ABSTRACT:Thirty-two structurally diverse drugs used for the treatment of various conditions of the central nervous system (CNS), along with two active metabolites, and eight non-CNS drugs were measured in brain, plasma, and cerebrospinal fluid in the P-glycoprotein (P-gp) knockout mouse model after subcutaneous administration, and the data were compared with corresponding data obtained in wild-type mice. Total brain-to-plasma (B/P) ratios for the CNS agents ranged from 0.060 to 24. Of the 34 CNS-active agents, only 7 demonstrated B/P area under the plasma concentration curve ratios between P-gp knockout and wild-type mice that did not differ significantly from unity. Most of the remaining drugs demonstrated 1.1-to 2.6-fold greater B/P ratios in P-gp knockout mice versus wild-type mice. Three, risperidone, its active metabolite 9-hydroxyrisperidone, and metoclopramide, showed marked differences in B/P ratios between knockout and wild-type mice (6.6-to 17-fold). Differences in B/P ratios and cerebrospinal fluid/ plasma ratios between wild-type and knockout animals were correlated. Through the use of this model, it appears that most CNSactive agents demonstrate at least some P-gp-mediated transport that can affect brain concentrations. However, the impact for the majority of agents is probably minor. The example of risperidone illustrates that even good P-gp substrates can still be clinically useful CNS-active agents. However, for such agents, unbound plasma concentrations may need to be greater than values projected using receptor affinity data to achieve adequate receptor occupancy for effect.Active transport mechanisms as determinants of drug absorption, distribution, and clearance have been the focus of considerable research effort over the past decade. Of the numerous transporter proteins recently investigated, the one for which the greatest amount of knowledge exists is P-glycoprotein (MDR1). Originally described as a transporter involved in imparting drug resistance to tumor cells, P-glycoprotein has been demonstrated to be important in reducing absorption of drugs from the intestinal lumen, in active secretion of drugs into urine and bile, and in extrusion of drugs from vital organs such as the brain and reproductive tissues (Troutman et al., 2002). As such, P-glycoprotein-mediated transport has become an important issue in the discovery and development of new drugs. For example, new compounds that are promising with regard to target receptor/ enzyme activity can be severely hampered in their ability to elicit pharmacological effects in vivo should they be good substrates for P-glycoprotein, especially if the route of administration is intended to be oral or the target tissues is one rich in P-glycoprotein activity. Furthermore, the potential for drug-drug interactions arises in the event that the P-glycoprotein substrate is coadministered with another agent that can inhibit P-glycoprotein.Several models have been developed to assess drugs as P-glycoprotein substrates. In vitro models have included the Caco...

This article is available online at http://dmd.aspetjournals.org ABSTRACT:The objectives of this study were to generate a data set of bloodbrain barrier (BBB) permeability values for drug-like compounds and to develop a computational model to predict BBB permeability from structure. The BBB permeability, expressed as permeabilitysurface area product (PS, quantified as logPS), was determined for 28 structurally diverse drug-like compounds using the in situ rat brain perfusion technique. A linear model containing three descriptors, logD, van der Waals surface area of basic atoms, and polar surface area, was developed based on 23 compounds in our data set, where the penetration across the BBB was assumed to occur primarily by passive diffusion The blood-brain barrier (BBB 1 ) consists of a continuous layer of endothelial cells joined by tight junctions at the cerebral vasculature. It represents a physical and enzymatic barrier to restrict and regulate the penetration of compounds into and out of the brain and maintain the homeostasis of the brain microenvironment (Davson and Segal, 1995). Brain penetration is essential for compounds where the site of action is within the central nervous system, whereas BBB penetration needs to be minimized for compounds that target peripheral sites to reduce potential central nervous system-related side effects. Therefore, it is critical during the drug discovery phase to select compounds that have appropriate brain penetration properties. Brain penetration is commonly assessed by two experimental approaches, namely equilibrium distribution between brain and blood and BBB permeability. The equilibrium distribution is defined as the ratio of concentrations in brain and blood (BB, quantified as logBB). LogBB is determined at

