This review addresses questions on how to accomplish successful central nervous system (CNS) drug delivery (i.e., having the right concentration at the right CNS site, at the right time), by ...understanding the rate and extent of blood‐brain barrier (BBB) transport and intra‐CNS distribution in relation to CNS target site(s) exposure. To this end, we need to obtain and integrate quantitative and connected data on BBB using the Combinatory Mapping Approach that includes in vivo and ex vivo animal measurements, and the physiologically based comprehensive LEICNSPK3.0 mathematical model that can translate from animals to humans. For small molecules, slow diffusional BBB transport and active influx and efflux BBB transport determine the differences between plasma and CNS pharmacokinetics. Obviously, active efflux is important for limiting CNS drug delivery. Furthermore, liposomal formulations of small molecules may to a certain extent circumvent active influx and efflux at the BBB. Interestingly, for CNS pathologies, despite all reported disease associated BBB and CNS functional changes in animals and humans, integrative studies typically show a lack of changes on CNS drug delivery for the small molecules. In contrast, the understanding of the complex vesicle‐based BBB transport modes that are important for CNS delivery of large molecules is in progress, and their BBB transport seems to be significantly affected by CNS diseases. In conclusion, today, CNS drug delivery of small drugs can be well assessed and understood by integrative approaches, although there is still quite a long way to go to understand CNS drug delivery of large molecules.
: Active‐site concentrations can be defined as the concentrations of unbound, pharmacologically active substances at the site of action. In contrast, the total concentrations of the drug in ...plasma/organ/tissue also include the protein‐ or tissue‐bound molecules that are pharmacologically inactive. Plasma and whole tissue concentrations are used as predictors of effects and side effects because of their ease of sampling, while the concentrations of unbound drug in tissue are more difficult to measure. However, with the introduction of microdialysis and subsequently developed techniques, it has become possible to test the free drug hypothesis. The brain is an interesting organ in this regard because of the presence of the blood‐brain barrier with its tight junctions and active efflux and influx transporters. We have proposed that research into brain drug delivery be divided into three main areas: the rate of delivery (PS, CLin), the extent of delivery (Kp,uu) and the non‐specific affinity of the drug to brain tissue, described by the volume of distribution of unbound drug in the brain (Vu,brain). In this way, the concentration of unbound drug at the target site can be estimated from the total brain concentration and the plasma concentration after measuring the fraction of unbound drug. Results so far fully support the theory that active site concentrations are the best predictors when active transport is present. However, there is an urgent need to collect more relevant data for predicting active site concentrations in tissues with active transporters in their plasma membranes.
Comprehensive determination of the extent of drug transport across the region-specific blood-brain barrier (BBB) is a major challenge in preclinical studies. Multiple approaches are needed to ...determine the regional free (unbound) drug concentration at which a drug engages with its therapeutic target. We present an approach that merges in vivo and in vitro neuropharmacokinetic investigations with mass spectrometry imaging to quantify and visualize both the extent of unbound drug BBB transport and the post-BBB cerebral distribution of drugs at regional and subregional levels. Direct imaging of the antipsychotic drugs risperidone, clozapine, and olanzapine using this approach enabled differentiation of regional and subregional BBB transport characteristics at 20-µm resolution in small brain regions, which could not be achieved by other means. Our approach allows investigation of heterogeneity in BBB transport and presents new possibilities for molecular psychiatrists by facilitating interpretation of regional target-site exposure results and decision-making.
l-Type amino acid transporter 1 (LAT1), selectively expressed at the blood–brain barrier (BBB) and brain parenchymal cells, mediates brain delivery of drugs and prodrugs such as l-dopa and ...gabapentin. Although knowledge about BBB transport of LAT1-utilizing prodrugs is available, there is a lack of quantitative information about brain intracellular delivery and influence of prodrugs on the transporter’s physiological state. We studied the LAT1-mediated intrabrain distribution of a recently developed prodrug of the cyclooxygenase inhibitor ketoprofen as well as its impact on transporter protein expression and function (i.e., amino acid exchange) using brain slice method in mice and rats. The intrabrain distribution of the prodrug was 16 times higher than that of ketoprofen. LAT1 involvement in brain cellular barrier uptake of the prodrug was confirmed, reflected by a higher unbound brain intracellular compared to brain extracellular fluid concentration. The prodrug did not alter LAT1 protein expression and amino acid exchange. Integration of derived parameters with previously performed in vivo pharmacokinetic study using the Combinatory Mapping Approach allowed to estimate the brain extra- and intracellular levels of unbound ketoprofen, prodrug, and released parent drug. The overall efficiency of plasma to brain intracellular delivery of prodrug-released ketoprofen was 11 times higher than after ketoprofen dosing. In summary, this study provides quantitative information supporting the use of the LAT1-mediated prodrug approach for enhanced brain delivery of drugs with intracellular targets.
Microdialysis has contributed with very important knowledge to the understanding of target-specific concentrations and their relationship to pharmacodynamic effects from a systems pharmacology ...perspective, aiding in the global understanding of drug effects. This review focuses on the historical development of microdialysis as a method to quantify the pharmacologically very important unbound tissue concentrations and of recent findings relating to modeling microdialysis data to extrapolate from rodents to humans, understanding distribution of drugs in different tissues and disease conditions. Quantitative microdialysis developed very rapidly during the early 1990s. Method development was in focus in the early years including development of quantitative microdialysis, to be able to estimate true extracellular concentrations. Microdialysis has significantly contributed to the understanding of active transport at the blood-brain barrier and in other organs. Examples are presented where microdialysis together with modeling has increased the knowledge on tissue distribution between species, in overweight patients and in tumors, and in metabolite contribution to drug effects. More integrated metabolomic studies are still sparse within the microdialysis field, although a great potential for tissue and disease-specific measurements is evident.
