Impact of P-Glycoprotein-Mediated Active Efflux on Drug Distribution into Lumbar Cerebrospinal Fluid in Nonhuman Primates

Molecular Mechanisms of Organic Anion Transporting Polypeptide-Mediated Organic Anion Clearance at the Blood-Cerebrospinal Fluid Barrier

The blood-cerebrospinal fluid barrier (BCSFB), formed by the choroid plexus epithelial (CPE) cells, plays an active role in removing drugs and metabolic wastes from the brain. Recent functional studies in isolated mouse choroid plexus (CP) tissues suggested the presence of organic anion transporting polypeptides (OATPs, encoded by SLCOs) at the apical membrane of BCSFB, which may clear large organic anions from the cerebrospinal fluid (CSF). However, the specific OATP isoform involved is unclear. Using quantitative fluorescence imaging, we showed that the fluorescent anions sulforhodamine 101 (SR101), fluorescein methotrexate (FL-MTX), and 8-fluorescein-cAMP (fluo-cAMP) are actively transported from the CSF to the subepithelial space in CP tissues isolated from wild-type mice. In contrast, transepithelial transport of these compounds across the CPE cells was abolished in Oatp1a/1b-/- mice due to impaired apical uptake. Using transporter-expressing cell lines, SR101, FL-MTX, and fluo-cAMP were additionally shown to be transported by mouse OATP1A5 and its human counterpart OATP1A2. Kinetic analysis showed that estrone-3-sulfate and SR101 are transported by OATP1A2 and OATP1A5 with similar Michaelis-Menten constants (Km). Immunofluorescence staining further revealed the presence of OATP1A2 protein in human CP tissues. Together, our results suggest that large organic anions in the CSF are actively transported into CPE cells by apical OATP1A2 (OATP1A5 in mice), then subsequently effluxed into the blood by basolateral multidrug resistance-associated proteins (MRPs). As OATP1A2 transports a wide array of endogenous compounds and xenobiotics, the presence of this transporter at the BCSFB may imply a novel clearance route for drugs and neurohormones from the CSF. SIGNIFICANCE STATEMENT: Drug transporters at the blood-cerebrospinal fluid (CSF) barrier play an important but understudied role in brain drug disposition. This study revealed a functional contribution of rodent organic anion transporting polypeptide (OATP) 1A5 towards the CSF clearance of organic anions and suggested a similar role for OATP1A2 in humans. Delineating the molecular mechanisms governing CSF organic anion clearance may help to improve the prediction of central nervous system (CNS) pharmacokinetics and identify drug candidates with favorable CNS pharmacokinetic properties.

Current Strategies to Enhance Delivery of Drugs across the Blood–Brain Barrier

The blood–brain barrier (BBB) has shown to be a significant obstacle to brain medication delivery. The BBB in a healthy brain is a diffusion barrier that prevents most substances from passing from the blood to the brain; only tiny molecules can pass across the BBB. The BBB is disturbed in specific pathological illnesses such as stroke, diabetes, seizures, multiple sclerosis, Parkinson’s disease, and Alzheimer’s disease. The goal of this study is to offer a general overview of current brain medication delivery techniques and associated topics from the last five years. It is anticipated that this review will stimulate readers to look into new ways to deliver medications to the brain. Following an introduction of the construction and function of the BBB in both healthy and pathological conditions, this review revisits certain contested questions, such as whether nanoparticles may cross the BBB on their own and if medications are selectively delivered to the brain by deliberately targeted nanoparticles. Current non-nanoparticle options are also discussed, including drug delivery via the permeable BBB under pathological circumstances and the use of non-invasive approaches to improve brain medication absorption.

Regional Differences in the Absolute Abundance of Transporters, Receptors and Tight Junction Molecules at the Blood-Arachnoid Barrier and Blood-Spinal Cord Barrier among Cervical, Thoracic and Lumbar Spines in Dogs

PURPOSE

The purpose of the present study was to quantitatively determine the expression of transporters, receptors and tight junction molecules at the blood-arachnoid barrier (BAB) and blood-spinal cord barrier (BSCB) in cervical, thoracic and lumbar spines from dogs.

