Enantiomeric methadone quantitation on real post-mortem dried matrix spots samples: Comparison of liquid chromatography and supercritical fluid chromatography coupled to mass spectrometry

This study describes two bioanalytical methods for the quantitation of the two methadone enantiomers in dried matrix spots using high performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) and high performance supercritical chromatography tandem mass spectrometry (HPSFC-MS/MS). Dried matrix spots were obtained by spotting 10 µL of each sample fluid on a Whatman paper. Methadone and its main metabolite, EDDP, were extracted with 100 µL methanol and subsequently injected into the LC-MS/MS and SFC-MS/MS systems. Enantiomeric separation was achieved with AGP-column for the LC conditions and with Chiralpak IH-3 in SFC. The two methods were fully validated and 93 post-mortem samples were analysed with both analytical methods. Results from validation parameters and results obtained for all post-mortem samples were compared with a significant spearman correlation of r_s = 0.9978 for R-methadone and r_s = 0.9981 for S-methadone. The LC method provided better results in terms of uncertainty, retention factor and resolution, whereas SFC provides better sensitivity, with lower LOD. Median R-/S-methadone ratio in peripheral blood was found equal to 1.60 (N = 32), varying from 0.79 to 4.23. The reported values were in good agreement with previously published results. Based on the results obtained here, SFC-MS/MS can be considered a reliable alternative to the widely used LC-MS/MS for the quantitation of methadone enantiomers in bioanalysis and should be evaluated for other bioanalytical methods. Both methods can be easily and quickly used in toxicological routine analysis for the methadone quantitation in human fluids matrices, even if considering that the polysaccharide coated column IH-3 used in SFC does not allow the enantiomeric EDDP separation.

Atomic-Level Characterization of the Methadone-Stabilized Active Conformation of µ-Opioid Receptor

Methadone is a synthetic opioid agonist with notoriously unique properties, such as lower abuse liability and induced relief of withdrawal symptoms and drug cravings, despite acting on the same opioid receptors triggered by classic opioids-in particular the µ-opioid receptor (MOR). Its distinct pharmacologic properties, which have recently been attributed to the preferential activation of β-arrestin over G proteins, make methadone a standard-of-care maintenance medication for opioid addiction. Although a recent biophysical study suggests that methadone stabilizes different MOR active conformations from those stabilized by classic opioid drugs or G protein-biased agonists, how this drug modulates the conformational equilibrium of MOR and what specific active conformation of the receptor it stabilizes are unknown. Here, we report the results of submillisecond adaptive sampling molecular dynamics simulations of a predicted methadone-bound MOR complex and compare them with analogous data obtained for the classic opioid morphine and the G protein-biased ligand TRV130. The model, which is supported by existing experimental data, is analyzed using Markov state models and transfer entropy analysis to provide testable hypotheses of methadone-specific conformational dynamics and activation kinetics of MOR. SIGNIFICANCE STATEMENT: Opioid addiction has reached epidemic proportions in both industrialized and developing countries. Although methadone maintenance treatment represents an effective therapeutic approach for opioid addiction, it is not as widely used as needed. In this study, we contribute an atomic-level understanding of how methadone exerts its unique function in pursuit of more accessible treatments for opioid addiction. In particular, we present details of a methadone-specific active conformation of the µ-opioid receptor that has thus far eluded experimental structural characterization.

Sigma-1 receptor antagonists: promising players in fighting neuropathic pain

Sigma-1 receptors (S1Rs) are strongly correlated to neuropathic pain (NP), since their inactivation may decrease allodynia or dysesthesia, promoting analgesic effects. In the recent patent landscape, S1R antagonists endowed with nanomolar S1Rs affinity emerged as potent antinociceptive agents. So far, three patented compounds have been proposed for counteracting NP. Particularly PV-752 and AV1066, disclosed by the University of Pavia (Italy) and Anavex, respectively, showed good analgesic activity in preclinical studies. Moreover, E-52862 developed by Esteve (Spain) has been proved to be effective, both in preclinical and Phase II clinical trials, against several symptoms of NP. These patents ascertain S1R antagonists as potential drugs, alone or in combination with other analgesic drugs, for managing NP in humans.

Enantioselective capillary electrophoresis for pharmacokinetic analysis of methadone and 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine in equines anesthetized with ketamine and isoflurane

An enantioselective assay for the determination of methadone and its main metabolite 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine in equine plasma based on capillary electrophoresis with highly sulfated γ-cyclodextrin as chiral selector and electrokinetic analyte injection is described. The assay is based on liquid/liquid extraction of the analytes at alkaline pH from 0.1 mL plasma followed by electrokinetic sample injection of the analytes from the extract across a buffer plug without chiral selector. Separation occurs cationically at normal polarity in a pH 3 phosphate buffer containing 0.16% (w/v) of highly sulfated γ-cyclodextrin. The developed assay is precise (intra- and interday RSD < 4% and < 7%, respectively), is capable to determine enantiomer levels of methadone and 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine in plasma down to 2.5 ng/mL, and was successfully applied to monitor enantiomer drug and metabolite levels in plasma of a pony that was anesthetized with racemic ketamine and isoflurane and received a bolus of racemic methadone and a bolus followed by constant rate infusion of racemic methadone. The data suggest that the assay is well suited for pharmacokinetic purposes.

Biodegradation study of methadone by adapted activated sludge: Elimination kinetics, transformation products and ecotoxicological evaluation

The biotransformation study of difficult-to-degrade opioid analgesic methadone (MTHD) was performed by activated sludge culture adapted to high concentration of methadone (10 mg/L). The study included determination of elimination kinetics of the parent compound, taxonomic characterization of microbial culture, identification of biotransformation products (TPs) and assessment of ecotoxicological effects of biotransformation processes. The chemical analyses were performed by ultra-performance liquid chromatography/quadrupole-time-of-flight mass spectrometry, whereas the ecotoxicological assessment was made based on determinations of toxicity to freshwater algae. Changes of the adapted sludge culture during the experiment were followed using the 16S rRNA gene amplicon sequencing. Depending on the experimental conditions, the elimination efficiency of methadone (10 mg/L) varied from 9% to 93% with the corresponding half-lives from 11.4 days to 1.5 days. A significantly faster elimination (t_1/2 from 1.5 days to 5.8 days) was achieved at cometabolic conditions, using glucose-containing media, as compared to the experiments with MTHD as a single organic carbon source (t_1/2 = 11.4 days). Moreover, increased biotransformation rate following the additional supplementation of ammonia, revealed a possible importance of nitrogen availability for the transformation at cometabolic conditions. The elimination of parent compound was associated with the formation of 3 different TPs, two of which were identical to main human metabolites of MTHD, 2-Ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) and 2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline (EMDP). EDDP represented over 90% of the total TP concentration at the end of experiment. The biodegradation of MTHD was associated with a pronounced drop in algal toxicity, confirming a rather positive ecotoxicological outcome of the achieved biotransformation processes.

