Effect of omeprazole on oral and intravenous RS-methadone pharmacokinetics and pharmacodynamics in the rat

Simultaneous determination of methadone and tramadol in serum samples by ultrasonic-assisted micro solid phase extraction and gas chromatography-mass spectrometry

Ultrasonic-assisted dispersive micro solid phase extraction (UA-DMSPE) is proposed as a fast and easy technique for the extraction and preconcentration of methadone and tramadol from serum samples. Different sorbents including carbon nanotubes, oxidized carbon nanotubes, and TiO2 nanoparticles were compared to extract methadone and tramadol. The best performance was obtained using oxidized carbon nanotubes due to the strong affinity between the drugs and carbon nanotube adsorbents. Final analysis of drugs performed by using gas chromatography-mass spectrometric detection. Different parameters affecting the extraction efficiency, such as the sample volume, amount of adsorbent, desorption solvent type and volume, centrifugation time, and speed were investigated and optimized. The striking features of this technique are correlated to its speed and the small volumes of sample (about 1 mL), desorption solvent (about 50 μL), and adsorbent (about 0.001 g) for analysis of drugs, and finally, milder centrifugation conditions relative to the previously reported adsorbent. The optimal parameters were achieved as follows: pH value was set at 9, the sample volume was adjusted to 1200 µL, the amount of adsorbent used was 1 mg, the extraction time was set at 5 min, and the volume of the desorption solvent was adjusted to 50 µL. The limits of detections (0.5 and 0.8 ng mL-1) and quantifications (1.5 and 2.5 ng mL-1) were obtained for methadone and tramadol, respectively. The developed method also showed good repeatability, relative standard deviation (RSD) of 9.49 % and 7.47 % (n = 5), for the spiked aqueous solution at the concentration level of 10, 50, and 100 ng mL-1 for analytes, and linearity, R ≥ 0.9809. The results showed that UA-DMSPE is a quick, relatively inexpensive, and environmentally friendly alternative technique for the extraction of opiate drugs from serum samples.

A Novel Maintenance Therapeutic for Opioid Use Disorder

Opioid use disorder (OUD) is a major socioeconomic burden. An ideal OUD pharmacotherapy will mitigate the suffering associated with opioid-withdrawal, inhibit the effects of high efficacy opioids, and minimize opioid-cravings while being safe and accessible to a diverse patient population. Although current OUD pharmacotherapies inhibit the euphoric effects of opioids of abuse, the extent to which they safely alleviate withdrawal and opioid-cravings corresponds with their intrinsic µ opioid receptor (MOR) efficacy. In addition to inhibiting the euphoric effects of opioids of abuse, the medium efficacy MOR agonist buprenorphine alleviates withdrawal and opioid-cravings, but its intrinsic MOR efficacy is sufficient such that its utility is limited by abuse and safety liabilities. Although the MOR antagonist naltrexone minimizes euphoria and has no abuse liability, it exacerbates suffering associated with withdrawal and opioid cravings. Therefore, a therapeutic with intrinsic MOR activity between the partial agonist (buprenorphine) and the antagonist (naltrexone) would strike a balance between the benefits and liabilities of these two therapeutics. To address this need, we derived RM1490, an MOR agonist based on a nonmorphinan scaffold that exhibits approximately half the intrinsic MOR efficacy of buprenorphine. In a series of preclinical assays, we compared RM1490 with buprenorphine and naltrexone at doses that achieve therapeutic levels of central nervous system MOR occupancy. RM1490 exhibited a behavioral profile consistent with reduced reward, dependence, and precipitated withdrawal liabilities. RM1490 was also more effective than buprenorphine at reversing the respiratory depressant effects of fentanyl and did not suppress respiration when combined with diazepam. SIGNIFICANCE STATEMENT: In preclinical studies, RM1490 has a physiological and behavioral profile suitable for opioid use disorder maintenance therapy.

