Characterization of Inositol Transporters As a Method for Drug Delivery to the Central Nervous System
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Characterization of Inositol Transporters as a Method for Drug Delivery to the Central Nervous System by Daniela Fenili A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Laboratory Medicine and Pathobiology University of Toronto © Copyright by Daniela Fenili 2010 Characterization of Inositol Transporters as a Method for Drug Delivery to the Central Nervous System Daniela Fenili Doctor of Philosophy Department of Laboratory Medicine and Pathobiology University of Toronto 2010 ABSTRACT A challenge in the treatment of central nervous system (CNS) diseases is the transport of drug candidates into the brain. Inositol stereoisomers have show promise as therapeutic agents for CNS disorders. scyllo-Inositol was an effective prophylactic and therapeutic for Alzheimer’s disease (AD) in TgCRND8 mice, a model of AD. This suggests inositol stereoisomers have excellent CNS bioavailability. They enter the brain through inositol transporters, of which there are three: one hydrogen myo-inositol transporter (HMIT) and two sodium myo-inositol transporters (SMIT1, SMIT2). HYPOTHESIS: Given the high CNS bioavailability of inositol stereoisomers, it may be possible to use inositol transporters to shuttle other compounds into the CNS. OBJECTIVES: 1. To confirm the CNS bioavailability of the two main inositol stereoisomers, myo- and scyllo-inositol, in both TgCRND8 and wild-type mice. 2. To examine inositol transporter expression in the brains, as a function of time and disease pathology, in both groups. 3. To evaluate the flexibility of the inositol transporters for transporting compounds by determining the substrate structural features required for active transport. RESULTS: myo-Inositol and scyllo-inositol accumulated in the brain following oral administration. Disease pathology did not alter baseline inositol levels or uptake. Brain subregional transporter expression was unaltered as a function of age or disease pathology. In vitro cell culture ii experiments found HMIT inactive and therefore not a contender for drug transport. In contrast SMIT1 and SMIT2 were both active and competitive transport assays, revealed distinct criteria for active transport through each system. However, both were stringent in the substitutions to the structure of myo-inositol possible to maintain active transport. CONCLUSION: Active transport through the inositol transporters is very sensitive to changes in the structure of myo- inositol and only conservative changes are possible. Therefore, these transporters would not make effective shuttling systems for drug transport into the brain. iii ACKNOWLEDGEMENTS I would like to thank JoAnne for her help and guidance throughout my PhD, both towards designing and interpreting my experiments and during the preparation of my thesis. I would like to thank my PhD committees, both past and present for their helpful suggestions and questions. I’d like to acknowledge my fellow lab members, both past and present for their help with interpreting and understanding my experiments, as well as for creating a great working environment full of fun, laughter and inappropriate music (you know who you are). Finally, I would like to thank my family and friends for their support and understanding whenever my experiment and thesis took away from our quality time. Without all of your support, this thesis would not have been possible. iv TABLE OF CONTENTS ABSTRACT..................................................................................................................................ii LIST OF TABLES .....................................................................................................................vii LIST OF FIGURES ...................................................................................................................vii LIST OF ABBREVIATIONS.....................................................................................................ix CHAPTER 1 .................................................................................................................................1 Introduction 1.1 Brain Barriers......................................................................................................................2 1.1.1 The blood-brain barrier ................................................................................................2 1.1.2 The Blood-CSF Barrier................................................................................................5 1.2 Strategies for transport of drugs across brain barriers ........................................................6 1.2.1 Barrier Circumvention .................................................................................................7 1.2.2 Barrier Navigation........................................................................................................9 1.3 Inositol Transporters as a Therapeutic Strategy................................................................13 1.3.1 Inositol in Health and Disease....................................................................................13 1.3.2 scyllo-Inositol as a Therapeutic for Alzheimer’s Disease..........................................16 1.3.3 The Inositol Stereoisomers.........................................................................................23 1.3.4 Inositol in Nature........................................................................................................25 1.3.5 Inositol Synthesis and Degradation Pathways ...........................................................25 1.3.6 The Inositol Transporters ...........................................................................................28 1.3.7 Inositol Efflux ............................................................................................................34 1.3.8 Inositol Pools..............................................................................................................35 CHAPTER 2 ...............................................................................................................................36 Rationale, Hypothesis and Objectives 2.1 Rationale ...........................................................................................................................37 2.2 Hypothesis.........................................................................................................................37 2.3 Objectives..........................................................................................................................38 CHAPTER 3 ...............................................................................................................................39 Materials and Methods.............................................................................................................39 CHAPTER 4 ...............................................................................................................................53 myo- and scyllo-Inositol Levels and Equilibrium in the Brain Abstract ....................................................................................................................................54 Introduction..............................................................................................................................55 Results......................................................................................................................................58 Discussion ................................................................................................................................77 CHAPTER 5 ...............................................................................................................................81 Quantification of Inositol Transporter Expression Levels Abstract ....................................................................................................................................82 Introduction..............................................................................................................................83 Results......................................................................................................................................86 Discussion ..............................................................................................................................100 v CHAPTER 6 .............................................................................................................................104 Substrate Structural Requirements for Inositol Transport Abstract ..................................................................................................................................105 Introduction............................................................................................................................106 Results....................................................................................................................................110 Discussion ..............................................................................................................................141 CHAPTER 7 .............................................................................................................................145 Discussion, Conclusions and Future Directions Discussion ..............................................................................................................................146 Conclusions............................................................................................................................162