Compatibility of Baclofen, Carvedilol, Hydrochlorothiazide, Mercaptopurine, Methadone Hydrochloride, Oseltamivir Phosphate, Phenobarbital, Propranolol Hydrochloride, Pyrazinamide, Sotalol Hydrochloride, Spironolactone, Tacrolimus Monohydrate, Ursodeoxycholic Acid, and Vancomycin Hydrochloride Oral Suspensions Compounded with SyrSpend SF pH4

Compounded liquid medication is frequently required in children to allow easy dose adjustment and overcome swallowing difficulties. The objective of this study was to evaluate the stability of oral suspensions compounded with SyrSpend SF PH4 and the commonly used active pharmaceutical ingredients baclofen 2.0 mg/mL, carvedilol 5.0 mg/mL, hydrochlorothiazide 2.0 mg/mL, mercaptopurine 10.0 mg/mL, methadone hydrochloride 10.0 mg/mL, oseltamivir phosphate 6.0 mg/mL, phenobarbital 9.0 mg/mL and 15.0 mg/mL, propranolol hydrochloride 0.5 mg/mL and 5.0 mg/mL, pyrazinamide 100.0 mg/mL, spironolactone 2.0 mg/mL and 2.5 mg/mL, sotalol hydrochloride 5.0 mg/mL, tacrolimus monohydrate 0.5 mg/mL, ursodeoxycholic acid 20.0 mg/mL, and vancomycin hydrochloride 25.0 mg/mL. Suspensions were compounded with raw powders, except for mercaptopurine, pyrazinamide, and sotalol hydrochloride, which were made from commercial tablets. Stability was assessed by measuring the percentage recovery at 0 (baseline), 60 days, and 90 days after compounding for suspensions made with raw powders, which were stored at 2ÅãC to 8ÅãC. The stability of tablets, which were stored at 2ÅãC to 8ÅãC and 20ÅãC to 25ÅãC, was assessed by measuring the percentage recovery at 0 (baseline), 7 days, 14 days, 30 days, 60 days, and 90 days. Active pharmaceutical ingredients quantification was performed by ultraviolet high-performance liquid chromatography via a stability-indicating method. Given the percentage of recovery of the active pharmaceutical ingredients within the suspensions, the beyond-use date of the final products (active pharmaceutical ingredients + vehicle) was at least 90 days for all suspensions in the conditions tested. This suggests that SyrSpend SF PH4 is suitable for compounding active pharmaceutical ingredients from different pharmacological classes.

Degradation of methadone by the sunlight/FC process: Kinetics, radical species participation and influence of the water matrix

Free chlorine sunlight photolysis (sunlight/FC) markedly enhances the degradation rate of methadone, a synthetic opioid used medically, over that obtained using sunlight alone. The pseudo-first-order rate constants of methadone degradation under acidic conditions ([methadone] = 0.2 μM, [free chlorine] = 4 μM, and pH = 4) for sunlight/FC and sunlight photolysis are 7.0 ± 1.1 × 10^-2 min^-1 and 1.4 ± 0.2 × 10^-2 min^-1, respectively. The improved methadone degradation can be attributed to the production of HO and reactive chlorine species (RCS) during sunlight/FC photolysis. HO and RCS predominantly accounted for degradation during sunlight/FC photolysis under acidic and neutral conditions, while direct photolysis was the major contributor towards methadone degradation in alkaline conditions. The initial pH (pH 4-11) and free chlorine concentration (1-6 μM) significantly influenced the overall degradation efficiency of methadone. The presence of HCO₃^-, Cl^- and dissolved organic matters, which may competitively react with HO and RCS, retard the degradation of methadone in synthetic wastewater. Consequently, a 50% lower methadone degradation rate was observed when deionized (DI) water was replaced with tap water. These results emphasize the need to consider different water matrices when applying sunlight/FC photolysis for water treatment.

Increases from 2002 to 2015 in prescription opioid overdose deaths in combination with other substances

BACKGROUND

Prescription opioid (PO) overdose deaths increased sharply over the last decade. Changes in PO deaths in combination with other psychoactive substances may provide a partial explanation.

METHODS

PO deaths from the National Multiple-Cause-of-Death Files for 2002-03 (N=15,973) and 2014-15 (N=41,491) were analyzed. We calculated (1) changes in proportions of deaths in combination with benzodiazepines, antidepressants, heroin, alcohol, cocaine between the two periods, and (2) proportions of increase in deaths attributable to each substance among PO and synthetic opioids other than methadone (SO-M) deaths, by age, gender, race/ethnicity.

RESULTS

Between 2002-03 and 2014-15, PO deaths increased 2.6 times; SO-M deaths 5.6 times, especially for ages 18-34, males, African-Americans. For PO deaths, most frequent combinations at both periods were with benzodiazepines; for SO-M, benzodiazepines, antidepressants in 2002-03, heroin, benzodiazepines in 2014-15. The largest increases occurred in combination with heroin among all PO (4.6% to 15.4%, change ratio=3.3[95%CI=3.1-3.6]), but especially SO-M deaths (1.2% to 24.5%, change ratio=21.3[95%CI=15.0-30.3]). Deaths involving cocaine decreased among PO, increased among SO-M deaths. One-fifth of increased PO or SO-M deaths were attributable to any of the five substances. Increased PO deaths were equally attributable to benzodiazepines and heroin; deaths attributable to heroin were higher among ages 18-49, males, and non-Hispanic whites. Increased SO-M deaths were attributable mostly to heroin among all groups.

CONCLUSIONS

Increased PO overdose deaths over the last decade may be partially explained by increased deaths in combination with other psychoactive substances. Use of other substances should be considered in efforts toward reducing prescription opioid overdoses.

Occurrence of drugs of abuse in surface water from four Spanish river basins: Spatial and temporal variations and environmental risk assessment

The present work investigates the occurrence of 22 drugs of abuse (DAs) and metabolites in surface water from four Spanish River basins: Llobregat, Ebro, Jucar and Guadalquivir. To this end, samples were collected from 77 selected sites in two sampling campaigns conducted in 2010 and 2011. Analyses were performed by means of on-line solid phase extraction-liquid chromatography-electrospray-tandem mass spectrometry (on-line SPE-LC-ESI-MS/MS). Cocaine, its metabolite benzoylecgonine, ephedrine, MDMA (ecstasy), and methadone and its metabolite EDDP were the most ubiquitous compounds, being present in more than 50% of the samples. Overall, DAs were present at low ng/L levels, showing comparatively higher concentrations in small tributary rivers impacted by wastewater treatment plant effluents. The results obtained did not show a clear relationship between the concentrations of DAs and the hydrological conditions of the river basins. Differences in the occurrence of DAs between river basins were observed, but only for MDMA this finding could be confirmed in both sampling campaigns. These results overall suggest similar drugs consumption in the studied areas. Estimation of potential ecotoxicological effects using Hazard Quotient (HQ) ratios and a concentration addition model showed that in four samples the presence of DAs could represent certain risk to aquatic organisms.

Transformation of methadone and its main human metabolite, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), during water chlorination

The reaction kinetics and reaction pathway of methadone and its main human metabolite, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) in chlorine containing waters were investigated by direct injection of individual reaction time aliquots in a liquid chromatography-quadrupole-time-of-flight-mass spectrometry (LC-QTOF-MS) system. Factors potentially affecting the rate of the reaction were evaluated in detail by means of a Box-Behnken experimental design in which methadone and EDDP were considered separately. Sample pH and chlorine concentration turned out to be the two significant variables, enhancing the kinetics with an increase in their values. Transformation products (TPs) were first searched over sample chromatograms by comparing control, blank and time zero samples to aliquots stopped at different reaction times. Their tentative identity was further inferred by generating their empirical formulae from their accurate single MS spectra and, subsequently, by interpreting their fragmentation pattern from their tandem MS (MS/MS) spectra. In total, 8 TPs, arising from intra-molecular cyclation, dehydrogenation, oxidation and chlorination, could be detected in the case of methadone, one of them being the EDDP and another 3 coming from EDDP, so being common to both the precursor drug and its metabolite. A tentative transformation pathway was proposed, and the reaction was evaluated under potential real circumstances by chlorinating two different river samples. In this way, it was possible to demonstrate that its extension is highly affected by the content of dissolved organic matter, so both compounds were highly or completely transformed in samples with a low anthropogenic impact, whereas they were considerably more stable in waters with a high concentration of organic matter. Finally, the ecotoxicity of precursors and transformation species was predicted by software tools, revealing that, in some cases, the toxicological responses displayed by the TPs were up to 100 times higher than those of methadone.

The Effects of Once-Daily Saquinavir/Minidose Ritonavir on the Pharmacokinetics of Methadone

Twelve methadone-maintained HIV-negative subjects were given saquinavir/ritonavir (SQV/rtv) 1600 mg/100 mg once daily for 14 days. Pharmacokinetic evaluations of total and unbound methadone enantiomers (R and S) were conducted before and after SQV/rtv. SQV/rtv was well tolerated, with no ACTG Grade 3-4 adverse events, no evidence of sedation, and no changes in methadone dose. For R-methadone (active isomer), C(max), AUC(0-24 h), and C(min) were unchanged, but percent unbound 4 hours after dosing was reduced by 12%. For S-methadone, no differences in pharmacokinetic parameters of total drug were seen, but unbound concentrations were reduced by 15% and 21% at 4 and 24 hours after dosing, respectively. SQV trough concentrations exceeded the anticipated EC(50) (50 ng/mL) in 10/12 subjects, persisting for at least 6 hours after the final dose in 4/6 subjects. Once-daily SQV/rtv in methadone-maintained subjects is safe and not associated with any clinically significant interaction with methadone during 14 days of concomitant administration.

