Physicochemical and Biomimetic Properties in Drug Discovery: Chromatographic Techniques for Lead Optimization

Explains how to take full advantage of chromatography to find successful drug candidates Based on a course taught by the author for many years, this book demonstrates how to use chromatography to measure the physicochemical and biomimetic properties of drug candidates in the early stages of drug discovery in order to fully optimize drug leads. Readers will learn how to interpret chromatographic retention data using various models in order to estimate the in vivo distribution behavior of drugs. The property data obtained by these chromatographic measurements can then be used to select and prioritize compounds for further in vitro and in vivo studies at the early stages of the drug discovery process. Physicochemical and Biomimetic Properties in Drug Discovery provides clear explanations and guidance to facilitate reproduction of the chromatographic measurements discussed in the book in any laboratory equipped with HPLC. It begins with a quick review of the drug discovery process. Next, it covers: Drug–likeness and the physicochemical property space of known drugs Basic pharmacokinetic properties Principles and methods of chromatography for the application of property measurements Molecular physicochemical properties that influence absorption and distribution: lipophilicity, solubility, permeability, and acid dissociation constant (p K a ) Models with measured physicochemical and biomimetic chromatographic descriptors: absorption, distribution, and drug efficiency The final chapter is dedicated to applications and examples of the use of chromatography in drug discovery. Throughout the book, real–life examples demonstrate how to optimize the use of chromatography to obtain successful drug candidates. Each chapter concludes with a series of questions, with answers in the appendix, making this book ideal for students as well as researchers in drug discovery.

Preface xi 1 The Drug Discovery Process 1 Summary 7 Question for Review 7 References 7 2 Drug–Likeness and Physicochemical Property Space of Known Drugs 9 Summary 12 Questions for Review 13 References 13 3 Basic Pharmacokinetic Properties 15 Absorption 17 Plasma Protein Binding 20 Distribution 22 Volume of Distribution 23 Unbound Volume of Distribution 29 Half–Life 30 Metabolism and Clearance 30 Free Drug Hypothesis 31 Summary 31 Questions for Review 32 References 33 4 Principles and Methods of Chromatography for the Application of Property Measurements 34 Theoretical Background of Chromatography 35 Retention Factor and Its Relation to the Distribution Constant Between the Mobile and the Stationary Phases 37 Measure of Separation Efficiency 40 Resolution and Separation Time 42 Gradient Elution 44 Applicability of Chromatography for Measurements of Molecular Properties 47 Summary 49 Questions for Review 50 References 50 5 Molecular Physicochemical Properties that Influence Absorption and Distribution—Lipophilicity 52 Partition Coefficient 52 Lipophilicity Measurements by Reversed Phase Chromatography with Isocratic Elution 58 Lipophilicity Measurements by Reversed Phase Chromatography with Gradient Elution 68 Lipophilicity of Charged Molecules—pH Dependence of Lipophilicity 72 Biomimetic Lipophilicity Measurements by Chromatography 78 Comparing Various Lipophilicity Measures by the Solvation Equation Model 90 Summary 102 Questions for Review 105 References 105 6 Molecular Physicochemical Properties that Influence Absorption and Distribution—Solubility 112 Definition of Solubility 112 Molecular Interactions with Water 116 Various Solubility Measurements that can be Applied During the Drug Discovery Process 119 Conditions that Affect Solubility 121 Solubility–pH Profile 132 Solubility and Dissolution in Biorelevant Media 134 Composition of Fasted State Simulated Intestinal Fluid (FaSSIF) 136 Preparation of FaSSIF Solution 136 Composition of Fed State Simulated Intestinal Fluid (FeSSIF) 136 Preparation of FeSSIF solution 136 Summary 143 Questions for Review 146 References 146 7 Molecular Physicochemical Properties that Influence Absorption and Distribution—Permeability 150 Biological Membranes 150 Artificial Membranes 153 Physicochemical Principles of Permeability 155 Experimental Methods to Measure Artificial Membrane Permeability 159 Relationships Between Permeability, Lipophilicity, and Solubility 166 Chromatography as a Potential Tool for Measuring the Rate of Permeation 171 Summary 175 Questions for Review 178 References 178 8 Molecular Physicochemical Properties that Influence Absorption and Distribution—Acid Dissociation Constant—pKa 182 Definition of pKa 182 Methods for Determining pKa 188 Spectrophotometric Determination of pKa 192 Determination of pKa by Capillary Electrophoresis 195 Chromatographic Approaches for the Determination of pKa 197 Summary 207 Questions for Review 209 References 209 9 Models with Measured Physicochemical and Biomimetic Chromatographic Descriptors—Absorption 213 Lipinski Rule of Five 214 Absorption Models with Lipophilicity and Size 217 Biopharmaceutics Classification System (BCS) 221 Absorption Potential—Maximum Absorbable Dose 227 Abraham Solvation Equations for Modeling Absorption 232 Effect of Active Transport and Metabolizing Enzymes on Oral Absorption and Bioavailability 235 Summary 236 Questions for Review 238 References 238 10 Models with Measured Physicochemical and Biomimetic Chromatographic Descriptors—Distribution 242 Models for Volume of Distribution 249 Plasma Protein Binding 276 Blood/Brain Distributions 288 Tissue Distribution 294 Summary 296 Questions for Review 298 References 298 11 Models with Measured Physicochemical and Biomimetic Chromatographic Descriptors—Drug Efficiency 303 Drug Efficiency 303 Summary 327 Questions for Review 327 References 328 12 Applications and Examples in Drug Discovery 330 Structure–Lipophilicity Relationships 330 Structure–Solubility Relationships 346 Structure–Permeability Relationships 354 Structure–Charge State Relationships 362 Structure–Protein Binding Relationships 366 Structure–Phospholipid Binding Relationships 371 Summary 376 Questions for Review 379 References 379 Appendix A Answers to the Questions for Review 387 Appendix B List of Abbreviations and Symbols 427 Index 433

KLARA VALKO is an Investigator in the Analytical Chemistry Department of Molecular Discovery Research at GlaxoSmithKline. Dr. Valko has more than thirty years of experience in HPLC, GC, and LC/MS. She also has more than ten years of experience teaching physicochemical measurements and ADME optimization tools at the UCL School of Pharmacy. Currently, Dr. Valko is an Honorary Professor, leading the Physchem/ADME module for a drug discovery MSc course.