Introduction

Since the thalidomide disaster, ensuring that new medicines are safe when used therapeutically has been one of the main responsibilities of regulatory agencies. Of course, there is no such thing as 100% safety, much as the public would like reassurance of this. Any medical intervention – or for that matter, any human activity – carries risks as well as benefits, and the aim of drug safety assessment is to ensure, as far as possible, that the risks are commensurate with the benefits.

Safety is addressed at all stages in the life history of a drug, from the earliest stages of design, through pre-clinical investigations (discussed in this chapter) and preregistration clinical trials (Chapter 17), to the entire post-marketing history of the drug. The ultimate test comes only after the drug has been marketed and used in a clinical setting in many thousands of patients, during the period of Phase IV clinical trials (post-marketing surveillance). It is unfortunately not uncommon for drugs to be withdrawn for safety reasons after being in clinical use for some time (for example practolol, because of a rare but dangerous oculomucocutaneous reaction, troglitazone because of liver damage, cerivastatin because of skeletal muscle damage, terfenadine because of drug interactions, rofecoxib because of increased risk of heart attacks), reflecting the fact that safety assessment is fallible. It always will be fallible, because there are no bounds to what may emerge as harmful effects. Can we be sure that drug X will not cause kidney damage in a particular inbred tribe in a remote part of the world? The answer is, of course, ‘no’, any more than we could have been sure that various antipsychotic drugs – now withdrawn – would not cause sudden cardiac deaths through a hitherto unsuspected mechanism, hERG channel block (see later). What is not hypothesized cannot be tested. For this reason, the problem of safety assessment is fundamentally different from that of efficacy assessment, where we can define exactly what we are looking for.

Here we focus on non-clinical safety assessment – often called preclinical, even though much of the work is done in parallel with clinical development. We describe the various types of in vitro and in vivo tests that are used to predict adverse and toxic effects in humans, and which form an important part of the data submitted to the regulatory authorites when approval is sought (a) for the new compound to be administered to humans for the first time (IND approval in the USA; see Chapter 20), and (b) for permission to market the drug (NDA approval in the USA, MAA approval in Europe; see Chapter 20).

The programme of preclinical safety assessment for a new synthetic compound can be divided into the following main chronological phases, linked to the clinical trials programme (Figure 15.1):

Fig. 15.1 Timing of the main safety assessment studies during drug discovery and development.

Regulatory toxicology studies in group (a) include 28-day repeated-dose toxicology studies in two species (including one non-rodent, usually dog but sometimes monkey especially if the drug is a biological), in vitro and in vivo genoxocity tests, safety pharmacology and reproductive toxicity assessment. In vitro genotoxicity tests, which are cheap and quick to perform, will often have been performed much earlier in the compound selection phase of the project, as may safety pharmacology studies.

The nature of the tests in group (b) depends greatly on the nature and intended use of the drug, but they will include chronic 3–12-month toxicological studies in two or more species, long-term (18–24 months) carcinogenicity tests and reproductive toxicology, and often interaction studies involving other drugs that are likely to be used for the same indications.

The basic procedures for safety assessment of a single new synthetic compound are fairly standard, although the regulatory authorities have avoided applying a defined checklist of tests and criteria required for regulatory approval. Instead they issue guidance notes (available on relevant websites – USA: www.fda.gov/cder/; EU: www.emea.eu.int; international harmonization guidelines: www.ich.org), but the onus is on the pharmaceutical company to anticipate and exclude any unwanted effects based on the specific chemistry, pharmacology and intended therapeutic use of the compound in question. The regulatory authorities will often ask companies developing second or third entrants in a new class to perform tests based on findings from the class leading compound.

There are many types of new drug applications that do not fall into the standard category of synthetic small molecules, where the safety assessment standards are different. These include most biopharmaceuticals (see Chapter 12), as well as vaccines, cell and gene therapy products (see below). Drug combinations, and non-standard delivery systems and routes of administration are also examples of special cases where safety assessment requirements differ from those used for conventional drugs. These special cases are not discussed in detail here; Gad (2002) gives a comprehensive account.

Types of adverse drug effect

Adverse reactions in man are of four general types:

Safety pharmacology testing and dose range-finding studies are designed to detect pharmacological adverse effects; chronic toxicology testing is designed to detect dose-related toxic effects, as well as the long-term consequences of pharmacological side effects; idiosyncratic reactions may be revealed in Phase III clinical trials, but are likely to remain undetected until the compound enters clinical use.