How can a methadone and an opiate-positive immunoassay result be reconciled in a patient prescribed only OxyContin and Wellbutrin?

Appropriate management of patients in pain clinics can be complex and sometimes confusing because providers must correctly interpret multiple sources of patient information. The correct interpretation of laboratory results is essential to guide subsequent patient treatment and management; however, laboratory and clinical pictures can appear to be conflicting in cases of substance abuse. Incorrect interpretation of laboratory results can multiply negative impacts on clinical outcomes and may lead to patient harm or death. This report introduces the complex nature involved in understanding and interpreting urine drug testing (UDT) results in pain patients who are prescribed opioid medications. Laboratory testing examples of UDT results are provided to illustrate the sometimes discordant nature of UDT interpretation. This case study describes one method of approaching cases where laboratory result interpretation in pain clinic patients is essential for medical treatment and management. The case presented in this manuscript illustrates a reconciliation of an opiate positive immunoassay result in a patient who was prescribed only OxyContin and Wellbutrin after traumatic amputations.

Liquid chromatography-tandem mass spectrometry for analysis of intestinal permeability of loperamide in physiological buffer

Analysis of in vitro samples with high salt concentrations represents a major challenge for fast and specific quantification with liquid chromatography-tandem mass spectrometry (LC-MS/MS). To investigate the intestinal permeability of opioids in vitro employing the Ussing chamber technique, we developed and validated a fast, sensitive and selective method based on LC–MS/MS for the determination of loperamide in HEPES-buffered Ringer's solution. Chromatographic separation was achieved with an Atlantis dC18 column, 2.1 mm×20 mm, 3 µm particle size and a gradient consisting of methanol/0.1% formic acid and ammonium acetate. The flow rate was 0.7 ml/min, and the total run time was 3 min. For quantification, two mass transitions for loperamide and a deuterated internal standard (methadone-d₃) were used. The lower limit of loperamide quantification was 0.2 ng/ml. This new LC-MS/MS method can be used for the detection of loperamide in any experimental setup using HEPES-buffered Ringer's solution as a matrix compound.

Estimated infant exposure to enantiomer-specific methadone levels in breastmilk

BACKGROUND AND OBJECTIVES

Breastfeeding, a public health priority, improves outcomes for infants. Methadone is dispensed as a racemic mixture; R-methadone is the active enantiomer. Pharmacologic data for R-methadone in breastmilk could improve risk-benefit decision-making for treatment of lactating women. This study estimated infant exposure to R- and S-methadone via breastmilk by theoretic infant dose (TID) and relative infant dose (RID) and reported the milk-to-maternal plasma (M/P) ratio.

METHODS

Women treated with methadone doses of 40-200 mg/day (mean, 102 mg/day) provided concomitantly collected plasma and breastmilk samples 1-6 days after delivery. Most (16 of 20) samples were taken at the time of peak maternal plasma levels; thus infant exposure estimates are for maximum possible exposure. Concentrations of R- and S-methadone were measured in maternal plasma and breastmilk; M/P ratio, TID, and RID were calculated for each enantiomer and total methadone.

RESULTS

The 20 participants were 18-38 years old and publicly insured; a quarter did not complete high school, and only one was not white. R-Methadone concentration was 1.3-3.0 times that of S-methadone in all breastmilk samples. The mean (SD) R-, S-, and total methadone M/P ratios were 0.52 (0.28), 0.28 (0.15), and 0.40 (0.21), respectively. Mean (range) R-, S-, and total methadone TID were 0.02 mg/kg/day (0.004-0.099), 0.013 mg/kg/day (0.002-0.071), and 0.033 mg/kg/day (0.006-0.170), respectively. Mean (range) RID of R-, S-, and total methadone were 2.7% (0.7-10.1%), 1.6% (0.3-7.2%), and 2.1% (0.52-8.8%), respectively.

CONCLUSIONS

R-Methadone is found in higher concentrations than S-methadone in breastmilk. Even at high methadone doses, breastmilk methadone concentrations were relatively low and support American Academy of Pediatrics recommendations that dose should not be a factor in determining whether women on methadone breastfeed.

Activation of µ-opioid receptors and block of Kir3 potassium channels and NMDA receptor conductance by L- and D-methadone in rat locus coeruleus

BACKGROUND AND PURPOSE

Methadone activates opioid receptors to increase a potassium conductance mediated by G-protein-coupled, inwardly rectifying, potassium (K(IR) 3) channels. Methadone also blocks K(IR) 3 channels and N-methyl-D-aspartic acid (NMDA) receptors. However, the concentration dependence and stereospecificity of receptor activation and channel blockade by methadone on single neurons has not been characterized.

EXPERIMENTAL APPROACH

Intracellular and whole-cell recording were made from locus coeruleus neurons in brain slices and the activation of µ-opioid receptors and blockade of K(IR) 3 and NMDA channels with L- and D-methadone was examined.

KEY RESULTS

The potency of L-methadone, measured by the amplitude of hyperpolarization was 16.5-fold higher than with D-methadone. A maximum hyperpolarization was caused by both enantiomers (∼30 mV); however, the maximum outward current measured with whole-cell voltage-clamp recording was smaller than the current induced by [Met](5) enkephalin. The K(IR) 3 conductance induced by activation of α(2) -adrenoceptors was decreased with high concentrations of L- and D-methadone (10-30 µM). In addition, methadone blocked the resting inward rectifying conductance (K(IR) ). Both L- and D-methadone blocked the NMDA receptor-dependent current. The block of NMDA receptor-dependent current was voltage-dependent suggesting that methadone acted as a channel blocker.

CONCLUSIONS AND IMPLICATIONS

Methadone activated µ-opioid receptors at low concentrations in a stereospecific manner. K(IR) 3 and NMDA receptor channel block was not stereospecific and required substantially higher concentrations. The separation in the concentration range suggests that the activation of µ-opioid receptors rather than the channel blocking properties mediate both the therapeutic and toxic actions of methadone.

Methadone: does stigma play a role as a barrier to treatment of chronic pain?

INTRODUCTION

The synthetic opioid methadone is a promising analgesic for the management of chronic neuropathic pain. Methadone therapy is increasing as its advantages are being realized over other opioids. Methadone's lack of known active metabolites, high oral bioavailability, low cost, and its additional receptor activity as an antagonist of N-methyl-D-aspartate receptors make it an attractive analgesic.

METHODS

We surveyed 550 pain physicians to determine their prescribing practices of methadone. The study was approved by our Institutional Review Board. A list of 550 pain physicians, which included practitioners in private practice, university settings, and community hospitals, were obtained and surveys sent via mail. The list was obtained through the American Pain Society's membership list. Out of 550 surveys sent, 124 replies were returned.

RESULTS

The 124 surveys that were returned included pain physicians from various settings: 20 responses from physicians practicing at a university setting, 16 responses from a community setting, 54 responses from a private setting, one from university and community settings, 7 from community and private settings, 3 from university and community and private settings; 23 did not specify. Of the 124 physicians, 111 prescribe methadone in their pain practice. Of the 13 physicians who do not prescribe methadone, the main reason for not using the drug for 5 physicians was because of social stigma, 2 because of minimal experience with the drug, 2 because the drug was not effective, one because of lack of knowledge, and one because of potential adverse effects. Of the 111 physicians who use methadone, 55 stated that social stigma was the most common reason patients refuse to take methadone for the treatment of pain, 44 because of adverse effects, and 5 stated "other" as the reason patients refuse to take methadone. Of 111 physicians who prescribe methadone, 100 prescribed it for neuropathic pain, 101 for somatic pain, 80 for visceral pain, 78 for cancer pain, and 34 for sickle cell pain. Also, 21 stated that methadone was the primary opioid they prescribed. Of the 111 physicians who prescribe methadone, 86 start methadone at low dose and titrate up to minimize side effects. Fourteen clinicians load methadone and titrate down to minimize adverse effects while maintaining analgesia.

