Cerebrospinal fluid can be used as a surrogate to assess brain exposures of breast cancer resistance protein and P-glycoprotein substrates

Discovery of Potent and Brain-Penetrant Bicyclic NLRP3 Inhibitors with Peripheral and Central In Vivo Activity

NLRP3 is a danger sensor protein responsible for inflammasome activation. This leads to pro-inflammatory cytokines release, like IL-1β, and pyroptosis, a regulated cell death. Mounting evidence associates excessive NLRP3 activation to neurodegenerative conditions, such as Alzheimer's and Parkinson's diseases. Thus, NLRP3 inhibitors could potentially provide therapeutic benefit for these disorders. We describe here the evolution of inhibitors relying on a pyridazine-based motif for their key interactions with NLRP3. A Cryo-EM structure helped optimizing protein-ligand complementarity. Subsequently, conformational NMR studies pointed the efforts toward 5,6-bicyclic cores that allowed a balance between brain penetration and undesirable properties, such as hERG inhibition. The effort culminated in compound 19, which showed moderate (mouse) to good (rat) brain penetration and was active at low dose in an LPS challenge model. Importantly, an earlier compound was active in a central neuroinflammation model providing a valuable proof of concept for NLRP3 inhibition.

Preclinical characterization of the absorption and disposition of the brain penetrant PI3K/mTOR inhibitor paxalisib and prediction of its pharmacokinetics and efficacy in human

Glioblastoma multiforme (GBM) is the most common primary brain tumour in adults. Available treatments have not markedly improved patient survival in the last twenty years. However, genomic investigations have showed that the PI3K pathway is frequently altered in this glioma, making it a potential therapeutic target.Paxalisib is a brain penetrant PI3K/mTOR inhibitor (mouse Kp,uu 0.31) specifically developed for the treatment of GBM. We characterised the preclinical pharmacokinetics and efficacy of paxalisib and predicted its pharmacokinetics and efficacious dose in humans.Plasma protein binding of paxalisib was low, with the fraction unbound ranging from 0.25 to 0.43 across species. The hepatic clearance of paxalisib was predicted to be low in mice, rats, dogs and humans, and high in monkeys, from hepatocytes incubations. The plasma clearance was low in mice, moderate in rats and high in dogs and monkeys. Oral bioavailability ranged from 6% in monkeys to 76% in rats.The parameters estimated from the pharmacokinetic/pharmacodynamic modelling of the efficacy in the subcutaneous U87 xenograft model combined with the human pharmacokinetics profile predicted by PBPK modelling suggested that a dose of 56 mg may be efficacious in humans. Paxalisib is currently tested in Phase III clinical trials.

Metabolomic and transcriptomic analysis reveals endogenous substrates and metabolic adaptation in rats lacking Abcg2 and Abcb1a transporters

Abcg2/Bcrp and Abcb1a/Pgp are xenobiotic efflux transporters limiting substrate permeability in the gastrointestinal system and brain, and increasing renal and hepatic drug clearance. The systemic impact of Bcrp and Pgp ablation on metabolic homeostasis of endogenous substrates is incompletely understood. We performed untargeted metabolomics of cerebrospinal fluid (CSF) and plasma, transcriptomics of brain, liver and kidney from male Sprague Dawley rats (WT) and Bcrp/Pgp double knock-out (dKO) rats, and integrated metabolomic/transcriptomic analysis to identify putative substrates and perturbations in canonical metabolic pathways. A predictive Bayesian machine learning model was used to predict in silico those metabolites with greater substrate-like features for either transporters. The CSF and plasma levels of 169 metabolites, nutrients, signaling molecules, antioxidants and lipids were significantly altered in dKO rats, compared to WT rats. These metabolite changes suggested alterations in histidine, branched chain amino acid, purine and pyrimidine metabolism in the dKO rats. Levels of methylated and sulfated metabolites and some primary bile acids were increased in dKO CSF or plasma. Elevated uric acid levels appeared to be a primary driver of changes in purine and pyrimidine biosynthesis. Alterations in Bcrp/Pgp dKO CSF levels of antioxidants, precursors of neurotransmitters, and uric acid suggests the transporters may contribute to the regulation of a healthy central nervous system in rats. Microbiome-generated metabolites were found to be elevated in dKO rat plasma and CSF. The altered dKO metabolome appeared to cause compensatory transcriptional change in urate biosynthesis and response to lipopolysaccharide in brain, oxidation-reduction processes and response to oxidative stress and porphyrin biosynthesis in kidney, and circadian rhythm genes in liver. These findings present insight into endogenous functions of Bcrp and Pgp, the impact that transporter substrates, inhibitors or polymorphisms may have on metabolism, how transporter inhibition could rewire drug sensitivity indirectly through metabolic changes, and identify functional Bcrp biomarkers.

