Investigation of utility of cerebrospinal fluid drug concentration as a surrogate for interstitial fluid concentration using microdialysis coupled with cisternal cerebrospinal fluid sampling in wild-type and Mdr1a(-/-) rats

Applicability of MDR1 Overexpressing Abcb1KO-MDCKII Cell Lines for Investigating In Vitro Species Differences and Brain Penetration Prediction

Implementing the 3R initiative to reduce animal experiments in brain penetration prediction for CNS-targeting drugs requires more predictive in vitro and in silico models. However, animal studies are still indispensable to obtaining brain concentration and determining the prediction performance of in vitro models. To reveal species differences and provide reliable data for IVIVE, in vitro models are required. Systems overexpressing MDR1 and BCRP are widely used to predict BBB penetration, highlighting the impact of the in vitro system on predictive performance. In this study, endogenous Abcb1 knock-out MDCKII cells overexpressing MDR1 of human, mouse, rat or cynomolgus monkey origin were used. Good correlations between ERs of 83 drugs determined in each cell line suggest limited species specificities. All cell lines differentiated CNS-penetrating compounds based on ERs with high efficiency and sensitivity. The correlation between in vivo and predicted Kp,uu,brain was the highest using total ER of human MDR1 and BCRP and optimized scaling factors. MDR1 interactors were tested on all MDR1 orthologs using digoxin and quinidine as substrates. We found several examples of inhibition dependent on either substrate or transporter abundance. In summary, this assay system has the potential for early-stage brain penetration screening. IC50 comparison between orthologs is complex; correlation with transporter abundance data is not necessarily proportional and requires the understanding of modes of transporter inhibition.

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.

Investigation of the role and quantitative impact of breast cancer resistance protein on drug distribution into brain and CSF in rats

Breast cancer resistance protein (BCRP) expressed in the blood-brain barrier plays a major role in limiting drug distribution into the central nervous system (CNS). However, functional involvement of BCRP in drug distribution into the brain and cerebrospinal fluid (CSF) remains unclear. The aim of present study was to reveal the role and quantitative impact of BCRP on CNS distribution. The brain-to-plasma unbound concentration ratio (K_p,uu,brain) and CSF-to-plasma unbound concentration ratio (K_p,uu,CSF) values of BCRP-specific substrates were determined in rats. The K_p,uu,brain values decreased, as the in vitro BCRP corrected flux ratio (CFR) increased. The K_p,uu,CSF values of BCRP-specific substrates were greater than the K_p,uu,brain values. Increase in the K_p,uu,brain values induced by co-administration of BCRP inhibitor correlated with the in vitro BCRP CFR and were greater than the increase in K_p,uu,CSF values induced by BCRP inhibitor except nebicapone. The contribution of BCRP to the brain and CSF distribution of the dual P-glycoprotein/BCRP substrates, imatinib and prazosin, was similar to that of BCRP-specific substrates. Thus, we revealed that the impact of in vivo BCRP on CNS distribution is correlated with in vitro BCRP CFR, and that BCRP limits drug distribution into the brain more strongly than into the CSF.

Impact of CNS Diseases on Drug Delivery to Brain Extracellular and Intracellular Target Sites in Human: A "WHAT-IF" Simulation Study

The blood-brain barrier (BBB) is equipped with unique physical and functional processes that control central nervous system (CNS) drug transport and the resulting concentration-time profiles (PK). In CNS diseases, the altered BBB and CNS pathophysiology may affect the CNS PK at the drug target sites in the brain extracellular fluid (brainECF) and intracellular fluid (brainICF) that may result in changes in CNS drug effects. Here, we used our human CNS physiologically-based PK model (LeiCNS-PK3.0) to investigate the impact of altered cerebral blood flow (CBF), tight junction paracellular pore radius (pararadius), brainECF volume, and pH of brainECF (pHECF) and of brainICF (pHICF) on brainECF and brainICF PK for 46 small drugs with distinct physicochemical properties. LeiCNS-PK3.0 simulations showed a drug-dependent effect of the pathophysiological changes on the rate and extent of BBB transport and on brainECF and brainICF PK. Altered pararadius, pHECF, and pHICF affected both the rate and extent of BBB drug transport, whereas changes in CBF and brainECF volume modestly affected the rate of BBB drug transport. While the focus is often on BBB paracellular and active transport processes, this study indicates that also changes in pH should be considered for their important implications on brainECF and brainICF target site PK.

