Evidence for sex differences in the impact of cytochrome P450 genotypes on early subjective effects of cannabis

Early positive subjective effects of cannabis predict the development of cannabis use disorder (CUD). Genetic factors, such as the presence of cytochrome P450 genetic variants that are associated with reduced Δ9-tetrahydrocannabinol (THC) metabolism, may contribute to individual differences in subjective effects of cannabis. Young adults (N = 54) with CUD or a non-CUD substance use disorder (control) provided a blood sample for DNA analysis and self-reported their early (i.e., effects upon initial uses) and past-year positive and negative subjective cannabis effects. Participants were classified as slow metabolizers if they had at least one CYP2C9 or CYP3A4 allele associated with reduced activity. Though the CUD group and control group did not differ in terms of metabolizer status, slow metabolizer status was more prevalent among females in the CUD group than females in the control group. Slow metabolizers reported greater past year negative THC effects compared to normal metabolizers; however, slow metabolizer status did not predict early subjective cannabis effects (positive or negative) or past year positive effects. Post-hoc analyses suggested males who were slow metabolizers reported more negative early subjective effects of cannabis than female slow metabolizers. Other sex-by-genotype interactions were not significant. These initial findings suggest that genetic variation in CYP2C9 and CYP3A4 may have sex-specific associations with cannabis-related outcomes. Slow metabolizer genes may serve as a risk factor for CUD for females independent of subjective effects. Male slow metabolizers may instead be particularly susceptible to the negative subjective effects of cannabis.

Pharmacokinetic Variability of Oral Cannabidiol and Its Major Metabolites after Short-Term High-Dose Exposure in Healthy Subjects

Introduction

Cannabidiol (CBD) is a widely utilized nonpsychoactive cannabinoid available as a prescriptive drug treatment and over-the-counter supplement. In humans, CBD is metabolized and forms the major active metabolite 7-hydroxy-cannabidiol (7-OH-CBD), which is further metabolized to 7-carboxy-cannabidiol (7-COOH-CBD). In the current study, plasma concentrations of CBD, 7-OH-CBD, and 7-COOH-CBD were measured, and the potential influences of sex, race, and body mass index (BMI) on the pharmacokinetic variability were assessed.

Methods

Blood samples from a previously conducted CBD drug interaction study in healthy volunteers (n = 12) were utilized. The subjects received orally administered CBD (Epiodiolex®), 750 mg twice daily for 3 days and a single dose on the 4th day. Nine plasma samples were collected, and plasma concentrations of CBD, 7-OH-CBD, and 7-COOH-CBD were analyzed by LC-MS/MS. Peak plasma concentration (Cmax), time to Cmax (Tmax), area under the curve (AUC), and metabolite-to-parent drug exposure ratios (MPR) were calculated. Statistical analysis was performed to determine the correlations of Cmax, AUC, and MPR of CBD, 7-OH-CBD, and 7-COOH-CBD in different sex, race, BMI, and body weight.

Results

For CBD, the mean Cmax was 389.17 ± 153.23 ng/mL, and the mean AUC was 1,542.19 ± 488.04 ng/mL*h. For 7-OH-CBD, the mean Cmax was 81.35 ± 36.64 ng/mL, the mean AUC was 364.70 ± 105.59 ng/mL*h, and the mean MPR was 0.25 ± 0.07. For 7-COOH-CBD, the mean Cmax was 1,717.33 ± 769.22 ng/mL, the mean AUC was 9,888.42 ± 3,961.47 ng/mL*h, and the mean MPR was 7.11 ± 3.48. For 7-COOH-CBD, a 2.25-fold higher Cmax was observed in female subjects (p = 0.0155) and a 1.97-fold higher AUC for female subjects (p = 0.0285) with the normalization of body weight. A significant linearity (p = 0.0135) of 7-OH-CBD AUC with body weight in females was observed. No significant differences were identified in Cmax, AUC, and PMR with race and BMI.

Conclusion

Observed differences in sex were in agreement with previously reported findings. A larger population pharmacokinetics study is warranted to validate the observed higher Cmax and AUC in females and significant linearity with body weight in females from the current study.

An in vitro evaluation of kratom (Mitragyna speciosa) on the catalytic activity of carboxylesterase 1 (CES1)

Kratom, (Mitragyna Speciosa Korth.) is a plant indigenous to Southeast Asia whose leaves are cultivated for a variety of medicinal purposes and mostly consumed as powders or tea in the United States. Kratom use has surged in popularity with the lay public and is currently being investigated for possible therapeutic benefits including as a treatment for opioid withdrawal due to the pharmacologic effects of its indole alkaloids. A wide array of psychoactive compounds are found in kratom, with mitragynine being the most abundant alkaloid. The drug-drug interaction (DDI) potential of mitragynine and related alkaloids have been evaluated for effects on the major cytochrome P450s (CYPs) via in vitro assays and limited clinical investigations. However, no thorough assessment of their potential to inhibit the major hepatic hydrolase, carboxylesterase 1 (CES1), exists. The purpose of this study was to evaluate the in vitro inhibitory potential of kratom extracts and its individual major alkaloids using an established CES1 assay and incubation system. Three separate kratom extracts and the major kratom alkaloids mitragynine, speciogynine, speciociliatine, paynantheine, and corynantheidine displayed a concentration-dependent reversible inhibition of CES1. The experimental Ki values were determined as follows for mitragynine, speciociliatine, paynantheine, and corynantheidine: 20.6, 8.6, 26.1, and 12.5 μM respectively. Speciociliatine, paynantheine, and corynantheidine were all determined to be mixed-type reversible inhibitors of CES1, while mitragynine was a purely competitive inhibitor. Based on available pharmacokinetic data, determined Ki values, and a physiologically based inhibition screen mimicking alkaloid exposures in humans, a DDI mediated via CES1 inhibition appears unlikely across a spectrum of doses (i.e., 2-20g per dose). However, further clinical studies need to be conducted to exclude the possibility of a DDI at higher and extreme doses of kratom and those who are chronic users.

In vitro evaluation of the impact of Covid-19 therapeutic agents on the hydrolysis of the antiviral prodrug remdesivir

Remdesivir (RDV, Veklury®) is an FDA-approved prodrug for the treatment of hospitalized patients with COVID-19. Recent in vitro studies have indicated that human carboxylesterase 1 (CES1) is the major metabolic enzyme catalyzing RDV activation. COVID-19 treatment for hospitalized patients typically also involves a number of antibiotics and anti-inflammatory drugs. Further, individuals who are carriers of a CES1 variant (polymorphism in exon 4 codon 143 [G143E]) may experience impairment in their ability to metabolize therapeutic agents which are CES1 substrates. The present study assessed the potential influence of nine therapeutic agents (hydroxychloroquine, ivermectin, erythromycin, clarithromycin, roxithromycin, trimethoprim, ciprofloxacin, vancomycin, and dexamethasone) commonly used in treating COVID-19 and 5 known CES1 inhibitors on the metabolism of RDV. Additionally, we further analyzed the mechanism of inhibition of cannabidiol (CBD), as well as the impact of the G143E polymorphism on RDV metabolism. An in vitro S9 fraction incubation method and in vitro to in vivo pharmacokinetic scaling were utilized. None of the nine therapeutic agents evaluated produced significant inhibition of RDV hydrolysis; CBD was found to inhibit RDV hydrolysis by a mixed type of competitive and noncompetitive partial inhibition mechanism. In vitro to in vivo modeling suggested a possible reduction of RDV clearance and increase of AUC when coadministration with CBD. The same scaling method also suggested a potentially lower clearance and higher AUC in the presence of the G143E variant. In conclusion, a potential CES1-mediated DDI between RDV and the nine assessed medications appears unlikely. However, a potential CES1-mediated DDI between RDV and CBD may be possible with sufficient exposure to the cannabinoid. Patients carrying the CES1 G143E variant may exhibit a slower biotransformation and clearance of RDV. Further clinical studies would be required to evaluate and characterize the clinical significance of a CBD-RDV interaction.

