Drug Metabolism and Pharmacokinetics in the Lead Optimisation of Novel Positive Allosteric Modulators of Α1 Strychnine Sensitive Glycine Receptors
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Drug Metabolism and Pharmacokinetics in the Lead Optimisation of Novel Positive Allosteric Modulators of α1 Strychnine Sensitive Glycine Receptors Thesis submitted in accordance with the requirements of the University of Liverpool for the degree of Doctor of Philosophy by Elinor Wylde October 2015 Declaration This thesis is the result of my own work. The material contained in the thesis has not been presented, nor is currently being presented, either wholly or in part for any other degree or other qualification. Elinor Wylde This research was carried out in the Liverpool School of Tropical Medicine and in the Department of Pharmacology at the University of Liverpool Acknowledgments I would like to thank my supervisors Prof. Martin Leuwer and Prof. Paul O’Neill for the opportunity to take part in this research and for all the help and guidance they have given over the course of my PhD. I would like to thank the members of the research group and our collaborators. A special thanks to Dr Chandra Pidathala and Dr Lee Taylor of the Liverpool University, Chemistry Department for sharing their research with me. Thank you to the staff and students at the Liverpool School of Tropical Medicine and in the Department of Pharmacology, who helped me set up my research and provided me with an excellent working environment. Particularly to Dr Ally Shone, who kindly offered to take me under her wing and to whom I am extremely grateful. Big thanks to my family, especially my parents, Kate and John for supporting me over throughout my undergraduate and postgraduate study. Thank you to my Aunt, Lynda who has offered me a place to stay during my studies. Finally, I would like to express gratitude all my friends who have provided me with a great deal of emotional support. To my best friend Laura, who has always been with me from primary school to postgraduate study. Lastly to Emma, Sammy, Tom and Jamie, who have been there to encourage me and have given me some of my favourite moments over the last four years. Dedication This thesis is dedicated to my friends and family Thank you for your support Abstract Chronic pain is a condition that is thought to affect roughly 8 million people in the UK. It is classified as pain that persists for more than 6 months. Chronic pain is commonly associated with depression, insomnia, anxiety and poor quality of life. Many treatments for chronic pain are accompanied by numerous debilitating side-effects, this in combination with insufficient pain relief means that approximately 50% of patients will discontinue their treatment. Most sufferers choose to live with the pain rather than deal with numerous adverse-effects. There is a great need for new therapeutics that are specifically designed to target the underlying mechanisms of chronic pain, therefore providing safer and more effective treatments. One such mechanism is the down-regulation of strychnine-sensitive glycine receptors (SSGRs) localised in the dorsal horn. Glycinergic activity is known to be inhibitory and artificial stimulation can produce analgesia. Positive allosteric modulators acting on α1 SSGRs may able to compensate for the inhibitory glycinergic activity that is reduced in chronic pain. Previous work within the group lead into the identification of propofol analogues designed to be novel positive allosteric modulators of α1 SSGRs. Work presented in this thesis describes the generation and optimisation of these analogues with a focus of drug metabolism and pharmacokinetics. The hit to lead process has resulted in the development of a lead compound that is highly potent at the target, has excellent pharmacokinetic and safety profiles and is able to produce high levels of analgesia in an animal model of neuropathic pain. Abbreviations °C Degrees Celsius Å Angstrom(s) AC Adenylyl cyclase ACN Acrylonitrile Ala Alanine aM Attomolar AUC Area under the curve cAMP Cyclic adenosine monophosphate ClCYP Cytochrome P450 clearance Clint Intrinsic clearance ClogD Calculated distribution coefficient ClogP Calculated partition coefficient cm Centimetre(s) d. H20 Distilled water Da Daltons DMPK Drug metabolism and pharmacokinetics DMSO Dimethyl sulfoxide EC50 Half maximal effective concentration EtOAc Ethyl acetate FA Formic acid FDA Food and Drug Administration g Gram(s) G G-force GABA Gamma aminobutyric acid GABAAR Gamma aminobutyric acid (subtype A) receptor GlyR Glycine receptor HCN Hyperpolarization-activated cyclic nucleotide-gated channel HEPES 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer HLM Human liver microsomes hr(s) Hour(s) IC50 Half maximal inhibitory concentration KD Knock-down animal model Kg Kilogram(s) L Litre(s) LC Liquid