Use of Intravenous Infusion Study Design to Simultaneously Determine Brain Penetration and Systemic Pharmacokinetic Parameters in Rats

In drug discovery, the extent of brain penetration as measured by free brain/plasma concentration ratio (K_p,uu) is normally determined from one experiment after constant intravenous infusion, and pharmacokinetics (PK) parameters, including clearance (CL), volume of distribution at steady state (Vss), and effective half-life (t _{1/2 ,eff}) are determined from another experiment after a single intravenous bolus injection. The objective of the present study was to develop and verify a method to simultaneously determine K_p,uu and PK parameters from a single intravenous infusion experiment. In this study, nine compounds (atenolol, loperamide, minoxidil, N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine, sulpiride, and four proprietary compounds) were intravenously infused for 4 hours at 1 mg/kg or 24 hours at 1 or 6 mg/kg or bolus injected at 1 mg/kg. Plasma samples were serially collected, and brain and cerebrospinal fluid samples were collected at the end of infusion. The PK parameters were obtained using noncompartmental analysis (NCA) and compartmental analysis. The K_p,uu,brain values of those compounds increased up to 2.86-fold from 4 to 24 hours. The CL calculated from infusion rate over steady-state concentration from the 24-hour infusion studies was more consistent with the CL from the intravenous bolus studies than that from 4-hour infusion studies (CL avg. fold of difference 1.19-1.44 vs. 2.10). The compartmental analysis using one- and two-compartment models demonstrated better performance than NCA regardless of study design. In addition, volume of distribution at steady state and t _1/2,eff could be accurately obtained by one-compartment analysis within 2-fold difference. In conclusion, both unbound brain-to-plasma ratio and PK parameters can be successfully estimated from a 24-hour intravenous infusion study design. SIGNIFICANCE STATEMENT: We demonstrated that the extent of brain penetration and pharmacokinetic parameters (such as clearance, V_ss, and effective t _1/2) can be determined from a single constant intravenous infusion study in rats.

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

Estimation of unbound drug concentration in the brain (C_u,brain) is an essential part of central nervous system (CNS) drug development. As a surrogate for C_u,brain in humans and nonhuman primates, drug concentration in cerebrospinal fluid (C_CSF) collected by lumbar puncture is often used; however, the predictability of C_u,brain by lumbar C_CSF is unclear, particularly for substrates of the active efflux transporter P-glycoprotein (P-gp). Here, we measured lumbar C_CSF in cynomolgus monkey after single intravenous administration of 10 test compounds with varying P-gp transport activities. The in vivo lumbar cerebrospinal fluid (CSF)-to-plasma unbound drug concentration ratios (K_{p,uu,lumbar CSF}) of nonsubstrates or weak substrates of P-gp were in the range 0.885-1.34, whereas those of good substrates of P-gp were in the range 0.195-0.458 and were strongly negatively correlated with in vitro P-gp transport activity. Moreover, concomitant treatment with a P-gp inhibitor, zosuquidar, increased the K_{p,uu,lumbar CSF} values of the good P-gp substrates, indicating that P-gp-mediated active efflux contributed to the low K_{p,uu,lumbar CSF} values of these compounds. Compared with the drug concentrations in the cisternal CSF and interstitial fluid (ISF) that we previously determined in cynomolgus monkeys, the lumbar C_CSF were more than triple for two and all of the good P-gp substrates examined, respectively. Although lumbar C_CSF may overestimate cisternal CSF and ISF concentrations of good P-gp substrates, lumbar C_CSF allowed discrimination of good P-gp substrates from the weak and nonsubstrates and can be used to estimate the impact of P-gp-mediated active efflux on drug CNS penetration. SIGNIFICANCE STATEMENT: This is the first study to systematically evaluate the penetration of various P-glycoprotein (P-gp) substrates into lumbar cerebrospinal fluid (CSF) in nonhuman primates. Lumbar CSF may contain >3-fold higher concentrations of good P-gp substrates than interstitial fluid (ISF) and cisternal CSF but was able to discriminate the good substrates from the weak or nonsubstrates. Because lumbar CSF is more accessible than ISF and cisternal CSF in nonhuman primates, these findings will help increase our understanding of drug central nervous system penetration at the nonclinical stage.