Mitragyna speciosa is reported to be beneficial for the management of chronic pain and opioid withdrawal in the evolving opioid epidemic. Data on the blood–brain barrier (BBB) transport of ...mitragynine and 7‐hydroxymitragynine, the active compounds of the plant, are still lacking and inconclusive. Here, we present for the first time the rate and the extent of mitragynine and 7‐hydroxymitragynine transport across the BBB, with an investigation of their post‐BBB intra‐brain distribution. We utilized an in vitro BBB model to study the rate of BBB permeation of the compounds and their interaction with efflux transporter P‐glycoprotein (P‐gp). Mitragynine showed higher apical‐to‐basolateral (A‐B, i.e. blood‐to‐brain side) permeability than 7‐hydroxymitragynine. 7‐Hydroxymitragynine showed a tendency to efflux, with efflux ratio (B‐A/A‐B) of 1.39. Both were found to inhibit the P‐gp and are also subject to efflux by the P‐gp. Assessment of the extent of BBB transport in vivo in rats from unbound brain to plasma concentration ratios (Kp,uu,brain) revealed extensive efflux of both compounds, with less than 10 percent of unbound mitragynine and 7‐hydroxymitragynine in plasma crossing the BBB. By contrast, the extent of intra‐brain distribution was significantly different, with mitragynine having 18‐fold higher brain tissue uptake in brain slice assay compared with 7‐hydroxymitragynine. Mitragynine showed a moderate capacity to accumulate inside brain parenchymal cells, while 7‐hydroxymitragynine showed restricted cellular barrier transport. The presented findings from this systematic investigation of brain pharmacokinetics of mitragynine and 7‐hydroxymitragynine are essential for design and interpretation of in vivo experiments aiming to establish exposure–response relationship.
To define and differentiate relevant aspects of blood–brain barrier transport and distribution in order to aid research methodology in brain drug delivery. Pharmacokinetic parameters relative to the ...rate and extent of brain drug delivery are described and illustrated with relevant data, with special emphasis on the unbound, pharmacologically active drug molecule. Drug delivery to the brain can be comprehensively described using three parameters:
K
p,uu
(concentration ratio of unbound drug in brain to blood), CL
in
(permeability clearance into the brain), and
V
u,brain
(intra-brain distribution). The permeability of the blood–brain barrier is less relevant to drug action within the CNS than the extent of drug delivery, as most drugs are administered on a continuous (repeated) basis.
K
p,uu
can differ between CNS-active drugs by a factor of up to 150-fold. This range is much smaller than that for log BB ratios (
K
p
), which can differ by up to at least 2,000-fold, or for BBB permeabilities, which span an even larger range (up to at least 20,000-fold difference). Methods that measure the three parameters
K
p,uu
, CL
in
, and
V
u,brain
can give clinically valuable estimates of brain drug delivery in early drug discovery programmes.
This study provides a new method for quantifying the cyclotide kalata B1 in both plasma and brain homogenate. Cyclotides are ultra‐stable peptides with three disulfide bonds that are interesting from ...a drug development perspective as they can be used as scaffolds. In this study we describe a new validated LC‐MS/MS method with high sensitivity and specificity for kalata B1. The limit of quantification was 2 ng/mL in plasma and 5 ng/gmL in brain homogenate. The method was linear in the range 2–10,000 ng/mL for plasma and 5–2000 ng/g for brain. Liquid Chromatographic separation was performed on a HyPurity C18 column, 50 × 4.6 mm, 3 µm particle size. The method had inter‐ and intra‐day precision and accuracy levels <15% and 12% respectively. Applying the method to in vivo plasma samples and brain homogenate samples from equilibrium dialysis yielded satisfying results and was able to describe the plasma pharmacokinetics and brain tissue binding of kalata B1. The described method is quick, reproducible and well suited to quantifying kalata B1 in biological matrices.
The aim of this study was to characterize the blood–brain barrier (BBB) transport and pharmacokinetics–pharmacodynamics (PKPD) relationship of oxymorphone and to further elucidate its possible ...contribution to oxycodone analgesia. The BBB transport of oxymorphone was studied using microdialysis in male Sprague–Dawley rats. Samples from microdialysis blood and brain probes, brain tissue, and plasma were analyzed by liquid chromatography with tandem mass spectrometry. The effect was measured as tail-flick latency. The study consisted of a PKPD experiment with combined microdialysis and antinociceptive measurements (n=8), and another antinociceptive effect experiment (n=9) using a 10 times lower dose. The combined data were analyzed with an integrated PKPD model in nonlinear mixed effect modeling utilizing a specific method (M3) for handling missing PD observations. The concentration of unbound oxymorphone was higher in brain than in blood, with a ratio of 1.9 (RSE, 9.7%), indicating active uptake at the BBB. The integrated PKPD model described the oxymorphone BBB transport and PKPD relationship successfully, with an EC50 in the brain of 63ng/mL, and the M3 method was able to address the issue of censored observations. Oxymorphone has active uptake transport at the BBB in rats, with moderate uptake clearance to the brain. Its contribution to analgesia after oxycodone administration is not significant.
PDF