The Pharmacokinetics and Pharmacogenomics of Psychostimulants

The psychostimulants-amphetamine and methylphenidate-have been in clinical use for well more than 60 years. In general, both stimulants are rapidly absorbed with relatively poor bioavailability and short half-lives. The pharmacokinetics of both stimulants are generally linear and dose proportional although substantial interindividual variability in pharmacokinetics is in evidence. Amphetamine (AMP) is highly metabolized by several oxidative enzymes forming multiple metabolites while methylphenidate (MPH) is primarily metabolized by hydrolysis to the inactive metabolite ritalinic acid. At present, pharmacogenomic testing as an aid to guide dosing and personalized treatment cannot be recommended for either agent. Few pharmacokinetically based drug-drug interactions (DDIs) have been documented for either stimulant.

Prediction of Carboxylesterase 1-mediated In Vivo Drug Interaction between Methylphenidate and Cannabinoids using Static and Physiologically Based Pharmacokinetic Models

The use of cannabis products has increased substantially. Cannabis products have been perceived and investigated as potential treatments for attention-deficit/hyperactivity disorder (ADHD). Accordingly, co-administration of cannabis products and methylphenidate (MPH), a first-line medication for ADHD, is possible. Oral MPH undergoes extensive presystemic metabolism by carboxylesterase 1 (CES1), a hepatic enzyme which can be inhibited by two prominent cannabinoids, Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD). This prompts further investigation into the likelihood of clinical interactions between MPH and these two cannabinoids through CES1 inhibition. In the present study, inhibition parameters were obtained from a human liver S9 system and then incorporated into static and physiologically-based pharmacokinetic (PBPK) models for prediction of potential clinical significance. The inhibition of MPH hydrolysis by THC and CBD was reversible, with estimated unbound inhibition constants (Ki,u) of 0.031 and 0.091 µM, respectively. The static model predicted a mild increase in MPH exposure by concurrent THC (34%) and CBD (94%) from smoking a cannabis cigarette and ingestion of prescriptive CBD, respectively. PBPK models suggested no significant interactions between single doses of MPH and CBD (2.5 - 10 mg/kg) when administered simultaneously, while a mild interaction (area under drug concentration-time curve increased by up to 55% and maximum concentration by up to 45%) is likely if multiple doses of CBD (10 mg/kg twice daily) are administered. In conclusion, the pharmacokinetic disposition of MPH can be potentially influenced by THC and CBD under certain clinical scenarios. Whether the magnitude of predicted interactions translates into clinically relevant outcomes requires verification in an appropriately designed clinical study. SIGNIFICANCE STATEMENT: This work demonstrated a potential mechanism of drug-drug interactions between methylphenidate (MPH) and two major cannabinoids (Δ9-tetrahydrocannabinol [THC] and cannabidiol [CBD]) not previously reported. We predicted a mild interaction between MPH and THC when the cannabinoid exposure occurred via cannabis smoking. Mild interactions between MPH and CBD were predicted with multiple oral administrations of CBD.

Non-effectiveness of Ivermectin on Inpatients and Outpatients With COVID-19; Results of Two Randomized, Double-Blinded, Placebo-Controlled Clinical Trials

Background

Ivermectin which was widely considered as a potential treatment for COVID-19, showed uncertain clinical benefit in many clinical trials. Performing large-scale clinical trials to evaluate the effectiveness of this drug in the midst of the pandemic, while difficult, has been urgently needed.

Methods

We performed two large multicenter randomized, double-blind, placebo-controlled clinical trials evaluating the effectiveness of ivermectin in treating inpatients and outpatients with COVID-19 infection. The intervention group received ivermectin, 0.4mg/kg of body weight per day for 3 days. In the control group, placebo tablets were used for 3 days.

Results

Data for 609 inpatients and 549 outpatients were analyzed. In hospitalized patients, complete recovery was significantly higher in the ivermectin group (37%) compared to placebo group (28%; RR, 1.32 [95% CI, 1.04–1.66]; p-value = 0.02). On the other hand, the length of hospital stay was significantly longer in the ivermectin group with a mean of 7.98 ± 4.4 days compared to the placebo receiving group with a mean of 7.16 ± 3.2 days (RR, 0.80 [95% CI, 0.15–1.45]; p-value = 0.02). In outpatients, the mean duration of fever was significantly shorter (2.02 ± 0.11 days) in the ivermectin group versus (2.41 ± 0.13 days) placebo group with p value = 0.020. On the day seventh of treatment, fever (p-value = 0.040), cough (p-value = 0.019), and weakness (p-value = 0.002) were significantly higher in the placebo group compared to the ivermectin group. Among all outpatients, 7% in ivermectin group and 5% in placebo group needed to be hospitalized (RR, 1.36 [95% CI, 0.65–2.84]; p-value = 0.41). Also, the result of RT-PCR on day five after treatment was negative for 26% of patients in the ivermectin group versus 32% in the placebo group (RR, 0.81 [95% CI, 0.60–1.09]; p-value = 0.16).

Conclusion

Our data showed, ivermectin, compared with placebo, did not have a significant potential effect on clinical improvement, reduced admission in ICU, need for invasive ventilation, and death in hospitalized patients; likewise, no evidence was found to support the prescription of ivermectin on recovery, reduced hospitalization and increased negative RT-PCR assay for SARS-CoV-2 5 days after treatment in outpatients. Our findings do not support the use of ivermectin to treat mild to severe forms of COVID-19.

Clinical Trial Registration

www.irct.ir IRCT20111224008507N5 and IRCT20111224008507N4.

The Influence of Cannabidiol on the Pharmacokinetics of Methylphenidate in Healthy Subjects

INTRODUCTION

Cannabidiol (CBD) is a widely utilized nonpsychoactive cannabinoid available as an over-the-counter supplement, a component of medical cannabis, and a prescriptive treatment of childhood epilepsies. In vitro studies suggest CBD may inhibit a number of drug-metabolizing enzymes, including carboxylesterase 1 (CES1). The aim of this study was to evaluate effect of CBD on the disposition of the CES1 substrate methylphenidate (MPH).