ABSTRACT:The present study examined the interaction of four 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (atorvastatin, lovastatin, and simvastatin in acid and lactone forms, and pravastatin in acid form only) with multidrug resistance gene 1 (MDR1, ABCB1) P-glycoprotein, multidrug resistance-associated protein 2 (MRP2, ABCC2), and organic anion-transporting polypeptide 1B1 (OATP1B1, SLCO21A6). P-glycoprotein substrate assays were performed using Madin-Darby canine kidney (MDCK) cells expressing MDR1, and the efflux ratios [the ratio of the ratio of basolateralto-apical apparent permeability and apical-to-basolateral permeability between MDR1 and MDCK] were 1.87, 2.32/4.46, 2.17/3.17, and 0.93/2.00 for pravastatin, atorvastatin (lactone/acid), lovastatin (lactone/acid), and simvastatin (lactone/acid), respectively, indicating that these compounds are weak or moderate substrates of P-glycoprotein. In the inhibition assays (MDR1, MRP2, Mrp2, and OATP1B1), the IC 50 values for efflux transporters (MDR1, MRP2, and Mrp2) were >100 M for all statins in acid form except lovastatin acid (>33 M), and the IC 50 values were up to 10-fold lower for the corresponding lactone forms. In contrast, the IC 50 values for the uptake transporter OATP1B1 were 3-to 7-fold lower for statins in the acid form compared with the corresponding lactone form. These data demonstrate that lactone and acid forms of statins exhibit differential substrate and inhibitor activities toward efflux and uptake transporters. The interconversion between the lactone and acid forms of most statins exists in the body and will potentially influence drug-transporter interactions, and may ultimately contribute to the differences in pharmacokinetic profiles observed between statins.

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.

ABSTRACT:The present study evaluates the impact of P-glycoprotein (P-gp) on plasma-brain disposition and transepithelial transport of sedating versus nonsedating H1-antagonists using multidrug-resistant (mdr) gene 1a and 1b (mdr1a/b) knockout (KO) mice and human MDR1-transfected Madin-Darby canine kidney (MDCK) cells. Three nonsedating (cetirizine, loratadine, and desloratadine) and three sedating (diphenhydramine, hydroxyzine, and triprolidine) H1-antagonists were tested. Each compound was administered to KO and wild-type (WT) mice intravenously at 5 mg/kg. Plasma and brain drug concentrations were determined by liquid chromatography-mass spectrometry analysis. Mean pharmacokinetic parameters (CL, V ss , and t 1/2 ) were obtained using WinNonlin. In addition, certirizine, desloratadine, diphenhydramine, and triprolidine (2 M) were tested as substrates for MDR1 using MDR1-MDCK cells. The bidirectional apparent permeability was determined by measuring the amount of compound at the receiving side at 5 h. The brain-to-plasma area under the curve (AUC) ratio was 4-, 2-, and >14-fold higher in KO compared with WT mice for cetirizine, loratadine, and desloratadine, respectively. In contrast, the brain-to-plasma AUC ratio between KO and WT was comparable for hydroxyzine, diphenhydramine, and triprolidine. Likewise, the efflux ratio between basolateral to apical and apical to basolateral was 4.6-and 6.6-fold higher in MDR1-MDCK than the parental MDCK for certirizine and desloratadine, respectively, whereas it was approximately 1 for diphenhydramine and triprolidine. Our results demonstrate that sedating H1-antagonists hydroxyzine, diphenhydramine, and triprolidine are not P-gp substrates. In contrast, nonsedating H1-antagonists cetirizine, loratadine, and desloratadine are P-gp substrates. Affinity for P-gp at BBB may explain the lack of central nervous system side effects of modern H1-antagonists.Antagonists of H1 histamine receptors (H1-antagonists) are the mainstays of treatment for a number of allergic disorders, particularly rhinitis, conjunctivitis, dermatitis, urticaria, and asthma. Two generations of H1-antagonists have been developed so far. The first generation H1-antagonists such as diphenhydramine (Benadryl), triprolidine (Actifed), or hydroxyzine (Atarx) produce histamine blockade at H1-receptors in the central nervous system (CNS 1 ) and frequently cause somnolence or other CNS adverse effects (Simons, 1999). Therefore, the first generation H1-antagonists are also referred to as sedating antihistamines. The second generation H1-antagonists such as cetirizine (Zyrtec), loratadine (Claritin), fexofenadine (Allegra), or desloratadine (Clarinex) represent an advance in therapeutics; in manufacturers' recommended doses, they produce relatively little somnolence or other CNS side effects (Kay and Harris, 1999). Therefore, the second generation H1-antagonists are frequently referred as nonsedating antihistamines. Evidence for this improvement in tolerance profile resulting from reduced CNS penetration has been li...