Applicability of free drug hypothesis to drugs with good membrane permeability that are not efflux transporter substrates: A microdialysis study in rats

In clinical pharmacology, the free drug hypothesis has been widely applied in the interpretation of the relationship between pharmacokinetics and pharmacodynamics (PK/PD). The free drug hypothesis assumes that the unbound drug concentration in blood is the same as that in the site of action at steady state. The objective of this study is to demonstrate whether the free drug hypothesis is universally applicable for all drugs. The unbound concentrations of the 18 compounds in blood and in brain interstitial fluids (ISF) at steady state following constant intravenous infusion were simultaneously monitored up to 6 hours via in vivo microdialysis technique. Based on the permeability and efflux ratio (ER), the test compounds can be divided into two classes. Class I includes the compounds with good membrane permeability that are not substrates of efflux transporters (eg, P-gp, BCRP, and MRPs), whereas Class II includes the compounds that are substrates of efflux transporters. The steady-state unbound drug concentrations in blood, brain, and CSF are quantitatively very similar for Class I compounds, whereas the steady-state unbound concentrations in the brain and CSF are significantly lower than those in blood for Class II compounds. These results strongly suggest that the free drug hypothesis is not universal for all drugs but is only applicable for drugs with good permeability that are not substrates of efflux transporters.

Physiologically Based Modeling Approach to Predict Dopamine D2 Receptor Occupancy of Antipsychotics in Brain: Translation From Rat to Human

Receptor occupancy (RO) is a translational biomarker for assessing drug efficacy and safety. We aimed to apply a physiologically based pharmacokinetic (PBPK) modeling approach to predict the brain dopamine D2 RO time profiles of antipsychotics. Clozapine and risperidone were modeled together with their active metabolites, norclozapine and paliperidone, First, in PK‐Sim a rat PBPK model was developed and optimized using literature plasma PK data. Then, blood‐brain barrier parameters including the expression and efflux transport kinetics of P‐glycoprotein were optimized using literature microdialysis data on brain extracellular fluid (brainECF), which were further adapted when translating the rat PBPK model into the human PBPK model. Based on the simulated drug and metabolite concentrations in brainECF, drug‐D2 receptor binding kinetics (association and dissociation rates) were incorporated in MoBi to predict RO. From an extensive literature search, 32 plasma PK data sets (16 from rat and 16 from human studies) and 23 striatum RO data sets (13 from rat and 10 from human studies) were prepared and compared with the model predictions. The rat PBPK‐RO model adequately predicted the plasma concentrations of the parent drugs and metabolites and the RO levels. The human PBPK‐RO model also captured the plasma PK and RO levels despite the large interindividual and interstudy variability, although it tended to underestimate the plasma concentrations and RO measured at late time points after risperidone dosing. The developed human PBPK‐RO model was successfully applied to predict the plasma PK and RO changes observed after risperidone dose reduction in a clinical trial in schizophrenic patients.

Prediction of Human Brain Penetration of P-glycoprotein and Breast Cancer Resistance Protein Substrates Using In Vitro Transporter Studies and Animal Models