Methadone: a review of drug-drug and pathophysiological interactions

Numerous established and potential drug interactions with methadone are clinically important in people treated with methadone either for addiction or for chronic pain. Methadone users often have comorbidities and are prescribed drugs that may interact with methadone. Methadone is extensively metabolized by cytochrome P450 (CYP) 3A4 and to a lesser extent by CYP 1A2, 2D6, 2D8, 2C9/2C8, 2C19, and 2B6. Eighty-six percent of methadone is protein bound, predominately to α1-acid glycoprotein (AGP). Polymorphisms in or interactions with CYPs that metabolize methadone, changes in protein binding, and other pathophysiological conditions affect the pharmacokinetic properties of methadone. It is critical for health care providers who treat patients on methadone to have adequate information on the interactions of methadone with other drugs of abuse and other medications. We set out to describe drug-drug interactions as well as physiological and pathophysiological factors that may impact the pharmacokinetics of methadone. Using MEDLINE, we conducted a systematic search for papers and related abstracts published between 1966 and June 2010. Keywords that included methadone, drug-drug interactions, CYP P450 and AGP identified a total of 7709 papers. Other databases, including the Cochrane Database of Systematic Reviews and Scopus, were also searched; an additional 929 papers were found. Final selection of 286 publications was based on the relevance of each paper to the topic. Over 50 such interactions were found. Interactions of methadone with other drugs can lead to increased or decreased methadone drug levels in patients and result in potential overdose or withdrawal, respectively. The former can contribute to methadone's fatality. Prescribers of methadone and pharmacists should enquire about any new medications (including natural products and over-the-counter medications) periodically, and especially when an otherwise stable patient suddenly experiences drug craving, withdrawal or intoxication.

Treatment with double dose of omeprazole increases β-endorphin plasma level in patients with coronary artery disease

INTRODUCTION

The proton pump inhibitor empirical trial, besides the analysis of symptoms, is the main method in the diagnosis of gastro-oesophageal reflux disease-related chest pain. β-Endorphin acts as an endogenous analgesia system. The aim of the study was verify whether β-endorphin plasma level is affected by omeprazole administration and influences the severity of anginal symptoms and outcome of the "omeprazole test" in patients with coronary artery disease (CAD) and chest pain of suspected non-cardiac origin.

MATERIAL AND METHODS

Omeprazole was administered to 48 patients with CAD in a randomized, placebo-controlled, crossover study design. At the beginning of the study, and again after the 14-day omeprazole and placebo treatment, the β-endorphin plasma concentration was determined.

RESULTS

The level of plasma β-endorphin after the administration of omeprazole was significantly greater than at the start of the study and following the placebo. Responders to omeprazole had an average lower β-endorphin plasma concentration than subjects who failed to respond to this therapy. Subjects with symptoms in class III (according to the Canadian Cardiovascular Society classification) after omeprazole administration had a greater β-endorphin plasma level than subjects in class II for anginal symptom severity.

CONCLUSIONS

Fourteen-day therapy with a double omeprazole dose significantly increases the β-endorphin plasma concentration in patients with CAD. Circulating β-endorphin does not seem to be involved in the mechanism for the "omeprazole test" outcome, although an individually different effect on pain threshold cannot be excluded.

Effect of P-glycoprotein inhibition on methadone analgesia and brain distribution in the rat

Methadone is an opiate drug that has been identified as an in-vitro substrate of the efflux pump P-glycoprotein (P-gp), active in the intestinal epithelium and in the blood–brain barrier (BBB), among other sites. The objective of this study was to test in vivo, in the rat model, the role of P-gp modulation on the analgesic effect and brain uptake of methadone, as well as identify the most relevant site via dual oral and intravenous (i.v.) experiments. The P-gp specific inhibitor (valspodar or PSC833) was preadministered (10 mg kg−1 i.v.) to test groups. Analgesia was measured using the tailflick test. The ED50 for oral methadone (2, 3, 6 and 8 mg kg−1) decreased three-fold in valspodar groups compared with controls (2.23 + 0.002 mg kg−1 and 6.07 + 0.07 mg kg−1; P < 0.0001). The overall analgesic effect (% antinociception) was elevated 3.1 times in pretreated compared with control rats (90.65% + 0.22 vs 29.23% + 14.0; P < 0.01) after 6 mg kg−1 oral methadone and 2.8 times after i.v. (0.35 mg kg−1) administration (91.75% + 4.27 vs 32.45% + 9.0; P < 0.01). The brain:plasma distribution ratio was higher in pretreated animals and AUCbrain (overall brain concentration) was 6 times higher after oral methadone and 4 times higher after i.v. compared with controls, disproportionally increased relative to plasma, implying an active process at the BBB. P-gp, and hence substrate comedication, plays a critical role in the evolution of the methadone analgesic effect and in its brain uptake, independent of the administration route.

Modeling methadone pharmacokinetics in rats in presence of P-glycoprotein inhibitor valspodar

PURPOSE

To quantify the in vivo role of P-glycoprotein (P-gp) in the pharmacokinetics of methadone after intravenous and oral administration, using valspodar as a P-gp inhibitor.

