Jeannie M. Padowski

Pharmacokinetic and Pharmacodynamic Implications of P-Glycoprotein Modulation

Modulation of P-glycoprotein (Pgp)-mediated transport has significant pharmacokinetic implications for Pgp substrates. Pharmacokinetic alterations may be at the systemic (blood concentrations), regional (organ or tissue concentrations), or local (intracellular concentrations) level. Regardless of the particular location of Pgp modulation, changes in substrate pharmacokinetics will have the potential to alter the magnitude and duration of pharmacologic effect (pharmacodynamics). It is important to understand each of the aspects of Pgp modulation for a given Pgp substrate in order to predict the degree to which Pgp modulation may affect that substrate, to minimize untoward effects associated with that modulation, or to exploit that modulation for specific therapeutic advantage.

Pharmacokinetic analysis and comparison of caffeine administered rapidly or slowly in coffee chilled or hot versus chilled energy drink in healthy young adults

Abstract Context: There is a paucity of data describing the impact of type of beverage (coffee versus energy drink), different rates of consumption and different temperature of beverages on the pharmacokinetic disposition of caffeine. Additionally, there is concern that inordinately high levels of caffeine may result from the rapid consumption of cold energy drinks. Objective: The objective of this study was to compare the pharmacokinetics of caffeine under various drink temperature, rate of consumption and vehicle (coffee versus energy drink) conditions. Materials: Five caffeine (dose = 160 mg) conditions were evaluated in an open-label, group-randomized, crossover fashion. After the administration of each caffeine dose, 10 serial plasma samples were harvested. Caffeine concentration was measured via liquid chromatography–mass spectrometry (LC–MS), and those concentrations were assessed by non-compartmental pharmacokinetic analysis. The calculated mean pharmacokinetic parameters were analyzed statistically by one-way repeated measures analysis of variance (RM ANOVA). If differences were found, each group was compared to the other by all pair-wise multiple comparison. Results: Twenty-four healthy subjects ranging in age from 18 to 30 completed the study. The mean caffeine concentration time profiles were similar with overlapping SDs at all measured time points. The ANOVA revealed significant differences in mean Cmax and Vd ss/F, but no pair-wise comparisons reached statistical significance. No other differences in pharmacokinetic parameters were found. Discussion: The results of this study are consistent with previous caffeine pharmacokinetic studies and suggest that while rate of consumption, temperature of beverage and vehicle (coffee versus energy drink) may be associated with slightly different pharmacokinetic parameters, the overall impact of these variables is small. Conclusion: This study suggests that caffeine absorption and exposure from coffee and energy drink is similar irrespective of beverage temperature or rate of consumption.

Neurochemical correlates of caudate atrophy in Huntington's disease.

The precise pathogenic mechanisms of Huntingtons disease (HD) are unknown but can be tested in vivo using proton magnetic resonance spectroscopy (1H MRS) to measure neurochemical changes. The objective of this study was to evaluate neurochemical differences in HD gene mutation carriers (HGMCs) versus controls and to investigate relationships among function, brain structure, and neurochemistry in HD. Because previous 1H MRS studies have yielded varied conclusions about HD neurochemical changes, an additional goal was to compare two 1H MRS data analysis approaches. HGMCs with premanifest to early HD and controls underwent evaluation of motor function, magnetic resonance imaging, and localized 1H MRS in the caudate and the frontal lobe. Analytical approaches that were tested included absolute quantitation (unsuppressed water signal as an internal reference) and relative quantification (calculating ratios of all neurochemical signals within a voxel). We identified a suite of neurochemicals that were reduced in concentration proportionally to loss of caudate volume in HGMCs. Caudate concentrations of N‐acetylaspartate (NAA), creatine, choline, and caudate and frontal lobe concentrations of glutamate plus glutamine (Glx) and glutamate were correlated with caudate volume in HGMCs. The relative, but not the absolute, quantitation approach revealed disease‐related differences; the Glx signal was decreased relative to other neurochemicals in the caudate of HGMCs versus controls. This is the first study to demonstrate a correlation among structure, function, and chemical measures in HD brain. Additionally, we demonstrate that a relative quantitation approach may enable the magnification of subtle differences between groups. Observation of decreased Glx suggests that glutamate signaling may be disrupted relatively early in HD, which has important implications for therapeutic approaches.

