Xenobiotic Metabolism and Disposition

The Design of Studies on Novel Compounds

Harry P.A. Illing

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781000722383
• Edition: 1
• Topic: Medicina
• Subtopic: Farmacologia

This publication explains the principles of the techniques used in disposition and metabolism studies in animals, in man, and in vitro. It includes methods for studying routes and rates of absorption, distribution, metabolism, and excretion of novel compounds and their metabolites. Relevant surgical, whole-body autoradiographic and pharmacokinetic procedures are discussed, together with techniques for separating and identifying metabolites and metabolic path-ways. Information on when the procedures are appropriate, and examples of what they represent, are presented in order to illustrate the value of such studies.

Section 1: Introduction

Chapter 1

THE RATIONALE FOR AND TIMING OF DRUG METABOLISM STUDIES

P. Johnson and R. Metcalf

TABLE OF CONTENTS

I.	Introduction A. Definitions
II.	Drug Discovery
III.	Metabolism Studies in the Research Phase
IV.	Early Preclinical and Clinical Phase A. Preclinical Studies 1. Study Design 2. Analytical Methods 3. Balance-Excretion Studies 4. Pharmacokinetics 5. Tissue Distribution Studies 6. Dose Level and Frequency of Dosing 7. Metabolic Profiles 8. Miscellaneous Studies B. Early Clinical Studies 1. Preliminary Assessments 2. Pharmacokinetics 3. Radiolabeled Metabolism Study
V.	Late Preclinical and Clinical Phase A. Preclinical Studies 1. Long-Term Toxicology 2. Reproductive Toxicology 3. Investigative Toxicology 4. Miscellaneous Studies B. Clinical Studies
VI.	Future
VII.	Conclusion

I. INTRODUCTION

The study of the metabolism of foreign compounds in mammalian systems began as long ago as the 19th century with the discovery of pathways of conjugation and aryl and alkyl hydroxylation. During the next hundred years major advances were made in various laboratories which provided the necessary background against which industrial pharmaceutical companies built their present approaches to drug metabolism studies. Some companies were already active in this area by the 1960s, but it was not until 1968, when the thalidomide tragedy had led to a general tightening up of regulatory requirements of new drugs, that the U.S. government declared its objective of requesting metabolic data prior to licensing new drug products, a move which was followed by regulatory authorities around the world.

Although regulatory authorities have still not subjected drug metabolism to the close scrutiny which they have applied to toxicology, there are now signs of an increased awareness of the value of good drug metabolism information in the assessment of a new product’s safety and efficacy. However, the licensing authorities’ regulations do vary from country to country and do change from time to time. Furthermore, the incorporation of some standard metabolism studies into the drug development process, only as a routine measure to satisfy regulatory authorities, has long been replaced by the use of such studies to contribute to the fundamental knowledge about a drug, its mechanism of action, the optimum dosage regime, and to the discovery of new chemical entities. Thus, this chapter will not consider regulatory issues in the design of drug metabolism studies, but will outline the integration of drug metabolism investigations into programs aimed at the discovery and development of novel drugs.

A. Definitions

In an area which attracts specialists from a wide range of disciplines (biology, pharmacy, biochemistry, pharmacology, chemistry, physics), it is not surprising that there is considerable confusion over terminology. Thus, metabolism is sometimes considered to be a subgroup within the area of pharmacokinetics, whereas to others, pharmacokinetics is the study of rates of processes within the area of metabolism.

For the purposes of this chapter, drug metabolism is understood to be the sum of the processes affecting the fate of drugs in mammalian systems and pharmacokinetics will be used to refer to the rates of all metabolic processes involving foreign compounds. Drug discovery is used to describe the entire process from the initial chemical synthesis of a novel compound through to its successful introduction as a new medicine. Three major phases are identified in this process: research, early preclinical and clinical, and late preclinical and clinical.

II. DRUG DISCOVERY

The process of drug discovery, which may take over 10 years and cost in excess of $100 million, should be considered prior to any discussion of the timing of drug metabolism studies. The activities which are involved in the successful discovery of a novel drug are outlined in Figure 1. No discrete drug metabolism activities are shown in this figure, because, as will become apparent later in this chapter and throughout this volume, drug metabolism studies are conducted during the entire period of drug discovery, from early research studies to late clinical investigations.

For a new drug to be approved for marketing it must have been shown to be a safe and effective therapy in the course of intensive testing in animals and man.

In drug discovery, the research phase represents a period in which chemists and biologists (including biochemists, pharmacologists, etc.) investigate the action of a number of chemicals on biological processes which may have a direct relationship to a diseased state, or may be concerned with the modification of fundamental physiological or biochemical processes remote from potential therapy. Chemical modifications are used to enhance the specificity and/or potency of molecules in a particular test or series of tests, with a view to selecting a limited number of compounds with the desired profile of activities as candidate compounds for development.

FIGURE 1. Schematic diagram of major activities in drug development.

The early preclinical phase is primarily concerned with the evaluation in a number of animal tests of the compound’s toxic potential together with additional studies of mode of action, preparation of commercially viable routes of synthesis, preparation of dosage forms suitable for animal studies and early studies in man, etc. The early clinical phase is designed to assess the safety and efficacy of the compound in man. Relatively small numbers of healthy subjects are used initially before the studies are extended to include patients.