Lumbar cerebrospinal fluid-to-brain extracellular fluid surrogacy is context-specific: insights from LeiCNS-PK3.0 simulations

Predicting brain pharmacokinetics is critical for central nervous system (CNS) drug development yet difficult due to ethical restrictions of human brain sampling. CNS pharmacokinetic (PK) profiles are often altered in CNS diseases due to disease-specific pathophysiology. We previously published a comprehensive CNS physiologically-based PK (PBPK) model that predicted the PK profiles of small drugs at brain and cerebrospinal fluid compartments. Here, we improved this model with brain non-specific binding and pH effect on drug ionization and passive transport. We refer to this improved model as Leiden CNS PBPK predictor V3.0 (LeiCNS-PK3.0). LeiCNS-PK3.0 predicted the unbound drug concentrations of brain ECF and CSF compartments in rats and humans with less than two-fold error. We then applied LeiCNS-PK3.0 to study the effect of altered cerebrospinal fluid (CSF) dynamics, CSF volume and flow, on brain extracellular fluid (ECF) pharmacokinetics. The effect of altered CSF dynamics was simulated using LeiCNS-PK3.0 for six drugs and the resulting drug exposure at brain ECF and lumbar CSF were compared. Simulation results showed that altered CSF dynamics changed the CSF PK profiles, but not the brain ECF profiles, irrespective of the drug's physicochemical properties. Our analysis supports the notion that lumbar CSF drug concentration is not an accurate surrogate of brain ECF, particularly in CNS diseases. Systems approaches account for multiple levels of CNS complexity and are better suited to predict brain PK.

A Practical Perspective on the Evaluation of Small Molecule CNS Penetration in Drug Discovery

The separation of the brain from blood by the blood-brain barrier and the bloodcerebrospinal fluid (CSF) barrier poses unique challenges for the discovery and development of drugs targeting the central nervous system (CNS). This review will describe the role of transporters in CNS penetration and examine the relationship between unbound brain (Cu-brain) and unbound plasma (Cu-plasma) or CSF (CCSF) concentration. Published data demonstrate that the relationship between Cu-brain and Cu-plasma or CCSF can be affected by transporter status and passive permeability of a drug and CCSF may not be a reliable surrogate for CNS penetration. Indeed, CCSF usually over-estimates Cu-brain for efflux substrates and it provides no additional value over Cu-plasma as the surrogate of Cu-brain for highly permeable non-efflux substrates. A strategy described here for the evaluation of CNS penetration is to use in vitro permeability, P-glycoprotein (Pgp) and breast cancer resistance protein efflux assays and Cu-brain/Cu-plasma in preclinical species. Cu-plasma should be used as the surrogate of Cu-brain for highly permeable non-efflux substrates with no evidence of impaired distribution into the brain. When drug penetration into the brain is impaired, we recommend using (total brain concentration * unbound fraction in the brain) as Cu-brain in preclinical species or Cu-plasma/in vitro Pgp efflux ratio if Pgp is the major limiting mechanism for brain penetration.

Pharmacological Optimization for Successful Traumatic Brain Injury Drug Development

The purpose of this review is to highlight the pharmacological barrier to drug development for traumatic brain injury (TBI) and to discuss best practice strategies to overcome such barriers. Specifically, this article will review the pharmacological considerations of moving from the disease target "hit" to the "lead" compound with drug-like and central nervous system (CNS) penetrant properties. In vitro assessment of drug-like properties will be detailed, followed by pre-clinical studies to ensure adequate pharmacokinetic and pharmacodynamic characteristics of response. The importance of biomarker development and utilization in both pre-clinical and clinical studies will be detailed, along with the importance of identifying diagnostic, pharmacodynamic/response, and prognostic biomarkers of injury type or severity, drug target engagement, and disease progression. This review will detail the important considerations in determining in vivo pre-clinical dose selection, as well as cross-species and human equivalent dose selection. Specific use of allometric scaling, pharmacokinetic and pharmacodynamic criteria, as well as incorporation of biomarker assessments in human dose selection for clinical trial design will also be discussed. The overarching goal of this review is to detail the pharmacological considerations in the drug development process as a method to improve both pre-clinical and clinical study design as we evaluate novel therapies to improve outcomes in patients with TBI.