Chromatography m/z Mass-to-charge ratio M+/M- Molecular ion MeOH Methanol mg Milligram(s) MgCl2 Magnesium chloride min(s) Minute(s) mL Millilitre(s) mM Millimolar mm Millimetre(s) MS Mass spectrometer mV Millivolt(s) n Sample size NADP+ Nicotinamide adenine dinucleotide phosphate NADPH Reduced nicotinamide adenine dinucleotide phosphate NaOH Sodium hydroxide NET Norepinephrine transporter ng Nanogram(s) nM Nanomolar nm Nanometre(s) NMDA N-methyl-D-aspartate OR Opioid Receptor P p-value PB Phosphate Buffer PBS Phosphate Buffer Saline PK/PD Pharmacokinetic and pharmacodynamic pKa Negative logarithm of the acid dissociation constant pM Picomolar RLM Rat liver microsomes rpm Revolutions per minute s Second(s) SEM Standard error of the mean SERT Serotonin transporter T1/2 Half-life V Voltage/Volt(s) α Alpha β Beta δ Delta κ Kappa μ Mu μg Microgram(s) μL Microliter(s) μM Micromolar Contents Declaration ........................................................................................................... 1 Acknowledgments ................................................................................................ 2 Dedication ............................................................................................................ 3 Abstract ............................................................................................................... 4 Abbreviations ...................................................................................................... 5 Contents............................................................................................................... 8 Chapter 1 : Chronic Pain ....................................................................................... 9 Chapter 2 : The Glycine Receptor as a Drug Target ............................................ 58 Chapter 3 : Lead Optimisation ........................................................................... 96 Chapter 4 : Conclusions ................................................................................... 191 Chapter 5: Experimental .......................................................................................................... 217 Appendix 231 Chapter 1 Chronic Pain Table of Contents 1.1 Pain ......................................................................................................... 11 1.1.1 Nociception ........................................................................................... 11 1.1.2 Inflammatory Pain ................................................................................. 13 1.1.3 Neuropathic Pain ................................................................................... 16 1.1.4 Chronic Pain .......................................................................................... 17 1.2 Managing Chronic Pain ............................................................................ 19 1.2.1 Antidepressants and Anticonvulsants .................................................... 19 1.2.2 Non-Steroidal Anti-inflammatory Drugs and Opioids 22 1.2.3 Adverse Effects ...................................................................................... 26 1.2.4 Gabapentin and Pregabalin ................................................................... 28 1.3 Mechanisms of Chronic Pain .................................................................... 33 1.3.1 Peripheral and Central Sensitisation ..................................................... 33 1.3.2 Disinhibition .......................................................................................... 37 1.4 Novel Therapeutic Targets ....................................................................... 41 1.4.1 M-Current ............................................................................................. 41 1.4.2 Sodium and Calcium Channels ............................................................... 43 1.4.3 Glutamate ............................................................................................. 45 1.4.4 Glycine .................................................................................................. 47 1.5 Project Aims ............................................................................................. 49 1.6 References ............................................................................................... 51 1.1 Pain 1.1.1 Nociception Pain is an unpleasant sensation that occurs when contact is made with noxious stimuli or when tissues, nerves or organs are damaged. Although uncomfortable, pain serves a protective purpose; it warns us of potential dangers and encourages us to guard injuries whilst they heal. Throughout history opium, in varying forms, has been used as a go-to for analgesia (pain relief)1. From the 1800s onwards, analgesic drugs such as morphine and aspirin have become more widely used. Improved understanding