METHODS

The expression levels of 31 transporters, 3 receptors, 1 tight junction protein, and 3 marker proteins in leptomeninges and capillaries isolated from spines (3 male and 2 female dogs) were determined by quantitative Targeted Absolute Proteomics (qTAP). The units were converted from fmol/μg protein to pmol/cm (absolute abundance at the BAB and the BSCB in a 1 cm section of spine).

RESULTS

The expression of MDR1 and BCRP were greater at the BSCB compared to the BAB (especially in the cervical cord), and the expressions at the lumbar BSCB were lower than that for the cervical BSCB. Among the organic anionic and cationic drug transporters, OAT1, OAT3, MRP1, OCT2 and MATE1/2 were detected only in the BAB, and not at the BSCB). The expression of these transporters was higher in the order: lumbar > thoracic > cervical BAB. The expressions of GLUT1, 4F2hc, EAAT1, 2, PEPT2, CTL1, and MCT1 at the BSCB of the cervical cord were higher than the corresponding values for the cervical BAB, and these values decreased in going down the spinal cord.

CONCLUSION

These results provide a better understanding of the molecular mechanisms underlying the concentration gradients of drugs and endogenous substances in the cerebrospinal fluid and parenchyma of the spinal cord.

Application of a high‐resolution in vitro human MDR1‐MDCK assay and in vivo studies in preclinical species to improve prediction of CNS drug penetration

P‐glycoprotein (P‐gp, MDR1) is expressed at the blood–brain barrier (BBB) and restricts penetration of its substrates into the central nervous system (CNS). In vitro MDR1 assays are frequently used to predict the in vivo relevance of MDR1‐mediated efflux at the BBB. It has been well established that drug candidates with high MDR1 efflux ratios (ERs) display poor CNS penetration. Following a comparison of MDR1 transporter function between the MDR1‐MDCKI cell line from National Institutes of Health (NIH) and our internal MDR1‐MDCKII cell line, the former was found to provide better predictions of in vivo brain penetration than our in‐house MDR1‐MDCKII cell line. In particular, the NIH MDR1 assay has an improved sensitivity to differentiate the compounds with ERs of <3 in our internal cell line and is able to reduce the risk of false negatives. A better correlation between NIH MDR1 ERs and brain penetration in rat and non‐human primate (NHP) was demonstrated. Additionally, a comparison of brain penetration time course of MDR1 substrates and an MDR1 non‐substrate in NHP demonstrated that MDR1 interaction can delay the time to equilibrium of drug concentration in the brain with plasma. It is recommended to select highly permeable compounds without MDR1 interaction for rapid brain penetration to produce the maximal pharmacological effect in the CNS with a quicker onset.

Quantitative prediction of pharmacokinetic properties of drugs in humans: Recent advance in in vitro models to predict the impact of efflux transporters in the small intestine and blood-brain barrier

Efflux transport systems are essential to suppress the absorption of xenobiotics from the intestinal lumen and protect the critical tissues at the blood-tissue barriers, such as the blood-brain barrier. The function of drug efflux transport is dominated by various transporters. Accumulated clinical evidences have revealed that genetic variations of the transporters, together with coadministered drugs, affect the expression and/or function of transporters and subsequently the pharmacokinetics of substrate drugs. Thus, in the preclinical stage of drug development, quantitative prediction of the impact of efflux transporters as well as that of uptake transporters and metabolic enzymes on the pharmacokinetics of drugs in humans has been performed using various in vitro experimental tools. Various kinds of human-derived cell systems can be applied to the precise prediction of drug transport in humans. Mathematical modeling consisting of each intrinsic metabolic or transport process enables us to understand the disposition of drugs both at the organ level and at the level of the whole body by integrating a variety of experimental results into model parameters. This review focuses on the role of efflux transporters in the intestinal absorption and brain distribution of drugs, in addition to recent advances in predictive tools and methodologies.