ABCG2/BCRP: variants, transporter interaction profile of substrates and inhibitors

INTRODUCTION

ABCG2 has a broad substrate specificity and is one of the most important efflux proteins modulating pharmacokinetics of drugs, nutrients and toxicokinetics of toxicants. ABCG2 is an important player in transporter-mediated drug-drug interactions (tDDI). Areas covered: The aims of the review are i) to cover transporter interaction profile of substrates and inhibitors that can be utilized to test interaction of drug candidates with ABCG2, ii) to highlight main characteristics of in vitro testing and iii) to describe the structural basis of the broad substrate specificity of the protein. Preclinical data utilizing Abcg2/Bcrp1 knockouts and clinical studies showing effect of ABCG2 c.421C>A polymorphism on pharmacokinetics of drugs have provided evidence for a broad array of drug substrates and support drug - ABCG2 interaction testing. A consensus on using rosuvastatin and sulfasalazine as intestinal substrates for clinical studies is in the formation. Other substrates relevant to the therapeutic area can be considered. Monolayer efflux assays and vesicular transport assays have been extensively utilized in vitro. Expert opinion: Clinical substrates display complex pharmacokinetics due to broad interaction profiles with multiple transporters and metabolic enzymes. Substrate-dependent inhibition has been observed for several inhibitors. Harmonization of in vitro and in vivo testing makes sense. However, rosuvastatin and sulfasalazine are not efficiently transported in either MDCKII or LLC-PK1-based monolayers. Caco-2 monolayer assays and vesicular transport assays are potential alternatives.

Quantification of Transporter and Receptor Proteins in Dog Brain Capillaries and Choroid Plexus: Relevance for the Distribution in Brain and CSF of Selected BCRP and P-gp Substrates

Transporters at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) play a pivotal role as gatekeepers for efflux or uptake of endogenous and exogenous molecules. The protein expression of a number of them has already been determined in the brains of rodents, nonhuman primates, and humans using quantitative targeted absolute proteomics (QTAP). The dog is an important animal model for drug discovery and development, especially for safety evaluations. The purpose of the present study was to clarify the relevance of the transporter protein expression for drug distribution in the dog brain and CSF. We used QTAP to examine the protein expression of 17 selected transporters and receptors at the dog BBB and BCSFB. For the first time, we directly linked the expression of two efflux transporters, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), to regional brain and CSF distribution using specific substrates. Two cocktails, each containing one P-gp substrate (quinidine or apafant) and one BCRP substrate (dantrolene or daidzein) were infused intravenously prior to collection of the brain. Transporter expression varied only slightly between the capillaries of different brain regions and did not result in region-specific distribution of the investigated substrates. There were, however, distinct differences between brain capillaries and choroid plexus. Largest differences were observed for BCRP and P-gp: both were highly expressed in brain capillaries, but no BCRP and only low amounts of P-gp were detected in the choroid plexus. K_p,uu,brain and K_p,uu,CSF of both P-gp substrates were indicative of drug efflux. Also, K_p,uu,brain for the BCRP substrates was low. In contrast, K_p,uu,CSF for both BCRP substrates was close to unity, resulting in K_p,uu,CSF/K_p,uu,brain ratios of 7 and 8, respectively. We conclude that the drug transporter expression profiles differ between the BBB and BCSFB in dogs, that there are species differences in the expression profiles, and that CSF is not a suitable surrogate for unbound brain concentrations of BCRP substrates in dogs.