METHODS

In a randomized, placebo-controlled, crossover study, 12 subjects ingested 750 mg of CBD solution, or alternatively, a placebo solution twice daily for a 3-day run-in period followed by an additional CBD dose (or placebo) and a single 10 mg dose of MPH and completed serial blood sampling for pharmacokinetic analysis. MPH and CBD concentrations were measured by liquid chromatography with tandem mass spectrometry.

RESULTS

The Cmax (mean ± CV) for the CBD group and placebo group was 13.5 ± 43.7% ng/mL and 12.2 ± 36.4% ng/mL, respectively. AUCinf (ng/mL*h) for the CBD group and placebo group was 70.7 ± 32.5% and 63.6 ± 25.4%, respectively. The CBD AUC0-8h (mean ± CV) was 1,542.2 ± 32% ng/mL*h, and Cmax was 389.2 ± 39% ng/mL. When compared to MPH only, the geometric mean ratio (CBD/control, 90% CI) for AUCinf and Cmax with CBD co-administration was 1.09 (0.89, 1.32) and 1.08 (0.85, 1.37), respectively.

DISCUSSION/CONCLUSION

Although the upper bound of bioequivalence was not met, the mean estimates of AUC and Cmax ratios were generally small and unlikely to be of clinical significance.

Plasma Carboxylesterase 1 Predicts Methylphenidate Exposure: A Proof-of-Concept Study Using Plasma Protein Biomarker for Hepatic Drug Metabolism

Hepatic drug-metabolizing enzymes (DMEs) play critical roles in determining the pharmacokinetics and pharmacodynamics of numerous therapeutic agents. As such, noninvasive biomarkers capable of predicting DME expression in the liver have the potential to be used to personalize pharmacotherapy and improve drug treatment outcomes. In the present study, we quantified carboxylesterase 1 (CES1) protein concentrations in plasma samples collected during a methylphenidate pharmacokinetics study. CES1 is a prominent hepatic enzyme responsible for the metabolism of many medications containing small ester moieties, including methylphenidate. The results revealed a significant inverse correlation between plasma CES1 protein concentrations and the area under the concentration-time curves (AUCs) of plasma d-methylphenidate (P = 0.014, r = -0.617). In addition, when plasma CES1 protein levels were normalized to the plasma concentrations of 24 liver-enriched proteins to account for potential interindividual differences in hepatic protein release rate, the correlation was further improved (P = 0.003, r = -0.703), suggesting that plasma CES1 protein could explain ~ 50% of the variability in d-methylphenidate AUCs in the study participants. A physiologically-based pharmacokinetic modeling simulation revealed that the CES1-based individualized dosing strategy might significantly reduce d-methylphenidate exposure variability in pediatric patients relative to conventional trial and error fixed dosing regimens. This proof-of-concept study indicates that the plasma protein of a hepatic DME may serve as a biomarker for predicting its metabolic function and the pharmacokinetics of its substrate drugs.

Medical Foods-A Closer Look at the Menu: A Brief Review and Commentary

The notion of a medical food-a foodstuff that, by definition, must be obtained and used while under medical supervision, and regulated by the US Food and Drug Administration, is a source of considerable confusion to the lay public as well as many-if not most-in the health professions community. Such restrictions are more often associated with pharmaceutical agents or medical devices. Additionally, specific regulatory aspects of medical foods are overseen by the US Food and Drug Administration, and these specifics appear to overlap with other foods and dietary supplements in terms of requirements and allowances. Furthermore, these requirements and allowances have changed over time and are likely to continue to evolve via federal regulatory action, or the introduction of newer formulations that defy current categorization. The present review attempts to bring some clarity to the definition of medical foods, to parse out the differences from related products, and to review the terminology surrounding medical foods, other foods, and dietary supplements.l.

Natural Products as Modulators of CES1 Activity

Carboxylesterase (CES) 1 is the predominant esterase expressed in the human liver and is capable of catalyzing the hydrolysis of a wide range of therapeutic agents, toxins, and endogenous compounds. Accumulating studies have demonstrated associations between the expression and activity of CES1 and the pharmacokinetics and/or pharmacodynamics of CES1 substrate medications (e.g., methylphenidate, clopidogrel, oseltamivir). Therefore, any perturbation of CES1 by coingested xenobiotics could potentially compromise treatment. Natural products are known to alter drug disposition by modulating cytochrome P450 and UDP-glucuronosyltransferase enzymes, but this issue is less thoroughly explored with CES1. We report the results of a systematic literature search and discuss natural products as potential modulators of CES1 activity. The majority of research reports reviewed were in vitro investigations that require further confirmation through clinical study. Cannabis products (Δ ⁹-tetrahydrocannabinol, cannabidiol, cannabinol); supplements from various plant sources containing naringenin, quercetin, luteolin, oleanolic acid, and asiatic acid; and certain traditional medicines (danshen and zhizhuwan) appear to pose the highest inhibition potential. In addition, ursolic acid, gambogic acid, and glycyrrhetic acid, if delivered intravenously, may attain high enough systemic concentrations to significantly inhibit CES1. The provision of a translational interpretation of in vitro assessments of natural product actions and interactions is limited by the dearth of basic pharmacokinetic data of the natural compounds exhibiting potent in vitro influences on CES1 activity. This is a major impediment to assigning even potential clinical significance. The modulatory effects on CES1 expression after chronic exposure to natural products warrants further investigation. SIGNIFICANCE STATEMENT: Modulation of CES1 activity by natural products may alter the course of treatment and clinical outcome. In this review, we have summarized the natural products that can potentially interact with CES1 substrate medications. We have also noted the limitations of existing reports and outlined challenges and future directions in this field.

The Potential for Pharmacokinetic Interactions Between Cannabis Products and Conventional Medications

PURPOSE

Increased cannabis use and recent drug approvals pose new challenges for avoiding drug interactions between cannabis products and conventional medications. This review aims to identify drug-metabolizing enzymes and drug transporters that are affected by concurrent cannabis use and, conversely, those co-prescribed medications that may alter the exposure to one or more cannabinoids.

METHODS

A systematic literature search was conducted utilizing the Google Scholar search engine and MEDLINE (PubMed) database through March 2019. All articles describing in vitro or clinical studies of cannabis drug interaction potential were retrieved for review. Additional articles of interest were obtained through cross-referencing of published bibliographies.