Safety pharmacology

The pharmacological studies described in Chapter 11 are exploratory (i.e. surveying the effects of the compound with respect to selectivity against a wide range of possible targets) or hypothesis driven (checking whether the expected effects of the drug, based on its target selectivity, are actually produced). In contrast, safety pharmacology comprises a series of protocol-driven studies, aimed specifically at detecting possible undesirable or dangerous effects of exposure to the drug in therapeutic doses (see ICH Guideline S7A). The emphasis is on acute effects produced by single-dose administration, as distinct from toxicology studies, which focus mainly on the effects of chronic exposure. Safety pharmacology evaluation forms an important part of the dossier submitted to the regulatory authorities.

ICH Guideline S7A defines a core battery of safety pharmacology tests, and a series of follow-up and supplementary tests (Table 15.1). The core battery is normally performed on all compounds intended for systemic use. Where they are not appropriate (e.g. for preparations given topically) their omission has to be justified on the basis of information about the extent of systemic exposure that may occur when the drug is given by the intended route. Follow-up studies are required if the core battery of tests reveals effects whose mechanism needs to be determined. Supplementary tests need to be performed if the known chemistry or pharmacology of the compound gives any reason to expect that it may produce side effects (e.g. a compound with a thiazide-like structure should be tested for possible inhibition of insulin secretion, this being a known side effect of thiazide diuretics; similarly, an opioid needs to be tested for dependence liability and effects on gastrointestinal motility). Where there is a likelihood of significant drug interactions, this may also need to be tested as part of the supplementary programme.

Table 15.1 Safety pharmacology

The core battery of tests listed in Table 15.1 focuses on acute effects on cardiovascular, respiratory and nervous systems, based on standard physiological measurements.

The follow-up and supplementary tests are less clearly defined, and the list given in Table 15.1 is neither prescriptive nor complete. It is the responsibility of the team to decide what tests are relevant and how the studies should be performed, and to justify these decisions in the submission to the regulatory authority.

Exploratory (dose range-finding) toxicology studies

The first stage of toxicological evaluation usually takes the form of a dose range-finding study in a rodent and/or a non-rodent species. The species commonly used in toxicology are mice, rats, guinea pigs, hamsters, rabbits, dogs, minipigs and non-human primates. Usually two species (rat and mouse) are tested initially, but others may be used if there are reasons for thinking that the drug may exert species-specific effects. A single dose is given to each test animal, preferably by the intended route of administration in the clinic, and in a formulation shown by previous pharmacokinetic studies to produce satisfactory absorption and duration of action (see also Chapter 10). Generally, widely spaced doses (e.g. 10, 100, 1000 mg/kg) will be tested first, on groups of three to four rodents, and the animals will be observed over 14 days for obvious signs of toxicity. Alternatively, a dose escalation protocol may be used, in which each animal is treated with increasing doses of the drug at intervals (e.g. every 2 days) until signs of toxicity appear, or until a dose of 2000 mg/kg is reached. With either protocol, the animals are killed at the end of the experiment and autopsied to determine if any target organs are grossly affected. The results of such dose range-finding studies provide a rough estimate of the no toxic effect level (NTEL, see Toxicity measures, below) in the species tested, and the nature of the gross effects seen is often a useful pointer to the main target tissues and organs.

The dose range-finding study will normally be followed by a more detailed single-dose toxicological study in two or more species, the doses tested being chosen to span the estimated NTEL. Usually four or five doses will be tested, ranging from a dose in the expected therapeutic range to doses well above the estimated NTEL. A typical protocol for such an acute toxicity study is shown in Figure 15.2. The data collected consist of regular systematic assessment of the animals for a range of clinical signs on the basis of a standardized checklist, together with gross autopsy findings of animals dying during a 2-week observation period, or killed at the end. The main signs that are monitored are shown in Table 15.2.

Fig. 15.2 Typical protocal for single-dose toxicity study.

Table 15.2 Clinical observations in acute toxicity tests

Single-dose studies are followed by a multiple dose-ranging study in which the drug is given daily or twice daily, normally for 2 weeks, with the same observation and autopsy procedure as in the single-dose study, in order to give preliminary information about the toxicity after chronic treatment.

The results of these preliminary in vivo toxicity studies will help in the planning and design of the next steps of the development programme; they will also help to decide whether or not it is worthwhile to continue the research effort on a given chemical class.

As mentioned above, these toxicology studies are preliminary, and usually they are not sufficient for supporting the first human evaluation of the medicine. Very often, they are not conducted under good laboratory practice (GLP) conditions, nor are they conducted with test material which has been produced according to good manufacturing practice (GMP).

Subsequent work is guided by regulatory requirements elaborated by the International Conference of Harmonization or by national regulatory authorities. Their published guidelines specify/recommend the type of toxicological evaluation needed to support applications for carrying out studies in humans. These documents provide guidance, for example, on the duration of administration, on the design of the toxicological study, including the number of animals to be studied, and stipulate that the work must be carried out under GLP conditions and that the test material must be of GMP standard. The test substance for the toxicology evaluation has to be identical in terms of quality and characteristics to the substance given to humans.