing drug utilisation review programme, and provide education and training. The move to consider allocative decision making (rationing) will become increasingly necessary as limits on structural and technical efficiency measures are reached. An institution will then need to decide whether to treat a particular group of patients, or provide a certain form of treatment. Improved methods for community consultation need to be explored and there must be a partnership between the health provider and the consumer in enunciating the existence of scarcity, determining priorities and ensuring that ethics and equity are not abandoned through this process.

The influence of CYP2D6 polymorphism and quinidine on the disposition and antitussive effect of dextromethorphan in humans.

OBJECTIVES: We studied the disposition of dextromethorphan in extensive and poor metabolizer subjects, as well as the effect of this polymorphism on the antitussive action of dextromethorphan. METHODS: Six extensive metabolizers were studied on four occasions: (1) after 30 mg dextromethorphan, (2) after 30 mg dextromethorphan 1 hour before 50 mg quinidine, (3) after placebo, and (4) after 50 mg quinidine. Six poor metabolizers were studied on two occasions: (1) after 30 mg dextromethorphan and (2) after placebo. Blood and urine were collected over 168 hours and assayed for dextromethorphan, total (conjugated and unconjugated) dextrorphan, 3-methoxymorphinan, and total 3-hydroxymorphinan. On each occasion at each blood sampling time, capsaicin was administered as an aerosol to provoke cough. RESULTS: Dextromethorphan area under the plasma concentration-time curve (AUC) was 150-fold greater in the poor metabolizers than in the extensive metabolizers, and quinidine increased the AUC in extensive metabolizers 43-fold. The median dextromethorphan half-life was 19.1 hours in poor metabolizers, 5.6 hours in extensive metabolizers given quinidine, and 2.4 hours in extensive metabolizers. For dextrorphan (as total), the AUC was reduced 8.6-fold in poor metabolizers; quinidine had no effect on the AUC. The median half-life was 10.1 hours in poor metabolizers, 6.6 hours in extensive metabolizers given quinidine, and 1.4 hours in extensive metabolizers. The apparent partial clearance of dextromethorphan to dextrorphan was 1.2 L/hr in poor metabolizers, 78.5 L/hr in extensive metabolizers given quinidine, and 970 L/hr in extensive metabolizers. There was a strong (r2 = 0.82) and significant (p < 0.01) positive correlation between the prestudy urinary metabolic ratios and the partial clearances of dextromethorphan to dextrorphan. There was very large intersubject variability in responsiveness to capsaicin. There was no difference in the capsaicin-induced cough frequency in the three groups. Dextromethorphan had no antitussive effect in this experimental cough model. CONCLUSION: The disposition of dextromethorphan was substantially influenced by CYP2D6 status. Capsaicin may not be an ideal agent in experimental cough studies.

Pharmacokinetic evaluation of proguanil: a probe phenotyping drug for the mephenytoin hydroxylase polymorphism.

1. Proguanil (PG) oxidative metabolism to cycloguanil (CG) has been linked to the CYP2C19-mediated genetic polymorphism in S-mephenytoin oxidative metabolism. In many countries, rac-mephenytoin can no longer be administered to humans and hence proguanil may be a more suitable probe for phenotyping purposes. 2. There are limited data on the pharmacokinetics of PG and CG and in particular, whether there is a relationship between the urinary metabolic ratio of PG and its partial intrinsic clearance to CG. 3. The disposition of a 100 mg oral dose of PG was investigated in 10 subjects with widely varying metabolic ratios (pre-study urinary metabolic ratio CG to PG = 0.068 to 1.11). Blood samples and all urine were collected for 96 h and assayed for PG and CG by h.p.l.c. 4. The urinary recovery of PG ranged from 30 to 69% of the dose and for CG from 2.8 to 32% of the dose. The overall urinary recovery of PG plus CG ranged from 54 to 77% of the dose. The AUC for PG ranged from 3.2 to 9.5 mg l-1 h whereas for CG it was from 0.02 to 0.71 mg l-1 h. The partial intrinsic clearance to CG ranged 25-fold from 0.41 to 10.1 l h-1. 5. There was a highly significant (r2 = 0.96, P < 0.001) relationship between the urinary metabolic ratio for PG (as CG/PG) and its partial intrinsic clearance to CG. 6. These data have provided evidence for the justification of the use of the urinary metabolic ratio of proguanil for population phenotyping purposes, provided systematic variation in renal drug clearance between populations is considered.