FINDINGS

After comparing the in vitro inhibition parameters to physiologically achievable cannabinoid concentrations, it was concluded that CYP2C9, CYP1A1/2, and CYP1B1 are likely to be inhibited by all 3 major cannabinoids Δ-tetrahydrocannabinol (THC), cannabidiol (CBD), and cannabinol (CBN). The isoforms CYP2D6, CYP2C19, CYP2B6, and CYP2J2 are inhibited by THC and CBD. CYP3A4/5/7 is potentially inhibited by CBD. Δ-Tetrahydrocannabinol also activates CYP2C9 and induces CYP1A1. For non-CYP drug-metabolizing enzymes, UGT1A9 is inhibited by CBD and CBN, whereas UGT2B7 is inhibited by CBD but activated by CBN. Carboxylesterase 1 (CES1) is potentially inhibited by THC and CBD. Clinical studies suggest inhibition of CYP2C19 by CBD, inhibition of CYP2C9 by various cannabis products, and induction of CYP1A2 through cannabis smoking. Evidence of CBD inhibition of UGTs and CES1 has been shown in some studies, but the data are limited at present. We did not identify any clinical studies suggesting an influence of cannabinoids on drug transporters, and in vitro results suggest that a clinical interaction is unlikely.

CONCLUSIONS

Medications that are prominent substrates for CYP2C19, CYP2C9, and CYP1A2 may be particularly at risk of altered disposition by concomitant use of cannabis or 1 or more of its constituents. Caution should also be given when coadministered drugs are metabolized by UGT or CES1, on which subject the information remains limited and further investigation is warranted. Conversely, conventional drugs with strong inhibitory or inductive effects on CYP3A4 are expected to affect CBD disposition.

New Formulations of Stimulants: An Update for Clinicians

In the last 15 years, there has been a marked increase in the number of available stimulant formulations with the emphasis on long-acting formulations, and the introduction of several novel delivery systems such as orally dissolving tablets, chewable tablets, extended-release liquid formulations, transdermal patches, and novel "beaded" technology. All of these formulations involve changes to the pharmaceutical delivery systems of the two existing compounds most commonly employed to treat attention-deficit/hyperactivity disorder (ADHD), amphetamine (AMP) and methylphenidate (MPH). In addition to these new formulations, our knowledge about the individual differences in response has advanced and contributes to a more nuanced approach to treatment. The clinician can now make increasingly informed choices about these formulations and more effectively individualize treatment in a way that had not been possible before. In the absence of reliable biomarkers that can predict individualized response to ADHD treatment, clinical knowledge about differences in MPH and AMP pharmacodynamics, pharmacokinetics, and metabolism can be utilized to personalize treatment and optimize response. Different properties of these new formulations (delivery modality, onset of action, duration of response, safety, and tolerability) will most likely weigh heavily into the clinician's choice of formulation. To manage the broad range of options that are now available, clinicians should familiarize themselves in each of these categories for both stimulant compounds. This review is meant to serve as an update and a guide to newer stimulant formulations and includes a brief review of ADHD and stimulant properties.

Relative Bioavailability of Silybin A and Silybin B From 2 Multiconstituent Dietary Supplement Formulations Containing Milk Thistle Extract: A Single-dose Study

PURPOSE

The purpose of this study was to compare the bioavailability between 2 milk thistle-containing dietary supplements, Product B and IsaGenesis, in healthy volunteers.

METHODS

Bioavailability between Product B, originally formulated as a powdered capsule, and IsaGenesis, reformulated as a soft gel, were compared by measuring silybin A and silybin B as surrogate pharmacokinetic markers for differences in absorption and bioavailability. For this randomized, open-label, crossover pharmacokinetic study, 12 healthy volunteers consumed a single-dose serving of each supplement separated by at least a 7-day washout period. Serial blood samples were obtained at 0, 0.5, 1, 1.5, 2, 3, 4, 6, and 8 hours and analyzed via LC-MS/MS.

FINDINGS

Rapid absorption and elimination of silybin A and silybin B have been observed after oral administration of both Product B and IsaGenesis. However, the absorption rate and extent, as indicated by mean the Cmax and mean plasma AUC, were significantly higher for the IsaGenesis soft gel formulation. The dose-corrected mean Cmax was 365% and 450% greater for silybin A and B, respectively, relative to powdered Product B. The time to Tmax was reached, on average, at least 1 hour earlier with IsaGenesis relative to Product B for both silybin A and silybin B.

IMPLICATIONS

The IsaGenesis soft gel formulation provided substantially greater absorption and bioavailability of silybin A and silybin B relative to the powdered Product B supplement. ClinicalTrials.gov Identifier: NCT02529605.

The Clinical Pharmacokinetics of Amphetamines Utilized in the Treatment of Attention-Deficit/Hyperactivity Disorder

Amphetamine (AMP), an indirectly acting psychostimulant approved for the treatment of attention-deficit/hyperactivity disorder (ADHD) in children, adolescents, and adults, is among the most long-standing therapeutic agents in all of clinical psychopharmacology. This review focuses on AMP absorption, metabolism, and elimination brought to bear on comparative pharmacokinetics in its various formulations. A comprehensive search of the published literature was conducted using MEDLINE (PubMed) and Google Scholar databases through April 2017 to retrieve all pertinent in vitro and human studies for review and synthesis. Additionally, Food and Drug Administration (FDA) databases were accessed for otherwise unavailable data when possible. Initially available as racemic (dl)-AMP, this drug was later supplanted by enantiopure (d)-AMPH or enantioenriched (75:25 dl)-AMP formulations; although racemic AMP returned as an approved drug to treat ADHD in 2014. Presently, there are several immediate-release (IR) formulations available, including d-AMP, dl-AMP, and mixed amphetamine salts, which are neither racemic nor the pure d-enantiomer (i.e., a 3:1 mixture of d-AMP and l-AMP). Furthermore, new modified-release AMP formulations, including an oral suspension and an orally disintegrating tablet, are now available. A lysine-bonded prodrug form of d-AMP also serves as a treatment option. Oral AMP is rapidly absorbed, with high absolute bioavailability, followed by extensive metabolism involving multiple enzymes. Some metabolic pathways exhibit stereoselective biotransformations favoring the l-isomer substrate. Drug exposure exhibits dose-proportional pharmacokinetics. Body weight is a fundamental determinant of differences in observed AMP plasma concentrations. IR formulations typically provide a T_max from 2 to 3 hours. In replicated studies, children exhibit a shorter plasma T_1/2 (∼7 hours) relative to adults (∼10 to 12 hours). There are few documented pharmacokinetic drug interactions of clinical significance beyond influences of drug-induced alteration of urinary pH. The array of AMP formulations addressed in this review offer flexibility in dosing, drug onset, and offset to assist in individualized pharmacotherapy of ADHD.

