Central nervous system drug disposition: the relationship between in situ brain permeability and brain free fraction

Unbound brain concentration determines receptor occupancy: a correlation of drug concentration and brain serotonin and dopamine reuptake transporter occupancy for eighteen compounds in rats

It is a commonly accepted hypothesis that central nervous system (CNS) activity is determined by the unbound brain drug concentration. However, limited experimental data are available in the literature to support this hypothesis. The objective of this study was to test this hypothesis by examining the relationship between in vitro binding affinity (K(I)) and in vivo activity quantified as the drug concentration occupying 50% of the transporters (OC(50)) for 18 serotonin (SERT) and dopamine transporter (DAT) inhibitors. In vivo rat OC(50) was determined by autoradiography using [(3)H]N,N-dimethyl-2,2-amino-4-cyanophenylthiobenzylamine and [(3)H](-)-2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane-1,5-napthalenedisulfonate (WIN35,428) as the ligands to assess SERT and DAT occupancy, respectively. The unbound brain concentrations were calculated from total brain concentrations and the unbound brain fraction, which was determined by the brain homogenate method. The in vivo total brain SERT and DAT OC(50) values (mean +/- S.D.) were 408 +/- 368- and 410 +/- 395-fold greater than the K(I) values, respectively. In contrast, the in vivo unbound brain SERT and DAT OC(50) values were only 3.3 +/- 2.1- and 4.1 +/- 4.0-fold different from the K(I) values. Therefore, prediction of the biophase drug concentration by using the unbound brain concentration rather than the total brain concentration results in an approximately 100-fold improvement for the accuracy. In the present study, a 10-fold improvement was also observed by using the unbound plasma concentration rather than the total plasma concentration to predict the biophase concentration in the brain. This study supports the hypothesis that CNS activity is more accurately determined by the unbound brain drug concentration and not by the total brain drug concentration.

Breast cancer resistance protein interacts with various compounds in vitro, but plays a minor role in substrate efflux at the blood-brain barrier

Expression of breast cancer resistance protein (Bcrp) at the blood-brain barrier (BBB) has been revealed recently. To investigate comprehensively the potential role of Bcrp at the murine BBB, a chemically diverse set of model compounds (cimetidine, alfuzosin, dipyridamole, and LY2228820) was evaluated using a multiexperimental design. Bcrp1 stably transfected MDCKII cell monolayer transport studies demonstrated that each compound had affinity for Bcrp and that polarized transport by Bcrp was abolished completely by the Bcrp inhibitor chrysin. However, none of the compounds differed in brain uptake between Bcrp wild-type and knockout mice under either an in situ brain perfusion or a 24-h subcutaneous osmotic minipump continuous infusion experimental paradigm. In addition, alfuzosin and dipyridamole were shown to undergo transport by P-glycoprotein (P-gp) in an MDCKII-MDR1 cell monolayer model. Alfuzosin brain uptake was 4-fold higher in mdr1a(-/-) mice than in mdr1a(+/+) mice in in situ and in vivo studies, demonstrating for the first time that it undergoes P-gp-mediated efflux at the BBB. In contrast, P-gp had no effect on dipyridamole brain penetration in situ or in vivo. In fact, in situ BBB permeability of these solutes appeared to be primarily dependent on their lipophilicity in the absence of efflux transport, and in situ brain uptake clearance correlated with the intrinsic transcellular passive permeability from in vitro transport and cellular accumulation studies. In summary, Bcrp mediates in vitro transport of various compounds, but seems to play a minimal role at the BBB in vivo.

Microdialysis evaluation of atomoxetine brain penetration and central nervous system pharmacokinetics in rats