MATERIALS AND METHODS

Methadone plasma concentrations after intravenous (0.35 mg/kg) and oral (6 mg/kg) administration were analyzed, in absence and presence of valspodar, using nonlinear mixed effects modeling (NONMEM V). Non-parametric bootstrap analysis and posterior predictive check were employed as model evaluation techniques.

RESULTS

The pharmacokinetics of methadone in the rat was successfully modeled using a two-compartmental model with a linear elimination from the central compartment and a first-order absorption process with lag time. Valspodar increased methadone F by 122% (95%CI: 34-269%) and decreased the V ( c ) and V ( p ) by 35% (95%CI: 16-49%) and 81% (95%CI: 63-93%), respectively. No effect of valspodar on other pharmacokinetic parameters was discernible. The non-parametric bootstrap analysis confirmed the absence of bias on the parameter estimates, and visual predictive check evidence the adequacy of the model to reproduce the observed time course of methadone plasma concentrations.

CONCLUSION

Valspodar increased methadone's bioavailability as consequence of P-gp inhibition, which resulted in an increased analgesic effect of methadone.

Méthadone : de la pharmacocinétique à la pharmacologie clinique

In January 2002, the official French methadone legislation prescription was modified. Thus, the number of clinicians authorized to introduce methadone substitution was increased. Knowledge of the pharmacokinetic and pharmacological properties of this compound remains particularly important for its appropriate prescription. Bearing this in mind, we linked methadone pharmacokinetics to its pharmacological use in this article. METHADONE PHARMACOLOGY: Methadone is a synthetic opiate. Its mean bioavailability is around 75%. Cytochrome P450 3A4 and 2D6 are involved in its hepatic metabolism. Its volume of distribution is of around 4 L/kg. The value of half-life elimination is of around 22 hours. These pharmacokinetic properties (long half-life, steady state concentration) are in favour of substitution use of this opiate. In practice, clinicians progressively introduce this substitution therapy to reach 80 mg +/- 20 mg per day, once daily. Therapeutic clinical goals are mainly to reduce craving, withdrawal symptoms, and to manage psychosocial problems and psychiatric co-morbidity. Practitioners should bear the latter in mind once substitution therapy has been appropriately initiated and stabilized. However, wide, interpatient, interindividual variability impacts on pharmacokinetic parameters. Subjects may be either high or poor metabolizers. Thus, bioavibility ranges from 36 to 100%. Induction or inhibition of CYP450 significantly modifies methadone pharmacodynamic properties. Genetic variability and medication can induce non response to substitution, craving, or withdrawal symptoms. PHARMACOLOGICAL INTERACTIONS: We describe here a large number of medications involved in pharmacokinetic or pharmacological interactions. Classical enzymatic inductors, such as antiepileptic molecules (phenobarbital, carbamazepin), antituberculosis compounds (rifampicin), or antiretroviral therapy (efavirenz, nevirapin, ritonavir), could possibly lead to respiratory depression for example. Metabolism inhibitors such as selective serotonin reuptake inhibitors (fluvoxamine, fluoxetine, paroxetine, sertraline) or antifungals of the azol groups could enhance plasma concentration and may sometimes lead to respiratory depression or death. Nevertheless, clinicians should know methadone pharmacokinetic properties and pharmacological interactions for the optimal opiate-dependant patients' management.

CLINICAL USE

Clinicians can use plasma concentrations as a useful indicator to reach substitution goals. The methadone plasmatic target value of 400 microg/ml can be recommended for therapeutic drug monitoring. This dosage not only facilitates interaction detection, but also hand encourages communication with the patient.

The effects of inhibiting cytochrome P450 3A, p-glycoprotein, and gastric acid secretion on the oral bioavailability of methadone in dogs

Methadone is an opioid, which has a high oral bioavailability (>70%) and a long elimination half-life (>20 h) in human beings. The purpose of this study was to evaluate the effects of ketoconazole [a CYP3A and p-glycoprotein (p-gp) inhibitor] and omeprazole (an H+,K(+)-ATPase proton-pump inhibitor) on oral methadone bioavailability in dogs. Six healthy dogs were used in a crossover design. Methadone was administered i.v. (1 mg/kg), orally (2 mg/kg), again orally following oral ketoconazole (10 mg/kg q12 h for two doses), and following omeprazole (1 mg/kg p.o. q12 h for five doses). Plasma concentrations of methadone were analyzed by high-pressure liquid chromatography or fluorescence polarization immunoassay. The mean +/- SD for the elimination half-life, volume of distribution, and clearance were 1.75 +/- 0.25 h, 3.46 +/- 1.09 L/kg, and 25.14 +/- 9.79 mL/min.kg, respectively following i.v. administration. Methadone was not detected in any sample following oral administration alone or following oral administration with omeprazole. Following administration with ketoconazole, detectable concentrations of methadone were present in one dog with a 29% bioavailability. MDR-1 genotyping, encoding p-gp, was normal in all dogs. In contrast to its pharmacokinetics humans, methadone has a short elimination half-life, rapid clearance, and low oral bioavailability in dogs and the extent of absorption is not affected by inhibition of CYP3A, p-gp, and gastric acid secretion.