In the late preclinical phase, long-term, chronic toxicology studies are conducted in animals, in order to monitor the effects of repeated exposure to the compound. These studies include daily administration for 6 or 12 months, dependent on the particular regulatory authority, and daily administration to rodents for up to 2 years or longer in carcinogenicity studies. During this period manufacturing and formulation processes are developed to production scale. The late clinical phase includes large-scale patient studies.

Although these three phases have been used in this chapter to categorize various aspects of drug discovery, it should be appreciated that this is done out of convenience and should not be interpreted too literally. Indeed, good development strategies require that each compound be considered on its own merits and studies scheduled accordingly, rather than on the basis of historical precedent or by following standard packages.

Historically, the major industrial commitment to drug metabolism studies has been during the preclinical phase. Once a compound had been selected for development from a discovery program and early toxicology studies had been initiated, metabolism studies were conducted to investigate the absorption, distribution, metabolism, and excretion (familiarly known as ADME) of the compound in animals (usually those species used in the toxicology studies). The climax of these metabolism studies was a study involving the administration of a radiolabeled form of the compound to volunteers in order to study its ADME in man. More recently, there has been an increased involvement of drug metabolism scientists in clinical studies and this trend will continue, with greater examination of the influence of disease on metabolism.

However, experience shows that, while in the research area scientists have become increasingly proficient at identifying novel compounds with attractive pharmacological properties in vitro and in animal models, too many compounds are found to be unacceptable in the course of their preclinical and clinical phases of development. Although there may be many reasons for the high failure rate in these phases, it is arguable that the selection of compounds for development, on the basis of pharmacological activity alone and in the absence of even preliminary metabolic data, has contributed to the relative lack of success.

Thus, it is anticipated that, although preclinical drug metabolism studies will continue to form a major part of the industrial approach, there will be an increased involvement of drug metabolism groups in research as well as in the clinical phases.

III. METABOLISM STUDIES IN THE RESEARCH PHASE

In this phase of drug discovery it is probable that only limited amounts of compound are available and that no radioactive material has been prepared. The emphasis of metabolism studies must therefore be on deriving relevant data using small amounts of material, in order to assist in the selection of one or more compounds from a series for investigation in man or for further development.

In vitro techniques using cell cultures, microsomal, and isolated organ preparations may be used to investigate a number of properties of novel compounds, while requiring minimal material (typically <10 mg of each compound).

The effects of compounds on hepatic enzyme functions (e.g., hydroxylation and dealkylation processes) are now widely used to detect likely enzyme inhibitors. When slightly more material is available these studies may be extended to include screening for enzyme-inducing potential. Unfortunately, the heterogeneity and distribution of (for example) mixed function oxygenase systems have meant that it has been difficult to interpret data from small studies of this type, with the added complication that, even within the same animal species, in vitro and in vivo studies do not always correlate. However, recent advances in applying the principles of molecular biology to drug metabolizing enzyme systems have created the potential for more definitive studies in this area.

Using similar metabolizing systems it is possible to investigate interspecies differences in the metabolism of novel compounds in vitro with a variety of organ systems. For example, hepatocytes or hepatic microsomes from a number of animal species may be used to rapidly assess the metabolic stability of novel compounds and to highlight interspecies differences in metabolism. Such information can be of great benefit in the interpretation of species differences in biological activity and, if extended to include human hepatocytes, can prevent unnecessary animal studies in inappropriate species. In the absence of radiolabeled material or a sensitive, mass-proportional detector, these investigations are limited by the analytical techniques necessary to monitor compound and/or metabolites in biological media. Nevertheless, the combination of, for example, high-pressure liquid chromatography with diode array detection or mass spectrometry promises to greatly extend the usefulness of these studies.

In order to avoid ambiguous information from cell culture work, it is desirable to include some assessments of cell function and viability when novel compounds are first used with cell preparations. These assessments may take the form of using foreign compounds as probes to investigate the viability of a range of metabolic pathways, monitoring protein biosynthesis, glucose metabolism, hormone secretion, membrane leakage, and so on. While no one result taken in isolation can be definitive, a number of complementary tests may be used to demonstrate the biochemical competence of the cultured cells as well as to indicate possible cytotoxic effects of novel compounds.

Other studies likely to be of value in the research phase include those in which passage across membranes is assessed. Thus, intestinal perfusion techniques may be an aid in the selection of compounds with high oral absorption, and the study of the passage of material across the blood-brain barrier will be of value in the selection of compounds with either central activity or no central activity. These investigations inevitably require more material than in vitro studies and also require appropriate analytical techniques, if radiolabeled material is not available.

It should be appreciated that, when carried out in the research phase, all the described studies will produce information which is incomplete in its relevance to drug discovery. While this is no reason to avoid the types of studies described in this section, the results must be interpreted with caution. It is always preferable to study a series of structurally or pharmacologically related compounds, rather than one compound in isolation, so that results may be compared and contrasted between members of the series. In this way it is possible, for example, to select from a series of compounds with similar pharmacological profiles only those compounds which appear (for example) to be metabolically stable or to have no enzyme-inducing or -inhibitory activity.

The goal of metabolism work in the research phase is to contribute to the optimal selection of compounds to be taken into the preclinical phase. Thus, any information on rates and routes of metabolism can provide feedback to pharmacologists and medicinal chemists so as to optimize the structure of the candidate drug.

IV. EARLY PRECLINICAL AND CLINICAL PHASE

The major objectives of all drug development studies carried out in this phase are to establish the toxicological profile and safety margin of the compound...