Ingrid F. Metzger

Chiral Plasma Pharmacokinetics and Urinary Excretion of Bupropion and Metabolites in Healthy Volunteers

Bupropion, widely used as an antidepressant and smoking cessation aid, undergoes complex metabolism to yield numerous metabolites with unique disposition, effect, and drug–drug interactions (DDIs) in humans. The stereoselective plasma and urinary pharmacokinetics of bupropion and its metabolites were evaluated to understand their potential contributions to bupropion effects. Healthy human volunteers (n = 15) were administered a single oral dose of racemic bupropion (100 mg), which was followed by collection of plasma and urine samples and determination of bupropion and metabolite concentrations using novel liquid chromatography–tandem mass spectrometry assays. Time-dependent, elimination rate–limited, stereoselective pharmacokinetics were observed for all bupropion metabolites. Area under the plasma concentration-time curve from zero to infinity ratios were on average approximately 65, 6, 6, and 4 and Cmax ratios were approximately 35, 6, 3, and 0.5 for (2R,3R)-/(2S,3S)-hydroxybupropion, R-/S-bupropion, (1S,2R)-/(1R,2S)-erythrohydrobupropion, and (1R,2R)-/(1S,2S)-threohydrobupropion, respectively. The R-/S-bupropion and (1R,2R)-/(1S,2S)-threohydrobupropion ratios are likely indicative of higher presystemic metabolism of S- versus R-bupropion by carbonyl reductases. Interestingly, the apparent renal clearance of (2S,3S)-hydroxybupropion was almost 10-fold higher than that of (2R,3R)-hydroxybupropion. The prediction of steady-state pharmacokinetics demonstrated differential stereospecific accumulation [partial area under the plasma concentration-time curve after the final simulated bupropion dose (300–312 hours) from 185 to 37,447 nM⋅h] and elimination [terminal half-life of approximately 7–46 hours] of bupropion metabolites, which may explain observed stereoselective differences in bupropion effect and DDI risk with CYP2D6 at steady state. Further elucidation of bupropion and metabolite disposition suggests that bupropion is not a reliable in vivo marker of CYP2B6 activity. In summary, to our knowledge, this is the first comprehensive report to provide novel insight into mechanisms underlying bupropion disposition by detailing the stereoselective pharmacokinetics of individual bupropion metabolites, which will enhance clinical understanding of bupropion’s effects and DDIs with CYP2D6.

Variants in the CYP2B6 3'UTR alter in vitro and in vivo CYP2B6 activity: Potential role of microRNAs

CYP2B6*6 and CYP2B6*18 are the most clinically important variants causing reduced CYP2B6 protein expression and activity. However, these variants do not account for all variability in CYP2B6 activity. Emerging evidence has shown that genetic variants in the 3′UTR may explain variable drug response by altering microRNA regulation. Five 3′UTR variants were associated with significantly altered efavirenz AUC0‐48 (8‐OH‐EFV/EFV) ratios in healthy human volunteers. The rs70950385 (AG>CA) variant, predicted to create a microRNA binding site for miR‐1275, was associated with a 33% decreased CYP2B6 activity among normal metabolizers (AG/AG vs. CA/CA (P < 0.05)). In vitro luciferase assays were used to confirm that the CA on the variant allele created a microRNA binding site causing an 11.3% decrease in activity compared to the AG allele when treated with miR‐1275 (P = 0.0035). Our results show that a 3′UTR variant contributes to variability in CYP2B6 activity.

Population pharmacokinetic modeling to estimate the contributions of genetic and nongenetic factors to efavirenz disposition

ABSTRACT Efavirenz pharmacokinetics is characterized by large between-subject variability, which determines both therapeutic response and adverse effects. Some of the variability in efavirenz pharmacokinetics has been attributed to genetic variability in cytochrome P450 genes that alter efavirenz metabolism, such as CYP2B6 and CYP2A6. While the effects of additional patient factors have been studied, such as sex, weight, and body mass index, the extent to which they contribute to variability in efavirenz exposure is inconsistently reported. The aim of this analysis was to develop a pharmacometric model to quantify the contribution of genetic and nongenetic factors to efavirenz pharmacokinetics. A population-based pharmacokinetic model was developed using 1,132 plasma efavirenz concentrations obtained from 73 HIV-seronegative volunteers administered a single oral dose of 600 mg efavirenz. A two-compartment structural model with absorption occurring by zero- and first-order processes described the data. Allometric scaling adequately described the relationship between fat-free mass and apparent oral clearance, as well as fat mass and apparent peripheral volume of distribution. Inclusion of fat-free mass and fat mass in the model mechanistically accounted for correlation between these disposition parameters and sex, weight, and body mass index. Apparent oral clearance of efavirenz was reduced by 25% and 51% in subjects predicted to have intermediate and slow CYP2B6 metabolizer status, respectively. The final pharmacokinetic model accounting for fat-free mass, fat mass, and CYP2B6 metabolizer status was consistent with known mechanisms of efavirenz disposition, efavirenz physiochemical properties, and pharmacokinetic theory. (This study has been registered at ClinicalTrials.gov under identifier NCT00668395.)

THE MODEL-AD CONSORTIUM PRECLINICAL TESTING PIPELINE: PHARMACOKINETICS AND PHARMACODYNAMICS OF PROPHYLACTIC TREATMENT WITH LEVETIRACETAM IN THE 5XFAD MOUSE MODEL OF ALZHEIMER’S DISEASE

restrictive barrier properties controlled by tight junctions and polarized expression of selective transporters, the endothelial cells that form the BBB effectively regulates movement of metabolites and nutrients between blood and brain parenchyma. Any changes in the BBB may impair the clearance of neurotoxic molecules allowing their accumulation and deposition in brain parenchyma and vasculature, leading to neuronal dysfunction and degeneration, and contribute to the onset and progression of Alzheimer’s disease (AD). In AD and CAA, accumulation of amyloid-b (Ab) on microvessels results in a rupture of vessels wall and cerebral hemorrhage, which contribute to, and aggravate, dementia. Accumulation of Ab depends on the imbalance between the production and clearance of Ab. Several pathways for Ab clearance from the brain have been reported including transport across the BBB and enzymatic degradation. Despite our understanding of the pathways responsible for BBB dysfunction and clearance of Ab, the availability of drugs to treat CAA or AD remains lacking. Identifying strategies to rectify BBB integrity and function, and maximize clearance of Ab from the brain is of high clinical importance for the development of interventions, which prevent or delay onset of CAA and AD.Methods:In my lab, we developed a novel BBB model consisting of cerebrovascular endothelial cells and high-throughput screening (HTS) methodologies to screen for hit compounds that ameliorate Ab induced increases in endothelial cell permeability and enhance Ab clearance. Identified hits were then tested in vivo in AD mouse model for BBB tightness, Ab brain levels and Ab related pathology. Results:Multiple hit compounds were identified from the screening that were ranked for their potencies. Most potent compounds were in vivo evaluated in ADmouse model for their therapeutic effect against AD pathology. Our in vitro to in vivo studies have successfully identified candidate therapeutic molecules to test in future clinical studies. Conclusions:Our findings demonstrated the BBB as a therapeutic target to prevent and/or slow the progression of the amyloid pathogenesis disorders CAA and AD.