Central nervous system uptake of intranasal glutathione in Parkinson’s disease

Glutathione (GSH) is depleted early in the course of Parkinson’s disease (PD), and deficiency has been shown to perpetuate oxidative stress, mitochondrial dysfunction, impaired autophagy, and cell death. GSH repletion has been proposed as a therapeutic intervention. The objective of this study was to evaluate whether intranasally administered reduced GSH, (in)GSH, is capable of augmenting central nervous system GSH concentrations, as determined by magnetic resonance spectroscopy in 15 participants with mid-stage PD. After baseline GSH measurement, 200 mg (in)GSH was self-administered inside the scanner without repositioning, then serial GSH levels were obtained over ~1 h. Statistical significance was determined by one-way repeated measures analysis of variance. Overall, (in)GSH increased brain GSH relative to baseline (P<0.001). There was no increase in GSH 8 min after administration, although it was significantly higher than baseline at all of the remaining time points (P<0.01). This study is the first to demonstrate that intranasal administration of GSH elevates brain GSH levels. This increase persists at least 1 h in subjects with PD. Further dose–response and steady-state administration studies will be required to optimize the dosing schedule for future trials to evaluate therapeutic efficacy.

Examination of the ability of the nasal administration route to confer a brain exposure advantage for three chemical inhibitors of P-glycoprotein

The central nervous system (CNS), efficiently isolated from the systemic circulation by the blood-brain barrier (BBB), represents a challenging therapeutic target. For CNS-targeted agents, augmenting brain exposure by increasing blood drug concentrations often is prohibited by systemic toxicity. Therefore, a means for selectively increasing brain exposure, while minimizing systemic exposure, would be desirable. Limited evidence has indicated that nasally-administered compounds can penetrate into brain, although the selectivity of this approach is unclear. This study demonstrated a distinct, but compound-specific, advantage of the nasal administration route in conferring selective CNS delivery (defined as a brain exposure advantage; BEA). Brain and systemic concentrations of three P-glycoprotein-inhibiting agents were evaluated following single nasal or systemic doses to mice, and the influence of administration route on brain exposure (absolute BEA) and on brain-to-blood partitioning (relative BEA) was calculated. Relative and absolute BEA differed markedly among rifampin, quinidine, and GF120918, with relative BEA ranging between 1.53- and 809-fold and absolute BEA between 0.114- and 9.19-fold. Although substantial increases in brain exposure and partitioning in conjunction with nasal administration were demonstrated, the utility of this approach may be limited by inability to deliver a therapeutically relevant mass of drug to the brain.

Glutathione as a Biomarker in Parkinson’s Disease: Associations with Aging and Disease Severity

Objectives. Oxidative stress contributes to Parkinsons disease (PD) pathophysiology and progression. The objective was to describe central and peripheral metabolites of redox metabolism and to describe correlations between glutathione (Glu) status, age, and disease severity. Methods. 58 otherwise healthy individuals with PD were examined during a single study visit. Descriptive statistics and scatterplots were used to evaluate normality and distribution of this cross-sectional sample. Blood tests and magnetic resonance spectroscopy (MRS) were used to collect biologic data. Spearmans rank-order correlation coefficients were used to evaluate the strength and direction of the association. The Unified PD Rating Scale (UPDRS) and the Patient-Reported Outcomes in PD (PRO-PD) were used to rate disease severity using regression analysis. Results. Blood measures of Glu decreased with age, although there was no age-related decline in MRS Glu. The lower the blood Glu concentration, the more severe the UPDRS (P = 0.02, 95% CI: −13.96, −1.14) and the PRO-PD (P = 0.01, 95% CI: −0.83, −0.11) scores. Discussion. These data suggest whole blood Glu may have utility as a biomarker in PD. Future studies should evaluate whether it is a modifiable risk factor for PD progression and whether Glu fortification improves PD outcomes.

Influence of time to achieve substrate distribution equilibrium between brain tissue and blood on quantitation of the blood-brain barrier P-glycoprotein effect.

Active efflux transport processes at the blood-brain barrier (BBB), such as P-glycoprotein (P-gp)-mediated efflux, can limit brain uptake of therapeutics. Accurate determination of the consequent impact on brain uptake is assumed to require sampling post-attainment of brain-to-blood distribution equilibrium. Because this approach is not always feasible, understanding the relationship between apparent degree of efflux (e.g., calculated BBB P-gp effect) and the fraction of time remaining until distribution equilibrium is achieved (FTDE) would be advantageous. This study employed simulation strategies to explore this relationship in the simplest relevant system (absence of protein binding, saturable uptake, or metabolism at the BBB). Concentration-time profiles were simulated with a 4-compartment system (blood, peripheral tissues, BBB endothelium and brain parenchyma). A unidirectional endothelium-to-blood rate constant, PS(e), represented P-gp-mediated efflux. A parameter space was selected to simulate an 18-fold P-gp effect, (K(p,brain) at distribution equilibrium in the absence [K(p,brain)=82] vs. presence [K(p,brain)=4.5] of P-gp-mediated flux), as observed for paclitaxel in P-gp-deficient vs. P-gp-competent mice. Hypothetical compounds with different P-gp effects, peripheral compartment distribution kinetics, or times to achieve distribution equilibrium were simulated by perturbing the values of relevant model parameters. P-gp effects calculated prior to attainment of distribution equilibrium may be substantially erroneous. However, reasonably accurate estimates can be obtained relatively early in the net distributional phase (under 20% error at FTDE>0.36 or 0.11 for bolus or infusion administration, respectively). Potential errors associated with non-equilibrium calculations are dependent on both P-gp-mediated and P-gp-independent components of flux across the BBB.

