In Vitro Methods for Estimating Unbound Drug Concentrations in the Brain Interstitial and Intracellular Fluids

Fridén, Markus; Gupta, Anubha; Antonsson, Madeleine; Bredberg, Ulf; Hammarlund‐Udenaes, Margareta

doi:10.1124/dmd.107.015222

Cited by 179 publications

(199 citation statements)

References 37 publications

(33 reference statements)

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“…A potential caveat in using brain tissue homogenate to estimate f u,brain is that homogenization may change binding properties by unmasking binding sites that are not accessible to a drug in vivo. These concerns may be addressed by using a brain slice approach in which the brain structure remains intact Fridén et al, 2007).…”

Section: How To Use K P To Assess Brain Penetration?mentioning

confidence: 99%

Progress in Brain Penetration Evaluation in Drug Discovery and Development

Liu¹,

Chen²,

Smith³

2008

J Pharmacol Exp Ther

165

156

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This review discusses strategies to optimize brain penetration from the perspective of drug discovery and development. Brain penetration kinetics can be described by the extent and time to reach brain equilibrium. The extent is defined as the ratio of free brain concentration to free plasma concentration at steady state. For all central nervous system (CNS) drug discovery programs, optimization of the extent of brain penetration should focus on designing and selecting compounds having low efflux transport at the blood-brain barrier (BBB). The time to reach brain equilibrium is determined by both BBB permeability and brain tissue binding. Rapid brain penetration can be achieved by increasing passive permeability and reducing brain tissue binding. Although many drug transporters have been identified at the BBB, the available literature demonstrates only the in vivo functional importance of P-glycoprotein (P-gp) in limiting brain penetration of its substrates. Drug-drug interactions mediated by P-gp at the BBB are possible due to inhibition or induction of P-gp. For newly identified drug transporters at the BBB, more research is needed to reveal their in vivo significance. We propose the following strategies for addressing drug transporters at the BBB. 1) Drug discovery screens should be used to eliminate good P-gp substrates for CNS targets. Special consideration could be given to moderate P-gp substrates as potential CNS drugs based on a high unmet medical need and the presence of a large safety margin. 2) Selection of P-gp substrates as drug candidates for non-CNS targets can reduce their CNS-mediated side effects.Brain is separated from the systemic circulation by two barriers: the blood-brain barrier (BBB) and the blood-cerebrospinal-fluid barrier (BCSFB). The BBB is composed of cerebral endothelial cells that differ from those in the rest of the body by the presence of extensive tight junctions, absence of fenestrations, and sparse pinocytotic vesicular transport. The BCSFB is formed by a continuous layer of polarized epithelial cells that line the choroid plexus. The BBB and BCSFB exhibit very low paracellular permeability and express multiple drug transporters. These characteristics restrict the entry of hydrophilic compounds or efflux transport substrates into brain (Davson and Segal, 1995). In this review, we will summarize recent published data relevant to assess drug brain penetration and present the authors' opinions on how to effectively address BBB issues in drug discovery and development.

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Section: How To Use K P To Assess Brain Penetration?mentioning

confidence: 99%

Progress in Brain Penetration Evaluation in Drug Discovery and Development

Liu¹,

Chen²,

Smith³

2008

J Pharmacol Exp Ther

165

156

View full text Add to dashboard Cite

show abstract

“…Therefore, alternative methods for estimating C u,brainISF have been developed; these methods combine measuring the amount of drugs in the whole brain, excluding the vascular spaces, (A brain ) with in vitro estimates of the unbound brain volume of distribution (V u,brain ). C u,brainISF is obtained by dividing A brain (mmol/g brain) by V u,brain (mL/g brain) for the drug (Fridén et al, 2009(Fridén et al, , 2007. Similarly, the unbound drug concentration in the plasma is obtained by multiplying the measured total plasma concentration by the estimated unbound fraction in the plasma (f u,p ).…”

Section: Introductionmentioning

confidence: 99%

“…Although the design of drugs restricted to peripheral tissues is an issue for medicinal chemistry, drug efflux at the BBB also needs to be studied in vivo using reliable models . This is generally done in rats by estimating the steady-state unbound brain-to-plasma concentration ratio K p,uu,brain (C u,brainISF /C u,p ) (Fridén et al, 2009(Fridén et al, , 2007Gupta et al, 2006). The K p,uu,brain parameter describes the difference between the concentration of pharmacologically active unbound drug in the brain interstitial fluid (C u,brainISF ) and that in the plasma (C u,p ).…”

Section: Introductionmentioning

confidence: 99%

Improved Measurement of Drug Exposure in the Brain Using Drug-Specific Correction for Residual Blood

Fridén

Ljungqvist

Middleton

et al. 2009

J Cereb Blood Flow Metab

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101

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A major challenge associated with the determination of the unbound brain-to-plasma concentration ratio of a drug (K p,uu,brain ), is the error associated with correction for the drug in various vascular spaces of the brain, i.e., in residual blood. The apparent brain vascular spaces of plasma water (V water , 10.3 lL/g brain), plasma proteins (V protein , 7.99 lL/g brain), and the volume of erythrocytes (V er , 2.13 lL/g brain) were determined and incorporated into a novel, drug-specific correction model that took the drug-unbound fraction in the plasma (f u,p ) into account. The correction model was successfully applied for the determination of K p,uu,brain for indomethacin, loperamide, and moxalactam, which had potential problems associated with correction. The influence on correction of the drug associated with erythrocytes was shown to be minimal. Therefore, it is proposed that correction for residual blood can be performed using an effective plasma space in the brain (V eff ), which is calculated from the measured f u,p of the particular drug as well as from the estimates of V water and V protein , which are provided in this study. Furthermore, the results highlight the value of determining K p,uu,brain with statistical precision to enable appropriate interpretation of brain exposure for drugs that appear to be restricted to the brain vascular spaces.