Metabolism and Distribution of Clozapine-N-oxide: Implications for Nonhuman Primate Chemogenetics

The use of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) in neuroscience has rapidly expanded in rodent studies but has lagged behind in nonhuman primate (NHP) experiments, slowing the development of this method for therapeutic use in humans. One reason for the slow adoption of DREADD technology in primates is that the pharmacokinetic properties and bioavailability of clozapine-n-oxide (CNO), the most commonly used ligand for human muscarinic (hM) DREADDs, are not fully described in primates. We report an extensive pharmacokinetic study using subcutaneous (SC) administration of CNO in five adult rhesus monkeys. CNO reached maximal plasma and cerebrospinal fluid (CSF) concentrations within 2 h after injection, with an observed dose-dependent increase in levels following a 3 and 10 mg/kg SC dose. Since CSF concentrations were below values predicted from unbound plasma concentrations, we investigated whether CNO was restricted from the CNS through active transport at the blood-brain barrier. In vitro assessment demonstrated that CNO is a substrate for P-glycoprotein (Pgp; efflux ratio, 20), thus providing a likely mechanism limiting CNO levels in the CNS. Furthermore, CNO is metabolized to the psychoactive compounds clozapine and n-desmethylclozapine in monkeys. The concentrations of clozapine detected in the CSF are sufficient to activate several types of receptor (including the hM-DREADDs). Our results suggest that CNO metabolism and distribution may interfere with reproducibility and interpretation of DREADD-related experiments in NHPs and calls for a re-evaluation of the use of CNO in DREADD-related experiments in NHPs along with the need to test alternative compounds.

Unmasking the Role of Uptake Transporters for Digoxin Uptake Across the Barriers of the Central Nervous System in Rat

The role of uptake transporter (organic anion–transporting polypeptide [Oatp]) in the disposition of a P-glycoprotein (P-gp) substrate (digoxin) at the barriers of central nervous system, namely, the blood-brain barrier (BBB), blood-spinal cord barrier (BSCB), and brain-cerebrospinal fluid barrier (BCSFB), was studied using rat as a preclinical species. In vivo chemical inhibition of P-gp and Oatp was achieved using elacridar and rifampicin, respectively. Our findings show that (1) digoxin had a low brain-to-plasma concentration ratio (B/P) (0.07) in rat; (2) in the presence of elacridar, the B/P of digoxin increased by about 12-fold; (3) rifampicin administration alone did not change the digoxin B/P significantly when compared with digoxin B/P alone; (4) rifampicin administration along with elacridar resulted only in 6-fold increase in the B/P of digoxin; (5) similar fold changes and trends were seen with the spinal cord-to-plasma concentration ratio of digoxin, indicating the similarity between BBB and the BSCB; and (6) unlike BBB and BSCB, the presence of rifampicin further increased the cerebrospinal fluid-to-plasma concentration ratio (CSF/P) for digoxin, suggesting a differential orientation of the uptake transporters at the BCSFB (CSF to blood) compared with the BBB (blood to brain). The observations for digoxin uptake, at least at the BBB and the BSCB, advocate the importance of uptake transporters (Oatps). However, the activity of such uptake transporters became evident only after inhibition of the efflux transporter (P-gp).

Natural flavonoids silymarin and quercetin improve the brain distribution of co-administered P-gp substrate drugs

P-glycoprotein (P-gp), a well known efflux transporter in the blood brain barrier inhibits the uptake of substrate drugs into brain. The main aim of this study is to evaluate the effect of natural product based P-gp inhibitors on brain penetration of various CNS drugs which are P-gp substrates. In this study, we have evaluated the inhibitory effects of natural bioflavonoids (quercetin and silymarin) on P-gp by using digoxin and quinidine as model P-gp model substrate drugs. In vitro inhibitory effects were evaluated in Caco-2 cell lines using digoxin as a model drug and in vivo P-gp inhibiting effect was evaluated in mice model using quinidine as model drug. The accumulation and bidirectional transport of digoxin in Caco-2 cells was determined in presence and absence of quercetin and silymarin. Elacridar was used as standard P-gp inhibitor and used to compare the inhibitory effects of test compounds. The apical to basolateral transport of digoxin was increased where as basolateral to apical transport of digoxin was decreased in concentration dependent manner in the presence of elacridar, quercetin and silymarin. After intravenous administration of P-gp inhibitors, brain levels of quinidine were estimated using LC-MS method. Increased brain uptake was observed with quercetin (2.5-fold) and silymarin (3.5-fold). Though the brain penetration potential of P-gp substrates was lower than that observed in elacridar, both quercetin and silymarin improved plasma quinidine levels. Caco-2 permeability studies and brain uptake indicate that both quercetin and silymarin can inhibit P-gp mediated efflux of drug into brain. Our results suggest that both silymarin and quercetin could potentially increase the brain distribution of co-administered drugs that are P-gp substrates.