Comparison of a New Intranasal Naloxone Formulation to Intramuscular Naloxone: Results from Hypothesis-generating Small Clinical Studies

Easy‐to‐use naloxone formulations are needed to help address the opioid overdose epidemic. The pharmacokinetics of i.v., i.m., and a new i.n. naloxone formulation (2 mg) were compared in six healthy volunteers. Relative to i.m. naloxone, geometric mean (90% confidence interval [CI]) absolute bioavailability of i.n. naloxone was modestly lower (55%; 90% CI, 43–70% vs. 41%; 90% CI, 27–62%), whereas average (±SE) mean absorption time was substantially shorter (74 ± 8.8 vs. 6.7 ± 4.9 min). The opioid‐attenuating effects of i.n. naloxone were compared with i.m. naloxone (2 mg) after administration of oral alfentanil (4 mg) to a separate group of six healthy volunteers pretreated with 240 mL of water or grapefruit juice. The i.m. and i.n. naloxone attenuated miosis by similar extents after water (40 ± 15 vs. 41 ± 21 h*%) and grapefruit juice (49 ± 18 vs. 50 ± 22 h*%) pretreatment. Results merit further testing of this new naloxone formulation.

Phase IIb Study of Intranasal Glutathione in Parkinson’s Disease

Background: Reduced glutathione (GSH) is an endogenously synthesized tripeptide depleted early in the course of Parkinson’s disease (PD) and GSH augmentation has been proposed as a therapeutic strategy in PD. Objective: This Phase IIb study was designed to evaluate whether a Phase III study of intranasal GSH, (in)GSH, for symptomatic relief is warranted and to determine the most appropriate trial design for a disease-modification study. Methods: This was a double-blind, placebo-controlled trial of 45 individuals with Hoehn & Yahr Stage 1–3 PD. Participants were randomized to receive intranasal placebo (saline), 100 mg GSH, or 200 mg GSH thrice daily for three months, and were observed over a one-month washout period. Results: All cohorts improved over the intervention period, including placebo. The high-dose group demonstrated improvement in total Unified PD Rating Scale (UPDRS) (–4.6 (4.7), P = 0.0025) and UPDRS motor subscore (–2.2 (3.8), P = 0.0485) over baseline, although neither treatment group was superior to placebo. One participant in the high-dose GSH cohort developed cardiomyopathy. Conclusions: Although predicted improvements in PD total and motor scores were observed, these data do not suggest (in)GSH is superior to placebo after a three month intervention. The symptomatic effects are sufficient to warrant a delayed-start or wash-out design study for disease-modification trials. Whether long-term use of (in)GSH leads to clinical improvements that are sustained and significantly different than placebo will require appropriately-powered longer-duration studies in larger cohorts. The improvement in the placebo arm was more robust than has been observed in previous PD studies and warrants further investigation.

Influence of Enterohepatic Recycling on the Time Course of Brain-to-Blood Partitioning of Valproic Acid in Rats

A widely used metric of substrate exposure in brain is the brain-to-serum partition coefficient (Kp,brain; Cbrain/Cserum), most appropriately determined at distribution equilibrium between brain tissue and serum. In some cases, Cbrain/Cserum can peak and then decrease, as opposed to monotonically increasing to a plateau, precluding accurate estimation of partitioning. This “overshoot” has been observed with compounds that undergo enterohepatic recycling (ER), such as valproic acid (VPA). Previous simulation experiments identified a relationship between overshoot in the Cbrain/Cserum versus time profile and distribution into a peripheral “compartment” (e.g., the ER loop). This study was conducted to evaluate model predictions of that relationship. Initial experiments tested the ability of activated charcoal, antibiotics, or Mrp2 deficiency to impair VPA ER in rats, thereby limiting the apparent volume of distribution associated with ER. Mrp2 deficiency (significantly) and antibiotics (moderately) interrupted VPA ER. Subsequently, brain partitioning was evaluated in the presence versus absence of ER modulation. Although overshoot was not eliminated completely, deconvolution revealed that overshoot was reduced in Mrp2-deficient and antibiotic-treated rats. Consistent with model predictions, overshoot was higher after antibiotic treatment (moderate ER interruption) than in Mrp2 deficiency (substantial ER interruption). Steady-state Kp,brain was unaffected by experimental manipulation, also consistent with model predictions. These data support the hypothesis that Cbrain/Cserum may overshoot Kp,brain based on the extent of peripheral sequestration. Consideration of this information, particularly for compounds that undergo significant extravascular distribution, may be necessary to avoid erroneous estimation of Kp,brain.