CONCLUSION

The majority of survey responders (90%) prescribed methadone in their pain practice, but on a very limited basis; 59% state <20% of their patients are on methadone. Three times a day dosing schedule was the most typical regimen (57%) while 77% prefer to titrate up on the dosage. It seems interesting that many clinicians do not prescribe methadone as a primary analgesic. One reason for this is due to the social stigma of its use in treatment of heroin addicts. Also, a lack of widely recognized treatment algorithms or guidelines to assist clinicians with opioid conversions and maintenance might be playing a role. The role of stigma as a barrier to adequate treatment of chronic pain among pain physicians prescribing practices is a fundamental, yet unexplored issue.

Stability of methadone and its main metabolite in oral fluid

Saliva is a readily available specimen that can be collected by non-invasive procedures and contains many drugs of interest in screening and diagnosis. It is obtained by a painless and non invasive method of sampling; it contains the free fraction of drugs and therefore it is a good indicator of intoxication state. Inspite of its usefulness, only a few studies on long-term storage have been conducted for some drugs of abuse, while methadone stability have not been investigated yet. This lack in standardization and the scarcity of analytical protocols actually restrict saliva applications. Authors studied methadone stability on saliva during 12 months. Fifty-nine saliva samples were collected from heroin addicts in methadone treatment with the Cozart Rapiscan Collection procedure. The samples, spiked with tri-deuterated internal standards analogs of methadone and 2-ethyl-1,5-dimethyl-3,3-diphenyl-pyrrolinium perchlorate (EDDP), were submitted to Solid Phase MicroExtraction-Gas Chromatography/Mass Spectrometry (SPME-GC/MS) technique. Quantitative determinations of methadone and EDDP were performed immediately and after various intervals (one month, two months, twelve months). The results obtained from this experiment show that methadone is sufficiently stable at 4 degrees C until 2 months, while a decrease have been observed for EDDP. These preliminary data prove the need to perform the analysis in a brief time, to avoid loss of EDDP. For the correct use of this biological matrix, more research and guidelines are recommended for drug testing on saliva.

Lack of influence of CYP2D6 genotype on the clearance of (R)-, (S)- and racemic-methadone

OBJECTIVE

To investigate the influence of CYP2D6 genotype on the oral clearance of (R)-, (S)- and rac-methadone.

METHODS

In this retrospective study, CYP2D6 genotypes were identified in 56 methadone maintained subjects. Plasma concentrations of (R)-, (S)- and rac-methadone were determined by stereoselective HPLC and sufficient data were available to estimate the apparent oral clearances of (R)-, (S)- and rac-methadone using a population kinetic model in 37 of the genotyped subjects.

RESULTS

The CYP2D6 allele frequencies were similar to those previously reported in Caucasians, the most common being: CYP2D6*1 (35.2%), CYP2D6*2 (12.0%) and CYP2D6*4 (22.2%). Three unknown SNPs were found in four subjects: 1811G > A (n = 1), 1834C > T (n = 1) and 2720G > C (n = 2). The oral clearances of (R)-, (S)- and rac-methadone varied 5.4-, 6.8- and 6.1-fold, respectively. No significant differences in methadone oral clearance were found between CYP2D6 genotypic PM, IM and EM (p = 0.57, 0.40 and 0.43 for (R)-, (S)- and rac-methadone, respectively). Only 1 subject had duplication of functional CYP2D6 alleles and the oral clearance of the three analytes was not markedly altered.

CONCLUSIONS

CYP2D6 poor, intermediate and extensive metabolizer genotypes did not appear to impact on the oral clearance of (R)-, (S)- or rac-methadone. In addition, methadone dosage requirements were not influenced by CYP2D6 genotypes in these subjects. However, the impact of duplication of functional CYP2D6 alleles on oral clearance and dosage requirements requires further investigation.

Stereoselective Block of hERG Channel by (S)-Methadone and QT Interval Prolongation in CYP2B6 Slow Metabolizers

Methadone inhibits the cardiac potassium channel hERG and can cause a prolonged QT interval. Methadone is chiral but its therapeutic activity is mainly due to (R)-methadone. Whole-cell patch-clamp experiments using cells expressing hERG showed that (S)-methadone blocked the hERG current 3.5-fold more potently than (R)-methadone (IC50s (half-maximal inhibitory concentrations) at 37 degrees C: 2 and 7 microM). As CYP2B6 slow metabolizer (SM) status results in a reduced ability to metabolize (S)-methadone, electrocardiograms, CYP2B6 genotypes, and (R)- and (S)-methadone plasma concentrations were obtained for 179 patients receiving (R,S)-methadone. The mean heart-rate-corrected QT (QTc) was higher in CYP2B6 SMs (*6/*6 genotype; 439+/-25 ms; n=11) than in extensive metabolizers (non *6/*6; 421+/-25 ms; n=168; P=0.017). CYP2B6 SM status was associated with an increased risk of prolonged QTc (odds ratio=4.5, 95% confidence interval=1.2-17.7; P=0.03). This study reports the first genetic factor implicated in methadone metabolism that may increase the risk of cardiac arrhythmias and sudden death. This risk could be reduced by the administration of (R)-methadone.

Enantiomeric Metabolic Interactions and Stereoselective Human Methadone Metabolism

Methadone is administered as a racemate, although opioid activity resides in the R-enantiomer. Methadone disposition is stereoselective, with considerable unexplained variability in clearance and plasma R/S ratios. N-Demethylation of methadone in vitro is predominantly mediated by cytochrome P450 CYP3A4 and CYP2B6 and somewhat by CYP2C19. This investigation evaluated stereoselectivity, models, and kinetic parameters for methadone N-demethylation by recombinant CYP2B6, CYP3A4, and CYP2C19, and the potential for interactions between enantiomers during racemate metabolism. CYP2B6 metabolism was stereoselective. CYP2C19 was less active, and stereoselectivity was opposite that for CYP2B6. CYP3A4 was not stereoselective. With all three isoforms, enantiomer N-dealkylation rates in the racemate were lower than those of (R)-(6-dimethyamino-4,4-diphenyl-heptan-3-one) hydrochloride (R-methadone) or (S)-(6-dimethyamino-4,4-diphenyl-heptan-3-one) hydrochloride (S-methadone) alone, suggesting an enantiomeric interaction and mutual metabolic inhibition. For CYP2B6, the interaction between enantiomers was stereoselective, with S-methadone as a more potent inhibitor of R-methadone N-demethylation than R-of S-methadone. In contrast, enantiomer interactions were not stereoselective with CYP2C19 or CYP3A4. For all three cytochromes P450, methadone N-demethylation was best described by two-site enzyme models with competitive inhibition. There were minor model differences between cytochromes P450 to account for stereoselectivity of metabolism and enantiomeric interactions. Changes in plasma R/S methadone ratios observed after rifampin or troleandomycin pretreatment in humans in vivo were successfully predicted by CYP2B6- but not CYP3A4-catalyzed methadone N-demethylation. CYP2B6 is a predominant catalyst of stereoselective methadone metabolism in vitro. In vivo, CYP2B6 may be a major determinant of methadone metabolism and disposition, and CYP2B6 activity and stereoselective metabolic interactions may confer variability in methadone disposition.

Stereoselective determination of methadone and its main metabolite in serum and urine from methadone maintenance patients

OBJECTIVES

A stereoselective HPLC method was developed to separate and quantify both enantiomers of methadone and its main metabolite EDDP in serum and urine. The method was used to establish that there is a relationship between the dose of methadone prescribed and its serum concentration as well as urine excretion of methadone and its metabolite enantiomers.

METHODS

The chiral alpha1-glycoprotein stationary phase was used for enantioseparation of (R)-methadone, (S)-methadone and (R)-EDDP (S)-EDDP. The enantiomers of methadone and EDDP were extracted from urine and serum by a simple solidphase procedure.