A proposed role for efflux transporters in the pathogenesis of hydrocephalus

Hydrocephalus is a common brain disorder that is treated only with surgery. The basis for surgical treatment rests on the circulation theory. However, clinical and experimental data to substantiate circulation theory have remained inconclusive. In brain tissue and in the ventricles, we see that osmotic gradients drive water diffusion in water-permeable tissue. As the osmolarity of ventricular CSF increases within the cerebral ventricles, water movement into the ventricles increases and causes hydrocephalus. Macromolecular clearance from the ventricles is a mechanism to establish the normal CSF osmolarity, and therefore ventricular volume. Efflux transporters, (p-glycoprotein), are located along the blood brain barrier and play an important role in the clearance of macromolecules (endobiotics and xenobiotics) from the brain to the blood. There is clinical and experimental data to show that macromolecules are cleared out of the brain in normal and hydrocephalic brains. This article summarizes the existing evidence to support the role of efflux transporters in the pathogenesis of hydrocephalus. The location of p-gp along the pathways of macromolecular clearance and the broad substrate specificity of this abundant transporter to a variety of different macromolecules are reviewed. Involvement of p-gp in the transport of amyloid beta in Alzheimer disease and its relation to normal pressure hydrocephalus is reviewed. Finally, individual variability of p-gp expression might explain the variability in the development of hydrocephalus following intraventricular hemorrhage.

Breast cancer resistance protein (ABCG2) in clinical pharmacokinetics and drug interactions: practical recommendations for clinical victim and perpetrator drug-drug interaction study design

Breast cancer resistance protein (BCRP; ABCG2) limits intestinal absorption of low-permeability substrate drugs and mediates biliary excretion of drugs and metabolites. Based on clinical evidence of BCRP-mediated drug-drug interactions (DDIs) and the c.421C>A functional polymorphism affecting drug efficacy and safety, both the US Food and Drug Administration and European Medicines Agency recommend preclinical evaluation and, when appropriate, clinical assessment of BCRP-mediated DDIs. Although many BCRP substrates and inhibitors have been identified in vitro, clinical translation has been confounded by overlap with other transporters and metabolic enzymes. Regulatory recommendations for BCRP-mediated clinical DDI studies are challenging, as consensus is lacking on the choice of the most robust and specific human BCRP substrates and inhibitors and optimal study design. This review proposes a path forward based on a comprehensive analysis of available data. Oral sulfasalazine (1000 mg, immediate-release tablet) is the best available clinical substrate for intestinal BCRP, oral rosuvastatin (20 mg) for both intestinal and hepatic BCRP, and intravenous rosuvastatin (4 mg) for hepatic BCRP. Oral curcumin (2000 mg) and lapatinib (250 mg) are the best available clinical BCRP inhibitors. To interrogate the worst-case clinical BCRP DDI scenario, study subjects harboring the BCRP c.421C/C reference genotype are recommended. In addition, if sulfasalazine is selected as the substrate, subjects having the rapid acetylator phenotype are recommended. In the case of rosuvastatin, subjects with the organic anion-transporting polypeptide 1B1 c.521T/T genotype are recommended, together with monitoring of rosuvastatin's cholesterol-lowering effect at baseline and DDI phase. A proof-of-concept clinical study is being planned by a collaborative consortium to evaluate the proposed BCRP DDI study design.

Differential role of P-glycoprotein and breast cancer resistance protein in drug distribution into brain, CSF and peripheral nerve tissues in rats