Four P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) substrates with human cerebrospinal fluid (CSF) concentrations and preclinical neuropharmacokinetics were used to assess in vitro-in vivo extrapolation of brain penetration in preclinical species and the ability to predict human brain penetration. Unbound brain (Cb,u), unbound plasma (Cp,u), and CSF compound concentrations (CCSF) were measured in rats and nonhuman primates (NHPs), and the unbound partition coefficients (Cb,u/Cp,u and CCSF/Cp,u) were used to assess brain penetration. The results indicated that for P-gp and BCRP dual substrates, brain penetration was severally impaired in all species. In comparison, for P-gp substrates that are weak or non-BCRP substrates, improved brain penetration was observed in NHPs and humans than in rats. Overall, NHP appears to be more predictive of human brain penetration for P-gp substrates with weak or no interaction with BCRP than rat. Although CCSF does not quantitatively correspond to Cb,u for efflux transporter substrates, it is mostly within 3-fold higher of Cb,u in rat and NHP, suggesting that CCSF can be used as a surrogate for Cb,u. Taken together, a holistic approach including both in vitro transporter and in vivo neuropharmacokinetics data enables a better estimation of human brain penetration of P-gp/BCRP substrates.

Quantification of Bisoprolol and Metoprolol in Simultaneous Human Serum and Cerebrospinal Fluid Samples

Background: Bisoprolol and metoprolol are moderately lipophilic, beta(1)-selective betablockers reported to cause adverse effects in the central nervous system (CNS), such as sleep disturbance, suggesting that both drugs may reach relevant concentrations in the brain. CNS beta(2)-receptor blockade has been suspected to be related to such effects. The higher molecular size of bisoprolol (325 Dalton) and the higher beta(1)-selectivity compared to metoprolol (267 Dalton) would suggest a lower rate of CNS effects. Methods: To address the pharmacokinetic background of this assumption, we quantified to which extent these beta(1)-blockers are able to enter the cerebrospinal fluid (CSF) in 9 (bisoprolol group) and 10 (metoprolol group) neurological patients who had received one of the drugs orally for therapeutic purposes prior to lumbar puncture. We quantified their total concentrations by liquid chromatography/tandem mass spectrometry in paired serum and CSF samples. Results: Median (interquartile range) in CSF reached 55% (47-64%) of total serum concentrations for bisoprolol and 43% (27-81%) for metoprolol, corresponding to 78% (67-92%) and 48% (30-91%) of respective unbound serum concentrations. Conclusion: The extent of penetration of bisoprolol and metoprolol into the CSF is similar and compatible with the assumption that both drugs may exert direct effects in the CNS.

Enhanced Brain Delivery of 2-(Phosphonomethyl)pentanedioic Acid Following Intranasal Administration of Its γ-Substituted Ester Prodrugs

2-(Phosphonomethyl)pentanedioic acid (2-PMPA) is a potent and selective inhibitor of glutamate carboxypeptidase-II (GCPII) with efficacy in multiple neurological and psychiatric disease models, but its clinical utility is hampered by low brain penetration due to the inclusion of multiple acidic functionalities. We recently reported an improvement in the brain-to-plasma ratio of 2-PMPA after intranasal (IN) dosing in both rodents and primates. Herein, we describe the synthesis of several 2-PMPA prodrugs with further improved brain delivery of 2-PMPA after IN administration by masking of the γ-carboxylate. When compared to IN 2-PMPA in rats at 1 h post dose, γ-(4-acetoxybenzyl)-2-PMPA (compound 1) resulted in significantly higher 2-PMPA delivery to both plasma (4.1-fold) and brain (11-fold). Subsequent time-dependent evaluation of 1 also showed high brain as well as plasma 2-PMPA exposures with brain-to-plasma ratios of 2.2, 0.48, and 0.26 for olfactory bulb, cortex, and cerebellum, respectively, as well as an improved sciatic nerve to plasma ratio of 0.84. In contrast, IV administration of compound 1 resulted in similar plasma exposure of 2-PMPA versus the IN route (AUC_IV: 76 ± 9 h·nmol/mL versus AUC_IN: 99 ± 24 h·nmol/mL); but significantly lower nerve and brain tissue exposures with tissue-to-plasma ratios of 0.21, 0.03, 0.04, and 0.04 in nerve, olfactory bulb, cortex, and cerebellum, respectively. In primates, IN administration of 1 more than doubled 2-PMPA concentrations in the cerebrospinal fluid relative to previously reported levels following IN 2-PMPA. The results of these experiments provide a promising strategy for testing GCPII inhibition in neurological and psychiatric disorders.