RESULTS

The validated method was applied to the analysis of 31 serum and urine samples obtained from methadone-maintained outpatients (65% male, age 28.8+/-4; methadone dose 146+/-47 mg). A significant correlation (Pearson) r=0.67 (p<0.001) between methadone dose and serum concentration of (R)-methadone was found. Due to the large variation in results obtained from analysis of the subjects' urine specimens, no statistically significant relationship between methadone dose and urine excretion of methadone and EDDP enantiomers was established. The rate of R/S methadone (1.38 in serum, 2.43 in urine) and R/S EDDP (0.83 in urine) confirmed stereoselectivity in methadone metabolism with high individual variability.

CONCLUSIONS

The enantioselective evaluation of serum methadone concentration might be an interesting tool in methadone maintenance programme. On the other hand, the urinary excretion of methadone and EDDP enantiomers is not reliable as marker of methadone compliance but could be useful for monitoring individual metabolism or for studying the stereoselectivity in pharmacokinetics and metabolism of methadone.

[Molecular simulations of the loading of methadone and buprenorphine into carbon nanotubes]

AIM

To simulate the inhalation of the C21H27NO and C29H41NO4 molecules, the effective components of methadone and buprenorphine, into carbon nanotubes, and discuss the feasibility of the loading of methadone and buprenorphine into carbon nanotubes.

METHODS

The MM + force-field based molecular dynamics (MD) method uas used.

RESULTS

The ends-opened carbon nanotubes with diameter larger than 1 or 1.25 nm can initiatively inhale the C21H27 NO or C29H41NO4 molecule, and both two molecules have higher potential energy at the open ends of the carbon tubes than that at the middle of the tubes; the present single-walled nanotubes are very suitable for the loading of methadone and buprenorphine.

CONCLUSION

It is possible to make sustained-release detoxification agents with methadone- or buprenorphine-loaded carbon nanotubes.

Population-based analysis of methadone distribution and metabolism using an age-dependent physiologically based pharmacokinetic model

Limited pharmacokinetic (PK) and pharmacodynamic (PD) data are available to use in methadone dosing recommendations in pediatric patients for either opioid abstinence or analgesia. Considering the extreme inter-individual variability of absorption and metabolism of methadone, population-based PK would be useful to provide insight into the relationship between dose, blood concentrations, and clinical effects of methadone. To address this need, an age-dependent physiologically based pharmacokinetic (PBPK) model has been constructed to systematically study methadone metabolism and PK. The model will facilitate the design of cost-effective studies that will evaluate methadone PK and PD relationships, and may be useful to guide methadone dosing in children. The PBPK model, which includes whole-body multi-organ distribution, plasma protein binding, metabolism, and clearance, is parameterized based on a database of pediatric PK parameters and data collected from clinical experiments. The model is further tailored and verified based on PK data from individual adults, then scaled appropriately to apply to children aged 0-24 months. Based on measured variability in CYP3A enzyme expression levels and plasma orosomucoid (ORM2) concentrations, a Monte-Carlo-based simulation of methadone kinetics in a pediatric population was performed. The simulation predicts extreme variability in plasma concentrations and clearance kinetics for methadone in the pediatric population, based on standard dosing protocols. In addition, it is shown that when doses are designed for individuals based on prior protein expression information, inter-individual variability in methadone kinetics may be greatly reduced.

Pharmacokinetics and Pharmacodynamics of Methadone Enantiomers in Hospice Patients With Cancer Pain

Racemic methadone is increasingly used to manage cancer pain. The authors studied 13 terminally ill patients with cancer pain, who underwent switching (rotation) from morphine to methadone. The relationship between initial morphine dose and final methadone dose, the pharmacokinetics of R- and S- methadone, and the degree of pain control and side effects were investigated. Preswitching serum morphine concentrations and second daily plasma concentrations of methadone were measured. The brief pain inventory (BPI) was used to assess pain every second day. "Worst pain" as measured by the BPI improved by >/=20% in 6 of the 13 patients. The mean morphine to methadone conversion ratio was 5.2 with wide interpatient variability (range 1.3 to 11). Average steady-state concentrations were 197 (98 to 379) mug/L and 272 (55 to 378) mug/L for R- and S-methadone, respectively. Mean population pharmacokinetic parameters for a 1-compartment model were 455 L and 338 L for apparent volume of distribution and 53.3 hours and 31.5 hours for half-life for R- and S- methadone, respectively. Bayesian estimates of apparent oral clearance for individual patients were 0.082 (0.052 to 0.112) L/kg/h and 0.117 (0.061 to 0.173) L/kg/h for R- and S- methadone, respectively (mean and 95% confidence interval). The low and variable clearance values generally resulted in slow achievement of steady-state concentrations over several days; inappropriately high plasma methadone levels occurred in 1 patient. Whereas optimal pain control was achieved in 46% of patients, there was no relationship with plasma concentrations of methadone. Best practice for methadone use in this patient group should include monitoring of both pain and methadone concentration.

Methadone enantiomer plasma levels, CYP2B6, CYP2C19, and CYP2C9 genotypes, and response to treatment

BACKGROUND AND OBJECTIVE

Recent in vitro studies have suggested an important role of cytochrome P450 (CYP) 2B6 and CYP2C19 in methadone metabolism. We aimed to determine the influence of CYP2B6, CYP2C9, and CYP2C19 genetic polymorphism on methadone pharmacokinetics and on the response to treatment.

METHODS

We included 209 patients in methadone maintenance treatment on the basis of their response to treatment and their daily methadone dose. Patients were genotyped for CYP2B6, CYP2C9, and CYP2C19. Steady-state trough and peak (R)-, (S)-, and (R,S)-plasma levels and peak-to-trough plasma level ratios were measured.

RESULTS

CYP2B6 genotype influences (S)-methadone and, to a lesser extent, (R)-methadone plasma levels, with the median trough (S)-methadone plasma levels being 105, 122, and 209 ng . kg/mL . mg for the noncarriers of allele *6, heterozygous carriers, and homozygous carriers (*6/*6), respectively (P = .0004). CYP2C9 and CYP2C19 genotypes do not influence methadone plasma levels. Lower peak and trough plasma levels of methadone and higher peak-to-trough ratios were measured in patients considered as nonresponders [median (R,S)-methadone trough plasma levels of 183 and 249 ng . kg/mL . mg (P = .0004) and median peak-to-trough ratios of 1.82 and 1.58 for high-dose nonresponders and high-dose responders, respectively (P = .0003)].

CONCLUSION

Although CYP2B6 influences (S)-methadone plasma levels, given that only (R)-methadone contributes to the opioid effect of this drug, a major influence of CYP2B6 genotype on response to treatment is unlikely and has not been shown in this study. Lower plasma levels of methadone in nonresponders, suggesting a higher clearance, and higher peak-to-trough ratios, suggesting a shorter elimination half-life, are in agreement with the usual clinical measures taken for such patients, which are to increase methadone dosages and to split the daily dose into several intakes.

A review of the use of methadone for the treatment of chronic noncancer pain

Methadone, although having been available for approximately half a century, is now receiving increasing attention in the management of chronic pain. This is due to recent research showing that methadone exhibits at least three different mechanisms of action including potent opioid agonism, N-methyl-D-aspartate antagonism and monoaminergic effects. This, along with methadone's excellent oral and rectal absorption, high bioavailability, long duration of action and low cost, make it a very attractive option for the treatment of chronic pain. The disadvantages of significant interindividual variation in pharmacokinetics, graduated dose equivalency ratios based on prerotation opioid dose when switching from another opioid, and the requirement for special exemption for prescribing methadone make it more complicated to use. The present review is intended to educate physicians interested in adding methadone to their armamentarium for assisting patients with moderate to severe pain.