The effect of milk thistle (Silybum marianum) and its main flavonolignans on CYP2C8 enzyme activity in human liver microsomes

Milk thistle is a widely-consumed botanical used for an array of purported health benefits. The primary extract of milk thistle is termed silymarin, a complex mixture that contains a number of structurally-related flavonolignans, the flavonoid, taxifolin, and a number of other constituents. The major flavonolignans present in most extracts are silybin A, silybin B, isosilybin A and isosilybin B, silydianin, silychristin and isosilychristin. Silymarin itself has been reported to inhibit CYP2C8 activity in vitro, but the effect of the individual flavonolignans on this enzyme has not been studied. To investigate the effects of milk thistle extract and its main flavonolignans (silybin A, silybin B, isosilybin A and isosilybin B) on CYP2C8 activity at relevant concentrations, the effect of milk thistle extract and the flavonolignans on CYP2C8 enzyme activity was studied in vitro using human liver microsomes (HLM) incorporating an enzyme-selective substrate for CYP2C8, amodiaquine. Metabolite formation was analyzed using liquid chromatography-tandem mass spectrometry (LC/MS-MS). The concentration causing 50% inhibition of enzyme activity (IC50) was used to express the degree of inhibition. Isosilibinin, a mixture of the diastereoisomers isosilybin A and isosilybin B, was found to be the most potent inhibitor, followed by isosilybin B with IC50 values (mean ± SE) of 1.64 ± 0.66 μg/mL and 2.67 ± 1.18 μg/mL, respectively. The rank order of observed inhibitory potency after isosilibinin was silibinin > isosilybin A > silybin A > milk thistle extract > and silybin B. These in vitro results suggest a potentially significant inhibitory effect of isosilibinin and isosilybin B on CYP2C8 activity. However, the observed IC50 values are unlikely to be achieved in humans supplemented with orally administered milk thistle extracts due to the poor bioavailability of flavonolignans documented with most commercially available formulations.

An Appraisal of Drug-Drug Interactions with Green Tea (Camellia sinensis)

This review summarizes published in vitro, animal, and clinical studies investigating the effects of green tea (Camellia sinensis) extract and associated catechins on drug-metabolizing enzymes and drug transporters. In vitro studies suggest that green tea extract and its main catechin, (-)-epigallocatechin-3-gallate, to varying degrees, inhibit the activity of CYP1A1, CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2D6, and CYP3A4. UGT1A1 and UGT1A4 isoforms were also inhibited by (-)-epigallocatechin-3-gallate. Animal studies suggest green tea extract and/or (-)-epigallocatechin-3-gallate significantly increase the bioavailability of diltazem, verapamil, tamoxifen simvastatin, 5-fluorouracil, and nicardipine. Conversely, green tea extract and/or (-)-epigallocatechin-3-gallate reduce the bioavailability of quetiapine, sunitinib, clozapine, and nadolol. Of the few clinical studies available for review, it appears neither green tea extract nor (-)-epigallocatechin-3-gallate inhibit any major cytochrome P450 enzyme. Regarding drug transporters, in vitro studies indicate P-glycoprotein, organic anion transporting polypeptide 1A1, organic anion transporting polypeptide 1B1, organic anion transporting polypeptide 1B3, organic anion transporting polypeptide 2B1, organic cation transporter 1, organic cation transporter 2, multidrug and toxin extrusion 1, and multidrug and toxin extrusion 2-K are potentially inhibited by green tea extract. A clinical study indicates the organic anion transporting polypeptide 1A1 transporter is inhibited by (-)-epigallocatechin-3-gallate while P-glycoprotein is unaffected. In conclusion, the ingestion of green tea extract or its associated catechins is not expected to result in clinically significant influences on major cytochrome P450 or uridine 5'-diphospho-glucuronosyltransferase enzyme substrates or drugs serving as substrates of P-glycoprotein. However, some caution is advised in the consumption of significant amounts of green tea beverages or green tea extract in patients prescribed known substrates of organic anion transporting polypeptide, particularly those with a narrow therapeutic index.

Opioid Drug-Drug Interactions: A Review

Opioid analgesics are among the most commonly prescribed medications. Frequently, they are combined with other therapeutic agents and pharmacodynamic or pharmacokinetic interactions may ensue. This review summarizes published case reports and studies of potential opioid drug interactions. A MED-LINE computer literature search (1966-1998) was undertaken to retrieve all pertinent case reports and studies of opioid drug interactions published in the English language. The results of the search indicate that numerous compounds from various therapeutic classes may participate in clinically significant pharmacodynamic and pharmacokinetic drug-drug interactions. Pharmacodynamic interactions usually involved additive central nervous system depression. Additionally, propoxyphene and tramadol can potentiate a hyperserotonergic state when coadministered with the SSRIs and MAOIs. Pharmacokinetic interactions typically involved inhibition or induction by specific hepatic cytochrome P-450 isoenzymes. Agents with enzyme inhibiting ability such as erythromycin, cimetidine, and selective serotonin reuptake inhibitors have been shown to potentiate the effects of certain opioid analgesics while codeine, which requires metabolic conversion via CYP 2D6 for pharmacological effectiveness, has reduced analgesic efficacy in the presence of inhibitors. The enzyme inducers rifampin and several anticonvulsants have been involved in the emergence of methadone withdrawal when added to existing methadone treatment. Additionally, enzyme inducers can increase the formation of the toxic metabolite of meperidine. Genetic polymorphism also potentially impacts the effectiveness of agents such as codeine since reduced active metabolite formation and analgesic efficacy has been demonstrated in individuals who lack CYP 2D6 activity.

Psychiatric Side Effects of Herbal Medicinals

The use of herbal medicinals has grown tremendously in the last several years. Unfortunately, there is little knowledge about the safety of these products. There have been several reports of psychiatric side effects associated with the use of herbal medicinals. Manic symptoms have been reported with the use of St. John's wort, yohimbe, ginseng, and ma huang or its synthetically produced alkaloids. Ma huang or its alkaloids have also been associated with psychotic symptoms. Worsening anxiety and panic attacks have been reported with yohimbe. Recommendations for the use of these phytomedicinals and for future research are included.

Plasma Antipsychotic Drug Concentrations: Practical Clinical Considerations

The practical application of plasma antipsychotic moni toring to achieve dose optimization remains problematic. Despite numerous studies attempting to establish antipsychotic dosing guidelines based on target plasma concentration ranges, a therapeutic window for most neuroleptics remains elusive and drug individualiza tion is still most often empirically determined. Drug concentration testing for all antipsychotics is commercially available, but at present, only haloperidol and clozapine appear to provide clinically useful plasma values. Reasons for obtaining levels include suspected noncompliance, pharmacokinetic drug interactions, toxicity, and cases in which difficulty arises in distinguishing between drug-induced toxicity and underlying pathology. Accordingly, antipsychotic plasma concentration monitoring offers potential to improve therapeutic outcomes only when interpreted carefully in conjunction with the patient's overall clinical status. Copyright © 1996 by W.B. Saunders Company

Atomoxetine: A Review of Its Pharmacokinetics and Pharmacogenomics Relative to Drug Disposition

Atomoxetine is a selective norepinephrine (NE) reuptake inhibitor approved for the treatment of attention-deficit/hyperactivity disorder (ADHD) in children (≥6 years of age), adolescents, and adults. Its metabolism and disposition are fairly complex, and primarily governed by cytochrome P450 (CYP) 2D6 (CYP2D6), whose protein expression varies substantially from person to person, and by race and ethnicity because of genetic polymorphism. These differences can be substantial, resulting in 8-10-fold differences in atomoxetine exposure between CYP2D6 poor metabolizers and extensive metabolizers. In this review, we have attempted to revisit and analyze all published clinical pharmacokinetic data on atomoxetine inclusive of public access documents from the new drug application submitted to the United States Food and Drug Administration (FDA). The present review focuses on atomoxetine metabolism, disposition, and genetic polymorphisms of CYP2D6 as they specifically relate to atomoxetine, and provides an in-depth discussion of the fundamental pharmacokinetics of the drug including its absorption, distribution, metabolism, and excretion in pediatric and adult populations. Further, a summary of relationships between genetic variants of CYP2D6 and to some degree, CYP2C19, are provided with respect to atomoxetine plasma concentrations, central nervous system (CNS) pharmacokinetics, and associated clinical implications for pharmacotherapy. Lastly, dosage adjustments based on pharmacokinetic principles are discussed.