1. This study was designed to evaluate how the absence of P-glycoprotein (Pgp, Mdr1a), breast cancer-resistance protein (Bcrp, Abcg2) or both affects drug distribution into sciatic nerves, brain and cerebrospinal fluid (CSF) in rats. 2. Pgp substrate (loperamide), BCRP substrates (dantrolene and proprietary compound X) and dual substrates (imatinib and proprietary compound Y) were well distributed into sciatic nerves with comparable nerve to plasma concentration ratios between wild-type and knockout (KO) rats. 3. Brain exposure increased substantially in Mdr1a(-/-) rats for loperamide and in Mdr1a(-/-)/Abcg2(-/-) rats for imatinib and compound Y, but minimally to modestly in Abcg2(-/-) rats for dantrolene and compound X. The deletion of Mdr1a or Abcg2 alone had little effect on brain distribution of compound Y. 4. While CSF to unbound brain concentration ratio remained ≥3 in the KO animals for dantrolene, compounds X and Y, it was reduced to 1 in the Mdr1a(-/-)/Abcg2(-/-) rats for imatinib. 5. The data indicate that Pgp and Bcrp do not play significant roles in drug distribution into peripheral nerve tissues in rats, while working in concert to regulate brain penetration. Our results further support that CSF concentration may not be a good surrogate for unbound brain concentration of efflux substrates.

Brain penetration of WEB 2086 (Apafant) and dantrolene in Mdr1a (P-glycoprotein) and Bcrp knockout rats

Transporter gene knockout rat models are attracting increasing interest for mechanistic studies of new drugs as transporter substrates or inhibitors in vivo. However, limited data are available on the functional validity of such models at the blood-brain barrier. Therefore, the present study evaluated Mdr1a [P-glycoprotein (P-gp)], Bcrp, and combined Mdr1a/Bcrp knockout rat strains for the influence of P-gp and breast cancer resistance protein (BCRP) transport proteins on brain penetration of the selective test substrates [(14)C]WEB 2086 (3-[4-(2-chlorophenyl)-9-methyl-6H-thieno[3,2-f][1,2,4]triazolo-[4,3-a][1,4]-diazepin-2-yl]-1-(4-morpholinyl)-1-propanon) for P-gp and dantrolene for BCRP. Brain-to-plasma concentration ratios (BPR) were measured after intravenous coinfusions of 5.5 µmol/kg per hour [(14)C]WEB 2086 and 2 µmol/kg per hour dantrolene for 2 hours in groups of knockout or wild-type rats. Compared with wild-type controls, mean BPR of [(14)C]WEB 2086 increased 8-fold in Mdr1a knockouts, 9.5-fold in double Mdr1a/Bcrp knockouts, and 7.3-fold in zosuquidar-treated wild-type rats, but was unchanged in Bcrp knockout rats. Mean BPR of dantrolene increased 3.3-fold in Bcrp knockouts and 3.9-fold in double Mdr1a/Bcrp knockouts compared with wild type, but was unchanged in the Mdr1a knockouts. The human intestinal CaCo-2 cell bidirectional transport system in vitro confirmed the in vivo finding that [(14)C]WEB 2086 is a substrate of P-gp but not of BCRP. Therefore, Mdr1a, Bcrp, and combined Mdr1a/Bcrp knockout rats provide functional absence of these efflux transporters at the blood-brain barrier and are a suitable model for mechanistic studies on the brain penetration of drug candidates.

[¹¹C]befloxatone brain kinetics is not influenced by Bcrp function at the blood-brain barrier: a PET study using Bcrp TGEM knockout rats

Knockout (KO) animals are useful tools with which to assess the interplay between P-glycoprotein (P-gp; Abcb1) and the breast cancer resistance protein (Bcrp, Abcg2), two major ABC-transporters expressed at the blood-brain barrier (BBB). However, one major drawback of such deficient models is the possible involvement of compensation between transporters. In the present study, P-gp and Bcrp distribution in the brain as well as P-gp expression levels at the BBB were compared between the Bcrp TGEM KO rat model and the wild-type (WT) strain. Therefore, we used confocal microscopy of brain slices and western blot analysis of the isolated brain microvessels forming the BBB. This deficient rat model was used to assess the influence of Bcrp on the brain and peripheral kinetics of its substrate [(11)C]befloxatone using positron emission tomography (PET). The influence of additional P-gp inhibition was tested using elacridar (GF120918) 2 mg/kg in Bcrp KO rats. The distribution pattern of P-gp in the brain as well as P-gp expression levels at the BBB was similar in Bcrp-deficient and WT rats. Brain and peripheral kinetics of [(11)C]befloxatone were not influenced by the lack of Bcrp. Neither was the brain uptake of [(11)C]befloxatone in Bcrp-deficient rats influenced by the inhibition of P-gp. In conclusion, the Bcrp-deficient rat strain, in which we detected no compensatory mechanism or modification of P-gp expression as compared to WT rats, is a suitable model to study Bcrp function separately from that of P-gp at the BBB. However, although selectively transported by BCRP in vitro, our results suggest that [(11)C]befloxatone PET imaging might not be biased by impaired function of this transporter in vivo.