Alpha‐1‐acid glycoprotein directly affects the pharmacokinetics and the analgesic effect of methadone in the rat beyond protein binding

Methadone is a basic drug highly bound to alpha1-acid glycoprotein (AGP), a plasma protein that increases in several pathological situations. Our aims were to evaluate the processes (pharmacokinetics-PK and/or pharmacodynamics-PD) associated with changes of methadone analgesia under conditions of increased AGP, and whether these changes are dependent on binding, secondary to a pathology, or directly attributable to AGP. AGP levels, in rats, were increased by two different methods: (a) experimental inflammation with turpentine oil (TP), and (b) by directly infusing the protein (exo-AGP). Both had a corresponding control group. Tail-flick analgesia and PK were evaluated after methadone dose (0.35 mg/kg i.v.). Bicompartmental PK parameters as well as interanimal and assay variabilities were estimated using NONMEM. The relationship between Cp and analgesic effect (PD) was analyzed with WINNONLIN. AGP levels in both pretreated groups (TP and exo-AGP) were significantly increased, and the unbound fraction (fu) was decreased, compared to controls. All PK parameters were lower in the pretreated groups, but in exo-AGP the difference was maintained even when corrected by fu. Paradoxically, also in exo-AGP the analgesic effect was practically nonexistent, although the unbound Cp remained high, possibly associated to a change in the PD. AGP appears responsible for alterations in both PK and PD, beyond protein binding and inflammatory processes.

Sex specificity in methadone analgesia in the rat: a population pharmacokinetic and pharmacodynamic approach

PURPOSE

To quantify the extent to which a sex-specific dichotomy in the temporal evolution of the analgesic effect, after intravenous (i.v.) methadone injection in the rat, relates to the pharmacokinetics (PK) and pharmacodynamics (PD) that mediate the dose-to-effect pathway.

METHODS

Tail-flick analgesia was measured after i.v. methadone injection (0.35 mg/kg) in female (n = 16) and male (n = 16) Sprague-Dawley rats. The PK were evaluated in separate female (n = 56) and male (n = 56) rats after they had received the same dose of methadone i.v. (0.35 mg/kg). A bicompartmental model described the kinetics and a sigmoid Emax model-related drug effect vs. simulated concentrations (pharmacodynamics) at the times of effect measurement. All model parameters as well as interanimal and assay variabilities were estimated with a mixed-effects population method using the program NONMEM.

RESULTS

The area under the effect-time curve (AUCE0-120) was (mean +/- interanimal SD) 1859+/-346 min in the females, which was significantly lower than the 4871+/-393 min in the males (P < 0.0001). On the contrary, the profiles of concentration vs. time were higher in females and, therefore, corresponded inversely to the effect vs. time-relative magnitudes. The central volume of distribution, V1, was 1.94+/-0.37 l/kg for female rats and 3.01+/-0.33 l/kg for male rats. Also, the central clearance was 0.077+/-0.006 l/min/kg and 0.102+/-0.005 l/min/ kg, respectively, for female and male rats. Both parameters differed significantly between sexes (P < 0.0001). The pharmacodynamic maximum observed effect parameter (Emax) was 37%+/-29% in female rats and 85%+/-16% in male rats, and these values were significantly different (P < 0.0001). The parameter for the concentration eliciting half of Emax (EC50) was 24.1+/-7.5 microg/l in female rats and 20.3+/-2.9 microg/l in male rats, and the Hill-related exponent, gamma, was 6.3+/-3.9 in female rats and 5.5+/-4.1 in male rats. These parameters did not differ significantly (at the P < 0.05 level).

CONCLUSIONS

A sex-specific dichotomy in the methadone antinociceptive effect, in the rat, was not proportionally related to plasma concentrations. Each sex corresponded to a distinct subpopulation of the PK parameters and one of the pharmacodynamic parameters (Emax). When the course of a drug involves PK or PD subpopulations, PK/PD modeling can afford the safest prediction of the effect-time evolution for a particular dose.