CES1P1 variant -816A>C is not associated with hepatic carboxylesterase 1 expression and activity or antihypertensive effect of trandolapril

PURPOSE

The majority of angiotensin-converting enzyme inhibitors (ACEIs) are synthesized as ester prodrugs that must be converted to their active forms in vivo in order to exert therapeutic effects. Hepatic carboxylesterase 1 (CES1) is the primary enzyme responsible for the bioactivation of ACEI prodrugs in humans. The genetic variant -816A>C (rs3785161) is a common variant located in the promoter region of the CES1P1 gene. Previous studies report conflicting results with regard to the association of this variant and therapeutic outcomes of CES1 substrate drugs. The purpose of this study was to determine the effect of the variant -816A>C on the activation of the ACEI prodrug trandolapril in human livers and the blood pressure (BP)-lowering effect of trandolapril in hypertensive patients.

METHODS

The -816A>C genotypes and CES1 expression and activity on trandolapril activation were determined in 100 individual human liver samples. Furthermore, the association of the -816A>C variant and the BP lowering effect of trandolapril was evaluated in hypertensive patients who participated in the International Verapamil SR Trandolapril Study (INVEST).

RESULTS

Our in vitro study demonstrated that hepatic CES1 expression and activity did not differ among different -816A>C genotypes. Moreover, we were unable to identify a clinical association between the BP lowering effects of trandolapril and -816A>C genotypes.

CONCLUSIONS

We conclude that the -816A>C variant is not associated with interindividual variability in CES1 expression and activity or therapeutic response to ACEI prodrugs.

An ex vivo approach to botanical-drug interactions: a proof of concept study

ETHNOPHARMACOLOGICAL RELEVANCE

Botanical medicines are frequently used in combination with therapeutic drugs, imposing a risk for harmful botanical-drug interactions (BDIs). Among the existing BDI evaluation methods, clinical studies are the most desirable, but due to their expense and protracted time-line for completion, conventional in vitro methodologies remain the most frequently used BDI assessment tools. However, many predictions generated from in vitro studies are inconsistent with clinical findings. Accordingly, the present study aimed to develop a novel ex vivo approach for BDI assessment and expand the safety evaluation methodology in applied ethnopharmacological research.

MATERIALS AND METHODS

This approach differs from conventional in vitro methods in that rather than botanical extracts or individual phytochemicals being prepared in artificial buffers, human plasma/serum collected from a limited number of subjects administered botanical supplements was utilized to assess BDIs. To validate the methodology, human plasma/serum samples collected from healthy subjects administered either milk thistle or goldenseal extracts were utilized in incubation studies to determine their potential inhibitory effects on CYP2C9 and CYP3A4/5, respectively. Silybin A and B, two principal milk thistle phytochemicals, and hydrastine and berberine, the purported active constituents in goldenseal, were evaluated in both phosphate buffer and human plasma based in vitro incubation systems.

RESULTS

Ex vivo study results were consistent with formal clinical study findings for the effect of milk thistle on the disposition of tolbutamide, a CYP2C9 substrate, and for goldenseal׳s influence on the pharmacokinetics of midazolam, a widely accepted CYP3A4/5 substrate. Compared to conventional in vitro BDI methodologies of assessment, the introduction of human plasma into the in vitro study model changed the observed inhibitory effect of silybin A, silybin B and hydrastine and berberine on CYP2C9 and CYP3A4/5, respectively, results which more closely mirrored those generated in clinical study.

CONCLUSIONS

Data from conventional buffer-based in vitro studies were less predictive than the ex vivo assessments. Thus, this novel ex vivo approach may be more effective at predicting clinically relevant BDIs than conventional in vitro methods.

The effects of milk thistle (Silybum marianum) on human cytochrome P450 activity

Milk thistle (Silybum marianum) extracts are widely used as a complementary and alternative treatment of various hepatic conditions and a host of other diseases/disorders. The active constituents of milk thistle supplements are believed to be the flavonolignans contained within the extracts. In vitro studies have suggested that some milk thistle components may significantly inhibit specific cytochrome P450 (P450) enzymes. However, determining the potential for clinically significant drug interactions with milk thistle products has been complicated by inconsistencies between in vitro and in vivo study results. The aim of the present study was to determine the effect of a standardized milk thistle supplement on major P450 drug-metabolizing enzymes after a 14-day exposure period. CYP1A2, CYP2C9, CYP2D6, and CYP3A4/5 activities were measured by simultaneously administering the four probe drugs, caffeine, tolbutamide, dextromethorphan, and midazolam, to nine healthy volunteers before and after exposure to a standardized milk thistle extract given thrice daily for 14 days. The three most abundant falvonolignans found in plasma, following exposure to milk thistle extracts, were silybin A, silybin B, and isosilybin B. The concentrations of these three major constituents were individually measured in study subjects as potential perpetrators. The peak concentrations and areas under the time-concentration curves of the four probe drugs were determined with the milk thistle administration. Exposure to milk thistle extract produced no significant influence on CYP1A2, CYP2C9, CYP2D6, or CYP3A4/5 activities.

ABC transporters in multi-drug resistance and ADME-Tox of small molecule tyrosine kinase inhibitors

The past decade has seen tremendous efforts in the research and development of new chemotherapeutic drugs using target-based approaches. These efforts have led to the discovery of small molecule tyrosine kinase inhibitors (TKIs). Following the initial approval of imatinib by the US FDA in 2001, more than 15 TKIs targeting different tyrosine kinases have been approved, and numerous others are in various phases of clinical evaluation. Unlike conventional chemotherapy that can cause non-discriminating damage to both normal and cancerous cells, TKIs attack cancer-specific targets and therefore have a more favorable safety profile. However, although TKIs have had outstanding success in cancer therapy, there has been increasing evidence of resistance to TKIs. The enhanced efflux of TKIs by ATP-binding cassette (ABC) transporters over-expressed in cancer cells has been found to be one such important resistance mechanism. Another major drawback of TKI therapies that has been increasingly recognized is the extensive inter-individual pharmacokinetic variability, in which ABC transporters seem to play a major role as well. This review covers recent findings on the interactions of small molecule TKIs with ABC transporters. The effects of ABC transporters on anticancer efficacy and the absorption, distribution, metabolism, excretion, and toxicity (ADME-Tox) of the small molecule TKIs are summarized in detail. Since TKIs have been found to not only serve as substrates of ABC transporters, but also as modulators of these proteins via inhibition or induction, their influence upon ABC transporters and potential role on TKI-drug interactions are discussed as well.