Physiologically based pharmacokinetic modelling of drug penetration across the blood-brain barrier--towards a mechanistic IVIVE-based approach

Predicting the penetration of drugs across the human blood-brain barrier (BBB) is a significant challenge during their development. A variety of in vitro systems representing the BBB have been described, but the optimal use of these data in terms of extrapolation to human unbound brain concentration profiles remains to be fully exploited. Physiologically based pharmacokinetic (PBPK) modelling of drug disposition in the central nervous system (CNS) currently consists of fitting preclinical in vivo data to compartmental models in order to estimate the permeability and efflux of drugs across the BBB. The increasingly popular approach of using in vitro-in vivo extrapolation (IVIVE) to generate PBPK model input parameters could provide a more mechanistic basis for the interspecies translation of preclinical models of the CNS. However, a major hurdle exists in verifying these predictions with observed data, since human brain concentrations can't be directly measured. Therefore a combination of IVIVE-based and empirical modelling approaches based on preclinical data are currently required. In this review, we summarise the existing PBPK models of the CNS in the literature, and we evaluate the current opportunities and limitations of potential IVIVE strategies for PBPK modelling of BBB penetration.

Drug transporters on arachnoid barrier cells contribute to the blood-cerebrospinal fluid barrier

The subarachnoid space, where cerebrospinal fluid (CSF) flows over the brain and spinal cord, is lined on one side by arachnoid barrier (AB) cells that form part of the blood-CSF barrier. However, despite the fact that drugs are administered into the CSF and CSF drug concentrations are used as a surrogate for brain drug concentration following systemic drug administration, the tight-junctioned AB cells have never been examined for whether they express drug transporters that would influence CSF and central nervous system drug disposition. Hence, we characterized drug transporter expression and function in AB cells. Immunohistochemical analysis showed P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) in mouse AB cells but not other meningeal tissue. The Gene Expression Nervous System Atlas (GENSAT) database and the Allen Mouse Brain Atlas confirmed these observations. Microarray analysis of mouse and human arachnoidal tissue revealed expression of many drug transporters and some drug-metabolizing enzymes. Immortalized mouse AB cells express functional P-gp on the apical (dura-facing) membrane and BCRP on both apical and basal (CSF-facing) membranes. Thus, like blood-brain barrier cells and choroid plexus cells, AB cells highly express drug transport proteins and likely contribute to the blood-CSF drug permeation barrier.

A novel PET protocol for visualization of breast cancer resistance protein function at the blood-brain barrier

Breast cancer resistance protein (BCRP) is the most abundant multidrug efflux transporter at the human blood-brain barrier (BBB), restricting brain distribution of various drugs. In this study, we developed a positron emission tomography (PET) protocol to visualize Bcrp function at the murine BBB, based on the dual P-glycoprotein (P-gp)/Bcrp substrate radiotracer [(11)C]tariquidar in combination with the Bcrp inhibitor Ko143. To eliminate the contribution of P-gp efflux to [(11)C]tariquidar brain distribution, we studied mice in which P-gp was genetically knocked out (Mdr1a/b((-/-)) mice) or chemically knocked out by pretreatment with cold tariquidar. We found that [(11)C]tariquidar brain uptake increased dose dependently after administration of escalating doses of Ko143, both in Mdr1a/b((-/-)) mice and in tariquidar pretreated wild-type mice. After 15 mg/kg Ko143, the maximum increase in [(11)C]tariquidar brain uptake relative to baseline scans was 6.3-fold in Mdr1a/b((-/-)) mice with a half-maximum effect dose of 4.98 mg/kg and 3.6-fold in tariquidar (8 mg/kg) pretreated wild-type mice, suggesting that the presented protocol is sensitive to visualize a range of different functional Bcrp activities at the murine BBB. We expect that this protocol can be translated to the clinic, because tariquidar can be safely administered to humans at doses that completely inhibit cerebral P-gp.