Oral etoposide and zosuquidar bioavailability in rats: Effect of co-administration and in vitro-in vivo correlation of P-glycoprotein inhibition

P-glycoprotein inhibitors, like zosuquidar, have widely been used to study the role of P-glycoprotein in oral absorption. Still, systematic studies on the inhibitor dose-response relationship on intestinal drug permeation are lacking. In the present study, we investigated the effect of 0.79 nM-2.5 μM zosuquidar on etoposide permeability across Caco-2 cell monolayers. We also investigated etoposide pharmacokinetics after oral or IV administration to Sprague Dawley rats with co-administration of 0.063–63 mg/kg zosuquidar, as well as the pharmacokinetics of zosuquidar itself. Oral zosuquidar bioavailability was 2.6–4.2%, while oral etoposide bioavailability was 5.5 ± 0.9%, which increased with increasing zosuquidar doses to 35 ± 5%. The intestinal zosuquidar concentration required to induce a half-maximal increase in bioavailability was estimated to 180 μM. In contrast, the IC₅₀ of zosuquidar on etoposide permeability in vitro was only 5–10 nM, and a substantial in vitro-in vivo discrepancy of at least four orders of magnitude was thereby identified. Overall, the present study provides valuable insights for future formulation development that applies fixed dose combinations of P-glycoprotein inhibitors to increase the absorption of poorly permeable P-glycoprotein substrate drugs.

Translational CNS Steady-State Drug Disposition Model in Rats, Monkeys, and Humans for Quantitative Prediction of Brain-to-Plasma and Cerebrospinal Fluid-to-Plasma Unbound Concentration Ratios

Capturing unbound drug exposure in the brain is crucial to evaluate pharmacological effects for drugs acting on the central nervous system. However, to date, there are no reports of validated prediction models to determine the brain-to-plasma unbound concentration ratio (K_p,uu,brain) as well as the cerebrospinal fluid (CSF)-to-plasma unbound concentration ratio (K_p,uu,CSF) between humans and other species. Here, we developed a translational CNS steady-state drug disposition model to predict K_p,uu,brain and K_p,uu,CSF across rats, monkeys, and humans by estimating the relative activity factors (RAF) for MDR1 and BCRP in addition to scaling factors (γ and σ) using the molecular weight, logD, CSF bulk flow, and in vitro transport activities of these transporters. In this study, 68, 26, and 28 compounds were tested in the rat, monkey, and human models, respectively. Both the predicted K_p,uu,brain and K_p,uu,CSF values were within the 3-fold range of the observed values (71, 73, and 79%; 79, 88, and 78% of the compounds, respectively), indicating successful prediction of K_p,uu,brain and K_p,uu,CSF in the three species. The overall predictivity of the RAF approach is consistent with that of the relative expression factor (REF) approach. As the established model can predict K_p,uu,brain and K_p,uu,CSF using only in vitro and physicochemical data, this model would help avoid ethical issues related to animal use and improve CNS drug discovery workflow.

Choroid Plexus and Drug Removal Mechanisms

Timely and efficient removal of xenobiotics and metabolites from the brain is crucial in maintaining the homeostasis and normal function of the brain. The choroid plexus (CP) forms the blood-cerebrospinal fluid barrier and vitally removes drugs and wastes from the brain through several co-existing clearance mechanisms. The CP epithelial (CPE) cells synthesize and secrete the cerebrospinal fluid (CSF). As the CSF passes through the ventricular and subarachnoid spaces and eventually drains into the general circulation, it collects and removes drugs, toxins, and metabolic wastes from the brain. This bulk flow of the CSF serves as a default and non-selective pathway for the removal of solutes and macromolecules from the brain interstitium. Besides clearance by CSF bulk flow, the CPE cells express several multispecific membrane transporters to actively transport substrates from the CSF side into the blood side. In addition, several phase I and II drug-metabolizing enzymes are expressed in the CPE cells, which enzymatically inactivate a broad spectrum of reactive or toxic substances. This review summarizes our current knowledge of the functional characteristics and key contributors to the various clearance pathways in the CP-CSF system, overviewing recent developments in our understanding of CSF flow dynamics and the functional roles of CP uptake and efflux transporters in influencing CSF drug concentrations.