Pharmacology of methadone and its isomers

Methadone is a synthetic opioid analgesic that is used as an alternate to morphine and hydromorphone for patients with severe pain. It is increasingly being used in opioid rotation schedules. Methadone has an asymmetric carbon atom resulting in 2 enantiomeric forms, the d and l isomers. The racemic mixture (dl-methadone) is the form commonly used clinically. Recent studies have revealed the pharmacological activity of the d-methadone isomer. We found that the d isomer of methadone has N-methyl-D-aspartate (NMDA) receptor antagonist activity both in vitro and in vivo. Studies were designed to examine the ability of d-methadone to attenuate the development of morphine tolerance and to modify NMDA-induced hyperalgesia in rats. Repeated dosing with intrathecal morphine produced a 38-fold increase in the morphine ED50 value. This decrease in the potency of morphine was completely prevented by the coadministration of intrathecal d-methadone at 160 microg/rat. In addition, the decrease in thermal paw withdrawal latency induced by the intrathecal administration of 1.64 microg/rat NMDA was completely blocked by pretreatment with 160 microg/rat d-methadone. Thus, the same dose of intrathecal d-methadone that attenuates the development of spinal morphine tolerance blocks NMDA-induced hyperalgesia in rats. These results support the

CONCLUSIONS

that d-metha-done affects the development of morphine tolerance and NMDA-induced hyperalgesia by virtue of its NMDA receptor antagonist activity.

(S)-(+)-methadone is more immunosuppressive than the potent analgesic (R)-(--)-methadone

Methadone is a widely used synthetic opioid which is administered as a racemic mixture of (R)-(--)- and (S)-(+)-enantiomers, with only (R)-(--)-methadone possessing mu opioid receptor agonist activity. Methadone inhibits numerous immune functions in vitro at concentrations above 10 microM in a nonstereoselective and naloxone-insensitive fashion, suggesting the presence of nonclassical opioid receptors on immune cells. No in vivo data on the effects of methadone's enantiomers on immune function are available. Therefore, the stereoselectivity of methadone's analgesia (hot plate latency) in vivo and immune suppression ex vivo (splenocyte proliferation) was investigated in groups of Balb/c mice. Significant analgesia was observed in animals that received racemic methadone (P=0.0012, 52% MPE) and (R)-(--)-methadone (P=0.0002, 70% MPE) when compared to saline-treated controls, while (S)-(+)-methadone was devoid of any such effect (-4% MPE). In vivo (R)-(--)- and racemic methadone caused significant inhibition (P<0.001, greater than -70%) of basal proliferation compared to saline control. In stark contrast to analgesia, in vivo (S)-(+)-methadone caused significantly greater inhibition of basal proliferation (P<0.001, -130%) than (R)-(--)- and racemic methadone. The immune suppression caused by methadone is not purely a classical opioid response but involves nonclassical opioid receptors located at the central level, which have yet to be characterised. Moreover, the dose at which immune suppression occurred could be achieved clinically.

Definite peak identification of (R)-and (S)-methadone and (R)- and (S)-EDDP using established HPLC and CE methods

Methadone is widely used for the treatment of opioid dependence. HPLC and CE are widespread methods for drug monitoring and metabolism studies. Although the methods are widely used for methadone and its main metabolite EDDP [1, 2], a definite direct peak identification for EDDP enantiomers is not described. This study describes a method for specific identification of each peak in the chromatogram and electropherogram of methadone analysis. The result of the study demonstrates differences in the elution order of the enantiomers of methadone and EDDP due to the technique used for analysis. The elution order of EDDP using HPLC is interchanged with respect to the order of the peaks in the electropherogram.

Functional inhibition by methadone of N-methyl-D-aspartate receptors expressed in Xenopus oocytes: stereospecific and subunit effects

Methadone is a strong opioid analgesic that is finding increasing use in chronic pain therapeutics. We explored its reported efficacy for inhibiting N-methyl-D-aspartate (NMDA) receptors in a functional electrophysiologic assay (Xenopus laevis oocyte expression). Racemic methadone inhibited all subtypes of rat NMDA receptors with derived 50% inhibitory concentrations in the low micromolar range. These concentrations overlap with clinically achievable concentrations reported in pharmacokinetic studies. In contrast, morphine inhibited these functional ion channels only at 8-16 times larger concentrations. The NR1/2A and NR1/2B subtype combinations were in general significantly more sensitive to inhibition by methadone and morphine compared with the NR1/2C and NR1/2D subtypes. In the presence of racemic methadone, the maximum NMDA-stimulated currents were markedly decreased, but the NMDA concentration producing 50% of maximal activation was altered only slightly, indicating that methadone blocks by a noncompetitive mechanism. Although stereoisomers of methadone showed minimal stereoselectivity in most subtypes, R(-) methadone was highly selective in its inhibition of the NR1/2A combination. These results provide further functional data describing the NMDA receptor inhibitory actions of methadone and support the hypothesis that methadone acts through both opioid and NMDA receptor mechanisms.

IMPLICATIONS

At clinically achievable concentrations, methadone inhibits functional N-methyl-D-aspartate receptors. These results indicate a unique mode of action by this opioid that may enhance its ability to treat chronic pain and to limit opioid tolerance.

Monolithic silica columns with chemically bonded ?-cyclodextrin as a stationary phase for enantiomer separations of chiral pharmaceuticals

An enantioselective silica rod type chiral stationary phase (CSP) is presented; a novel combination of the well known enantiomer separation properties of beta-cyclodextrin and the unique properties concerning the flow behavior of silica monoliths. Two different synthesis routes are described, and it was found that the in situ modification of a plain silica rod column turned out to be the best. The chromatographic behaviour of the beta-cyclodextrin silica rod was studied and compared with a very similar commercially available beta-cyclodextrin bonded particulate material (ChiraDex). Even if the amount of beta-cyclodextrin bound to the silica rod was only about half of the amount of beta-cyclodextrin bound to ChiraDex) particles, good resolutions were achieved for a set of chiral test components like Chromakalin, Prominal, Oxazepam, Methadone and some other drugs. By taking advantage of the unique features of the silica rods relating to their flat H/u (Van Deemter) curves, fast enantiomer separations could be demonstrated.

Nicotine and drug interaction expectancies among methadone maintained cigarette smokers

Smoking rates among persons being treated with methadone for opiate dependence are exceptionally high. Nevertheless, there is debate about whether smoking cessation has a negative effect on substance abuse treatment outcomes. To understand patient perceptions of the interaction between smoking and illicit drug use, we administered the Nicotine and Other Substances Interaction Expectancies questionnaire (NOSIE) to 168 smokers-79% Caucasian and 48% female-at two methadone programs in Providence, Rhode Island. The four summed-rating scales of this 20-item instrument exhibited good to excellent internal-consistency reliabilities. The scales measured the effect of drug use on smoking (5 items, alpha =.84), the effect of smoking on drug use (3 items, alpha =.73), smoking to cope with drug urges (9 items, alpha =.91), and the difficulty of concomitantly quitting smoking and drugs (3 items, alpha =.72).Most subjects believed drug use increased smoking, but that smoking did not trigger drug use. Relatively few reported smoking to cope with drug urges and most did not consider quitting smoking and quitting drug use to be incompatible goals. NOSIE was a reliable instrument that offers insight into patient expectations of the interaction of smoking and drug use.