A comprehensive in vivo evaluation of brain penetrability and central nervous system (CNS) pharmacokinetics of atomoxetine in rats was conducted using brain microdialysis. We sought to determine the nature and extent of transport at the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCB) and to characterize brain extracellular and cellular disposition. The steady-state extracellular fluid (ECF) to plasma unbound (uP) concentration ratio (C(ECF)/C(uP)=0.7) and the cerebrospinal fluid (CSF) to plasma unbound concentration ratio (C(CSF)/C(uP)=1.7) were both near unity, indicating that atomoxetine transport across the BBB and BCB is primarily passive. On the basis of the ratios of whole brain concentration to C(ECF) (C(B)/C(ECF)=170), brain cell (BC) concentration to C(ECF) (C(BC)/C(ECF)=219), and unbound brain cell concentration to C(ECF) (C(uBC)/C(ECF)=2.9), we conclude that whole brain concentration does not represent the concentration in the biophase and atomoxetine primarily partitions into brain cells. The distributional clearance at the BBB (Q(BBB)=0.00110 l/h) was estimated to be 12 times more rapid than that at the BCB (Q(BCB)=0.0000909 l/h) and similar to the clearances across brain parenchyma (CL(ECF-BC)=0.00216 l/h; CL(BC-ECF)=0.000934 l/h). In summary, the first detailed examination using a quantitative microdialysis technique to understand the brain disposition of atomoxetine was conducted. We determined that atomoxetine brain penetration is high, movements across the BBB and BCB occur predominantly by a passive mechanism, and rapid equilibration of ECF and CSF with plasma occurs.

Striatal and extrastriatal D2/D3-receptor-binding properties of ziprasidone: a positron emission tomography study with [18F]Fallypride and [11C]raclopride (D2/D3-receptor occupancy of ziprasidone)

To elucidate the "atypicality" of ziprasidone, its striatal and extrastriatal D2/D3-receptor binding was characterized in patients with schizophrenia under steady-state conditions. These data were compared with striatal receptor occupancy values after single-dose ziprasidone ingestion in healthy controls. [F]fallypride positron emission tomography (PET) recordings were obtained in 15 patients under steady-state ziprasidone treatment at varying time points after the last dose. Binding potentials were calculated for striatal and extrastriatal regions. D2/D3-receptor occupancies were expressed relative to binding potentials in 8 unmedicated patients. In a parallel [C]raclopride-PET study, striatal D2/D3-receptor occupancy was measured in healthy subjects after single oral doses of 40 mg ziprasidone or 7.5 mg haloperidol. Ziprasidone plasma concentrations correlated significantly with D2/D3-receptor occupancies in all volumes of interests. Occupancy in extrastriatal regions was approximately 10% higher than in striatal regions. Half maximal effective concentration values were consistently higher in striatal than in extrastriatal regions (temporal cortex: 39 ng/mL; putamen: 64 ng/mL), irrespective of the time between last dosing and scan. Single ziprasidone doses resulted in higher occupancies exceeding the 95% prediction limits of the occupancy versus plasma concentrations for chronic dosing. Ziprasidone shares moderate preferential extrastriatal D2/D3-receptor binding with some other atypicals. D2/D3-receptor occupancy is rapidly attuning to the daily course of ziprasidone plasma levels, suggesting relatively high intraday variations of D2/D3-receptor binding. The discrepancies between single-dose and steady-state results are important for the future design of dose-finding PET occupancy studies of novel antipsychotics. Single-dose studies may not be totally relied on for final dose selection.

Progress in brain penetration evaluation in drug discovery and development

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.

Carrier-mediated cellular uptake of pharmaceutical drugs: an exception or the rule?

It is generally thought that many drug molecules are transported across biological membranes via passive diffusion at a rate related to their lipophilicity. However, the types of biophysical forces involved in the interaction of drugs with lipid membranes are no different from those involved in their interaction with proteins, and so arguments based on lipophilicity could also be applied to drug uptake by membrane transporters or carriers. In this article, we discuss the evidence supporting the idea that rather than being an exception, carrier-mediated and active uptake of drugs may be more common than is usually assumed - including a summary of specific cases in which drugs are known to be taken up into cells via defined carriers - and consider the implications for drug discovery and development.

Challenges for blood-brain barrier (BBB) screening

Whilst blood-brain barrier permeability is an important determinant in achieving efficacious central nervous system drug concentrations, it should not be viewed or measured in isolation. Recent studies have highlighted the need for an integrated approach where optimal central nervous system penetration is achieved through the correct balance of permeability, a low potential for active efflux, and the appropriate physicochemical properties that allow for drug partitioning and distribution into brain tissue. Integrating data from permeability studies performed incorporating an assessment of active efflux by P-glycoprotein in combination with drug-free fraction measurements in blood and brain has furthered the understanding of the impact of the blood-brain barrier on central nervous system uptake and the underlying physicochemical properties that contribute to central nervous system drug disposition. This approach moves away from screening and ranking compounds in assays designed to measure or predict central nervous system penetration in the somewhat arbitrary units of brain-blood (or plasma) ratios.

On the rate and extent of drug delivery to the brain

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.