Isopropylphenidate: an ester homolog of methylphenidate with sustained and selective dopaminergic activity and reduced drug interaction liability

OBJECTIVE

The most widely utilized pharmacological treatment of attention-deficit/hyperactivity disorder (ADHD) is the psychostimulant methylphenidate (MPH). Most MPH formulations consist of the racemic mixture of d-threo-(R, R)-MPH and l-threo-(S, S)-MPH isomers. MPH is characterized by its low bioavailability and short half-life (2-3 hours). Additionally, significant inter-individual variability in MPH pharmacokinetics has been consistently documented. Accordingly, efforts have been directed at developing alternatives to MPH as therapeutic agents. A wide range of MPH analogues (dl-α-[2-piperidyl]-phenylacetic acid esters) have been synthesized with the dopamine transporter (DAT) and norepinephrine transporter (NET) as principle neuropharmacological targets. The present study investigated the metabolic profiles and pharmacological activity of the isopropyl ester derivative of MPH, dl-isopropylphenidate (IPH), both in vitro and in vivo.

METHODS

The synthesis, monoaminergic transporter binding, cellular uptake profiles, and assessment of metabolic hydrolysis and transesterification in the presence of ethanol are described using MPH as a comparator. Additionally, an in vivo assessment of IPH stimulant effects (vs. saline) in rats was performed with locomotor activity as a pharmacodynamic outcome.

RESULTS

IPH displayed unique pharmacological characteristics including greater DAT than NET binding and cellular uptake activity, and greater resistance to hydrolysis and transesterification via carboxylesterase 1 relative to MPH. Further, sustained psychostimulant properties offer the prospect of an enhanced duration of action.

CONCLUSIONS

Our findings are consistent with IPH exhibiting attributes distinguishing it from MPH and warranting further study and development of IPH as a novel psychotherapeutic agent.

An assessment of pharmacokinetics and antioxidant activity of free silymarin flavonolignans in healthy volunteers: a dose escalation study

Milk thistle (Silybum marianum) extracts, one of the most widely used dietary supplements, contain a mixture of six major flavonolignans (silybin A, silybin B, isosilybin A, isosilybin B, silychristin, and silydianin) and other components. However, the pharmacokinetics of the free individual flavonolignans have been only partially investigated in humans. Furthermore, antioxidant effects of the extract, which may underlie the basis of many therapeutic effects, have not been thoroughly assessed. The present study evaluated the pharmacokinetics of the six major flavonolignans in healthy volunteers receiving single doses of either one (175 mg), two (350 mg), or three (525 mg) milk thistle capsule(s) on three separate study visits. Additionally, the steady-state pharmacokinetic parameters were determined after the subjects were administered one capsule three times daily for 28 consecutive days. Our results demonstrated that all six flavonolignans were rapidly absorbed and eliminated. In order of abundance, the exposure to free flavonolignans was greatest for silybin A followed by silybin B, isosilybin B, isosilybin A, silychristin, and silydianin. The systemic exposure to these compounds appeared linear and dose proportional. The disposition of flavonolignans was stereoselective, as evidenced by the apparent clearance of silybin B, which was significantly greater than silybin A, whereas the apparent clearance of isosilybin B was significantly lower than isosilybin A. The concentrations of urinary 8-epi-prostaglandin F2α, a commonly used biomarker of oxidative status in humans, were considerably decreased in study subjects after a 28-day exposure to the extract (1.3 ± 0.9 versus 0.8 ± 0.9 ng/mg creatinine) but failed to reach statistical significance (P = 0.076).

Pharmacokinetics of Olanzapine After Single-Dose Oral Administration of Standard Tablet Versus Normal and Sublingual Administration of an Orally Disintegrating Tablet in Normal Volunteers

Olanzapine (OLZ) is a second-generation antipsychotic agent available in 2 solid oral dosage forms, a standard oral tablet (SOT) and an orally disintegrating tablet (ODT). This study assessed the absorption of each by different routes of administration. Secondarily, the influence of P-glycoprotein (P-gp) genotype was assessed. It was hypothesized that more rapid absorption of the OLZ ODT would occur when administered sublingually versus standard oral administration. A randomized, 3-way crossover study assessed the 5-mg OLZ formulations in healthy volunteers (n = 10). Blood was collected (0-8 hours) to assess OLZ pharmacokinetics using liquid chromatography/mass spectrometry. Both routes of ODT administration resulted in more measurable early concentrations relative to SOT. However, there were no statistically significant differences observed between any of the OLZ exposures for observed pharmacokinetic parameters (C(max), T(max), AUC(0-8h)). The homozygous TT genotype for P-gp resulted in an increased AUC of OLZ for SOT administration but not for either condition where sublingual absorption could occur.

Limitations of in vitro assessments of the drug interaction potential of botanical supplements

Although there are inherent and recognized limitations of in vitro screening methodologies to assess conventional drug-drug interactions (DDIs) per industry guidelines and those adopted by independent laboratories, further limitations are being appreciated which are unique to the evaluation of botanical products and potential DDIs in which they may participate. Among the larger issues faced are the uncertainty in assigning hepatic concentrations of multiple constituents and their potential metabolites, accounting for oral bioavailability, distribution, first-pass metabolism and active metabolites. Furthermore, the wide variability in the chemical composition of commercially available botanical supplement formulations continues to be a major concern, and manufacturing standards or enforcement thereof is essentially nonexistent in most countries. Differing formulations, unspecified product excipients, administration and absorption of the therapeutic ingredient(s) of a standardized dosage form, the very presence and/or concentration of one or more phytoconstituents within a supplement are typically unknown and nontarget entities. A further issue is the absence of authentic analytical standards, and the inability to accurately screen the entities as mixtures to even approximate typical scenarios, which may occur following the ingestion of dietary supplements, adds additional layers of complexity to experimental design and difficulty in interpreting experimental results. Multiple challenges exist in experimental methodologies employed in performing in vitro research with conventional pharmaceuticals and those unique to botanical extracts. These obstacles prevent the investigators from effectively utilizing high-throughput models to accomplish more than essentially "flag" suspected sources of drug interactions which must be further evaluated in vivo, at present, in order to confirm clinical significance. This review is intended to discuss the problems and challenges in evaluating botanical-drug interactions using in vitro methodologies.