Stereoselective metabolism of methadone N-demethylation by cytochrome P4502B6 and 2C19

Methadone is a clinically used opioid agonist that is oxidatively metabolized by cytochrome P450 (CYP) isoforms to a stable metabolite, EDDP. Methadone is a chiral drug administered as the racemic mixture of (R)-(-)- and (S)-(+)-methadone, but (R)-methadone is the active isomer. The cytochrome P450 (CYP) isoform involved in methadone's metabolism is thought to be CYP3A4, but human drug-drug interaction studies are not consistent with this. The ability of the common human drug-metabolizing CYPs (obtained from baculovirus-infected insect cell supersomes) to generate 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrilidine (EDDP) from racemic methadone was examined and then determined if the CYP isoforms metabolized methadone stereoselectively. Only CYP2B6, 2C19, and 3A4 generated measurable EDDP from 1 microg/ml of racemic methadone. The hierarchy of EDDP generation was CYP2B6 > CYP2C19 >/= CYP3A4. At 10 microg/ml of methadone, CYP2C9 and CYP2D6 also generated EDDP, but in at least 10-fold lower quantities than CYP2B6. Michaelis-Menten kinetic data demonstrated that CYP2B6 had the highest V(max) (44 ng/min/10pmol) and the lowest K(m) (12.6 microg/ml) for EDDP formation of all the CYP isoforms. In human liver microsomes with high and low CYP2B6 expression but equivalent CYP3A4 expression, high CYP2B6 expression microsomes generated twice the amount of EDDP from 10 microg/ml of methadone than low CYP2B6 expression microsomes. When stereoselective metabolism of racemic methadone by CYP2B6, 2C19, and 3A4 was examined using an enantiospecific methadone assay, CYP2B6 preferentially metabolized (S)-methadone, CYP2C19 preferentially metabolized (R)-methadone, and CYP3A4 showed no preference. These data suggest that multiple CYPs metabolized methadone but CYP2B6 had the highest V(max)/K(m). In addition, only CYP2B6 and 2C19 showed stereoselective metabolism. Our data could explain why the plasma concentration ratio of R/S methadone is variable and why drugs that induce CYP2B6 such as nevirapine and efavirenz also induce methadone metabolism, while the CYP3A4 inducer rifabutin has no effect on methadone pharmacokinetics.

Ionization enhancement in atmospheric pressure chemical ionization and suppression in electrospray ionization between target drugs and stable-isotope-labeled internal standards in quantitative liquid chromatography/tandem mass spectrometry

The phenomena of ionization suppression in electrospray ionization (ESI) and enhancement in atmospheric pressure chemical ionization (APCI) were investigated in selected-ion monitoring and selected-reaction monitoring modes for nine drugs and their corresponding stable-isotope-labeled internal standards (IS). The results showed that all investigated target drugs and their co-eluting isotope-labeled IS suppress each other's ionization responses in ESI. The factors affecting the extent of suppression in ESI were investigated, including structures and concentrations of drugs, matrix effects, and flow rate. In contrast to the ESI results, APCI caused seven of the nine investigated target drugs and their co-eluting isotope-labeled IS to enhance each other's ionization responses. The mutual ionization suppression or enhancement between drugs and their isotope-labeled IS could possibly influence assay sensitivity, reproducibility, accuracy and linearity in quantitative liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS). However, calibration curves were linear if an appropriate IS concentration was selected for a desired calibration range to keep the response factors constant.

The importance of stereoselective determination of drugs in the clinical laboratory

About 56% of the drugs currently in use are chiral compounds, and 88% of these chiral synthetic drugs are used therapeutically as racemates. Only a few of these drugs qualify for a stereospecific determination in a clinical laboratory for therapeutic drug monitoring of patients. If the qualitative and quantitative pharmacokinetic and pharmacodynamic effects are similar, the enantiomers do not need to be separated. However, if the metabolism of the different stereoisomers is handled by different enzymes which are either polymorphic or can be induced or inhibited, and if their pharmacodynamic effects have differences either in strength or in quality, enantiospecific analysis is urgently needed. Unfortunately, there are many racemic drugs where the stereospecificity of the metabolism and/or the pharmacodynamic effects of the enantiomers is not known today. For these drugs, there is a great need for studies concentrating on these differences to improve treatment of the patients.

Heroin discriminative stimulus effects of methadone, LAAM and other isomers of acetylmethadol in rats

RATIONALE

LAAM (alpha- l-acetylmethadol) is a derivative of the synthetic mu-opiate agonist methadone and is one of the four isomers of acetylmethadol. Methadone and LAAM have similar pharmacological properties and both are approved medications for the treatment of heroin dependency disorders. Few studies have reported on the pharmacology of acetylmethadol's other isomers and most of these have focused on their potential analgesic activity.

OBJECTIVES

The purpose of the present investigation was to examine the discriminative stimulus effects of LAAM, the other isomers of acetylmethadol, and methadone in rats trained to discriminate heroin from water, and to compare the duration of the discriminative stimulus effects of heroin, methadone, and LAAM.

METHODS

Long-Evans rats were trained to discriminate 0.3 mg/kg heroin from water under a fixed ratio 10 (FR10) schedule of food reinforcement. Dose-response functions for heroin, methadone, LAAM, three other isomers of acetylmethadol: alpha- d-acetylmethadol, beta- d-acetylmethadol, beta- l-acetylmethadol, and its precursor, beta- l-methadol were examined. Additionally, the time course effects for heroin, methadone, and LAAM were examined.

RESULTS

LAAM and methadone dose-dependently occasioned heroin-like discriminative stimulus effects. Two of acetylmethadol's isomers, alpha- d-acetylmethadol and beta- d-acetylmethadol, were more potent than LAAM in producing heroin-like effects. The beta- l-methadol precursor and beta- l-acetylmethadol did not fully substitute for heroin's discriminative stimulus. LAAM elicited heroin-like discriminative stimulus effects for at least 6 h and generated partial generalization up to 36 h following administration.

CONCLUSIONS

Methadone, LAAM, beta- d-acetylmethadol and alpha- d-acetylmethadol, but not beta- l-acetylmethadol and beta- l-methadol evoke heroin-like discriminative stimulus effects.

The use of methadone for cancer pain

Methadone is not a new analgesic drug [69]. Several studies have demonstrated that methadone is a valid alternative to morphine, hydromorphone, and fentanyl for the treatment of cancer-related pain, and extensive reviews on the subject have been published in recent years [10,23,25,64,70,71]. Most people involved in pain therapy, however, are not well informed about the properties of methadone. The authors believe that the low cost of methadone paradoxically contributes to the limited knowledge of its characteristics and to the restricted therapeutic use of this drug. The low cost of methadone means there is little financial incentive for pharmaceutical companies to invest in research or to disseminate scientific information. Unfortunately, the lack of scientific information from pharmaceutical companies frequently results in a lack of knowledge on the part of physicians. Unless the existing approach changes, both culturally and politically, ignorance about methadone will persist among medical experts. The low cost of methadone, rather than being an advantage, will result in the limited exploitation of an effective drug.

Chiral capillary electrophoresis as predictor for separation of drug enantiomers in continuous flow zone electrophoresis

Separation of the enantiomers of chlorpheniramine and methadone in acidic buffers containing carboxymethyl-betacyclodextrin (CMCD) as chiral selector was investigated by capillary zone electrophoresis. For a range of pH and CMCD concentrations, the mobility difference and resolution of the enantiomers were determined. Then, conditions known to provide well resolved enantiomers and optimized chiral separation were applied to chiral continuous flow electrophoresis. In that approach, a thin film of fluid flowing between two parallel plates is employed as carrier for electrophoresis. The electrolytes and the sample are continuously admitted at one end of the electrophoresis chamber and are fractionated by an array of outlet tubes at the other. The number of pure enantiomeric fractions obtained by chiral continuous flow electrophoresis was found to be directly dependent on the enantiomeric mobility difference. For racemic chlorpheniramine separated in a betaine-acetic acid buffer at a total throughput of 5 mg/h, complete enantiomeric separation is shown to require a mobility difference of about 3 x 10(-9) m2/V s. Furthermore, compared to the previous investigations with hydroxypropyl-beta-cyclodextrin, CMCD was found to permit improved fractionation of methadone enantiomers. With a total racemic drug throughput of about 15 mg/h, continuous flow zone electrophoresis processing with CMCD as chiral selector is shown to have the potential of providing pure enantiomers on a mg/h scale. The results indicate that chiral capillary zone electrophoresis data can be employed as predictor for preparative scale chiral separations based upon continuous flow zone electrophoresis.