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“…Several indirect validation studies have been published, such as comparing the projected brain to plasma concentration ratios with the observed ratios, or comparing the unbound brain fraction determined by brain homogenate method with that determined by brain slice method, or by CSF method (Kalvass and Maurer, 2002;Maurer et al, 2005;Becker and Liu, 2006;Liu et al, 2006;Summerfield et al, 2006Summerfield et al, , 2008. Recently, Fridén et al (2007) showed that C ub predicted C m within 3-fold of error for 10 of 15 compounds. In that study, the microdialysis for 14 of the 15 compounds was compiled from the literature, and all the data were generated using different methods or in different species.…”

mentioning

confidence: 99%

Unbound Drug Concentration in Brain Homogenate and Cerebral Spinal Fluid at Steady State as a Surrogate for Unbound Concentration in Brain Interstitial Fluid

Liu

Natta²,

Yeo³

et al. 2008

Drug Metab Dispos

167

141

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ABSTRACT:The objective of the present study was to examine the accuracy of using unbound brain concentration determined by a brain homogenate method (C ub ), cerebral spinal fluid concentration (C CSF ), and unbound plasma concentration (C up ) as a surrogate for brain interstitial fluid concentration determined by brain microdialysis (C m ). Nine compounds-carbamazepine, citalopram, ganciclovir, metoclopramide, N-desmethylclozapine, quinidine, risperidone, 9-hydroxyrisperidone, and thiopental-were selected, and each was administered as an intravenous bolus (up to 5 mg/kg) followed by a constant intravenous infusion (1-9 mg/kg/h) for 6 h in rats. For eight of the nine compounds, the C ub s were within 3-fold of their C m ; thiopental had a C m 4-fold of its C ub . The C CSF s of eight of the nine compounds were within 3-fold of their corresponding C m ; 9-hydroxyrisperidone showed a C CSF 5-fold of its C m . The C up s of five of the nine compounds were within 3-fold of their C m ; four compounds (ganciclovir, metoclopramide, quinidine, and 9-hydroxyrisperidone) had C up s 6-to 14-fold of their C m . In conclusion, the C ub and C CSF were within 3-fold of the C m for the majority of the compounds tested. The C up s were within 3-fold of C m for lipophilic non-P-glycoprotein (؊P-gp) substrates and greater than 3-fold of C m for hydrophilic or P-gp substrates. The present study indicates that the brain homogenate and cerebral spinal fluid methods may be used as surrogate methods to predict brain interstitial fluid concentrations within 3-fold of error in drug discovery and development settings.For drugs with an intended action in the central nervous system it is assumed that unbound drug in brain interstitial fluid is in direct contact or in equilibrium with the site of action (de Lange and Danhof, 2002). Therefore, in preclinical and clinical pharmacokinetic/pharmacodynamic studies, it is critical to determine the concentration in the interstitial fluid for brain-targeted compounds. Unbound plasma concentration (C up ) has been used to represent the unbound concentration in tissue (Wilkinson, 2001). Because the brain is separated from the plasma by the blood-brain barrier (BBB) and the blood cerebrospinal fluid barrier (BCSFB), C up may not represent the interstitial fluid concentration (Davson and Segal, 1995;Hammarlund-Udenaes et al., 2008;Liu et al., 2008).Microdialysis has been considered as a standard approach to measure interstitial fluid concentration (C m ) (Joukhadar and Müller, 2005;Chaurasia et al., 2007). Although this technique has been developed for more than 2 decades, it is primarily used for determination of neurotransmitters and not drug concentrations in the brain. The main limitations of this technique include high resource requirements, low throughput, and special surgical skills to set up the experiment. In addition, many compounds in the discovery stage are often very lipophilic, and it is difficult to apply microdialysis technique to study these compounds because of high nonspecific bindi...

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In Vitro Methods for Estimating Unbound Drug Concentrations in the Brain Interstitial and Intracellular Fluids

Cited by 179 publications

References 37 publications

Progress in Brain Penetration Evaluation in Drug Discovery and Development

Progress in Brain Penetration Evaluation in Drug Discovery and Development

Improved Measurement of Drug Exposure in the Brain Using Drug-Specific Correction for Residual Blood

Unbound Drug Concentration in Brain Homogenate and Cerebral Spinal Fluid at Steady State as a Surrogate for Unbound Concentration in Brain Interstitial Fluid

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