A sensitive LC-MS/MS assay for the simultaneous analysis of the major active components of silymarin in human plasma

Silymarin, an extract of crushed achenes of the milk thistle plant Silybum marianum is a multi-constituent mixture, 70-80% of which consists of a complex assortment containing the flavonolignans silybin A and B, isosilybin A and B, silydianin, and silychristin, and the flavonoid taxifolin. To date, numerous pharmacological actions of the silymarin extract have been documented in the biomedical literature, including hepatoprotective, anti-inflammatory, anti-tumor, and anti-fibrotic activities. The present study describes a novel liquid chromatographic-tandem mass spectrometric method for simultaneous analysis of silychristin, silydianin, silybin A and silybin B, isosilybin A and isosilybin B, and taxifolin in human plasma employing liquid-liquid extraction. This assay provides excellent resolution of the individual silymarin constituents via utilization of a 100 A 250 mm × 2 mm, 5 μm C(18) column with the mobile phase consisting of 51% methanol, 0.1% formic acid, and 10mM ammonium acetate. The lower limit of quantification was 2 ng/ml for each constituent. Calibration curves were linear over the range from 2 ng/ml to 100 ng/ml for all analytes (r(2)>0.99). The intra- and inter-day accuracies were 91-106.5% and 95.1-111.9%, respectively. The intra- and inter-day precision was within 10.5%. Additionally, recovery, stability, and matrix effects were fully validated as well. This method was successfully applied to human plasma samples from subjects treated with the milk thistle extract Legalon(®).

Evaluation of organic cation transporter 3 (SLC22A3) inhibition as a potential mechanism of antidepressant action

Organic cation transporter 3 (OCT3, SLC22A3) is a low-affinity, high-capacity transporter widely expressed in the central nervous system (CNS) and other major organs in both humans and rodents. It is postulated that OCT3 has a role in the overall regulation of neurotransmission and maintenance of homeostasis within the CNS. It is generally believed that all antidepressant drugs in current clinical use exert their primary therapeutic effects through inhibition of one or more of the high-affinity neuronal plasma membrane monoamine transporters, such as the norepinephrine transporter and the serotonin transporter. In the present study, we investigated the inhibitory effects of selected antidepressants on OCT3 activity in OCT3-transfected cells to evaluate whether OCT3 inhibition may at least in part contribute to the pharmacological effects of tested antidepressants. The studies demonstrated that all examined antidepressants inhibited OCT3-mediated uptake of the established OCT3 substrate 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (4-Di-1-ASP) in a concentration-dependent manner. The IC(50) values were determined to be 4.7 μM, 7.4 μM, 12.0 μM, 18.6 μM, 11.2 μM, and 21.9 μM for desipramine, sertraline, paroxetine, amitriptyline, imipramine, and fluoxetine, respectively. Additionally, desipramine had an IC(50) value of 0.7 μM for the uptake of NE by OCT3, while the IC(50) value of sertraline was 2.3 μM for 5-HT uptake. Both desipramine and sertraline appeared to inhibit OCT3 activity via a non-competitive mechanism. In vivo studies are warranted to determine whether such effects on OCT3 inhibition are of sufficient magnitude to contribute to the overall therapeutic effects of antidepressants.

A liquid chromatography/tandem mass spectrometry assay for the analysis of atomoxetine in human plasma and in vitro cellular samples

A simple, rapid and sensitive method for quantification of atomoxetine by liquid chromatography-tandem mass spectrometry (LC-MS/MS) was developed. This assay represents the first LC-MS/MS quantification method for atomoxetine utilizing electrospray ionization. Deuterated atomoxetine (d3-atomoxetine) was adopted as the internal standard. Direct protein precipitation was utilized for sample preparation. This method was validated for both human plasma and in vitro cellular samples. The lower limit of quantification was 3 ng/mL and 10 nm for human plasma and cellular samples, respectively. The calibration curves were linear within the ranges of 3-900 ng/mL and 10 nm to 10 µm for human plasma and cellular samples, respectively (r(2)  >0.999). The intra- and inter-day assay accuracy and precision were evaluated using quality control samples at three different concentrations in both human plasma and cellular lysate. Sample run stability, assay selectivity, matrix effect and recovery were also successfully demonstrated. The present assay is superior to previously published LC-MS and LC-MS/MS methods in terms of sensitivity or the simplicity of sample preparation. This assay is applicable to the analysis of atomoxetine in both human plasma and in vitro cellular samples.

Enantiospecific determination of DL-methylphenidate and DL-ethylphenidate in plasma by liquid chromatography-tandem mass spectrometry: application to human ethanol interactions

In humans, concomitant DL-methylphenidate (DL-MPH) and ethanol results in the carboxylesterase 1 (hCES1) mediated biotransformation of MPH to the transesterification metabolite DL-ethylphenidate (DL-EPH). The separate enantiomers of MPH and EPH are found at low ng/ml to pg/ml plasma concentrations. Substantial pharmacological differences exist between D- and L-isomers of MPH and EPH, both in terms of pharmacological potencies and receptor selectivity, as well as in pharmacokinetic properties. Accordingly, a sensitive, accurate and precise enantiospecific analytical method is required in order to fully explore pharmacokinetic-pharmacodynamic correlations regarding the MPH-ethanol interaction. The present study describes a novel liquid chromatographic-tandem mass spectrometric method for simultaneous analysis of D- and L-MPH as well as D- and L-EPH concentrations from human plasma. This assay provides baseline resolution of the individual MPH and EPH isomers utilizing a vancomycin-based chiral column. The lower limit of quantification was 0.025 ng/ml for each isomer when extracting 0.5 ml plasma aliquots. Calibration curves were linear over the range from 0.025 ng/ml to 25 ng/ml for all analytes (r(2)>0.995). Assay accuracy and precision were excellent and stability studies and assessment of potential matrix effects contributed to the validation of the method. Application of the method to human plasma samples collected after the administration of dl-MPH with or without ethanol is included, and the implications of this pharmacokinetic drug interaction discussed.

An in vitro evaluation of guanfacine as a substrate for P-glycoprotein

BACKGROUND

With a US Food and Drug Administration-labeled indication to treat attention-deficit/hyperactivity disorder (ADHD), the nonstimulant guanfacine has become the preferred α(2)-agonist for ADHD treatment. However, significant interindividual variability has been observed in response to guanfacine. Consequently, hypotheses of a contributing interaction with the ubiquitously expressed drug transporter, P-glycoprotein (P-gp), have arisen. We performed an in vitro study to determine if guanfacine is indeed a substrate of P-gp.

METHODS

Intracellular accumulation of guanfacine was compared between P-gp expressing LLC-PK1/MDR1 cells and P-gp-negative LLC-PK1 cells to evaluate the potential interaction between P-gp and guanfacine. Cellular retention of guanfacine was analyzed using a high-performance liquid chromatographic-ultraviolet method. Rhodamine6G, a known P-gp substrate, was included in the study as a positive control.

RESULTS

At guanfacine concentrations of 50 μM and 5 μM, intracellular accumulation of guanfacine in LLC-PK1/MDR1 cells was, 35.9% ± 4.8% and 49.0% ± 28.3% respectively, of that in LLC-PK1 cells. In comparison, the concentration of rhodamine6G, the positive P-gp substrate, in LLC-PK1/MDR1 cells was only 5% of that in LLC-PK1 cells.

CONCLUSION

The results of the intracellular accumulation study suggest that guanfacine is, at best, a weak P-gp substrate. Therefore, it is unlikely that P-gp, or any genetic variants thereof, are a determining factor in the interindividual variability of response observed with guanfacine therapy.