Development and application of a chiral high performance liquid chromatography assay for pharmacokinetic studies of methadone

Methadone enantiomers and EDDP, the main metabolite of methadone, were separated (R(s) = 2.0 for methadone enantiomers) following liquid-liquid extraction from human serum and urine followed by reverse-phase high-performance liquid chromatography on a derivatized beta-cyclodextrin column and quantified at therapeutic concentrations with ultraviolet detection. Detector response was linear (r(2) > 0.98) to 1,000 and 2,500 ng x mL(-1) for methadone enantiomers and EDDP, respectively. The limit of quantification from a 1-mL biological sample was 2.5 and 5 ng x mL(-1) for methadone enantiomers and EDDP, respectively. Interday variation was <13% and intraday variation was <8% for the analytes of interest. The assay was applied to plasma protein and erythrocyte binding studies and a 96-h pharmacokinetic study in two healthy female volunteers following oral dosing with rac-methadone. The binding of methadone to plasma proteins was enantioselective with the active (-)-(R) enantiomer having the highest free fraction (mean +/- SD: 21.2+/-7.6% vs. 13.3+/-6.2% for (+)-(S)-methadone, n = 8). Binding of methadone to erythrocytes was not apparently enantioselective (38.6+/-1.3% and 38.1+/-1.4% bound for (-)-(R)- and (+)-(S)-methadone, respectively). The pharmacokinetic study revealed enantioselective disposition of methadone in one volunteer but not in the other. EDDP was observed in urine but was only in small or undetectable concentrations in serum. The method is applicable to in vitro and pharmacokinetic studies of rac-methadone disposition in humans.

Chiral stationary phases in HPLC for the stereoselective determination of methadone

An extensive study of the behavior of three chiral stationary phases (CSP) is presented for the stereoselective determination of methadone. The following chromatographic columns were selected: a cellulose, Chiralcel OJ; a modified cyclodextrin. Cyclobond I 2000 RSP, and a protein, Chiral-AGP. Retention factors, enantioselectivity, efficiency, and resolution were tested by modifying the composition of the mobile phase as well as the temperature. The mechanism for the chiral recognition of methadone on each support was discussed. Optimal chromatographic parameters were obtained for the three supports tested, and methadone enantiomers were separated in less than 20 minutes. The cellulose-based column gave the best resolution, but this CSP was not adapted to clinical analyses of methadone. Under optimized conditions, the cyclodextrin- and protein-based columns allowed an excellent separation of methadone enantiomers, but no interference with the primary metabolite was found only with Chiral-AGP.

Experimental verification of a predicted, hitherto unseen separation selectivity pattern in the nonaqueous capillary electrophoretic separation of weak base enantiomers by octakis (2,3-diacetyl-6-sulfato)-gamma-cyclodextrin

The capillary electrophoretic separation of cationic enantiomers with single-isomer multivalent anionic resolving agents was reexamined with the help of the charged resolving agent migration model. Three general model parameters were identified that influence the shape of the separation selectivity and enantiomer mobility difference curves: parameter b, the binding selectivity (K(RCD)/K(SCD)), parameter s, the size selectivity (mu0(RCD)/mu0(SCD)), and parameter a, the complexation-induced alteration of the analyte's mobility (mu0(RCD)/mu0). In addition to the previously observed discontinuity in separation selectivity that occurs as mu(eff) of the less mobile enantiomer changes from cationic to anionic, a new feature, a separation selectivity maximum was predicted to occur in the resolving agent concentration range where both enantiomers migrate cationically provided that (i) K(RCD)/K(SCD) <1 and mu0(RCD)/mu0(SCD) >1 and (K(RCD)mu0(RCD))/(K(SCD)mu0(SCD)) > 1, or (ii) K(RCD)/K(SCD) >1 and mu0(RCD)/mu0(SCD) <1 and (K(RCD)mu0(RCD))/(K(SCD)mu0(SCD)) <1. This hitherto unseen separation selectivity pattern was experimentally verified during the nonaqueous capillary electrophoretic separation of the enantiomers of four weak base analytes in acidic methanol background electrolytes with octakis(2,3-diacetyl-6-sulfato)-gamma-cyclodextrin (ODAS-gammaCD) as resolving agent.

[New psychopharmacologic studies on methadone: implications for the treatment of opiate dependency]

New psychopharmacological studies performed these last few years should allow, in future, to improve methadone prescription. In particular, thanks to the identification of enzymes involved in the metabolism of this molecule, it is now possible to better understand the problems of metabolic interactions, to foresee and to avoid them. Studies showing the influence of genetic and environmental factors on blood concentrations of methadone, as well as a recent clinical study performed with methadone doses largely in excess of 100 mg/day, stress the importance of adapting methadone treatment individually, particularly as regards to the choice of the dose. This choice must not result from an administrative standardization, as it is essential for patients that, with an adequate social and psychological treatment, an adequate pharmacological treatment should be associated.

[Methadone as an analgesic]

Methadone is a synthetic opioid agonist which has been available for more than 40 years. Although its main use has been in the maintenance therapy of opioid addicts, it has a number of unique characteristics including excellent analgesic effect, a high bioavailability, no known active metabolites, long duration of action during long-term treatment, high potency, and low cost. However, its use is limited by the complexity of the pharmacokinetics, with a risk of drug accumulation with toxic effects. Recent findings suggest that standard equianalgesic tables are unreliable, because methadone is much more potent when used for long-term treatment. Therefore methadone should be titrated carefully and increases in dosage should be performed every third or fourth day.

Methadone conversion to EDDP during GC-MS analysis of urine samples

During validation of a gas chromatography-mass spectrometry (GC-MS) method for the methadone metabolite 2-ethylidine-1,5dimethyl-3,3-diphenylpyrrolidine (EDDP), it was noted that detectable levels of EDDP were found during analysis of extracts from drug-free urine samples spiked with methadone. Different amounts of EDDP were detected by GC-MS during confirmation analysis; however, levels consistently exceeded 50 ng/mL at methadone concentrations > 10,000 ng/mL. Quantitation of EDDP was determined by the addition of EDDP-d3 to methadone-spiked urine samples. Subsequent analysis of methadone-spiked urine extracts by high-performance liquid chromatography (HPLC) indicated no EDDP as a result of contaminated standard or conversion during solid-phase extraction. Reducing the GC injector-port temperature from 260 degrees C to 180 degrees C reduced the observed EDDP concentration in one sample from 201 ng/mL to 53 ng/mL at the initial methadone concentration of 10,000 ng/mL. These results indicate GC injector-port temperature induces thermal conversion of methadone to EDDP as an artifact. When confirmation of methadone and EDDP is critical to determining individual compliance with maintenance programs, alternative chromatographic methods (e.g., capillary electrophoresis, HPLC, or liquid chromatography-mass spectrometry) should be considered.

L-Methadone and D,L-methadone in methadone maintenance treatment: a comparison of therapeutic effectiveness and plasma concentrations

The clinical effectiveness of l-methadone maintenance treatment (LMMT) carried out using d,l-methadone or l-methadone have been compared with ambulatory heroin-dependent subjects. A total of 40 heroin-dependent subjects, previously maintained on l-methadone in Frankfurt am Main, were divided into two groups under randomised double-blind conditions and received either an equivalent dose of l-methadone as d,l-methadone or remained on the previous l-methadone treatment. Requests for a change in the dose of d,l-methadone and l-methadone were recorded, urine samples for determination of illicit drug use were collected and the individual level of opiate craving was determined over a 22-day observation period. There was no significant difference between the two groups in the number requests for a dose change (dose increase <10%). However, there was a significant increase in heroin use in the group which continued to receive l-methadone. Although there was less variability in opiate craving in the group receiving d,l-methadone, the mean intensity of opiate craving did not differ between the two groups. The mean l-methadone dose:l-methadone plasma concentration ratio, an index of the bioavailability of l-methadone in individual subjects, showed no significant change when the treatment was changed to d,l-methadone. The mean d-methadone:l-methadone plasma concentration ratio was 1.17. There was no significant difference between these ratios for day 15 and day 22. The mean l-methadone:EDDP plasma concentration ratio in the l-methadone group was 22.2 and the d,l-methadone:EDDP plasma concentration ratio was 18.4 . The plasma EDDP concentration in the d,l-methadone group increased 3-fold after starting treatment with d, l-methadone. These findings suggest that d,l-methadone can be used in methadone maintenance treatment of heroin-dependent subjects but that further studies are required to evaluate pharmacokinetic interactions between methadone enantiomers.

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.